mechanical seminar low heat engine

Low Heat Rejection Engine for Automotive Use - An Inceptive Study

ABSTRACT

An unconventional low heat rejection (LHR) engine design is described. Previous research efforts into LHR concepts, including high temperature materials and tribology, are reviewed. The continued reliance on conventional compression ignition designs is identified as a significant roadblock. Therefore, a set of design criteria is established, and an original design is formulated. The powerplant consists of an independent positive displacement compressor and expander, a high pressure combustor, and a compressed air accumulator. Compatibility with ceramic materials and high temperature combustion were primary goals. Inherent characteristics include full torque from zero RPM, enhanced combustion, improved efficiency under part load operation, and increased volumetric and thermodynamic efficiencies. These potential improvements warrant the design and construction of a prototype for further study.

INTRODUCTION

This paper describes the initial, inceptive design of an optimized low heat rejection engine. Included are a discussion of past LHR engine research, a brief description of the deficiencies and roadblocks which were uncovered, and a discussion of an avenue toward progress in LHR design. A set of basic criteria is established which supports the original design of an unconventional powerplant. Next, some relevant advances in material science are discussed, including ceramics and high temperature tribology. Lastly, the structure of the design is presented and described.

This design will more fully utilize the potential of the LHR concept and advanced ceramic materials. Additionally, the design provides for maximum torque from rest, efficient part load operation, enhanced volumetric efficiency, and improved combustion.

A follow-up to this inceptive study would include detailed computer modeling of the thermodynamic and gas flow process. Specific materials should be selected, and a more detailed design should be developed to aid in the construction of a working prototype.

BACKGROUND

THE LOW HEAT REJECTION ENGINE CONCEPT is not new. Engine designers have long been aware that both maximizing the temperature of combustion and minimizing the losses of combustion energy prior to the expansion cycle will increase thermodynamic efficiency. However, material limitations have restrained the development of such interests. The development and introduction of advanced ceramic materials in the past few decades has enabled low heat rejection research to progress, principally in the field of compression ignition engines.

In 1975, the U.S. Army Tank-Automotive Command (TACOM) and Cummins Engine Company began an “adiabatic” engine program [1], and significant progress has been made. [2,3] In addition, Isuzu Motors [4,5], Ford Motor Company [6,7], General Motors Corporation [8], NKG Insulators/Mitsubishi Motors Corporation [9], Integral Technologies [10,11], Massachusetts Institute of Technology [12], and others have conducted LHR engine experimentation. Most of these studies investigated modified conventional direct injection diesel engines.

Although promising, the results of the above investigations have been somewhat mixed. While most have concluded that insulation reduces heat transfer, none have produced exceptional gains in efficiency or performance. In fact, a few have shown that the addition of insulation and the elimination of cylinder coolant flow, degrades performance and increases cylinder wall heat flux. [12, 13] A helpful discussion of several of these studies, which also included several numerical simulations not discussed here, was done by Union College and Oak Ridge National Laboratory. [14]

An observational study of LHR combustion using a high speed camera was done by Tiajin University. It was found that ignition delay following fuel injection was reduced, impairing air-fuel mixing, and ultimately prolonging combustion duration. [15] A reduced fuel injector nozzle orifice diameter or the employment of special impingement plates to enhance air-fuel mixing was suggested.

THE SHORTCOMINGS OF LHR RESEARCH are mainly the result of the improvisation of conventional engines to LHR designs. The mere substitution of ceramic components or the addition of insulating coatings fails to account for increased combustion temperatures and an altered combustion process. [14] There is no inherent utilization of the resulting retained heat energy. Consequently, complex turbocompounding devices or bottoming cycles must be employed to capture some portion of the retained heat energy from the exhaust stream.

In addition, conventional piston-crank designs produce piston side-wall stresses which make the direct substitution of ceramic for metallic materials troublesome. New designs which reduce piston side forces, or slapping forces, would be more compatible with ceramics. In a LHR review done by West Virginia University it was concluded that, “It may be helpful to return to the basics of engine design in formulating models of LHR engines.” [16] This is the fundamental principle which this paper will address.

While “returning to the basics” of engine design for the purpose of accommodating LHR concepts, this paper will next suggest factors which further optimize this powerplant with regard to its compatibility with its intended application, the passenger vehicle.

DESIGN CRITERIA

Below are several sets of design criteria which are the framework for the design process producing the engine described in this paper. While any engineering endeavor is fraught with compromises and undesirable practicalities, no individual criterion has been completely eliminated at the expense of another in this design.

CRITERIA SATISFYING TO THE VEHICLE OPERATOR which will produce a marketable product:

• Quiet operation.

• Smooth operation, lacking in vibration.

• Quick response to control input.

• Ample torque and power.

• High reliability and durability.

• Ease of operation and maintenance.

ECONOMIC CRITERIA pertaining to cost effective production and maintenance while in service:

• Simple in design.

• Manufactured from the minimum amount of materials by economical methods.

• Modular and/or compact in construction for ease of repair and ease of placement in compact engine

compartments.

• Long service life.

PASSENGER VEHICLE COMPATIBILITY CRITERIA relating to typical automotive power

requirements and driving cycles:

• High torque at low speeds and less at high speeds.

• Efficient part load operation.

• Operable at temperatures between -30 to

50 °C, and unaffected by altitude.

EFFICIENCY CRITERIA pertaining to fuel consumption and to the challenges we face with our

environment:

• Multi-fuel capable.

• Highly fuel efficient.

• Low in emissions of incomplete combustion products.

These criteria bear upon the basic design of any automotive power plant. However, the wealth of

practical knowledge contained in the above cited references identifies several key pitfalls. These can be

utilized as additional, invaluable criteria which must be avoided, or accounted for in the design process.

• Low tolerance of ceramic materials to mechanical stress and thermal shock.

• Failure of liquid lubricants at high temperatures.

• Degraded combustion process due to high temperatures.

• Reduced volumetric efficiency due to increased component temperatures encountered during

induction.

CERAMIC MATERIALS

Two major obstacles of the LHR engine are component strength and tribology at high temperatures. Where conventional metals and lubricants fail to perform at elevated temperatures, advanced ceramic materials provide an alternative. These materials have provided the major impetus to LHR research and development in recent decades. High hardness, high elastic moduli, resistance to corrosion and wear, strength at high temperatures, and often low thermal conductivity are physical properties of ceramics. Principal substances of interest include nitrides and carbides of silicon (Si3N4 and SiC); oxides of chromium, aluminum, and iron (Cr2O3, A1203, and Fe203); and partially stabilized oxide of zirconium, (Zr02, or PSZ).

Low ductility, low tensile strength, and low bending strength have impeded the direct replacement of metals with ceramics in conventional engine designs. Conventional piston and cylinder stresses make the application of ceramics extremely challenging. Large piston ring loading forces produce large stresses and large friction forces. To reduce these forces, modifications to piston and connecting rod mechanics would decrease the demands which are currently placed upon tribological research.

Both monolithic ceramic components and ceramic coatings have been used by various LHR engine researchers. The work done by Adiabatics and the U.S. Army makes extensive use of titanium and ceramic coated steel components. Compared to monolithic ceramics, coatings “can be attributed to lower cost, better reliability and durability, and the ease in design of the ceramic coated diesel engine.” [17] A 100 hour endurance test of a partially cooled prototype was successfully completed. Liquid lubricant and a “self- cleaning” top piston ring were utilized. [3] Subsequent to this test, solid lubricant top piston rings are under development. [18]

Most experimental LHR diesel engines to date employ liquid lubricants. Although the performance of liquid lubricants in the above studies has been improved, their continued use will severely limit further increases in combustion temperatures. The design developed in this paper will utilize expander temperatures of 1200 to 1500

°C, precluding the use of any known liquid lubricant. In fact, one of the design goals is to develop a structure that could operate with ceramic tribolical surfaces in the absence of liquid lubrication. Therefore, an investigation into other means of lubrication is essential.

HIGH TEMPERATURE TRIBOLOGY

The best liquid lubricants decompose at temperatures in excess of 350 °C. Therefore, a piston/cylinder tribological system operating in an environment in excess of 1000 °C will need to utilize some other form of lubrication. Three alternatives are discussed below: solid lubricants, solids formed via gaseous dissociation, and gaseous lubricants.

SOLID LUBRICATION has been widely investigated by material scientists and LHR diesel researchers. In the absence of a liquid transport medium, the solid lubricant must be included in the piston ring or cylinder liner materials. Such materials are often referred to as self-lubricating composites. Significant research efforts have been undertaken by Adiabatics Inc., et al. [18,19], the Midwest Research Institute [20], and Battelle. [21 ,22,23] Recent work done by Battelle [24] describes the successful application of a well known “Boes” compact to temperatures of 850 °C. [25,26] The NASA Lewis Research Center has shown the effectiveness of coatings of some oxides and fluorides to 900 °C. [27,28] Nickel/molybdenum-bonded titanium carbide cermets [29] and nickel/molybdenum- bonded chromium carbide cermets [30] have also shown to be effective. Additional research must be directed toward the development of the most promising materials.

DISLOCATION OF CARBONACEOUS GASES has been presented as a means to continuously replenish a lubricating carbon on metallic or ceramic surfaces undergoing friction. [31,32] “Graphitic” carbon on nickel-containing and Sialon (Si3N4 +10% A1203) surfaces are produced in an ethylene gas atmosphere at 350 to 650 °C. Coefficients of friction well below 0.1 were observed. However, it is questionable whether such techniques could be applied to this design. The high temperature pre-combustion which is utilized would reduce any carbonaceous gases to carbon dioxide prior the expansion cylinders.

GASEOUS LUBRICATION offers the greatest potential for low friction at high temperatures. A ringless piston/cylinder with very small and precise clearances would allow small amounts of combustion gases to pass, forming a gaseous film. Although the low viscosity of such a film will not support large loads, significant reductions in friction are probable. Major impediments exist to implementation of gaseous lubrication in conventional piston/crank engines. However, due to its structure, this optimized LHR design is particularly suited to gas lubrication.

First, main piston side forces have been greatly reduced by the employment of a crosshead piston and cylinder. Second, the combustion products which blow- by the piston, producing the gaseous film, would not escape the main cylinder. There is no issue of contamination of a crankcase lubricant. Third, thermal cycling is reduced due to the pre-combustion, external to the cylinder. The expansion cylinder will only be exposed to products of combustion, and not to cool atmospheric air. Therefore, tight tolerances can me maintained.

A low friction, unlubricated diesel engine was investigated by S. Timoney at the University College of Dublin. [33] It consisted of a ringless, valveless, opposed piston, two stroke diesel engine constructed of silicon carbide. Although the study was rather limited, it successfully demonstrated the operation of a piston/cylinder lubricated by a gaseous film. A second investigation into this same design was conducted with similar results. [34]

Regardless of the type of in-cylinder lubrication, the crosshead shaft seal must utilize a solid lubricant. Refer to expander description below. It is a relatively small interface to seal, and will only experience linear sliding forces. Therefore, the application of a solid lubricant will be less involved.

POWERPLANT STRUCTURE

The powerplant consists of a several separate subsystems. Figure 1 illustrates a schematic of the engine design, including a positive displacement compressor, a compressed air accumulator, a fuel combustor (or burner), and a positive displacement expander. It is similar in design to George Brayton’s (1 839-1 892) “Ready Motor” gas engine. [35,36] Brayton’s engines were commercially available gas or oil burning “hot air” designs from which the Brayton, or isobaric combustion, thermodynamic cycle originated. Various engines containing some of the features which will be described below may be found in numerous patents. Recent investigations into a coal burning engine by West Virginia University have produced a related design. [37] However, all of the above designs have lacked critical components or the materials required to become successful.

THE COMPRESSOR is a conventional, positive displacement piston device. It is driven by the expander via a suitable fixed-ratio, power transmission, but is a separate device. Attention is given to isolate the compressor from the high temperatures present in the burner and expander. Actuated poppet type valves contribute to complexity and cost, but also reduce noise and improve air flow when compared to standard air compressor reed type valves. A conventional air or liquid cooling system will be employed to maintain low compressor temperatures and enhance volumetric efficiency. The amount of heat rejected will be significantly less than that rejected from an IC engine cooling system. For a prototype powerplant, a small motorcycle engine could be modified to operate as a compressor. The intake to the compressor may be throttled to vary induction air flow. Induced air flow will modulate to maintain the desired compressed air accumulator pressure. The additional work encountered during throttling, which is required to induce air at pressures below atmospheric, may be eliminated by eliminating the throttle. Alternatively, intake valve timing may be varied or individual cylinders may simply be idled. Any of the above techniques will significantly reduce part load compressor work, increasing the overall, part load efficiency.

AN INSULATED ACCUMULATOR will receive and briefly store the compressed air supply. All losses of heat energy from the compressed air stream will be minimized after exiting the compressor. The hot, compressed air will flow from the accumulator via a main throttle to the burner. The accumulator will be the largest component in the powerplant. It is not practical to construct an accumulator with sufficient volume to allow for prolonged operation without a functioning compressor or to allow for regenerative braking. However, it must be of sufficient volume to provide an energy reserve to the burner/expander during acceleration while the induction controls respond and maintain pressure. For example, a 5 second reserve serving an expander at 0.02 kg/sec would require 0.1 kg of compressed air. If a volumetric compression ratio of 6 and a compressor efficiency of 85% are assumed, an accumulator pressure of approximately 1300 kPa is produced. If the 5 second air flow is allowed to isothermally reduce the accumulator pressure by 20% to 1040 kPa, a volume of 0.065 m3 is required. A 2.5 m long cylinder with a diameter of 18 cm contains a similar volume. Higher compression ratios, reduced mass flow rates, or larger acceptable pressure drops will reduce accumulator size requirements. This is a substantial volume for an insulated, mobile pressure vessel. However, it is this energy reserve which will produce the favorable torque versus engine speed required of a typical passenger vehicle. Full torque is instantly available at zero RPM, and less is produced at higher engine speeds. Relatively slower operating speeds will be seen, and idling requirements are reduced or eliminated. Additionally, power transmission requirements are greatly reduced. This characteristic result of reserve power is common to electric motors and steam engines.

Possible locations for the large accumulator are the engine compartment, somewhere in the rear of the vehicle, or in an elongated form underneath the vehicle similar to a driveshaft. The independent compressor and expander arrangement make the use of an accumulator possible, and the torque/power requirements of the typical passenger vehicle make an accumulator essential.

THE BURNER AND FUEL INJECTION SYSTEM will combust a variable flow of compressed air with a high pressure fuel supply. The structure is similar in design to a small gas turbine combustor. For a prototype powerplant, it would be practical to modify and utilize a small gas turbine combustion system. However, the ignition system must be capable of reliable and repeated re-lighting. This is a condition not encountered by gas turbines. The final burner design will utilize ceramic materials throughout to allow exit temperatures of 1200 to 1500°C.

With the high combustion temperatures, the formation of nitrogen dioxide (NO2) increases exponentially. Increased residence time of the combustion products within the combustor provides complete combustion, but it also contributes to NO2 formation. An effective after-treatment with a reducing catalyst or the limitation of maximum flame temperatures must be investigated.

THE EXPANDER consists of an insulated cylinder housing a double acting piston. The cylinder walls and heads are constructed to retain as much heat as possible. Insulating air gaps within the cylinder wall will provide additional insulation. An appropriate tribolical system encompassing piston rings (if used), the cylinder wall, and crosshead shaft seal will provide dry, high temperature lubrication as discussed above. The absence of liquid lubricants has eliminated the need for an oil sump and accompanying vertical piston/cylinder arrangement. This permits various cylinder arrangements which are not feasible with conventional piston over oil sump designs. Spatial efficiency may be improved, particularly in the flexibility of component placement. As discussed above, hot gas blow-by passing the piston will be contained within the opposite end of the cylinder and not vented. A ventilation system, such as crankcase ventilation, for emissions control is unnecessary, nor is there an issue of oil sump contamination of combustion products. The crosshead shaft passes through one cylinder head to a crosshead piston/cylinder via a packing gland as shown in Figure 2. The packing material will be a solid lubricant as discussed above. This arrangement will remove the majority of lateral piston/cylinder forces from the high temperature expander.

The crosshead is removed from the high tribolic system and contains them at the crosshead temperature combustion products and may be conventionally lubricated in conjunction with a conventional crankarm and crankshaft power take-off.

Figures 1 and 2 illustrate poppet style valves on the expander intake and exhaust ports. The wide apertures attainable via poppet valves make them a good choice, but the use of rotary or sliding type valves should not be overlooked.

As the maximum cycle temperature is increased, the powerplant thermodynamic efficiency will increase, as will the specific work output. This temperature will be seen at the exit of the burner, throughout the expander intake manifold, at the intake valves and valve guides, and into the expander itself. The materials used for these components, and possibly NO2 control, will dictate the maximum allowable temperature. Once selected, this temperature may then be used to find an appropriate volumetric compression ratio. A computer simulation utilizing the standard Brayton cycle was performed by this author in order to maximize the specific work output of an engine with respect to the compression ratio. Utilizing a 1200 to 1500 °C maximum cycle, assuming compressor and expander efficiencies of 85 percent, and neglecting heat rejection, a volumetric compression ratio of 6 to 8 was derived. Further increases in the compression ratio will incrementally increase the compressor to expander back-work ratio, decreasing overall specific work output. The simulation also assumed that the volumetric expansion ratio was equal to the compression ratio. In reality, this is not required. The ability to increase the expansion ratio independently of the compression ratio has the potential to more fully expand the products of combustion. This concept is well known in conventional engine design, the Atkinson cycle, and has actually been used to improve efficiency. An engine made by James Atkinson (1 846-1 91 4) in England was first to utilize this concept.

Another consideration in the selection of compression ratio is the potential use of exhaust gas regeneration, and an accompanying potential for further increases in cycle efficiency. While the previously discussed improvements in efficiency without regeneration are sufficient to justify continued development of this design, a brief discussion of regeneration is warranted. As the compression ratio increases, the compressor exiting temperature also increases. It is this gas stream which must accept the heat energy from the hot exhaust gases. The higher the compressed air temperature, the lower the potential for heat transfer from exhaust gases. Therefore, for a given, fixed maximum cycle temperature, a lower compression ratio will facilitate regeneration. However, a reduction in compression ratio will reduce the specific work output. An increased mass flow rate, and hence increased cylinder volume, must be then be utilized to maintain an equivalent net work output. The added complexity and cost of a regenerator will be a major consideration as well. Finally, one last option is an integral accumulator/regenerator design. A regenerator is a heat exchanger requiring large surface areas in order to be effective. An accumulator could be developed to contain an integral heat exchanging conduit through which exhaust gases could pass, providing a regenerative effect.

POWERPLANT CONTROL

Several control sequences will operate independently. Induction, power, and combustion control are briefly described below.

INDUCTION will vary as required to maintain the desired accumulator pressure. As the compressor operates via a fixed transmission ratio with the expander, the intake to the compressor will be throttled to modulate induction. Part load compressor work will be reduced in proportion.

ENGINE POWER will be delivered in response to an operator input. Figure 1 illustrates a main throttle at the inlet to the burner for modulation of compressed air flow. Alternatively, the passage between the accumulator and the burner could be left unrestricted, and compressed air flow could be controlled via a variable intake valve period. This approach would provide the full accumulator pressure at the intake valves, and losses associated with throttling would be avoided. The art of steam engine design has dealt with this issue extensively. It was previously termed “variable cut-off”. A good comparison of flow control methods was made by GM Research Labs with regard to the SE- 101 steam powerplant [38].

A contrast between these two control methods can be seen via their ideal indicator diagrams. Figure 3a illustrates an ideal diagram of a throttled expander. Both full and part load conditions rather fully expand the combustion products. The associated throttling losses are not shown. Figure 3b illustrates an ideal diagram with variable intake period control (variable valve timing). During full load, a substantial portion of the expansion process is wasted, while during part load, expansion is more complete. Since the majority of automotive powerplant operation is part load, this is a significant concern. However, the added complexity of a variable valve mechanism may be the overriding factor.

COMBUSTION will be controlled by the modulation of fuel flow to the burner. Ideally, fuel would be delivered in direct proportion to the compressed air flow. Two methods for determining the compressed air flow rate are possible. First is the direct measurement of flow via a high temperature, high pressure mass flow sensor. The availability of such a sensor “off the shelf” is questionable. Therefore, the empirical calculation via more readily measurable operating parameters such as throttle position, accumulator pressure and temperature, and engine speed would be more practical.

With semi-continuous combustion, very lean air/fuel ratios are possible. The main limitation on excess air would be satisfactory ignition and re-ignition under all operating conditions. A wide variety of fuels could be utilized. The increased time allowed for combustion, in the confines of a high temperature burner and expander, allows for complete oxidation of the fuel. Although complete combustion of hydrocarbons is possible, the high temperatures will surely increase the formation of NO₂. Attention must be given to maintaining acceptable levels in the exhaust products.

CONCLUSIONS

This exercise in engine design has contrived a powerplant design that takes advantage of continuous high temperature combustion and advances in ceramic materials. The potential of the LHR concept can be incorporated more readily if an adherence to conventional engine structures and designs is forgone.

A preliminary design has been developed with the “potential” for the following:

• Smooth and quiet power delivery due to non- impulsive combustion, relatively slower engine speeds,

and a favorable torque response.

• Enhanced part load efficiency due to variable compressor induction.

• Reduced fuel consumption due to lean air/fuel ratios.

• Improved combustion efficiency due to a semi-continuous combustion prior to the expander.

• Increased volumetric efficiency due to reduced induction component temperatures.

• Increased thermodynamic efficiency due to more complete expansion of combustion products.

• Increased thermodynamic efficiency due to increased combustion temperatures.

Although many issues with the application of materials remain, this design merits further

development. The combined effect of the above improvements could produce a powerplant with

significant reductions in emissions of incomplete combustion products as well as significant

improvements in

fuel economy.

REFERENCES

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[2] P. Badgley, R. Kamo, W. Bryzik, and E. Schwarz, “NATO Durability Test of an Adiabatic Truck Engine”; SAE Paper 900621, Detroit, Ml (1990)

[3] M. Woods, W. Bryzik, and E. Schwarz, “100 Hour Endurance Testing of a High Output Adiabatic Diesel Engine”; SAE Paper 940951, Detroit, Ml (1994)

[4] H. Kawamura and H. Matsuoka, “Low Heat Rejection Engine with Thermose Structure”; SAE Paper 950978, Detroit, Ml (1995)

[5] H. Kawamura, “Development Status of Isuzu Ceramic Engine”; SAE Paper 880011, Detroit, Ml (1988)

[6] W.R. Wade, P.H. Havstad, V.D. Rao, M.G. Aimone, and C.M. Jones, “A Structural Ceramic Diesel Engine - The Critical Elements”; SAE Paper 870651, Detroit, Ml (1987)

[7] P.H. Havstad, l.J. Garwin, and W.R. Wade, “A Ceramic Insert Uncooled Diesel Engine”; SAE Paper 860447, Detroit, Ml (1986)

[8] A.C. Alkidas, “Performance and Emissions Achievements with an Uncooled Heavy-Duty Single-Cylinder Diesel Engine”; SAE Paper890144, Detroit, Ml (1989)

[9] Y. Miyairi, T. Matsuhisa, T. Ozawa, H. Oikawa, and N. Nakashima, “Selective Heat Insulation of Combustion Chamber Walls for a Dl Diesel Engine with Monolithic Ceramics”; SAE Paper 890141, Detroit, Ml (1989)

[10] T. Morel, S. Wahiduzzaman, E.F. Fort, D.R. Tree, D.P. Dewitt, and K.G. Kreider, “Heat Transfer in a Cooled and an Insulated Diesel Engine”; SAE Paper 890572, Detroit, Ml (1989)

[11] T. Morel, S. Wahiduzzaman, E.F. Fort, “Heat Transfer Experiments in an Insulated Diesel”; SAE Paper 8801 86, Detroit, Ml (1988)

[12] W.K. Cheng, V.W. Wong, F. Gao, “Heat Transfer Measurement Comparisons in Insulated and Non-Insulated Diesel Engines”; SAE Paper 890570, Detroit, Ml (1989)

[13] G. Woschni, W. Spindler, and K. Kolesa, “Heat Insulation of Combustion Chamber Walls – A Measure to Decrease Fuel Consumption of IC. Engines?”; SAE Paper 870339, Detroit, Ml (1987)

[14] C.S. Reddy, N. Domingo, R.L. Graves, “Low Rejection Engine Research Status: Where Do We Go from Here?”; SAE Paper 900620, Detroit Ml (1990)

[15] Y. Wang, C. Yang, G. Shu, Y. Ju, and K. Zhao, “An Observation of High Temperature Combustion Phenomenon in Low-Heat- Rejection Diesel Engines”; SAE Paper 940949, Detroit, Ml (1994)

[16] R.A. Churchill, J.E. Smith, N.N. Clark, and R.A. Turton, “Low-Heat Rejection Engines – A Concept Review”; SAE Paper 880014, Detroit, Ml (1988)

[17] W. Bryzik, and E. Schwarz, R. Kamo, and M. Woods, “Low Heat Rejection From High Output Ceramic Coated Diesel Engine and Its Impact on Future Design”; SAE Paper 931021, Detroit, Ml, (1993) [18] L. Kamo, A. Kleyman, W. Bryzik, and E. Schwarz, “Recent Development of Tribological Coatings for High Temperature Engines”; SAE Paper 950979, Detroit, Ml (1995)

[19] P. Moorhouse, B. Mortimer, R. Kamo, “Solid Lubrication Studies for Adiabatic Diesel Engines”; SAE Paper 850508, Detroit, Ml (1985)

[20] P. Sutor an W. Bryzik, “Tribological Systems for High Temperatue Diesel Engines”; SAE Paper 870157, Detroit, Ml (1987)

[21] P.A. Gaydos and K.F. Dufrane, “Lubrication of Ceramics in Ring/Cylinder Applications”; SAE Paper 890888, Detroit, Ml (1988)

[22] K.F. Dufrane, W.A. Glaeser, and A.R. Rosenfield, “Studies of Dynamic Contact of Ceramics and Alloys for Advanced Heat Engines”; ORNL/Su b-84-0021 6/1 [NTIS #DE88 008996], (March 1988)

[23] K.F. Dufrane, W.A. Glaeser, “Performance of Ceramics in Ring/Cylinder Applications”; SAE Paper 870416, Detroit, Ml (1987)

[24] P.A. Gaydos, “Self-Lubricating Materials for High Temperature Ring/Cylinder Application”; SAE Paper 910455, Detroit, Ml (1991)

[25] M.N. Gardos, “An Analysis of the GaIlnIWSe2 Lubricant Compact”; ASLE Transactions 28, (1984), pp. 231-238

[26] D.J. Boes and B. Chamberlain, “Chemical Interactions Involved in the Formation of Oxidation-Resistant Solid Lubricant Composites”; ASLE Transactions 11, pp. 131- 139, (1968)

[27] H.E. Sliney, “Solid Lubricant Materials for High Temperatures - A Review”; Tribology International 15, (October 1982), pp. 303-315

[28] H.E. Sliney, T.P. Jacobson, D. Deadmore, andK. Miyoshi, “Tribology of Selected Ceramics at Temperatures to 900 °C”; Ceramic Engr. & Sci. Proceedings 7, (1986), pp. 1039-1051

[29] E.F. Finkin, S.J. Calabrese, and M.B. Peterson, “Evaluation of Materials for Sliding at 600 °F to 1800°F in Air”; Lubrication Engineering 29, (1973), pp. 197-204

[30] T.M. Yonushonis and G.W. Wolter, “Evaluation of Wear Resistant Chrome Oxide and Plasma- Spray Coatings”; Ceramic Engr. & Sci. Proceedings 6, (1985), pp.1067-i 076

[31] J.L. Lauer and SR. Dwyer, “High Temperature Lubrication by Carbon Continuously Replenished by Surface Reaction with Carbonaceous Gases - Comparison of Metallic (Ni) and Ceramic (Si3N4) Surfaces”; SAE Paper 900686, Detroit, Ml (1990)

[32] J.L. Lauer and B.G. Bunting, “Catalytic Generation of Lubricants from Carbonaceous Gases on Surface Undergoing Friction at High Temperatures”; SAE Paper 88001 9, Detroit, Ml (1988)

[33] 5. Timoney and G. Flynn, “A Low Friction, Unlubricated SiC Diesel Engine”; SAE Paper 830313, Detroit, Ml (1983)

[34] G. Flynn and J.W. MacBeth, “A Low Friction, Unlubricated, Uncooled Ceramic Diesel Engine - Chapter II”; SAE Paper 860448, Detroit, Ml (1986)

[35] C.L. Cummins, Internal Fire, pp. 186-1 99, SAE, Warrendale, PA, 1989.

[36] G.B. Brayton, U.S. Patent 125,166, “Improvements in Gas Engines”, April 2, 1872.

[37] Craven, N. Clark, and J. E. Smith, “Initial Investigations of a Novel Engine Concept for Use with a Wide Range of Fuel Types”; SAE Paper 920057, Detroit, Ml (1992)

[38] P.T. Vickers, C.A. Amann, H.R. Mitchell, and W. Cornelius, “The Design Features of the GM SE-lOl -A Vapor-Cycle Powerplant”; SAEPaper 700163, Detroit, Ml (1970)

CONTENTS

1. ABSTRACT

2. INTRODUCTION

3. BACKGROUND

4. DESIGN CRITERIA

5. CERAMIC MATERIALS

6. HIGH TEMPERATURE TRIBOLOGY

7. POWERPLANT STRUCTURE

8. POWERPLANT CONTROL

9. CONCLUSIONS

10. REFERENCES

Acknowledgement

I take this opportunity to express my deep sense of gratitude and sincere thanks to all who helped me to complete this work successfully. My first and foremost thanks go to The God Almighty who showered his immense blessings on my effort.

I deeply indebted to my guide Dr: Jayakumar for his excellent guidance, positive criticisms, valuable comments, suggestions, and constructive feedback. I also extend my hearty thanks to my Head of the department Prof. K.T.Subhramanian and other teachers of the Dept. of Mechanical Engineering for their valuable help.

Finally, a big thanks to my parents and friends, dear and near whom directly and indirectly contributed to the completion of this work.

mechanical seminar hydrofoil ship design

1.Introduction

A Hydrofoil is a specially designed hydrodynamic surface that creates lift significantly exceeding drag. The main function of the hydrofoil is to lift the ships hull out side the water. At low speeds the ships hull sits on the water and the hydrofoils are totally submerged in water, but as the speed increases the hydrofoils create lift, bringing the hull outside the water surface.

The basic principle of the hydrofoil concept is simply to lift a ship's hull out of the water and support it dynamically on wing-like lifting surfaces, i.e. hydrofoils, to reduce the effect of waves on the ship and to reduce the power required to attain modestly high speeds. Engineers and naval architects have been intrigued with the possibilities of this concept for many years. A United States patent for a hydrofoil was defined in the late 1880s, about the same time as the early airplane and airfoil patents. The earliest record of a successful hydrofoil flight is 1894 when the Meacham brothers demonstrated their 14 foot test craft at Chicago, Illinois. This compares with the Wright brothers' first airplane flight in 1903. The early attempts to exploit the hydrofoil concept were frustrated by lack of suitable structural materials and power plants. However, advancement in these areas, much of it stemming from aircraft developments, has permitted development over the past 30 to 40 years of the technology necessary to achieve and demonstrate reliable and effective hydrofoil ships for both military and commercial below.

2.Hydrofoil Basics

Many people are familiar with airfoils. Foil is simply another word for the wing (such as the wing on an airplane). A hydrofoil is a wing that 'flies' in water. Hydrofoil is also used to refer to the boat to which the water wings are attached. A hydrofoil boat has two modes of operation:

(1) as a normal boat with a hull that displaces water and

(2) with the hull completely out of the water and only the foils submerged.

Hydrofoils let a boat go faster by getting the hull out of the water. When a normal boat moves forward, most of the energy expended goes into moving the water in front of the boat out of the way (by pushing the hull through it). Hydrofoils lift the hull out of the water so that you only have to overcome the drag on the foils instead of all of the drag on the hull.

The foils on a hydrofoil boat are much smaller than the wings (foils) on an airplane. This is because water is about 1000 times as dense as air. The higher density also means that the foils do not have to move anywhere near as fast as a plane before they generate enough lift to push the boat out of the water.

The hydrofoils generate lift only when they are in the water; if they leave the water, the boat will crash down onto the surface of the water (and thus submerge the foils) until the foils generate enough lift to lift it back out. Like an airplane, a hydrofoil must be controllable in terms of pitch, roll, and yaw. Unlike an airplane, a hydrofoil must also maintain a consistent depth. Whereas an airplane has a range of about 40,000 feet in which to maintain its altitude, a hydrofoil is limited to the length of the struts, which support the boat above the foils.

3.working

HOW DOES A HYDROFOIL(AEROFOIL, WING) WORK ?

MAIN FUNCTIONAL REQUIREMENT: Lift the boat’s hull outside the water.

DESIGN PARAMETER: Hydrofoil (It is a foil or wing under water used to lift the boat’s hull until it is totally outside the water.)

GEOMETRY/STRUCTURE:

Detail of Hydrofoil Geometry

EXPLANATION OF HOW IT WORKS/ IS USED:

1. At low speeds the hull (body of ship) sits in the water and the hydrofoils are totally submerged in the water.

2. As the boat’s speed increases, the hydrofoils create lift.

3. At a certain speed, the lift produced by the hydrofoils equals the sum of of the boat and cargo weights. Therefore the hull comes out of the water.

4. Instead of having an increase in drag with increasing speed because the hull is lifted out of the water (contrary to what happens in traditional boats due to pressure drag), the hydrofoils provide a more efficient way of cruising. Decreasing the drag contributes to the better use of the power needed for the movement of the boat.

4.DOMINANT PHYSICS:

How is the lift produced - Fluid Dynamics.

Bernoulli’s Equation: Po = P₁ + ½rv₁² + rgy₁ = P₂ + ½rv₂² + rgy₂

Variables	Units
Po Stagnation Pressure	[Pa] or [lbf/ft2]
P Pressure	[Pa] or [lbf/ft2]
r Density	[kg/m3] or [lbf/ft3]
V Velocity	[m/s] or [ft/s]
g Gravitational-constant	[m/s2] or [ft/s2]

5.PRINCIPLE

This equation applies to flows along a streamline which can be modeled as: inviscid, incompressible, steady, irrotational and for which the body forces are conservative. Also the difference on the height of the foil (the distance from the bottom section to the upper one) is small enough so that the difference pgy₂ - pgy₁ is negligible compared to the difference of the rest of the terms. What is left is that the pressure plus one half the density times the velocity squared equals a constant (the stagnation pressure).

As the speed along these streamlines increases, the pressure drops (this will become important shortly). The fluid that moves over the upper surface of the foil moves faster than the fluid on the bottom. This is due in part to viscous effects, which lead to formation of vertices at the end of the foil. In order to conserve angular momentum caused by the counter-clockwise rotation of the vortices, there has to be an equal but opposite momentum exchange to the vortex at the trailing edge of the foil. This leads to circulation of the fluid around the foil. The vector summation of the velocities results on a higher speed on the top surface and a lower speed on the bottom surface. Applying this to Bernoulli’s it is observed that, as the foil cuts through fluid, the change in velocity produces the pressure drop needed for the lift. As it is presented in the diagram, the resulting or net force (force= (pressure)(area)) is upward.

This explanation can be enriched with the Principle of Conservation of Momentum. (Momentum = (mass)(velocity)) If the velocity of a particle with an initial momentum is increased, then there is a reactant momentum equal in magnitude and opposite in direction to the difference of the momentums. (See diagram).(Mi = Mf + M)

Detail of Hydrofoil:

a) Pressure Profile

b) Momentum Transfer

c) Circulation

d) Streamlines

6.FOIL LIFT AND DRAG

FOIL LIFT IS CALCULATED USING THE EQN:

Z=(1/2) p*V 2*Cl*S

FOIL DRAG IS CALCULATED USING THE EQN:

Z=(1/2) p*V 2*Cd*S

WHERE p=DENSITY, V=VELOCIYY, C=DRAG OR LIFT COEFF, S=PROFILE C.S.AREA

7.ANGLE OF ATTACK

As it has been presented, lift comes from the dynamics of the fluid in the area surrounding the foil. But the lift can be optimized by positioning the hydrofoil at an angle (relative to the incoming fluid flow) called the angle of attack (See diagram). The goal is to optimize the lift to drag ratio. This ratio depends on the shape of the foil, which in this case is considered to be a thin foil. With a small angle of attack, the lift increases rapidly while the drag increases at a small rate. After an angle of ~10° the lift increases slowly until ~15° where it reaches a maximum. After ~15° stall can set in. When the angle of attack is 3° to 4° the ratio of lift: drag is at it’s maximum. So the foil is more efficient at those angles (3°and 4°) with lift to drag ratios of ~ 20 to 25:1

8.LIMITING PHYSICS:

Detail of Hydrofoil Geometry

At first, people can think that stalling is likely to be a problem in hydrofoils as it is in airfoils, but surprisingly it is not. A steep angle of attack is not needed in the design of the hydrofoil. On the contrary, small angles of attack are used on hydrofoils to optimize the lift to drag ratio as explained before.

What is a primary concern is the design of the foil, the struts/supports, and their positioning. All these features have to be taken in consideration. So the features are designed to

produce a minimum speed that will lift the boat of certain weight and keep it foil borne.

One problem that a hydrofoil craft can experience is the height of the waves being greater than the struts. Also, if the craft is traveling faster than the waves, the foils could break to the surface and outside of the water, resulting in a loss of lift and a negative angle of attack when the foil dives into the next wave, making the craft crash into the sea. Engineers have designed hydrofoils to minimize these limitations and better the ship’s performance

There are two particularly persistent problems faced by designers of hydrofoils:

Cavitation and ventilation.

Ventilation occurs when part of a hydrofoil pierces the surface of the water and air gets sucked down the lifting surface of the foil. Since air is much less dense than water, the foil generates much less lift and the boat crashes down. Ventilation can occur at any air-water interface.

Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.

Cavitation occurs when the water pressure is lowered to the point where the water starts to boil. This frequently happens with propellers. When a propeller is turned fast enough, the blades generate so much lift (i.e. the pressure on the lifting surface of the blades goes down) that the water flowing over the propeller blades begins to boil. When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water.

Note that a hydrofoil is not a hovercraft. Hydrofoils fly on wings in the water that generate lift whereas hovercraft floats above the water on a layer of air. In both cases the boat's hull leaves the water, but the mechanisms by which this is achieved are completely different

9.configurations

Hydrofoil configurations can be divided into two general classifications, surface piercing and fully submerged which describe how the lifting surfaces are arranged and operate (see Figure 1). In the surface piercing concept, portions of the foils are designed to extend through the air/sea interface when foil borne. Struts connect the foils to the hull of the ship with sufficient length to support the hull free of the water surface when operating at design speeds. As speed is increased, the lifting force generated by the water flow over the submerged portion of the foils increases causing the ship to rise and the submerged area of the foils to decrease. For a given speed the ship will rise until the lifting force equals the weight carried by the foils. As indicated by the terminology, the foils of the fully submerged concept are designed...

Figure 1-- Surface-Piercing (Left) & Fully Submerged (Right) Foil Configurations

to operate at all times under the water surface. The struts which connect the foils to hull and support it when the ship is foilborne generally do not contribute to the total hydrofoil system lifting force. In this configuration, the hydrofoil system is not self-stabilizing. Means must be provided to vary the effective angle of attack of the foils to change the lifting force in response to changing conditions of ship speed, weight and sea conditions. The principal and unique operational capability of hydrofoils with fully-submerged foils is the ability to uncouple the ship to a substantial degree from the

effect of waves. This permits a relatively small hydrofoil ship to operate foilborne at high speed in sea conditions normally encountered while maintaining a comfortable motion environment for the crew and passengers and permitting effective employment of military equipment. It is this desirable characteristic which has caused the hydrofoil ship development in the United States to concentrate on the fully-submerged foil concept.

The basic choices in foil and strut arrangement are canard, conventional or tandem as shown in Figure 2. Generally ships are considered conventional or canard if 65% or more of the weight is supported on the front or the aft foil respectively. If the weight were distributed relatively evenly on the fore and aft foils, the configuration would be described as tandem.

SPLIT	NON-SPLIT	ARRANGEMENT
		CONVENTIONAL 0.00 <>
		CANARD 0.65 <>
		TANDEM 0.35 <>

Figure 2 - Foil/Strut Arrangements

10.Features

10.1)Weight Limitations

Like the airplane designer, the hydrofoil designer must, at all times, be extremely conscious of weight. The hydrofoil type of craft is weight critical, and every pound of weight saved in structure, outfit, or machinery means weight available for payload and fuel.

The structural engineer, in designing hydrofoils to conserve weight, uses aircraft techniques. Relative to conventional ships, hydrofoil craft are subject to very high loadings, as caused by high operating speeds. Likewise, lightweight, high strength materials are used. He also must contend with fatigue and problems of hydroelasiticity, including both divergence and flutter.

10.2)Hull Considerations

The development of a satisfactory hull form for hydrofoil application represents a significant challenge to the designer. The hull should perform well in the hullborne mode but also during takeoff and during foilborne operation where impacts with waves are involved. In addition, the hull configuration of a hydrofoil ship must satisfy all of the requirements for strength, freeboard, and intact and damaged stability for any other ship.

Relatively high power requirements for high-speed operation, in common with other high performance systems, pay a high performance dividend for achieving a minimum weight structure. Therefore, hydrofoil ship hulls are generally constructed using high-grade aluminum alloys, 5000 series weldable alloy being typical. Structurally, the hull must have the strength to resist wave impact at high speed as well as distribute the concentrated load at the strut attachment points. Although hydrofoil hulls may appear quite conventional, the required compromises are more complex than for a monohull because of the many operating modes of the ship. An efficient hull form for a lower speed operation requires a narrow beam. However, a righting moment large enough to satisfy the stability criteria of reference [6] with the foils retracted generally dictates a wide beam. Cresting the tops of waves while foilborne points toward the use of a deep vee forward and high deadrise.

Another major consideration in hydrofoil hull design is the requirement for good seakeeping characteristics in a heavy sea. If hydrofoil craft are to operate unrestricted in open ocean, they must be capable of surviving storm seas in the hullborne condition. Furthermore, in certain missions, it may be expected that the hydrofoil ship will spend the greater portion of its operating lifetime in the hullborne mode. Thus, it is essential that close attention be given to the hull seakeeping characteristics. With the foils extended during hullborne operation, which is normal operation at sea, there is a significant reduction of craft motion in both the roll and pitch modes which is normally not heavily damped. Thus the strut/foil system gives hydrofoil craft hullborne motion characteristics of ships having much larger displacement.

10.3)Foil Systems

Foil variable lift is obtained by either trailing edge flaps or variable incidence of the entire foil as illustrated in Figure 5.

Figure 5 - Hydrodynamic Force Control

10.4)Weight Trend

A fundamental limitation is imposed by the so-called "square-cube" law, which impacts the growth potential of hydrofoil ships. The lift developed by the foils is proportional to their planform area (the square of a linear dimension), whereas the weight to be supported is proportional to a volume (the cube of a linear dimension). It follows that as size of the hydrofoil is increased, the foils tend to outgrow the hull. Aircraft solve this problem by increasing speed and wing loading as size is increased, but practical hydrofoil speeds are limited by cavitation.

In the early period of hydrofoil development it was felt that an increase in the foil and strut weight fraction by direct application of the square-cube law would inherently limit hydrofoil size. More detailed design studies show that foil system weight fractions increase only slightly with displacement, Figure 6.

Figure 6 - Strut and Foil System Weight Trend

The principal reasons why the weight fraction does not increase as might be expected is that required strut length varies with design sea state, not ship size, and larger foils are structurally more efficient. For hydrodynamic efficiency, it is desirable to use as high a foil aspect ratio (span/chord) as possible. The PHM aft foil extends almost 10 feet on either side of the hull. Thus, a camel is normally used to hold the ship away from the pier for mooring. When no camel is available the ship must be moored across the end of a pier or the transom of a larger ship with the stern overhanging. PHMs have occasionally nested bow to stern. As ship size increases and foils grow relative to the hull and in actual dimension, practical considerations dictate efforts to limit the span. The trend will be to move toward tandem foil configurations to divide the weight more evenly between the forward and aft foils.

11.Propulsion Systems

Modern hydrofoil ships have been made possible by the development of lightweight diesel engines and marinized gas turbine engines. Most of the European commercial ships using fixed surface-piercing foil systems have used lightweight diesel engines driving subcavitating propellers by means of an angled transmission system. This combination provides simplified construction, relative ease of maintenance and low cost. However, the comparatively high specific weight (6-8 pounds per horsepower) of the diesel engines and higher overall drag have resulted in practical design speeds of these ships of about 35 to 40 knots.

Existing aircraft gas turbine engines slightly modified and coupled with specially designed free powered turbines are available in sizes with power ratings up to about 30,000

horsepower and specific weights of around 0.5 pounds per horsepower. The newer large engines employing blade cooling techniques have specific fuel consumption rates at their design power about equal to diesel engines. Gas turbine engines have been used in all major U.S. military and commercial hydrofoil ships permitting practical design speeds greater than 40 knots. Propellers are the most efficient propulsion device available for operating over the subcavitating speed range of current hydrofoil ships. The power transmission systems required when using fully submerged foil systems consist of right angle bevel gears, flexible shafts and possibly a speed reduction gearbox in the propeller transmission pod. See Figure 7 as an example.

Figure 7 - PGH-1 FLAGSTAFF Propulsion System

Problems encountered with gear transmission systems in early hydrofoil ships led to interest in waterjet propulsion systems. While not entirely eliminating the need for gearboxes, these systems consist of underwater inlets, water ducts in the struts, a pump located in the machinery spaces and an above-water exhaust nozzle. The U.S. Navy's PHM waterjet system is shown on Figure 8. The price paid to achieve these less complex waterjet systems is a decrease in propulsive efficiency of about 20% at 45-50 knots and considerably more at takeoff speeds along with an increase in propulsion system weight due to the water carried in the system.

Figure 8 - PHM Waterjet System

12.Automatic Control System

As noted earlier, surface-piercing hydrofoil configurations are self-stabilizing in both pitch and roll and thus do not require an automatic control system. However, to reduce the inherent reaction to rough seas, a number of ships have added trailing-edge flaps to the surface-piercing foils and have used autopilots for ride improvement.

In the United States, full automatic control of submerged foils has been deemed necessary to attain the seaway performance desired for ocean-going hydrofoil ships. Typically, control is accomplished by positioning trailing-edge flaps on the forward and after foils and by rotating the swiveled forward strut (rudder), or by moving the entire foil surface and by using the power driven aft strut as a rudder. See Figures 9 and 10 for schematic and pictoral diagrams of a control system. The control surfaces are positioned by means of conventional electro- hydraulic servos. The control system motion sensors consist of: 1) a vertical gyro which measures craft pitch and roll angular motion, 2) a rate gyro which measures craft yaw rate, 3) three vertical accelerometers, one accelerometer being located approximately on top of each strut (the two aft accelerations work differentially to provide roll angular acceleration feedback, and they work in unison to provide pitch and heave acceleration feedback), and 4) a height sensor which measures the height of the bow above the water surface. The manual inputs consist of a foil depth command, which the helmsman uses to select any desired foil depth (or flying height), and the helm, which introduces the craft turning commands.

Figure 9 - Hydrofoil ACS Schematic

Figure 10 - Typical Hydrofoil Automatic Control System (ACS )

The ACS provides continuous control during takeoff, landing, and all foilborne operations. The pitch, roll, and height feedback loops provide automatic stabilization of the craft. The craft is automatically trimmed in pitch by the pitch feedback, and roll trim is accomplished by helm inputs. To steer the ship, the helmsman simply turns the helm, and the ACS automatically maintains a coordinated turn, with turn rate being proportional to helm deflection. ACS system requirements and operation are discussed in detail in References (7), (8), and (9).

13.Hydraulic System

The hydraulic and automatic control systems are worthy of mention because: 1) they have proven reliable and functionally well suited for a hydrofoil ship, 2) they combine proven aircraft system equipment applications, and 3) they are essential to all operations: foilborne, hullborne, and docking. Because the hydraulic systems are crucial to both foilborne and hullborne operation, the design should employ multiple levels of redundancy to assure continued operation in the event of system failures.

On the PHM, for instance, four separate systems supply the required power to the various hydraulic equipment users which include the foilborne and hullborne control actuators, strut retraction and lock actuators, bow thruster, anchor windlass, and emergency fuel pump. Systems No. 1 and No. 2 supply hydraulics to the forward part of the ship while systems No. 3 and No. 4 supply the aft part. Two separate supply systems feed each user with provisions included to transfer (shuttle) the user from its primary supply to its alternate supply in the event of loss of primary supply pressure. The hydraulic systems of the PHM operate at a standard 3,000 psi (20.68 MN/m2) constant pressure. Proven aircraft hardware, mostly from the Boeing 747 aircraft, was used where possible. The hydraulic pumps, tube fittings, tubing material, and filters were all taken directly from the 747. In the case of the foilborne and hullborne steering actuators, an automatic shuttle valve was specifically developed for the hydrofoil program which rapidly transfers the user actuator from a failed supply to the alternate, thus assuring continued safe foilborne operation. The hydraulic actuators on the PHM were for the most part specifically designed and developed for this program. The four foilborne control actuators, the hullborne steering actuator, two hullborne thrust reverser actuators and the strut retraction actuators all were designed, manufactured and qualified to military specifications including rigorous environmental and life testing.

The PHM hydrofoil program pioneered the use of a new hydraulic fluid, a synthetic hydrocarbon. This new fluid provides a much greater resistance to fire and explosion than its predecessor. At the same time it overcomes the serious shortcomings of phosphate ester base fluids which have proven to be incompatible with the saltwater environment.

14.Characteristics

14.1 Resistance and Powering

Although the major reason for the employment of hydrofoils is to lift the hull out of the water to reduce the effect of waves and to reduce the drag at high speed, a naval hydrofoil ship spends a considerable portion of its life hullborne and must have an efficient hull form to keep the drag low at low speed and through takeoff. Total drag just prior to takeoff is a significant factor in establishing the power requirement. Careful attention must be paid to the hull design to minimize this effect. Figure 3 shows a generalized smooth water drag curve for a hydrofoil craft with its significant "hump" prior to takeoff. Comparison is also made with a typical planing craft to illustrate the high-speed advantage of the hydrofoil even in smooth water. To overcome additional takeoff drag which results from rough water, a power margin

over the smooth-water takeoff drag is required. Since the magnitude of this margin is a prime factor in the sizing of the propulsion system, it is essential that it not be arbitrarily overspecified. Tests in design sea states on well-instrumented U.S. Navy hydrofoils show that 20 to 25 percent margin is ample to permit takeoff in rough water in any direction.

Figure 3 - Typical Calm Water Thrust

14.2 Seakeeping

Some of the principle advantages of hydrofoil ships, over all other monohull or alternative ship types are: (1) the ability of a ship, which is small by conventional ship standards, to operate effectively in nearly all sea environments, and (2) an improved ratio of power to displacement in the 30 to 50 knot speed range permitting economical operation at these higher speeds. The submerged-foil ship can maintain its speed and maneuverability in heavy seas while simultaneously providing a comfortable working environment for the crew. The ship's automatic control system (ACS) provides continuous dynamic control of the ship during takeoff, landing, and all foilborne operation. In addition to providing ship roll and pitch stability, the ACS controls the hull height above the water surface, provides the proper amount of banking in turns and all but eliminates ship motions caused by the orbital particle motion of waves. Foilborne operations only become limited as wave height exceeds the hydrofoil's strut length. Figure 4 shows operating data points for three submerged-foil hydrofoil ships in actual sea conditions. The data clearly show only a modest reduction in speed as wave heights increase. A hypothetical operating envelope is drawn to represent hydrofoils designed to have a 50-knot speed capability in calm water.

Figure 4 - Effect Of Sea State On Hydrofoil Speed and Drag Curves

14.3 Maneuvering

Besides a significant speed advantage, hydrofoils are more maneuverable and provide a more stable platform than conventional ships. Foilborne turns are accomplished in a banked (coordinated) fashion. This causes the centrifugal force required in turns to be provided predominantly by the reliable lift capability of the submerged foils rather than by the unpredictable side forces from the struts. Turn coordination enhances crew comfort during high-rate turns because the accelerations due to turning are felt primarily as slightly greater vertical forces rather than lateral forces. For example, a 0.4g turn is felt as only 0.08g vertical acceleration increase while the lateral acceleration is zero. Therefore, hydrofoil ships have design turn rates of 6 to 12 degrees per second, two to four times those of conventional ships, and they can maintain these rates in both calm and rough seas. This makes the hydrofoil ship a more difficult target for enemy missiles, guns, or torpedoes. The exceptional stability of the hydrofoil ship makes it a superior platform in which to mount surveillance equipment and weapons while maintaining crew comfort and proficiency.

15.Advantages

1) A hydrofoil requires only 50% of the power of a displacement vessel of comparable size, for a given speed

2) The hydrofoil due to their small size and maneuverability, are target less vulnerable to tactical military weapons like missiles

3) Greater platform stability and high speed

4) Can be maintained even in seaway due to better sea keeping ability

SOME HYDROFOILS AND THEIR USE: Hydrofoils have become very popular. They are used in various kind of sea traveling, from military use to water sports. The high speed, smooth cruise and better turns delivered by hydrofoils have been used in military ships. Sailing has also adopted the hydrofoils to gain more speed. They enable new inventions that can satisfy people’s desire to challenge danger, like the sky ski. It is a water ski with a hydrofoil attached, which permits people to fly above the water surface. Every day more hydrofoils are used, and in the future, they may be the dominate method of sea traveling

16.Summary

Although the basic concept of hydrofoils has been around for 85 years, it has only been in the last 35 years through advances in materials, light weight propulsion plants, and control theory, they have become a viable open ocean concept. Involved. The design of a hydrofoil demonstrates the very essence of engineering that is the trade-off and compromise among often-conflicting requirements of many disciplines to arrive at a good balanced design.

17.Conclusion

Although the basic concept of hydrofoils has been around for 85 years, it has only been in the last 35 years through advances in materials, light weight propulsion plants, and control theory, they have become a viable open ocean concept. Involved. The design of a hydrofoil demonstrates the very essence of engineering that is the trade-off and compromise among often-conflicting requirements of many disciplines to arrive at a good balanced design.

18.REFERENCES

www.foils.org

www.howstuffworks.com

www.