Author: Paul A. McLachlan



The development of the water-cooled pivotal piston was undertaken to overcome the shortcomings of the conventional sliding piston as applied to a two-stroke engine. It is well recognized that these shortcomings restrict the high power density two-stroke engine from being adopted for automotive power units. A number of two-stroke engine development projects were undertaken by automotive OEMs in the early 1990s. Although companies such as Orbital advanced the potential of the two-stroke engine by delivering finely atomized fuel after the exhaust port was closed no new fundamental mechanical design was introduced to mitigate the poor piston support, poor thermal control and loss of lubricating oil out the exhaust.

The transition to hydrogen fuel significantly changes the design criteria for an IC engine and this discussion paper sets out to explore the key differences of the water-cooled pivotal piston engine and how these differences affect the suitability of this design to meet a multitude of needs in a hydrogen fuel age. The potential of the water-cooled pivotal piston technology to advance hydrogen IC engine viability is now considered to be strategic and good reason to optimize this engine on hydrogen fuel.

1. Introduction

The water-cooled pivotal piston overcomes the inherent mechanical deficiencies of the conventional sliding piston in a two-stroke engine.

These deficiencies were noted as;-

Poor restraint and control of piston movement.

High requirement for piston lubrication with a consequent loss of oil into the exhaust system.

Poor thermal control of the piston in a high performance two-stroke engine.

The best means of overcoming these deficiencies was to locate and control piston movement with a pivot bearing and so remove the need to rely on the cylinder walls to restrain the piston. This resulted in a low friction mechanism which needs lubrication for the compression sealing components only. With the greater stability gained through restraining the piston with a pivot bearing the engine becomes a mechanically more quiet design with the potential to achieve a long service life.

This cross section of the pivotal piston engine shows the piston pivot point at the back of the piston and the piston pin position connected to the crankshaft via a connecting rod. This provides a direct load path to the crankshaft. The side surface of the piston arc seal is visible as is the grove in the head which carries the head seal which seals to the arc seal radial surface. The combustion cavity is in the head and piston bowl (unseen). The piston water-cooling gallery is visible as a hole through the piston (blocked at each end). The exhaust block houses the transfer port passages and the exhaust port (unseen) which is shielded off from the primary chamber below the piston by the piston skirt. The piston skirt is a floating seal attached to the piston at the bottom and free to extend out to the exhaust block surface at the top.

Water-cooled pivotal piston engine.

The thermal control of the piston was achieved by utilizing the center of the pivot shaft to access the piston for internal water-cooling. This provided an independent means of controlling piston temperature without needing to excessively over-cool the engine. The thermally smooth combustion chamber surfaces of a fully water-cooled two-stroke engine opened up new IC engine development options.


Cut-away of a water-cooled pivotal piston.

The development of this technology has matured at a time when the necessity for a suitable hydrogen fuel ICE is increasingly evident. The prospect of the hydrogen age presents an opportunity to take a fresh look at new IC engine design to maximize the characteristics of hydrogen combustion. The need to explore the ultimate hydrogen IC engine design is considerable as the degree to which fuel cell powered vehicles might penetrate the global vehicle market is unknown. The hydrogen IC engine and hybrid hydrogen/electric systems may yet become the ubiquitous means of powering vehicles.

2. Current H2ICE development.

The objective in recent projects to convert gasoline engines to operate on hydrogen has been to achieve a similar power output to gasoline. The difficulties encountered in this endeavour come from the intention to ‘convert’ a gasoline engine to operate on hydrogen. The modern automotive engine has been incrementally developed to operate on gasoline over a long period and is therefore unlikely to be the ideal basis for a new generation of hydrogen fueled IC engines. Four/stroke automotive engines with poppet exhaust valves do not make full use of the potential power or efficiency that could be achieved from a hydrogen fuel ICE.

The work of BMW and Ford is still at an early stage and further development will raise the performance level of hydrogen fuel automotive engines. However the limiting factor is the thermal difficulties when operating poppet valve four-stroke engines on hydrogen and these difficulties are only overcome with a reduction in engine performance. The resulting lower power density inevitably results in a lower total vehicle efficiency. It is however, necessary to carry out such development projects and establish the boundaries which need to be extended. These projects form the information background needed to guide development in new directions and ultimately arrive at the ideal hydrogen IC engine.

The search for comparable power from four-stroke automotive engines converted to hydrogen fuel has added complexity to the power unit by the addition of compressors and inter-coolers. A specifically designed H2ICE is needed to maximize the potential for high power density with the minimum of mechanical complexity. This requires engineers to look at the characteristics of hydrogen fuel and to assemble a criteria for a hydrogen engine. With this criteria prioritized it then requires a step back from established design to ponder ‘what if’.


3. What are the relevant characteristics of hydrogen compared to gasoline as an IC engine fuel?

1. high power density by weight

2. low power density by volume

3. fast burning

4. high combustion temperatures

  1. low ignition temperature
  2. broad range of air/fuel ratio with good ignition capability
  3. high auto ignition temperature
  4. no evaporative engine cooling
  5. no lubricating qualities

4. How do these differences effect the design criteria for a hydrogen fuel internal combustion engine?

  1. The high specific energy by weight of hydrogen gas is an advantage which should be extended to the utmost in a hydrogen powered vehicle. This suggests that every effort should be made to minimize mass and bulk throughout the vehicle with every structural feature be designed to enhance light weight and where practical to carry fuel.

  1. While hydrogen has a high specific energy by weight it is a bulky fuel by volume. It is this characteristic which creates the technological barrier to utilizing hydrogen in transportation. Hydrogen gas displaces the incoming charge of air in a naturally aspirated engine reducing the volume of air tapped in the combustion chamber. This can be overcome by direct injecting hydrogen into the combustion chamber at the end of the air induction phase. It is known that a direct to chamber fuel delivery is essential for good BSFC in a two/stroke engine and so a DI system would achieve both objectives of fuel economy and high output in a two-stroke hydrogen engine.
  2. Fast combustion aids thermal efficiency in an internal combustion engine. The objective being to exert pressure on the piston after TDC with a minimum of pressure build up before TDC. A fast combustion also improves engine efficiency by reducing time for quenching heat from the combustion and dissipating into the engine. High equivalency ratios are needed to benefit from this characteristic while at low equivalency ratios twin spark plugs would be advantageous in reducing the time for flame propagation.
  1. Hydrogen combustion temperatures are higher than with gasoline fuel when burning a high air/hydrogen equivalency ratio. A new hydrogen IC engine will need to extend the fuel/air ratio beyond current boundaries in order to optimize engine output. This will require particular attention to a wide range of thermal management issues.
  2. Because a relatively low ignition energy is required particular attention will need to be paid to controlling the temperature of the fresh charge, avoiding any hot spots in the combustion chamber. Hot exhaust valves or carbon deposits can cause engine damaging pre-ignition. It is difficult to achieve full engine output by utilizing induction boosting or high compression ratios with out also employing inter-cooling systems.
  1. The ability to ignite a low air/fuel ratio presents the potential to reduce pumping losses by controlling the output of the engine through the fuel delivery instead of relying fully on throttling the engine.
  2. The high auto ignition temperature or high octane rating has the potential to improve thermal efficiency through the use of high compression ratios. To utilize this characteristic it will be necessary to control pre-ignition and ultimately combustion quenching limitations.
  3. With no evaporative cooling it is again important to minimize the heat buildup of engine components with a well designed comprehensive engine thermal management system designed to cope with the environment in which the engine operates.
  4. The absence of lubrication will present durability difficulties with combustion sealing components. A low level of independent oil feed to the injector may also be required.

5. Key factors in the design of an efficient hydrogen IC engine;-

To achieve the most output for the minimum input of fuel in an IC engine the critical issues are ;-

  1. High level of fuel trapping
  2. Fast and complete combustion
  3. Minimal effect from combustion quenching
  4. Minimal parasitic losses from friction and pumping

Beyond these fundamental requirements there is the potential to optimize the efficiency of an H2ICE with a hydrogen/electric hybrid system. However this will only provide sufficient advantage in a stop start city environment and so for reasons of vehicle usage patterns and additional cost there will remain a large sector of the market which prefers a simple and efficient hydrogen IC engine.

Thermal control- Complete thermal control of the combustion environment is fundamental to the efficient operation of the engine. Control of head and piston surface temperature is essential as well as the surfaces that the induction air or air/fuel charge comes in contact with as it enters the working chamber of the engine. The characteristics of hydrogen fuel weigh in favour of a two/stroke engine design with it’s inherently low peak combustion temperatures and absence of hot spots in the combustion chamber, as long as the piston is thermally controlled.

High level of fuel trapping- A high level of fuel trapping will be required to ensure that the efficiency and range of the vehicle is optimized. It will be necessary to utilize a direct to chamber fuel delivery system. It is accepted that DI is required for a two-stroke engine to achieve optimum fuel efficiency and in a hydrogen engine it will be needed for this reason as well as to obtain good volumetric efficiency for high performance.

Low friction and pumping losses- Up to 40% of the cranking energy in a current automotive is spent turning the engine over against friction and pumping losses. The opportunity to make gains in this area with a new hydrogen IC engine design can not be ignored. The elimination of a valve drive train and sliding piston friction accounts for a significant reduction in engine friction. The wide range of fuel-air equivalency ratio at which hydrogen can be ignited presents the potential to control engine output with a minimum of engine throttling thereby reducing pumping losses. The fast combustion of hydrogen is a significant advantage in that it allows for later ignition and therefore less pressure buildup BTDC reducing pumping losses and increasing BMEP.

Long service life and low manufacturing cost- There is always a durability/cost ratio trade off in the manufacture of engine components. An engine with few components is more cost effective to develop and manufacture to meet a set standard of reliability and durability. Modular assembly minimizes product development time and manufacturing cost. It is also desirable to avoid the added cost and weight of power boosting mechanisms such as superchargers and inter-coolers.

Minimum size and weight- A compact and lightweight engine is needed to reduce total vehicle mass and extend vehicle range or to allow more room for passengers and cargo along with perhaps multi- tank hydrogen storage.

6. Considerations for the new H2ICE option?

If hydrogen becomes the common fuel for transportation it is unlikely that all applications will be met by fuel cell technology. A prime consideration when selecting a power unit for use in a mobility application is a satisfactory level of power density. In a race car or an aircraft this requirement is extreme while in a train it is not so important but in an automobile it is still very important. The ultimate hydrogen IC engine will undoubtedly have a lower thermal efficiency than a fuel cell yet it may perform efficiently in a vehicle. This is because high power density initiates a ‘flow on’ of weight savings which extend throughout the vehicle to the suspension, brakes and wheels. A measure of total vehicle efficiency must also include the life of the car from manufacture through operation servicing and finally recycling of materials.

To deliver optimum power density an air/hydrogen ratio as near as possible to a stoichiometric level is sort. This will perhaps remain a matter of degree as it presents very challenging thermal management difficulties. All combustion chamber surfaces will need to be thermally uniform, devoid of hot spots in order to operate on a high hydrogen/air equivalency ratio. A high BMEP from a minimum peak combustion pressure/temperature is also needed to minimize NOx emission and maximize BSFC. With these considerations paramount a direct injected, water-cooled, two/stroke engine with an internally water-cooled piston could well be the most suitable blend of criteria to meet these demands.

Along with the development of the fuel cell has come a greater understanding that a power unit can be assessed as if it were a box which when fed with fuel, delivers power. It is the output, cost, size, weight, efficiency and harmful emissions from this box which are important. Any potential hydrogen ICE should also be compared using this objective assessment.

The opportunity here is to develop new IC engines designs which exploit the inherent low emission and high power potential of hydrogen fuel and so move into the hydrogen age with lighter, stronger and better performing vehicles. The identification of a suitable hydrogen internal combustion engine for mobility applications must be thoroughly researched if the simple and cost effective solution is to be found.

It has been largely overlooked that the pending hydrogen age presents an opportunity to design new and higher performance, more compact IC engines. This is because it is difficult to put aside the technology of the gasoline age and take a clean sheet approach to the design of the ideal hydrogen IC engine.

7. The advantages of the water-cooled pivotal piston are key factors in meeting performance and emissions criteria for a hydrogen IC engine.

High power density becomes possible through utilizing the simplicity and compact design of the two/stroke engine principle. The internally water-cooled pivotal piston is an essential element in the design of a hydrogen fueled two/stroke IC engine. Without this level of thermal control power density would be compromised.

Because there is no longer a sliding piston to lubricate oil is specifically metered directly to the compression seals, crankshaft and connecting rod bearings. The oil requirement for the compression seals is no more than is needed to lubricate the compression rings of a modern four-stroke engine where the oil control ring keeps excess oil from the compression rings. Oil is metered directly to the crankshaft, connecting rod bearings at a low rate. Equal at this point to 300/1 fuel/oil ratio.

Low NOx emission is possible due to the use of the low peak temperature of the two/stroke system. The thermal control of all combustion chamber surfaces including the piston crown assists in the control of peak NOx creating combustion temperatures. With the piston independently cooled it is possible to maintain uniform piston and head surface temperatures thereby avoiding pre-ignition and detonation.

8. Opposed four chamber of two litre gasoline pivotal piston engine.

Weight 65 kg.

Size 400x450x580mm

Power 170 kW

This represents a power density of 2.6 kW per kg of engine weight.( 1.5 hp per lb.)

Optimized for hydrogen fuel at an air/fuel equivalency ratio of >.6 the estimated output is in the region of 120 - 130 kW. This output represents about 1.8 kW per kg of engine weight which is still an excellent level of power density when compared to one of the best gasoline, naturally aspirated, automotive engines - the V10 BMW at 1.55kW per kg.

This comparison suggests that it will be possible to develop hydrogen fueled pivotal piston engines which are significantly smaller in size and lighter weight than current gasoline automotive engines.

The water-cooled pivotal piston technology has been developed to meet the requirements of high power density applications such as light aircraft, snowmobiles, jet skis, motorcycles, portable power generation, automotive internal combustion/electric hybrid systems. The current twin chamber, one litre, pivotal piston engine delivers in excess of 1hp per lb. of engine weight operating on pump gasoline.

The development of this technology is timely to provide compact and efficient H2ICEs for transportation as well as decentralized power generation and CHP applications. It offers a blend of sustained high power operation, low cost of manufacture and low NOx emission that will be valuable in a hydrogen fueled economy.

9. Conclusion

A significant improvement in total vehicle efficiency through an advanced hydrogen ICE design will impact on the volume of H2ICE powered vehicles in operation in a future hydrogen economy. The development of a suitable IC engine design specifically for hydrogen fuel is yet to be undertaken and the potential to achieve a viable, longer term outcome, is therefore unknown. The lower thermal efficiency of even the ideal hydrogen IC engine as compared to current fuel cell technologies must be weighed against the full life cycle cost from manufacture to disposal of the H2ICE.

The hydrogen internal combustion engine has a long way to go and converting current four-stroke automotive engines to operate on hydrogen will not provide us with a benchmark that gives an indication of the full potential of the hydrogen ICE. The important decision is to invest R&D dollars into a design which has an inherent capability of meeting the target requirements. The bright side is that it could be relatively inexpensive to explore and the result can be better than is currently anticipated.


Comparison of Hydrogen Characteristics



Natural Gas


Lower heating value kJ/g




Self-ignition temperature (ºC)




Flame temperature (ºC)




Flammability limits in air (vol%)

4 – 75

5.3 – 15

1.0 – 7.6

Minimum ignition energy in air (uJ)




Detonability limits in air (vol%)

18 – 59

6.3 – 13.5

1.1 – 3.3

Theoretical explosive energy (kg TNT/m3 gas)




Diffusion coefficient in air (cm2/s)





BELLONA Report 6:2002


Hydrogen IC Engine Boosting Performance & NOx Study. 2003-01-0631

Robert J. Natkin, Xiaoguo Tang, Brad Boyer, Bret Oltmans, Adam Denliger, James Heffel.

Ford P2000 Hydrogen Engine Dynamometer Development. 2002-01-0242

Xiaguo Tang. Daniel M. Kabat, Robert J. Natkin, William F. Stockhausen, James Heffel.

Survey of Hydrogen Combustion Properties NASA 1383

Isadore L. Drell Frank E. Belles

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