Two extra strokes are added to the customary internal combustion engine four stroke Otto cycle, which makes a six stroke engine. A third down-stroke is a "steam stroke" and a third up-stroke exhausts the expanded steam while venting heat from the engine.
The engine cold starts on the Otto cycle, coasting through the fifth and sixth strokes for a short period. After the combustion chamber temperature reaches approximately 400 degrees Fahrenheit (200 °C), a mechanical operation phases in the fifth and sixth strokes. Just before the fifth stroke, water is injected directly into the hot combustion chamber via the engine's fuel injector pump, creating steam and another power stroke. The phase change from liquid to steam removes the excess heat of the combustion stroke forcing the piston down (a second power stroke). As a substantial portion of engine heat now leaves the cylinder in the form of steam, no cooling system radiator is required. Energy that is dissipated in conventional arrangements by the radiation cooling system has been converted into additional power strokes.
In Crower's prototype, the water for the steam portion of the cycle is consumed at a rate approximately equal to that of the fuel, but in production models, the steam will be recaptured in a condenser for re-use. Heat will be available from the condenser to provide interior heating of the vehicle, much as a conventional heater core works in cars and trucks today.
The principal advantage present in the six-stroke design is the engine's ability to extract work from heat that is otherwise lost through the cooling system of a conventional four-stroke engine. Since the steam strokes have the side-effect of cooling the engine internally , this will allow the use of much higher compression ratios, allowing the full potential of a fuel to be extracted. Since heat that previously would have been drawn out of the engine via the cooling system can be harnessed, compression ratios once useable for only short term applications (such as race engines) could conceivably be used in regular, long-running time scenarios without environmentally harmful anti-knock chemicals. Ultra-lean air/fuel mixtures, desirable for low emissions and high efficiency, may be used since excess heat, undesirable in other engine architectures, can likewise be harnessed in six-stroke applications. Under these circumstances, far more energy from fuel could be converted to useful output.
The weight and power loss of most conventional cooling system parts, such as the fan, radiator, and coolant pump, can be eliminated. On a large diesel truck, these parts may weigh as much as a thousand pounds. At least a portion of this advantage is lost if water is recovered from the exhaust through a condenser.
The mechanical modifications needed to "six-stroke" a small air-cooled industrial diesel already being manufactured are far less complicated than any hybrid system. Many maintenance features of this engine would be parallel or identical to the knowledge base of mechanics well-versed with gasoline, diesel, and racing engines.
Physical engine size reduction (per a designated power rating) is possible as one third of the engine strokes produce power (in the Crower six-stroke), instead of one quarter (in the Otto cycle). This, and the extra power stroke being provided by using water instead of fuel, means there is a significant improvement to the fuel efficiency and pollution within a given power range, and this is in a field where small improvements create great interest.
The higher percentage of power strokes may allow lower working speeds, with higher torque output at lower and broader rpm ranges. Lower working speed might allow designs with greater crankshaft diameter, for engine dimensions with inherently more torque potential.
As a high pressure steam engine that does not need a certified pressure boiler, the related hardware complexities, dangers, and weight penalties and certification requirements are removed.
A steam-free cool down period is needed to clear water/steam from the engine.
Cold climate anti-freezing measures would be needed in the water reservoir.
Oil contamination, from the water/steam portion of the cycle, is an obstacle to be dealt with, though additional piston/cylinder sealing rings can be easily added and special oils used. Also, data from lubrication oil engineering systems for steam turbines is readily available to help identify likely concerns and possible "well-researched" remedies.
The weight of an oil separator and a water condenser are likely additions, although these will be far smaller and lighter than a conventional cooling system.
Endurance testing will likely identify components that may need to have upgraded materials designated, such as possibly using stainless steel for the valves, cylinders, and rings.
Prior art exist to Crower's invention, but no preexisting six-stroke patents were mentioned in his patent application. As of May 2008, no patent has been awarded.
The "steam" power stroke and the "burning hydrocarbon" stroke may not produce the same amount of force as each other. A one-cylinder engine would run much smoother with a relatively heavy flywheel to smooth pulsations, much like the early large one-cylinder diesel industrial factory engines.
One down-stroke does not provide any power, and, as mentioned above, the other two down-strokes may each provide different levels of power. This suggests the most compact configuration that will provide an inherently smooth running operation is an in-line three-cylinder engine. Of course, many other configurations and cylinder quantities may work.
Although the Crower cycle could be made functional with a variety of fuels and RPM ranges, steam theory suggests it would be very useful when coupled with a diesel cycle, which performs well in low-rpm long-stroke applications.
A Crower cycle engine may prove to be useful as a passenger vehicle power plant. If meeting automobile emissions standards proves troublesome, the design may prove useful in heavy engine applications such as transport diesel trucks, heavy equipment (bulldozers, etc.), buses, and stationary power generators.