Variable Stroke Count Engine

This engine concept is derived from the six stroke steam hybrid engine concept [link], but should have more power when the engine is cool, and consume less water, and less fuel, when the engine is hot.

Each engine head contains the following components, all of which are computer controlled: A fuel injector, a water injector, an air intake valve, an air exhaust valve, and a steam exhaust valve. There might be a spark plug or a glow plug.

There are also a number of high-speed electronic thermometers which measure cylinder temperature, head temperature, and water temperature at the injector.

Near the end of every engine cycle, a weighted average of the temperatures is calculated, and compared to a threshold.

Whenever the temperature is below the threshold (which will always be true when the engine is starting), the cylinder will be operated with a conventional four stroke Otto cycle.

Whenever the temp is above the threshold, the cylinder will operate as two stroke steam engine.

In steam mode, water is injected into the cylinder, flash-boiled into steam by the hot piston, cylinder walls, and valves, expanded to produce power, then expelled through the steam exhaust valve into the steam manifold, which leads to a condenser.

Furthermore, the amount of water injected is precisely controlled, so that the average torque of the two stroke cycle exactly equals the average torque of the most recent four stroke cycle.

When the engine is cold, the engine will run just like a normal four stroke engine. At higher temperatures, each four stroke cycle will be followed by one or more two stroke cycles; the number of two stroke cycles which take place between adjacent four stroke cycles will vary... hence the idea's name.

The advantages of this idea over a regular four stroke engine are the same as those of a six stroke steam hybrid, namely:

The engine isn't water-cooled, thus the engine block can be lightened by eliminating the coolant passages through it.

Each steam cycle produces power without consuming extra fuel.

In-cylinder hot spots are reduced or eliminated, which reduces engine knocking, and allows higher compression ratios.

There are also advantages over a regular six stroke steam hybrid:

Because the amount of power that must be produced by the steam expansion stroke is approximately one half of the power produced by the Otto-cycle combustion stroke minus the power consumed by the Otto-cycle compression stroke, we don't need very high steam pressures.

Since the maximum steam pressure is less than the maximum compressed air pressure, the variable stroke engine engine doesn't need to be any sturdier than a regular internal combustion engine.

Compare this with Bruce Crower's 6 stroke steam hybrid, whose prototype he built out of a diesel engine block, in order to contain the high steam pressures.

In addition, by injecting less water at the beginning of each steam expansion cycle, the steam pressure at the end of the expansion cycle is lowered, implying that water is being used more efficiently.

Last but not least, when the engine is warming up, we don't lose any power due to "coasting" through an unused fifth and six stroke; instead the engine runs as a normal 4 stroke engine.