Over the years a great number of reciprocating engines have been developed, each one having advantages or improvements over previous models.
The fossil fuel reciprocating engines have largely used a two or a four stroke cycle. The two stroke cycle engines having the advantage of one power stroke per revolution, as opposed to one power stroke for every two revolutions in the four stroke cycle. However, the two stroke cycle engines have two distinct disadvantages. The first being that their breathing is not always efficient, some unburnt fuel and/or oxygen is lost to the exhaust system, and some exhaust gases remain after the exhaust cycle. And secondly, two stroke engines often require oil to be added to the fuel for lubrication of the crankshaft and piston. Both of these factors combined mean that two stroke engines often produce higher levels of pollution.
A further problem with present technology engines is the conversion of forces felt by the piston into rotary motion. This is traditionally performed using a crank shaft. However, a crankshaft is most efficient when the crank arm is at right angles to the direction of motion of the piston, but is less efficient at other angles. And since the largest forces are felt on the piston around top dead centre, when the crank arm is almost aligned with the direction of motion of the piston, the result is an inefficient transfer of forces into the crank shaft.