Instead of a cylindrical piston being slid inside a cylinder, how about a pivoting square piston engine?
A New Zealand company has redesigned the two-stroke combustion engine with the intention not only of overcoming the shortcomings of the conventional sliding-piston two-stroke engines, but also to produce a combustion engine that can better exploit hydrogen as a fuel.
The Pivotal engine replaces the conventional sliding piston with a pivoting piston, the pivot point of which is at the back of the piston. (See cutaway diagram) The piston pin position connects to the crankshaft via a connecting rod. The side surface of the piston forms the inner wall of the combustion chamber when the unit pivots up in the compression cycle.
The result, according to the company, is the elimination of piston rock or slap, improved compression sealing and reduced lubrication requirements.
Sounds like a really clever idea. They have built a 4 cylinder, 2.1 L prototype of the engine:
Pivotal joined with Mace Engineering to develop a prototype 2.1-liter four-chamber gasoline pivotal-piston engine, which produced 170 kW of power while weighing 65 kg — a power density of 2.6 kW per kg of engine weight.
If optimized for hydrogen fuel at an air/fuel equivalency ratio of >.6, Pivotal estimates the output to be in the region of 120kW–130 kW. This hydrogen output represents about 1.8 kW per kg of engine weight—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.
Out of a 2.1 L engine without forced induction, 170 kW is an amazing figure. But 2-strokes do have a much better power to weight and power to capacity ratio.
Flash Animation: http://www.pivotalengine.com/flashversion.html