[confusious says (OP)]

Hi folks, I just want to settle this notion that I was told years ago regarding the humble two-stroke engine, is it true, that it is still the most efficient fosil fuel engine for power output for it's weight? and if so, why aren't manufacturers getting in on the act?...my idea for a very efficient car, would be to build it with a powerful electric motor and have the two-stroke engine charging the batteries

[CliffordK]

Two stroke gasoline engines are typically the most efficient for size & weight.  However, they are often the least efficient for fuel efficiency & SMOG.  At least one compact car, the Subaru 360 used a 2 stroke engine.

Detroit Diesel makes two stroke supercharged diesel engines.  There would be no oil needed to be added, and since the engine is injected during the compression stroke, one can blow air past the cylinder without wasting fuel.  The supercharger may be necessary to blow out the cylinder.  I'm not sure about efficiency as they were often used in military vehicles such as the gama goat.

[SeanB]

You can make a very fuel efficient 2 stroke engine, but it has to be fuel injection and supercharged, and will be quite complex, especially with regards to valving and timing, especially if you are looking for high RPM out of it.

The majority of big ship diesel engines are 2 stroke though, they rely on having massive superchargers to scavenge the cylinder, and slow enough that valve timing is easy to do if rather large and slow. They inject hot fuel into the cylinders, which is the same stuff used to line roads, just without the gravel. Used engine oil is burned as well in them, it is the same oil that is used coming into port and leaving, outside they burn high sulphur crude bottoms. Those have no electric start, more like a 2000kW starter diesel motor to provide oil pressure and turn it over. You know it is a big engine when you see a person standing inside a cylinder and using a ladder to get out.

[wolfekeeper]

Actually rocket engines are the most efficient engines, at least at their optimum speed... about Mach 15... which is excessively fast!!!

It's because they have a huge expansion ratio, they're able to extract about 60-70% of the energy in the fuel and turn it into fast moving exhaust. When the vehicle speed matches the exhaust speed, then the mechanical transfer between the exhaust and the vehicle is virtually 100%, so you end up with an overall efficiency of over 60%.

CHP systems can do better still, you can get 90+% if you use the waste heat for something. Not sure, but wind turbines should be over 70% efficient as well, but you care less about how much you're slowing the air down.