ScottG (guest) View all posts in this topic  
Fri Jun 20, 2003 9:08 pm
Thermodynamics, the short lesson        

A parameter that is used to describe the efficiency of a reciprocating piston engine is the Mean Effective Pressure (MEP). MEP is the theoretical constant pressure, if it acted on the piston during the power stroke, that would produce the same net work that is actually produced in the cycle. You can increase this MEP to increase the thermal efficiency of the engine, look it up and you'll learn some techniques for increasing MEP.

Another thing that is useful to know is that Thermal Efficiency increases with increasing compression ratio. This is why diesel engines are more efficient. This increase in efficiency however is limited by the cutoff ratio of your piston cylinder assembly. The cutoff ratio is the volume in the cylinder just after ignition divded by the volume in the cylinder just after compression and before ignition. The smaller the cutoff ratio, the higher the engine efficiency. The compression ratio increase is also limited by "autoignition", which is fuel combusting under compression rather than spark in spark-ignition engines. The upper limit is needed to prevent knocking.

Here are some graphs to help you see the relations I just discussed. Rc is the cutoff ratio. Please remember these are curves that do not account for engine friction. That is why the curves seem high, friction is what brings them way down to the efficiencies we know and hate.

I won't go into crazy amounts of more detail from a thermodynamics perspective, but hopefully you know a little more about engines now. If you want to know more about what YOU can actually do to increase the efficiency, a simple google search will give you thousands of ideas. The best thing you can do to increase efficiency is reduce friction inside your engine, but that's not so easy. For those of you that care though, the Ninjette has a thermal efficiency of 20.4%, not too bad really. I am sure you can increase it if you want though.

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