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Originally Posted by stoinkythepig  Lots of small pistons moving fast are better for generating HP for a given displacement (due to a higher rev ceiling), not necessarily for better fuel efficiency. For fuel efficiency in today's engines, you want a slow turning engine operating at WOT under heavy load, at the engine's torque peak, all the time to reduce pumping losses. This is one of the reasons why diesels are so efficient, they run slowly (compared to gas engines) and have no throttle to restrict airflow. They also run best under a heavy load (at least 70% of their rated output)
The latest Cycle World magazine has a great Kevin Cameron essay about improving efficiency of gasoline engines. He's a MA native (I'm pretty sure) and one of my favorite writers. |
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I understand but you are a little off track. I was trying to keep it simple and assume people were driving about the same vehicle. The Diesel is an entire different cycle and from what I remember makes great gains from high compression. I was trying to give the guy some guidelines to making an informded decision about what to buy. Diesels are a great choice and I personally would consider any VW 2.0l turbo if I could.
Yes, small pistions have less mass and can change direction faster. Their valve trains have less mass and can turn faster. This is why more small pistons turning faster will have more hp compared to less big ones that can not turn as fast per the same displacement, they simply move more air and therefore can burn more gas which has nothing to do with mpg if you lose it all to heat.
So since no thermal guys jumped in I did a quick look to refresh my memory. If the surface to volume ratio is large you lose more heat, hence your mpg goes down as your engine can not convert this heat lose to motion. I then wrote a small program to calculate this ratio and found that really big cylinders should save you a lot of heat.
So this guy should buy a small car, with great cd that has an engine with one huge cylinder. If I'm right. That should reduce internal friction as well. I wonder what the mpg of my tractor is?