Quote:
Originally Posted by drmiller100
Hey Ken,
I'm trying to wrap my head around some things as you know. So we all "know" a smaller engine is more efficient then a big engine, right?
But, if take a 500cc engine, in theory we could find an RPM and run it at WOT to get our 22 horsepower.
In theory, we could take a 1500cc engine, and run it at 1/3 the RPM, at WOT, and get our 22 horsepower. So, given this example, out 1500 cc engine would be as efficient as the 500cc engine, right????
of course the bigger engine would be a lot heavier, as would the rest of the drive train.
The CX/VX honda engines were designed for this I believe - 2 valve per cylinder, teeny intake and exhaust, and higher compression.
Hey. Another question. Back to the monster diesels sort of. If I have a 1500cc engine at low rpm, will I lose less power to the cylinder walls? Or does the extra time because of the low rpm cancel that out?
maybe I'm over analyzing this - just looking for opportunities.
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Larger engines are more efficient because they have less cylinder wall surface area for the same displacement.
The larger engine would be less efficient at 22 HP than the smaller engine due to the fact that 22 HP would be low compared to the best bsfc map of the larger engine. The smaller engine even though it would be less efficient than the larger engine would be more efficient because it would be closer to it's ideal torque output. The larger engine would need to be at a higher HP load to reach it's best efficiency.
The Honda CX engine was 8 valves. The VX was 16 valves. In lean burn the VX engine had one intake valve that was barely open, which helped to create the swirl in the combustion chamber that was an essential component of a properly functioning lean burn. The turbulence helped to better mix the fuel and air to reduce the potential for certain areas in the mixture to be too rich while other areas were too lean.
The VX engine would pull fine from 1000 RPM. Not sure about the CX engine, but my VX would cruise at 1000 RPM, 30 MPG in 5th gear. For acceleration it would do it but it was better if you downshifted.
To answer the 1500 CC engine question. The ideal RPM is probably between 1300 and 2100 RPM. Above or below those RPM levels and you have losses increase due to different factors. That is a general rule with exceptions that would be due to engine design parameters like stroke, valve timing, weight of the reciprocating masses, intake and exhaust tuning, etc.
regards
Mech