Quote:
Originally Posted by drmiller100
you pump three times the air, you have 3 times the pumping losses with a diesel.
friction is mouse nuts in terms of efficiency (mouse nuts are so small as to be really hard to measure).
Diesels have a slow burn rate, so they can't turn high RPM. To make a diesel compete with a gasoline engine, the only way to keep the weight of the engine similar is to put a turbo on the diesel.
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3x the air doesn't mean 3x the pumping losses. Pumping losses depend on lots of things, but primarily the velocity of the air. The turbo increases the density of the air so, at the same engine speed you can put in 3x the air at roughly the same air velocity, because the density is 3x higher. Now, obviously higher density increases friction somewhat, but velocity is the main driver.
Friction is typically a lesser deal than pumping, but only slightly less. Friction within the engine can be very significant at high speeds and light loads (where it's actually more significant than pumping sometimes). Friction is roughly proportional to speed to, so if the speed is half, the friction is half.
As far as the power density vs. gasoline you're right--turbos are a big factor in trying to compete with gasoline. The burn rate is part of it. Another factor is the higher air-fuel ratios.