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Old 03-09-2011, 11:07 PM   #12
amanichen
Scooby Newbie
 
Member#: 163999
Join Date: Nov 2007
Chapter/Region: MAIC
Location: Centreville, VA
Vehicle:
07 WRX

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Quote:
Originally Posted by bal00 View Post
No, that's not what it means. Think about it for a second.

Gas engines have throttles, and whether you restrict the intake air flow at the throttle or at the air filter makes no difference.

If you lose 50 mbar across the air filter, you can simply open the throttle a bit further, get the same amount of air into the cylinders and burn the same amount of fuel.

edit: aschen was quicker.
The more closed the throttle is, the lower the pressure in the intake is, and the greater the pressure differential between the inlet and outlet of the engine. Maintaining a greater pressure differential requires more work, and therefore requires more fuel. The closer to ambient pressure you can get the intake manifold, the less hard the engine has to work. I really hate to use the term "pumping losses" but that's it in a nutshell.

However, as most people have observed after adding a hugeass cone filter to their turbocharged vehicles, the increase in fuel economy and power associated with "high flow" (lower pressure drop) air filter is generally insignificant, as is the reduction with a dirty vs clean filter. There are so many energy losses associated with ICEs and the overall vehicle, that the pumping loss is relatively small fraction of the total energy used.

A more efficient configuration is having no throttle, and controlling volumetric efficiency using valve timing only. If you lower the intake pressure drop you can get the same amount of air to flow into the engine in a shorter time, and reduce the time spent "pumping" against a higher pressure differential between the intake and exhaust manifolds.

VVVVVVV

Quote:
Originally Posted by aschen View Post
Even if you throttle with valve timeing you still have "pumping losses" I think for a fixed discplacment engine.
That is correct - you cannot completely eliminate "pumping losses" because engines are air pumps by nature - they cause a lower pressure on the inlet and a higher pressure on the outlet.

Quote:
The way to get around this inherent compromise for spark ignition engine is to have one that can operate in lean burn (throttle with amount of fuel injected) or variable discplacement where the intake volume is always atmospheric but you vary the intake stroke or whatever to adjust the power level.
Exactly, such as on a diesel =) or an as yet to be made truly variable displacement engine =(
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Last edited by amanichen; 03-09-2011 at 11:43 PM.
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