a technical discussion, engine efficiency |
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a technical discussion, engine efficiency |
r_towle |
Aug 15 2005, 08:27 PM
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#1
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Custom Member Group: Members Posts: 24,624 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
so, I have been thinking (not always a good thing)
If an engine is basically an air pump, and the objective is to make it as close to 100 percent efficient, why is the exhaust valve so much smaller than the intake valve? Forget emmisions, forget fuel delivery, just want to understand why this is so...it is so on most engines... Rich |
lapuwali |
Aug 18 2005, 08:35 PM
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#2
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Not another one! Group: Benefactors Posts: 4,526 Joined: 1-March 04 From: San Mateo, CA Member No.: 1,743 |
For a long time, the theoretical maximum was held to be 500cc per cylinder, which is why you see so many 2.0 fours. However, these rules were engraved in stone in the 1950s, when metallurgy was nowhere near as advanced as it is today. British designers, in particular, were convinced of the soundness of this rule. The key factor is partly down to balance, but mostly it's the sheer forces of stopping and starting a piston bigger than that as revs increase. The forces go up as the square of the engine speed, so they get very serious indeed. That said, there have been a very large number of engines made with cylinders well over 500cc, including the ubiquitous 350cu in (5.7L) small-block Chevy, weighing in at 712cc per cylinder. There are 800-900cc single cylinder bike engines, too. Porsche themselves made 3.0 fours in the later 944s and 968s, at 750cc per cylinder. The balance factor on inline fours isn't very good, and Porsche did use balance shafts in these engines, mostly to keep them from shaking themselves and the occupants of the car to death. The balance factor on a flat engine is much better, so there's little reason to no go larger, and the 2.5 flat-4 Subaru engine is proof of that. The 3.2, 3.4, and 3.6 flat-6s in later 911s and Boxsters also thumb their noses at this "rule". |
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