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Old 01-02-2009, 05:31 PM   #5 (permalink)
amateur mech. engineer
Join Date: May 2008
Location: New York City
Posts: 112

Sporty Accord - '88 Honda Accord LX-i
90 day: 23.25 mpg (US)

Dad's Camry - '01 Toyota Camry CE
90 day: 22.81 mpg (US)

Artie's Camry - '98 Toyota Camry
90 day: 37.3 mpg (US)
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What you're missing is that the compression ratio will be high during low loads and low during high loads. The compression pressure will remain almost constant while the engine load changes. The higher compression ratio will give much better efficiency during low power operation. The peak pressures will be limited to avoid problems with knocking and preignition.

Variable valve timing will change the effective compression ratio but it can't give the engine high compression during light loads. A really efficient engine would have both variable compression ratio and variable valve timing. It could stay efficient over a wide range of power levels, unlike normal engines. The low combustion pressure and high pumping loss makes them less efficient at low load operation. Variable valve timing reduces pumping loss and variable compression ratio allows higher combustion pressure at low loads.

The disadvantage of the variable compression ratio is that it might be too expensive for some economy cars. I think that a reasonable compromise is to use a high fixed compression ratio such as 14:1 with variable valve timing, variable intake air temperature, and variable fuel mixture. The engine could use high octane fuel only for high power levels and low octane fuel most of the time. Some water injection may also be good for eliminating knock when making high power.

Last edited by Andyman; 01-02-2009 at 05:38 PM..
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