Toymods Car Club Inc: Tech & Conversions => Fuel usage at same RPM levels in different gears

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oldcorollas
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Kita-Ku, Sapporo, Japan
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January 2003

Re: Fuel usage at same RPM levels in different gears	Tue, 17 June 2003 12:23

just another slant on it.. an engine will use (assuming injected and equal AFR's) an amount of fuel that is directly proportional to the air being consumed. since torque is directly proportional to air consumed (or volumetric efficiency), disregarding internal losses for different rpm, the amount of fuel for a given torque output will be equal. given that for cruising at a given speed will have the same air resistance, the torque at the wheels will need to be the same, so fuel consumed is equal. from this you add in losses...so in a higher gear (like 3rd instead of 4th) you have more engine thrashing around, so more losses.... but it also depends on your torque curve... if you are below your peak, and the car slows a little because of a rise in the road, you have to apply more throttle, cos oyu have less torque at lower rpm with same throttle if you are higher than your torque peak, then if the car slows, the torque increases, so car speeds up etc, all with your foot in same place.... this is of course assuming that the WOT curve applies to part throttle... also in a higher gear, if you want to speed up, you have more torque advantage, so takes less fuel than in a lower gear. all depend son a lot of things, but for a given road speed on a dyno, you will have more fuel used at higher rpm. same goes for on the road with wind resistance... but the difference may not be very much at all (and is basically the mechanical losses between the engine speeds). if you have EFI, borrow an oscilliscope and measure the pulse widths of the injectors on a dyno at same speed with different engine rpm... longer pulsewidth = more fuel... Cya, Stewart PS, if ya had a Megasquirt you could just look at the datalog to see what the pulsewidth was in milliseconds, then multiply by the number of injection events, based on rpm and you'd know exactly.. [Report message to a moderator]

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