Fuel Economy vs RPM

I have an AT Toyota Comrey 1999.

I want to know why the RPM stays high when I do not press the accelrator and since fule consumption is directly proportinal to RPM. When I am going downhill I don't depress the accelrator but still the RPM stays above 2500 ( 85 MPH). Is the fuel consumption high even in this case?

Reply to
nat2k5us
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Reply to
karinhall

First, hi RPM does cause a degradation in economy, but not as much as to be a great problem. If you are towing or have a hill to climb, your engine needs RPM to deliver torque.

At 85 mph. 2500 rpm on a down-hill grade is to be expected. I'm surprised its not higher.

Jason

Reply to
Jason James

I also read about a fuel cut off when going downhill .. i.e only the gas required to keep the oxygen sensors warm is pumped.

Reply to
nat2k5us

Normally, that is called the throttle.

If you cut off fuel completely, the engine, of course, stops running (even though the drive shaft may be causing the parts to rotate) and must be restarted.

Merritt

Reply to
Merritt Mullen

The computer controlled electronic fuel injection system maximizes fuel economy. If you're coasting downhill, regardless of engine speed, with the throttle closed, fuel consumption is minimal. Fuel consumption occurs when fuel is sprayed through the injectors. Injector "pulse width" (how long they are open) is controlled (in addition to engine speed) by temperature, throttle position and engine load (intake manifold vacuum). To go one step further, with the throttle closed, manifold vacuum is high, so fuel consumption would be low. At larger throttle openings, vacuum drops, fuel consumption increases. At highway speeds, the lockup torque converter engages to improve fuel economy by eliminating drive line inefficiency. That is why the engine speed (RPM) is constant coasting downhill at 85 mph.

Reply to
Daniel

Thanks Daniel and Merritt .. just by not keeping the a leg on the gas when the car is travelling with the desired speed I was able to increse my mileage by 20%.

Reply to
nat2k5us

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