Quote:
Originally Posted by ST2008
I always think that the car is more efficient if I can drive it with less engine load for the same speed. Am I wrong ?
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Higher load is better for any given speed. Here's the explanation...
There are 2 ways to make more power:
1. bigger explosions (more torque)
2. more frequent explosions (more RPM)
Consider that the biggest explosion the combustion chamber is able to produce happens when the throttle is wide open. With no restriction, the intake stroke fills the cylinder with the same density of air as the open atmosphere, and there is no vacuum (pressure difference between the open air, and the inside of the cylinder). This is the maximum amount of oxygen the combustion chamber can hold without forcing more air into it with a supercharger. Fuel is added to the combustion chamber in proportion to the amount of air drawn in, and since the cylinder has the maximum amount of air, the maximum amount of fuel that can be burned cleanly is injected.
Since we know that restricting the intake by closing the throttle is not efficient, it follows that bigger explosions are a more efficient use of fuel than smaller, more frequent explosions.
If low load were efficient, then it would be best to run in 1st gear on the highway, with the engine screaming away, and the accelerator barely being pressed. Instead, reducing the RPM and pushing the accelerator pedal more improves efficiency. You accomplish this by shifting to a higher gear.
Like I said earlier, none of this is of much practical use unless you pulse and glide, or drive a manual transmission. An automatic car is going to select the appropriate gear given the speed and engine load. There is a trick to get an automatic to shift to a higher gear sooner by briefly backing off the throttle a little, letting the car shift to the next gear, and then applying more throttle again. It's not going to save much fuel in the grand scheme of things, and it's too much effort for most drivers to be concerned with.
*In case anyone is curious, the horsepower equation is:
Horsepower = Torque (how big are the explosions) * Engine RPM (how frequent are the explosions) / 5252
You can arrive at the same horsepower by increasing torque and reducing RPM, or by increasing RPM and reducing torque, the former being more efficient than the latter.