Engineering Question #358

Robert, a 29 year old male from the Internet asks on March 11, 1998,

What is more efficient: to push or pull a car (front wheel vs. rear wheel drive)?

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The answer

John Jones answered on March 11, 1998

To first approximation, it makes no difference. The major inefficiencies in a car are the engine's losses in energy: about a third of the fuel energy goes to heating the exhaust, and another third goes to heating the radiator water. After that, the biggest inefficiency is mechanical friction between the piston and the cylinder, and friction in the drive train (connecting rod, big end bearings, crankshaft, gearbox). The choice of front-wheel versus rear-wheel drive has no effect on any of these inefficiencies.

If the engine is in the front of the car, rear-wheel drive will need a longer drive shaft; this will add a bit to the weight of the car, but it's a minor factor. Shifting to four-wheel drive WILL reduce efficiency, because you now have a more complicated drivetrain, with more sources of friction.

>From a mechanical viewpoint, a pull is as efficient as a push.

In looking for inefficiencies, you need to look for wasted work -- is there work being done which doesn't contribute to the forward motion of the car?

I don't see any major sources of wasted work in the shift from front-wheel drive to rear-wheel drive.

The important differences between front-wheel drive and rear-wheel drive are more in the ease of steering the car, particularly in slippery conditions, than in the efficiency.

One way of investigating this would be to search in auto magazines and find fuel consumption per km for half-a-dozen front-wheel drive cars and half-a-dozen rear-wheel drive cars, where all the cars are the same age and in the same weight range. I would predict that no significant difference will be found in fuel consumption.

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