Where is extra fuel consumption in RWD and AWD cars dissipated?

Question

I have understood that rear-wheel drive (RWD) and all-wheel drive (AWD) cars are less fuel efficient than front-wheel drive (FWD) cars, and that for this reason most cars sold today are FWD. I'm assuming front engine here, as is typical these days. I'm also assuming the AWD is permanent and not on-demand, and that the rear wheels are mechanically, not electrically driven.

However, if RWD and AWD cars are less fuel efficient, the extra fuel consumption must be dissipated as heat somewhere. Where is the extra fuel consumption dissipated? In the driveshaft? In the center/rear differential? Or perhaps in the brakes (due to increased mass of the RWD/AWD vehicle)?

Answer 1

The short, easy answer: heat.

That energy is being converted to heat in the driveline components for various reasons. It's important to note that these numbers are aggregates, however, and a really high end well engineered AWD or RWD system could outperform a cheaply manufactured FWD system.

Differential losses tend to be considerably larger, especially in the case of RWD and AWD vehicles where the torque path is turned 90 degrees as it enters the rear diff and exits it toward the rear wheels. In the case of hypoid-type gearsets (where the gear tooth profile is both curved and oblique) that are commonly used in RWD differentials, losses in the 6 to 10 percent range are the norm, while loss from the driveshaft(s) and prop shaft(s) tend to account for about 0.5 to 1 percent of total loss, depending on how well they're balanced and how many the vehicle is equipped with. In the case of FWD vehicles, the torque path is more direct to the front wheels and the use of efficient helical final drive gears means that drivetrain losses can be as much as 50 percent lower than on RWD and AWD vehicles.

Source: http://www.superstreetonline.com/how-to/engine/modp-1005-drivetrain-power-loss/

Answer 2

There is no fundamental reason for RWD to be less fuel efficient than FWD. There are many advantages to FWD, fuel efficiency is not a driver.

Most rear drive cars these days are sport(y) , performance , luxury, or SUV's that are not optimized for maximal fuel efficiency. That is the reason you will typically see lower economy numbers.

AWD is another matter as there are additional gears and bearings in the drive train that come with an economy cost.

Answer 3

Your premise is wrong. The main reasons FWD cars are more commonplace today are:

1. easier assembly: the entire drivetrain goes into the chassis as one complete assembly.
2. unibody construction: FWD cars don't need a transmission and driveshaft tunnel under the car, making the unibody construction optimized for its own sake.

Answer 4

It's difficult to reach a conclusive answer, but you're on the right track with the extra mass of a RWD transmission - the propeller shaft taking power to the rear wheels adds some considerable weight. The gearbox is also traditionally attached directly behind the engine, and this means it impinges on the passenger compartment. A tunnel has to be constructed over both the gearbox and prop shaft, which requires more metal versus a flat passenger compartment floor.

More moving parts also means more bearings, which means more sources of heat loss. Compared to FWD, there are bearings at either end of the prop shaft, and generally (on cars with two-piece shafts) a 'carrier' bearing in the middle. Some bearing designs require periodic lubrication, which is easy to neglect.

The differential on a RWD vehicle also has to rotate the drive through 90 degrees - the rear drive shafts are perpendicular to the prop shaft. Some energy is lost as heat in the differential doing this. By contrast, FWD cars use a more linear differential design (usually built directly into the gearbox) which has most of the gears rotating along the same axis.

AWD vehicles is much of the same - extra weight (second differential, second pair of drive shafts, prop shaft) and extra moving parts needing more bearings. Some designs use a simple mechanical central differential, others (such as Subaru) use a stack of clutches and vary the slip to transfer power between front and rear. Any time two components are not being driven in sync (such as clutch slip), there will be energy loss through heat.

In general, all the extra energy is being lost as heat, either through friction in the drive components, or through burning more fuel to compensate for the weight addition.