Drivetrain loss isn't a loss, it's just a necessary expenditure

Percentage estimates of power used in drivetrains are wildly inaccurate. Photo: Shutterstock.
Percentage estimates of power used in drivetrains are wildly inaccurate. Photo: Shutterstock.

Power loss through the drivetrain (the gearbox, diff, hubs and so forth) is a common phrase heard and used by motoring enthusiasts when explaining the difference between the power at the engine and the power at the wheels, but it's a misnomer.

Looking at it like an accountant does, it's not a loss but a necessary expenditure, like wages or rent in a business.

If you have a power source that needs to go through some sort of gear reduction, then it's an essential part of the process of motivating the driven wheels.

It's not just internal combustion engines that use them either. Electric vehicles also benefit from having gears. Even though they can make all their torque from zero RPM, using a first gear significantly reduces the amount of electrical energy required to initiate movement (thereby increasing the range from the battery or fuel cell).

Either way, that drivetrain will provide resistance, such as friction and inertia (an object's resistance to a change in motion, and they have many rotating parts).

So, in the same way that a business has run at a loss when its total expenditure exceeds its total revenue generated during the same set period, the powertrain (engine and drivetrain combined) could only be running at a loss if it took more power to turn the drivetrain than the power unit can produce.

It is also very difficult to determine the drivetrain's expenditure without knowing both the figure at the flywheel and the figure at the driven wheels. Then it's a simple matter of A (engine power) minus B (power at wheels) equals C (drivetrain expenditure).

However, without knowing both A and B, there's no good way to estimate C.

It's not a percentage of power. The drivetrain doesn't care what engine is in front of it.

Take, as an example, a standard 190kW XR6 Falcon versus a very-modified 750kW XR6 Turbo, each using a version of the Barra 6-cyl engine, and both using fundamentally the same factory 6-speed manual.

The standard XR6 might see roughly 140kW at the wheels if it's really healthy (as people saw at dyno days when they were fairly new). That's a drivetrain expenditure of about 26 percent. But using the same percentage to estimate the modified turbo's usage, it would be spending 195kW turning the same drivetrain, more than the normal XR6 makes at the engine. Yes, it will spend noticeably more power, but don't try to account for that 145kW difference with heat. Just 1.45kW would keep your bedroom warm, so think what 145kW of heat would do.

It's also not a set kilowatt figure for any particular drivetrain, because it can vary with factors such as rotational speed.

From physics we know energy cannot be created or destroyed, merely converted from one form to another. That energy expenditure will be primarily turned into the two things we alluded to already, rotational kinetic energy (a drivetrain would keep spinning if allowed to) and heat (from friction).

It takes more power to spin something faster, or to reach a target speed sooner (with a greater rate of acceleration), and there are different drivetrain designs. They have moving components of different mass to one another, and of different size. And various size components have their mass at different distances to the axis of rotation, and if the outer part of that mass travels a greater distance it requires more effort to get it going.

We also know that the difference between the engine power and the power at the wheels is usually greater for all-wheel-drives compared to two-wheel drives. Again, this is due to the likelihood that the AWD drivetrain's moving components will add up to more weight as a consequence of there simply being more of them (more diffs, driveshafts, hubs, and other moving parts). That also means more surfaces interacting and therefore more sources of friction.

We haven't even discussed the torque multiplication effect of gear ratios on measurements at the wheels, so it's not hard to see that it can get really complicated, to the point where we just completely give up and talk in meaningless rough estimates instead.

Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.