What are you torquing about?

How many times have you heard claims that some diesels can accelerate better than their petrol counterparts because they've got more torque? Ever wondered if it's true? Let's try thinking about it a little (without getting too bogged-down or pedantic), with the help of some basic concepts and terminology:

Power

The rate at which energy is converted to mechanical work (typically at the flywheel)

Torque

The turning force, often in ftlbs, equivalent to the force in lbs at a distance of 1 ft

Speed

The rate at which the car moves, often in mph

The two figures that you usually see for engines are peak power and peak torque. Neither tells anything like the whole story of how “fast” it could make a car. For the engine's contribution, you need a full graph of its power or torque against its rotational speed (revs). Either graph can be derived from the other, as power is torque times revs[1].This is why they always cross at ~5250 rpm when using bhp, ftlbs and revs per minute.

Now, let's think about the following question, the answers to which are hopefully instructive:

What accelerates a car?

Sounds simple, but the (technical) answer is slightly surprising - it's the road! The torque from the drivetrain through the wheels pushes the road backwards, so the road pushes the car forwards[2]!

To accelerate as fast as possible, should you select the gear that maximises the engine power or torque at your current speed?

This is what enthusiastic driver care about! The answer is somewhat complicated by gearbox loses. If we were to assume that the possible choices of gears are similar in efficiency, then the answer to the above is power. This can seem counter-intuitive, as it's the wheel torque that is often seen as more directly linked to the force at the wheel. However, that depends upon the gear selected, which is what we’re trying to work-out!

As we're asking about accelerating from a given speed, it's actually the power that is more easily converted to acceleration - power being force times speed[3], thus making the selected gear irrelevant[4].

Are people wrong in thinking that torquey engines make for good acceleration?

In so far as you can make sweeping generalisations, they're not. Engines with high peak torque typically have a wider usable power range, so can average better acceleration throughout a road speed range. Again, it's the graph of torque (or power) against revs that's important, not just the peak.

Diesel engines usually have a more limited rev range than their petrol counterparts, but if you measure the percentage of the rev range over which you have more than, say, 75% of the peak power, you'll usually find it to be larger than that of a typical petrol engine. That reduces the number of gear changes required, and so the time lost there.

So, the next time that someone tells you that a high-torque diesel car accelerates quickly because it's got so much torque, ask them why modern Formula 1 cars have peaks of over 700 bhp, with only around 200 ftlbs of torque (unsurprisingly, I haven’t seen precise figures). It's not quite that simple, but they certainly look to maximise peak power, not torque. They're not after a relaxed driving style though :)

© 2010 Roger Nixon. All rights reserved.