Despite the fact that speed limits are the same for everyone, one of the figures that everybody wants to know when researching their new car is ‘how fast is it?’.
0-62mph times are all well and good, and they’re a good overall indicator of how a car feels to drive. But what happens when you put your foot on the throttle? And what else should you take into account when shopping for your car?
How acceleration works
In a conventional fuel-injection engine, when you press the throttle, fuel is forced into the engine, where it mixes with air. The spark plugs ignite the compressed fuel/air mix, and this – what is essentially a controlled explosion – forces up a piston. Many pistons working together driving the crankshaft, which is connected to one (or both) of the axles, turning the wheels. The harder you press the throttle, the faster the pistons fire up and down, and the faster the crankshaft spins. It’s that simple.
Turbochargers work to force more air into the engine. Engines normally rely on atmospheric pressure to force air in. At high revs, engines without a turbocharger won’t get enough air from atmospheric pressure alone, meaning it runs inefficiently.
The turbocharger uses exhaust gases to drive a turbine that draws in additional air, increasing the amount of air being forced into the engine and increasing efficiency. It’s because of this strive for efficiency (and fuel economy) that diesel cars have had turbos fitted for years, and an increasing number of petrol-powered cars are being fitted with them too.
Power and torque
In short, torque is how strong your engine is, and horsepower is a measure of how fast it can put that power to work. Torque is often known as ‘pulling power’.
A dynamometer is a machine used to calculate horsepower. It measures torque and revs, and these are inserted into a complex mathematical equation, which spits out the power figure. Engines with the same ‘peak torque’ can have different power, if the revs at which that peak torque is delivered are different.
If your car’s peak torque is delivered at high revs, you may find you have to change down a gear (and send the revs higher) to get the power you need. You’ve probably found yourself doing this when going up hill or overtaking a slower car.
Therefore, considering we spend most of our time driving at low or medium revs, cars that deliver their peak torque at lower RPM will feel more useably powerful in everyday driving. Historically, diesel engines have done this more often than petrol engines, which is why most HGVs, buses and tow cars are diesel-powered.
BHP, PS and kW
Car makers often express the power in one of three figures.
- BHP is generally considered the industry standard means of expressing power.
- PS (or Pferdestärke, horse-strength in German) is commonly used by German car manufacturers. 1PS is 0.986BHP.
- kW (kilowatts) is the EU’s ‘legal’ measurement of engine power, and this is how an increasing number of carmakers are expressing their engines’ power – especially in electric vehicles. 1kW is approximately 1.34BHP
On this basis, the same car could be described as having 100BHP, 101.4PS or 74.6kW of power.
Torque refers to the amount of pulling power an engine delivers when working at different speeds. The more torque an engine has, the more pull (or acceleration) it offers at lower revs.
Torque is nearly always discussed alongside brake horsepower, as it gives clues to how quickly the engine will be able to shift the car’s weight when overtaking or pulling away from traffic lights.
All of the above assumes we are talking about a combustion engine, which uses mechanical power to move the wheels. Electric cars have very few moving parts, with no pistons revolving. The upshot of this is that all of its power is available from a standstill – meaning electric cars are often enormously powerful from 0-62mph, and on the move.
This is great news for hybrid drivers, as the electric part of the drivetrain is often used to support the engine at low revs – typically when pulling away from junctions, for instance.
BHP, torque and 0-62mph figures often aren’t especially helpful to normal drivers – they’ve been calculated in a laboratory setting, and peak torque is sometimes delivered at very high revs that you are unlikely to experience in typical driving.
The best way to get a feel for a car’s acceleration is to physically drive the car. Get behind the wheel and feel how quickly it reacts in a real world environment – powering up a steep hill, for instance, or acceleration from 40mph to 60mph to simulate an overtaking manoeuvre. It’s why you shouldn’t discount a car with a lower peak torque figure, if that torque is delivered at lower revs.Arrange a test drive of any turbocharged or hybrid Lexus vehicle at your nearest Inchcape Lexus location