“Power is how fast your car hits a wall at full throttle; torque is how far it moves the wall after impact,” is one description of the two units I recently heard. Unfortunately, this explanation is not entirely accurate, but it is a way of making sense of the concepts in simple terms. Although car fanatics grow up with these engine performance indicators, few understand their meaning or how they are measured. With this article, we intend to settle the confusion.
The idea of force in a straight line is an easy concept to grasp. Ever pushed a vehicle that ran out of fuel? You have to exert a force on the vehicle to move it. Force is measured in newton (N). To understand newton, you only have to step on the bathroom scale (see figure 1). Take the mass reading in kilograms (say 80 kg) and multiply it with Earth’s gravitational constant (9,81 m2) and you know what force (80 x 9,81 = 784,8 N) you are exerting on the measuring device.
Torque is essentially just a force exerted on an object tangentially to a centre point resulting in a torque moment trying to rotate the object. Although this may sound foreign, we all use torque daily; for example, opening the cap of a bottle. A torque (force creating a moment around the centre of the cap) is needed to break the seal and release the cap.
The difference to force in a straight line is that the distance of the tangential force to the centre contributes to the magnitude of the torque. Take, for example, a special wheel spanner one metre in length (figure 2). If a force of 784,8 N is exerted at the end as shown (same 80 kg person standing on the tip), the wheel nut would experience a torque moment of 784,8 newton metre (N.m). The tangential force is therefore multiplied with the distance to calculate the torque value and this also explains the unit N.m.
When it comes to internal-combustion engines, the torque output is measured at the output shaft (or crankshaft) of the engine. This torque moment is the result of the in-cylinder pressure on top of the piston during combustion forcing it down the cylinder. This linear motion is converted to rotation by the con-rod and crank-throw connection (figure 3). On an engine dynamometer (above), a brake – usually an eddy current device or electric AC motor – connected to the output shaft is used to brake the engine and keep the speed constant. The torque reaction is then measured with a load cell connected to the brake (dyno). The entire maximum torque curve of an engine can be generated by braking the engine at incremental intervals from idle to maximum engine speed while the engine is at full throttle.
When we explained torque, time did not enter the equation. We never asked how long it took to unscrew the cap of the bottle or how quickly the wheel nut was tightened/removed. The link between torque and power is time. Therefore, it makes sense that, when a stopwatch is involved, power is the important quantity (actually, power-to-weight ratio, as this is key to acceleration). In the case of an internal-combustion engine, time refers to the engine speed or rotational speed of the output shaft. Engine power can easily be calculated if the torque figure and engine speed at the specific load point are known (see below). This is what happens on an engine dyno when the torque curve is produced. Power is not directly measured; it is calculated. It helps to think of power as the rate of energy rather than force. The reason is the unit for energy is joule and the unit for power is joule per second (J/s).
An engine produces 426 N.m at 8 400 r/min.
How much power is developed?
P = 2NπT/60
P = power (W)
π = Pi’s constant
N = engine speed (r/min)
T = torque (N.m)
= 2(8 400)(3,14)(426)/60 = 374 730 W or 375 kW (the maximum power of the 4,0-litre Porsche 911 Speedster engine)
Gearing vs. torque and power
A transmission is sometimes called a torque-multiplier for good reason. If you’re a cyclist, you already know this fact. By gearing down for an incline, the torque in the pedal crank (as a result of your force on the pedals) is multiplied to the rear-wheel hub. It has to do with the size of the gear pair linked together. If the cyclist is able to produce 300 N.m at the pedal crank and the gearing is such that the front gear is half the size of the rear (also implying the number of teeth), the rear wheel hub will experience 600 N.m.
In a vehicle’s transmission, the first-gear ratio can easily be 3 to 1 with a final-drive ratio of again 3 to 1. If the maximum engine torque is 200 N.m, the driven axle will experience
200 N.m x 3 x 3 = 1 800 N.m. If second gear has a 2 to 1 ratio, the drive axle torque drops to 1 200 N.m when second gear is selected.
However, power is not multiplied over gearing and stays constant (accepting some efficiency losses). This is easy to understand because, when the torque is multiplied through gearing, the rotational speed is reduced by the same ratio. Again, the cyclist (with a fixed maximum power output) would appreciate that the “granny gear” lowers the cycling speed dramatically. This also explains why a 30 kW engine (producing only 150 N.m) in a tractor can pull out tree stumps. The torque can be hugely multiplied through very low gearing, resulting in a massive stump-pulling force, but the speed at which the exercise happens is extremely slow as the 30 kW stays constant.
Petrol vs. diesel torque
As explained earlier, the torque developed by an ICE is as a result of in-cylinder pressures. Petrol engines are knock-limited, meaning if the in-cylinder pressures rise too high, the engine experiences the harmful event of auto-ignition where fuel combusts spontaneously without spark initiation. On the other hand, the in-cylinder pressures of a diesel engine are limited only by the strength of the components because combustion takes place using compression ignition. Turbopetrol engines have closed the gap on turbodiesels but ultimately a diesel engine with similar capacity (and induction; for example, turbocharging) would triumph in the torque competition because of this.
The reason why similar capacity (and induction) petrol engines can produce more power is because of higher possible engine speeds. Diesels are speed limited to around 5 000 r/min because of the time needed for the diesel injection and the compression-ignition process. Petrol engines, however, can easily exceed 5 000 r/min (the Porsche Speedster’s engine can rev to 9 000). Below is a comparison of the torque and power curves of two theoretical engines of the same capacity. One is diesel and can deliver 400 N.m, but only up to 5 000 r/min. The other is a petrol engine that can deliver only 300 N.m, but up to 8 000 r/min. It is clear the petrol engine delivers more power (251 kW versus 209 kW) and ultimately higher acceleration
in a similar vehicle. The diesel version with higher initial acceleration has to shift to second gear at 5 000 r/min (with the lower torque multiplication factor resulting in less tractive force on the driven axle) while the petrol engine can continue in first gear up to 8 000 r/min and gain the upper hand.
Electric motor torque
Electric vehicles (EVs) are the future of motoring. This is good news from the point of view of torque availability. The moment at the motor shaft is produced by magnetic forces already present at zero engine speed. Therefore, in contrast to an ICE, which needs to run to produce torque, maximum torque is available almost from the word go. The typical torque curve of an electric motor depicts this trait with the torque dropping off only at higher speeds owing to back electromotive force (EMF). Without going into the details of back EMF, it is essentially a force which opposes the rotor rotation at speed because of the voltage generated in the coil opposing the supply voltage in polarity. The most satisfying aspect of electric motors is there is no lag when the driver requests a torque demand. This would please even diehard petrolheads.