Performance check: torque and power

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When talking about automobiles, especially sportscars and trucks, a lot of stock is put into the torque its engine provides and the amount of horsepower. Comparing the performance of one car with another’s often comes down to which has the bigger numbers, and for anyone who wants to get a better understanding of automobiles in general would need to understand these two concepts.

How are these numbers tied to performance, what is the science behind engine torque and horsepower, and how do these affect how a car feels?

When you turn the key and press the accelerator in an automobile, the engine produces torque and horsepower. The ignition of air and fuel in the car’s combustion chambers sets the complicated machinery of the crankshaft, transmission, and drive axles into motion, and this is what drives the automobile forward. To put it scientifically, the potential energy stored in fuel becomes the kinetic energy needed for driving. In this example, engines perform the work needed to move the car.

Power in physics denotes how fast work is accomplished or the rate of which a force acts over some distance. Power produced by an automobile engine is called horsepower. Eighteenth century scientist James Watt, who invented the steam engine, developed the concept. In mathematical terms, one horsepower is needed to move 550 pounds one foot in one second, or the power needed to move 33,000 pounds one foot in one minute.

Torque, or moment of force, is the rotating force produced by the engine’s crankshaft. Scientifically speaking, it is the measure of the turning force of an object. Torque can be generated without moving an object. When it does move an object, it then becomes “work,” and this is what most people think of when they think of torque. The more torque produced by an engine, the more work potential it has.

Simply put, horsepower is how fast an engine completes work, or applies torque, in a given amount of time.

Extending this to cars, a sports car has a lot of power due to how fast it can move itself over a given time period. A semi truck similarly has a lot of power because it can complete a great amount of work by hauling heavy cargo. The semi truck, however, likely has a lot more torque, because it has to work harder to get itself moving.

The relationship between torque and power reveal much about a vehicle: high torque tends to result in high power, but the load on the vehicle, and the way that it moves, make a big difference. This is the reason tractors and light construction equipment often have power ratings of under a hundred horsepower. In this case, the engines are built to be strong and slow, producing higher torque steadily over a slower period of time. Meanwhile, sports cars with a more than 1000 horsepower can use their engines to get to impressive speeds.

With a lot of torque, when you put your foot down, you can feel an immediate response even at low revolutions per minute. This is from acceleration.

Torque at the wheels is the real operator that causes the forces for acceleration, but a car needs to have high enough horsepower to weight ratio for the driver to feel that work being done and feel acceleration. If you increase engine power at any given speed, you increase acceleration regardless of engine torque.

For instance, a freight train, which has a large amount of torque in its turbo diesel engines as well as a lot of horsepower, also has immense mass. Passengers on the train barely ever feel the acceleration because the horsepower to weight ratio is not very good. A motorcycle does not have high torque with a smaller engine, and it does not make much horsepower either. But its weight is minimal so the horsepower to weight ratio is excellent, allowing it to accelerate very quickly.

Another thing to consider when talking about car performance is gearing. As gears are torque multipliers, gearing can have a huge effect on how much acceleration a car can have. A big engine with high torque and low horsepower, alongside a small high revving engine with low torque-high horsepower can accelerate the same with the right gearing.

This is because one engine can complete its work with pure strength, while the other can take advantage of leverage. This is why a 4-cylinder car and a V8 car can both accelerate equally fast if the 4-cylinder is set up properly.

With the right gearing, a small engine could pull a freight train. It would just take many revolutions of the engine before the train noticeably moved. You reach a point where the limiting factor is not the torque or horsepower of the engines, but the grip of the tires. — Bjorn Biel M. Beltran