The reason for using big engines that have high torque when we can use a smaller, low torque engine and use a transmission to get the torque needed?


Smaller engines should have better fuel consumption so this should be better, right?

In: Engineering

Transmission converts speed into torque. That is, a small engine would take a *long ass time* to do the same job as a larger one. Not much point in building a truck if it can only do 3km/h on that sewing machine motor, no?

Also, larger engines are usually more fuel efficient. The small engine uses less fuel each second but if you take all the fuel it needs to move the same thing the same distance, it’ll come up as more in total.

Torque is only half of the equation when it comes to towing a lot of weight. The other half is weight, since that increases the traction that the wheels get. So having a big engine is a bit of an advantage in that regard.

That said: If you need a lot of weight anyway and you want to use as little fuel as possible, the best option is a diesel engine. That is for example why you never see a big semi truck using a gasoline engine.

We already do that. Low gears are exactly there to increase the torque when the vehicle is starting from a stop.

You can get any torque you want from any engine size you want. Torque can be arbitrarily changed by using gears. That isn’t a problem.

What is a problem is engine power. This cannot be gamed using tricks like transmission, it has to come from fuel – the more fuel burned the more power. You can increase the amount of fuel going through the engine by increasing the cylinder size or by turning the engine faster. That’s what you do when you press the gas pedal – you increase the power by increasing engine revs.

So, you can have roughly the same power at 500cc engine at 10k rpm or 2000cc engine at 2500rpm. But a smaller engine will wear out faster and is less efficient for reasons related to fuel-burning process. It’s acceptable for a motorbike that wants a lightweight engine, but a truck needs much more power and doesn’t mind to carry the weight if it comes with better fuel efficiency and longevity.