First, let me define relation between Power, Torque and RPM
(The petrol vs diesel engine comparison is written after this explanation)
In the automotive world, lots of things move in circles rather than in straight lines. Think of the wheels or engine. Distance is no longer meters, it is revolutions measured in radians. Speed is revolutions(or radians) over time instead of distance over time, for example, revolutions/minute(RPM) or radians/second. Mass is called inertia. We still need some kind of’push’ to get something spinning. Instead of force, we have torque which is’twisting’ force so to speak. Like force, torque must be applied to get something to spin faster or slower(acceleration).
This scenario is analogous with lever system. We know, for a lever F1d1= F2d2. Where F1 >> F2 but d1 << d2. So by moving the lever a greater distance we can lift enough load though the work done in both cases are same. As I already said, in automotive world, the force is equivalent to torque and distance is equivalent to rotation. That means, by rotating a wheel more, we can generate higher torque on it(though power delivered to wheel remains the same in fact a bit less because of frictional loss)! Using gears, we just supply more torque to road wheels, so that the car can move. As engine revs faster and faster, torque rises(to a certain limit and then it falls). Using gearbox, the torque at engine’s flywheel is multiplied at road wheels. When this torque produces enough force at the wheels, the car moves. Thus the main function of a gearbox is to act as a torque multiplier so that road wheels get enough power to pull the car.
Torque is measured by the amount of force applied tangentially at a given distance and that is force * distance(Newton meter).
Work in the rotational world is still basically force*distance. Torque is our force and distance is our revolutions in radians(1 revolution= 6.28 radians roughly 2 pi). We use the same units such as Nm. The reason the units do not change is that the distance(radians) is a dimensionless quantity.
Power= Work / time= Force * distance / time= Torque(Nm) * revs(2pi radian distance per second)
= Torque * revs/min * min/60s *(2 * pi)= Torque * RPM * 0.1
Usually max power occurs at a higher RPM than max torque in most car engines.
Now let us see what happens when you start car from rest.
A car is moved gradually when started. At this state, engine revs slowly – thus it can’t produce enough power to road wheels to move the car. But, even at low rev, engine does develop some torque. Using gearbox(say 1st gear), this torque is multiplied several times(appx. 15-20 times in most cars). Now the torque produces sufficient power to the road wheels to enable the car to move. As engine revs higher, torque becomes less dominant and engine itself can generated enough power to make the car move. So for this reason, you usually shift to higher gears as speed rises. After 60 km/h you can use top gear in most cars.
Thus we note following points.
Engine’s power becomes dominant in high rev and high speed. In fact, what will be the car’s max speed depends on engine’s power.
At low speed, it is torque, which moves the car.
For this reason, all car engines produce max torque at lower RPM and max power at higher RPM(just examine any engine’s power torque curve)
Torque is the deciding factor for a vehicle’s traction – i.e. how much load it can pull. For this reason, truck engines are designed to produce very high torque at comparatively low rev. Also, diesel engines produce more torque than petrol engines(of same size). This also explains why truck engines are usually diesel engines.
It is possible for a car engine to pull huge load(increasing torque using several gears) as of a truck engine. However, to produce the required torque, the car engine has to rev at very high RPM, which will make the engine seize very soon and will use too much fuel(which is thus not economical)!
Usually an engine’s idle speed is around 800 – 1000 RPM and a car’s starting(just moving from rest) rev is around 1200 – 1500 RPM.
An engine’s torque peak will be found when it finds its maximum volumetric efficiency. This is the point when the bores are most completely filled with mixture. If you can accomplish this at a low RPM(as most engines do) you can start applying your force sooner. You have more power, more twisting force on the axle.
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NOW PETROL VS DIESEL ENGINE
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From the above discussion, we can conclude which engine is better.
Well, better for which purpose?
If your sole aim is to pull load - definitely diesel engines are better because of their higher torque generation at low RPM. That’s why lorries and buses are all run by diesel. Also, large vehicles(lorry, bus etc.) don’t need to travel fast but they need to carry huge load. So, a low RPM yet high torque engine is required for them.
So, first conclusion is, torque is associated with high load carrying capacity. So, bueses and lorries use diesel engines.
Now for a passenger car, good acceleration(pick up) is more desirable rather than load carrying capacity. Small cars should always(and remain) have petrol engines. You can rev the petrol engine high and thus can go faster. Diesel engine has more NVH(Noise Vibration Harshness) than petrol engines. Of course, you can reduce these by using appropriate technologies - but it will then increase the size and cost of the car.
So, second conclusion is, if you buy a small car, go for petrol engine.
Myth: Diesel engines last longer than petrol engines.
Fact: Diesel engines operate at much lower RPM range than petrol engine’s. If you don’t rev your petrol engine too much, your petrol engine WILL last as long as a standard diesel engines! That’s why formula 1 car engines last only for few races(because they are revved at 15000 RPM).
Myth: Diesel car is cheaper to run.
Fact: Only if you use it commercially. Only in India diesel costs much less than petrol. In most other countries, where petrol costs nearly as much as diesel, diesel cars are not that cheaper to run.
So, the last conclusions are:
If you want a small car which you will not use too much everyday, always choose a petrol car.
If you want to transport heavy goods, or will travel extensively everyday, then choose a diesel car.