Why petrol engines outperform diesel engines, assuming they produce the same torque?


I get the basic understanding. More torque = you can pull more. More horsepower = more power output.

When I get it on paper, I don’t get it in real life performance. Why does the same Kia Optima with 163HP / 196 Nm petrol engine accelerates faster (9.4 seconds) than 141HP / 340 Nm engine (11 seconds) when it has more torque, so it can pull faster?

Why does a petrol engine outperforms diesel, assuming they both have the same torque, so on paper, they both should pull similarly (assuming they both need to shift into third gear to reach 100km/h)?

In: 0

You have the basics wrong.

The following has started to become my sort of general explanation of (engine) torque and power, and how they related to the acceleration of a vehicle:

Engine torque, which is what is meant when one talks about torque and cars, tells you absolutely nothing on its own. What actually accelerates a car is the torque at the wheels. In fact, that is not even true. What accelerates the car is the force at the wheels. And the force at the wheels is basically the engine torque, multiplied by the gear ratio and then finally divided by the tire radius.

It is that multiplied by the gear ratio that is extremely important. What this means, is illustrated using the following example.

Take two cars. I’m going to give them extremely hypothetical engines. Car A produces 300 Nm from 0 RPM to 2000 RPM, and 0 Nm at all other RPMs. Car B produces only 200 Nm from 0 RPM to 4000 RPM and 0 at all other RPMs. Both cars have transmissions suited for each engine, so that each car hits the same speed at maximum RPM in each gear. So both cars can go 60 km/h in first gear for example. Car A and B are otherwise completely identical. Identical mass, identical air resistance, identical tires etc.

Even though Car B produces less torque from the engine, it will at any speed whatsoever always be able to use a twice as high a gear ratio as Car A. I.e, if Car A uses a 1:2 gear ratio, it means that the torque at the wheels will be twice as high as the engine torque, i.e. 600 Nm. However, in all circumstances where Car A uses a 1:2 gear ratio, Car B can use a 1:4 gear ratio (because it produces its torque at twice the RPM) while achieving the same speed as Car A. The wheel torque of Car B will then be 4 times its engine torque, i.e. 800 Nm. Car B produces a higher torque at the wheels than Car A, despite having lower engine torque.

So, Car B will accelerate faster, because it revs higher, and can therefore use a higher gear ratio while still achieving the same speed. In this case, the higher gear ratio more than makes up for the lower engine torque.

So in order to achieve maximum acceleration, you want the best combination of engine torque and a high gear ratio. So for any given speed, you want to drive at an RPM where the torque at that RPM multiplied with the gear ratio gives the highest number. So in a normal car, this usually means not driving at the RPM where you have the highest engine torque, but a somewhat higher RPM, producing less engine torque, but since you are at a higher RPM, it means you have a lower gear, and therefore a higher gear ratio, and the wheel torque will be higher for that speed. If you would downshift even further, you’d get an even higher gear ratio, sure, but then the engine torque would drop off so much that the higher gear ratio no longer makes up for it.

Sounds complicated? Perhaps. Luckily, there is a number that perfectly captures all of this. Horsepower (or power). Horsepower basically tells you how good the engine torque multiplied by the gear ratio is. So the maximum power of the car tells you the best combination of engine torque and gear ratio the car has. In mathematical terms: P = T*RPM/7127, where P is power in HP, T is torque in Nm and RPM is, well, RPM. The number 7127 comes from converting revolutions per minute to radians per second and converting watts to HP. If you use ft-lb insteand of Nm, the number is 5252 instead of 7127.

How much power the car produces at any instant will tell you exactly the force it produces for any given speed. F = P/v, where F is the force (determining you acceleration), P is the power (given in watts, not HP) and v is the speed (given in meters/s). Note that the power is not peak power, it is the power the engine produces at whichever RPM it is currently running.

So for maximum acceleration, you basically want a CVT that constantly keeps the RPMs at maximum power. This ensures that it you will always have the perfect balance between engine torque and a high gear ratio.

But most normal ICEs do not have CVTs. Instead they have a limited number of gears, which means you end up spending most of the time driving at non-optimal RPMs (even when trying to maximize acceleration). And this is why peak power does not tell you everything either. Instead, it is the average power that the car produces when you accelerate from whatever speed to whatever other speed. Having more gears compensates for a more narrow power band, since having more gears allows you to stay in a narrower RPM range at all times and speeds. And a CVT can basically be seen as a gearbox with an infinite number of gears.

And then there is of course all the practical things. Most normal cars, or even sports cars, are not necessarily geared towards maximum 0-100 km/h acceleration (which is what most people look at). Race cars, for example, accelerate from standstill only once during a race, so they are usually geared towards having maximum acceleration out of corners, which often means having a long first gear (this is true for sports bikes at least, which are often limited to 6 gears). Then there is of course traction, and the fact that different vehicles do have different mass, air resistance etc.

Further, there are often things to take into account other than acceleration as well. Fuel consumption, reliability, robustness, noise etc. An engine that produces little torque but revs high will produce power, yes, but constantly running on high revs means high engine wear and generally bad fuel consumption. Semis, for example, want to run on low RPMs for these reasons, but they also need high power (high wheel torque). But since they need to run at low RPM for fuel efficiency and reliability/engine wear reasons, they can’t use a high gear ratio. So they must have massive engine torque to produce the required power (wheel torque) instead.

I don’t know if this is eli5, but I’ve tried to hone this explanstion into something that should be somewhat easy to grasp for most people who have some interest in vehicles.

Edit: So to answer your actual question, the car with more maximum power tends to accelerate faster. Peteol engines usually rev so much higher than diesels that they produce more power despite producing less (engine) torque.

Bla bla.

Diesel Burns slower, where petrol has a more instant explosion kind or burning, diesel takes ‘ slightly ‘ more time: diesel keeps burning and building up pressure against the piston, while the piston is already traveling down.

Hence, diesel has more torque, as the burning/exploding keeps on going while the piston is already traveling downwards: pressure is still there
Petrol has an explosion and pressure is going down while the piston travels downwards.

Diesel has a lower burning speed, thus the rpm is limited. If the rpm gets to high, there simply isn’t enough time to burn all the diesel before the piston hits bottom. You can’t burn more diesel, so you cant get more power

Petrol has a higher rpm, and thus more petrol can be burnt per minute. More power. But still, not as much torque,.