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I know this isn't exactly a Motor Vehicle Maintenance & Repair related question but this seems the most appropriate StackExchange community to ask it. I apologize if I am being wrong in my assumption.

Given 2 cars that are identical in every aspect except torque produced by the engine ( identical amount of power, regardless of the unit of measurement - HP, kW, etc. - , identical weight, identical weight distribution, identical number of driven wheels, same final drive and identical aerodynamic characteristics) and both cars are equipped with a Continuously Variable Transmission (CVT), which of these 2 cars would have a greater acceleration and a greater top speed if one car is to be equipped with a diesel engine (more torque but the same HP) and the other with a petrol engine (less torque but the same HP)?

Note the fact that both cars use a CVT, eliminating gear numbers, gearing ratios and other aspects pertaining to a traditional gearbox. Furthermore, whether or not it is possible for such 2 vehicles to exist is not up for debate.

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  • Different engines changes a lot. If the CI (compression ignition aka diesel) is turbocharged as they almost always are, is the SI (spark ignition) turbo charged as well? This will dramatically effect the torque curve for both engines. CI uses heavy steel engine blocks, what about the cars will change to allow equal weight since the SI engine would have an Aluminum block?
    – MooseLucifer
    Jun 12, 2016 at 16:11
  • Thanks for the input. Actually, I didn't want to get into these details. I know that it's probably highly unlikely, if at all possible, to build these cars as I mentioned in my post. I chose a diesel v petrol scenario because they seem to be 2 faces of the same coin and people can relate to them more easily. I could've very well said a kerosene powered turbine jet engine versus a 2 stroke petrol but it would make for an even more unlikely scenario. The point was to understand the relationship between torque and power.
    – user1969903
    Jun 13, 2016 at 5:53

2 Answers 2

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Torque doesn't matter. It's power. The difference is that diesel engines have their peak power at lower RPM compared to petrol engines. Same power at lower RPM means more torque.

(Think about: The motor of my car has max. torque of 95Nm. I torque the wheel lugs with 110Nm by hand. So why can the motor accelerate my car to 150km/h, while I can push it to walking speed only, though I can apply more torque?)

The capability to accelerate seems to be higher for a diesel, because people tend to not accelerate near the max. RPM, where a petrol engine has max. power. They mostly accelerate with moderately high RPM, where a diesel engine has its max. power.

A gearbox is a torque converter. If the first gear has a ratio of 1:4 and the last of 1:1, the output shaft will rev with 1/4 of the RPM, but 4 times the torque of the input shaft. In contrast, power is conserved (minus some losses). And power is energy per time - transformed into the kinetic energy of the car.

So: Since your cars have a CVT where motor RPM is kept constant, the CVT will be set to the individual RPM of max. power for max. acceleration.

The car with more power will accelerate faster!

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  • This post sounds good. It is a challenge to dismiss experience and common mechanical limitations and think theoretically. Thanks!
    – john D.
    Jun 12, 2016 at 14:55
  • So in the end it all boils down to power output. In this scenario, I take it that the CVT will keep the engine RPM at a constant value that produces the most power regardless of the torque produced at that specific RPM for both cars. Since the power output of both these vehicles is equal, then both will accelerate at the same rate. Thanks for the answer, it helped me wrap my head around the concept. 'Power is energy per time': This leads me to believe that the torque of the engine is the energy produced but power is a measure of how well it can sustain that energy output, correct?
    – user1969903
    Jun 13, 2016 at 5:45
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    Yes, both cars will accelerate equally. No, torque is not the energy produced. It is the "rotary force" produced, and force times speed is power. (and force times distance is energy) The motor and I can generate the same torque, but as soon as the wheel is moving, my torque decreases due to low power. The concept of torque/force and power is difficult to understand, and the term power is often used as strength and not that physical measure given in in kW or HP.
    – sweber
    Jun 13, 2016 at 6:07
  • You may also have a look at mechanics.stackexchange.com/questions/25419/…
    – sweber
    Jun 13, 2016 at 6:07
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Gas engines are capable of much higher rpm than diesels, so your top speed with gas would be higher. I don't think if it is possible to build equivalent gas and diesel engines that have similar weight and HP.

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  • Max motor RPM alone doesn't define top speed. The transmission plays a role (and has different rations for diesel and petrol) and the power (torque) vs. RPM curve plays a role, too.
    – sweber
    Jun 12, 2016 at 14:48

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