If I have a vehicle (V1) with one 100 kW engine and an identical vehicle (V2) with 2 of these 100 kW engines, will this mean that V2 will have double the torque of V1 and double the top speed of V1?

Suspected Answer

It's not like in a series circuit in which, when you have two identical batteries in series, the the voltage doubles.

I'm guessing that there will be some increase (but not double) in torque, but the top speed would remain the same between V1 and V2 whilst pulling an identical load. I suspect that the difference between V1 and V2 will be that V2 may be quicker to achieve the top speed, but the top speed won't increase by much. I.E. V1's max speed pulling 30 kg will be 20 kph but might take 10 seconds, but V2s max speed pulling 30 kg will be 20 kph but might take 8 seconds.

  • $\begingroup$ The question mentions "a vehicle" and asks about the top speed of the vehicle. Therefore, the question is asking a trivially impossible thing (that the second engine does not add any mass to the vehicle). It should be rephrased to ask the benefits of having two engines and its application tricks, maybe. $\endgroup$ Jul 1, 2015 at 18:48
  • $\begingroup$ @GurkanCetin, I asked about the torque and speed, how these two would change depending on having two identical engines vs just a single engine. $\endgroup$
    – 3kstc
    Jul 2, 2015 at 0:27

2 Answers 2


The simple answer is yes. In Australia they have freight trucks that they call road trains. They have a second engine in the middle primary to start up which they switch off when they reach a certain speed limit. They use this type of topology when transporting freight across the outback. I thing most of them use a 400 hp Cummins diesel engine in the middle. Application is already in use but not exactly as per your question.

I believe this type of topology is very common in the locomotive space.

Here are some images to enjoy.

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A 600 hp (450 kW) 19 L (1,200 cu in) Cummins engine powers the prime mover, while a 400 hp (300 kW) Cummins engine is installed in the rear trailer of the B-double, driving through an automatic transmission, giving a total of 1,000 hp (750 kW). Weights of 460 t (453 long tons; 507 short tons) are achieved with ore loading in side-tipper bodies on a 100 km (62 mi) round trip.

As these trucks operate on private property, they are not subject to governed weight and length rulings, but instead are used in the most efficient way possible

Also to below is an image from a patent to indicate active interest in you idea by the automotive - Power-train interest group. I have attached the patent for your reference.

power trailer


  • 2
    $\begingroup$ The last statement about the trucks operating on private roads & not being subject to weight & length rulings only applies to certain road trains. Generally in the northern part of the country road trains on public roads can be up to 50 m in length, whereas in southern parts the max length is 36 m. The green truck in the last picture is 50 m long & it hauls Pb-Zn concentrate 120 km from McArthur River Mine to a port on the coast. $\endgroup$
    – Fred
    Jun 30, 2015 at 13:19
  • 2
    $\begingroup$ About 20 years ago I used to have to overtake that green road train. Fortunately there were enough stretches of long straight bit of road & little traffic from the other direction. If the last trailer was swaying severely then overtaking was delayed until the trailer settled. $\endgroup$
    – Fred
    Jul 1, 2015 at 3:56

There is no simple answer. What is the top speed limiting factor? For a truck, it's mostly handling so no amount of power can increase that. For a sports car, it's mostly aerodynamic resistance. While the aerodynamic resistance (more precisely it's called drag) increases proportionally to velocity squared, the power needed to overcome it increases proportionally to the velocity cubed. So 8x the power would yield 2x top speed and 2x more power means a 1.26x faster vehicle (when considering aerodynamics alone).

The torque will be definitely double, if the transmission and wheels can handle the doubled amount. At wost, you can add secondary set of entire propulsion set, just as they did with Citroen 2CV Sahara.

You should also research trainsets, as there are some real-life examples of almost identical vehicles differentiated only by amount of power plants.

  • 1
    $\begingroup$ Power dissipation by drag is proportional to velocity cubed, so doubling your power only gives you 2^(1/3) = 1.26 times the top speed. $\endgroup$ Jul 3, 2015 at 3:32

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