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How is the engine of a tractor different from a car? So far, all I know, is, that a tractor has a rather high torque $ M $ and a car engine has rather high power $ P $. But physically, they are quite connected: $ P = M * \omega $.

Finally, I know, what matters is lastly what the engine can deliver to the tyres but nevertheless, I guess, the engines are quite different?

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3 Answers 3

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The engine is different, but what is significantly different and what really determines the exact force on the tires is the gearbox and the differential.

The same engine, which has a Torque at certain rpm (and therefore delivers a certain amount of power), can deliver different torque at different rpm (but at most the same power).

At given rpm, you can have a certain amount o Torque and power. You can use the gearbox to increase the speed at the expense of the torque. Or you can increase the torque at the expense of rpm. (IMHO it makes more sense to design a new design a gearbox to meet certain loading requirements, than redesign an engine around a gearbox).


At first glance, you might say that the power is constant, however the power output of an engine is heavily influenced by the rpm. You can see an example below.

enter image description here

Figure: : Power and Torque curve for Tractor Engine (source: iasri.res.in)

It is interesting to see, that the maximum torque and maximum power don't really coincide. Usually the maximum torque is at lower rpm compared to the maximum power.


enter image description here

figure: torque, power vs rpm for a sedan (source:Car and driver)

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  • $\begingroup$ Actually the gearing will be designed to get certain groundspeeds while matching certain engine speeds with pto speeds... $\endgroup$
    – Solar Mike
    Aug 3, 2021 at 9:07
  • $\begingroup$ @SolarMike I think we are in perfect agreement on that. $\endgroup$
    – NMech
    Aug 3, 2021 at 9:09
  • $\begingroup$ Thanks a lot, that was well explained. I looked for a plot to show a comparison between a car and a tractor engine but couldn't find one. In case you have one at hand, I would appreciate it if you would add it. Otherwise, thanks nevertheless! $\endgroup$
    – Ben
    Aug 3, 2021 at 10:01
  • $\begingroup$ Because cars can be very different, it might be best if you can just google for "torque rpm power curve" for the type of car you are interested in. $\endgroup$
    – NMech
    Aug 3, 2021 at 10:04
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    $\begingroup$ Another big difference is that tractor engines (like aircraft piston engines) are expected to run at 70% power or higher all day, every day. Car engines are not. The maximum power of a car engine is only used for high acceleration, which is irrelevant for a tractor. $\endgroup$
    – alephzero
    Aug 3, 2021 at 14:41
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As pointed out by alephzero, a tractor engine runs at its full rated load or nearly so all day long, whereas a car engine only runs at full power for short bursts. This requires the internal components of a tractor engine to be much stouter and more resistant to abuse. For example, where a car engine might have a cast crankshaft, a tractor engine would use a forged crankshaft instead. The tractor will also have a significantly larger-capacity cooling system for this same reason.

Engine weight is immaterial for a tractor while long life under heavy loads and day-to-day reliability are both essential. For this reason, tractor engines are significantly "overbuilt" with extra-thick cylinder walls to support being rebored when the engine has reached its rebuild time.

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You optimize an engine for a tractor by looking at how it's used. This is not the same as looking at torque versus power, because in the end, it is really only power that is required. Torque can be gained through gears.

Tractors:

  • Move rather slowly compared to road vehicles
  • do not need acceleration or drivability like a car
  • operate external equipment via a Power Take-off (PTO). This means you can't run the engine slowly since the driven equipment needs power regardless of the speed over ground of the tractor
  • are operated continuously for long periods. This affects operating cost (tends towards diesel engines because a diesel is more efficient) and design considerations for longevity.

What you typically end up with is a diesel engine that is relatively small for the given size of the tractor, and is built with a very heavy duty rating, so it will last a long time. And very short gearing.

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