I became very curious about this. Petrol engines run at higher RPMs to generate the required torque. Also, petrol burns well in a short amount of time.

Diesel engines generate high torque at low speeds and generate soot if they are made to burn too fast.

So why don't we have diesel engines run at very low speeds and use a large number of gears or a CVT? This combination can allow the flywheels to rotate at the same speeds as a petrol engine as well as cleaner emissions because sufficient time is given to fully burn the diesel at very low rpms.

Why isn't this used? What are the difficulties?

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    Just because diesel engines generate soot when the rpm's are too high (do they? I am not sure) it doesn't mean than they generate less soot when the rpm's are overly low. The most important thing is that a diesel need to run "lean", it must burn with an excess of oxygen. Unfortunately it burns hotter, that means more nitrous oxides – Martin Mar 7 '17 at 16:53
  • What vehicle are you repairing? – cory Mar 7 '17 at 18:03
  • In general you want any kind of engine to run as fast as possible. Engines (including electric motors) generate a more or less fixed amount of energy for each revolution (at full throttle), so the more revs the more power, or the smaller/cheaper your engine can be for a set amount of power. Also the amount of soot does not become lower and lower at very low speeds, it's just a matter of there being enough oxygen and time to fully combust the fuel. – JanKanis Mar 3 '18 at 15:43

Practical considerations and cost are the most likely cause. In the US, few cars are diesel to begin with. The public simply does not buy them. In Europe or rest of world, diesels are more popular. Also, manual transmission cars are more popular in EU/World than USA, so it would be the best market for a diesel. A diesel with 8+ gears requires a lot of shifting. Even experienced drivers may become tired of so much shifting. Plus, the more gears, the larger the transmission physically would be. Considering even an automatic transmission, only in the past 4 years have 8+ gear transmissions been created for commuter vehicles. It would also cost more for all the moving parts in a high gear count transmission. If we consider a CVT, it has only been in the last 10 years that they have even become practical for use in cars with modest horsepower or torque. So their adoption is still extremely small.

Now with the advent of electric hybrids, with just as much or more torque than diesel engines, there is no incentive for any company to make a diesel engine paired with a high count gearbox/CVT when better fuel economy and emissions can be realized with electric power.

I'm assuming you're talking about consumer vehicles and not well established tech like big-rigs or commercial ships (ocean cargo vessels).

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