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Given the following post, respond as if you're commenting on Reddit:
human
Why are engines that use the Diesel cycle able to produce more torque than engines using the Otto cycle?
gpt
Former caterpillar Diesel engineer here. Here is an ELI15.  


The torque from an engine is a result of the cylinder pressure and the geometry of the engine. This is intuitive because torque is simply a force multiplied by the length of a lever arm.  


Geometry.

Highly undersquare engines tend to have higher torque but lower RPM.  An underquare engine has a longer stroke than its cylinder diameter. This gives the piston a long lever arm to work the crankshaft. An engine like this has less surface area in relation to its cylinder volume near top dead center and thus has less heat loss and better fuel economy. The trade off is that peak rpm for this engine is lower because the amount of area in the Cylinder head is less for a given cylinder volume.    


Diesel engines tend to be undersquare. This is because diesel fuel burns slowly with air, so high RPMs are not going to be achievable even with lots of valve area. Diesel engines are also often designed with reducing fuel costs in mind, so there is added incentive for undersquare designs. Diesel engines being undersquare helps them achieve high levels of torque at low rpm.   


Cylinder pressure.  


Diesel engines cannot knock. Knock is a process where fuel begins to explode in a cylinder before a spark plug fires. This sets up destructive pressure waves in the cylinder. Diesels do not premix fuel with air before the combustion event in the cylinder and thus avoid this phenomena. This allows for very high compression ratios.  In addition, Diesel engines tend to be turbocharged. Turbocharging greatly improves fuel economy, power and drivability with few downsides in a diesel. All of these things contribute to diesels having relatively high cylinder pressures.  Put together, these factors allow for very high cylinder pressures to be achieved.  Thus this helps these engines achieve high torque.   


That being said, Gasolines engines are also capable of achieving very high torque figures if they are designed along similar lines. And increasingly, with the trend towards direct injection, turbocharging and under square design in gasoline engines for the sake of fuel economy, we are seeing gasoline engines with diesel-like torque figures.