In an ideal diesel cycle, the cut-off ratio is set by the maximum gas temperature after combustion. That's all fine and good, but in a real-world engine what controls the cut-off ratio, and what are the limits to its flexibility? I feel like it must be the valve timing, but perhaps it is controlled by the fuel injection? Perhaps both.

Is it common to have variable cut-off ratios based on ambient temperature to achieve higher power ratings at lower temperatures? Or is this not possible in a real diesel cycle?

An example: If an engine runs both during the summer (35 °C) and the winter (-10 °C) in the Midwest can its cut-off ratio be adjusted such that the final post-combustion gas temperature is the same in both cases leading to much more power in the winter than in the summer?


2 Answers 2


OK, fair warning: I am answering my own question and am not a engines person. So this could be wrong.

The real limit in the engine is how hot certain parts can get without breaking. This temperature is related to the gas temperature after combustion via the cooling system and the cylinder design (convective heat transfer between the gas and the cylinder wall). This proportionality will vary from engine to engine, but for arguments sake lets just assume that all engines have some magic gas temperature limit that is governing the cut-off ratio.

In a real engine there is not a clear cut-off ratio because the compression, combustion, and expansion steps all bleed into each other. However, the final gas temperature still sets the volume at which combustion stops which sets a cut-off ratio analogy.

The thing that is governing this is not really the valve timing as the rpm of the engine is remaining the same and the timing during each revolution does not change. What does change is the Air to Fuel Ratio (AFR). This is controlled by the fuel injection system based on how much oxygen is coming into the engine and at what temperature.

Lowering the inlet temperature does two things:

  1. It increases the mass flow of air through the engine.
  2. It decreases the temperature of the air prior to combustion.

Both of these things lead to a higher AFR at the same limiting gas temperature after combustion. The AFR is only controlled by the quantity of gas injected which is possible to dynamically control.

So, my answer is yes, it can be adjusted on the fly.

Again, that could be wrong.


I agree with the answer given by Murenrb but for one thing. Engines aren't in danger of breaking as much as they're in danger of losing lubrication. Engine oil in the piston rings forms a bead that seals the gap between the piston and the wall. Hotter oil has lower viscosity, which means that the bead may blow out, breaking the seal. This is why engines are liquid cooled, and why towing capacity and radiator size are so strongly correlated.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.