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btostes said:
Does anyone know typical values of cutoff ratio in diesel engines? And how have they evolved through time?
The cutoff ratio in a diesel engine determines the immediate balance of efficiency against power.
The range of cutoff ratio available, is determined by the manufacturer of the engine, in combination with the fuel injection system technology. The operating cutoff ratio is changed dynamically by the operator of the engine, depending on the power required at the time.
For maximum efficiency, the ratio will be close to one. For maximum power, the RPM will be raised as much as possible. The cutoff ratio will then be less important, limited by the compression ratio.
There is a design compromise between maximum power, charge-air boost-pressure, and engine capacity. That needs to be matched by the fuel injection technology.
Injecting the maximum volume of fuel quickly, improves efficiency. That allows an increase in maximum RPM, with the associated increase in power. But increasing the RPM increases the cutoff ratio employed.
Improvements in common rail injection may have lowered the typical cutoff ratio used over time, that has increased diesel engine efficiency.
I would say that cutoff ratio has been lowered for economic reasons, as the price of fuel has increased, and the exhaust particulate emissions permitted, have been lowered.
The "typical value of cutoff ratio", is dependent largely on the driver of the vehicle, and their immediate economic and time constraints.