In text-books, we are taught that 2 stroke internal combustion engines are inefficient and are used only in small motors where weight is more important than efficiency or power. However what people don’t realize is that most of the big, high power, high efficiency diesel engines are also two stroke engines. This includes most of the locomotives, ships and submarines. (Gas turbines have higher efficiencies and power to weight ratio, but their high maintenance restricts its use as continuous power supply)
So why the role reversal for Diesel engine?
1. In a petrol engine the intake is a mixture of fuel (petrol) and air (oxygen). The major source of lower efficiency of 2 stroke IC engine is that either some of the fuel leaks away with the exhaust or the exhaust fumes are not fully drained out from the cylinder. However in diesel engine the intake is only air. The fuel is sprayed into the combustion chamber during the combustion phase only. So this takes care of the major source of inefficiency.
2. Diesel is traditionally regarded as a slow and bulky engine. However 2 stroke engine means that for the same rpm (and to the large extend weight and size), there is twice as much combustion and twice as much power.
However all said, 2 stroke engine crams up the intake and exhaust so much that a lot of aerodynamic and thermal stress problems emerge due to close proximity of cold intake with hot exhausts.