Others have pretty much covered the main reasons: * Diesel engines have higher thermal efficiency. A very large marine diesel can exceed 50% thermal efficiency. read more
The air and fuel mixture limits fuel compression, and hence the overall efficiency. A diesel engine compresses only air, and the ratio can be much higher. A diesel engine compresses at the ratio of 14:1 up to 25:1, whereas in a gasoline engine the compression ratio is between 8:1 and 12:1. read more