I found a website that deals in commercial diesel engines and its performance intake air temperature is reported to be 25*c which would suggest to me that warm air(not hot air) is better for combustion. It has been a challenge to find info on diesel intake temperatures.
Diesel engines for fast vessels with low load factors
Maximum performance rating
e.g. fast yachts, corvettes and frigates
Power ratings classifiable
Intake air temperature 25°C
Sea water temperature 25°C
Power reduction at 45°C/32°C
- Series 8000: none
- Series 1163: 3.0%
Diesel engines for fast vessels with high load factors
Maximum continuous rating
e.g. fast ferries, yachts, corvettes and frigates
Power ratings classifiable
Intake air temperature 25°C
Sea water temperature 25°C
Power reduction at 45°C/32°C
- Series 1163: 4.6%
- Series 8000: none
Combined propulsion plants with diesel engines and gas turbines: MTU Online