What is the main reason why the actual thermal efficiency of an internal combustion engine is much lower than its theoretical air-standard efficiency?

1. The working fluid is not air but a mixture of air and fuel with variable specific heats, along with heat losses, friction, and incomplete combustion.
2. The engine runs at variable speeds and uses mechanical gears.
3. The friction between the piston and the cylinder is completely negligible in theoretical cycles.
4. The combustion process is modeled as a simple heat transfer process in real engines.