Wrong again.
It's simple really.
When air is cold, it has little motion. At absolute zero, the molecules are not moving at all. When hot, the molecules are excited and fast moving.
What does this mean? Cold air takes up less volume than the same quantity of hot air.
Compression of engines is entirely different. When you take the air/fuel mixture into an engine, it's at standard air pressure on the outside. This leads to a very slow burn of the fuel. By increasing the compression, the flame front travels a smaller distance between molecules of air/fuel, resulting in a much quicker expansion of gases.
Typically unleaded fuel engines have anywhere between 9 to 10.5:1 compression when naturally aspirated, versus 16 to 20:1 for diesel. The reason diesel doesn't need spark plugs is NOT because higher density air is hotter, but because when you compress the air and fuel that close together, it'll spontaneously combust.
But those compression ratios have ABSOLUTELY nothing to do with the temperature of the air. In fact, the colder air you can intake, the more fuel you can add to it, and the more power you can make. Hence intercoolers on turboed and supercharged cars, and cold air intake kits on N/A engines.
