Inside the cylinders of a diesel engine, the mixture of fuel and air is compressed to about 1/20 of its original volume. Though diesel engines have no spark plugs to ignite this mixture, it ignites anyway because...
1) diesel fuel and air ignite at room temperature
2) increases in temperature decreases molecular motion.
3) change in temperature is controlled by the cooling system
4) rapid increase in volume raise the temperature of the mixture
5) rapid increases in volume raise the temperature of the mixture.