a diesel engine uses heat and high compression (compression ratios commonly 22:1 versus 9:1 for gasoline) to ignite the fuel. when normally starting a modern diesel engine, it typically uses glowplugs to preheat the cylinder(s). if a battery is completely discharged then it may not provide the necessary electrical power to "glow" the glowplugs, making the push starting of a diesel vehicle with a dead battery almost impossible.