77) The maximum propulsive efficiency of a turbo-jet engine is at a speed of

78) In order to mix air and petrol in the required proportion and to supply it to the engine during suction stroke, then __________ is employed.

79) The advancing of spark timing in spark ignition engines will __________ knocking tendency.

80) The knocking in spark ignition engines can be reduced by

81) An atom bomb utilises uncontrolled nuclear fission.