SYNOPSISCalculating the heat balance of Otto engines one can recognize that the ratio between heat transfer and power increases in the part load range, i.e. city driving. To improve the efficiency of such engines it is necessary to reduce the quantity of heat transfer and also of friction losses. Both effects can be achieved by increasing the temperatures of the coolant and the lubricant. This gives a reduction of fuel consumption as well as of exhaust emissions. Both effects are valid even in the warm-up phase as well as under steady state operating temperature as are confirmed by the results of our experiments.