Mechanical Losses

ENGINEMAN 1 & C (to the atmosphere) during the exhaust stroke; and some is absorbed by the walls of the exhaust passages. Heat losses to the atmosphere through the exhaust are unavoidable. This is because the engine cylinder must be cleared of the hot exhaust gases before the next air intake charge can be made. The heat lost to the exhaust is determined by the temperature within the cylinder when ex- haust begins. The amount of fuel injected and the weight of air compressed within the cylinder are the controlling factors. Improper timing of the exhaust valves, whether too early or too late, will result in increased heat losses. If too early, the valve releases the pressure in the cylinder before all the available work is obtained; if too late, the necessary amount of air for complete combustion of the next charge cannot be realized, although a small amount of additional work may be ob- tained. Proper timing and seating of the valves is essential in order to maintain heat loss to the exhaust at a minimum. Heat losses due to imperfect or incomplete combustion have a serious effect on the power that can be developed in the cylinder. Because of the short interval of time necessary for the cycle in modern engines, complete combustion is not possible; but heat losses can be kept to a minimum if the engine is kept in proper adjustment. It is often possible to detect incomplete combustion by watching for abnormal exhaust temperatures and changes in the exhaust color, and by being alert for unusual noises in the engine. Mechanical Losses There are several kinds of mechanical losses, but all are not present in every engine. The mechanical or friction losses of an engine include bearing friction; piston and piston ring friction; pumping losses caused by operation of water pumps, lubricating pumps, and scavenging air blowers; power required to operate valves; etc. Friction losses cannot be eliminated, but they can be kept to a minimum by maintaining the engine in its best mechanical condition. Bearings, pistons, and piston rings should be properly installed and fitted, shafts must be in alignment, and lubricating and cooling systems should be at their highest operating efficiency. 5-10