ENGINEMAN 1 & C(to the atmosphere) during the exhaust stroke; andsome is absorbed by the walls of the exhaustpassages.Heat losses to the atmosphere through theexhaust are unavoidable. This is because theengine cylinder must be cleared of the hot exhaustgases before the next air intake charge can bemade. The heat lost to the exhaust is determinedby the temperature within the cylinder when ex-haust begins. The amount of fuel injected and theweight of air compressed within the cylinder arethe controlling factors. Improper timing of theexhaust valves, whether too early or too late, willresult in increased heat losses. If too early, thevalve releases the pressure in the cylinder beforeall the available work is obtained; if too late, thenecessary amount of air for complete combustionof the next charge cannot be realized, althougha small amount of additional work may be ob-tained. Proper timing and seating of the valvesis essential in order to maintain heat loss to theexhaust at a minimum.Heat losses due to imperfect or incompletecombustion have a serious effect on the powerthat can be developed in the cylinder. Because ofthe short interval of time necessary for the cyclein modern engines, complete combustion is notpossible; but heat losses can be kept to a minimumif the engine is kept in proper adjustment. It isoften possible to detect incomplete combustionby watching for abnormal exhaust temperaturesand changes in the exhaust color, and by beingalert for unusual noises in the engine.Mechanical LossesThere are several kinds of mechanical losses,but all are not present in every engine. Themechanical or friction losses of an engine includebearing friction; piston and piston ring friction;pumping losses caused by operation of waterpumps, lubricating pumps, and scavenging airblowers; power required to operate valves; etc.Friction losses cannot be eliminated, but they canbe kept to a minimum by maintaining the enginein its best mechanical condition. Bearings, pistons,and piston rings should be properly installed andfitted, shafts must be in alignment, and lubricatingand cooling systems should be at their highestoperating efficiency.5-10
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