indicator cock on each individual cylinder andcheck the color of the exhaust.2. High exhaust temperatures. If thetemperatures of exhaust gases from individualcylinders become higher than normal, it is an in-dication of an overload within the cylinder. If thetemperature of the gases in the exhaust headerbecomes higher than usual, it is an indication thatall cylinders are probably overloaded. Frequentchecks on the pyrometer will indicate whethereach cylinder is firing properly and carrying itsshare of the load. Any sudden change in theexhaust temperature of any cylinder should beinvestigated immediately. The difference in ex-haust temperatures between any two cylindersshould not exceed the limits prescribed in theengine manufacturer’s technical manual.3. High lubricating oil and cooling watertemperatures. If the temperature gages for thesesystems show an abnormal rise in temperature,an overloaded condition may exist. The causes ofthe abnormal temperature in these systems shouldbe determined and corrected immediately if engineefficiency is to be maintained.4. Excessive heat. In general, excessive heatin any part of the engine may indicateoverloading. An overheated bearing may be theresult of an overloaded cylinder; or an abnormallyhot crankcase may be the result of overloadingthe engine as a whole.5. Excessive vibration or unusual sound. Ifall cylinders are not developing an equal amountof power, the forces exerted by individual pistonswill be unequal. When this occurs, the unequalforces cause an uneven turning movement to beexerted on the crankshaft, and vibrations are setup. Through experience, you will learn to tell bythe vibrations and sound of an engine when a poordistribution of load exists. You should use everyopportunity to observe and listen to enginesrunning under all conditions of loading andperformance.Causes of UnbalanceAn engine must be kept in excellentmechanical condition to prevent unbalance. Aleaky valve or fuel injector, leaky compressionrings, or any other mechanical difficulties willmake it impossible for you to balance the loadunless you secure the engine and dismantle at leasta part of it.To obtain equal load distribution betweenindividual cylinders, the clearances, tolerances,and the general condition of all parts that affectthe cycle must be maintained so that very little,if any, variation exists between individualcylinders. Unbalance will occur unless the follow-ing conditions are as nearly alike as possible forall cylinders:1. Compression pressures2. Fuel injection timing3. Quantity and quality of fuel injected4. Firing pressures5. Valve timing and liftWhen unbalance occurs, correction usually in-volves repair, replacement, or adjustment of theaffected part or system. Before any adjustmentsare made to eliminate unbalance, it must be deter-mined beyond any doubt that the engine is inproper mechanical condition. When an engine isin good mechanical condition, few adjustmentswill be required. However, after an overhaul inwhich piston rings or cylinder liners have beenrenewed,considerable adjustment may benecessary. Until the rings become properly seated,some lubricating oil will leak past the rings intothe combustion space. This excess oil will burnin the cylinder, giving an incorrect indication offuel oil combustion. If the fuel pump is set fornormal compression, and the rings have not seatedproperly, the engine will become overloaded. Asthe compression rises to normal pressures, therewill be an increase in the power developed, as wellas in the pressure and temperature under whichthe combustion takes place. Therefore, when anoverhaul has been completed, the engineinstruments must be carefully watched until therings are seated, and all necessary adjustments aremade. Frequent compression tests will serve as ahelpful aid in making the necessary adjustments.Unless an engine is so equipped that compressioncan be readily varied, the engine should beoperated under light load until the rings areproperly seated.Chapter 5—ENGINE PERFORMANCE AND EFFICIENCY5-5
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