ENGINEMAN 1 & CEffect of UnbalanceFrom the preceding discussion, it can bereadily seen that, in general, the result of un-balance will be overheating of the engine. Theclearances established by the engine designer allowfor sufficient expansion of the moving parts whenthe engine is operating at the designedtemperatures, but an engine operating attemperatures in excess of those for which it wasdesigned is subject to many casualties. Excessiveexpansion soon leads to seizure and burning ofthe engine parts. Should the temperatures in thecrankcase rise above the flash point of thelubricating oil vapors, an explosion may result.High temperature may destroy the oil film be-tween adjacent parts, and the resulting increasedfriction will further increase the temperature.4. Keep cylinder temperatures and pressuresas evenly distributed as possible.5. Train yourself to recognize the symptomsof serious engine conditions.ENGINE EFFICIENCYEngine efficiency is the amount of powerdeveloped as compared to the energy input whichis measured by the heating value of the fuel con-sumed. The term “efficiency” is used to designatethe relationship between the result obtained andthe effort expended to produce the result.Since power is directly proportional to the mepdeveloped in a cylinder, any increase in mep willcause a corresponding increase in power. If themeps in the individual cylinders vary, power willnot be evenly distributed among the cylinders.The term “compression ratio” is frequentlyused in connection with engine performance.From your study of the principles of internal com-bustion, you will recall that compression ratio isthe ratio of the volume of air above the piston,when the piston is at the BDC position, to thevolume of air above the piston when the pistonis at the TDC position.EFFICIENCIESThe quality of combustion obtained dependsupon the heat content of the fuel. The amountof heat available for power depends upontemperature. Temperature varies directly aspressure; therefore a decrease in pressure willresult in a decrease in temperature, and in poorcombustion. Poor combustion will cause loweredthermal efficiency and reduced engine output.The principal efficiencies which must beconsidered in the internal combustion process arecycle, thermal, mechanical, and volumetric.Cycle EfficiencyCylinder load balance is essential if the desiredefficiency and performance of an engine is to beobtained. To avoid the harmful effects ofoverloading and unbalancing of load, the load onan engine should be properly distributed amongthe working cylinders; and no cylinder, or theengine itself, should ever be overloaded.In general, load balance in an engine can bemaintained if the following procedures areobserved:1. Maintain the engine in proper mechanicalcondition.2. Adjust the fuel system according to themanufacturer’s instructions.3. Operate the engine within the temperaturelimits specified in appropriate instructions.The efficiency of any cycle is equal to the out-put divided by the input. The efficiency of thediesel cycle is considerably higher than the Ottoor constant volume cycle because of higher com-pression ratio and because combustion starts ata higher temperature. In other words, the heatinput in a diesel engine is at a higher averagetemperature. Theoretically, a gasoline engineusing the Otto cycle would be more efficient thanthe diesel engine if equivalent compression ratioscould be used. However, engines operating on theOtto cycle cannot use a compression ratio com-parable to that of diesel engines because fuel andair are drawn together into the cylinder and com-pressed. If comparable compression ratios wereused, the fuel would fire or detonate before thepiston reached the correct firing position.Since temperature and amount of heat con-tent which is available for power are proportional5-6
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