CHAPTER 5ENGINE PERFORMANCE AND EFFICIENCYYour prime concern as an Engineman is tokeep the machinery for which you are responsi-ble operating in the most efficient manner possi-ble. From your past experience and training, youknow that engine efficiency and performancedepend upon much more than just operating thethrottle and changing oil at prescribed intervals.The preceding chapters have covered many of thecasualties which may occur to reduce the poweroutput of an engine. You have learned how to pre-vent the occurrence of many of these casualties.As you gain experience and understanding, youwill probably have to train other people. The peo-ple you will train will frequently come up withmany questions about why an engine does or doesnot perform efficiently. Will you be able to answertheir questions?To understand the various factors thatinfluence engine performance and efficiency, athorough knowledge of the internal combustionprocess is necessary. Once the combustionprocess is understood, it will be much easier foryou to appreciate the part played by such factorsas engine design, engine operating conditions, fuelcharacteristics, fuel injection, ignition, pressuresand temperatures, and compression ratios. Thischapter provides some of the informationnecessary for a better understanding of the manyfactors that affect engine performance and effi-ciency. As an Engineman, you will be able to gaincomplete understanding of such factors onlythrough continued study and practical experience.You should know how the power which anengine can develop is limited by such factors asthe mean effective pressure, the length of pistonstroke, the cylinder bore, and the engine speed.You must also know how these factors are usedin determining the power developed by an engine.You must learn how heat losses, efficiency ofcombustion,volumetric efficiency, and theproper mixing of fuel and air limit the powerwhich a given engine cylinder can develop. Youmust become familiar with the factors which causeoverloading of an engine and unbalance betweenengine cylinders. You should know the symptoms,causes, and effects of cylinder load unbalance andthe steps that are necessary to maintain an equalload on each cylinder.You must know what is meant by engineefficiency and know how the various types ofefficiencies and losses are used in analyzing theinternal combustion process. You must also befamiliar with those factors which may cause theefficiencies to increase or decrease, and with theways these variations affect engine performance.Parts of this chapter may serve as a briefreview, but most of the information provideddeals with those factors that influence engine per-formance and efficiency.ENGINE PERFORMANCEIn addition to mechanical difficulties, anyengine performance may be affected by othercauses, such as engine design and operator’s per-formance. A comparison of the principal condi-tions which influence the performance ofinternal combustion engines is given in table 5-1.Note that the performance conditions for the twotypes of engines (diesel and gasoline) aresomewhat similar, except for some differences dueto factors dealing with fuel and ignition.POWER LIMITATIONSThe design of an engine limits the amount ofpower that an engine can develop. Other limiting5-1
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