Chapter 5—ENGINE PERFORMANCE AND EFFICIENCYindicated mean effective pressure by imep in theexpression, one can also show:bmep = imep × mechanical efficiencytype meter) the amount of air taken in by theengine, converting the amount to volume, andcomparing this volume to the piston displacement.Volumetric efficiency =To illustrate this, the bmep for the engine inpreceding examples at full load and three-fourthsload is computed as follows:Volume of air admitted to cylinderVolume of air equal to piston displacement× 10033,000 × bhp33,000 × 900bmep1212=L × A × N=10 × 56.14 × 74412= 70 psiorThe concept of volumetric efficiency does notapply to 2-stroke cycle engines. Instead, the term“scavenge efficiency” is used. Scavenge efficiencyshows how thoroughly the burned gases areremoved and the cylinder filled with fresh air. Asin the case of a 4-stroke cycle engine, it is desirablethat the air supply be sufficiently cool. Scavengeefficiency depends largely upon the arrangementof the exhaust, scavenge air ports, and valves.bmep = imep × mechanical efficiency= 105 × 67, or 70 psiENGINE LOSSESBmep gives an indication of the load an enginecarries, and what the output is for piston displace-ment. As the bmep for an engine increases, theengine develops greater horsepower per pound ofweight. For a given engine, bmep changes in directproportion with the load.Volumetric EfficiencyAs the heat content of a fuel is transformedinto useful work, during the combustion process,many different losses take place. These losses canbe divided into two general classifications: ther-modynamic and mechanical. The net useful workdelivered by an engine is the result obtained bydeducting the total losses from the heat energyinput.The volumetric efficiency of a 4-stroke engineis the relationship between the quantity of intakeair and the piston displacement. In other words,volumetric efficiency is the ratio between thecharge that actually enters the cylinder and theamount that could enter under ideal conditions.Piston displacement is used since it is difficult tomeasure the amount of charge that would enterthe cylinder under ideal conditions. An enginewould have 100% volumetric efficiency if, at at-mospheric pressure and normal temperature, anamount of air exactly equal to piston displacementcould be drawn into the cylinder. This is not possi-ble, except by supercharging, because the passagesthrough which the air must flow offer a resistance,the force pushing the air into the cylinder is onlyatmospheric, and the air absorbs heat during theprocess. Therefore, volumetric efficiency is deter-mined by measuring (with an orifice or venturiThermodynamic LossesLosses of this nature are a result of the follow-ing: loss to the cooling and lubricating systems;loss to the surrounding air; loss to the exhaust;and loss due to imperfect combustion.Heat energy losses from both the cooling watersystems and the lubricating oil system are alwayspresent. Some heat is conducted through theengine parts and radiated to the atmosphere orpicked up by the surrounding air by convection.The effect of these losses varies according to thepart of the cycle in which they occur. The heatof the jacket cooling water cannot be taken as atrue measure of heat losses, since all this heat isnot absorbed by the water. Some heat is lost tothe jackets during the compression, combustion,and expansion phases of the cycle; some is lost5-9
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