Chapter  5—ENGINE  PERFORMANCE  AND  EFFICIENCY indicated mean effective pressure by imep in the expression, one can also show: bmep = imep × mechanical efficiency type  meter)  the  amount  of  air  taken  in  by  the engine, converting the amount to volume, and comparing this volume to the piston displacement. Volumetric  efficiency  = To illustrate this, the bmep for the engine in preceding examples at full load and three-fourths load is computed as follows: Volume of air admitted to cylinder Volume of air equal to piston displacement ×  100 33,000 × bhp 33,000 × 900 bmep 12 12 = L  ×  A  ×  N ⁼10  ×  56.14  ×  744 12 = 70 psi or The concept of volumetric efficiency does not apply to 2-stroke cycle engines. Instead, the term “scavenge efficiency” is used. Scavenge efficiency shows  how  thoroughly  the  burned  gases  are removed and the cylinder filled with fresh air. As in the case of a 4-stroke cycle engine, it is desirable that the air supply be sufficiently cool. Scavenge efficiency  depends  largely  upon  the  arrangement of the exhaust, scavenge air ports, and valves. bmep = imep × mechanical efficiency = 105 × 67, or 70 psi ENGINE  LOSSES Bmep gives an indication of the load an engine carries, and what the output is for piston displace- ment. As the bmep for an engine increases, the engine develops greater horsepower per pound of weight. For a given engine, bmep changes in direct proportion with the load. Volumetric  Efficiency As the heat content of a fuel is transformed into useful work, during the combustion process, many different losses take place. These losses can be  divided  into  two  general  classifications:  ther- modynamic and mechanical. The net useful work delivered by an engine is the result obtained by deducting the total losses from the heat energy input. The volumetric efficiency of a 4-stroke engine is the relationship between the quantity of intake air and the piston displacement. In other words, volumetric  efficiency  is  the  ratio  between  the charge that actually enters the cylinder and the amount that could enter under ideal conditions. Piston displacement is used since it is difficult to measure the amount of charge that would enter the cylinder under ideal conditions. An engine would have 100% volumetric efficiency if, at at- mospheric pressure and normal temperature, an amount of air exactly equal to piston displacement could be drawn into the cylinder. This is not possi- ble, except by supercharging, because the passages through which the air must flow offer a resistance, the force pushing the air into the cylinder is only atmospheric,  and  the  air  absorbs  heat  during  the process.  Therefore,  volumetric  efficiency  is  deter- mined by measuring (with an orifice or venturi Thermodynamic  Losses Losses 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 water systems and the lubricating oil system are always present.  Some  heat  is  conducted  through  the engine parts and radiated to the atmosphere or picked  up  by  the  surrounding  air  by  convection. The effect of these losses varies according to the part of the cycle in which they occur. The heat of the jacket cooling water cannot be taken as a true measure of heat losses, since all this heat is not absorbed by the water. Some heat is lost to the  jackets  during  the  compression,  combustion, and expansion phases of the cycle; some is lost 5-9