Engine Testing Theory and Practice P2

The closed engine test cell system makes a suitable case for students to study an example of the flow of heat and change in entropy. In almost all engine test cells the vast majority of the energy comes into the system as highly concentrated ‘chemical energy’ entering the cell via the smallest penetration in the cell wall, the fuel line. It leaves the cell as lower grade heat energy via the largest penetrations: the ventilation duct, engine exhaust pipe and the cooling water pipes. In the case of cells fitted with electrically regenerative dynamometers, almost one-third of the energy supplied...
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Engine Testing Theory and Practice P22 The test cell as a thermodynamic system The energy of the world is constant; the entropy strives towards a maximum.1 Rudolph Clausius (1822–1888)IntroductionThe closed engine test cell system makes a suitable case for students to study anexample of the flow of heat and change in entropy. In almost all engine test cells thevast majority of the energy comes into the system as highly concentrated ‘chemicalenergy’ entering the cell via the smallest penetration in the cell wall, the fuel line.It leaves the cell as lower grade heat energy via the largest penetrations: the ventilationduct, engine exhaust pipe and the cooling water pipes. In the case of cells fittedwith electrically regenerative dynamometers, almost one-third of the energy suppliedby fuel will leave the cell as electrical energy able to slow down the electricalenergy supply meter. Many problems are experienced in test cells worldwide whenthe thermodynamics of the cell have not been correctly catered for in the design ofcooling systems. The most common problem is high air temperature within the testcell, either generally or in critical areas. The practical effects of such problems willbe covered in detail in Chapter 5, Ventilation and air conditioning, but it is vital forthe cell designer to have a general appreciation of the contribution of the variousheat sources and the strategies for their control. In the development of the theory of thermodynamics much use is made of theconcept of the open system. This is a powerful tool and can be very helpful inconsidering the total behaviour of a test cell. It is linked to the idea of the controlvolume, a space enclosing the system and surrounded by an imaginary surface, thecontrol surface (Fig. 2.1). The great advantage of this concept is that once one has identified all the massand energy flows into and out of the system it is not necessary to know exactly whatis going on inside the system in order to draw up a ‘balance sheet’ of inflows andoutflows. The test cell as a thermodynamic system 15 Control surface In Out Control volume Fuel Exhaust Air Machine Power Water WaterFigure 2.1 An open thermodynamic systemThe various inflows and outflows to and from a test cell are as follows:In∗ OutFuelVentilation air (some may be used by Ventilation airthe engine as combustion air)Combustion air (treated) Exhaust (includes air used by engine) Engine cooling waterCharge air (when separately supplied) Dynamometer cooling waterCooling water Electricity from dynamometerElectricity for services Losses through walls and ceiling Balance sheets may be drawn up for fuel, air, water and electricity, but by farthe most important is the energy balance, since every one of these quantities hasassociated with it a certain quantity of energy. The same concept may be applied tothe engine within the cell. This may be pictured as surrounded by its own controlsurface, through which the following flows take place:In OutFuel PowerAir used by the engine ExhaustCooling water Cooling waterCooling air Cooling air Convection and radiation∗ Compressed air may be a further energy input; however, usage is generally intermittent and unlikely to make a significant contribution.16 Engine TestingMeasurement of thermal losses from the engine is dealt with in Chapter 13, Thermalefficiency, measurement of heat and mechanical losses, where the value of the methodin the analysis of engine performance is made clear.The energy balance of the engineTable 2.1 shows a possible energy balance sheet for a cell in which a gasoline engineis developing a steady power output of 100 kW. Note that where fluids (air, water,exhaust) are concerned, the energy content is referred to an arbitrary zero, the choiceof which is unimportant: we are only interested in the difference between the variousenergy flows into and out of the cell. Given sufficient detailed information on a fixed engine/cell system it is possibleto carry out a very detailed energy balance calculation (see Chapter 5, Ventilationand air conditioning, for a more detailed treatment). Alternatively there are somecommonly used ‘rule of thumb’ calculations available to the cell designer; the mostcommon of these relates to the energy balance of the engine which is known as the‘30–30–30–10 rule’. This refers to the energy balance shown in Table 2.2. The key lesson to be learnt by the non-specialist reader is that any engine testcell has to be designed to deal with energy flows that are at least three times greaterthan the ‘headline’ engine rating. To many this will sound obvious but a commonfixation on engine power and a casual familiarity with, but lack of appreciation of, theenergy density of petroleum fuels has misled many people in the past to significantlyTable 2.1 Simplified energy flows for a test cell fitted with a hydraulicdynamometer and 100 kW gasoline engineEnergy balanceIn OutFuel 300 kW Exhaust gas 60 kWVentilating fan power ...