IC Engines Supercharging - RMIT UNIVERSITY ASSOCIATE DEGREE...

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Unformatted text preview: RMIT UNIVERSITY ASSOCIATE DEGREE IN ENGINEERING TECHNOLOGY & ADVANCED DIPLOMA IN MECHANICAL ENGINEERING (Principal Technical Officer) THERMO-FLUIDS 2 & THERMODYNAMICS 3 I. C. ENGINES Supercharging & Turbocharging EX TRACTS FROM APPLIED THERMODYNAMIC —Eastop & McConkey r ' ' Document HandouI Cover Sheel.doc F unlverSIW Author: Prasanna Desilva Save Dale: 051134201? RMIT Universin Page 1 of} Turbocharger From Wikipedia, the free encyciopedia A turbocharger, or turbo (colloquialism), is a Centrifugal compressor powered by a turbine which is driven by an engine‘s exhaust gases. Its benefit lies with the compressor increasing the pressure of air entering the engine (forced induction) thus resulting in greater performance (for either, or both, power & efficiency). Popularly used with internal combustion engines (6. g. four—stroke engines like Otto cycles and Diesel cycles). Turbochargers have also been found useful compounding external combustion engines such as automotive fuel cellsm Cut-away view of an air foil bearing-supported turbocharger made by Mohawk Innovative Technology Supercharging Figure 13.14 (p. 439) shows how the power output of an engine is affected by the reduction in volumetric efiiciency at increased engine speed. The purpose of supercharging is to raise the volumetric efficiency above that obtained with normal aspiration. The main attraction of supercharging engines is to obtain a high power output from a small engine hence giving a good power—weight ratio with a corresponding saving in space which is important in some applications such as road and rail vehicles. In this case the engine is designed as a supercharged engine to withstand the higher loads and temperatures reached in supercharging compared with normal aspiration. Greater benefits are to be expected from supercharging the diesel engine than from the petrol engine because of the diiferent methods of charging the cylinders Reciprocating internalucombustion Engines Fig. 13.42 Pressure— volume diagram for a four-stroke CI engine with supercharging (a) and without supercharging (b) Fig. 13.43 Diagrams of a four-stroke, four-cylinder CI engine with mechanical supercharging (a) and with turbocharging (b) (see p. 466) and the quite different combustion characteristics of the two types of engine, The diesel induces air only and the fuel is injected under pressure into the cylinder with self~igniti0n of the fuel in the air; the petrol engine induces a mixture of air and fuel which is spark ignited and burns as described insections 13.6 and 13.? giving fundamental combustion problems which do not occur with the diesel engine. To avoid charge detonation or ‘knocking’ in the petrol engine, giving uncontrolled combustion, the compression ratio may have to be reduced, an action which adversely affects the thermal efficiency of the engine_ Alternatively a fuel of higher octane rating may be necessary. With the diesel engine higher boost pressures can give more satisfactory combustion conditions with a wider range of usable fuels, reduced delay periods, controlled pressure rise and an engine which is smoother and quieter in operation. j The main features of supercharging are illustrated in the p—V diagrams for;g the idealized constant-volume four-stroke Cycle in Fig. 13.42 and the plant line; diagrams in Fig. 13.43. Figure l3.42(a) shows the normally aspirated cycle line l—-5 representing both the induction and exhaust strokes at about ambient air pressure pa. The early applications of supercharging were fol piston-engined aircraft in which the ‘blower’ was driven mechanically from t engine as shown in Fig. 13.43( a). The power output of the engine was increas by the higher flow of air, and hence the fuel consumed, but part of this increa ' in power was required to drive the blower. The effects on the p—V diagram," shown in Fig. 13.42( b), are to increase the pressures (and temperatures) reach" during the cycle and to give a positive pumping loop, 15671, to add to the ma working loop 12341. Reversible adiabatic processes Reversible adiabatic processes Inlet manifold pi, 7; Com * i @193 Exhaust manifold a. T. (3) (hi Example 13.3 e a blower mechanically connected to the engine The power required to driv n the net bp of the must be subtracted from the engine output to obtai supercharged engine. Then , area 12341 + area 15671 1mep = W x constant (13.16) length of diagram (13.17) and bp = (17M x ip) —— (power to drive blower) (for mechanically driven blowers only). gine is 12.9 kW/rn3 of free air induced per minute. The engine is a 3-litre four-stroke engine running at 3500 rev/min, and has a volumetric efficiency of 80 0/0, referred to free air conditions of 1.013 bar and 15 c’C. It is prOposed to fit a blower, driven mechanically from the engine. The blower has an isentropic efficiency of 75 % and works through a pressure ratio of 1.7. Assume that at the end of induction the cylinders contain a volume of charge equal to the swept volume, at the pressure and temperature of the delivery from the blower. Calculate the increase in bp to be expected from theengine. Take all mechanical efficiencies as 80%. The average ip developed in a CI en Figure 13.42(a) shows the start of the exhaust process at 4 at a pressu substantially greater than the ambient, pa. This means that over 60% of th‘ cylinder charge is suddenly exhausted by a free expansion which constitutes-"41 considerable loss of the energy released on combustion; of about 30—4 ' between diesel and petrol engines. The attraction of ‘turbocharging’ is evider‘i as the energy lost in this way is used to drive a turbine Wheel integral wi compressor wheel which delivers compressed air or charge to the cylinder. additional work available from the gas is indicated, after continuing reversible adiabatic expansion line 3—4 down to the pressure pa at 6, by 4614. The physical arrangement is shown in Fig. 13.43(b) and there i mechanical connection” to the engine. The turbocharger combination; free~running unit with approximately equal mass flow rates over the mi and compressor wheels reaching an equilibrium speed in the r. 20 000—80 000 rev / min. The simplest form of the supercharged cycle shows constant pressures Cd: in the inlet manifold, pi, and in the exhaust manifold, pa, and it is essent pi > pc, see Fig. 13.44(a). This pressure difference, pi — pe, can be util scavenge residual gas from the combustion chamber if there is some : between the exhaust and inlet valve operation and particularly so for th engine. This is called constant pressure supercharging and requires a larg exhaust manifold to create a constant pressure supply to the turbin highly pulsating delivery from the engine cylinders. The T—s diagra turbocharger is shown in Fig. l3.44(b) and, using the methods of CH the energy balance for the unit is obtained as follows: ' The compressor power input _ p. {ran Uh; W,3 = macpa I; ——i — 1 11C Pa The turbine power output, ‘ p {T3 — “I'VE,— _" WT S Illccche 1 _‘ J X 11-!“ Pa E E e isentropic efiiciencies of the compressor and turbine where 110 and 117 are th respectively. For a balance of mass flow rates . . . mr me=ma+mh —=l+-_—=1+F/A ina ma Where F/A = fuel to air ratio = inf/vita. Also We = FWT x W, where W = therefore I (is—1W, (vs—1W3 T llfl) *HI—(t) semi ' pe Cpa Ta where no = 11M x 11»; x 130 = the overall efficiency of the turbocharger. This expression shows how the manifold pressure pi depends mainly upon no and Te as the effect of the F/A ratio is small. A set of characteristics can be drawn of pi/p21 against 110 for different values of Te. A set is obtained for each value of pa and the minimum requirement to sustain the limit is p‘.= : pi as shown in Fig. 13.45. For example for pi/pa = 2, and T3 = 773 K (500°C) m, = 42%. The overall efficiency is higher for lower values of pa and higher values of pi and T3. The study of turbocharging should continue into the design of the turbocharger unit to meet its service requirements, but this is outside the scope of this book and specialist references such as 13.8 should be consulted. The usual arrangement for a turbocharger is a single-stage centrifugal compressor driven by a single—stage axial-flow turbine for the medium and large-size engines for industrial, rail, and marine applications, and by a radial—flow turbine for the smaller engines used in automotive applications, transport vehicles and cars. It is somewhat against earlier expectations that most car manufacturers now include supercharged petrol—engined cars in their product range with apparent overlap with their normally aspirated engines of difi‘erent capacities. This is in spite of the fact that cars spend a great deal of Is and that additional control is necessary to restrict vent the onset of knock by retarding the ignition. mechanical efficiency of the drive, the boost pressure and pre AG"! ...
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IC Engines Supercharging - RMIT UNIVERSITY ASSOCIATE DEGREE...

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