INTRODUCTION • The power out put of an engine depends upon the amount of air inducted per unit time and the degree of utilization of this air , and the thermal efficiency of the engine. Indicated engine Power IP=P*L*A*n*K/60000 ……………..(1) Where, IP= indicated power (kW) P=indicated mean effective pressure(N/m2) L=length of stroke A= area of piston n= no of power stroke, for 2-s engine-N and for 4-s engine N/2, N= rpm K= No of cylinders
Three possible methods utilized to increase the air consumption of an engine are as follows: • Increasing the piston displacement: This increases the size and weight of the engine, and introduces additional cooling problems. • Running the engine at higher speeds: This results in increased mechanical friction losses and imposes greater inertia stresses on engine parts. • Increasing the density of the charge: This allows a greater mass of the charge to be inducted into the same volume.
Definition The most efficient method of increasing the power of an engine is by supercharging, i.e. increasing the flow of air into the engine to enable more fuel to be burnt. • A Supercharger is run by the mechanical drive, powered by engine power . • A turbocharger uses the otherwise unused energy in the exhaust gases to drive a turbine directly connected by a co-axial shaft to a rotary compressor in the air intake system.
COMPRESSED AIR Air inlet Fig.1 Supercharger
Types Fig. 2 Turbocharger
Need of turbocharger and super charger • For ground installations, it is used to produce a gain in the power out put of the engine. • For aircraft installations, in addition to produce a gain in the power out put at sea-level, it also enables the engine to maintain a higher power out put as altitude is increased.
Working principle of a turbocharger: • A turbocharger is a small radial fan pump driven by the energy of the exhaust gases of an engine. • A turbocharger consists of a turbine and a compressor on a shared shaft. • The turbine converts exhaust to rotational force, which is in turn used to drive the compressor. • The compressor draws in ambient air and pumps it in to the intake manifold at increased pressure, resulting in a greater mass of air entering the cylinders on each intake stroke.
Types of super charger: Based on the use of compressor • Centrifugal type • Roots type • Vane type • Components of turbocharger • Air compressor • Turbine • Intercooler
Thermodynamic analysis of turbocharged engine cycle 3 2 4 1 0 FIG. 6 Four-stroke cycle of an SI engine equipped with a supercharger turbocharger, plotted on p-v coordinates.
Net work output Wnet= work done by piston + Gas exchange work = area A + area Area A= .......................(2) Area B= work done by turbocharger= ..............(3) Wnet = Work done per unit of air mass. Where, p0 = atmospheric pressure, p1= pressure after compression, T0= atmospheric air temperature, V1= volume of boosted air, rp =pressure ratio, r = compression ratio, cp=Specific heat of air and η = turbocharger efficiency,
Selection process of turbocharger • The concept of turbocharger is illustrated in Figure 7. • Compressor air inlet,Point1- p1, T1 • Compressor air out let, point2-p2, T2 • Turbine exhaust gas inlet, point 3-p3,T3 • Turbine exhaust gas outlet- • P4, T4 Figure7. Illustration of the concept of a turbocharger.
Terms essential for turbocharger selection Air Consumption and Air-Delivery Ratio: …………………….(4) Where mat = theoretical air consumption rate, kg/h atm & De = engine displacement, L Ne = engine speed, rpm ρa = density of air entering compressor, kg/m3 The air-delivery ratio is the ratio of the measured over the theoretical air consumption of an engine: …………………..(5) where ev = air-delivery ratio mat= theoretical air consumption of the engine, kg/h ma= actual air consumption of the engine, kg/h
……………………(5) • A turbocharger air delivery ratio. • The turbine pressure ratio is defined as , κpt = p3 / p4 ……………….(6) • Pressure ratio across the compressor, κpc, as ………………….(7) • The temperature ratio across the compressor ……………………..(8) Where ec = compressor efficiency, decimal.
The compressor efficiency = ( theoretical temperature rise across the compressor)/(the actual temperature rise). ec is always less than 1.0. • The turbine efficiency = ( the actual temperature drop across the turbine )/(the theoretical temperature drop). The turbine efficiency is also always less than 1.0.
The following procedure may be used in selecting a turbocharger for an engine. 1. Select the desired, achievable power output, Pb; verify that the chosen power level does not require an excessive pbme. Realistically, pbme ≤ 1250 kPa is achievable. 2. Calculate mf = Pb× BSFC, using an achievable value for BSFC. Typically, for a well-designed engine, it is possible to achieve , 0.2 < BSFC < 0.25 kg/kW h. 3. Calculate ma = mf× (A/F), using the desired A/F ratio of the turbocharged engine. For a CI engine running on diesel fuel, typically 25 < (A/F) < 32. 4. Select the compressor and the point on the compressor map (see Figure 8 for an example map) at which the compressor will operate at rated load and speed of the engine. Equations 3 through 4 can be reworked into
5. Select the turbine and the operating point on the turbine map. The turbine and compressor must rotate at the same speed, the turbine flow must equal the compressor flow times (1 + FA), and the turbine must supply enough power to drive the compressor while overcoming bearing friction. The mechanical efficiency of the turbocharger …………………..(9)
Equation 10 can be reworked into characteristic-value equations that incorporate the speed, flow and power constraints: …………….(10) ……………….(11) where τ avaiablel = characteristic value available τ required = characteristic value required u = − (k′ − 1)/k′ et = turbine efficiency, decimal em = turbocharger mechanical efficiency, decimal Cpc = constant-pressure specific heat of ambient air, kJ/kg·K Cpt = constant-pressure specific heat of heated air, kJ/kg·K The available characteristic value depends upon the FA ratio, the turbocharger efficiencies, and the temperature ratio across the engine.
Advantages of supercharger and turbocharger • The more increase the pressure of the intake air above the local atmospheric pressure (boost), the more power the engine produces. Automotive superchargers for street use typically produce a maximum boost pressure between 0.33 to 1.0 bar , providing a proportionate increase in power. • Engines burn air and fuel at an ideal (stoichiometric) ratio of about 14.7:1, which means that if you burn more air, you must also burn more fuel. • This is particularly useful at high altitudes: thinner air has less oxygen, reducing power by around 3% per 1,000 feet above sea level, but a supercharger can compensate for that loss, pressurizing the intake charge to something close to sea level pressure.
Disadvantages of turbocharger and supercharger • Cost and complexity • Detonation • Parasitic losses • Space • Turbo lag
Performance evaluation of the Turbo charged Agricultural Tractor Engine Place: Agricultural Machinery Research Centre, Massey University, Palmerston North, New Zealand in 1990. Tractor- John Deere 3140 No of cylinder-6 Compression ratio- 16.8: 1 Fuel – 10% tallow ester + 90% diesel
Experimental Setup • Monitor exhaust temperature with (Fe/ Cn thermocouple and oil sump temperature with (Cu/Cn thermocouple). • Before each run the engine was worked under load for 10-15 min to achieve normal operating conditions. • Using a calibrated A W Nebraska 200 p.t.o. dynamometer, a series of steady state measurements of p.t.o. speed, torque and hence power was taken • Settings an injection pressure of 210 bar and fuel pump calibration to provide 51 mm3 of fuel at rated speed and full load. • A Campbell 21X data logger
Experimental Conditions • Naturally aspirated engine 2. Naturally aspirated + servicing and 3. Turbocharged engine. In the experiment the following parameters were measured. • Torque • Power • Exhaust gas temperature • Turbocharger Oil Temperature.
Type of Compressor. 2. Vane type 1.Centrifugal type 3. Root’s type
Results and discussion: • Torque: Torque-rise percentage (from torque at maximum power at approximately 570 rev/min at the p.t.o. to maximum torque, which represents the torque back-up, or “lugging ability” of the tractor), was 18.9% for the original naturally aspirated mode, rose to 21.6% after servicing, and reached 33% after turbocharging. • Power:Due to the increased torque after servicing, maximum power increased from 63.1 kW to 65.9 kW at 570 rev/min and remained higher throughout the working speed range. The turbocharged version produced a maximum power of 77.1 kW • Exhaust gas temperature: • Oil temperatures
Conclusions: • Due to low speed of operation and less power in agricultural tractor, turbocharger is used not supercharger for more power generation and to operate it higher altitude. • Turbo-charging a tractor engine is an acceptable method of increasing its performance if carried out within manufacturers’ specifications. • Lower engine operating temperatures result which can be beneficial. • Since the engine lubricating oil is subjected to high temperatures as it passes through the turbocharger the correct oil must be used as specified for turbocharged engines.