Supercharging and Turbocharging in I C Engines 12
Supercharging and Turbocharging in I. C. Engines
• 12. 1 Introduction • The power output of an engine depends on the amount of air inducted per unit time, the degree of utilization of this air and thermal efficiency of the engine. The amount of air can be increased by increasing engine speed or by increasing air density at intake. The method of increasing air density called “Supercharging”. This is done by supplying a pressure higher than atmospheric pressure at which the engine naturally aspirates air from atmosphere. This is done by using a pressure boosting device called a supercharger. • • 12. 2 Objects of Supercharging • The increase in the amount of air inducted per time by supercharging is obtained mainly to burn a greater amount of fuel in a given engine and thus increases its power output. The objects of supercharging include one or more of the following: • To increase the power output for a given weight and bulk of the engine. This is important for aircraft, marine and automobile engines where weight and space are important • To compensate the loss of the power due to altitude. This is mainly relates to aircraft engines. • To obtain more power from an existing engine.
• 12. 3 Thermodynamic cycle with supercharging • Fig. shows (p-v) diagram from an ideal Otto cycle supercharged engine. Area (8 -6 -7 -0 -1 -8) represents the work done by supercharging in supplying air at a pressure P 1 while the area (1 -2 -3 -4 -1) is the output of the engine. Area (0 -16 -7 -0) represents a gain in work during as exchange process due to supercharging. Area (1 -6 -8 -1) represents the loss of work. • There are two important differences in the (p-v) diagram for supercharge engine and natural aspiration engine which are: • Increase in power over the natural aspirated cycle. • The pumping loop of a supercharged engine is positive instead of negative in natural aspirated engine. • 12. 4 Supercharging power • Power required for driving the supercharging can be calculated considering the steady flow process as given in figure below: • Writing down the steady flow energy equation for this process we get:
• • Supercharged petrol engines have a greater SFC than naturally aspirated engines. Knocking can be controlled in high supercharged engines by injection of water in combustion chamber. Another produce is to use to intercooling of the charge before it fed to the engine. So because of its poor fuel economy, supercharging is not very popular and is used only when more power is needed or when more power is needed to compensate altitude loss. 12. 6 Supercharging of Diesel Engine • The supercharging in diesel engine does not result in any combustion problem, rather it improve combustion. • • Hence the rate of pressure rise resulting in a better, quicker and smother combustion. • The degree of supercharging is limited by thermal and mechanical load on the engine and strongly depends on the type of supercharger used and the design of the engine. • The power can be supplied by a separate drive for the supercharger, or by connecting the supercharger directly to the engine shaft. In all cases the gain in power output of the engine would be many times more than the power required to drive the compressor. Q: what the supercharging limits in petrol and diesel engines?
• • • 12. 7 Supercharger types 1. Reciprocating compressor: rarely used nowadays except for some stationary installations, it’s quite bulky and heavy, high compression ratio can be obtained, its efficiency (75 -85) % 2. Van Blower When the blades move, out air is inducted between spaces. The air is discharged when these spaces near the exhaust side of the supercharger. It’s noisy and had a limit speed. 3. Lysholm compressor Air is admitted at one end of the compressor and trapped between helical rotors and casing. This type of compressor produces constant compression. The main disadvantage of this compressor is its mechanical complexity which has a limited use. 4. Roots Blower It is consisting of two cylindrical shaped lobes rotating in opposite directions in a common housing. Air enters the space between the rotor lobes at the inlet and is carried around the rotor to discharge port. Compression takes place only when discharge port is opened and the pressure rise almost instantaneously. It’s noisy, simplicity, cheapness and good mechanical efficiency, rotor does not require lubrication its suitable for low and medium speed diesel engine for stationary and marine applications.
• • 5. Centrifugal Compressor It consists of an impeller rotating in a close fitted housing. Air enters the hub of the impeller axially and is turned 90 o by the radial vanes. A high velocity is imparted to air due to centrifugal action. High velocity of air from the tip of the radial vanes is passes to the diffuser where the pressure is raised and the air is supplied to the engine it has a velocity of (10000 -15000) rpm for low speed and (15000 -30000) rpm for high speed engines. Centrifugal compressor is simple, small, cheap, good efficiency. It’s commonly used for supercharging • One important disadvantage of this type of compressor is the occurrence of surge. If the compressor working near surge limit and any reduction of flow occurs, this will reduce efficiency of the compressor. Further reduce of flow air to low values cause instability in the air supply. • • 12. 8 Turbochargers are centrifugal compressors driven by exhaust gas turbines. These are nowadays extensively used for supercharging all types of two stroke engines. By utilizing the exhaust energy of the engine, it recovers a substantial part of energy which would otherwise go waste. Thus turbocharger will not draw on the engine power. For the total heat input to an engine, there is about 27 -30% of the energy goes to exhaust. Apart of this energy can be used to run a gas turbine which in turn will supply more air to the engine by driving a compressor. Such of the exhaust energy boosts engine power and results in better thermal efficiency and fuel consumption. •
• • 12. 9 Methods of Turbocharging 1. Pulse operation 2. Constant pressure 3. Pulse converter • 12. 10 Limitation of Turbocharging • 1. The use of turbocharger requires special exhaust manifolds. • 2. Fuel injection has to be modified to inject more fuel per time; this requires either large pumping element or large nozzles • 3. The efficiency of turbine blades is very sensitive to gas velocity so that it’s very difficult to obtain a good efficiency over a wide range of operation. • 4. Naturally aspirated engine allow soild particles without any stress, but turbocharged engine pass only most minute materials particles without damage.
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