Supercharger
1 4/13/2019 Supercharger Air Flow Requirements • Naturally aspirated engines with throttle bodies rely on atmospheric pressure to push an air–fuel mixture into the combustion chamber vacuum created by the down stroke of a piston. • The mixture is then compressed before ignition to increase the force of the burning, expanding gases. • The greater the mixture compression, the greater the power resulting from combustion. Engineers calculate engine airflow requirements using these three factors: – Engine displacement – Engine revolutions per minute (RPM) – Volumetric efficiency Volumetric efficiency – is a comparison of the actual volume of air–fuel mixture drawn into an engine to the theoretical maximum volume that could be drawn in. – Volumetric efficiency decreases as engine speed increases.
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Principles of Power Increase: The power output of the engine depend up on the amount of air induced per unit time and thermal efficiency.
The amount of air induced per unit time can be increased by: Increasing the engine speed. the increase in engine speed calls for rigid and robust engine as the inertia load increase also the engine fraction and bearing load increase and also the volumetric efficiency decrease.
Increasing the density of air at inlet. The increase of inlet air density calls supercharging which usually used to increase the power output from engine due to increase the air pressure at the inlet of engine.
3 4/13/2019 Supercharger The Objects of Supercharging:
To increase the power output for a given weight and bulk of the engine. This is important for aircraft, marine and automotive engines where weight and space are important. To compensate for the loss of power due to altitude. ( this mainly relates to aircraft engine which loss power at an approximate rate of one percent 100 meters altitude. To obtain more power from an existing engine.
Special applications of supercharging:
Air density decreases with the altitude “ICE aircraft” & hence the power. This negative effect is controlled by using supercharged. Volumetric efficiency can be controlled by supercharging. Supercharging can be very effectively to improve scavenging efficiency that controls the performance of two‐stroke engines.
4 4/13/2019 Supercharger Thermodynamic Cycle with Supercharger:-
The difference between the p-v diagrams of natural aspirated and supercharged engine are: The suction pressure of supercharged engine is higher than atmospheric pressure. The pumping loop of supercharged engine is positive.
So that, the power of engine increased and also the mechanical efficiency and engine has better gas exhaust process. 5 4/13/2019 Supercharger Supercharging Power:- The power required for driving the supercharger can be calculated as shown:
Where:
T1 atmospheric air temperature P1 atmospheric pressure. T2 air temperature at end of compression P2 air pressure at end of compression.
6 4/13/2019 Supercharger Supercharging of spark ignition engines: For spark ignition engines , it is employed only to aircraft and racing car engine due to: increase in the intake temperature and pressure reducing ignition delay and increase flame speed but these factor also increase the tendency of detonation and pre-ignition. The supercharging petrol engine have greater fuel consumption than natural a aspired engine.
So that, the supercharging used in spark ignition engine only When large amount of power is needed. with engine has low compression ratio. for aircraft and racing care engines.
Note:- Knocking can be controlled in highly supercharged engine by injection of water in the combustion chamber or by intercooling of charge before fed to the engine.
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8 4/13/2019 Supercharger Supercharging of C.I. engines: for diesel engine, increase the intake temperature and pressure reduce ignition delay and the rate of pressure rise is better so, quieter and smoother combustion. By improving combustion allows a pour quality fuel to be used in diesel engine.
Limitation of supercharging in C.I. engines: For SIE is due to knock. For CIE is due to reached the thermal load for engine material. the main considerations in limiting the degree of supercharging of a C.I.E are: Durability. Reliability. Fuel economy.
9 4/13/2019 Supercharger Effect of Supercharging on Performance of Engines: Power output: The power output of a supercharged engine is higher than its naturally aspirated due to:
The amount of air induced per cycle for given swept volume is increased. The mechanical efficiency is slightly improved. During the gas exchange process some of the work done on the supercharger is recovered. Supercharging results in better scavenging and reduced exhaust gas temperatures in the engine.
10 4/13/2019 Supercharger Mechanical efficiency: An increase in the supercharging pressure increases the gas load and that increase the fraction forces, but the increase in bmep is much more than increase in fraction forces. So that; the mechanical efficiency of supercharged engine are slightly better than the natural aspirated engine.
11 4/13/2019 Supercharger Fuel Consumption The power required to run the supercharger varies with different arrangements of supercharger.
If the supercharger is directly driven by the engine some of the power developed by engine will be used in running the supercharger. At part load, the compression of supercharger is not fully utilized so more loss in power and specific fuel consumption for mechanical driven will be more. For Supercharged SI engine have specific fuel consumption higher than a natural aspired engine.(due to use rich mixture to avoid detonation) For Supercharged CI engine have specific fuel consumption less than a natural aspired engine.(due to better combustion and increase thermal efficiency) If the supercharger is driven by exhaust it does not required any power from the engine. This will given 5% better thermal efficiency and the specific fuel consumption lower than the natural aspired engine.
12 4/13/2019 Supercharger Modification of an Engine for Supercharging: these modification made engine more suitable to supercharging as: Increase the valve overlap period, to allow complete scavenging of the clearance volume. Increase in clearance volume by decrease the compression ratio. The injection system must be modified to supply the required amount of fuel, that required a greater nozzle area than the normal aspired engine.
Methods of Supercharging : a supercharger can be run in a number of ways. The supercharger may be run by the engine through gearing or alternatively an exhaust driven turbine can run the compressor. As shown in the figure.
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Generator Type Super
14 4/13/2019 Supercharger Superchargers: compressors that are mechanically driven by the engine crankshaft and this loss in engine power is up to 15% of engine output.
Since the power to drive the compressor must be deducted from the indicated power of the engine, the mechanical efficiency of the engine decreases.
There are two categories of compressors used for supercharging:
A) Positive Displacement B) Dynamic Compressors i) Sliding vane i) Centrifugal ii) Root's blower ii) Axial iii)Screw compressors iv) Reciprocating compressor
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Sliding Vane Compressor:
Deep slots are cut into the rotor to accommodate thin vanes which are free to move radially. The rotor is mounted eccentrically in the housing. As the rotor rotates, the centrifugal forces acting on the vanes force them outward against the housing. The trapped air is compressed as the compartment volume decreases, and is then discharged through the outlet port.
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Roots Compressor:
Two rotors are connected by gears. Air trapped in the recesses between the rotor lobes and the housing is carried toward the delivery port without significant change in volume. As these recesses open to the delivery line, since the suction side is closed, the trapped air is suddenly compressed by the backflow from the higher‐pressure delivery line. The most common use of this type supercharger or blower is on the Detroit two stroke diesel engines “ buses and fire trucks” It is not known as the most efficient type of supercharger.
17 4/13/2019 Supercharger Modified Roots Type:
To make the Roots type more efficient, Eaton has modified the impellers by adding a twist to them and moving the intake port to the end. This makes them more efficient and quieter. The modified roots type is the best type of supercharger to use until the boost pressure exceeds 13 PSI. Above 13 PSI the better supercharger would be the Lysholm type.
18 4/13/2019 Supercharger Screw “Lysholm” Compressor:
It has two helical rotors, male and female, which intermesh very closely without ever making contact. Air is drawn in normally at the open end of the rotors and is trapped as the rotors mesh together, gradually compressed between the rotor lobes and the closed end face as it is propelled along, then expelled from the delivery port at the required pressure. Hint :Saab and Mercedes uses the Lysholm superchargers
19 4/13/2019 Supercharger centrifugal supercharger:
It uses impeller to spin the air outward by centrifugal force. Diffuser vanes guide the air flow to the outlet. The simple construction leads to advantages of lightweight and compact size. It spins at very high speed (up to 60,000 rpm) to produce maximum boost pressure. To make this possible, it incorporates step‐up gears to multiply rev from crankshaft. The boost pressure grows exponentially with rev. As a result, centrifugal supercharger produces little boost at low to medium rev. It works best at high rpm. Because of its low inertia and friction, it has the highest efficiency among all superchargers
20 4/13/2019 Supercharger Turbochargers
Available energy exhaust Gasses Rotate Turbine. Turbine Rotates Compressor Shaft. Compressor shaft acts as centrifugal compressor. Air drawn into compressor, and is compressed and heated. Heated air can be cooled by intercooler. Cooling reduces temperature and pressure. May also have pressure losses due to friction.
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NOTE:- The power developed by the turbocharger is sufficient to drive the compressor and over come its friction resistance. The turbocharger is usually rather independent of the engine in that it is only connected to it by a simple exhaust pipe. The speed of turbocharger ranges from 20000 to 80000 rpm. In order to supply sufficient energy to the turbocharger the exhaust valve is open much before the bottom dead center as compared to the naturally aspirated engine. The power output of turbocharged engine range from 50 hp to 30000 hp. below 50 hp the cost of manufacture of an efficient turbine becomes excessive.
22 4/13/2019 Supercharger Waste Gate Valve Device installed on turbocharged engine to control manifold pressure and to regulate the maximum boost pressure in turbocharger. It is valve, when it opens, allows engine exhaust to bypass the turbocharger turbine, effectively reducing intake pressure. The waste‐gate valve is operated by a diaphragm that is operated by manifold pressure. The diaphragm will open the waste gate valve whenever manifold pressure reaches the desired maximum.
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Turbo lag
Lag is the delay in engine response upon demand for a rapid increase in power. One of the methods to improve transient response is to reduce turbine and compressor wheel inertia. Another way to eliminate turbo‐lag is to use a supercharger.
Turbo Lag Reduction: Twin Turbo
Two turbochargers: Smaller turbo for low rpm low load and a larger one for high load Smaller turbo gets up to speed faster so reduction in turbo lag
Supercharger/turbo: Supercharger used at low speed to eliminate turbo lag At higher rpm turbo charger used to eliminate parasitic load
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Methods Of Turbocharging Methods Of Turbocharging
Constant Pressure Turbocharging Pulse Turbocharging Pulse Converter Two Stage Turbocharger Miller Turbocharging Hyperbar Turbocharging
25 4/13/2019 Supercharger Arrangement of Exhaust Manifolds:
The clearance volume should be properly scavenged in a supercharged engine.
The scavenging takes place during the valve overlap period in which both inlet and exhaust valves remain open. This requires that there should be a definite pressure difference between the charging air pressure and the back pressure of the engine (pressure in the exhaust manifold).
If the period between successive exhausts into a single pipe differs by less than 204o crank angle in 4-stroke turbocharged engine interference with scavenging process with occur. For this reason not more than three cylinders firing successively can be exhausted into common manifold.
but for engine have more than three cylinders it must have more than one turbine or the turbine must be of multi enter type.
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The numbers of exhaust manifold required for engine with different number of cylinders be:
No. of cylinders No. of exhaust manifold Type of turbine used 4 - 6 2 Two-entry turbine 5 - 6 3 Three-entry turbine 7 - 8 4 Four-entry turbine
27 4/13/2019 Supercharger The arrangement of exhaust manifold :-
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Constant Pressure Turbocharging
The exhaust ports from all cylinders are connected to a single exhaust manifold whose volume is sufficiently large to ensure that its pressure is virtually constant. Major advantage of the constant pressure system is that turbine inlet conditions are steady and known, hence the turbine can be matched to operate at optimum efficiency at specified engine conditions. The main disadvantage is that the available energy entering the turbine is low compared to pulse type. When the engine load is suddenly increased or a rapid speed increase is required, the pressure in the large volume is slow to rise. This restrict such type from frequent response applications
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Advantages of constant pressure turbocharging High turbine efficiency, due to steady flow Good performance at high load Simple exhaust manifold
Disadvantages of constant pressure turbocharging: Low available energy at turbine Poor performance at low speed and load Poor turbocharger acceleration
Applications of constant pressure turbocharging: Large industrial and marine engines operating at steady speed and load, highly rated; two‐ and four‐stroke.
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Pulse Turbocharging
The majority of turbocharged engines use the pulse, not the constant . The objective is to make the maximum use of the high pressure and temperature which exists in the cylinder when the exhaust valve opens. In most cases the benefit from increasing the available energy will more than offset the loss in turbine efficiency due to unsteady flow. The pulse system requires the exhaust manifold to be so small. the exhaust ports are connected to the turbine by short, narrow diameter pipes.
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Advantages of pulse turbocharging High available energy at turbine Good performance at low speed and load Good turbocharger acceleration
Disadvantages of pulse turbocharging Poor turbine efficiency with one or two cylinders per turbine entry Poor turbine efficiency at very high ratings Complex exhaust manifold with large numbers of cylinders Possible pressure wave reflection problems (on some engines)
Applications of pulse turbocharging Automotive, truck, marine and industrial engines; 2 & 4 stroke. low and medium rating
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Limitations of turbocharging
The use of turbocharging requires special exhaust manifold.
Fuel injection has to be modified to inject more fuel per unit time. ( which requires either larger pumping elements and larger nozzles. This means overloading of the cams and other components)
A natural aspirated engine can digest solid particles in the inlet air without undue stress but turbocharged engines pass only the most minute material particles without damage.
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