Supercharging

A supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle, increasing the power output of the engine.

A supercharger can be powered mechanically by a belt, gear, shaft, or chain connected to the engine's crankshaft. It can also be powered by an exhaust gas turbine. A turbine-driven supercharger is known as a turbosupercharger or turbocharger. The term supercharging refers to any pump that forces air into an engine, but, in common usage, it refers to pumps that are driven directly by the engine, as opposed to turbochargers that are driven by the pressure of the exhaust gases.

Valve timing Diagram

Theoretically above cycle is well perfect but in actual practice,it is slightly modified by the opening of inlet valve and delayed closing of exhaust valve.The details are as below.

The inlet valve is opened 10 to 30 degree in advance to the top dead centre of the piston to facilitate the inrush of fresh charge and out rush of burnt gases.

The piston moves down during suction stroke which is continued up to 30 to 40 degree or even 60 degree after the bottom dead centre.The inlet valve is then closed and compression stroke starts.

To give some extra time to fuel to burn,the spark is produced at 30 to 40 degree before the top dead centre of piston.The pressure rises up and attains a maximum value when the piston is about 10 degree past to top dead centre.

The exhaust valve is open about 30 to 60 degree before piston reaches to bottom dead centre.The burnt exhaust gases pushed out of cylinder as the piston starts moving upward.This exhaust stroke continuous till the exhaust valve closed when the piston is about 8 to 10 degree or even 25 degree past the top dead centre.

The angle between the position of the crank at the inlet valve opening and that exhaust valve closing is known as valve overlap.

All this angular positions of crank can be plotted by a circular line corresponding to one vertical line;where top dead center can be taken at top of the line and bottom dead center at bottom of the vertical line.

Difference between Two stroke and Four Stroke

How the Engines Work

"Stroke" refers to the movement of the piston in the engine. 2 Stroke means one stroke in each direction. A 2 stoke engine will have a compression stroke followed by an explosion of the compressed fuel. On the return stroke new fuel mixture is inserted into the cylinder.

A 4 stroke engine has 1 compression stroke and 1 exhaust stoke. Each is followed by a return stroke. The compression stroke compresses the fuel air mixture prior to the gas explosion. The exhaust stroke simply pushes the burnt gases out the exhaust.

A 4 stroke engine usually has a distributor that supplies a spark to the cylinder only when its piston is near TDC (top dead center) on the fuel compression stroke, ie. one spark every two turns of the crank shaft. Some 4 stroke engines do away with the distributor and make sparks every turn of the crank. This means a spark happens in a cylinder that just has burnt gasses in it which just means the sparkplug wears out faster.

Animated picture goodness showing examples of these engines can be found at carbibles.com.

A Common List of Advantages and Disadvantages

Advantages of 2 Stroke Engines:
- Two-stroke engines do not have valves, simplifying their construction.
- Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Two-stroke engines are lighter, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

Disadvantages of 2 Stroke Engines:
- Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
From:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

Open Gas turbine Cycle

Open Gas Turbine Cycle

Open gas turbine cycle is the most basic gas turbine unit. The working fluid does not circulate through the system, therefore it is not a true cycle. It consists of a compressor, a combustion chamber and a gas turbine. The compressor and the gas turbine are mounted on the same shaft. The compressor unit is either centrifugal or axial flow type. The working fluid goes through the following processes:

• 1-2 irreversible but approximately adiabatic compression
• 2-3 constant pressure heat supply in the combustion chamber
• 3-4 irreversible but approximately adiabatic expansion of combustion gases

In the simplified T-s diagram, shown above, pressure loss at compressor inlet, in combustion chamber and at turbine outlet are neglected. Thermal efficiency of the cycle is:

Gas Turbines

A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. (Gas turbine may also refer to just the turbine element.)

Energy is added to the gas stream in the combustor, where air is mixed with fuel and ignited. Combustion increases the temperature, velocity and volume of the gas flow. This is directed through a nozzle over the turbine's blades, spinning the turbine and powering the compressor.

Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.