Lecture 2. Top Dead Center (TDC): Position of the piston when it stops at the extreme point away from the crankshaft. – Top because this position is at.

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Presentation transcript:

Lecture 2

Top Dead Center (TDC): Position of the piston when it stops at the extreme point away from the crankshaft. – Top because this position is at the top of the engines, and dead because the piston stops as this point. – When the piston is at TDC, the volume in the cylinder is a minimum called the clearance volume. Engine Terminology :

Bottom Dead Center (BDC): Position of the piston when it stops at the point closest to the crankshaft. Stroke : Distance traveled by the piston from one extreme position to the other : TDC to BDC or BDC to TDC. Engine Terminology :

Bore :It is defined as cylinder diameter or piston face diameter; piston face diameter is same as cylinder diameter. Swept volume/Displacement volume : Volume displaced by the piston as it travels through one stroke. – Swept volume is defined as stroke times bore. – Displacement can be given for one cylinder or entire engine (one cylinder times number of cylinders). Engine Terminology :

Clearance volume : It is the minimum volume of the cylinder available for the charge (air or air fuel mixture) when the piston reaches at its outermost point (top dead center or outer dead center) during compression stroke of the cycle. – Minimum volume of combustion chamber with piston at TDC. Compression ratio : The ratio of total volume to clearance volume of the cylinder is the compression ratio of the engine. – Typically compression ratio for SI engines varies form 8 to 12 and for CI engines it varies from 12 to 24 Engine Terminology :

Comparison of Two Stroke & Four Stroke Engine

Two Stroke Engine: The thermodynamic cycle is completed in two strokes of the piston. The turning moment is more uniform. So a lighter fly wheel is used in the case of a two stroke engine. In a two stroke engine, there is a power stroke in every rotation of the crank shaft. Because of the ability to obtain a power stroke in each rotation of the crank shaft, the power produced is twice compared to four stroke engine if the size is same. There are no valve system provided for two stroke engine, This made the working and design simpler. Due to the absence of valve system snd light weight, the initial production cost of two stroke engine is low.

In the case of two stroke engine, the volumetric efficiency is less due to lesser time for induction. There is chance for incomplete burning and causes environmental pollution. These internal combustion engines are used where low cost, compactness and light weight are important considerations. High power-to-weight ratio Examples are in scooters, motor cycles, ships etc..

Disadvantages: Two-stroke engines don't live as long as four-stroke engines. Due to the lack of a dedicated lubrication system 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. Two-stroke engines do not use fuel efficiently, Two-stroke engines produce more pollution.

Four Stroke Engine: In a four stroke engine, there is only a power stroke in every two rotations of the crank shaft since the thermodynamic cycle is completed in four strokes of the piston. Because of the above reason, the turning moment is less uniform. So a heavier fly wheel is used in the case of a four stroke engine. Because of the production of only a power stroke in each two rotations of the crank shaft, the power produced is half compared to two stroke engine if the size is same. There is valve system provided for four stroke engine. This made the working and design of this engine complex. Due to the presence of valve system and increased weight, the initial production cost of four strike engines is high.

In the case of four stroke engine, the volumetric efficiency is high due to larger time for induction. Here, thermal efficiency is higher. There is less chance for incomplete burning and so less chances for environmental pollution. These engines are used where efficiency is important. Examples are in cars, buses, trucks, etc..

Working of Four Stroke Engine: Intake stroke: The Piston is going down so the volume of the cylinder is getting larger. The Intake valve is open. A vacuum, or pressure of lessthan atmospheric is created in the cylinder. Atmospheric pressure forces air into the cylinder psi at sea level. Fuel is mixed with the air, and the cylinder is filled with the mixture of air and fuel.

Compression Stroke The Piston is going up so the volume of the cylinder is getting smaller. The Intake & exhaust both valves remain closed during compression stroke. Pressure of the mixture increased due to decrease in volume.

Power Stroke : The Piston is going down. Volume of the cylinder is increasing. Both valves closed. Spark plug fires, causing compressed air/fuel mixture to burn. The burning fuel creates tremendous pressure which forces the piston down. turning the crankshaft and producing power. This is not an explosion, but a smooth, controlled burn.

Exhaust Stroke: The Piston is going up, so the volume of the cylinder is getting smaller. Both valves are open for a short period of time at TDC. This is called "Valve Overlap"

Working of Two Stroke Engine: