The Two-Stroke Engine A two-stroke engine is a combustion engine that completes the thermodynamic cycle in two movements of the piston( one revolution of the crankshaft)

The two-stroke cycle 1- combined power and Exhaust stroke as the piston moves down. 2- induction and compression as it moves up again.

The two strokes are : 1 - compression stroke, which start by closing the inlet and exhaust ports. 2 - power or expansion stroke, which start by firing and the inlet and exhaust are still closed.

Mode of operation of the two-stroke engine 1 - The compression and fuel intake The fuel/air mixture is first drawn into the crankcase by the vacuum that is created during the upward At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward stroke of the piston.

Mode of operation of the two-stroke engine 2- Power Stroke As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel, as shown here:

loop scavening

The principle of the way the 2-stroke engine works: 1- Intake 2-Crankcase compression

3- Transfer/Exhaust4- Compression

5- Power

1- Two-stroke engines don't last nearly as long as four-stroke enginesdue to the lack of lubrication system. 2- Two-stroke oil is expensive 3- Two-stroke engines do not.use fuelefficiently 1- Two-stroke engines do not have valves, which simplifies their construction and lowers their weight. 2- Two-stroke engines fire once every revolution. This gives two-stroke engines a significant power boost. 3- Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A dvantages Of Two-Sroke Engines Disadvantages of the Two-stroke 4- Two-stroke engines produce a lot of pollution.

Comparison between two stroke and four stroke engine The Four-Stroke Engine The Two-Stroke Engine 1- the complete cycle requires two revolutions of the crankshaft to complete. 2- Higher (theoretically twice) because of one stroke every revolution. 1- the complete cycle requires one revolutions of the crankshaft to complete. 2- Lowe rpower because of one stroke in two revolutions.

Two-stroke Parts 1 - Piston The piston performs the following functions : (1) Forms a movable gas-tight plug to confine the charge in the cylinder. (2) Transmits to the connecting rod the forces generated by combustion of the charge. (3) Forms a guide and a bearing for the small end of the connecting rod.

Piston Materials * Pistons are made of aluminum in small engines or cast iron in.larger slower- speed engine * Aluminium alloy pistons almost used in all modern engines because :the aluminium alloy is of 1-lower strength than cast iron, 2- the light weight of this material 3- its higher Coefficient of thermal expansion 4- The thermal conductivity of aluminium is about three times that of iron. this, enables aluminium pistons to run at temperatures about 200°C lower than cast- iron ones.

The main parts of a piston are : 1 - The top,which may also called the Head or Crown. 2 - The Ring belt. 3 - The Pin bosses. 4 - The Skirt

2 - Piston rings The main functions of piston rings are : (1) To form a pressure seal, preventing blow-by of the gases, including combustion products at high temperatures. (2) Control of the flow of oil in enough quantity to the skirt and to the rings themselves, while preventing excessive amounts from entering the combustion chamber. (3) The transfer of heat from the piston to the.cylinder walls

The piston ring of an internal combustion engine must be designed with sufficient heat resistance to withstand exposure to high.temperature gas

3-Connecting rod: * The crankshaft connecting rod mechanism transforms reciprocative motion to rotational motion. * There are bearing portions at both ends,the piston side is called the small end,and The crankshaft side,the big end. * The bearing portions receive load from the.weight of the piston and the conneting rod

* The connecting rod, usually a steel or alloy forging ( though sometimes aluminum in small engines ). connecting rod materials 1- failure of the bearings,the conneting rod should be made as light as possible. 2- Buckling,the rod portion usually has an I-beam shape because of the high rigidity-to-weight ratio of this shape. : Problems that connecting rod face

4- Crankshaft The crankshaft converts reciprocative motion to rotational motion. It contains counter weights to smoothen the engine revolutions

types of crankshaft There are two types of crankshaft, 1- The monolithic type used for multi-cylinder engines, 2- the assembled type fabricated from separate Elements which is mainly used for motorcycles. The type of crankshaft determines what kind of Connecting rods are used. Connecting rods

Most of crankshaft has been a steel forging,nodular cast iron crankshafts are also accepted normal practice in automotive engines. the crankshaft is.supported in Main bearing Main bearing Crankshaft Materials The strength of the shaft depends primarily on that of the material from which it is made.

5 -Gudgeon pin connects the piston to the connecting rod and provides a bearing for the connecting rod to pivot upon as the piston moves.piston pivot

6- flywheel The flywheel is fastened to rear of the crankshaft. The energy imparted to it during the power stroke is sufficient to keep the crankshaft turning through the idle strokes until it receives another impulse thus producing asmooth running engine.

Indicated Work Per Cycle The indicated work per cycle /Cylinder is obtained by integrating around the curve to obtain the area enclosed on the diagram. With two-stroke cycle in the figure below, the enclosed integration is straightforward.

Mean Effective Pressure Mean Effective Pressure (MEP). It is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as.that produced during the actual cycle Wnet = MEP * Piston area * Stroke = MEP * Displacement volume Wne t MEP = Vmax - Vmin