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.

The Two-Stroke EngineA 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 intakeThe 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 StrokeAs 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/Exhaust 4- Compression

5- Power

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.

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

Advantages 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 Two-Stroke Engine The Four-Stroke Engine

1- the complete cycle requires one revolutions of the crankshaft to complete.

2- Lowe rpower because of one stroke in two revolutions.

1- the complete cycle requires two revolutions of the crankshaft to complete.2- Higher (theoretically twice) because of one stroke every revolution.

Two-stroke Parts1 - 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 of1-lower strength than cast iron , 2- the light weight of this material 3- its higher Coefficient of thermalexpansion 4- The thermal conductivityof aluminium is about three times that of iron.this , enables aluminium  pistons to  run attemperatures  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 ringsThe 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 mechanismtransforms  reciprocative motion to rotational motion.

* There are bearing portions at  both ends,thepiston side is  called the  small end,and Thecrankshaft 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- CrankshaftThe 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 Elementswhich is mainly used for motorcycles.

The type of crankshaft determines what kind of Connecting rods are used .

Most of crankshaft  has been a steel forging ,nodularcast iron  crankshafts  are  also  accepted normalpractice in automotive  engines . the crankshaft is.supported in Main bearing

Crankshaft Materials

The strength of the shaft depends primarily on thatof 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.

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 obtainedby integrating  around the curve to obtain the area enclosed on the diagram.

With two-stroke cycle in the figure below , theenclosed  integration is  straightforward .

Mean Effective PressureMean 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

                      Wnet                                     MEP = -----------------           Vmax - Vmin