Four Stroke Gas EnginesFour Stroke Gas EnginesThe four strokes of a internal combustion engine are:

•Intake

•Compression

•Power

•Exhaust

Each cycle requires two revolutions

of the crankshaft (720˚ rotation), and one revolution of the camshaft to complete

(360˚ rotation).

Each stroke = 180˚ of crankshaft revolution.

Intake StrokeIntake StrokeFirst Stroke

The piston moves down the cylinderfrom TDC (Top Dead Center) to BDC (Bottom Dead Center).

This movement of piston causes low air pressure in the cylinder (vacuum)

Mixture of Air and Fuel in the ratioof 14.7 : 1 (air : fuel) is drawn intothe cylinder.

Intake valve stays open and theExhaust valve stays closed duringthis stroke.

Compression strokeCompression strokeSecond stroke

The piston moves from BDC to TDC

Intake and exhaust valves stay closed

Air and fuel mixture is compressed8:1 to 12:1

The pressure in the cylinder is raised

Power strokePower strokeThird stroke

At the end of compression strokethe sparkplug fires, igniting the air/fuelmixture.

Both the valves stay closed in this stroke.

The expanding gases from the combustion in the cylinder(with no escape) push the pistondown.

The piston travels from TDC to BDC.

Exhaust strokeExhaust strokeFourth and last stroke

The momentum created by the Counter-weights on the crankshaft,move the piston from BDC to TDC.

The exhaust valve opens and the burned gases escape into theexhaust system.

Intake valve remains closed.

Four strokesFour strokes

All four strokes

Four Cylinder EngineFour Cylinder Engine

Valve TimingValve Timing

The ratio of cam to crank sprocket is 2:1

For every two revolutions of Crankshaft, the Camshaft turns once.

Belt driven

Chain driven

Gear driven

Carburetor Carburetor The job of the carburetor is to accurately meter extremely tiny quantities of fuel and mix it with the air entering the engine, so thatthe engine runs properly.

It works on the principle of High and low pressure

High and low pressure is created with the help of Venture

Ignition systemIgnition systemProvides high voltage surges – as high as 47,000 volts to the spark plug

Parts• permanent magnet. • armature assembly. • breaker points. • condenser. • spark plug.

LubricationLubricationIn the engine, dippers on the connecting-rod bearing caps enter the oil pan with each crankshaft revolution to produce the oil splash.A passage is drilled in each connecting rod from the dipper to the bearing to ensure lubrication.(used only on 4-stroke small engines)

CoolingCooling

Air cooledFins are used to dissipate heat

Liquid cooledCoolant is used to dissipate heat.