1. Four Stroke Gas Engines
The four strokes of a internal combustion engine are:
Intake
Compression
Power
Exhaust
Each stroke = 180˚ of
crankshaft revolution.
Each cycle requires two revolutions
of the crankshaft (720˚ rotation), and
one revolution of the camshaft to complete
(360˚ rotation).

2. Intake Stroke First Stroke The piston moves down the cylinder
from 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 ratio
of 14.7 : 1 (air : fuel) is drawn into
the cylinder.
Intake valve stays open and the
Exhaust valve stays closed during
this stroke.

3. Compression stroke Second stroke The piston moves from BDC to TDC
Intake and exhaust valves stay closed
Air and fuel mixture is compressed
8:1 to 12:1
The pressure in the cylinder is raised

4. Power stroke Third stroke At the end of compression stroke
the sparkplug fires, igniting the air/fuel
mixture.
Both the valves stay closed in
this stroke.
The expanding gases from the
combustion in the cylinder
(with no escape) push the piston
down.
The piston travels from TDC to BDC.

5. Exhaust stroke Fourth 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 the
exhaust system.
Intake valve remains closed.

6. Four strokes
All four strokes

7. Four Cylinder Engine

8. Valve Timing The ratio of cam to crank sprocket is 2:1
For every two revolutions of
Crankshaft, the Camshaft turns once.
Chain driven
Belt driven
Gear driven

9. 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 that
the engine runs properly.
It works on the principle of
High and low pressure
High and low pressure is created
with the help of Venture

10. Ignition system
Provides high voltage surges – as high as 47,000 volts to the spark plug
Parts
permanent magnet.
armature assembly.
breaker points.
condenser.
spark plug.

11. Lubrication
In 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)

12. Cooling Air cooled Fins are used to dissipate heat Liquid cooled
Coolant is used to dissipate heat.