Branch: : Aeronautical EngineeringSend Friend Request
The understanding of the different Bike mechanism is an important aspect
for a rider to be more familiar with the bike and get adjusting to the
different aspect of the Bike.
Let’s look at the different mechanism of a Bike:
1. Working of Engine and Valve:
There are two types of engine commonly used in Motorbikes. They are Two
Stroke Engine and Four stroke Engine.
Both the Engines come under the banner of “Internal Combustion (IC)
Engine”. In the two stroke engine, the working cycle is completed in two
strokes of type piston or one revolution of the crankshaft. This is achieved
by carrying out the suction and compression processes in one stroke (or
more precisely in inward stroke), expansion and exhaust process in the
second stroke (or more precisely in outward stroke).
In a four stroke engine, the working cycle is completed in four strokes of the
piston or two revolutions of the crankshaft. This is achieved by carrying out
suction, compression, expansion and exhaust processes in each stroke. The
four stroke Petrol Engine Cycle also known as Otto Cycle requires four
strokes of operation in the engine cylinder. The four strokes of a petrol
engine sucking fuel-air mixture (petrol mixed with proportionate quantity of
air in the carburetor known as charge) are described below.
* Suction or Charge Stroke: In this stroke, the inlet valve opens and pure
air is sucked into the cylinder as piston moves downwards from the Top
dead centre (TDC). It continues till the piston reaches its Bottom dead
* Compression Stroke: In this stroke, both the valves are closed and the
air is compressed as the piston moves upwards from the BDC to TDC. As a
result of compression, pressure and temperature of the air increases
considerably. This completes one revolution of the crank shaft.
* Expansion or Working Stroke: Shortly after the piston reaches the TDC
(during the compression stroke), fuel oil is injected in the form of very fine
spray into the engine cylinder, through the nozzle, known as fuel injection
valve. At this moment, temperature of the compressed air is sufficiently high
to ignite the fuel. It suddenly increases the pressure and temperature of the
products of combustion. The fuel oil continuously injected for a fraction of
the revolution. The fuel oil is assumed to be burnt at constant pressure. Due
to increased pressure, the piston is pushed down with a great force. The hot
burnt gases expand due to high speed of the piston. During this expansion,
some of the heat energy is transformed into mechanical work. It may be
noted that during this working stroke, both the valves are closed and the
piston moves from TDC to BDC.
* Exhaust Stroke: In this stroke, the exhaust valve is open as the piston
moves from BDC to TDC. This movement of the piston pushes out the
products of the combustion from the engine cylinder through the exhaust
valve into the atmosphere. This completes the cycle and the engine cylinder
is ready to suck the fresh air again.
It will be interesting to know that from the thermodynamic point of view,
there is no difference between two-stroke and four-stroke cycle engine. The
difference is purely mechanical.
Today all the major IC Engines of the world are running on the four stroke
2. The Transmission:
The engine converts the explosive energy to mechanical energy, through the
reciprocating motion to rotary motion. The power developed from rotary
motion is controlled by various systems in a bike. This system is known as
the Transmission. The Transmission consists of the clutch, gear-box & finaldrive
chain all the way upto the driving wheel.
The clutch is a very important “Link” in the transmission of the bike. Its
primary use is to allow the rider to engage and disengage the engine from
the wheels. It also takes up most of the load and vibrations from the engine
and does not allow it to pass onto the rest of the transmission.
After the clutch comes the gear-box. As the name suggests, a box having
set of gears. It allows the rider to use the optimum amount of power from
the engine as and how required. The main function of the gear-box is to
reduce strain on the engine by supplying efficient power at required time.
For Instance, the first gear is lowest and most sensitive gear, which helps in
moving of a stationary bike without putting excessive load on the engine. On
the other hand use of second gear requires relatively more acceleration and
gives more strain on the engine and clutch. A lower gear serves two
purposes. They serve as brake and also improve acceleration when required.
For instance, while going downhill on a lower gear provides “Braking Power”
and during uphill provides extra power.