All about IC Engines of Motorbikes: Carburetor and FI System

Find in this article everything you need to know about IC Engine of Motorbikes. Also, here you will find detailed information about the carburetor and FI System.

Prepared by:

Er. Ashok Subedi, Mechanical Engineer ( Institute of Engineering)

Dr. Durga Bastakoti, Ph.D. in Mechanical Engineering

What is IC Engine?

Internal Combustion(IC) Engine is a type of heat engine in which the ignition and combustion of fuel happen inside the engine itself. In other words, the fuel of engine combines with air inside the combustion chamber, and they build up to work as the engine’s fluid. The motorbikes have either two-stroke or four strokes IC engines.

The first internal combustion engine is said to be Otto Engine developed by Nikolaus Otto. Then gasoline and diesel engines have been developed too. In case of a bike, gasoline engines systems are used.

Piston-Cylinder System

In the IC engine system, the piston travels through a certain space inside the cylinder to inject the fluid, and there are four processes known as strokes. In an operating cycle, the piston completes two rounds inside the cylinder. The components are Piston, Cylinder, crankshaft, inlet valve, exhaust valve.

For bikes, petrol engines are sued, and it consists of a spark plug. The spark plug ignites the fuel/air mixture, and this process is called ignition. The piston is connected to the connecting rod, and during the process, the connecting rod turns the crankshaft.

The Process Involved in the Engine

Source: Sciencedirect

Intake stroke:

In this stroke, first, the inlet valve is opened, and a vacuum is created to let the mixture of petrol and air drew down.

Compression stroke:

The mixture of petrol and air is compressed in this stroke. The charge is ignited by an electric spark, i.e. Spark plug-in bikes. The compression causes heat energy to be generated and an increase in temperature as well as vaporization of the fuel. The compression ratio ranges from 6:1 to 10:1.  

Power stroke:

After the ignition cycle, both valves are closed, and the expansion of hot gases lead to the piston head. Then the force is transferred from the piston to the connecting rod, which eventually turns the crankshaft with certain torque.

Exhaust stroke:

At the last process, the exhaust valve is open whereas the inlet valve is closed so that the gases are expelled from the engine with the help of piston via exhaust valves.

Energy combustion in IC Engine

The heat pumps in IC engines depend on suitable electric supply and produce primary energy ratio up to 1.5. As a result, the efficiency is higher. The chemical energy is transformed into shaft work to mechanical energy, and it later leads to kinetic energy. The fluid flow circuit is in the combustion chamber. The fuel mixture provides Hydrogen-rich formats which are later used in energy form.

Energy Transmission Mechanism:

  1. Through the engine:  As discussed earlier, the piston crankshaft mechanism transfers the power through  the engine
  2. Through the motorbike: ­Energy transfer takes place in bike via series of structure to the rear wheel, i.e. from gearset, clutch and drives system sequentially.

Energy wastage

As per the studies, it is said about 60% of energy is lost due to friction, pumping, fuel injection, and the entire mechanism.  The petrol engine contains lots of moving parts due to which the friction is produce and energy is wastage to overcome the friction.

What does the CC signify in the engine?

The abbreviation “CC” means Cylinder Capacity (cubic centimeter capacity) of the combustion cylinder. The larger the cc in bikes, the bigger the bike will have a cylinder so that more ratio of air and fuel can be driven through.  This represents the total engine displacement of a motorbike.

The volume of the engine cylinder can be computed using the following formula;

V = pi * r^2 * (TDC - BDC)

Where, TDC = Top Dead Centre and BDC = Bottom Dead Centre

If any bike has higher CC, the number of burning fuel increases resulting in a higher power. However, the higher power consumption can decrease in mileage due to high fuel consumption. In general, the CC of the bike is proportional to the power generation by bikes.

The carburetor in bike helps to supply the IC engine with air/ fuel mixture by maintaining the flow of air in fuel through the main bore. When the throttle is given to bike, the air is allowed to flow through carburetor to control the flow of fluid.


Carburetor parts can be divided into the main bore, bowl, vents, jets, choke and accelerator pump.

  1. Choke has a butterfly valve that is pulled to control the intake of fuel, and then it is drawn into the cylinder and is often used during the winter season.
  2. JET: When the choke is gradually opened, the engine gets warmed up, and fuel moves from the jet to the air.
  3. Venturi tube: To decrease the pressure for drawing fuel, Venturi shaped airflow restrictions are built in the carburetor to provide the perfect ratio of mixture for combustion.
  4. Accelerator pump: almost every bike contains an accelerator pump which helps in the ignition of fuel when the throttle is started.
  5. Bowl: This is the area where the fuel hangs out and controls the needle of the carburetor.

Almost all the bike’s carburetor has side drafts. In general, we can say carburetor is based on Bernoulli’s Principle. When the air moves with high velocity, static pressure decreases, whereas dynamic pressure increases. The throttle and fuel jet sizes control the mixture of fuel and air. The resultant is then supplied to the chamber where combustion takes place.

Fuel injection system:

Modern bikes now have electronic fuel injection systems nowadays. This system is governed by sensors, fuel injectors, fuel tubing valves, fuel pumps, etc. The valves are controlled by the cam on the camshaft, which controls the injection of fluid to the manifold.

In most cases, there are two fuel injectors which are opened with valves and the cam present on the camshaft runs it. The governor controls the time period of opening of timing valves and period of injection of fuel to cylinders. Governor is either automatic or pneumatic style. The sensor used plays an important role in the control of engine operation and transmission. The transmission of data from the engine about the pumping of fuel is monitored using sensors to injectors.

Examples of Fuel injection bikes:

Suzuki Gixxer SF FI, Yamaha FZ, Suzuki Intruder FI, Hero Glamour series, TVS Apache 4v Series, KTM 200 Duke, Honda 190

How is the Fuel Injection System different from the Carburetor System?

  1. The main difference is the power available in the system where the fuel injection system directly draws the fuel and air.
  2. Along with this, the system also omits the issues like heating, icing of inlet due to which the power is better than that of the carburetor system.
  3. Secondly, the fuel economy is also better in the FI system as the air to fuel proportion is more accurate than that of the carburetor system. However, the maintenance of the FI system is complex than the carburetor as the FI system requires more advanced tools, which is expensive.
  4. Further, the carburetor is less expensive, which draws the fuel directly from the tank while the FI system requires a fuel pump that is attached inside the tank for the operation of fuel flow.
  5. But the response of the throttle is better in the FI system because of advanced technology and tools.

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