Efficiency Of Internal Combustion Engine: What, How, Different Types and Facts


In this article we shall discuss about the topic- Efficiency of internal combustion engine. Internal combustion engines are commonly used in locomotives to run them.

The internal combustion engines or IC engines use the thermal energy from burnt fuel and gets it converted to mechanical energy. The speed of locomotives depend on how fast the combustion cycle is repeating itself inside the engine. In this article we shall study about internal combustion engines in detail.

What is internal combustion engine?

Internal combustion engines are heat engines inside which the fuel is combusted with the help of an oxidizer. The resulting thermal energy is converted to mechanical energy by pushing the piston to the bottom most part of the cylinder

The piston moves back and forth performing a reciprocating motion. The reciprocating motion of piston is directed to the crankshaft. The crankshaft starts rotating as a result of which the wheels also rotating. We shall study more about internal combustion engines in detail in further sections of this article.

How does an internal combustion engine work?

Internal engines work by converting the thermal energy, produced after combusting fuel in the presence of an oxidizer, to mechanical energy. When the piston is at top dead center, fuel is burnt as a result of which some amount of thermal energy is released.

This energy pushes the piston to the bottom dead center. The piston moves to the top dead center again due to inertia, this time the exhaust gases are expelled out from the exit valve. The piston is connected to a crank through connecting rod which converts the reciprocating motion of piston to rotary motion.

Image: Carnot Cycle

Image credits: Cristian QuinzacaraCarnot cycle pV diagramCC BY-SA 4.0

What is the efficiency of internal combustion engine?

We now know the working of intrernal combustion engine works. We know that internal engine can produce its own work.

Now we shall discuss about the efficiency of an internal efficiency. It is defined as the amount of work that the engine has produced to the heat input to the engine.  

Mathematically, efficiency is given as-

Heat efficiency of internal combustion engine

Heat efficiency of internal combustion engine is the ratio of useful heat to the total heat input given to the system.

The heat source provides the heat to the heat engine and a heat sink absorb all the waste heat from the engine. The useful heat is used as the work output from the engine. The heat efficiency of an internal combustion engine is given in the section given below-

Volumetric efficiency of an internal combustion engine

The volume of the cylinder is called as the swept volume which is the maximum volume of air fuel mixture that can be taken in.

Volumetric efficiency is the ratio of total air fuel mixture taken inside the cylinder to the total volume of the cylinder or the swept volume. Mathematically, the volumetric efficiency of an internal combustion engine is given in the section below-

Overall efficiency of internal combustion engine

Overall efficiency of any heat engine takes into account all types of efficiencies namely, thermal efficiency, mechanical efficiency, volumetric efficiency etc.

Mathematically, the overall efficiency of internal combustion engine is –

What is mechanical efficiency of an internal combustion engine?

Mechanical efficiency is the efficiency with which the mechanical components inside the internal combustion engine work.

The mechanical components include piston, valves, piston pin cylinder etc. The engine works when these mechanical components work in harmony with each other. Mechanical efficiency is a ratio between two things, they are work output and work input.

What is the maximum efficiency of internal combustion engine?

Different internal combustion engines have different efficiencies. The efficiencies depend on the mechanical efficiency, type of fuel used, type of engine etc.

The most efficient internal combustion engines have an efficiency of 50% but the engines that are allowed on road have efficiencies between 20 to 30 percent. Combustion engines are not as efficient as we though them to be as the energy conversion rate is very low.

How to calculate the efficiency of internal combustion engine?

In above sections we saw the formula of calculating efficiency of internal combustion engine. In those sections we did not discuss about how to find the quantities used in that formula.

The work done is defined as the net useful heat taken from the heat input. So the net work can be found simply by subtracting Heat input and heat output. The difference between the two will give us the net work output. When we divide the work output by heat input swe get the value of efficiency. We have already discussed the formula in above sections.

Efficiency of internal combustion engine vs electric motor

We have already discussed about the maximum efficiencies of internal combustion engines. The electric motor vehicles do not burn fuel to run.

Electric motors on the other hand have higher efficiency than combustion engines. The energy conversion rate of electric motor is 85 % . So we can say that the electric motors have an efficiency of around 80%-85% whereas the maximum efficiency of an internal combustion engine can go nearly up to 50%.

Efficiency of internal combustion engine vs gas turbine

Gas turbines are mechanical devices that convert heat energy from steam to mechanical energy. The mechanical energy will be converted then to electrical energy which is done with the help of a generator.

Gas turbines have an efficiency of around 35-40%. That is more than most of the internal combustion engines. But we do not use gas turbines in locomotives because of the size. Internal combustion engines are compact in size whereas the gas turbines are huge and are used in applications with high speeds.

How to increase efficiency of internal combustion engine?

To increase the efficiency of internal combustion engines we use the following methods-

  • Lower heat rejection
  • High compression ratio
  • Using a lean fuel mixture
  • Running the engine at optimum conditions

Why are gas turbines not used in vehicles if they have higher efficiency?

The efficiency of gas turbines may be higher. We have discussed the numbers in the above sections already. But the reason why they are not used in vehicles is the size of a turbine.

The size of a gas turbine is very large. This is why turbines are used in aeroplanes and not in vehicles. The gas turbines are used where the application deals with high speed. Internal combustion engines are compact in size hence they are used in vehicles and gas turbines are not.

Abhishek

I, Abhishek Khambhata, have pursued Btech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth year project was based on performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like minded people.

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