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IC Engines - Natural Aspiration Vs Forced Induction

Snapshot: More number of manufacturers are going with turbocharged petrol engines nowadays, we have seen them in the EcoSport, Polo and many others. The Brezza would be the latest one to get a turbocharged engine. So what is the difference between a normally aspirated engine and a turbocharged / supercharged engine?

Since the invention of internal combustion engines, there has been a gradual increase in their efficiency. All internal combustion engines produce energy by burning fuel like petrol, CNG, diesel etc. inside a closed cylinder. Internal combustion engines compress the air / air-fuel mixture to the smallest possible space before igniting the fuel to create maximum pressure from working exhaust gases, this makes them more efficient.

While there’s always enough fuel available for burning, the amount of required air for complete combustion is limited by the way that engine breathes. There are essentially just two ways in which they breathe and they are classified under – Natural Aspiration and Forced Induction. The article discusses these two induction methods used in the internal combustion engines.

Natural Aspiration

A naturally aspirated engine relies on the downward stroke of the piston to suck air / air fuel mixture into the cylinder. While you can put as much fuel as you want but it’s of no use until you also pack enough air along with it so that complete combustion takes place and the engine can burn the fuel completely to produce energy. Therefore, a naturally aspirated engine is limited by its natural capacity to draw air into the cylinder. So the induction method that relies on the downward stroke of the piston to suck air / air fuel mixture is known as natural aspiration.

Can the Natural Aspiration be made more efficient?

Yes, there are a few ways to increase the volumetric efficiency in case of a normally aspirated engine, and it’s with the use of high-flow air filters so that there’s minimum obstruction in the air’s path and this helps increase the overall volumetric efficiency. Most manufacturers employ highly efficient intake manifolds, however if that’s not the case you can use an inlet manifold that has extremely smooth inner surface and smooth contours.

​Read - Five things you should never do in an automatic car!

Forced Induction

In case of forced induction, a mechanically driven compressor or an exhaust driven compressor is used to compress the charge, unlike the naturally aspirated engine that draws air into the combustion chamber at atmospheric pressure and then compresses it, the forced induction system compresses the air before putting it into the combustion chamber and it is above the atmospheric pressure and more dense which make the engine capable of burning more fuel.  

The two methods that have been used for forced induction are Supercharging and Turbocharging.

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Supercharging

In case of supercharging, a compressor driven by the engine directly is used for compressing the charge. Since they are driven directly from the engine and are speeds are proportional to the engine speed, there’s no lag between the throttle inputs and response. The only reason that they are not as popular as the turbochargers is that because they use the engine power and are therefore not as efficient. 

Turbocharging

A turbocharger is an exhaust driven compressor that utilises the high pressure gasses coming out of the engine to drive a turbine and in turn drive a compressor for charge compression. Since they use the waste energy of the spent gases for compression, they are more efficient. A pronounced effect of using turbocharging is that there is a lag between the throttle inputs and response, known as turbo lag. With advent of newer technologies and turbochargers (Twin-scroll turbo, variable geometry turbo etc.) the turbo lag is not as pronounced as it used to be in the past.

Read - Turbocharger - How does it work and why is there a lag?

A common side effect of both forced induction methods is that the compression raises the temperature, due to which the charge density decreases. Not only is it counterproductive it also increases the chances of detonation. Through an intercooling process that’s used in all forced induction methods, the charge is subsequently cooled after compression which makes it cool and denser and hence more useable.

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