Forced Induction 101

An engine produces power by igniting fuel and air inside a chamber. As the fuel and air are ignited, the pressure within the chamber increases, applying a force onto a piston or rotor. The generated force is applied on a crank shaft which causes rotation. How much power generated is mainly determined by how much fuel and air is ignited inside the chamber to produce the driving force.

There are two types of induction systems for combustion engines; naturally aspirated and force-fed (forced induction) engines.

Naturally aspirated (N/A) engines draw in air for combustion under atmospheric conditions. As the piston moves down, the intake valve opens allowing the piston to suck air into the chamber. How well the chamber is filled is called its volumetric efficiency.

A volumetric efficiency of 100% means that the chamber is completely filled with air compared to the chamber at static conditions. The volumetric efficiency of a naturally aspirated engine can never be higher than 100% the engine cannot fill the chamber more than under static conditions due to the air pressure being the same. Its ability to fill the chamber is also greatly affected by how long the intake valve is open for and how fast the engine is rotating.

Forced induction engines use a pump to increase the air pressure entering the engine and therefore increasing its volumetric efficiency. Higher air intake volume allows for more fuel to be burnt which increases the amount of power generated by the engine.

There are 2 different types of pumps in forced induction systems; a turbocharger and a supercharger.

A turbocharger is an air pump which is driven by exhaust gases exiting an engine. It consists of a compressor that is connected to a turbine via a common shaft. The compressor is essentially a fan within a housing which takes in air through the centre and forces it out of the housing at an increased pressure and velocity into the intake manifold of an engine. The turbine is another fan which has exhaust gas from the engine, pushing on the blades which cause it to spin. The exhaust gas then exits out of the centre of the turbine.

A supercharger is a compressor which is directly driven by the engine via a belt and pulley system. As the engine RPM increases, the compressor RPM increases, pushing more air into the engine. Supercharged engines tend to have linear power delivery compared to turbocharged engines as the rotational speed of the compressor of the supercharger increases linearly.

A small engine equipped with a turbocharger or supercharger has the ability of produce the same amount of power as a large capacity engine without the associated fuel consumption. For example, a turbocharged 2.0L engine at low RPM off boost will consume similar amounts of fuel to a naturally aspirated 2.0L engine. A large 6.0L engine even at low RPM will consume more fuel than the 2.0L engine but when the 2.0L engine is on boost, and then the power generated can be the same as the 6.0L engine which means the fuel consumption will be similar. The fuel saving is when the smaller engine is off boost, it performs like a small engine and consumes fuel like a small engine but when it is on boost, then it performs like a large engine.

Another benefit of a forced induction engine is the ability to increase its performance. To improve the performance of a N/A engine, its volumetric efficiency needs to be increased as this is what allows more air to be ingested by the engine to create more power. Increasing the maximum RPM will also increase its performance.

To increase its volumetric efficiency requires changes to cam shafts, inlet tracts, exhaust systems etc. The increase in power will only be marginal as most engines are fairly efficient from factory. In terms of ease and performance compared to cost, a forced induction engine is best.

The performance of a turbocharged engine can be increased greatly just by increasing the boost or controlling where maximum boost is reached. Simple modifications such as larger exhaust systems and free flowing air filters can yield large power increases compared to doing the same modification to a N/A engine.

Posted in Technical Articles