Why do we need to cool air after it leaves a turbocharger?

Question

When we are using a turbocharger to compress the incoming air, the air gets hotter. Usually this hot air is cooled by using an intercooler before it's passed to the engine.

What is the reason behind cooling this air?

Why can't we pass it as hot air, since inside the engine the air will be compressed which will heat it up anyway?

Answer 1

tl;dr

• To combat detonation (in SI engines)
• To increase power/efficiency

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Details

There are a few important factors at play here.

• Engine detonation is a real concern for SI engines

  A spark-ignition engine is more likely to experience premature ignition (aka knocking or detonation) with hotter air. In fact, the calculations in the example below can show that this is the primary reason why intercooling is such a Good Idea.

• Hot air rises, cold air sinks

  In physics-speak, hot air is less dense than cold air. This means that the volume occupied by 1 kg of hot air is greater than the volume occupied by 1 kg of cold air.

• The internal combustion engine is a volumetric device

  What this implies is that every time the engine turns over and completes a cycle, the volume of air that is admitted into the combustion chamber(s) is fixed.

• Power depends on mass, not volume

  The power developed by the engine is proportional to the mass of air admitted into the combustion chamber and not its volume. More air molecules = more bang.

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The reason why turbochargers (or any other forced-induction devices) are utilized is to increase the power and/or efficiency of the IC engine. At combustion chamber level, this is achieved by increasing the amount of air molecules present during combustion.

The turbocharger achieves this by pressurizing the incoming air. An unwanted by-product of this compression process is that the outgoing air is hot and less dense.

If this hot air is fed to the combustion chamber as-is, the likelihood of engine detonation is greater.

By cooling the air via an intercooler, engine operation being safer since engine knock is reduced.

As an added bonus the air becomes slightly denser, enabling more air molecules to be present during combustion.

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Bonus Example

This is one of those questions where numbers can speak louder than words:

Forums indicate that a stock Mitsubishi Evo X is capable of generating 22 psi boost at mid-range RPM.

At sea level, the turbo inlet conditions are as follows:

Air pressure @ turbo inlet      = 14.7 psi

Assumed inlet air temperature   = 25 °C
  => air density @ turbo inlet  = 1.184 kg/m^3

Assuming 85% turbocharger efficiency, engineering calculations¹ will yield a discharge temperature close to 92 °C:

Air pressure @ turbo outlet     = 14.7 + 22
                                = 36.7 psi 
Air density @ 36.7 psi, 92 °C   = 2.41 kg/m^3

Were it not for the fact that we care about detonation, the outlet density value looks rather tasty - it is more than double that of the inlet.

But look what happens when we run this hot discharge air through an intercooler.

Let's assume a 1 psi drop in pressure, and that the air is cooled down to 70 °C:

Air density @ 35.7 psi, 70 °C = 2.50 kg/m^3

Despite the fact that we lose precious boost through the intercooler, the cooling effect ends up increasing the density by over 3%, so now the air is denser, and, more importantly, safer from an engine knock/detonation standpoint.

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¹ - _{I have worked out a truly marvelous calculation for this, which this margin is too narrow to contain}

Answer 2

In short, there are two reasons:

1. Cooler air is more dense, so you get more power from the same boost pressure because you can inject more fuel at the same time.
2. Hotter air will make the air/fuel mixture detonate prematurely (the mix needs to burn at a constant rate, at the exact right moment. It shouldn't "explode").

In the second instance this will mean that you have to change the amount of ignition timing advance in an effort to prevent the mixture from exploding. It will cost you power because you're not firing the cylinder at the precise moment needed for optimal power delivery. You're losing power AND you get worse fuel consumption.

In addition to intercooling, another way of cooling down the air going into the cylinder is to inject either a water/methanol mix OR Nitrous Oxide (in this case called a low-pressure or slow-release NO2 system because it's used to cool down the charge, not directly to increase power) alongside the air/fuel mixture. This is a favourite tactic of Subaru owners because these cars HATE hot air and leaner (more powerful) air/fuel ratios and additional cooling helps you run leaner air/fuel mixtures and optimal timing.