Intercoolers

Intercoolers After yesterdays post on forced induction, I thought today I would go over intercoolers.  Since the whole point of forced induction is to get more air into an engine, the fact that compressing the air heats it up (which makes it less dense) is counter productive.

The job of an intercooler is to reduce the temperature of the compressed air making it more dense.  Since the real goal of forced induction, or any other way to make more power, is to get more oxygen into an engine, increasing the density of the intake air is a good thing.

Depending on the temperature, an engine with a lower boost level may actually be getting more oxygen into it that one with a higher boost level, but also a higher temperature.

An intercooler works by reducing the temperature of the air going into the engine.  There are basic types of intercoolers, air to air and liquid to air.  Air to air intercoolers are the simplest, they work basically like the radiator does on your car.  The compressed air flows through the intercooler and air passing across it cools the compressed air down which increases the density.

There are a couple of disadvantages to this setup.  One is that since there is a longer path for the air to flow, it can increase the lag before the boost reaches the engine, such as with turbo lag.  That is because there is more volume to fill to the PSI level that is being generated.  The same as how your air compressor takes longer to fill up your car tire than your bicycle tire.  The other disadvantage is that the intercooler needs to be designed so that it doesn’t restrict the airflow too much.  A poorly designed intercooler can reduce the boost pressure by 4 PSI or more.  This can negate the effectiveness of the intercooler.

With a liquid to air, there is what is basically a mini radiator that the compressed air flows across, and the cool liquid flowing through cools the compressed air.  The liquid is then cooled back down by a radiator.  The main advantages are that there isn’t the increased volume under boost, and that you can cool the liquid in the intercooler to below ambient temperature with ice water.  The drawbacks are that liquid to air intercoolers are more complicated with more points of failure.  There are fittings that can leak liquid, there are pumps that can fail etc.  With air to air intercoolers, as long as the intercooler doesn’t get a leak or a hose doesn’t come off, there really isn’t much to go wrong.  The other draw back to liquid to air is cooling the charge for a long period of time.  Over time the system isn’t able to dump the heat out fast enough and the temperature of the intake charge slowly increases.  So they are more effective in the short term, like for drag racing, but wouldn’t be as effective for road racing, or in a turbo diesel pickup pulling a load up a long hill.

The other advantage to a lower intake charge temperature (if it is from intercoolers or any other means) is that there is a lower tendency for detonation or knock.  The higher the boost pressure, the more likely it is to have detonation.  When you lower the temperature of the air, it helps to reduce the hot spots in the cylinder that can cause detonation.

While I am not sure that any intercooler is better than no intercooler, a properly designed one is definitely better than none.  Intercoolers that are not correct for the application can reduce flow by enough to reduce their effectiveness, and may not even cool the air enough to be helpful.

Like everything else, you need to make sure you are matching the entire system to what you are trying to accomplish.


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