Turbo Intercooler Theory

An intercooler simply exchanges heat between the charged air from a turbo and some cooling medium. In an air-air intercooler, the cooling medium is the outside air that blows through the fins of the intercooler. In an air-water intercooler, the cooling medium is water. Typically, an air-water intercooler is used in marine applications (where there is plenty of water that remains a constant temperature), but one could also be used at the strip. Often, ice water is used for air-water Intercoolers at the drag strip.

How an Intercooler Works

First, some definitions:
Cooling medium: The material which absorbs the heat from the charged air: usually aluminum.
Transfer medium: The material which actually transfers the heat from the charged air to the cooling medium.

The warm, compressed air from the turbo outlet is sent through the intercooler tubes. These tubes are typically made of aluminum and have aluminum fins attached to them. Aluminum is an excellent conductor, so it is able to transfer some of the heat of the charged air to the cooling medium which flows over the outside of the tubes and the fins. This cooling medium (usually air), first makes contact with the intercooler at one temperature, and exits on the other side of the intercooler at a higher temperature - successfully absorbing some of the heat of the charged air.

One is ultimately concerned with the efficiency of an intercooler - or what percentage of the difference in temperature between the charged air and the cooling medium it is able to absorb. If the charged air temperature is 300° and the cooling medium is 100°, the differnce is 200°. A 50% efficient intercooler will cool the charged air by 100°, bringing the temperature down to 200°. A 75% efficient intercooler will cool the charged air by 150°, bringing the temperature down to 150°.

A technical discussion of heat transfer, while interesting, is rather difficult to apply to determine the efficiency of a turbocharger. The reason is that steady-state heat transfer equations require more knowledge about the transfer environment than is readily obtainable. One could instead determine the efficiency of an intercooler by installing a thermocouple before and after the intercooler and calculating the temperature difference. Knowing this and the temperature of the cooling medium is all you need to calculate the efficiency of the intercooler. Note that the efficiency is different at different boost levels since denser air is harder to cool than thin air.

Advantages to Intercooling

Suppressing Engine Knock.
Compressed air leaving a turbocharger is often more than 200° F. This warm air is much more prone to predetonation and autodetonation (knocking) than air at ambient temperature and pressure. Knocking, if unresolved, can ultimately ruin an engine?. There are several approaches to eliminating ping, including using higher octane fuel and cooling the compressed intake air. An intercooler can achieve the latter, reducing the likelihood of ping. Increasing Intake Air Density. Another advantage to cooling the compressed air is that the cooler air will also be more dense, which is the objective of turbocharging. Water Injection. An alternative (or supplement) to an intercooler is a water/alcohol injection system. For details, have a look at our water/alcohol injection page.

Choosing an Intercooler

Bigger is Better. This is one area where the American addage, "the bigger the better", applies. When selecting an intercooler, choose (or have made) the largest intercooler you can fit into the space you have under your hood. Here's why: Efficiency and Size. Typically, a larger intercooler has more surface area over which the heat transfer can occur. The greater the surface area (given the same transfer medium), the greater the amount of heat transfered. As usual, better quality can make up for a smaller size. Having a better transfer medium, such as aluminum, will increase the drop in temperature. Also, surface area can increase without using a large intercooler, by using more and smaller fins. More fins, though, means less air which can fit between them. Many people report excellent results using intercoolers fabricated by Spearco. They are also capable of making custom intercoolers. Pressure Drop and Size. Smaller intercoolers can be a flow restriction. This flow restriction can increase the pressure drop across the intercooler. This causes the turbo to compress the air to a higher pressure in order to maintain the same pressure at the intake manifold (after the intercooler pressure drop). Greater compression, even if it's done at the same compressor efficiency, will result in higher temperatures leaving the turbocharger. Ultimately, this results in a warmer air temperature and lower air density at the intake manifold.

Often, using an OEM intercooler which was made for a turbocharged engine larger than yours will present an affordable and effective option.

Internal Fins. Some intercoolers have fins in the tubes. These serve to both swirl the air as well as provide more surface area to transfer heat through the transfer medium to the cooling medium.

Clean the Tubes. Dirt and oil coating the tubes forms an insulating layer, making it more difficult to transfer heat to through the transfer medium. You may experience slight increases in cooling efficiency if you clean the inside of your intercooler - especially if you are using one from a junkyard.

Original Article - Turbo Intercooler Theory

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The attached spreadsheet is a steady state model intended to provide an insight into the effects of chargecooling on an engine. As you can see, it has been approximated for an Esprit engine using a lotus type water/air intercooler. The specifications shown for the intercooler have not been taken from actual measurements but are an approximation based on visual estimates. Play with the numbers and see for yourself what you can expect

Attach:IC-Design.xls