Intercoolers Investigated

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    The large number of myths and misconceptions surrounding turbo-diesel intercoolers prompted me to create a device that could continuously:

    ·         Measure the air pressure in the duct between the air filter and the turbo,

    ·         Measure the air pressure coming out of the turbo,

    ·         Measure the ambient air temperature,

    ·         Measure the air temperature in the engine compartment, particularly directly below the top-mounted intercooler,

    ·         Measure the temperature of the compressed air coming out of the turbo,

    ·         Measure the temperature of the induction air coming out of the intercooler,

    ·         Calculate the efficiency of the intercooler based on its ability to cool the compressed air,

    ·         Send all this data to a mobile phone via Bluetooth,

    ·         When so commanded operate a pump to spray water onto the intercooler.

    I then created a mobile phone app to display all of this data while driving under highway conditions. Due to the high volume of vital info being delivered I gave the app the facility to save sequential screenshots for later review, and after each screenshot the facility to take a camera picture to place the screenshot in context, such as of road conditions or dashboard instrument status. I also gave it a button to spray water onto the intercooler.

    The significance of intercooler efficiency is its effect on engine power. A 100% efficient intercooler will approximately double engine power, while 0% of intercooler efficiency will be equivalent to a normally aspirated engine. Therefore 10% of intercooler efficiency translates to about 5% of total engine power.

    After using this device in several vehicles over hundreds of kilometres I learned many interesting facts:

    ·         Under non-towing operation at highway speed most intercoolers are about 90% efficient and little purpose would be served by making it bigger.

    ·         The first step towards maximising engine power is to keep the air filter scrupulously clean. At full speed a 2.5 litre turbo diesel engine sucks around 50 litres of air per second. To achieve this volume air is pulled through the filter at a speed of up to 10 metres per second. This creates a partial vacuum in the air duct of generally not less than 20mB, which can increase quickly to well over 50mB when the air filter is partially blocked by dust particles. This impediment to airflow makes it very hard for the turbocharger to do its job.

    ·         At normal highway speeds a front-mounted intercooler enjoys no advantage over a top-mounted one. This however can change dramatically to low-speed heavy climbs when  the fast-turning radiator fan and the low forwards speed can let hot air from the engine compartment flow upwards through the top-mounted intercooler, greatly reducing its efficiency. Since a front-mounted  intercooler is generally also served by the radiator fan, this puts it at a great advantage when maximum power is sorely needed. Once again increasing the size of the top-mounted intercooler would serve no purpose here since the intercooler is effectively not functioning.

    ·         While towing the demands placed on the intercooler increase and it will deliver decreased efficiency. This is to be expected when you consider that when fuel consumption increases, as it generally does while towing, the air required to burn that fuel will increase proportionally, and that increased volume of hot compressed air must be cooled be the same intercooler.  The loss of intercooler efficiency is proportional to the increase in fuel consumption and this is where an intercooler with a larger surface area would offer a distinct advantage.

    ·         Spraying water on the intercooler serves no purpose. At high forward speed when the evaporating water would enhance cooling, enhanced cooling is not needed, and at low speed when extra cooling is sorely needed the lack of airflow means the water does not evaporate significantly. Since the temperature of the intercooler at times like this can easily exceed 100 degrees centigrade the boiling of the water should make a difference but doesn’t since it just dribbles through to the engine.

    ·         A snorkel may be advantageous in that it sucks higher-level air that may be less dusty, but the ram-effect of a forward-facing snorkel is negligible – at most about 5mB, which may reduce the partial vacuum behind the air filter slightly but not enough to noticeably increase engine power. Rain water collected by the snorkel and carried to the air filter will cause the fibres of the filter element to swell and further restrict airflow. A snorkel system should have a water drain at its lowest point to get rid of this water.

    A waste-gate is an expensive addition to a turbocharger that allows extra exhaust gas to bypass the turbine when compression approaches the 1000mB maximum. To save a little money manufacturers like to avoid the need for a waste-gate by making the turbo build full boost only at very high exhaust-gas volumes, i.e. at high RPM and at full throttle. While these turbochargers will still deliver full boost at high speed, their lag will be significantly greater and any restriction to airflow offered by the air filter will be more keenly felt. Not ideal for towing since the acceleration to full speed will be much slower


    Conclusion: If you’re going to tow with a turbo-diesel get one with the greatest intercooler surface area per engine size, and get a turbocharger with a waste-gate.

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    Simon Tasman
    Simon Tasman on


    Could you explain something to me? Why do we even need an intercooler when the turbocharger compresses the induction air?

    I thought an intercooler just kept the engine cooler.



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    Fyko on

    Good questions.

    There’s a lot of misunderstanding out there about the concepts of pressure versus density. The turbocharger delivers increased pressure, but the engine only responds to increased density. The intercooler changes increased pressure into increased density.

    There are two laws of physics that govern what we are dealing with here – both are easy to understand. The first is Gay-Lussacs Law:

    P1/T1 = P2/T2

    P here stands for Pressure, and T represents Temperature in Kelvins. A Kelvin is the same as a degree Centigrade but the scale starts at -273.15 degrees C, which is absolute zero – the lowest possible temperature. An ambient temperature 20 C is the same as 293K.

    Imagine a container sealed filled with air at ambient pressure and a temperature of 20 C. That’s 293K. Gay-Lussac tells us that if we double the temperture (to 586K or 313 C) we will also double the pressure by approximately 1000mB. But since the number of molecules of air inside the container remain unchanged the density of the air remains the same. Cooling the container back to ambient restores the pressure also back to ambient.

    The Ideal Gas Law:

    PV = nKT tells us that number of moles of air given by n will double if the Pressure P doubles and the Temperature T remains the same. If T is allowed to rise then n will be left behind to keep the equation balanced. V(olume) and constant K remain unchanged and can be ignored.

    In short you need to bring the temperature back to ambient after doubling the pressure if you want the density (and hence engine output power) to likewise double.

    Since no intercooler can be expected to cool the induction air all the way back to ambient some loss of efficiency is inevitable. Induction air at 30 C above ambient is about as good as any intercooler can deliver under highway conditions and has to be acceptable.

    In a petrol engine all this changes because the evaporating fuel will draw a lot of heat out of the compressed induction air as it passes through the inlet port so temperatures there can only be calculated and not measured reliably.

    An efficient intercooler will never make an engine run cooler – in fact the reverse is true, it will make it run hotter.


    Because increased air density means more moles of air (which is 20% oxygen) are available to burn more fuel, releasing more energy. Since diesel engines can never be more than 26% efficient, the other 74% of this increased energy will be felt as heat, which either passes out through the exhaust pipe or remains in the engine and must be removed either through the coolant radiator or through the oil cooler.

    Best regards,


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