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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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.