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Need help understanding the thermodinamics implications of an intercooler

  1. Sep 1, 2009 #1
    Hello everyone,
    I am a car entusiast who likes to turn wrenches and modify things, but I like to do it first by understanding the teory behind each one of my projects.

    Here's the case study I would like to get some help on:

    The car is a BMW Z3 with a 4 cylinder 1.9 liters engine. Few months ago I supercharged the car with a kit based on an Eaton/Magnuson MP62 Gen 3 Roots type supercharger.

    Roots type superchargers are positive displacement - volume of air pumped per revolution is constant and in this case equal to 62cubic inches or 1 liter - and have no internal compression - which means it just moves air; positive pressure in the intake manifold is built through resonance: the volume pumped per revolution is more than the engine uses, thus pressure rises until the equilibrium is reached -.

    The current gear ratio (pulley on the SC / crank pulley) generates 8PSI or 0.54bar of positive pressure in the intake manifold. Being positive displacement it also cause such pressure to remain constant throughout the RPM range; due to losses when the SC is not spinning fast enough pressure actually reaches the 8PSI around 3K RPM (engine RPM) and remain constant all the way to engine redline (6500RPM).

    Because Roots type uses the resonance effect to build positive pressure they are also very inefficient. Efficiecy (IIRC) is around 70%. I haven't done any empirical testing on how much the temperature rises, but the manufacturer of the kit claims 21 deg C above ambient temperature - it seems a little low, but I don't have fact around it.

    The kit is not intercooled (adding an intercooler is actually the subject of this thread).

    Bottom line: we have 0.54bar of delta P and 21 deg C of delta T.

    So, let's assume I add an intercooler with an efficienct of 80%: this means I will be able to reduce the intake charge by about 13 deg C, thus making it 9 deg C above ambient.

    Now I have a cooler air that it's denser. Since I haven't changed the gearing ratio of the SC, volume is constant.

    Q1: what should I expect to happen to delta P (let's assume 0 losses from passing the air through the intercooler) based on the laws of thermodynamics?

    Q2: by simply cooling the air charge would I notice any change on the engine torque? If so why? Again I would like to understand what the laws of thermodynamics say on this regard.

    As a side note, a few details on how fueling is done. The car has electronic fuel injection. There is a mass air flow (MAF) sensor before the supercharger that senses how much air goes through. The engine control unit (ECU) uses such measurement to calculate the injector's cycle (how long they need to remain open) to achieve a 12:1 (12 unit of air per unit of fuel) air fuel ratio (AFR). Thus what happens to the air AFTER the MAF sensor doesn't affect the AFR.

    This should be enough details, but let me know if I'm missing something...

    Thanks for any idea!!!
  2. jcsd
  3. Feb 23, 2016 #2
    If you need any help or have any questions regarding the turbo charge air cooler maybe I can help. I work for a company who manufactures and designs charge air coolers for large diesel engines called Vestas aircoil.
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