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Turbo verses supercharging and thermodynamics

  1. Oct 5, 2011 #1
    This one puzzles me. Can an automotive engineer please correct my understanding? I am not an automotive engineer but would like to understand more (I am a bio engineer)

    Compressing the charge is a way to increase the volumetric efficiency by limiting pumping losses?

    So any method of compressing the charge is better than a normally aspirated engine?

    Turbos use ‘waste’ energy in the form of hot gas. They take some energy out to compress the intake air, hence volumetric efficiency increases?

    Superchargers do not use waste energy but actually place a drag on the engine so they are not as efficient as turbos. However, since they still increase volumetric efficiency, they are still better than nothing (and are more reliable)??

    One thing that really puzzles me: after power stroke BDC, does the exhaust gas ‘fly out’ of the cyclinder because it is still very hot/high pressure, or does it need to be 'pumped out'? If it is pumped out then the turbo is not recuperating ‘waste energy’ but is stealing energy rather like the supercharger does.... any enlightenment available!!!????
     
  2. jcsd
  3. Oct 5, 2011 #2

    brewnog

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    When the exhaust valve opens, the exhaust gas does 'fly out' of the cylinder for a while, but then some of it is pushed out by the rising motion of the piston. This last 'push' is at fairly low pressure (a few bars at most), so isn't causing that much 'energy theft' as you put it. However, it IS at high temperature! Try thinking of the turbo taking energy out of the exhaust gas by reducing its temperature by several hundred degrees, rather than just by being spun around by pressure. Of course, it's pressure that drives the turbine wheel, but when you consider the amount of energy the turbine takes out of the gas, you can see why they can give considerable engine efficiency improvements.

    Mechanical superchargers do directly 'steal' energy from the crankshaft.

    Pressure charged engines aren't necessarily 'better'.
     
  4. Oct 7, 2011 #3
    When the density of a charge is increased not only does the V.E. gets increased, if all the parts are adequate in quality our mechanical and thermal efficiencies increase as well. Once the exhaust valve lifts off the seat this begins out BLOW DOWN PHASE. This is going on when the piston is still moving towards BDC. Heat, cylinder pressure exert the evac. force and the seat and valve along with the port shapes influence how well our cylinders empty. Once ABDC has been reached we start our pumping phase.

    As Brewnog mentions the turbine wheel reduces the exh. gas flow efficiency just from being within the system. You also have to look at how well that turbo housing flows itself. Not all turbos flow the same. This can begin the aspect we know as pumping loss due to high residual pressure in the cylinder upon approaching TDC.

    What else to look at is the type of fuel and static & dynamic compression and if there are thermal coatings as well.
     
  5. Oct 7, 2011 #4
    Correct. Neither turbos nor supers are more efficient than normally aspirated engines. What they do is all the engine to produce a lot more horsepower for the overall weight of the engine. Superchargers are better at it (higher pressure ratios), and better suited to a fairly narrow RPM range, but they're not efficient at all. Dragsters use superchargers. Turbochargers are much more efficient than superchargers, and are thus better suited to sporty automobiles and aircraft, where both weight and efficiency is an issue.

    A straight diesel is the most efficient recip. Turbos are used for both sport diesels like the VW Jetta as well as 18-wheelers as they create minimal additional drag (energy consumption) at cruising speeds, but they provide a lot of extra HP when you need it for very little additional weight.

    I get at least 40 mpg highway in my Jetta TDI.
     
  6. Oct 8, 2011 #5

    brewnog

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    No, that wasn't my point. My point was that 'betterness' isn't a characteristic which can be defined well enough to make comparisons about aspiration type. Turbochargers do increase efficiency, primarily because the higher in cylinder pressure at start of compression gives a higher peak cylinder pressure, and thus higher Carnot efficiency.
     
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