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Automotive Powering a turbo with intake air

  1. Dec 20, 2016 #1
    I have a basic understanding of how a traditional turbo works. Exhaust gas spins turbine which powers compressor, which compresses intake air.
    General problem with turbos is lag. Why couldn't you have a separate air intake that fed directly to the turbo (via a filter obviously), for when off throttle, and use the power of the car moving through the air to keep the turbo spinning?
    It would also have the benefit of cooling the turbo.
    If this would work, could you keep the intake open when on throttle to increase boost, or would you lose pressure from the exhaust gas trying to exit via the intake?
     
  2. jcsd
  3. Dec 20, 2016 #2

    Baluncore

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    At the speed of a moving car the ram air pressure is very low and insufficient to spin a turbo.
    The exhaust is a much better source of energy to maintain turbo RPM.
    An electric motor would be an alternative way to spin up the turbo, like a supercharger.
     
  4. Dec 20, 2016 #3
    By putting a simple gear on the shaft connecting the turbine and compressor couldn't this be overcome? Since with intake air the volume of air is essentially limitless (within reason). I.e you could just put a bigger turbine on it than you would for exhaust gas.
     
  5. Dec 20, 2016 #4

    berkeman

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    If you are trying to use the energy of the incoming air as the vehicle moves forward through the air, what happens to the vehicle's air resistance when you try to tap the energy of the incoming air? :rolleyes:
     
  6. Dec 20, 2016 #5
    When you take your foot of the throttle of a turbo car the turbine blades stall, this can be mitigated by having a blow off valve to relieve the pressure or by recirculating the pressure from the intake after the turbo to the front of the turbo, this is better since it does keep the turbine spinning a bit longer but doesn't sound as cool. There are several anti-lag systems used in vehicles like rally cars where you want the turbos kept spinning for instant power. One is to retard the ignition timing so that fuel is still burning as the exhaust valve opens keeping the turbo spinning.
     
  7. Dec 20, 2016 #6

    jack action

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    Well, you could. This concept is called ram-air. You convert the incoming air velocity into air pressure.

    The thing is that the engine intake already needs air. The advantage of a compressor (like a turbo) is to pressurize the incoming air. So why not feed the pressurized 'rammed' air directly to the intake, whether the turbo is working or not? The pressure increase will compensate for the turbo lag just as well.

    Instead, you want to use the fresh pressurized 'rammed' air to run the turbine that powers the compressor which - like any machine - cannot convert the energy input with a 100% efficiency. You are just taking a simple concept and making it more complex for less performance.
     
  8. Dec 20, 2016 #7

    russ_watters

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    Turbo lag would be replaced by an even slower ram-air lag based on the speed of the car. Whereas the turbocharger is spun by exhaust gases even if the car isn't moving very fast (and indeed is still spinning when the car is idling), the ram air turbine requires the car to be moving at a certain speed before it can even spin-up at all.
     
  9. Dec 20, 2016 #8

    Baluncore

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    The maximum dynamic pressure of ram air in the standard atmosphere is proportional to the square of the speed. Any flow through a device would result in a lowering of these pressures. Here are the numbers, boost in psi against speed in miles per hour;
    10 mph = 0.002 psi, 20 mph = 0.007 psi, 30 mph = 0.016 psi, 40 mph = 0.028 psi, 50 mph = 0.044 psi,
    60 mph = 0.064 psi, 70 mph = 0.087 psi, 80 mph = 0.114 psi, 90 mph = 0.144 psi, 100 mph = 0.177 psi,
    150 mph = 0.399 psi, 200 mph = 0.710 psi, 250 mph = 1.109 psi, 300 mph = 1.597 psi, 350 mph = 2.174 psi,
    400 mph = 2.840 psi, 450 mph = 3.594 psi, 500 mph = 4.437 psi, 550 mph = 5.369 psi, 600 mph = 6.390 psi
    650 mph = 7.499 psi, 700 mph = 8.697 psi, 750 mph = 9.984 psi
    As you can see, below a couple of hundred mph, ram air is insufficient to spin a turbo.
     
  10. Dec 20, 2016 #9
    Although after 200 mph efficiency starts to ramp up sharply according to those figures,
    At that point though there will be more urgent considerations, such as preventing the car from taking off after hitting a small bump on the ground.
     
  11. Mar 7, 2017 #10
    Ram air intake is quite similar to cold air intake systems because it is designed to get more air.

    As for the turbo lag, it is important to put a properly sized turbo to keep heat from the engine. To reduce the surface area of the exhaust port to limit heat dissipation, put the turbo as close as possible to the valves. That will also shield the exhaust system between head to the turbine to keep heat.
     
  12. Mar 8, 2017 #11
    Turbo lag is the result of using an engine with two torque curves. BTW, seldom mentioned for some reason but superchargers have their lag also. Both take time to spin up and effect the load on the engine. Examining the torque curves of an engine shows there is not an appreciable power loss during torque curve transition so the "problem" is all about instantaneous boost.

    DJR91, Put the marketing hype and the writers easy topic to witch about aside and ask yourself this, "There are ways to that turbo lag could be greatly reduced or even done away with entirely but it isn't, why?" You are examining the potential of using the non-induced ram air intake which has been reliably shown to you to have little effect on turbo lag. Why not just examine ways to always have the turbo ready at a predetermined potential? Even the lag that would come from moving the induced boost from the turbo to the intake has workable solutions.

    Then again you can increase torque by increasing displacement but in racing Ferrari and GM have both proven the limits with this option.

    Have you considered that turbo lag may be a good condition? First consider this though, is lag really lag? Lag implies a time delay between action and reaction. The only time the compressor surge line flat lines is when it's induced to operate below the designed minimum airflow. Turbo lag is really the time it takes for the turbo to go from minimum design specs to deliver the boost the accelerator is demanding. Again when graphed the time taken to deliver all the boost demanded is a constant rise when transitioning between the engines two torque curves. Turbo response is almost instantaneous and adding boost, just not the full demand.

    What you need to look for is the studies done where the boost was instantaneously available when demanded. Consider the effects instantaneous boost might have on an engine. What's the added fatigue and failure rates of components effected by instantaneous boost? Would overcoming the added stresses of instantaneous boost require materials prohibited in most racing or would there be enough interest from consumers to justify offering production street cars with engines designed and built to meet the requirements of instantaneous boost?

    It might help to look in to NOS and how it's used. It's use is limited to full throttle conditions but most street cars with let's say a 250 HP system don't start with the full 250 HP shot. They'll start with maybe a 50 HP shot and build up to the full shot. Rebuilding engines is expensive for most people and it takes the car off the road. There are also limits to how many times an engine can reliably be rebuilt.

    Finally look into what is being done to maximize turbo lag efficeincy and overall turbine efficiency like variable geometry turbines and changing blade camber designs.

    It's cool that you are questioning what is because that's where the new and better ideas come from. The place to start sometimes though is with looking at what has been tried, what's being studied and tested now and then consider what's being missed. I guarantee there are ideas being missed.
     
  13. Mar 8, 2017 #12

    Baluncore

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    If turbo lag is the problem then you could compress air and store it for later. When you want to take off you dump the compressed air into the inlet manifold and run on that while the turbo charger spins up to take over from the compressed air. A flap valve can block airflow back through the turbo compressor until the turbo exceeds the pressure of the compressed air.
     
  14. Mar 8, 2017 #13

    russ_watters

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    You could also add a motor/generator to the axle (or even decouple them) to store extra energy from the turbine and use it to spin-up the compressor.
     
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