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Turbo alternator

  1. Mar 15, 2008 #1
    Im a mech engineering student and I am very interested in alternate fuels, and better mpg. So I was thinking and did some research on the topic im posting about and found very little so Im looking to experienced moders for some input.

    what if you were to hook up a turbocharger so that the exhaust spun the turbo shaft but instead of turning a compressor blade, it was used (through a gear reduction, or pulley system to slow the rpm to a usable rate) to turn the alternator? then you could use a smaller serp belt on the engine, and you could see some increased efficiency because of the decrease of parasitic loss. the most difficult parts of a project like this would be to make sure that at idle, the car was spinning the turbo at a fast enough speed to produce the amount of electricity needed, and that to reduce the gearing (i use the term loosely because it might not be best to use "gears" to do this) from the turbo spinning at very high rpm, to what the alternator is used to seeing.

    It would be best to be able to use the stock alternator, thus reducing cost and the pain of replacing it, and/or moding the wiring from it. other than that the game is on for ideas to improve this concept.

    I know BMW is using a similar idea involving capturing the heat loss from an engine/exhaust to run a small steam turbine to run the electrical system, but I think that the turbo idea might be a better one as far as a bolt on goes.

    as far as added back-pressure goes, you could put in a free flow exhaust, and the turbo all together and thus roughly maintain "stock" back-pressure.

    any input?
  2. jcsd
  3. Mar 15, 2008 #2
    I would think a hydrolic drive off the turbo would work better
    then a mess of gears or belts
    and you could then drive other systems too like
    water/cooling pump, A/C pump, power steering, in addition to the alt
    maybe some fans too
  4. Mar 15, 2008 #3


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    It's a good idea, but here are a few things to consider:

    - Parasitic losses to the battery charging alternator are low, and are only incurred when the alternator is actually generating power (not all the time).

    - The speed range of a turbo is huge. To gear this down to suit a normal alternator would require a very large turn-down ratio. This means that while the turbine isn't spooled up (a high proportion of the time) your alternator won't be able to generate a large amount of power, and (more importantly) the effective inertia of the system would be very high.
  5. Mar 15, 2008 #4
    actually I have a mazda3 (just got it woohoo awesome) and it already has the power steering and radiator fans totally electric, so doing the alternator would be such a great power saver for me. But the a/c pump would be great to take off the engine too.

    what do you mean exactly by a hydrolic drive? like a water pump basicly with hydrolic fluid?
  6. Mar 15, 2008 #5
    and to brewnog, i think that because of the way my car is set up (look at my last post) the demands of the alternator are higher. and I have seen reports of people disconnecting their alternator from the battery to see what kind of mpg improvements they get, low reports are something like an 8% increase from what I can tell, and that is huge over the lifetime of the car.

    and to compensate somewhat for the turn down ratio, maybe in combo with a bank of capacitors, you could use a small CVT like out of a snowmobile or even a large chainsaw?

    by the way, thanks for the "it's a good idea but here are some things to overcome" comment, its stuff like this that makes me love this forum. people helping people instead of telling them it cant be done :)
    Last edited: Mar 15, 2008
  7. Mar 15, 2008 #6
    hydraulic a small pump useing oil like fluid
    most are displacement types rather then vane so they work at higher pressure
    and lower volume
    a accumulator [ tank with air pressure ] can keep power up to motors
    to drive the accessories as needed when turbo is not at full speed
    but a accessorie drive turbo need not be sized for peak power like a compressor turbo is
    a much smaller unit could be used and partly bypassed at full power to cut back pressure
    and keep the unit spinning at lower RPM maybe even at idle
  8. Mar 16, 2008 #7
    that is true, it could be much smaller possibly. maybe a turbo that is meant for a motorcycle?
  9. Mar 16, 2008 #8
    It's a cool idea, I thought about that once before. You could definitely find a small enough turbo to do it, and the wastegate would keep it from over spinning. In reality though it would not be worth the extra weight of the turbo, the plumbing, the oiling system, the space it would take up, and the extra heat and parasitic effects on the motor. on the other hand though, it would sound badass. Especially when you turn your lights on and hear it whine :)
  10. Mar 20, 2008 #9


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    In principle, the idea is sound. There are plenty of people using turbines to generate eletricity, and exhaust pressure and temperature are where the biggest inefficiencies of cars are.

    In practice, adding a turbine means that the engine has to work harder to get the exhaust out - and that means less power at the wheels. With a turbocharger, this is going to be more than offset by the increased volumetric efficiency of the engine - but if you're dumping all the energy into the battery you may well notice the power loss.
  11. Mar 4, 2011 #10
    Having an extensive automotive background I thought of this as well. The battery powers the vehicle the alternator charges the battery as well as supplying some power to the car.It would work but would have to go under the vehicle where it is very wet so it would need to be put into a water resistant box.
  12. Mar 4, 2011 #11
    There is little loss of power with the turbo inline you would have to size it to your exhaust pipe. my truck would require a 3 " turbo.
  13. Mar 4, 2011 #12
    Loss from belt driven accessories is 10-15% then fan being the biggest at 20%.
  14. Mar 4, 2011 #13


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    Welcome to PF. You'll note that this thread is 3 years old, it's a good revival!

    Water resistance is the least of your worries. Do you have an idea what size battery is required to power a car? Have you looked at what this weighs and costs? Take a look at any of the 3rd generation hybrids in production at the moment and look at some of the engineering challenges they've faced.

    Turbochargers aren't 'sized' to the exhaust pipe diameter, they're sized principally to give a specific air mass flow at a given pressure ratio. A '3 inch turbo' doesn't mean anything. The turbo "size" is comprised of the frame size, compressor wheel size, turbine wheel size, compressor housing A/R ratio, turbine housing A/R ratio, compressor wheel trim, and turbine wheel trim.

    No, the biggest losses are thermal (heat to coolant and oil, heat to exhaust, and heat to intercooler where fitted). Losses to belt driven ancilliaries depends entirely on what's being driven and what conditions those driven equipment are being run at. The cooling fan losses are generally a couple of percent of heat in fuel, not 20% as quoted.

    I'm not sure what your 'extensive automotive background' comprises, but until people here know you, if you wish to throw figures around, you might wish to provide some references to back them up.
    Last edited: Mar 4, 2011
  15. Mar 4, 2011 #14
    What I meant is if your vehicle has 1.5" exhaust the turbo outlet will be 1.5" if you have 3" exhaust your turbo outlet will 3".
    I received a 25HP increase by eliminating my belt driven fan to an electric one. And throwing out the smog pump gave me a 10hp increase. Under-drive pulleys also gave me a 15hp increase. Which technically is not and increase just freeing up lost HP from the engine. So if the manufacturer say the vehicle has 225hp you need to subtract the belt driven accessories to have a true HP output. I also relocated my oil filter and the electric fan hardly comes on even in the hottest summer day's. I have done all my own work on my mustang which includes new heads intake cam 1.7 ratio rocker arms. With the new cam shaft I have .554 lift on intake and exhaust.Drive shaft loop, Airbags in the rear coil springs. B&M short throw shifter and a new Center line racing clutch. And the HP rating for vehicle's is from the flywheel not rear wheel unless specified by the manufacturer. By throwing out the smog pump I dropped about 25lbs off the car weight and received about 15% increase in mileage. And it still met emissions standards.
  16. Mar 4, 2011 #15
  17. Mar 4, 2011 #16
    What turbo manufactures have done to improve the peak efficiency around wide rpm is .
    1)variable geometry exhaust housing, this not used much in gas as temps to high but idea is a variable vanes/deflectors in the housing that can keep efficiency up over engine rpm band .
    2) twin scroll housing, this is were the exhaust housing has two different A/R compartments,. one for low speed response and larger for high rpm . there a flap to control when it opens (there still normal wastgate still used) .RX7 used this for many yrs on there turbo 13b engines (87-91).
    3)Sequential turbos , this is were there 2 turbos, its similar to twin scroll idea. One turbo is run at low speed/boost and then second kicks in . Later 92+ RX7 went to this system and so did Toyota supra turbo's at end.
  18. Mar 4, 2011 #17


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    Not true. The turbine outlet connection will have greater diameter than the duct running from the manifold, this ensures the back-pressure on the turbine wheel is not increased due to an inadequate pipe diameter, and promotes expansion over the turbine.
  19. Mar 4, 2011 #18
    My truck has a 3" exhaust and the outlet side of the turbo is 3" diameter. By outlet size I mean as the exhaust passes through the turbo. The boost side that goes to the intake on my application is also 3 ".
  20. Mar 4, 2011 #19


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    So turbine 'inlet' then, not 'outlet'.

    Anyway, I still don't see what a nominal pipe diameter has to do with anything. When turbocharging an engine, the existing exhaust pipe wouldn't even enter into the considerations as far as turbomachinery sizing would go.
  21. Mar 4, 2011 #20
    Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.
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