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Automotive Gas Turbines in cars

  1. Oct 3, 2016 #1
    I know that this topic has already been covered (kind of) in a separate forum post, but it became rather confused and there were many differing and contradicting opinions/supposed facts, so I just want to clarify a few things.

    Firstly, are gas turbines more or less efficient (useful mechanical energy yield per unit mass of fuel) than conventional piston engines? (Assuming that both were allowed to run at their optimum speeds, which I know are very different.)

    Incidentally, does anyone know the approximate rpm speeds at which gas turbines and piston engines are most efficient?

    Secondly, do gas turbines have a high or lower power to weight/size ratio? What effect would the addition of a regeneration device for recapturing heat energy from the exhaust gasses to the turbine have upon this ratio?
    (For clarification of roughly how this works see: http://www.turbinecar.com/mags/R-T1972.htm)

    Thirdly, do gas turbine engines have faster or slower throttle response than an equivalent piston engine? Most arguments seem to suggest it is slower but this (http://www.bladonjets.com/applications/automotive/) website seems to suggest otherwise in its penultimate paragraph. By how much would the two engine types differ?

    Finally, are gas turbine engines simpler and smoother when in action compared with piston engines? Are they easier or harder (as well as cheaper or more expensive) to build, maintain and fix? I know the man who set up and runs MadMax Race Team, and who built their "Streetfighter" jet bike (which uses direct drive from a turbofan jet engine that originally came out of a helicopter to turn its wheel). By all accounts it is as smooth to drive as any electric bike, but with a world record holding top speed and literally spine crushing acceleration.
    (see https://en-gb.facebook.com/Turbinebike/)

    Also, would noise from the gas turbine engine be a concern, or can they be silenced enough to make them street legal?

    If you have any other information regarding the advantages or disadvantages of gas turbines in cars please share it. It would be helpful (but by no account necessary) if you could support any statements with facts or references.

    Any insight you have to offer would be most helpful, and thank you for your time!
     
  2. jcsd
  3. Oct 4, 2016 #2

    cjl

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    This highly depends on the engine. Both gas turbines and piston engines are capable of efficiency approaching or even exceeding 50%, but in typical automotive use, you would never see anywhere close to this number. Turbines at small scale suffer from large amounts of tip leakage in the compressor and turbine, which severely limits efficiency, and turbines tend to have poor part-throttle efficiency, so at automotive sizes and usage scenarios, I'd probably give the edge to piston engines here, especially if you start looking at modern advancements such as direct injection, stratified charge, atkinson cycles, and super-high compression ratios.

    There's not really a rule of thumb here, since it's so design dependent. In addition, turbines can have multiple different "RPMs", since they can have multiple shafts at fairly dramatically different rotational speeds.

    Gas turbines have a substantial advantage over normal car engines in power to weight and power to size. This is a large part of why they have pretty much taken over in aviation - a 700hp turbine engine is much smaller and lighter than a 700hp piston engine.

    Slower. Much slower. That website doesn't talk about throttle response either - the paragraph you refer to is talking about warm up time.

    Smoother? Almost definitely. Simpler? Arguably - they have substantially fewer parts, but the thermal requirements are much tougher, and as a result, the component cost for a turbine can be very high. The thermal requirements are made even worse by the fact that efficiency pretty much directly relates to the temperature at which you can run the burner, and the hotter the better. I'd say they're probably harder to build, but easier to maintain.

    You could probably silence them, though I'd imagine you'd need a pretty extensive exhaust system to achieve the noise levels we're used to on modern cars.
     
  4. Oct 5, 2016 #3
    Thank you very much for such a detailed and quick response! I have just one more query: is it true that turbine engines are more compatible with a wider variety of fuels? Could I, for example, burn hydrogen as my fuel, but when it runs out, simply use petrol from a normal petrol station? (which are far more common and easier to find)
    Also, by coupling a turbine engine to an electric motor, to tackle the throttle-response and low-efficiency-at-low-speed problems, could it plausibly be used for automotive purposes, or is the technology still to far away?
     
  5. Oct 5, 2016 #4

    JBA

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    All of the above is well said; but, with regard to the silencing, since the primary noise is high frequency this can be controlled rather easily with sound damping insulation and exhaust silencers. I have worked around turbine powered high capacity pumps and even with two turbines running the sound level in and around the pump building was quite comfortable.

    With regard to the exhaust temperature the use of an exhaust to intake heat exchanger, similar to that used on the Chrysler Turbine prototype cars of the early 1960's, can reduce the exhaust discharge temperature. I had an opportunity to see one of those vehicles and at idle their exhaust temperature was about the same as a conventional auto engine (and, yes, I stuck my hand into the exhaust stream to determine that).

    With regard to the electric motor turbine coupling that is already being used on the turbochargers of the Formula 1 race cars and is also being incorporated into the next generation of a couple of European production automobiles, as best as I can remember, they are BMW and Volvo engines.
     
  6. Oct 5, 2016 #5

    mheslep

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    The concept goes back decades and is in common use, though not in automobiles.
    https://en.m.wikipedia.org/wiki/Gas_turbine-electric_locomotive
    https://en.m.wikipedia.org/wiki/Turbo-electric_transmission#Ships_with_turbo-electric_drive

    A few concept turbo electric autos have been built. Car collector Jay Leno owns one or two. For the reasons outlined by others above, especially high temperature turbine components, they are exorbitantly expensive.

    https://en.m.wikipedia.org/wiki/Jaguar_C-X75 (over $1.2 million).

    The gas turbine remains king of power density for the moment*, not efficiency, and not cost per unit power. Thus the turbine dominates aviation, sees some high performance marine use and powers the US main battle tank (1500 hp)

    *There's been some work on integrating superconductors into electric motors, in which case they might match or exceed the power density of turbines, and are far more efficient.
     
    Last edited: Oct 5, 2016
  7. Oct 5, 2016 #6

    JBA

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    Unfortunately, in order to keep pace with the fleet fuel economy reduction requirements such expensive measures are becoming necessary, a similar example is Mercedes Benz latest new engine development with active variable compression piston strokes. It is exactly all of this required expensive technology for advancing IC engines, not to mention the current 10 and 12 speed transmissions, that leads me to believe that given the focus on improving battery technology, driving range and charging infrastructure the electric powered automobile is going to win the automotive power war even as an economic alternative considering it only requires a battery, electric motor (no technology advances required), and a single or two speed, at most, transmission (no technology advances required).

    PS I am conceding this as a 45 year engineering veteran of the petroleum industry and life long avid car fan; who, also still prefers a manual transmission, unless the alternative is a multigear, paddle shifting, dual clutch, automatic engine/transmission speed matching unit (also expensive).
     
  8. Oct 5, 2016 #7
    Going off on a slightly different tangent here: I know that the temperature in the combustion stage of a gas turbine engine can easily exceed 1600 degrees C, but do you know roughly how hot it would be at the intake. I assume that it is highly design dependant, and it is being constantly cooled by the supply of fresh air, but would there really be a noticeable difference? Also, would constructing the engine predominantly out of non-metals have any noticeable effect on containing the heat? (as non-metals are normally far less thermally conductive than metals).

    Again, thank you all for sharing your expertise.
    : )
     
  9. Oct 5, 2016 #8

    JBA

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    I can only comment on the heat exchanger's benefit to point of saying that Chrysler decided that in spite of its complexity it was worth incorporating, I believe for both an efficiency and an exhaust cooling advantage.

    With regard to materials and heat transfer, the controlling factor in the gas turbine is turbine blades maximum allowable operating temperature and obtaining that with current burner designs is not really an issue since only a portion of inlet air passes through the combustion section of the burner. As a result, there is little benefit in incorporating other ceramic material elements to the turbine engine assembly unless they would be lighter than their metal counterparts and I suspect that any such substitutions are already incorporated into our high performance jet engines for performance in military service and efficiency in commercial aircraft applications.

    With the recent increasing interest in hybrid powered highway haulers there is one group that is considering utilizing a small turbine power unit in a turbine-electric drive system for those vehicles since the ability to operate the turbines within a more uniform speed range is of an advantage for operating efficiency.

    While there is not much notoriety on the subject, small turbines for portable generators are alive and well and many people are still always investigating where and how they might be effectively utilized in other applications as well because of their adaptability to operate on a wide variety of fuels.

    Look at it this way, it is not a long jump from a turbocharger to a simple turbine engine, so compressor and turbine technology development interchange between those two units is a given.
     
  10. Oct 5, 2016 #9

    cjl

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    Maximizing turbine inlet temperature is a significant consideration in aviation turbines, and it is pretty much limited by materials (hence actively cooled single crystal superalloy turbine blades, as well as research into ceramic turbine materials). Unfortunately, increases in combustion temperature do tend to increase production of nitrogen oxides, so there are two competing factors here where high turbine temperatures are beneficial for efficiency, but harmful for certain emissions.

    That having been said, the overall trend in turbine design has been to continue to push turbine inlet temperature as far as materials will allow, so I'd definitely not say that it is "not really an issue". Unfortunately, I suspect that any automotive turbine will need to make some pretty significant sacrifices in this area too, since I doubt that the budget will allow for single crystal superalloys in the turbine.
     
  11. Oct 5, 2016 #10

    rcgldr

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    It depends on the engine. The 1963 Chrysler turbine ran up to 44,500 rpm.

    Just about any fuel other than leaded gasoline, although from what I've read about turbines in general for boat, bikes or model aircraft, diesel is the most common fuel (full scale aircraft use jet fuel).

    http://en.wikipedia.org/wiki/Chrysler_Turbine_Car

    Gasoline engines are usually most efficient (power versus fuel flow) at peak torque rpm, but that is usually much more power than needed for a car to cruise, so the ideal for a conventional (non-hybrid) car is to get decent efficiency at lower rpm, for example, a 2006-2013 Corvette Z06 has a 505hp 7.0 liter engine that redlines at 7000 rpm, but gets about 25 mpg at 1500 rpm at 70 mph. Cars with smaller engines would involve somewhat higher rpm at cruise speeds.
     
  12. Oct 6, 2016 #11

    russ_watters

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    Staff: Mentor

    The wiki on gas turbines has a good section on pros and cons for surface vehicles. I think the biggest potential is probably for series hybrids, since they can be run at constant speed and at an efficient operating point. This is done in trains.

    https://en.m.wikipedia.org/wiki/Gas_turbine
     
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