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Diesel engines.

  1. Nov 19, 2004 #1
    Why don't car makers produce cars with diesel engines coupled with electric generator and then use only electric motors for propulsion?
    You know what I mean? :redface:
  2. jcsd
  3. Nov 19, 2004 #2


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    Generators and electric motors add expense to the car but consumers (with tax breaks and fuel savings) are starting to buy the hybrids so that trend may have reached a point where it can be justified.

    In the US the biggest issue is the sulfur in the diesel fuel where even the cleanest diesel cars are like 4x dirtier than a typical gasoline car. With catalytic converters and cleaner diesel this could be reduced to almost no difference from what I've read about.

    Given the increased efficiency and ability to use existing supply chains it could catch on quickly here but the public perception would be the biggest obstacle. Current owners of the VW diesels complain about trouble finding diesel inside large cities, not a problem near the outskirts or rural areas where truck stops always have diesel.

    The new high-revving quiet EFI diesels are already a $7,000 premium and a generator/motor would probably add as much. Its just a chicken-egg game to get the production up high enough to get those numbers down and something drastic like another oil embargo even would be needed. (unless you read the gloom-n-doom in the energy thread :smile:)

  4. Nov 19, 2004 #3


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    An electric motor alone has to be pretty big to accelerate a car: that's why a hybrid uses both a gas/diesel engine and an electric motor at the same time.

    When cruising, the diesel is preferable since its more efficient to use it directly than to use it to generate electricity.
    Last edited: Nov 19, 2004
  5. Nov 19, 2004 #4


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    Unlike internal combustion engines, electric motors produce roughly constant power, so:
    [tex]P=a \times v[/tex] is essentially constant. Specifically electric motors are very good at getting things moving from a standing start. AFAICT the big motivation for hybrid gasoline-electric designs is that the energy density of oil-based fuels is about 100 times that of chemical batteries.

    Regarding efficiency it really depends on what you're optimizing for. For example, it's readily concievable that there is an engine that achieves maximum fuel -> energy efficiency at some particular speed, so, to optimize for fuel efficiency, you would run the engine at this constant speed, and use electric motors for drive, and, this tactic is in fact used for some industrial vehicles. The mechanical-electric-mechanical conversion can be 95+% efficient, so that a 5% increase in engine efficiency leads to a net fuel efficiency increase. All-electical drive also has ancillary benefits such as easy independant all-wheel drive and flexible geometry.

    To answer the original question, diesel hybrids (with and without purely electrical drive) exist, and are on the drawing boards, or in production from major auto makers such as GM and Mercedes. I don't know if they're targeted at the private market though.
  6. Nov 21, 2004 #5
    Conventional submarines use dual diesel - electric propulsion underwater, some excavating machines and if I'm not mistaken all big locomotives we see on our railway tracks.
  7. Nov 23, 2004 #6
    This is true, but railway locomotives still use the diesel to drive a generator, and then use the generated electicity to drive the wheels. Why? It's because of the enormous engineering difficulty of driving many wheels, at all speeds from zero up to the locomotive's maximum.

    Big diesels are pretty much constant speed devices - they have a narrow operating speed range. It's very difficult to design gearboxes and transmissions that can provide the required gear range and also drive many axles. The best transmission that engineers have come up with so far is an electric one. Tap changers, and thyristor drives are easier to engineer than mechanical gear boxes.
  8. Nov 30, 2004 #7
    Additionally, the loss of energy...

    It's much more economical to convert chemical energy in the fuel to mechanical energy in an engine to the wheels than to convert chemical in the fuel to mechanical in the engine to electrical in the generator to mechanical in the motor to the wheels. Small losses at each step add up quick...
  9. Dec 1, 2004 #8


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    This was my basic point. There are, of course exceptions such as diesel locomotives, where low-speed torque is critical.

    The reason I'm commenting now is, NateTG, your numbers bother me -
    95% is about the limit of an electric motor on its own. I'm not sure about generators, but I doubt they are as efficient as motors - and using the same motor as a generator and a propulsor also likely reduces it. Last, excess energy needs to be stored in batteries for acceleration - an additional 95% on both sides of that.

    That gives a maximum of 90% efficiency for direct diesel->electric drive and 81% for diesel->electric generator->battery->electric drive.
  10. Dec 1, 2004 #9
    Give them time! Currently, the "hybrid electric" cars (like the Toyota Prius) use the principle, but with gasoline engines. The big advantage of the design is that you can use a fairly small, efficient IC engine, and get the required extra power for accelerating from the electric motor. Most cars have big engines which are required to put out full power only a very small percentage of the time. By the way, most railroad locomotives are diesel-electric hybrids.
  11. Dec 3, 2004 #10
    > The mechanical-electric-mechanical conversion can be 95+% efficient...

    That's extremely generous. A common 1 hp split phase motor draws 12 amps at 115 volts, which is 1380 watts. 1 hp of electrical energy is given as 746 watts, so we're down to 54%. Smaller motors are even worse, larger are marginally better. The physical construction of a motor or generator are similar, so, if we have a 1 hp diesel driving a generator, driving a motor, it's down to ¼ hp at the wheels. A 10 hp 600 volt 3 phase is 72% efficient (7460/10 A x 600v x 1.73) so it's down to 52% output after a double conversion.
  12. Dec 3, 2004 #11


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    wire2 - using only current and voltage tells you VA (apparent power) and only after applying the power factor could we calculate watts for the true power.

    But the point that cumulative efficiency drops at each conversion is quite valid. For the locomotive or heavy equipment using diesel/electric drivetrains the application demands its use, not the efficiency of it. All the available hybrids still have a mechanical drive system to the wheels directly from their IC engines.

  13. Dec 3, 2004 #12
    A valid point, Cliff. I left power factor out only to keep it simple. If applied, it would have made the conversions even less efficient. My intent was to answer the original post with actual examples.

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