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Could Hybrid Vehicles That Use An Engine to Power an Electric Motor Be Created?

  1. Jul 5, 2010 #1
    So with most of the hybrid vehicles around, they use a conventional engine and transmission with an electric motor to take over at certain points, and the engine recharges the electric motor. But could a hybrid be made that uses say an ordianry gasoline or diesel engine that directly powers an electric generator that then drives an electric motor to move the vehicle?

    This is how the diesel-electric freight trains are designed I believe, and such a vehicle would not require any transmission or batteries I'd think, no transmission because the engine only powers the generator, so it isn't linked to the wheels, and no batteries because the electric motor relies on power from the engine.

    Are there no diesel-electric or gas-electric cars/trucks such as this because the technology is too complicated, or expensive, or takes up too much room right now? Or the fuel usage just wouldn't be any different (it would take the same amount of fuel to go 300 miles with a gas or diesel-electric as it would with a conventional engine)?
     
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  3. Jul 5, 2010 #2
    I'm not at all sure, but I believe the Chevy Volt does just what you describe. I'll let you google it to verify.
     
  4. Jul 5, 2010 #3

    brewnog

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    There are two main drawbacks I can see here. If you do away with batteries, you lose the ability to store energy from regenerative braking which is an inherent benefit of a hybrid design. There are also considerable losses involved in the additional conversion processes in the powertrain (alternator, motors, and everything in between), and these are in addition to the usual engine losses which you will struggle to minimise across the vehicle's speed and load range.

    Nice thinking though.
     
  5. Jul 5, 2010 #4
    The Fisker-Karma it seems sort of uses this concept, however it still has batteries. The Chevy Volt also uses it, relying solely on battery power at first, then a small engine kicks in to provide power to a generator which powers the electric motors.
     
    Last edited: Jul 5, 2010
  6. Jul 5, 2010 #5

    russ_watters

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    The primary benefit of a typical hybrid comes via the ability of the engine to always operate at near its peak efficiency operating point. Eliminating the battery almost completely eliminates this benefit.
     
  7. Jul 6, 2010 #6

    jack action

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    I disagree with that. An "electric transmission" (a generator powering an electric motor) would be a true CVT even without batteries, thus making engine rpm independent of wheel rpm such that it can always operate at near its peak efficiency operating point.

    I think the true problem lies in size, weight and/or cost. If you have a combustion engine that puts out 200 hp, an electric transmission would require a generator that can handle 200 hp, AND an electric motor that can also handle 200 hp (or 2 X 100 hp or 4 X 50 hp). Although I'm no expert on generators or electric motors, simple ones of such power rating are huge. I think that even more hi-tech ones are still bigger than a comparable IC engine and, of course, are more expensive. You might be removing one transmission, but you're adding two motors.

    I would think locomotives can use that system because the weight of the locomotive is less of a factor considering the whole freight train. They also use huge resistances connected to the electric motor for converting braking power into heat (I'm guessing brake pads wouldn't last long to stop a whole freight train!)
     
  8. Jul 6, 2010 #7
    Yes. This type of hybrid is called a series hybrid. The Chevy Volt is a series hybrid which differs from current hybrids like the Prius which is a parallel hybrid. A series hybrid does offer many advantages over a parallel hybrid in that it is theoretically more efficient since the engine can operate at peak efficiency all the time as russ stated. You also save some weight in the drive train area but need a bigger more robust electrical power train.

    The main reason for the lack of series hybrids is cost. Big electric motors, inverters/converters and batteries cost a lot more than an ICE engine. The biggest advantages are efficiency and simpler "plug in" operation. Series hybrids generally go a lot further (and faster) on battery only power due to a bigger battery.

    If you want remove the battery and have the ICE and generator power an electric motor you will be taking a hit in efficiency. An entire engine plus electric powertrain will not be as efficient as just a typical ICE engine. Not only is the efficiency less but the cost will also be much higher.
     
  9. Jul 6, 2010 #8

    russ_watters

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    You're misunderstanding the issue. A substantial fraction of your power is lost just in making the engine spin, so if the engine isn't outputting much power, that can be a sizeable fraction of the power output of the engine. The peak efficiency operating point (of the car, not the engine) is 3d point, not a 2d point. It's not just an efficiency at an RPM, it is at an RPM and a power output. If you're running at the best rpm, but running at a very small (or zero) power output, then you're not going to be operating efficiently.

    For example, my car burns .3 gal/hr just sitting at idle, at idle rpm. I'll take a look at what it burns at higher rpm (with no output).

    At the very least, you'd want a battery with enough capacity to get you started from a red light so you don't have to leave the engine running. Then you have to calculate how much reserve capacity you need (how many red lights, how much time to start the engine). That's the type of calculation that went into the designing of the hybrid in the first place!
     
    Last edited: Jul 6, 2010
  10. Jul 6, 2010 #9

    jack action

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    Vehicles with start-stop technology have been around succesfully http://www.italiaspeed.com/2009/cars/fiat/02/regatta_es/1602.html" [Broken].

    The start-stop technology is one thing, the CVT (such as the electric transmission under discussion) is another. And they can both be used separately or simultaneously to reduce fuel consumption.

    The http://en.wikipedia.org/wiki/Chevrolet_Volt" [Broken] battery pack has nothing to do with the system the OP is referring to. It could be seen as another system, in parallel (not in series) of the one under discussion in this thread. Yes, the Chevy Volt has an ICE-powered generator connected to an electric motor. The car could theoretically work by itself just with that system. The engineers have just put an extra externally rechargeable battery pack that is used to power the vehicle for the first 40 miles (64 km), satisfying the daily commute for 75% of Americans, hence completely eliminating the use of an ICE. The ICE takes over after that, operating at its peak efficiency all the time, because it is physically unconnected from the wheels, just like I've mentioned before. Looking at efficiency alone, removing the battery pack wouldn't make it pointless to use the system, i.e. a CVT.
     
    Last edited by a moderator: May 4, 2017
  11. Jul 6, 2010 #10

    russ_watters

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    Not pointless, but it would eliminate a lot of the benefit, since if you remove the battery pack, the ICE is not going to be operating at peak efficiency anymore. You also lose half of the performance of the car by removing the battery, since the car has a 150hp electric motor and 71 hp generator. That's the tradeoff: by making the powerplant smaller you keep it closer to its most efficient operating point but lose power unless you store the energy.
     
    Last edited by a moderator: May 4, 2017
  12. Jul 6, 2010 #11

    mheslep

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    The Chevy Volt's drive train operation is more complicated than 'E-motor for some miles, deplete the battery, then run all engine.' Yes the Volt will run in an e-motor/battery only mode when the battery has ample charge, but the battery is never allowed to fully deplete. Once the battery reaches 30% charge or so, the motor kicks on/off intermittently in 'charge-sustaining' mode, allowing the gas engine to run only at a peak efficiency set point, as Russ suggested above. Also, as others have mentioned, the battery allows regen braking energy capture, though I doubt regen braking saves as much energy as allowing a reciprocating engine to run only at its peak efficiency point.

    voltchart.jpg
     
  13. Jul 6, 2010 #12

    russ_watters

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    I checked my car's computer this morning: the engine uses about 1.3 gal/hr of fuel at 3000 rpm while sitting still in my garage, my foot on the clutch and the air conditioning off. I have a 160hp engine. That's a lot of fuel just to make the engine spin, without even outputting any power.

    Analysis: 3000 rpm roughly equates 75 mph and as my car gets about 30mpg at 75 mpg, the fuel flow would be 2.5 gal/hr. As you can see, the loss before you even get through the transmission is a big fraction of the the total fuel consumption of the car. If you could cut that loss in half by, say, putting in an engine of half the size (not sure that follows, but go with it for an example...), you'd drop the fuel consumption by more than 25% to 1.85 gal/hr at 75 mph and increase the fuel economy to 40.5 mpg.
     
    Last edited: Jul 8, 2010
  14. Jul 7, 2010 #13
    I assume it certainly wasn't idling at 3000rpm, so you must have had your throttle open slightly.

    It's also pretty dodgy trying to take a reading like this as the fuelling map will be overfuelling when you have an open thottle and no load. You would likely be able to shut off 3/4 of the injectors and still get the engine to happily chug away.
     
  15. Jul 7, 2010 #14

    russ_watters

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    Yes.
    With the fuel flow that high, it is tough to believe there is that much error in it. The point is, there is load - it's just that it is all internal to the engine.

    And the car certainly sees situations like that regularly driving down a highway with a slight decline.
    Sure - with the rest of the injectors spitting 4x as much fuel!
     
  16. Jul 7, 2010 #15
    You'd be surprised at how massively overkill engine fuelling maps are to keep themselves turning, even big V8's only need 2 cylinders with normal fuelling on tickover to keep themselves spinning, it's a bit rough and lumpy but it certainly has enough to keep itsself going.

    It's very surprising that so much fuel is being used at 3000rpm when you aren't acutally moving anywhere. Obv, I don't acutally know this specific case, but my spidersense is telling me that that just isn't quite right.
     
  17. Jul 7, 2010 #16

    mheslep

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    Interesting, that's a waste power output (as heat) of 46 kW, or 62 HP*. That reminds me that the total power output of your engine, tractive plus rejected waste heat is more like 4X rated, or 640 HP.

    *using 1 gal gas = 128 MJ
     
    Last edited: Jul 8, 2010
  18. Jul 8, 2010 #17
    Not being an engineer (yet), this thread has gone a bit above my head! Good thread though. I have a lot to learn.
     
  19. Jul 8, 2010 #18

    russ_watters

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    Fair enough: lets explore the issue a little more.

    I'm using one of these to read my car's computer while I drive (recommended by someone on PF): http://www.scangauge.com/products/
    It is pretty accurate as far as I can tell. The actual fuel consumption readout is off by about 8-12% and this error is calibrated out before it gives me the numbers. The fuel consumption accuracy varies seasonally by about 4% (ie, the error is around 8% in winter, 12% in summer), but not by an identifiable amount from one tank to the next (I have to make corrections of +- around .2 gal from tank to tank, but inconsistency in fill amount could account for all of that). This combined with the fact that the idle fuel flow is almost certainly accurately derived from the lookup table tends to eliminate error generally associated with measured values. The overall point of all of this is that I see no inconsistencies anywhere that would make me doubt the accuracy of what it is telling me.

    My car is a 2004 Mazda 6. It has a 2.3L 4cyl engine that is rated by the mfg at 160 hp @ 6,000 rpm, with a peak torque of 155 ft-lb @ 4,000 rpm.

    Yesterday, I did a max power acceleration in 3rd then in 4th gear and it measured about 9 gal/hr @ 5,000 rpm. This number has an uncertainty of around .3 gal/hr due to the fact that it was changing quickly and the readout is only about once a second. I did just those two trials. Assuming the torque and efficiency only drop off a little from 5-6,000 that gives a calculated 10.8 gal/hr at max power. Ratioing back to 3,000 rpm yeilds 5.4 gal/hr (I'll record that one tomorrow - just realized I need it).

    Using a lower heating value of gas of 115,000 btu/hr and 33% efficiency yields 15.06 hp/gal. So 10.8 gal/hr is 162.6 hp - remarkably close to the mfg spec. 5.4 gal/hr yields 81.3 hp.

    Now 1.3 is 25% of 5.4, indicating 25% drive loss up to the transmission. Is that reasonable? I'm not sure - but you're right that it seems high. Googling, I find most people use 15-20% for "drive losses" on a fwd car. My 25% doesn't include the loss at the wheels and only includes part of the loss in the transmission - unless the loss in the transmission is worse when not in gear than in gear (which it may be, as you have parts slipping past each other). I don't remember if I had the clutch down or was just in neutral. But 15% of 5.4 is only 0.8.

    But with rpm constant, drive loss will be a constant horsepower, not a constant fraction of horsepower. 15% of peak horsepower is 32% of cruise horsepower (based on 2.5 gal/hr from the previous page = 37.7 hp at 75 mph).

    And now that I've gone through all of that, I thought of a reason why my 3,000 rpm "idle" may have been high: my engine was cold. I would think that in the absence of a warm and properly functioning set of sensors, the engine management system would err on the side of running rich. I'll run the same test again tomorrow when the car is warm to see if I get the same result.
     
  20. Jul 9, 2010 #19

    jack action

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    The Mazda 2,3 L has a stroke of 94 mm.

    @ 3000 rpm, it gives a mean piston speed of 9.4 m/s (= rpm * stroke / 30 000) which represents a friction MEP of about 2 bar. (= 1 + 0.105 * 9.4; empirical equation)

    If you add 0.75 bar for the vacuum created by the throttle closed, you need to produce a MEP of 2.75 bar just to keep the engine idling @ 3000 rpm.

    2.75 bar @ 3000 rpm for a 2261cc engine (the true size of the mazda engine) represents almost 21 hp. (= MEP * Vol * rpm / 895 200)

    https://www.physicsforums.com/showpost.php?p=2785976&postcount=55" and 2.9 MJ/kg for the energy of the fuel, with an AFR of 14.7, you need 0.84 gal/h. (= power / eff / AFR / ef * 0.9335)

    If your engine is cold and you are running rich (say an AFR of 11), your fuel consumption goes up to 1.13 gal/h.

    So the 1.3 gal/h makes sense.
     
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