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I Why are accelerating cars less fuel efficient?

  1. Sep 22, 2016 #1
    What is it about acceleration that makes it less fuel efficient?
     
  2. jcsd
  3. Sep 22, 2016 #2

    davenn

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    how about putting some thought into it .... what do you think the main reason would be ?
     
  4. Sep 22, 2016 #3

    Bystander

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    "Raw gas" through the carburetor in the "old days." Nowadays? Nothing but the sudden stops and starts. Everything burned in the engine goes up in smoke from the brakes.
     
  5. Sep 22, 2016 #4

    davenn

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    let him think about it before answering :wink:
     
  6. Sep 23, 2016 #5

    Khashishi

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    Don't get caught up in details of engine engineering. This question can be answered with basic principles and spherical cow assumptions.
     
  7. Sep 23, 2016 #6

    jbriggs444

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    How about firming up the question. Less efficient than what? And by what measure of "efficiency"?
     
  8. Sep 23, 2016 #7

    berkeman

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    LOL, I had to look that one up! :smile:
     
  9. Sep 23, 2016 #8
    physics is based on spherical cow assumptions
     
  10. Sep 23, 2016 #9

    sophiecentaur

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    The way you define 'Efficiency' will affect the result.
    Mpg is a particular meaning of efficiency which is not really in tune with the normal Energy out / Energy in definition. If the 'useful Energy' were to be defined as 'how well you warm up the brake discs' then frequent speeding up and slowing down could give you higher 'efficiency'. (Tongue firmly in my cheek, here.)
    As Dave has suggested, you need to be thinking about this on your own and then tell us your thoughts. (It's not straightforward School Physics.)
     
  11. Sep 23, 2016 #10
    I got tripped up by the question, less efficient than deceleration, coasting.... what??

    Maybe what is the most fuel efficient way to get from speed = 0 to speed = 100mph? There would be a clear answer to that I think.
     
  12. Sep 23, 2016 #11

    rcgldr

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    For most cars, which are overpowered, you can get somewhat better fuel efficiency (versus distance) by accelerating and coasting (with clutch in or neutral), then a continuous speed, but that would interfere with traffic, coasting neutral with an automatic is illegal in some states in the USA, and it creates extra wear on a vehicle.

    There's also the issue that for most cars, fuel efficiency (versus distance) is poor at speed below 40 mph == 64 kph, and there's a trade off on how fast you accelerate to 40 mph == 64 kph versus time spent at slower speed.
     
  13. Sep 23, 2016 #12
    Really??

    Accelerate/coast is efficient?

    I would have thought continuous small acceleration?
     
  14. Sep 23, 2016 #13
    If you didn't have enough fuel to reach your destination, how would you drive to maximize distance?

    Is increase of drag with speed a factor to consider?
     
  15. Sep 23, 2016 #14

    billy_joule

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    It's called the throttle for a reason. Would you expect an engine to be more efficient when it's throttled? (literally choking the fuel & air flow through the engine) or when the throttle is wide open and fuel & air flow is unrestricted? Modern injected engines are a bit smarter with how throttling is done but I suspect they're all still more efficient at WOT (wide open throttle).
    If were talking about fuel economy then the easiest way to improve distance/fuel is to use the brakes less. And when possible, travel at the peak point of your cars economy vs speed curve.
     
  16. Sep 23, 2016 #15
    So a drag car is fuel efficient??

    Throttles I assume are wide open.
     
  17. Sep 23, 2016 #16

    sophiecentaur

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    That's one way of looking at it. But, in the old days, you could buy a 'vacuum meter' which told you the pressure in the induction manifold. To help you get the best mpg, you avoided letting that pressure increase so you had to keep the butterfly valve as closed as possible - i.e. throttled.
    But, as I have already pointed out, the term 'efficiency' is not used in this thread, in the way it is used in 'proper' Physics and Engineering. So everything here is little more than a matter of opinion.
    As you say, use of brakes is wasteful of fuel and they are needed more often if you spend your time needlessly accelerating. Basically, you have to drive in such a way as to be a pain in the butt for everyone who's following you. :mad:
     
  18. Sep 23, 2016 #17

    sophiecentaur

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    I assume you are being ironic here. The stench of unburned fuel at drag meetings tells you that drag cars are not designed for efficiency (conventional or mpg type).
    I imagine that 'real' efficiency is an important criterion for F1 cars, which need to go as fast as possible and to eek out their fuel.
     
  19. Sep 23, 2016 #18
    F1 cars I would think are not efficient, they are grossly overpowered to compensate for their constant deceleration/acceleration around tight tracks and cornering. They are literally the opposite of aerodynamic. Their top speed is not great and doesn't have to be.

    But ya, the way efficient is used in the OP is ambiguous.


    Their was a hybrid fuel / human power race with a total fuel limit. Efficiency became important for competitors.
     
    Last edited: Sep 23, 2016
  20. Sep 23, 2016 #19

    davenn

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    it's one of the most inefficient uses of fuel on the planet
    they are not interested in efficiency, just in generating lots of power and doing that very quickly
     
  21. Sep 23, 2016 #20
    That may be true of an engine that is designed and built to run at WOT all the time, but an automobile engine is not. Up to 70% - 80% throttle, the fuel system is designed to run at stoichiometric air/fuel ratio. If it continued to run at stoichiometric ratio near WOT, the combustion chamber would develop too much pressure and heat and be damaged. To prevent this damage, additional fuel is added at and near WOT. This reduces combustion chamber temperature simply by absorbing heat to evaporate.

    In the real world, engines are not simple mechanical heat engines anymore. There is some pretty complex software that controls them, and effects how efficient they are under different conditions. Generally the engines and matching software should be designed so that peak efficiency happens under the most typical driving conditions. (unless it's a VW, in which case peak efficiency correlates to governmental testing conditions instead. ;-)

    Even at peak efficiency though, efficiency is pretty low because the engine needs to operate under a wide range of loads and speeds. You could make an engine that is very efficient at 2000 rpm at 30% throttle, that would get great highway gas mileage, and be absolutely onerous to drive under any other conditions. Nobody would buy that car.

    This is 1 reason that hybrids get better gas mileage. They design the engines to be very efficient in a narrow range of conditions, then keep them in those conditions, by supplementing them with electrical power when demand is higher, and drawing off excess power to save for later when demand is low.
     
    Last edited: Sep 23, 2016
  22. Sep 23, 2016 #21
  23. Sep 24, 2016 #22

    sophiecentaur

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    Max Power capability doesn't come into the definition of efficiency. A grossly underrun engine could still be run at high efficiency. The fact is that F1 cars need to do a job which involves loads of acceleration and braking. This uses up a lot of fuel. If they were not built with an eye to efficiency (to do that job), they would need to make more pit stops and, hence, lose races. I would say that means their engines need to be as efficient as possible so that the majority of the energy wasted in the brakes and by the tyres on corners can be replaced using as little fuel as possible. But, of course, we are being very cavalier in our definitions in this thread and the process of getting from A to B on the race track uses up a lot of fuel. The car starts at rest and ends at, say 150k/hr and that added Kinetic Energy is the only output energy that they have obtained from the whole exercise. (Plus I find motor racing noisy and boring - so I may be biased here)
     
  24. Sep 5, 2017 #23

    poe

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    Very simple question you ask, but I didn't find a good answer. First let's clarify your question, because it is poorly stated.

    Why is faster acceleration less efficient than slower acceleration?

    There are many variables that play into this, but there is a fundamental reason why, and I haven't seen it explained anywhere.

    The reason is time. Acceleration is a process, and fast acceleration keeps engine RPM high, and the higher the RPM the less time there is to extract work from the combustion event. What needs to be kept in mind is how energetic the gases of combustion are at beginning of combustion stroke as opposed to near the end of the combustion stroke! As a matter of fact the higher the RPM, the more useless the end of the stroke becomes, because at high RPMs the piston starts to move through the end of the stroke about as fast as the gases of combustion could make it move!

    A great way to think of this is the higher the RPM, the less of the stroke is used. Whereas low RPMs allow much more time for the lower energy gases of combustion to bounce off the piston crown and push on it even toward the end of the stroke.

    It's all about molecular collisions of the energetic gases with the piston that make it move! It takes time!
     
  25. Sep 6, 2017 #24
    While the cars as such may not be efficient (depending on your definition of an efficient car), their engines certainly are. Current F1 engines have 45-50% thermal efficiency, way more than regular road cars.

    I think the McLaren F1 team would like to have a word with you... In any case, 360-370 km/h can hardly be called "not great" for a car on a race track.

    Formula 1 is fuel limited, both in total quantity per race (105kg) and in maximum fuel flow (100kg/h). Engine efficiency is very important, otherwise you have to do fuel saving.
     
  26. Sep 6, 2017 #25

    I like Serena

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    A car computer will always show that your fuel consumption goes up significantly when accelerating. And the action radius goes down significantly.
    So it looks as if accelerating is really bad. But that is misleading, since the gained kinetic energy is not taken into account.
    It is only when we brake that we effectively throw away our fuel. Ironically the car computer will indicate extreme fuel efficiency when braking. :)
     
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