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Question about acceleration regarding Diesel vs. Gas

  1. Aug 25, 2013 #1
    I've read on many sites that diesel cars usually accelerate slower than gas cars.

    Why is that though? Diesel engines reach higher torque and at lower RPMs so why is it that gas cars accelerate faster? Are diesel cars just a lot heavier or something?
     
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  3. Aug 25, 2013 #2

    Pythagorean

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    No, it's just that acceleration comes from a combination of engine torque and rpm's, depending on gear ratios, not from torque alone. A very high torque with a very low RPM isn't going to turn the wheels very fast. Also note that wheel torque is not engine torque.
     
  4. Aug 25, 2013 #3
    I'm a little confused. Isn't torque the reason why RPM speeds up? The higher the torque, the faster the RPM goes up?
     
  5. Aug 25, 2013 #4

    Pythagorean

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    It depends on the load. A slow moving car is a bigger load than a fast moving car. So the same torque is going produce more or less RPM's depending on how much it's being resisted. Which is why we flip through gears. And the gear ratio to the tire is going to affect how much RPM you can get out of a certain torque too.
     
  6. Aug 25, 2013 #5
    You'll typically get similar road speeds in gear from a diesel compared to a petrol, which requires a much longer gearing.

    You can't take throw away comments and them try to apply reasoning to them. As the comment and insinuation that petrol powered cars accelerate faster because they powered by petrol, does not hold true.

    Case in point:
    BMW 120i vs 120d.
    Same displacement, very comparable power outputs (approx 170bhp), both have a 6 speed manual.
    Both have about a 7.5 second 0-60.

    Both are just as quick if driven with the intent of accelerating as fast as possible. In reality the 120d is probably the easier of the two to accelerate quickly in real world driving. Eg 40-70 for a motorway slip road.

    There are many, many more examples. It's just that BMW happen to have a diesel and petrol of the same power output and displacement.
     
    Last edited: Aug 25, 2013
  7. Aug 25, 2013 #6

    jack action

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    Power gives you faster acceleration, not torque. Diesel cars have usually less power than their petrol counterparts (Although this is less and less true with today's cars).

    That is another way of saying power: Power = Torque X RPM


    Low RPM means low power.

    Same power, same car, then same acceleration.

    That is because diesel usually have their maximum power at a lower RPM, which means you need to accelerate less the engine to reach you maximum power; energy that can be better used to accelerate the car. (Small difference though)
     
  8. Aug 25, 2013 #7
    The point I was getting at is that your typical diesel doesn't accelerate as quickly, because it's not designed to. Diesels are bought by people who don't care about driving, and want to move from A to B spending as little on fuel as possible. It's why they buy cars with tractor engines.

    If one was so inclined (as in the BMW case), one can make the diesel have exactly the same performance as the petrol equivalent, with better economy to boot. They are still not as exciting to drive.

    The point of maximum power is largely irrelevant, it'll be somewhere near the top of it's usable rev range just like the petrol. It's that the diesel has more low end power.

    The real crux of the comment was that the diesel will pull better from low rpm, so you don't have to bother changing gear to get the best out of it.
     
  9. Aug 26, 2013 #8

    Baluncore

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    Petrol engines were designed for maximum performance, diesel was designed for best economy.

    Petrol vehicles and diesel vehicles required quite different driving styles. As fuel injection and turbo technology advance, diesel will equal petrol in performance. But diesel will always be more economic because of it's higher compression ratio.

    The compromise is diesel, with a turbo-charger, tuned for performance.
     
  10. Aug 26, 2013 #9
    There are to elements of acceleration. Instant acceleration, and average acceleration.

    High torque can give you a high instant acceleration. For example you could generate say, .6g's of acceleration in a specific gear, which is pretty hard acceleration in a car. However, if you only manage that for 2 seconds before you need to change up to the next gear, is it a fast car? If we round up 1G to 10m/s, then the difference in speed would only be 12m/s.

    On the other hand, a petrol car might only manage .4g's of acceleration in a gear, BUT it will be able to maintain it for maybe 4 seconds. That would be 16m/s change in velocity for that gear. The rate of acceleration is lower, but the average acceleration is higher, and this is the effect of power. Being able to accelerate hard with lots of torque is meaningless if you cannot maintain it with power.

    So yes, diesel cars are generally nowhere near as fast as they feel.

    In the case of equally powered versions of either, then it ultimate still boils down to the thrust curve, which is dictated by power. You can plot a basic acceleration curve for a car, with velocity in m/s and power in KW, it is simply (P/V) / (Mass * 9.81). Acceleration will be on the Y axis and Velocity on the X. It will give you unrealistic values at low speeds as it will be limited by traction, but that curve is the acceleration capacity of any car and you see that torque does not factor in to it.
     
    Last edited: Aug 26, 2013
  11. Aug 26, 2013 #10

    Baluncore

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    I feel they are.


    The toque curve specifies the engine torque to RPM relationship. Power = Torque * RPM.
     
  12. Aug 27, 2013 #11
    Well that's subjective, depending on what you are used to. But I can assure you, diesels seldom deliver the warp speed acceleration that the initial surge of torque delivers.

    A typical rep-mobile 2.0 TDI might produce 300lbft, and at 2500rpm it will feel as fast as a petrol car with 300lbft at 2500rpm.

    The difference is, when the diesel runs out of lungs at 4500rpm and needs to change up, it will have produced maybe 200bhp. The petrol car can carry on to maybe 6500rpm, it might make 300bhp.

    So, are you honestly convinced that the diesel feels as fast as the petrol, even though at some set point, the rate of acceleration probably is equal?

    Thanks for clearing that up. I feel you missed the point somewhat though.
     
  13. Aug 28, 2013 #12
    I’ll have to dig out the charts somewhere off my old PC, but I took the torque curves and gear ratios for a BMW 120i and 120d (very close max power and 0-60 times) and calculated the max vehicle acceleration at each speed. IIRC the 120d always had higher acceleration when both in the same gear, but always had to shift up at a lower speed than the 120i, at which point the 120i had the higher acceleration. So you would conclude that the 120d might “feel” faster because in a given gear you can get more acceleration, but this doesn’t take into account that half of the time the petrol could be in a lower gear.

    The other thing is that people generally drive about at quite low rpm (to reduce noise, pollution, engine wear, or whatever other reasons), so it’s not surprising that a diesel would “feel” faster to the average driver. A race driver who always drives to get the most out of the engine with no regard to noise would have a different view.
     
  14. Aug 28, 2013 #13
    Ryuk1990, as xxChrisxx said, diesel cars are usually slower because they are designed that way. There may be exceptions on the market, but diesel cars aren't designed as performance machines. And it's not because they are tractor engines....if that were so Harleys would actually have some horsepower as they are based on aircraft engines. Diesel engines are heavier, and the general design of adding a diesel option to a vehicle adds quite a bit of wieght. Thus, they are slower. There is no real desire, or need, to make them perform on par with their gas siblings. People buy them for the mileage or simply because the fuel is more available or cheaper. It is possible to increase a diesels power to provide superior performance though. Look-up videos of diesel trucks at dragstrips, and if you really doubt a diesels performance potential, look-up an Audi R10 TDI. I'm sure the Audi driver never said "it doesn't feel as fast as the gas engine car I just passed." You can over-analyze the gearing, rpm, torque, horsepower, g-forces, and even the smell of the molecules if you like, but it comes down to design intent.
     
  15. Aug 28, 2013 #14

    Baluncore

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    Fundamentally it comes down to compression ratio. A spark ignition engine compresses air-fuel mix and so cannot have a compression ratio greater than about 10:1, without pre-ignition. A compression ignition engine has a compression ratio of about 20:1 (or 15:1 with turbo boost). Thermodynamic efficiency is a function of compression ratio, so diesel engines can, and will, always have significantly better fuel economy than petrol engines.

    A diesel vehicle is purchased for fuel economy reasons. There is a design trade-off with diesel. Is it preferable to accelerate 10% faster, or improve fuel economy by 10%. A diesel can be designed to have better acceleration than a petrol vehicle for the same fuel economy. It is just that rational professionals use diesel for the fuel economy and engine life advantages. The last thing they need is short term acceleration, since fuel economy is cumulative throughout the journey. Professional driving is not a competitive drag race.

    It takes skill to drive a diesel for performance, but it can be done. It is harder to acquire that skill for drivers of low intelligence, or those who first learned to drive petrol vehicles. Petrol teaches bad habits. Any petrol vehicle driver jumping into a diesel vehicle will miss the sheer exhilaration of fuel expenditure possible with petrol. Petrol is an addiction, diesel is a way of life.

    In the real world, it is not the magnitude of acceleration that is important, it is staying power.
     
  16. Aug 28, 2013 #15

    rcgldr

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    In the Lemans car racing series series, Audi's R10 turbo charged diesel engine based race cars have done well. Link to articles:

    http://en.wikipedia.org/wiki/Audi_R10_TDI

    http://audworld.com/r10-tdi-the-ultimate-race-car

    Trivia - some trucks use/used cummins superchargers on two stroke disel engines. The superchargers or "blowers" became popular for usage on hot rod and drag cars even though those use gasoline, alcohol, or nitro methane.

    http://nostalgiadr.wordpress.com/2010/04/26/blowers-by-the-numbers
     
  17. Aug 28, 2013 #16

    Baluncore

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    A supercharger will work equally well on SI or CI vehicles because the drive is mechanical and not from a turbine in the exhaust. Two stroke diesels need “blowing” because there is little time to get sufficient air for CI into the cylinder. The Junkers aircraft engines of the 1930s not only had opposed pistons, but also had square box pistons at the top of the motor. The square “cylinder” was used to pump air into the combustion chamber.

    A turbocharger is fuel dependent because there are differences of chemistry and temperature between SI and CI engines. Diesel runs a cooler exhaust, often with excessive oxygen. Petrol runs hotter exhaust without free oxygen present in the exhaust.

    You cannot swap turbochargers between CI and SI engines.
     
  18. Aug 29, 2013 #17
    My god, you are so right, it's all so clear now. Diesel is Zen. And of course diesel drivers must be better and more skilled.




    At the end of the day, you push the right pedal to go quicker, the middle one to stop, and the left one when you want to change gear. Wiggle the circle in the middle so you don't crash.

    It's all basically the same. Trying to make diesel out to be fundamentally different is slightly odd.
     
    Last edited: Aug 29, 2013
  19. Sep 12, 2013 #18

    462chevelle

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    good reading there.
     
  20. May 18, 2015 #19
    I'd like to point out that diesel cars have won the Le Mans 24 hour races in all of the last 9 years, beating gas cars which had previously dominated these races for many decades. Road going diesel cars may also be quick. Take the BMW M550d xDrive powered by a 3.0 liter turbo diesel 6-cylinder engine: it does 0-60 mph in 4.7 seconds. Not bad for a family sedan. Compare that to the BMW 535i xDrive powered by a 3.0 liter turbo 6-cylinder gas engine: it does 0-60 mph in 5.7 seconds. Not bad either. The botton line is if the design objective is performance instead of low fuel consumption, yes, the diesel engine can perform adequately.
     
  21. Jun 3, 2015 #20

    Randy Beikmann

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    The simplest way to look at this is to realize that torque in itself does not accelerate a vehicle. A forward traction force at the tire patch does, from simply using F=ma. It doesn't matter if you use a gasoline engine, diesel engine, electric motor, or turbine. The design details of any of them also don't matter.
    What does matter is how much power the engine/motor is producing at the vehicle's operating speed. If there is a high overall gear ratio between the engine and the axle, then a small engine torque will produce a large axle torque (and large traction force) - but the engine will spin pretty fast. The same force can be produced with a lower overall gear ratio, but only with a higher engine torque.
    No matter what, F*velocity = torque*omega(engine). That is, power at the tire patch equals power from the engine. (Obviously ignoring any friction losses).
     
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