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Increased mass (inertia) affect on wind drag

  1. Aug 18, 2013 #1
    Okay, so the scenario is a passenger car, traveling on the highway. As a general rule, I say that increased weight (mass) will have a beneficial effect on MPG (IF tire pressure is increased so that rolling resistance is reduced to compensate for the increased weight), ONCE the vehicle is up to speed. Essentially, (again, assuming tire pressure is increased to compensate for rolling resistance increases from weight), if a helicopter could swoop in and place a 500 lb weight into the trunk of a car already traveling 75 mph on flat ground......that weight could actually INCREASE mpg if the vehicle remained at precisely 75 mph.....over the course of 200 miles.

    But.....someone recently claimed that this increased weight, thus inertia, also would reduce the vehicle's energy consumption in order to overcome atmospheric drag....thus remain at 75mph. I said it would not have any affect on drag, and that the wind drag would remain a drain on efficiency, no matter how much the vehicle's mass / inertia was. I am certain that I'm right....otherwise we'd all put 400 kg weights in our trunk when taking road trips. However, I can't seem to conclusively prove this using math or better examples. Increased inertia requires energy as an input...which is used up over time for various reasons (hills, bumps, etc.).....but what about a constant wind drag? Seems to me that this initial inertia would be quickly used....and then replenished constantly in direct proportion with the atmospheric drag. Thus.....zero net benefit over time.

    That make any sense??
  2. jcsd
  3. Aug 18, 2013 #2


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    Staff: Mentor

    You are right. The increased mass just acts as a store of reserve energy - the motor had to work harder to bring the heavier mass up to speed, and it has to work just as hard to maintain a constant speed in the face of wind resistance. If the motor does slack off a bit at speed, the wind resistance will slow the car a bit less at first, but the motor will have t work harder to restore the lost speed than if the car were lighter, so we end up with no net gain.

    [Edi:t You are NOT right in the first paragraph of your post. Gotta read my own posts more carefully :smile:]
    Last edited: Aug 18, 2013
  4. Aug 18, 2013 #3
    Whoa wait...why would increased mass increase mpg? The "helicopter placing a weight on a moving car" might have a short-term benefit for the car, but at the expense of the helicopter, which had to burn more fuel to accelerate the mass.

    But as you say, the increased mass would NOT decrease wind drag or its effect. The amount of energy required to fight drag can be easily calculated, and nowhere in the formula is mass used. It might seem that a train is hard to stop because its heavy, and erroneously think the mass helps it fight drag. But the fact that it's harder to stop is offset exactly by the fact that it's harder to start. PLUS its higher mass results in greater friction and thus less efficiency.
  5. Aug 19, 2013 #4
    Every aspect of this scenario is fundamentally wrong.

    First of all, if you increase the weight of the vehicle you will increase, consequently, the kg/hp ratio. That means that in order to maintain the same speed (say 75mph, with the same gear in the gearbox) with the increased load the engine will have to work in higher rpm which in turn increases fuel consumption.

    Secondly, the aerodynamic drag has nothing to do with the mass of the vehicle. It has to do with the properties of the fluid (air in this circumstance), the speed, the wetted area and the shape of the vehicle. So increasing or decreasing the mass has no effect in the aerodynamic drag.
  6. Aug 19, 2013 #5


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    You're assuming your own conclusion here - this statement is based around the premise that a heavier vehicle would be a higher load on the engine, which you haven't yet established. Once a vehicle is at speed, mass is largely irrelevant to fuel consumption. It isn't entirely irrelevant, since rolling resistance is somewhat mass dependent, but the aerodynamic drag is a significantly larger factor, and it will depend only on the vehicle's shape and size. Also, the engine RPM (I'll assume a manual gearbox, or an auto with a locking toque converter here) will have a fixed ratio with wheel speed, so even if the load increased, the engine RPM would not necessarily change (nor would it have to, since most cars do not have the throttle open fully at highway speeds just to maintain speed).

    Realistically, for most cars (so long as the increased weight was well within what the car was able to carry), the increased mass should only slightly decrease the highway fuel efficiency, while it should substantially decrease fuel efficiency in a situation involving acceleration and deceleration. A modern semi truck is a good example of this - the highway fuel efficiency of one can approach 10mpg, despite weighing perhaps 20 times more than a typical car (but with significantly less than 20 times the aerodynamic drag).
  7. Aug 19, 2013 #6

    Exactly correct. hp/kg ONLY matters when overcoming inertia or the pull from gravity. On the highway, on flat ground.....it matters (essentially) zero. In certain cases, increased mass can INCREASE fuel efficiency though. But, you'd have to drive perfectly to achieve it....and be driving in fantasy land. The reason for this is that the extra mass (which is a 100% efficient store of energy) can be used to overcome a bump or small throttle adjustments. To not have this mass, the engine would have to be used to compensate. The engine is only about 20-25% efficient though. Now, there are down-sides too from mass too, such as rolling resistance and the real-world fact that hills exist. I'd rather have a MASSIVE fly-wheel on my car for x-country trips than extra mass anywhere else.
  8. Aug 19, 2013 #7

    Not sure it works that way....quite a bit on 'rolling resistance' here:


    too many factors for me to reach a firm conclusion one way or the other based on a quick read.

    road curves can also play a role...

    Seems like a tire would be linear viscoelastic and that temperature plays a role:

  9. Aug 19, 2013 #8


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  10. Aug 19, 2013 #9
    We agree on that.

    So, what's the point? Can you prove mathematically that increasing the mass increases fuel efficiency (taking into account everything of course)?
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