R.H. Allen wrote:
> Deputy Dumbya Dawg wrote:
>> "Bill Baka" <[email protected]> wrote in message
>> : It would probably be easy to get stretch limo up
>> : to 35 MPG as long as it stayed at 65 MPH.
>
> I don't know about THAT. You might be able to *design* one that does so,
> but you'd probably have to sacrifice a lot of things that are desirable
> in a stretch limo. Perhaps engine power, perhaps interior space (to
> allow for improved aerodynamics) ... just pulling things off the top of
> my head.
Almost. It isn't that hard once you get up to speed to just kick in an
overdrive gear where the engine is loafing along at about 900 RPM for a
V-8. The undercarriage of the limo might need some streamlining and the
tires pumped up to about 40 PSI but it can happen. A full sized car only
takes about 14--15HP to maintain 65 MPH and a V-8 can put out that much
at around 900 RPM. It would have to put out about 90 foot pounds of
torque to make that much power, but with a well tuned engine that should
be no problem. Every engine has an optimum HP Vs. efficiency that could
be graphed, but very few of the car magazines both to show below
2,000RPM. The magazines always put the power at 5,200 RPM and the peak
torque curves in the pages, but I haven't seen one (yet) that shows a
graph of efficiency versus power. I don't care if it makes 100 HP at
6,000 RPM because I almost never go that high unless I have to get out
of the way of a semi or something. The 14 HP thing was for my big
Chrysler so it should be much less for an economy car.
>
>> I may agree with you if this limo was in space but here on earth with
>> gravity your argument does not hold water. The more weight (mass
>> affected by the force of gravity) the more friction and the more
>> energy to maintain the velocity.
>
> No. You're neglecting inertia. A moving object tends to keep moving, and
> if it's heavy it's harder to stop than if it's light.
That may be over simplifying the problem, but Newton and those other
scientists did get it right, and that was taught way back in grade
school. Inertia makes the limo get crummy mileage in town but makes no
difference at all on the highway unless the driver is always speeding up
and slowing down. The cure for that is simple, just get another driver.
>
>> If you have your lime at 65mph
>> and you stop putting energy into it, it will slow down and stop.
>
> Yes.
Too obvious.
>
>> The more mass in the limo the faster it stops.
>
> No. Let's ignore aerodynamic drag for a moment and pretend that the only
> force slowing the car down is rolling resistance. The rolling resistance
> of a car tire on asphalt is about 3% of the car's weight. Thus, the
> deceleration force on a 1000kg car is 300N, and the deceleration force
> on a 2000kg car is 600N. By Newton's second law, the first car
> decelerates at a rate of 300N / 1000kg = 0.3 m/s^2, and the second
> decelerates at a rate of 600N / 2000kg = 0.3 m/s^2. In other words, both
> cars slow at the same rate.
If this is done in neutral it will determine the effects of weight on
the tires combined with the actual rolling resistance. Do it with the
transmission in gear and you will really notice the difference of engine
drag.
>
> Rolling resistance does increase with velocity, but on two identical
> vehicles it will increase by the same amount for each, so the result
> will be the same -- both vehicles will slow at the same rate.
>
> Now it *is* true that the more weight you put on a tire, the larger its
> contact patch with the ground. This *might* increase the coefficient of
> rolling resistance, but only very slightly if at all (the material the
> tire is made from and the surface it's rolling on have far greater
> influence on rolling resistance). Let's say it's 3.1% for the heavier
> car instead of 3% -- almost certainly an overestimate -- which would
> produce a force of 620N. Over the course of a mile, that would require
> 32,000J of extra energy compared to the lighter car to maintain constant
> speed.
>
> Let's say the lighter car gets 30 mpg and both cars transfer energy from
> the gasoline to the road at 25% efficiency. There are 120 million joules
> in a gallon of unleaded gasoline, so 40 million joules are burnt each
> mile. The extra 32,000J the heavier car requires each mile correspond to
> 128,000J/mile of extra gasoline. Therefore, the extra weight degrades
> the car's mileage to 29.9 mpg. The difference of 0.1 mpg may as well be
> zero considering that it's an overestimate to begin with, and that other
> factors such as driving habits and regular vehicle maintenance (or lack
> thereof) make a far greater difference in mileage than that.
I was initially talking about a constant 65 MPH on cruise control or a
very calculating driver who knows how to hold the speed with very little
throttle jockeying.
>
> Now if you factor in aerodynamic drag, both vehicles -- being identical
> aside from weight -- will face the same drag force. They will expend the
> same amount of energy overcoming it to maintain a constant speed.
> However, if you let your foot off the gas and coast to a stop, you'll
> find the heavier car coasts farther. I refer you back to Netwon's second
> law to understand why.
That applies very well to a car doing 65 MPH, but the place where you
will find the answer to the amount of rolling resistance by the tires is
around 20--30 MPH. The bleed off of speed with my big Chrysler was about
40 to 45 seconds with wind resistance being a lesser factor.
Anyone who doubts the effect of the air should take a car out and try it
for themselves.
BTW, the slick looking car may be less aerodynamic than the brick
looking car. Why? Take a look at the underside and see how much junk the
air on the bottom has to go through. Of course auto makers don't expect
people to look there.
Bill Baka