🚀 go-pugleaf

In article <701774624.F00006@blkcat.UUCP>, Don.Noll@p3425.f70.n109.z1.fidonet.org (Don Noll) writes:
>   
>   DL> Any speed higher than 55 mph MUST consume more gas!!! How much 
>   DL> more might be questionable; but it has to be more unless 
>   DL> someone has developed a machine that reverses the laws of 
>   DL> physics.. You never get something for nothing in nature... 
>   DL> 
>   DL> To "push" a car faster, requires more energy..i.e. more gas; 
>   DL> for the same car.. 
>   DL> 
>   DL> Mechanical and air friction increase with speed; therefor to go 
>   DL> faster, requires more energy. . . . . .. 
>  
> Not true really.  Alot depends on the frontal area of the car,
> The gearing and final drive ratio, and at what point the engine produces
> the most power for the least amount of gas in RPMS.
> 
> I own an 1983 FORD Mustang with a 5.0 liter V-8 that gets best mileage
> in 5th gear at about 2100 RPMS. (which with the current rear end or
> transaxel is about 65 miles an hour.  Lower speeds, epecially 55 actually
> reduce the gas milage about 2 miles per gallon. Testing was done with
> a decent (non FORD) cruise control and with trips of over an hour at
> a constant speed.  (Wife then girlfriend went to UVA, I lived in
> Springfield)  20+ trips on Weekends.  In traffick and stop and start
> is a different kettle of fish. Observed milage at 65 27 MPH, at 55 25 MPH.
> It also could have been the HILLS on 29 and some inertia, who knows??????
> 

Enough already!  Let's decompose the problem explicitly into cause and effect factors that can be quantified.  The main consumers of energy when an automobile is travelling down the highway are aerodynamic resistance, rolling resistance, and mechanical friction in the drive train components.   Hill climbing adds to energy consumption while going down hill reduces energy consumption at a given speed.  It is not a wash but let's only consider effects other than hills.  

The aerodynamic resistance is given by one half the product of air density times the square of the relative air speed times the frontal area times the drag coefficient.  For a given car the drag coefficient is a constant except for effect of wind direction and the frontal area is constant.  The other factors for sea level air density give 1.15 horsepower per square foot at 55 miles per hour and 1.89 horsepower per square foot at 65 miles per hour.  This is 65% more power to overcome the air resistance at 6




5 miles per hour than at 55 miles per hour.  However, distance is being covered at a faster rate at 65 mph than at 55 mph.  The result is that energy consumption in horsepower hours per mile per square foot of effective frontal area is .0291 at 65 mph and .02090 at 55 mph.   This is 39% more energy per mile at 65 mph than at 55 mph to overcome aerodynamic resistance.  

The other factors of rolling resistance and mechanical friction increase in a fashion more linear with speed.  If the actual resistance were linear with speed the power to overcome it would be proportional to the square of the speed and the energy per mile would go up proportional to speed or by  18% for 65 mph compared to 55 mph.  

Why aren't these kinds of percentages easy to observe?  If we calculate the actual power to propel Don Noll's 1983 Mustang against the air at 55 mph and at 65 mph we will get around 12 hp at 55 and around 17 at 65 mph.  These are less than 10% of the installed power of his 5 liter engine.  Gasoline engines generally supply the highest efficiency when operating near maximum power at wide open throttle.  Our difficulty is with the mismatch of the power plant with the demands of steady driving at highway spee




ds.  This is the dilemma (or at least the tradeoff) faced by automobile designers and buyers.  We like performance at least in acceleration even if we can't legally use the high end speed capability.  This means our engines are loafing in cruise and cannot be nearly as efficient as a much smaller engine in the typical highway cruise.  If a car with an efficient small engine of say 60 hp is run through a check of 55 mph versus 65 mph gas mileage, the result will easily favor the lower speed.  

It is impossible to obtain accurate relations between gas mileage and driving speed unless the wind speed is monitored throughout the test.  For example Don Noll's results would be confounded completely if he drove 55 mph on his way to UVA and 65 mph on his returns but there were an average 7mph southwest wind during his trips.  This would provide 62 mph relative wind during his "55 mph" trips and 58 mph relative wind during his "65 mph" trips.  This is not an unlikely scenario.  Cross winds also increase 




the drag over straight ahead conditions in a more complicated way.  Nationwide, the "average" cross wind is taken to be about 7 mph for overall fleet considerations.

I like powerful cars, favor higher speed limits, more stringent licensing requirments for drivers, separation of traffic more completely  by actually specifying speed ranges in particular lanes, etc.  However,  I know that there is a fundamental price in energy consumption compared to a system that is optimized for a lower speed.