It's been a while since the 1991 debut of the 100-mpg GM Ultralite.Now that the "wow" factor has dimmed,I spent some time with the calculator as an academic exercise,comparing GM's wunderwagon,with Basjoos'Aerocivic,which I feel is the benchmark for ecomodding to date. For the rolling-resistance of the calculations,I used data from my CRX testing at Chrysler,then double-interpolated for the difference in mass and RR between the CRX and Ultralite and between my "conventional" steel radial all-season tires,and the 50% less RR concept tires of the Ultralite.From this I was able to estimate RR road load horsepower requirements for the Ultralite at 50-mph,and 70-mph. ------------------------------------
For the aero portion I estimated the frontal area of the GM car at 19.416 feet squared (86.35% of it's gross frontal area,as compared to GM published % for the EV-1). GM published Cd0.19 for their car. ---------------------
GM also rated the 100-mpg at 50-mph.--------------------
Looking at the 50-mph environment,I estimate the power needs for RR at 1.192-horsepower and aero at 3.193-hp,for a road-load requirement of 4.385-hp.------------------------
Using a powertrain efficiency of 95%,then the engine load would be 4.615-hp at the flywheel.----------------------
At 100 mpg and 50 mph,the car consumes 0.5-gallons per hour.-------------------------
At 6.1626 pounds per gallon,the car burns 3.0817-pounds of gasoline per hour.---------------------
3.0817 pounds divided by 4.615 Bhp yields a BSFC of 0.66764 pounds per horsepower/hour.----------------
Moving to 70-mph,I calculate that power for RR would rise to 1.6708-hp,and aero power requirement would rise to 8.764,for a road-load hp requirement of 10.434-hp.--------------------
Again,dividing by 0.95(for mech.efficiency),the engine load would be 10.984-Bhp.--------------------
Multiplying by BSFC yields 7.3307 pounds of gas.------------------------ dividing by 6.1626 lbs per gallon yields 1.1895 gallons.------------------------
dividing 70-miles distance by 1.1895 gallons yields 58.845 mpg at 70-mph for the Ultralite.Not quite as dramatic as 100-mpg at 50-mph.-------------------
No doubt,the BSFC shifts with velocity so I admit all this is only a quess.That's all engineering is anyway.If anyone can shoot some big holes in my methodology I'd be appreciative,as I'm still trying to learn.----------------------
It does appear that basjoos has accomplished something quite remarkable when you compare the numbers.
