basjoos-Aerocivic beats GM's-Ultralite

[aerohead]

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.

[Frank Lee]

With that many estimates in there, going to 2 or 3 or 4 decimal places probably isn't accurate.

[DifferentPointofView]

All the numbers make my head hurt worse *stumbles*

nonetheless, its still impressive that a regular guy can make something from "scraps" and even COMPARE to a big automakers design. even beat it with numbers.

[jamesqf]

Originally Posted by aerohead

It's been a while since the 1991 debut of the 100-mpg GM Ultralite.

Never saw that one before. If they'd built it, I would have bought one (though I'd prefer a bit less side glass). Heck, if they'd build it today, I'd buy it.

Which, when you think about it, goes some way towards explaining why GM is doing so well these days :-)

[aerohead]

Originally Posted by Frank Lee

With that many estimates in there, going to 2 or 3 or 4 decimal places probably isn't accurate.

when you run the numbers,my calculator displays out to 13-significant figures,if I do not round,and use the "full display" values,it makes virtually no difference to the end results. The numbers I posted are at a higher resolution than those presented at engineering congresses and I feel that for estimating purposes,they will yield realistic values plus-or-minus say 5%.--------------------------- The whole purpose of books like Hucho's is so folks like us can use the formulas and relationships to interpolate or extrapolate values given some "knowns",to build cars on paper or within the computer,and gauge the merit of particular mods.----------------------- I wish GM had published figures for the Ultralite at a meaningful road speed.Short of that,we're forced to use the tools available to us to glimpse that world.