I saw an interesting episode of Mythbusters last night got me thinking. On that episode they filled footballs to 13 PSI first with air and launched them, and then deflated and filled them to 13 PSI with Helium and launched them. When the balls were thrown with equal force the one with Helium would float like a blimp and go farther, right? Well they showed the answer is NO ! The balls with air averaged an inch or so further (negligible give the total distance the ball flew). On a .scale the Helium filed balls were lighter (that makes sense). Bottom line, they determined because the helium balls were lighter, they had less momentum, so the fact they “floated” further was offset by less momentum.



So what does this have to do with Hybrids? Well I figure a hybrid (HCH with 12.3 Gal. fuel tank used here for an example) when full of gas and everything else is equal, weighs 76.875 (12.3 gal x 6.25 #/gal @ 72 degrees F ) pounds more then when it's gas tank is empty.



So does a lighter car necessarily go further then a heavier car, if both have the same amount of energy applied? I have not gone through my first full tank of gas yet, but from my crude experimenting (average MPG driving the same route to and from work) it seems the car is getting worst mileage the lighter is gets. I know we have some real good engineers in this forum that could probably apply some Static and Dynamics formulas to prove which way mileage trends as the car becomes lighter, but since auto makers try to make cars as light as possible, I would expect my results go opposite with what is expected.

Larry
'06 Magnetic Pearl HCH #7264

I'm an architect, not an engineer, but I would think that less fuel = less mass = less force to accelerate. Weight is the biggest enemy of FE. My 1700 lb 85 Civic Si got 40 mpg+.

I suppose with a hybrid more weight also means more energy regenerated from the braking, but with the inefficiency of that system your still going to loose mileage with added weight.

Are you making a case for a wireless "cable car" EV?

Not trying to be funny - that would not be a bad idea on urban tollroads....

61.5mpg lifetime - 82mpg in recent months

Best Run >
www.cleanmpg.com

"fanatic" is what the lazy call the dedicated

It's been an eternity since I took a physics class, but I think it's a matter of F=ma or Force equals mass x acceleration. Take two objects of different masses/weights and accelerate them at the same rate. It takes less force to accelerate the lighter object. In the hybrid's sense, it takes less horsepower (and less fuel to generate that horsepower) to propel the car.

That all being said, the football scenario has a couple things going against it (and I might be really pulling this one out of where the sun don't shine). It's possible that the wind higher up was greater than the wind on the ground. If you did the experiment in a vacuum, the helium football would probably go further than the air-filled football. Lighter objects have more inertia than heavier ones, so a wind will slow it down much quicker than a heavier object. The combination of factors probably added up to the helium football's distance not really surpassing the air-filled one. Mythbusters failure to measure windspeed at the altitude of the ball's trajectory somewhat puts the experiment into question.

Granted, this is from recollection of a Physics class I took 18 years ago, so I wouldn't quote me to Scientific American

Jeff

Umm. Did you guys watch the show. The Helium footballs actually went LESS distance because while it takes less energy to accelerate a lighter object, it also takes less energy to slow it down. Basicly the heavier footballs were not slowed down by wind resistance as much as the lighter one. Try throwing a whiffle ball vs a base ball. See which one you can throw further.

If there were zero wind resistance I can buy that less mass = longer distance, but with wind resistance I think you end up turning things around.

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"

Ikewin,
I am an engineer, but probably don't qualify as a good engineer.

I do agree with your hypothesis, but disagree with your testing methodology. There are so many variables that can occur during driving...... speed, temp, wind, rain, snow, trip length, mechanical break in, tire break in, tire pressure, # passengers, etc that it would be impossible to detect the effect of fuel weight on your mileage. Much less since you haven't gone through a single tank yet.

Even under well controlled testing conditions, the difference would IMO be so small that it would probably fall within the margin of error of the test.

It's a Wonderful Day for Science!

At highway speeds, weight affects mpgs more through "rolling resistance" than acceleration.

Every kilogram you add to your car increases the friction in the wheel bearings and the tyres on the road, regardless of acceleration or slowing down. Try pulling an office chair with the fattest person in the office sitting on it - I guarantee the friction will be higher!

So in every case, more mass equals worse miles per gallon.

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm

This is well documented in the racing industry.... where everything is exagerated compared to road cars. Teams try to use the least amount of fuel possible to help FE and speed.

Kevin is correct about wind resistance (drag) being by far the largest factor to overcome at highway speeds. Being a pilot, I did lots of flight testing, efficiency racing, and aircraft race preparation in the 1980's. Our single engine "Glasair" aircraft were going 242 miles per hour on only 160 hp. The faster one travels, the greater the effect of wind resistance; hence drag reduction is the key. The engineer's challenge is to reduce frontal area, wetted area (surface area) and parasite drag (things protuding).

In my Insights I have attempted to test MPG at varying weights, keeping test conditions identical, if possible. I have never observed a heavier Insight achieve higher FE than a lighter one. That is why I specifically ordered my last Insight without A/C, to save 38 lbs. Also I leave the passengers seat at home when on longer trips.

I believe that when a vehicle is ALREADY at cruising speed, or slowing down, a little higher weight is OK, and the car may actually "penetrate" more or roll longer (farther) under most conditions. But in typical driving, acceleration is so often a factor; therefore, a lighter car will win nearly every time............

2005 Insight

89.3 mpg lifetime

Video camera replaces outside mirrors