Author Topic: What factors play into automobile fuel efficiency?  (Read 2309 times)

Offline scikopas

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What factors play into automobile fuel efficiency?
« on: January 14, 2014, 01:35:36 PM »
I just saw this about a car Volkswagen designed (XL1) that gets 261 Miles per gallon with a 2 cylinder diesel engine and a 20kW electric motor.
http://www.popsci.com/article/cars/volkswagen-xl1-most-efficient-car-ever-0

The article mostly cites reducing weight and aerodynamic drag as to how they built such an efficient car, but I don't believe that those are the primary factors.  What goes into making an engine fuel efficient? I assume this is still a metal engine (as opposed to a high temperature ceramic), so the efficiency boost probably wouldn't be entirely from running the engine at a much hotter temperature like they taught us in thermodynamics class...

any insights from car/engine people?
PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
my (materials) science podcast: LASER (Let's Agree Science and Engineering are Rad!) twitter @scikopas

Offline adrive7

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Re: What factors play into automobile fuel efficiency?
« Reply #1 on: January 14, 2014, 09:05:31 PM »
ooooooooooh. This could be a good show topic....

But those are two fairly massive factors. Especially aerodynamics. Unfortunately engine tech has plateaued a bit with respect to efficiency, so the real gains are with things like hybrid powertrains, aerodynamic improvements, and weight loss. More to come on the show? We should be recording this weekend.
Joe
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Offline scikopas

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Re: What factors play into automobile fuel efficiency?
« Reply #2 on: January 15, 2014, 08:19:09 AM »
sweet, thats exactly the kind of answer I was looking for.

I only know a little about drag forces so I didn't realize aerodynamics actually mattered that much... in that case, my question is updated to ask why are cars (im looking at buses/trucks without those rear flaps particularly) so dang un-aerodynamic?

When I was in undergrad I remember that everybody was really excited about semi-ductile ceramics for engines, so that combustion could take place above 1000C instead of the 700C (maybe?) that it is at now. Because of the larger ΔT, the efficiency would be jacked way up, but ceramic materials that can handle the high stresses of an engine life are still a little out of reach.

anyway, i look forward to learning something from you guys if you decide to talk about it!
PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
my (materials) science podcast: LASER (Let's Agree Science and Engineering are Rad!) twitter @scikopas

Offline adrive7

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Offline scikopas

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Re: What factors play into automobile fuel efficiency?
« Reply #4 on: January 22, 2014, 02:09:10 PM »
Sweet! that was a good explanation, thanks!

PhD student in Materials Science at Arizona State University currently working on high-temperature superconductors and quantum computers or something.
my (materials) science podcast: LASER (Let's Agree Science and Engineering are Rad!) twitter @scikopas

Offline Tragic Story

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Re: What factors play into automobile fuel efficiency?
« Reply #5 on: January 27, 2014, 02:34:47 PM »
A few brainstorming ideas I had:

You mentioned the radiator grill being a huge aerodynamic drag.  What if we borrowed an idea from the computer world and built a water (or other highly thermo conductive liquid) cooling system to cool the engine?  Like a water cooled CPU.
Also, what about gears, gear ratio and tire size?  Does this have any real world effect on MPG?
Finally, my out of the box idea:  What if we just turned long stretches of highways into massive conveyor belts/airport people movers that moved along at a set speed, say 40 MPH, that way if you wanted to fly down the interstate at 80, you would only actually be doing 40MPH.  If you wanted to cruise at 65, you would only need a measly 25 MPH.  Not sure how many nuclear power plants we would need to power it but I can tell you right now...  Watching the squirrels trying to cross that will be hilarious.
[Insert witty comment here]

Offline adrive7

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Re: What factors play into automobile fuel efficiency?
« Reply #6 on: January 27, 2014, 10:45:31 PM »
So, all new cars (except some electrics) are water cooled. Coolant runs through the engine, absorbs heat then is run out through the radiator where it's cooled off, only to go right back into the engine. However, what some cars do to alleviate radiator drag is to have active grill shutters; motorized doors that open and close automatically depending on coolant temperature.

Gear ratios and tire size definitely play a role in fuel economy. Gear ratios are carefully selected by the engineering team to efficiently deliver engine power to the road. Depending on the performance you want out of a car, you can skew the ratio in that direction. For example, Trucks and Sports cars will skew towards a "shorter" rear end (differential) in order to pull heavier loads or accelerate more quickly. A sedan would want a "taller" gear ratio to improve fuel economy.

Similarly, a larger tire diameter will go farther per revolution than a small one. But like everything, there are lots of factors that go into tire size selection. Bigger wheels can improve fuel economy to a point, but eventually the larger wheel weighs too much and will start to hurt. Additionally that weight will start to degrade handling, comfort and performance. Not to mention big tires are more expensive.

The conveyor idea would likely save some gas, as the conveyor would be able to impart some amount of motion to the vehicle, but aerodynamic drag would still be a major player. Plus it would take boat loads of energy to move the conveyor. Better, I think, to have wireless charging embedded in the roadway and charge your electric car as you drive.
Joe
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Offline Psydotek

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Re: What factors play into automobile fuel efficiency?
« Reply #7 on: January 31, 2014, 12:42:43 PM »
One barricade to fuel efficiency is safety requirements that get more and more stringent every year and most of the time that means more weight.

You can have the most aerodynamically efficient vehicle but if it doesn't pass the safety tests it is not going to make it to the market, at least not in meaningful numbers.

Cheap, fuel efficient, safe.  Pick 2.