What makes cars fuel efficient?

[monroe74]

"WEIGHT is a bigger problem than HP"

I agree that weight is very important.

"Although HP has skyrocketed, it has really not outpaced weight by very much over the years."

I don't think that's true. I think our standards have shifted.

"170HP sounds like alot for a small car.... But a 3000 pound car needs this kind of HP just to be able to accelerate somewhat decently."

Here's an example of what I'm talking about. The Chevy Impala was (and still is) one of the best-selling cars in the US. Let's pick the 1980 model, just for the heck of it. That car weighed 3,344-3,924 lbs. The base engine had 115 hp. The optional engines had 120-155 hp.

Assuming the lightest Impala (3344 lbs) and the biggest engine (155 hp), that's 21.6 lbs/hp (and of course that's an overly generous assumption). 3000 lbs with 170 hp is 17.6 lbs/hp. That's what you're claiming is needed. But in 1980 lots of American buyers were willing to accept 21.6 lbs/hp, or something even worse than that.

A 2008 Impala weighs 3555 lbs, and has 211 hp. That's 16.8 lbs/hp. So the car weighs about the same as it did in 1980, but the stock engine is now 36% bigger (in hp) than the biggest optional engine that was offered in 1980.

Here's another interesting comparison. A stock 1980 Corvette had a 0-60 time of 7.1-7.7 seconds. The 2006 Impala has a 0-60 time of 5.7-8.4 sec. So the family sedan now has performance comparable to the 1980 Corvette.

"I think that performance/acceleration is not really the issue."

I've explained why I don't really agree. We have a problem because cars have gotten heavier, but we also have a problem because we've become greedy for hp. And cars with big motors still have relatively bad FE, even if you try to drive them efficiently and slowly.

The car companies don't have much incentive to sell small, light cars with small motors, because those cars also have smaller profits. And selling a hybrid is a lot more profitable than selling a car like a VX.

"even a VX looks sleek and aerodynamic compared to the overly tall small cars you see today"

The VX looks sleek and aerodynamic because it actually is. Its coefficient of drag is 0.32. That's pretty low.

"small cars tend to be MUCH less sleek these days than larger offerings"

I don't see any evidence of this. The new Civic and Prius (for example) are exceptionally sleek.

references:

http://auto.howstuffworks.com/1980-corvette.htm
http://auto.howstuffworks.com/chevrolet-impala23.htm
http://www.motortrend.com/roadtests/...ecs_price.html
http://www.edmunds.com/new/2008/chev...162/specs.html
http://en.wikipedia.org/wiki/Automob...g_coefficients

[VetteOwner]

dont forget those old boats were ment to be towing a camper going 80 on the interstate. While they didnt have much HP those boats had a buttload of torque.

nowadays its a virtual race to see who can get to walmart fastest, who can show off to their neighbors, and bragging rights about hp and 0-60 times, etc...

also something we haven't touched on is rim sizes, nowadays its who can have the biggest rims on your car. Back in the 80's- early 90's cars had narrow small tires. Nowadays a 12-14" rim is unheard of. The public is expecting a ride quality liek sitting on a lazyboy. Only way to do that is increase the tire size and width...Not to mention the interior layout(width, number of armrest, cupholders, etc) hell my chevette doesnt even have cup holders! had an AM only radio with ONE speaker(could get it without the radio) no ac, no power anything...Nowadays if you try to get a car without AC they look at you like your crazy...

i blame todays society really, were all expecting a Cadillac ride out of a pinto sized car, it just aint gonna happen!

[froggy81500]

I don't think cars in general have gotten heavier than what they used to be. They've gone to use a lot more lighter materials, like aluminum, plastics and composites. The difference between a complete cast iron motor and an aluminum one could be several hundred pounds.

The horsepower aspect of it goes with American culture. Its cool, hot and sexy, while econo boxes and ecofriendly cars traditionally aren't. Sure, I'd love to get back into another Mustang, but having a family, I'll stick with the two Saturn Ions we have now. If people bought vehicles that actually suited their needs and not their wants, you might actually see a different picture of vehicles on the road, and fuel costs accordingly. But as is the case with Americana, its always bigger, better, faster that sells. The automakers have known this all along, which is why fuel economy standards, the CAFE standards, hadn't increased in decades. No one will built cars that people won't buy. When you see gas hit $5/gallon just for regular 87, then you might actually see people change their minds about what they drive.

[monroe74]

"I don't think cars in general have gotten heavier than what they used to be."

It's true that certain lighter materials are used, but I think cars in general are heavier. For example, take into account that in many households, the family vehicle is now a pickup or SUV. These are obviously heavy. Minivans are popular, and heavy.

And lots of individual models have gained weight. A Civic now weighs more than an Accord used to weigh. A '77 Accord weighed about as much as my '95 VX. And it got by with 68 hp.

The original Civic (1973) weighed 1500 pounds, and had 50 hp. Civics today start at about 2600 pounds.

Makers have a large profit incentive to only sell cars loaded with toys. And buyers have been happy to go along with this.

[samandw]

I'm kinda surprised no-one has touched on the other important reason for low mpg's in modern cars: Ever tightening emissions regs.

People often equate low emissions with high efficiency, but that is simply not true. Lean burn technology like the Civic VX used is much much harder to do, due to tightening of NOX regulations.

Also, weight HAS gone up significantly in spite of the improvements in technology. Just compare the 2008 Challenger to the 1970 Challenger. The 1970 had drum brakes, steel body, cast iron V8, etc etc, and yet weighed 700-800 lbs LESS than the 2008. Safety regs and extra features are definitely a part of it, but I also wonder if modern automakers just aren't placing much value on weight reduction. . .

[R.I.D.E.]

here is some info from the EPA hydraulic hybrid document:

Losses of energy:

Fuel conversion-engine

25.56 in-7.788 out

Powertrain

7.788 in-6.359 out

Wheel slip

6.359 in-5.507 out

at wheel losses

4.965 Aero and Rolling out
Resist Energy .0962

This is for a military HMMWV but the figures are fairly close for most vehicles

Figures for vehicles (overall averages-passenger car)
EPA city cycle
Brakes-40%
Aero drag-29%
Rolling resistance-31%

EPA highway cycle
Brakes-9%
Aero drag-62%
Rolling resistance-29%

The esitmated potential for improvements
Mild hybrid-20-40%
Full hybrid (series) with conventional engine-60-80%
Future full series (advanced engines,improved aero, LRR tires)-100-120%

They have a test vehcile that weighs 3800 pounds that gets 80 MPG.

I have spent a considerable time over the last 8 years to develop a hybrid drive system that addresses all of the issues with the exception of aerodynamics and tire rolling resistance.

As you can see from these figures the greatest room for improvement is in the engines conversion of fuel energy into output power. The most obvious improvement would be to utilize the wasted heat energy in the engine itself.

From the first figures you can see that a 1% (from negative to positive) improvement at the engine translates to a 15% improvement at the wheels. The engine is where the greatest potential improvement would be possible, and this has been the focus of most R&D, which is really sad. I say this because the 60-80% quoted improvement could be accomplished with no change in the engine.

The simplest improvement would be to use the exhaust heat to generate energy, especially when the catalist needs to operate at 900 degrees to function properly. Put a small self contained steam turbine immediately behind the cat and use the power generated to run all the accessories, including those driven by belts directly from the engine.

My focus is on the powertrain, where the extreme improvements demonstrated by radical hypermiling prove a 100 % improvement over current EPA ratings is not just possible but completely doable. The present record for an Insight is 180MPG, at an average speed in the mid 30s MPH. With a truely infinitely variable transmission (my design incorporates the components in the wheels themselves (replacing the brakes on an equal weight basis- selective 4wheel drive) you have the potential for neck snapping acceleration as well as all wheel regeneration, without sacrificing efficiency). Another major advantage is a significant reduction in per vehicle manufactured components.

In summary I am trying to describe a car that cost less to produce than anything currently available in the US market, while providing acceleration that could reach the limit of the tires ability to maintain traction with the road. That right, acceleration equal to the 4 wheel drive rally cars on dry pavement. Accleeration superior to any 2 wheel drive vehicle on the planet.
In fact the rate of acceleration could be identical to the shortest braking distance of the same vehicle, with every acceleration event reusing over 85%of the energy consumed in braking from the high speed to a stop.

After six years of rejections by 100s of organizations, 6 trips to my congressman, etc,etc,etc,etc, I finally got one person to listen and this fall Va Tech will be building a prototype that will demonstrate the design . The threshold of efficiency is 82% wheel to storage to wheel regeneration. That is about 3 times the efficiency of current electric hybrids. The current state of the art is in the low to mid 70% range.

My design is not an adaptation of anything existing, it's purpose from the beginning was to allow the techniques of hypermiling to be applied in such a way that the engine is hypermiling (no idle-no low efficiency running) while the powertrain stores the engine produced energy to be applies gradually or virtually instantly to the vehicle depending on the operators desires.

How about 0-6 in twenty revolutions of the wheels, about 120 feet. hte weight of the vehicle is relatively insignificant (not totally of course) because you are utilizing that same weight to accumulate more energy storage , every time you decelerate.

In hypermiling you must sacrifice energy to reach your higher pre coast speed, due to the exponential increase in aero drag. This system allows you to select any desired speed, and the engine only needs to run to maintain suffecient reserves of energy to provide on rapid acceleration event.

No idling
No low efficiency operation
85%+regeneration capability

Combine this with minimal weight, reduced manufacturing costs, a much simpler and more reliable powertrain, and of course every possible aero and rolling resistance refinement and you can see the potential.

The perfect machine (of course unobtainable) would get 5 TIMES the current mileage. What I see is about half of that when the potential is fully realized.

regards
gary

[dosco]

Originally Posted by samandw

Also, weight HAS gone up significantly in spite of the improvements in technology. Just compare the 2008 Challenger to the 1970 Challenger. The 1970 had drum brakes, steel body, cast iron V8, etc etc, and yet weighed 700-800 lbs LESS than the 2008. Safety regs and extra features are definitely a part of it, but I also wonder if modern automakers just aren't placing much value on weight reduction. . .

I would disagree. Cost drivers in fabrication are materials and labor. Big companies always seek to reduce costs, and thus I think the carmakers have a very strong incentive to reduce weight.

Of course balanced with this is the need to meet safety requirements.

I do know that many carmakers make extensive use of modern CAE tools to predict crash performance of new car designs. One use of the tools are optimization routines to figure out ways to reduce weight but maintain crash performance.

[froggy81500]

Originally Posted by dosco

I would disagree. Cost drivers in fabrication are materials and labor. Big companies always seek to reduce costs, and thus I think the carmakers have a very strong incentive to reduce weight.

Of course balanced with this is the need to meet safety requirements.

I do know that many carmakers make extensive use of modern CAE tools to predict crash performance of new car designs. One use of the tools are optimization routines to figure out ways to reduce weight but maintain crash performance.

It cuts both ways. Automakers will reduce costs IF people won't buy at the selling price. So long as people keep paying, there really is no incentive to reduce anything. Basic supply and demand. We keep paying they keep making. People keep buying freighter sized SUV's they'll keep building them.

[monroe74]

gary: "Va Tech will be building a prototype"

That's excellent news. I look forward to hearing more news about that effort.

I think you're on the right track. The car should be able to automatically do what a good hypermiling driver does: maximize the amount of time the engine is running at high efficiency, and minimize the amount of time it spends at idle, and at low throttle openings (because that is inherently inefficient).

dosco: "the carmakers have a very strong incentive to reduce weight"

You're right that they save money when they use less material. But it's also true that lighter materials often cost more then heavier materials.

Anyway, more weight means more profit for them. When they make a model bigger, and add lots of extras, that adds both weight and profit.

[GasSavers_RoadWarrior]

Hey R.I.D.E. I like the idea of flywheels too, realised how much more efficient they are than batteries. I wanna build a storage flywheel that fits in a spare tire well, that works as a homopolar motor and generator, and fit two homopolar motor/generators to the rear wheels of FWD cars (Or use the wheel rims heh) brake and it dumps energy to the flywheel, then tap it again for acceleration. Thinking of using a simple mechanical commutator modulation device for power control. Also thinking that low voltage pyroelectric generators could be added to the system and could keep the "charge" topped up on long highway runs... such that you might be able to cut the IC motor for 10 minutes in the hour, cruise off the flywheel, then let it suck up waste heat again for 50 mins. Also there's some low voltage pyroelectric-solar tech that might make enough power to keep the flywheel spinning while you're parked... i.e. park at 9 AM with 6000RPM in the flywheel and at 5pm it's still got 6000RPM to get you going, 'coz it sucked up enough sun to overcome frictional losses. Having auto vehicles though, I might be more inclined to set it up to make enough HHO to run the motor off for a short period of time at highway speed, coz that's easier than killing the motor and figuring out how to run the accessories and transmission pump. (Could make for some interesting P+G variations, like accelerate up to 70 at most efficient BSFC, flywheel brake down to 55, HHO "coast", repeat.)