Fuel Efficiency

High efficiency regenerative braking minimizes the impact of vehicle mass on throughput efficiency.

The increased energy required to accelerate and translate through gravitational potential can be recovered and reused. Because of the relationship of prime mover efficiency to the efficiencies of regeneration significant mass and volume fraction increases can be compensated for through regeneration and power averaging.

Where combustion energy saved approximates regenerated energy / prime mover efficiency fraction.

Considerably less than 25% improvement.

depends on wind direction

Low Clearance Ahead--14 ft.

Reef sail, matey!

Thanks! "what if" the weight of the vehicle decreased as the speed increased?

rolling drag would decrease with weight (the drag associated with wheel flexure, oil drag. cog drag, drive train drag etc), but rolling drag and air drag would both increase with speed.

Rolling drag is fairly small in high pressure tires compared to air drag

"if the weight of the vehicle decreased as the speed increased" that'd be a horse of a different color and you'd be in Wonderland.

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What if the Regenerative Braking engaged a pump that feed an hydraulic accumulator whose release at acceleration powered an assist to the process? Therefor the recovered energy could be allocated to reducing fuel consumption at acceleration.

Engineers did indeed try this back in the early '80's.

They used off-the-shelf parts,available at the time.

They removed the transmission, replaced it with a hydraulic pump/motor,added a

spherical reservoir,added a clutch to engage,disengage the motor when pressure was

needed.

The motor was switched off when not needed and restarted using hydraulic pressure.

The switching was calculated and performed with a Z80 microprocessor(primitive by today's standards).

The motor was run at speed for maximum efficiency.

Overall mileage was much better in the city,and nominally better on the highway.

Total weight was less than standard tranny,and parts for modification were cheaper than the standard system.

As for why this never caught on, blame it on the cheap price of gas at the time,and inertia from Detroit.

I believe the article was in Popular Mechanics at the time,but I am not sure which year.

Anyone that has a collection of old PM mags could probably find it.

Considering the advance in engine technology and electronics,it could be a viable

alternative for public transportation,such as buses,etc.

Maybe even personal vehicles.

There is more than one way to pluck a Wood Duck,or skin a Woodchuck.

Way back when, we had a rule of thumb in drag racing...for every 100lbs you could take of the vehicle (and driver!) you could reduce your ET by approximately .01 seconds. I used to strip my everyday driver of seats, spare tire etc., and leave this big pile of stuff in the pits...then I'd race!

Find a half ton pickup truck and do a comparison....It would matter if it's diesel or gas...but not likely to find a 30 mpg in gas, and stop and go driving or highway cruising will make a difference...The Ram 1500 EcoDiesel 3.0L V-6 HFE comes close...29mpg half ton ...although the weight is a more realistic ~5200 lbs and 6200 lbs loaded, probably not more than a few mpg difference....

http://www.dieselhub.com/halfton/ecodiesel.html

If I was you and wanted to really get a feel for the relationship between weight and fuel efficiency, (power) find a private pilot that owns something like a *Cessna 150 (prefer high wing, less damage), that would be willing to join in your little experiment. Load the aircraft to max take of weight, then take off, noting power settings, RPMs, fuel burn, rate of climb and air speed. Then take note of the same settings at cruise trim and air speed, (less rate of climb).

Land, remove 25% of the max take off weight, (refueling will be required), take off and repeat noting power settings, RPMs, fuel burn, rate of climb and air speed. Note the same setting at cruise trim and air speed.

While in this configuration, find an open field and **start throwing excess weight out the door and note power settings to maintain the same cruise air speed.

Land, remove all excess weight with only 1 hour of fuel for the last flight. take off, noting all the same parameters.

Land, and plot your findings.

* Have your wallet readily available, av-gas isn't cheap!

** reason for preferring high wing aircraft

Large cargo ships have much better fuel efficiency than any car,and so do trains.

Newton's law say Force * Distance = Work done in Joules.

Force= Mass * Acceleration.

Distance traveled = Acceleration *(Time^2)+ Velocity *Time.

Put together all these equations and you can see how mass, work done are related. Energy in joules is same as energy you have to put in y burning fuel, assuming 100 % efficiency.

Hope this will tell you how reducing mass by 25% directly results in saving energy or fuel used to run the vehicle.

I studied these equations 55 years ago!!!!!

You left out the much more complicated, and at any significant speed, more important, friction components, especially air friction (drag).

Most of the auto mfrs are exploring to change over to plastic or aluminum components. So that weight of vehicle can be reduced and mileage is increased.

I've been having PMs with Bruce and he seems to be confusing a reduction in aero downforce with a reduction in mass.

I've explained the difference to him and the affect that reducing the weight would have on (mostly) traction and handling.

I have not heard back from him since last night.

I don't know if he has communicated with anyone else here.

I have to take exception here.

The effect of adding (or subtracting) the weight of one vehicle to a 30,000,000 pound train will be hard to determine.

And, since the OP does not plan to remove weight from his single vehicle, this is all moot anyway.

See #18

Dear Mr. Bruce Gardner,

There are several factors that have impact on Fuel Efficiency of a vehicle.

Once such factor is the Frictional Co-Efficient for the wheels. Under Identical conditions, the force required to over come the friction and the vehcile to move is governed by Equation F = µ x R,

where F = the Min force required to make a move,

µ = the Friction Co-Efficient and

R = the Reactive Force acting opposite to gravitational force

(I am sure about the equation, but the explanation for R - I do not correctly remember - CR4 Members may guide me in this matter)

From the above equation it is clear - higher the weight of the vehicle higher will be the fuel consumption

DHAYANANDHAN.S

I'm not quite sure why some have marked your comments as OT, except that the equation you provided is the equation for sliding friction, and a car is rarely (hopefully never) sliding. The R value you mention is the force normal to the surface. On a flat, Horizontal surface that force is the weight of the car. On a slope, it is the weight of the car multiplied by the cosine of the slope.

In any case, all this equation tells us is how much parallel force the tires can exert on the road before they slip. It has nothing to do with mileage of the vehicle.

If you were in an electric vehicle with "perfect" efficiency during acceleration and regenerative braking, then in theory the vehicle weight would be negated. Any additional power required to accelerate the additional mass would be recaptured during braking.

As others are saying, this is not a simple question. In automotive, the general rule of thumb is for a 10% weight savings, there is a 6-7% improvement in overall fuel economy. But, depending on city or highway driving, vehicle mass plays a differing role. Once at highway speeds, the frictional impact from tires, powertrain , aero and other become key factors.

If the weight savings is combined with the secondary benefits, including a reduction in powertrain size, this compounds the fuel economy savings. Smaller engines, which use less fuel, can be used with equivalent performance.

There is a good paper from NIST which explains many of the factors regarding weight savings for not only automobiles, but trucks also. Sorry, I was having trouble adding the link, but below is the title for the paper, which you can search.

Joost-W-DOE-VTP-NIST-ASP-AHSS-Workshop-R03.pdf

It helps , in your question is strictly an engineering one , to change the measurement of fuel efficiency from MPG to Hours per Gallon. Engine operating hours . A set for cruise engine setting , as you would find on a boat , or an airplane , means that the RPMs are constant . Starting here it's much easier to calculate fuel efficiency . Say start at a fuel consumption of 10 Gallons per hour ( of engine operation .) One hour will consume 10 gallons of fuel . If you want to increase efficiency , that number must be reduced . A 4000 # vehicle uses a different amount of fuel depending on what speed it's going. A lighter vehicle can indeed cut fuel consumption dramatically , and cost . There is a mathematical equation ( like Joules per Hour ) which will give you a hard number , but parameters all must be met . Speed , Distance travelled , OAW ( overall weight ).
Physics is a hard science , you provide hard numbers into a fixed energy equation , and you will get different answers . Your 4K pound vehicle will be most efficient at what speed ? 30 mph ? 40 ? you need a distinct velocity number to benefit from equating speed , weight and energy consumption . At 30 MPH , you may discover that the engine RPM is only 2000 RPM , relatively slow for a car . This , when plugged into a gallons per hour at a certain RPM , will show a constant number , so many gallons burned per hour of engine operating time .

Don't know how relevant it might be to *current* vehicles, but back in 1961-1967 a Ford Motor Company engineer came up with this logarithmic-form equation for estimating fuel consumption (161 car-sample):

c = K*(wt^0.3067)*(cid^0.3469)*(ar^0.3395); R^2 = 0.932

where:

c = gallons-per-mile

K = 5.248 x 10^-4

wt = weight in lbs.

cid = cubic-inch displacement

ar = axle ratio

• source: SAE Automotive Fuel Economy, Vol. 15 (PT-15), 1976, "Factors Affecting Vehicle Fuel Economy," by Clayton LaPointe, Ford Motor Company, page 105.

If you take off the aerodynamic structures from the vehicle, reducing its weight, the fuel consumption goes up, Mildred.

You can try this experiment with your own vehicle: place a known quantity of sandbags throughout your car, drive 50 miles and record the consumption, then reduce the load by 10% and record consumption, repeat until the vehicle is empty.

Your biggest problem is how to accurately measure the fuel consumption for each run, and in reality you would need multiple runs at each loading. However I'm certain that the heavier the vehicle the more fuel will be consumed.

The auto companies spend tons of money on this, and by way of comparison, you could look at the Chevy Cruze story where they lightened the vehicle by over two hundred pounds in order to boost the mpg ratings of their "eco-friendly" models, even to the point of reducing the fuel tank capacity by 3 gallons to reduce the weight of the fuel. They have also taken out the spare because carrying its weight consumes significant fuel over the car's lifetime.

In 1999-2000, I ran a new International truck. It was well equipped with fairings, a big high sleeper, Cummins N14 435 HP engine pilling a 48 ft reefer. It ran from Missoula, MT to Seattle and back and to Portland, OR and back to Missoula each week, grossing as close to 80,000 lbs as possible. This is tough mountain driving. It averaged a faction over 7 miles per gallon.

My 93 3/4 ton 454 Chev pickup gets about 9 miles per gallon. It weights 6600 lbs set up for towing. So it would take about 12.12 times of that pickup to equal the semi truck. Calculating that out, the pickup would have to get almost 85 miles per gallon to equal pound for pound what the semi truck gets.

Trains and ships are more fantastic. I'll let someone else figure that out. Will pickups ever get close to the pounds/miles of the semi trucks...probably never.

Not a big effect on highways, maybe if loading improves aerodynamics, you can reduce drag and increase mpg. But, if you design a 25% lighter vehicle, you reduce the size of tires, engine, gears,etc., all the drags associated, and that's where you increase mpg.

4000lb? 30mpg? Pull the other one; it's got bells on.

Please explain how a Chevy Cavalier can travel at all with 25% of its weight missing?

Very few Cavaliers travel at all.

I had one,and I used to call it my Dr. Pepper Car: 10-2-4.

(This reference may be lost on the newer generation,if so,look it up.)

Work on it 10 hours,drive it 2 hours,let it cool or 4 hours,the work on it for 10 more hours.

Worst car I ever bought.

Last GM vehicle I will ever buy.

Fool me once,shame on you.

Fool me twice,shame on me.

IMHO:GM is more interested in financing vehicles than in making them.

After GMAC was formed,quality went downhill.

Of course,as always,I could be wrong.

Simple:

Put it on the top of a cliff and push it over.

(Preferably with a full load of politicians or lawyers on board).

At least you will get the metal salvage weight for the car;

No redeemable value for the load.

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I just completed a 503.7 mile trip in my 4,500 lb diesel VW Passatt, and got 42.52 mpg. I bought fuel fuel at the beginning and end of the trip, so the 4-digit precision is warranted, if the odometer has that precision. It has the original tires at specified pressure, so the odometer should be pretty close. That trip included roughly 10,000 feet of altitude change (10,000 upward and 10,000 downward), many miles of very curvy narrow, but paved, roads, a tiny amount of driving on snow/ice, and also a considerable amount of driving at 75mph.

BTW, I had no problem getting it smogged last week.

Interesting question.

Back in 2007 I had a new company truck which was a 1/2 ton rated, GM product with the 5.3L V-8 and VVT fuel efficiency control.

The truck slow speed driving average mileage around town and on of-road conditions was 17.5 MPG and the on-highway average mileage at 65MPH average speed was 21-22MPG.

We have a very large number of different types and different sized vehicles in our motor pool.

The much smaller Colorado trucks with the in-line 5 cylinder engines driven under the same conditions turned in 18MPG slow speed and 20-21MPG on-highway mileage at the same 65MPH speed.

I do not understand this phenomenon as the larger truck that weighs at least 30% more than the smaller truck actually got better mileage at highway speeds and only suffered a 3.6% loss of mileage at slower speed operation.

If it were simply weight, the smaller truck should have at least 30% higher fuel mileage than the larger truck at all operating speeds and under all operating conditions.

I firmly believe that automotive manufacturers produce vehicles that meet whatever fuel efficiency numbers are required by the federal regulations and definitely could do much better in many instances.

I see the same phenomenon taking place in cars and larger trucks wherein the vehicle even though lighter and cheaper constructed does not significantly improve fuel consumption numbers.

While the four cylinder cars do get somewhat better mileage at slow speeds and city stop & go driving they do not do well when driven in mountainous and/or hilly terrain especially if the speed limit is above 55MPH.

My nephew's new Ford F250 diesel truck has less horsepower and less torque, yet it gets 7MPG less than his older (pre-low sulfur diesel) Ford F250. This coupled with the Urea use requirements has drastically increased his vehicle operating cost.

I am also quite certain that fuel quality, and fuel additives drastically affect engine efficiency.

I think aerodynamics, engine design, vehicle weight, vehicle service use, and fuel quality are a few of the items that affect fuel efficiency.

I noticed the same phenomenon with a smaller lighter truck getting poorer fuel economy at highway speeds than a bigger heavier one.

My thoughts are on the air resistance. You have to try harder to fight the air (increases exponentially) and the bigger trucks have more powerful engines that can simply do the work with less effort.

This could be a gear ratio effect too, that puts the transmission in the right spot for the engine's peak power efficiency.

I have been thinking for years we need hybrids that don't use the motor for locomotion, and only for generation. That way the motor could be optimized for the generator and only ran when needed. You might even replace it with a small turbine that burned many fuels.

Drew K

Mass is directly proportional to energy in gravitational potential. Therefore the effect of mass on energy consumption increases in non-regenerative systems directly as the number of grading cycles.

Mass is directly proportional to energy in acceleration. Therefore the effect of mass on energy consumption increases in non-regenerative systems directly with the number of stop/go cycles

Because increased mass implies increased volumes there is some effect on the aerodynamic cross-section variable of the air drag equation because the vehicle must be bigger.

Mass is a direct variable in road drag.

The combustion energy saved in regenerative capable systems approximates regenerated energy / prime mover efficiency fraction. This means that significant mass and volume fraction increases can be compensated for through high efficiency regeneration and power averaging.

The main increase in efficiency of the larger engine is in in thermal efficiency.

A lot of energy is lost in smaller engines is due to heat loss to the heads,etc.

The longer stroke of the huge diesels allows more time for complete combustion of the fuel.

The piston speed is also very slow,adding to the efficiency.

They also turn slower RPM,from around 100 to 200 rpm,and are usually 2 stroke

engines,producing power on every stroke.

The huge diesels also use #6 fuel oil,which is almost like tar,and has to be heated to

be pumped.

The most efficient of these is an opposed piston design with no head;pistons

compress the fuel between them,minimizing the heat loss.

Down sizing these features in not feasible due to weight and space considerations.

Also the ratio of weight to load is not feasible on small vehicles,like cars,etc.