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Fuel Efficiency

11/28/2015 7:01 AM

What is the relationship between vehicle weight and fuel efficiency? As an example: a 4,000lb vehicle may get 30 mpg in highway use. If the weight were reduced by 25% what effect would it have on mpg?

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#1

Re: Fuel efficiency

11/28/2015 7:24 AM

Simple. The more mass you have the more energy it takes to move it.

That is, all other things being equal, you still have to deal with friction whether it is aerodynamic drag, tires, bearings, road surface, etc.

You also have to consider the type of driving you are trying to compare it with.

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#43
In reply to #1

Re: Fuel efficiency

12/01/2015 7:46 PM

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.

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#2

Re: Fuel efficiency

11/28/2015 8:01 AM

At slow speeds, MASS rules both MPG and acceleration; but at highway speeds, it's AERODRAG that dominates both MPH and MPG.

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#3

Re: Fuel efficiency

11/28/2015 8:30 AM

Considerably less than 25% improvement.

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#4

Re: Fuel efficiency

11/28/2015 8:50 AM

depends on wind direction

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#7
In reply to #4

Re: Fuel efficiency

11/28/2015 9:08 AM

Wet your finger and hold it out the window

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#8
In reply to #4

Re: Fuel efficiency

11/28/2015 9:10 AM

Low Clearance Ahead--14 ft.

Reef sail, matey!

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#5

Re: Fuel efficiency

11/28/2015 8:54 AM

Drag increase is a square function. Double the speed = 4 times the drag.

Acceleration consumes energy that is consumed by the drag, and can not be recovered as the drag slows the speed to zero. Regenerative braking into a battery can recover a portion of the energy, but unless it is an electric car, the saved energy can not be used for subsequent acceleration.

A 25% reduction in weight would save 25% of the acceleration energy, but unless the shape was reduced to reduce the drag, the vehicle would have the same energy requirement for travel at a certain speed. It would reach a speed 25% faster upon acceleration - more or less. Acceleration into mounting drag, with constant drive, leads to an asymptotic approach to the maximum speed at which a certain power will drive the vehicle, at which all energy is consumed by drag and friction at a constant speed.

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#6
In reply to #5

Re: Fuel efficiency

11/28/2015 9:06 AM

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

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#9
In reply to #6

Re: Fuel efficiency

11/28/2015 9:28 AM

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

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#41
In reply to #5

Re: Fuel efficiency

12/01/2015 11:20 AM

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.

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#45
In reply to #41

Re: Fuel efficiency

12/02/2015 11:32 AM

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.

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#10

Re: Fuel efficiency

11/28/2015 9:51 AM

Unfortunately there are a few other factors that nowadays play a pretty significant role in fuel efficiency or lack thereof. Emissions compliance being near #1 in most vehicles.

For example look at the present VW issue.

After that consider that the average newer 3/4 ton diesel pickup trucks now getaround 10 - 12 MPG while weighing in at ~8000 pounds plus have surprisingly good aerodynamics as compared to an older non emissions 80,000+ pound semi truck that gets 5 - 7 MPG in the same driving condition despite having the aerodynamics of a small building.

10+ times the weight plus far worse aerodynamics using the same engine HP ratings and still they only use twice as much fuel.

Hmmm. Things to think about.

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#11

Re: Fuel efficiency

11/28/2015 9:53 AM

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!

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#13

Re: Fuel efficiency

11/28/2015 10:52 AM

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

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#14

Re: Fuel efficiency

11/28/2015 2:09 PM

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

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#15

Re: Fuel efficiency

11/28/2015 10:44 PM

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

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#16

Re: Fuel Efficiency

11/29/2015 4:10 AM

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!!!!!

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#36
In reply to #16

Re: Fuel Efficiency

11/30/2015 9:52 AM

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

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#17

Re: Fuel Efficiency

11/29/2015 7:28 AM

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.

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#18

Re: Fuel Efficiency

11/29/2015 9:52 AM

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.

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#19

Re: Fuel Efficiency

11/29/2015 10:27 AM

In small engines, such as cars and trucks, a reduced weight is a benefit in MPG.

As diesel engines become larger,they become more efficient.

In terms of tons moved per mile,per pound of fuel,trains are more efficient than cars or trucks.

Put your car on a flat bed train car and see how much more fuel it (the train) consumes going coast to coast because of the added weight.

Cargo ships are even more efficient movers of weight than trains.

According the the laws of physics,reduced mass will require less energy to move or accelerate,but in the real world,after a certain size,the efficiency of the prime mover becomes more efficient at converting fuel energy into movement as the engine becomes larger,and the load is matched to the output.

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#23
In reply to #19

Re: Fuel Efficiency

11/29/2015 11:41 AM

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

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#32
In reply to #23

Re: Fuel Efficiency

11/30/2015 8:07 AM

OK,another way to look at this is add the fuel consumption of 1000 cars,each

weighing 3000 pounds,and compare to the fuel consumption of the train,or 100,000

cars and compare to a ocean freighter.

Of course,the whole topic is academic anyway,and theoretically,by pure physics

calculation,a heavier weight will take more energy to move or accelerate.

I do not argue that point,but in the real world,there are more considerations than

pure mass.

Fuel efficiency is a different matter.

Weight is a factor,and rolling friction,and wind resistance,and up to a certain

point,these forces are dominant.

I agree reducing the weight of a personal vehicle will increase mileage if all other

factors are the same.

A single can of soda pop can cost many$$$ over the life of an airframe (20+ years,and millions of miles) if left on board a plane.

So weight does matter.

But after a certain weight, they become minor compared to the increased efficiency

of very large diesel engines.

The largest of these has a bore of over 3 feet,and a stroke of nearly 9 feet,and is a

turbocharged two stroke.(See link):

https://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96-C

Trains have very little relative rolling friction,compared to weight,and added weight

adds little to this friction.

The freight cars also draft one another,reducing total wind resistance.

Trains do not stop-and-go,so they waste less energy.

Once a train is up to speed,it takes a relatively small amount of energy to keep it

going on level ground.

A train traveling 60 mph can coast for miles with the engine off.

Changes in elevation are the main drag,because the engine has to "lift" the load over

the hill.

A railroad engineer has to know the total load and elevation changes of the route to

determine the number of engines required for the trip,not only to pull the train,but to

stop it.

But as you say, all of this is a moot point,and strictly academic.

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#31
In reply to #19

Re: Fuel Efficiency

11/30/2015 7:57 AM

I question your claim that ships are more efficient. I have rowed many boats and ridden many bicycles in my time. On a bike I can pedal and coast for a very long time but even in a tiny efficient kayak you paddle and only coast for a short distance.

I understand the physics that the water displaced by the front is replaced at the back but again, consider a train on level land will coast even at a slow speed for hours while a ship will coast only for minutes.

The problem with trains is they are not using their dynamic braking to generate and store energy.

Drew K

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#20

Re: Fuel Efficiency

11/29/2015 10:27 AM

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

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#42
In reply to #20

Re: Fuel Efficiency

12/01/2015 12:17 PM

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.

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#21

Re: Fuel Efficiency

11/29/2015 11:17 AM

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.

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#22

Re: Fuel Efficiency

11/29/2015 11:31 AM

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

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#24

Re: Fuel Efficiency

11/29/2015 12:29 PM

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 .

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#25

Re: Fuel Efficiency

11/29/2015 2:05 PM

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.

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#26

Re: Fuel Efficiency

11/29/2015 6:32 PM

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

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#27

Re: Fuel Efficiency

11/30/2015 12:06 AM

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.

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#28

Re: Fuel Efficiency

11/30/2015 1:14 AM

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.

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#29

Re: Fuel Efficiency

11/30/2015 3:22 AM

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.

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#30

Re: Fuel Efficiency

11/30/2015 6:27 AM

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?

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#33
In reply to #30

Re: Fuel Efficiency

11/30/2015 8:37 AM

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.

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#35
In reply to #30

Re: Fuel Efficiency

11/30/2015 8:56 AM

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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#34

Re: Fuel Efficiency

11/30/2015 8:42 AM

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#37

Re: Fuel Efficiency

11/30/2015 10:09 AM

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.

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#38

Re: Fuel Efficiency

11/30/2015 11:43 AM

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.

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#39
In reply to #38

Re: Fuel Efficiency

11/30/2015 12:24 PM

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

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#40
In reply to #39

Re: Fuel Efficiency

11/30/2015 3:05 PM

My friend bought a 1/2 ton Ford F150 extended-cab, long bed 4-wheel drive truck with a six cylinder engine and he was totally disgusted with it's performance especially when attempting to pull his 18ft travel trailer up into the mountains.

He took it back and traded for the same truck with a 6.2L v8 gas engine and the same gear ratio rear end which pulls the trailer well and actually gets better mileage than the 6 cylinder when pulling the trailer.

However the 6 cylinder truck unloaded got almost 3 miles per gallon better mileage than the V8 when driving unloaded and on flat ground.

I would think that the loss of efficiency is being caused by the engine failing to achieve and maintain the optimum fuel-to-air/O2 ratio required to maintain the exact required power output for overcoming vehicle rolling resistance under all conditions.

I have noticed every small truck powered by a 4 cylinder engine that I have looked at so far have very low gear ratios which I am sure is being done in an effort to maintain enough power to keep the vehicle owner(s) happy with the performance.

Some of the trucks have had 5:13 and others were 4:10 or 4:11 ratios.

For those that never pull or haul any significant load (empty) the vehicles perform ok on flat ground but when lightly loaded or when driven in mountainous or hilly country they fall far short of performing well.

I personally think that Variable Displacement Engines with Variable Valve Timing controls is the answer especially in the truck market as it allows the torque and horsepower needed for the current driving conditions yet provides the maximum fuel economy possible.

There is nothing more unsafe than an underpowered vehicle attempting to pass a slower moving vehicle on any incline especially on a 65-75MPH freeway in heavy traffic.

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#44

Re: Fuel Efficiency

12/01/2015 8:09 PM

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.

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#46

Re: Fuel Efficiency

12/02/2015 11:49 AM

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.

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