Which is best for fuel economy: Faster or slower acceleration

Power/Fuel
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21

I think that a comparison of the two is non productive. This entire discussion is more dependent upon the weight of the car, the actual rate of acceleration, the duration of cruising speed once obtained, the initial and terminal rates and the vehicle in question. Over a fixed distance, it takes more energy to accelerate. Case closed.

If you run around accelerating to legal speeds w/o regard to distance traveled at optimum efficient cruising speeds as OP never alludes to, no answer is right or wrong except in general, conservative driving wins out. Cars use more gas in general at higher engine rpms, which is more necessary during more rapid acceleration. If no mention is given to a prolonged 50 mph for example, cruising speed load carried or vehicle in question, you use more energy…go push a car.

22

I think it’s an interesting question.

Since the amount of energy needed to get a specific vehicle to a specific speed remains constant no matter whether you do it fast or slow, does accelerating faster really use more fuel. And, if so, why? As I explained in my post above, I believe it’s because when one accelerates faster the engine itself needs more energy to operate at the higher RPMs that it achieves.

Assuming we know something and explaining it are different things. Sometimes when we try to exlain something we discover that what we thought we knew wasn’t so accurate after all.

I like the challenge, the question “why?”.

23

Since the amount of energy needed to get a specific vehicle to a specific
speed remains constant no matter whether you do it fast or slow,

Here’s a stab at 70s physics memory. I welcome corrections to errors or omissions.

Work = Force x Distance
Force = Mass x Acceleration

That means for “work”, which is the energy transferred to the accelerating vehicle, we get:

Work = Mass x Acceleration x Distance

To me that says when you increase acceleration, the work input must also increase.
That increased work (energy) needs to come from increased fuel consumption.

What am I missing?

24

I am going out on a limb also and say acceleration to 60 mph in 1 second requires more energy than accelerating to 60 mph in 10 seconds. Something like changing speed from 100 mph to 110 mph requires more energy than moving from 10 to 20 mph, but the change was still 10 mph.

25

You’re missing velocity.
In figuring the amount of energy to get a specific mass to a specific speed, velocity is a by definition a constant and distance a variable.

If acceleration is greater the energy consumed in a given distance is greater, but the total energy to bring a spcific mass to a specific velocity remains the same. The distance changes. The energy is amortized over over a shorter distance with greater acceleration.

26

Back in the 1950’s and 60’s there was a publicity stunt performed on an annual basis called “The Mobilgas Economy Run”. Professional drivers extracted phenomenal fuel mileage from the Detroit sleds of that era…Their secret? Perfectly tuned cars and avoiding acceleration and braking if at all possible. Accelerating going uphill was always avoided. The cars were equipped with vacuum gauges and tachometers which let the drivers keep the car in the mileage sweet spot as much as possible…

27

Also being missed.

The more power you demand from a given engine, the more HEAT it generates and wastes. This heat can only come from one place…Every calorie that escapes from your car came from the fuel you paid money for…

28

That’s a great point. If the OP had to do the work himself, he would discover that rapid acceleration tires him out faster than gradual acceleration.

29

Also being missed.

The more power you demand from a given engine, the more HEAT it generates and wastes. This heat can only come from one place…Every calorie that escapes from your car came from the fuel you paid money for…

Really?

That certainly contradicts the dyno results taken from this particular engine which seems to achieve its lowest fuel used per horsepower-hour running at half throttle and around 2500 to 3000 rpm. Full throttle was only slightly less efficient and both half and full throttle was significantly more efficient than 1/4 throttle.


Anyway, I find that knowing when not to accelerate is way more important than exactly how one accelerates. Accelerating towards a red light is a waste no matter how efficiently one accelerates. I do everything possible to avoid unnecessary acceleration, and that means driving as if the brakes don’t work. But, acceleration is a necessary evil in every car trip and when I have to accelerate, I don’t hesitate to put my foot into it and put the engine in its most efficient operating point.

By the way, my 5 speed Yaris typically gets about 43-44 mpg on my commute and my best so far has been 46 mpg. Not bad for a car that’s EPA rated at 29/36 mpg.

30

While not scientific by any means, I’ve checked the mileage for a number of years on my Lincolns under both light and hard acceleration. Both are equipped with a digital readout that will provide real time mileage averages with changes shown every second or so.
This readout has also been verified many times to be amazingly accurate.

The car gets about 19 in town and about 27 on the open road. From a dead stop the cars appear to provide the following.
Under light acceleration the display will show anywhere from 12 to 15 MPG.
Under moderate acceleration it will show 9 to 11, etc.
Under hard acceleration it will drop as low as 5 to 6 MPG.

Very light acceleration at highway speeds (level ground) will drop the display down to the 23 to 25 MPG level with comparable drops under medium or hard acceleration.

Coasting to a stop from highway speeds seems to provide the best mileage under deceleration and oddly enough, as the speed drops down to say 30 MPH the fuel mileage deteriorates more than in the 60 MPH range.

Like I said, it’s not scientific but I’m a real stickler on mileage and pick it to death. Not just for fuel savings reasons but it keeps me aware of any subtle performance issues that may be starting to surface. My opinion would be that easy does it.

There can also be differences due to engine types. Some smaller engines with a narrow power band may really be lugging if one is babying them.

31

Coasting was against the rules in the Mobilgas economy runs. This prevented the drivers from using the more efficient “pulse and glide” driving technique commonly done in the Shell eco runs done on a closed course. In pulse and glide, the engine is run for only a few seconds at or close to full throttle and then the engine is switched off and the car coasts for a minute or so and then the process was repeated.

32

“Anyway, I find that knowing when not to accelerate is way more important than exactly how one accelerates. Accelerating towards a red light is a waste no matter how efficiently one accelerates.”

This is one in which I fully agree. Without stipulating the conditions surrounding acceleration, it becomes an argument and not a debate and theory goes out the window. Going directly from acceleration to deceleration is an example of one of those conditions. Not necessarily because of the loss of fuel during acceleration, but the loss of fuel “by” unnecessary acceleration when “coasting” would suffice and overall average speed may be unaffected.

33

You asked for a clear simple answer … sorry.

The answer is fuel, engine, drive train, vehicle, throttle position, weather, and terrain dependent … along with other things.

It requires a relatively complex set calculus equations to evaluate energy (fuel) consumption ?under the curve? ? a set of integral/differiential equations to generate OPTIMUM operating REGIONS for a specific vehicle (model) configuration.

Some manufacturers have already incorporated shift indicators in their FE manual transmission vehicles that allow the vehicle designer to ?advise? the driver when to shift for “best fuel economy” under conditions at that instant.

With NEW vehicle designs, it is possible for the manufacturer to include ?optimum? acceleration ?profiles? in engine and transmission controllers (letting the controllers do the math to determine the optimum operating point) potentially allowing driver selectable ?fuel frugality?, ?normal?, or ?power/performance?, particularly with automatic transmissions, ? assisting the driver to achieve desired results, whether fuel frugality or performance.

The other thing that I have observed is that generally the smallest displacement engine ?to do the job (including the slowest acceleration)? [i]seems[/i] to result in the lowest fuel consumption.

The rest is up to you and those doing VERY DETAILED vehicle fuel consumption analysis.

34

I ride bicycles quite often and it seems that in general, the faster you go, the sooner you get tired.
I have learned to budget my energy when pedaling a bike. During acceleration and climbing hills, I give it all I’ve got to keep the speed up. During level stretches of road, I pedal at a sustainable rate. During downhills, I stop pedaling and let myself catch my breath for the next uphill where I again give it all I got.
This has shaved minutes off of my trip times by allowing me to travel at a higher average speed.

Going 5 mph for one mile and then going 10 mph for a second mile does not average out to 7.5 mph, it averages out to 6.66 mph.
Going 5 mph for one hour and then going 10 mph for a second hour does average to 7.5 mph.

To obtain a high average speed, it is a lot more important to keep from going slow than it is to peak out the top speed. That’s why you see the pros stand on the pedals during accelerations from a stop and while climbing steep hills and letting their bikes coast on steep downhills. It’s not that they can’t increase the speed by pedaling, it’s that that energy is better saved for the next climb.

35

So there is some speed and load (torque) where the engine is most efficient.
Ignoring the rest of the drivetrain you would want the engine to stay at this “sweet spot” during acceleration.
You’d also want the engine at or near this point when cruising.
So there’s conflicting desires right there, at least with a single engine of fixed displacement.
Since cars normally spend a pretty small fraction of a trip accelerating that may only have a very small effect on overall MPG.

36

Years ago and auto magazine journalist took a drive with Jackie Stewart the racing driver in a Ford Taurus through city and suburban traffic. It was the most boring ride he ever had, according to him. Jackie Stewart, like Wayne Gretsky in hockey, looks ahead several blocks and hardly ever accelerates quickly.

Drivers like that get very good mileage and long engine and transmission life.

Forget the theoretical physics and engine curves, driving with an egg between your foot and the pedal yields the best gas mileage. Most automatics are programmed to upshift depending on throttle position. A light touch on the gas will cause a quicker upshift.

37

The idea that hard acceleration saves gas when compared with the time tested “drive like you have an egg under the gas pedal” reminds me of people who say that when low on fuel it is better to race to the gas station as this will mean the engine is running a shorter period of time.

38

VERY INTERESTING IDEA!

IF you have access to a vehicle that has instant and average trip readouts that can be reset to ZERO, cruise control, and a 3 to 5 mile level course with established start and end points without stop signs/lights in between so that uniform cruise speed can be maintained, then you could run your own experiment with several trials. I recommend a minimum of 3 trials using high, medium, and low accelerations.

Set cruise to the planned cruise speed, reset mpg readouts to zero at the start point, accelerate at maximum and just before you reach “planned cruise speed” activate “RESUME” to engage the cruise control for the remainder of the course, imediately stop and record the data from “trip mpg”. It is probably of greater value (more meaningful) to divide “trip mpg” into one (1) in order to get gallons/mile.

Reset the “trip mpg” and repeat the proceedure using very easy acceleration to “RESUME” cruise speed to the end of the course. Again, stop immediately, record “trip mpg”.

And when convenient, calculate low acceleration “gallons/mile” and compare to the high acceleration results.

IF … you wish … you could do a “trial” at an intermediate acceleration just as a check.

If you have someone with you that can measure your time from 0 to “RESUME” cruise speed then you can plot “gallons/mile” versus “acceleration time” on grid paper to see IF there is a linear (straight line) relationship between “gallons/mile” and “acceleration time”.

I suspect that HIGH, MEDUIM, and LOW points on the graph will actually be a curve instead of a straight line.

The experiment is certainly all doable.

However, keep in mind that these results apply specifically to that specific “test vehicle”, and [b]MAYBE[/b] that model configuration. Other makes/models will probably get different results. But the trends will probably be similar.

IF … someone suceeds in doing the experiment, I hope they will share the results!

It should be interesting …

39

You’re not taking the peculiarities of internal combustion engine into account. Each one has a different operating range where they are most efficient. Also for a gasoline powered engine, the engine must overcome the pumping losses that the throttle plates cause (at less than wide open throttle), which further hurts efficiency.

40

B.L.E. … GOOD POST!

First, using your dyno characterized ICE, it appears to me that 1/2 thottle between 2k and 3k is the “sweet spot” for minimum fuel consumption (LB/BHP/HR) which results in an output of 30 to 40 HP … probably more than adequate for “constaint speed cruising”.

Is the typical driver sufficiently skilled to hit those shift points with a manual transmission … particularly in the adsence of a tachometer?

Now, what happens to HP “demand” during aggressive accelleration? Don’t F = Ma and W =Fd come in to play? Isn’t higher HP required for higher accelleration? Isn’t that the main reason the “heavy pedal” folks like high HP high RPM engines?

Since this is a dyno test of (I assume) a bare engine … how does drive train efficiences interact during various levels of accelleration loads compared to steady state cruising?

Simplistically, F = Ma and W = Fd apply specifically to the total vehicle mass. During “cruise” (constant speed) accelleration is “zero”. How does this work into your thinking?