Which is best for fuel economy: Faster or slower acceleration

Power/Fuel
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41

Wow!

This has been a fascinating thread to follow and my sincere thanks to all who participated. You’ve given me much to think about and I’ve learned a lot. Thank you!

One contributer’s idea really stood out for me, though, in regard to my original specific question: That automatic transmissions (my interest) are designed to shift to the next higher gear based on optimum engine speed.

Duuuuhhhhh! I should have realized and considered that but I’m glad to have it called to my attention.

My personal observations indicate that accelerating too slowly causes the transmission to linger in lower gears longer and lower gears are not typically designed for fuel economy so much as for getting the mass underway.

Similarly, accelerating too fast does indeed keep the transmission in the lower gears longer because the open throttle sends a message to the computer that speed is more important than economy. The engine is allowed to rev higher to gain speed faster and that results in shifting later than if moderate acceleration is used.

So, for me, in my own simple way, the bottom line appears to be “moderate” acceleration will serve my needs the best.

Aha! But what is “moderate”?

BTW, all of this assumes level ground and all other factors being equal so I’m recogizing the variety of variables that could come into play but, for this exercise, I’m ignoring them and concentrating on just acceleration (which no doubt makes me a simpleton but at least then I can understand things a little better).

Back in the carb days, specifically the four-barrel variety, when you stepped on the gas hard enough to open the second set of barrels, you could both feel and hear the change in engine dynamics. It sounded like the engine was going to suck the hood right through the air filter and you had to climb back into the front seat to control the car.

EFI changes the rules, of course, and computer-driven components change them even more but, from what I’ve read here relating to my specific situation and vehicle, I think that “moderate” means listening to the engine to sense when the speed is just short of requiring the engine to take a deeper breath and push harder and that would be the optimum speed for acceleration.

Automatics can’t lug so that’s not an option or a concern but they can be made to shift sooner than later and it behooves the operator to strive to get the shifting to happen as quickly as possible, which precludes both extra-slow and extra-fast acceleration.

It is possible - although not necessarily practical all the time - to accelerate somewhat aggressively from first gear to second, purposely lifting the throttle to allow the transmission to shift gears, then do the same from second to third, etc. With an automatic, that’s probably optimum.

But, practically speaking, you’d come pretty close by just striving to work with mid-throttle so the transmission shifts as quickly as possible

That satisfies me, folks, and I thank you all for helping me to think through all of this.

bc

42

Why would a automatic transmission the “lingers in lower gears” be automatically bad for fuel economy? perhaps this lingering is happening because the load on the engine dictates it. It is not just engine speed that determins when a shift should occour, engine load is also a parameter.Lingering in a lower gear is better than lugging in a higher gear.

43

…automatic transmissions (my interest) are designed to shift to the next higher gear based on optimum engine speed.

Sorry, but there is more to it than that. On most cars, the throttle position also influences the shift points. With a wide open throttle, your transmission knows you are trying to accelerate rapidly, so it shifts at higher RPMs than it does if you are easy on the throttle. If you go easy on the throttle, the transmission figures you are looking for smooth economical acceleration, so it shifts at lower RPMs. Some cars, like my parents’ 1989 Nissan Maxima, have a switch that will allow you to alter the shift points. The “sport” setting will make the shift points at higher RPMs, and the “comfort” setting shifts at lower RPMs.

I am sure I am leaving out other factors that influence the shift points. Today’s automatic transmissions are more complex than you think.

44

Like I said in my earlier post: DI; dual clutch transmissions; better sensors; and VERY SMART engine/transmission controllers … really change the problem/solution … provided the OEM is willing to “program in” what they know about vehicle/power train characteristics as operator “selectable operating profiles” like “Sports/Power mode” and “Fuel Economy mode”.

I suspect an “off-the-street” driver with a vehicle equiped with this level of “smart control” system could best the desired objective over a “real driver” in an indentical vehicle with the same engine and transmission minus the smart controller with “selectable operating profiles” provided hyper-miling is not used.

Just my opinion …

45

I think that the point made earlier is the critical one here, that point is that so little time is spent in the acceleration mode that placing so much emphasis on its effect on mpg (like is being done here) is an error.

46

The only way to use a significantly lower amount of energy needed to accelerate to your cruising speed is to lower your cruising speed. At 55 mph, your vehicle has only 84% of the kinetic energy it has at 60 mph which means it takes only 84% as much energy to accelerate to that slightly lower cruising speed and you only throw away 84% as much energy when you brake to a stop.

When I drive down a road with a four way stop sign every couple of miles, instead of slowly creeping up to high cruising speed, I pulse up to a lower cruising speed. This works particularly well with motorcycles which have low mass but high drag.
The advantage of driving this way is that you achieve nearly the same average speed of people who are not particularly trying to get good gas mileage and it is a lot less likely to incite road rage from other drivers.

High peak speed is one of the world’s most overrated time savers. If you pass someone who is only going 60 mph so that you can drive the rest of the mile to your destination at 70 mph, you have only saved about 8 seconds of time. High peak speeds really guzzle the fuel without significantly raising the average speed of your trip.
Slow acceleration on the other hand, devastates your average speed. For that reason, I choose to accelerate rather rapidly to a lower top speed and my excellent gas mileage from both my motorcycle and car seems to bear out my theories.

47 
 B.L.E.'s post is excellent.  In an idealized system, it takes the same amount of power to accelerate to a certain speed -- but engines are not ideal, they have brake specific fuel consumption which complicates things, i.e. they use more fuel to generate the same power under various conditions.

 If you blip the throttle quickly, the fuel injection system injects a squirt of fuel (like a "accelerator pump" on a carbuereted car) which wastes fuel.  Full throttle runs a full throttle enrichment to gain a little extra power, but uses more gas.   So as you know, don't floor it.   Doing that super slow acceleration some people do, besides pissing everyone off behind you, is probably less efficient too -- (ignoring for now coasting down hill) cars tend to use the least fuel when cruising, so spending all this time slowly accelerating can use more gas then getting up to speed and cruising a larger percentage of the time.   Probably a moderate acceleration is best.

 Of course, the big exception -- if you're dealing with traffic lights, whatever acceleration gets you the most green lights is best.  Anything is more efficient than starting up from a dead stop over and over.
48

While we’re at it, why not compare a Prius and a fighter jet? It’d return just as useful results.

The OP wanted to know about acceleration changes on the same platform, not whether or not a sports car would out-do a van in fuel efficiency. BTW, here’s one that’ll blow your way out of the water. Compare a GT-40 to a Tesla. Similar weight and shape, right? :wink:

OP: JustDoIt is right. There are a lot of variables to consider when asking this question, not the least of which is “What car are we talking about here?”

A few additional thoughts: Some cars develop their power high up in the RPM range. That’s why you don’t see fuel savings when driving an old Honda “efficiently” by always shifting below 1500RPM. The engine is developing just about nil for power at that RPM, and so you’re burning gas with little return, whereas if you rev it higher, you’re actually getting some power out of that fuel use. On the other hand, if you drive around in a Bugatti Veyron with the pedal floored you’ll be out of fuel in about 12 minutes, while gentler driving extends your range considerably.

Those are just extreme examples of why your question does not have a simple answer. As you said “the odds of being able to drive consistantly one way or another” are low, the answer really doesn’t matter much for real world situations. Just accelerate without taking forever to get up to speed, and don’t floor it unless you have to.

49

“if you’re dealing with traffic lights, whatever acceleration gets you the most green lights is best. Anything is more efficient than starting up from a dead stop over and over.”

Bingo! Stop and go traffic is where this is really the issue. While it might not make much difference when one is accelerating up to highway speed, it make LOTS of difference if one is going from one light to another. If the lights are timed poorly, gentle acceleration and early braking saves LOTS of gas, as opposed to rapid acceleration and hard braking.

50

One of the magazine shows (I think 20/20) did this test…

The two identical cars…and tested them to ensure they both get about the same mpg…then they gave one to a guy who’s a very conservative driver and one who (lets just say spirited driver).

After one month gas mileage was checked…the conservative driver consistently achieved 20-30% BETTER gas mileage then the other guy.

51

B.L.E., Caddyman was right, but in truth heat is an end product. Heat is one of the energy forms that the engine turns the energy contained in the gasoline to, the other being motion (in the form of inertial energy in the spinning engine parts and torque). Some of the heat energy is sent out the tailpipe, some is dissipated via the cooling system and radiant energy, some is captured in the cat converter for use in converting baddies to goodies.

In short, while there is a direct relationship between the amount of fuel entering the cylinders and the generation of heat, there really is not a direct relationship between the heat generated and gas mileage. It’s more complex than that. It’s a question of how much of the energy in the gas molecules is converted to forward motion rather than other forms, and gear ratios and other factors affect that.

The curves you posted are an excellent illustration that relationsahips are not constant, and are nonlinear, but it’s also true that these curves will vary widely for different engines. Some engines will be more efficient at lower RPMs, others at higher RPMs. It’s also true that the efficiency of the engine itself is only part of the mileage equation. Gearing, rolling resistance, and aerodynamics are factors also.

52

And THAT is the bottom line. All factors considered, conservative driving has been proven time and time again to be more fuel efficient. But less FUN…WHEEEEEEE!!! (sorry, couldn’t resist).

53

This has been alluded to elsewhere, but I think it merits emphasizing. For city driving, the acceleration method that maximizes fuel economy is the one that minimizes braking. When you slow down and stop, you really lose fuel economy. You want to eliminate that. You want to cruise at a speed that hits every light green, so when you get off the interstate you hit the first light either red or green (that’s random), but from there you just cruise down the street at whatever speed the lights are set at, and you accelerate at whatever rate is necessary to cruise at that speed.

Unless you live in a low tax state that considers it unnecessary to provide such services to its citizens. Or are you high tax staters so jealous of us with low taxes that you are now doing away with such government waste as synchronized lights?

54

Since cars normally spend a pretty small fraction of a trip accelerating that may only have a very small effect on overall MPG.

Actually, except for city driving, that’s true. Where you get pounded is on repeated red lights, or stop signs. Then your small fraction could very easily be 10-20%, and since the penalty is in the neighborhood of 50-100% (maybe), your fuel loss for driving in the city can be up to 20-40%. (Mine is about 25%.)

55

“Of course, the big exception – if you’re dealing with traffic lights, whatever acceleration gets you the most green lights is best. Anything is more efficient than starting up from a dead stop over and over.”

The distinct advantage of hybrids where an electric motor running at 90% efficiency W/o need for gear changing and with regenerative braking, all work in it’s favor to help mitigate the problems of driving in stop and go traffic. I would like to hear from an EV owner on this topic. IMO, quicker moderate acceleration within reason using an electric motor “may” be more of an advantage than using ICE with a transmission.
Then, maybe not.

56

I can tell you what I have observed in my Prius: if I accelerate slowly, the engine uses 100% batttery; if more quickly, the gas kicks in. Does that tell anyone anything?

57

De ja vu’
A strikingly similar barrage of opposing posts as the hornets nest I poked a stick in a few months ago.
A similar theory query as my same distance trip ;
driving slower…for a longer period of time.
vs
driving faster… for a shorter period of time.

consumption rate countered by time.

58

I’ll put my spin on this. Lets say you had to pull an auto, make it a light one. You could lean into it slowly gently increase your speed until you get up to 5 mph without too much effort after say 100 yards. Now you want to get that car to 5mph after 25 yards, I guarantee you will be sweating, thus an example that greater acceleration requires greater energy.

59

Accelerating that car to 5 mph in 25 yards takes more power, not more energy than doing it in 100 yards.

Kinetic energy = mass X velocity squared / 2.

Notice that the formula does not ask for rate of acceleration to come up with an answer.

I would say that accelerating it to 5 mph in 100 yards had you supplying the same kinetic energy as doing it in 25 yards and in addition, you had to fight the tire’s rolling resistance for an additional 75 yards resulting in a higher total energy expenditure.

Actually the OP’s question is similar to another question that I remember being discussed on this board. It was climbing a tall hill with a steep and short route up the hill verses another winding and long route up the hill. While the long and winding road may have given you better gas mileage, the short and steep route most likely used less fuel. Both routes forced the engine to lift the car to the top of the hill but the longer route also forced the engine to overcome the rolling resistance and air resistance of the longer road in addition to still lifting the weight of the car to the top of the hill.

60

well it dosen’t take 50 to 100 posts to answer the question. The fact that sudden acceleration uses more fuel has been established many years ago. Also, hard acceleration puts a strain on every part of the car. I know I will get a lot of smart replies but I am not going to say anything more about it.