Why no fast acceleration?
 

I have just started reading some articles about fuel efficient driving. While I cannot say that I am committed to doing everything that hypermilers seem to do, I have already started backing off my speeds, and staying more off the gas pedal when I see that I need to break up ahead; in just a few days I am now automatically picking out ideal points from which I can coast. I am surprised by how much I’m enjoying this—its almost as if I’m working at a puzzle or something. It sure is making me a safer driver. And a more relaxed one--as well as saving some gas and $.

OK—down to my question. I saw one suggestion, that urged drivers not to accelerate too quickly. What is the reason for not accelerating too quickly? Is it simply that a rapid acceleration chews up more gas than a slower one? Or is it that a rapid acceleration will make it more likely that you will have to break soon?

For example, if I have a clean shot for 1/2 mile—no lights or cars ahead of me or anything—would I still be consuming gas unnecessarily by accelerating rapidly?

If you accelerate too quickly the fuel mixture will be enriched to prevent detonation, either by the carburetor acceleration pump or by the ECU operating off its load map. When the mixture drops to 14.7:1 or lower, the engine efficiency drops.

Don't let "don't accelerate too quickly" confuse you into thinking it means "accelerate slowly." It means not to so it too quickly.

Harder acceleration means less acceleration, and that's a good thing- until the computers decide to start dumping extra fuel into the mix.

OK--I realize each car/engine might be different, but are there some decent rules of the thumb---like take 5 seconds for accelerating each additional 10 MPH, or something like that? Articles discussing this have been incredibly detailed---a bit too much for my eyes, given that I'm not a mechanic or a physicist.

Let's keep it simple. I found this quote in another thread here:

It's been debated back and forth as fas as what rate of acceleration is best. Personaly, I've found it hard to notice a difference either way. What really kills your mileage is unecessary acceleration. Hard acceleration is generally thought of as bad becuase, in many cases, people accelerate hard only to have to brake hard at the stop that's right up ahead. It's really the braking that is wasting all the energy.

If this is the best policy to take, then the only thing I need to worry about is speeding up so fast that 5 secs later I'm breaking. Was that advice (in the quote) on target?

Haha, I startrd hypermiling the same way as you. Not to commited and now I try to find every way possible to ecomod my truck so I'll get better gas millage.

Best thing to have to get the best possible gas millage without heavy ecomods, just start with a Vacuum gauge. With it you'll know if you're accelerating to hard or to soft. There is the Ultragauge that is pretty awesome to monitor your car's efficiency.

High rpm acceleration is bad and consumes lots of gas. If you have a manual transmission you can do decent acceleration at not-high rpm, like 2000. If you press the pedal that same amount in an automatic it will push the rpm higher and consume more fuel. You can accelerate efficiently faster in a manual than an automatic.

High-throttle enrichment varies widely from car to car. Some you have to actually have the pedal to the floor and high rpm, like 4000+. Some cars, anything above 80% throttle will enrich the mix. So to be safe, I'd say keep the gas pedal at or below 3/4 if you don't know otherwise.

That's a nice rule of the thumb. Both vehicles are automatic. I'll try to get that in a sig on the left if I can figure out how.

Look up "BSFC charts". Most automotive engines are most efficient at lowish RPMs and high (but not 100%) loads. As others have said, the computer will often enrich the fuel mixture as you near 100% load, which is bad. In my 5MT, I get the best economy when I shoot for about 80% load (as reported by the computer) at less than 2500RPM. The pedal position required to accelerate at that load changes based on RPM, however, so you can't just rely on pushing it a set amount. You really need instrumentation to get close to "perfection ", but I figure a good rule in an automatic is to accelerate at near the max rate you can without causing a downshift, and let it upshift as soon as possible.

That said, so long as you aren't revving it up, the biggest gains are too be found in the art of slowing down.

One of the easiest examples to understand vehicle acceleration is with one of the least popular(at least in the beginning) automatic transmissions, a Continuously Variable Ratio Tranny(CVT). My own CVT was a Nissan CVT, that was adapted to be used in a 2007 Dodge Caliber. The CVT was said to be most efficient when driving at 2000 rpms. At 60mph, rpms were 2000. When accelerating, set the throttle so your rpms are 2000, & you could accelerate slowly to 60mph. When both conditions could be met, cruising at 2000rpm & slow acceleration at 2000rpm, Caliber would give pretty good mpg, despite the car itself, not aerodynamic or the engine, particularly thrifty. Any divergence from 2000rpms & you should expect less thrifty mpg.

Then there is my car, you can hoon it up as long as you don't need to waste energy to slow down afterwards without severely impacting fuel economy.

Here's a BSFC chart for a Toyota Corolla:
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http://i233.photobucket.com/albums/e...bsfc_SAE_9.jpg
The circles with numbers show the amount of fuel needed to produce a certain amount of power. Note how you get less from your fuel as RPM increases and load decreases. On this particular engine, you want to stay between maybe 1750 and 3000RPM.

Didn't see any for a 1998 Honda Civic, even without searching for the year.

OK I can keep it simple: accelerate but not too fast (I think I've trained myself to do that already), & learn how to coast, and approach intersections.

Yep. Brakes waste gas. Avoid needing them.

I haven't seen one either, but if you look at a bunch of different engines' charts, you start seeing strong similarities. Take that general knowledge and apply it and you WILL get results.

The differences between your engine and the general "other" engine is only a few percent this way or that, and only matters for a portion of your driving time. You've got the right idea - keep it simple and keep working to do better.

Wait a sec. Just remembered that my 98 civic doesn't have an indicator for RPMs. Some do, just not mine. Oh well at least I can continue accelerating slowly, to avoid breaking as much.

Get a Scangauge. It is the best tool for monitoring your engine and ECU. It will give instant feedback on mileage and it can display all sorts of engine fucntions including RPM.

This. So many times. I got my old Civic up to 42 mpg without a gauge. The first tank with a gauge was 46 mpg, and that was my new low - every one after that was even higher.

But you can roughly work out the rpm by calculating speed in each gear. For my 1996 Civic DX, it worked out like this:
1500 rpm - 2300 rpm (my road-tested ideal rpm range for the Civic)
2nd gear: 13 mph - 20 mph
3rd: 20 mph - 30 mph
4th: 26 mph - 40 mph
5th: 33 mph - 50 mph

Or to keep it simple, when you reach (gear x 10) mph, shift. When you're in 3rd gear, shift when you get to 30 mph and don't take 3rd gear beyond that speed.

It gets troublesome with 5th gear. What are you going to do? You don't have a 6th gear to shift into, and you can't limit yourself to 50 mph always. You just grit your teeth and deal with the high rpm cruising and grumble at the engineers who built it this way.

Remember that neutral is an option as well. It takes a good amount of energy just to keep the engine spinning at 2000 rpm at a steady speed. If you shift to neutral it takes a lot less fuel to spin it at idle rpm instead.

In physics, faster acceleration=higher energy requirement .

Right, per unit time, but you spend less time accelerating to a given velocity if you do it more quickly.

Nick666 brings up a good point that I'd like to riff on:

Ib like to think of everything in terms of energy, and use comparisons that easily demonstrate that idea.

Acceleration: faster acceleration takes more energy, and therefore more gas (engineering of your car notwithstanding). Think of riding a bicycle and accelerating to 20 mph from a dead stop.

It can be done easily if done slowly, but your legs would be on fire if you went flat-out to get to 20 mph. Same goes for the car.

This also applies for coasting, braking, conserving momentum, etc.

Of course faster acceleration takes more energy, but less time to reach the target velocity. So the most efficient way to get to the target velocity is to operate the engine in the part of the BSFC where its more efficient, and that is generally at higher loads and torque near the torque peak. So "brisk" acceleration with short shifting at the torque peak is usually more efficient, avoiding fuel enrichment and higher rpm.

Moving with a sense of purpose isn't drag racing.

Faster Acceleration ... reaches higher amounts of wind resistance sooner.

If traveling the same distance .. and the same cruising speed .. The faster acceleration will have a higher average speed .. will require more energy.

If traveling the same distance .. and the same average speed .. the faster acceleration would than have to travel at a slower cruising speed in order to average the same speed .. a slower cruising speed would reduce the energy needed.

I suspect there is also a psychological aspect to the average type of person who accelerates quickly ... vs the type that does so slowly .. and I suspect that the net overall beneficial psychological type for energy or fuel efficiency .. is the slower accelerating psychology... Even if the specific causes of that net benefit may not necessarily be in the acceleration rate itself .. But in the overall method that type of psychology approaches numerous types of driving conditions/options.

- - - -
I would Ditto the BSFC points made previously .. but with the caveat .. there can be variation from that +/- caused by the differences between energy efficiency vs fuel efficiency .. for different contexts.

Faster Acceleration... reaches gliding speed sooner.

You're exactly right about the difference between energy efficiency and fuel efficiency. All of this is vehicle specific and situation specific, and pointless once the driver has a gauge and can learn the car and how best to apply it.

Overall, I'm with the "accelerate harder" crowd in most situations because pulsing longer than is called for is burning gas longer than is called for. Even "harder" is realtive- this isn't exactly a crowd that bounces off the rev limiter.

I ran some tests with my truck. Starting from work, pulling out onto the road, and accelerating to 35 MPH (the speed limit) at various rates and shifting at different RPM's. The Scangauge trip MPG started at zero, and I read it at the same point down the road. Accelerating like the proverbial grandma got me to 35 MPH just before the reference point, while accelerating harder got me to 35 MPH sooner. If I got to 35 MPH before the reference point, I would maintain 35 MPH to the reference point. One test per day.

Results: My truck does best if I tromp the gas pedal about 3/4 the way down, and shift at 2500 RPM. Coincidentally, that has me accelerating at about the same rate as most people.

With my manual Forester I like to accelerate at about 3/4 throttle and shift at 2500 rpm until I get in 5th and then just finish accelerating at 3/4 throttle until at desired speed. With my Town and Country and automatic, I use the "economy" button on and hold about 3/4 throttle which seems to let it naturally shift at 2500-3000 rpm. Without that "economy" button which most drivers seem to hate, it lets the van rev higher and use it's almost 300 hp which is great for drag racing but bad for economy. On my Hemi Aspen, any throttle and the thing accelerates fast. My key there is to get to the speed and back off the throttle so it will go into 4 cylinder deactivation mode faster. Then it's key to hold acceleration slow enough not to kick it into 8 cylinder mode which I wish it were possible to lock in 4 cylinder mode. In all cases I seem to accelerate with most traffic, faster then some, slower then others.

On newer cars the enrichment kicks in frighteningly early. My FR-S' stock fuel mapping enriches at 60% load for most rpms. I changed the map, but it's very easy to accidentally kick it into enrichment.

On cars with no wideband sensor you need a bit more load to get enrichment, and old school cable throttle engines would just not enrich the mix until higher rpm (you also get very crap to nonexistent power output below 2000rpm as a result).

You mean on sports and or turbo cars.

My (turbo) Fiat will do enrichment pretty early. My 2014 Renault is very reluctant to go into enrichment and my old school cable throttled Jeep has heaps of power below 2000 and won't do enrichment at all unless road speed is over 50mph.

Since I red that thread I tried with my Vue short brisk accel (1-3 Vac = 60-65% throttle) vs steady accel (5-10 vac= 40-45% throttle) vs slow accel (12-15 vac=35-40% throttle) always shifting at 1800-2000 RPM and I got the best results with steady acceleration.

With short brisk accel my fuel economy went south about 1mpg, so I started with Lighter loads and my it went back up. With slow accel, the problem is that every acceleration takes forever to get to speed with barely any mpg improvement.

Mpgs now hoovering near 27mpg. Outside cool températures and high winds are killing my Mpgs on the highway.

Im doing every thing I can to get better Mpgs but cant seem to get anything better than 27mpg.

Oh well...

That's a Jeep though, I was thinking more like Corolla. My MR2 had a 1ZZ-FE and even pulling a 2200lb car, it groans loudly but doesn't really go below 2000rpm. Newer economy cars can go open loop at lower rpms these days too. The manufacturer is incentivized to make the engine run rich if they don't need the excess power while running government tests since the consumer is the one paying for the extra fuel, and it helps keep the replacement rates on the cats under warranty down.

I also suspect that a Renault (who doesn't sell cars in the US) sold in Australia would have a different fueling map than a US market car for regulation or cost reasons.

Now that you bring a counterexample up though, I feel like I need to go put the OBDII scanner on the other cars in the family and see...heh.

My Fit has electronic throttle and it doesn't go into Open Loop enrichment until the pedal is basically on the floor. I had to be more careful to avoid it with my old cable-throttle Civic. It really depends on the specific design.

In the past, small gasoline engines, carbureted, the best fuel economy was always full throttle and coast. "pulse and glide" as told here I guess.

Depends on the car. My maths that that my dad's corolla could get 55-65 MPG while drafting a truck at 60 MPH and it was a 1.6L carbed engine with a 4 speed lockup TC only in 4th. ~2300 RPM at 60 MPH in 4th with locked TC. Math was needed due to stop and go traffic at the end of trip, I assumed pre-2007 city fuel economy since the traffic was bad enough to resemble it

If the target speed is going to be optimal, say 45~50 MPH, the sooner you can start reaping the benefit of that speed the better, IMO. Diesel favor a more brisk acceleration for this reason. Poking along with the beetle in slow acceleration just prolongs the agony of the scangauge being down in the ~30 MPG range. I can go briskly at ~20 MPG instant, for a few seconds. Then I'm cruising at ~60 MPG at those optimal speeds.

Similar thread: http://ecomodder.com/forum/showthrea...ate-28617.html

I'll share my thoughts from it with a few updates:

As an a instructor and a tuner this is something I get asked alot.

The short an simple answer is that disregarding air resistance or losses due to great average speed, the ideal eco-acceleration should be the maximum acceleration available prior to entering open loop, or pulling ignition timing due to knock, or increasing AFR, in the highest gear possible (or lowest RPM). And... that acceleration is only useful if upon reaching the intended speed one immediately starts to Pulse and Glide, at least in a 5spd. This is because the gear ratios are locked for a particular gear, so in order to take advantage of the higher efficiency near WOT one must provide a lower RPM return, via the P&G technique.

I tend to accelerate under heavy load in 5th gear (as soon as its reasonable to take), but reduce the acceleration enough to stay in closed loop.

This minimizes frictional losses in the engine by using the piston stroke, valve duration and pumping losses to their maximum ability and since rotational losses are not load dependent the greater power output increases the efficiency of each engine power stroke by reducing the loss %.

Its the same idea as having to withdraw $400 from a ATM, would you do it all in one hit or in 10 little hits? Your gonna get dinged $1.00 for each transaction so might as well minimize the number of transactions by maximizing the available amount in each one.

Its the same reason why the same engine in a turbo diesel variation can get better mileage then in that engine in non turbo diesel variation. The extra RPM needed in the non turbo causes unnecessary frictional losses.

If you want to give your brain a stretch on this material, take a look on these 1960 tests on tractors, in particular, HP-Hr/Gallon, and note the better efficiency at higher load, of course these did not have a WOT enrichment device on them, so the most efficient engine is the one that took the biggest advantage of the combustion stroke. Think of the HP-Hr/Gallon as comparable to KiloWatt Hours/Gallon on an electric generator, if you can get more power (KwH) out for the same gallon, your doing better.

TractorData.com Oliver 88 tractor tests information

Hope that helps,

Steve

Not if they had an accelerator pump...

i find this true in my smart, we call it "drive it like you stole it". accelerating for 8-10 seconds at 1.8-2 GPH uses less fuel than accelerating 1.3-1.5 GPH for 15-20 seconds. (i use an ultra gauge) of course these figures are for smart fortwo's, your consumption may vary by make and model ;)

well we know that fast starts and fast stops does consume more fuel, so it would make sense to accelerate at the most fuel efficient speed.

Standard engine design
 

I would say some testing needs to be done. It's too easy to generalize and what may be the best way to drive one car may not be true for another.

In my reading about car engine design, I was surprised to find that the design point for car engines is 85% throttle. This means that car engines are, or were, most efficient at 85% throttle. That's probably volumetric efficiency. With the advent of computer control, this may no longer be relevant. From a theoretical viewpoint, the factors we want to look at are instantaneous fuel consumption, speed and aerodynamics. It's relatively easy to hook up some sensors and measure or calculate fuel consumption, speed, distance covered and acceleration over a test run but not so easy to figure in the negative force from air resistance. Finding the most fuel efficient way to drive a car may require a number of test runs, but if the test runs do not give consistent results, it may be problematic.

Recently I drove my car over 200 miles at freeway speeds and found that the mpg of my next tank of gas around town was much better than before the run to the big city and back. I attributed this to the distance run "cleaning out the engine" or burning up the combustion byproducts such as carbon buildup, temporarily making the engine more efficient until the small errands around town builds up the deposits again. It reminded me of earlier days when I lived with racers who would change the jets in the carburetors to give the best air-fuel mixture. The criterion at the time was the color of the deposits in the exhaust pipe. Black was too rich, nearly white was dangerously lean and gray was ideal. How gray? That was a matter of experience and the color had to examined at the end of a good long run. For some that was just 10 miles on the freeway; for others it was at least 30 miles.