1993 Civic automatic - featherfoot in all gears, or briskly get past 2nd and then featherfoot ?

[Cd]

On a low powered automatic, is it better to gently featherfoot the gas until you reach 2 or 3 gear, or is it better to give it a good push ( maybe 1/3 to 1/2 the way fully pressed ) and then featherfoot it from there ?
I have always thought that driving with the least amount of pressure on the gas pedal was the way to get food fuel economy.
But driving like this is driving me nuts.
Going from a standstill to 2nd gear takes perhaps 4 seconds.
Besides being rude to other drivers, it's just dangerous to drive like that.

[Blacktree]

There are different schools of thought on that. I'm not sure if there's definitive proof either way. But here's my opinion, based on experience.

If you do the "slow and steady" thing, you may never get up to cruising speed, depending on road conditions. And cruising is where you save gas. If you're always accelerating (even lightly), you're spending more gas than if you're cruising.

You also have to consider the time component. If you accelerate briskly for a few seconds, then settle into a cruising speed, you were only wasting gas for a few seconds. If you accelerate slowly for a much longer time, the fuel is burned a lower rate (compared to brisk accel), but for a much longer time period. So the total amount of fuel used may not be much different.

I'm sure you're familiar with pulse & glide. It's a similar idea. And it's proven to work. The logistics of pulse & glide vs steady-state cruise are similar to brisk acceleration vs slow acceleration. Plus if you accelerate briskly, you can get into the P&G zone sooner.

Those are my thoughts on the subject. But if you have the time, maybe you can test it and find out!

Side note: P&G is even more effective with a diesel engine. Because diesel engines burn minuscule amounts of fuel at idle (much less than a gas engine). So maximizing idle time will pay big dividends with a diesel. This has nothing to do with your situation, but may help some of the viewers.

[freebeard]

I feel some obligation to get out of the way for the person behind me.

[Ecky]

Lower average speed improves fuel economy, all else equal - though in the case of a car accelerating through fixed gear ratios, all else is not equal.

Probably the best metric to use is the average brake specific fuel consumption. I'll set up a hypothetical situation.

If (just as an example) peak BSFC happens at 75% load between 2000rpm and 3000rpm, and cruising on the highway will be at 2500rpm @ 50% load, you have a few ways to burn fuel:

1) Accelerate at 50% load, targeting 2500rpm as the midpoint between shifts. This will be slow acceleration that will taper as you approach your cruising speed. You'll have the same BSFC as cruising (minus torque converter losses) but none of it will be in the peak BSFC zone. Economy may be better just by virtue of lower average trip speed. The torque converter may also stay locked up even during acceleration.

2) Accelerate at 95% load at 3000-4000rpm up to cruising speed, then settle into cruising at 50% load / 2500rpm. This will be brisk acceleration and will maximize time spent cruising. At no point (other than transitionally) is the engine producing the most useful work. It shoots past peak BSFC during acceleration, then drops below it to cruise. Also, average trip speed will be higher. Although it will reduce the absolute time where the torque converter is locked up, more relative power will be sent through an unlocked torque converter.

3) Accelerate at 75% load targeting 2500rpm as the midpoint between shifts, then drop down to 50% load once cruising speed is achieved. To do this, you may need to back off the throttle to induce an early shift, as the computer may want to shift at a higher RPM due to the relatively high load. This technique targets staying in peak BSFC the most - essentially, the engine gets the most useful work out of each unit of fuel.


Generally speaking, #3 ought to be the most efficient. The only way it wouldn't be, is by increasing the average speed of the trip.

[aerohead]

I agree that, a BSFC map of your engine is your friend.
* Some BSFC 'islands' can be as 'short' as 300-rpm.
* A delta-300 rpm can alter mpg by 2.85%
* Friction horsepower can increase by 32% within a 300-rpm increase!
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Many decades ago, BMW published the only paper which ever, directly addressed 'transient'-losses associated with acceleration phase resident time.
The paper was entitled something like, 'Good News for Jackrabbit Starts.'
The crux of the matter was that, most fuel is lost during transient engine operation during acceleration up to 'steady-speed' cruising, and that judicious use of the throttle, which gets you 'quickly' up to speed, actually costs you less in the long-run, as you get to the most fuel efficient driving regime the soonest, and for the 'longest'. I think Blacktree and Ecky have spoken to this already.
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* And I believe that, around 80%-load engine operation gets you near max-torque, which also falls at minimum BSFC ( HONDA Motors used this metric for their CIVIC VX reporting ).
* And the 2,500-rpm 'HAS' been reported on with respect to efficiency, as it has to do with the surface-speed of the piston ring/cylinder friction component of the engine's friction-horsepower ( fhp ) losses, which varies as the square of rpm ( why a change of only 300-rpm can increase lower mpg by 2.85% ) [ something not accounted for in the so-called 'throttle stop' engine technique ].

[freebeard]

The paper was entitled something like, 'Good News for Jackrabbit Starts.'
...
I think Blacktree and Ecky have spoken to this already.

Permalink #3 wasn't explicit enough. Sixty foot times are approximately the width of the intersection.

I tend to hold back so I can roll up on the light just as it changes.

[Blacktree]

Edit: The following is an FYI. It's not a response to anyone in particular.

Just to avoid confusion, load percentage and throttle position aren't necessarily the same. The ECU uses things like throttle position, engine RPM, MAP / MAF data, etc to calculate the load. The load calculation is constantly being updated, and can change from one second to the next, even at the same throttle position. This is where it helps to have something like a Scangauge. Because there's no practical way of knowing the load percentage without instrumentation that can tease that data from the ECU.

Based on what I've read, automobile engines tend to be more efficient at higher load. And this plays into the pulse & glide technique. Basically, you're alternating between peak efficiency or idle. So the engine doesn't transition through the lower load / lower efficiency zones. The brisk acceleration is similar, but you're alternating between peak efficiency or cruise. Although you could P&G when you reach cruising speed.

But there's a catch. Most ECUs will switch to "power enrichment" mode at or near 100% load. The exact amount can vary from one car to the next. As the name suggests, power enrichment mode dumps extra gas into the engine. So if you're doing P&G or brisk acceleration, you may have to walk a fine line between peak efficiency and power enrichment.

That said, I think it should be safe to stay below 90% load. If you have a Scangauge or similar, you can keep an eye on that.

[hayden55]

Go for the highest load you can get with staying in closed loop so it doesn't go into enrichment. Typically try to target 2100 as your mid point in rpms. Between those rpms and high load it should reduce a lot of pumping losses associated with the throttle

[Nissandriver]

This might explain why my several blasts per tank to 6000-6500 rpm in 2nd gear in my Sentra doesn´t ruin my gas mileage. I often drive it starting in 2nd gear and shifting to 5th when I hit 50-60 mph. I wish I knew how to consistently accelerate at 75% load.

[hayden55]

Yep. It feels odd. In my 3.0 5 speed Ranger using like 90% throttle to 2100rpm felt odd as i was accelerating pretty quick. But it was the difference of like 25mpg vs 19mpg average.