Driving TOO slow?

[Sidakers]

Ok, so I've been using Torque for my android phone for the past week or so, and I've got both MPG data and Engine load data on my screen at all times. Most of my travels are on the highway system in wichita, because most all of my main stops are only a stones throw off the highway, two that I can literally coast to off the highway. I'm in an automatic saturn sl2, and for anyone unfamiliar with wichita, most of it's highway system consists of rolling hills.


I've noticed, when I match "normal" acceleration patterns of other traffic (be up to the speed limit when merging) My engine load sits at about 80% when accelerating. I usually get 29-30 mpg on these trips. When I drive a bit slower, and limit myself to 60%, with pulse and glide (coast uphill, accelerate downhill), I can if I try reaaaallly hard, get 43-45 mpg. When I barely touch the gas though, keeping my engine load around 40% when accelerating (I only do this in the weee hours of the morning when no one is out) I can only squeeze out 35 mpg, but I simply can't get up to a speed to where I can let off the gas for the glide part of pulse and glide.

So my question is this..

Is it possible that My engine just operates better under a 60% load than a 40% load, or Is it that pulse and glide is just that important to good fuel economy, even in an automatic?

Thanks,
Sidney

[skyl4rk]

It is the coasting part of your trip where you are gaining mpg numbers. It probably has to do with matching your acceleration to uphill runs and using downhill runs for long coasts.

[piers.singer]

I don't think that you're driving too slowly, rather that you are ACCELERATING too slowly. Your engine will be more efficient at higher loads. When you accelerate at a higher load you reach your target speed more quickly, keeping your engine in a more efficient mode for a short time before allowing you to coast or cruise. By accelerating at lower loads you reach your target speed less quickly, keeping your engine in a less efficient mode for longer. This, as you have noted, will give you less time to coast so don't be afraid to step on the load pedal a bit, it's good for you.

As an aside, I raise an eyebrow at your hill technique. If you can coast up a hill whilst retaining appropriate speed then good on you but you shouldn't be accelerating downhill, this is expending petrol when you could just be letting gravity do the work. Once again, don't be timid about giving it some boot on the way uphill, your FE numbers will thank you.

[jakobnev]

Is it possible that My engine just operates better under a 60% load than a 40% load,*snip*

It's not only possible, it's probable. (In fact the whole point of P&G is to keep engine load up in the efficient range while the engine is actually used.)

I suggest you look up the threads about BSFC-charts.

[Sidakers]

Originally Posted by piers.singer

I don't think that you're driving too slowly, rather that you are ACCELERATING too slowly. Your engine will be more efficient at higher loads. When you accelerate at a higher load you reach your target speed more quickly, keeping your engine in a more efficient mode for a short time before allowing you to coast or cruise. By accelerating at lower loads you reach your target speed less quickly, keeping your engine in a less efficient mode for longer. This, as you have noted, will give you less time to coast so don't be afraid to step on the load pedal a bit, it's good for you.

As an aside, I raise an eyebrow at your hill technique. If you can coast up a hill whilst retaining appropriate speed then good on you but you shouldn't be accelerating downhill, this is expending petrol when you could just be letting gravity do the work. Once again, don't be timid about giving it some boot on the way uphill, your FE numbers will thank you.

Well that sorts it out. I was always stressed on not jack-rabbiting (still a no-no), but unfortunately (and apparently) went to the other extreme. I hadn't thought my engine would do better at 60% than 40%. My motivation for hills was also the same, try and not work it to hard up hills and let it go down hills easy. Thanks for the clarification.

[Joenavy85]

One of these days i'll get around to redoing my testing in this area. a while back my buddy and I did an acceleration comparison test on my Jeep, and it showed that for city driving (less predictable stops) it is better to accelerate slowly (tested on route 41 and turning left at the next light from where we started) where as if you are accelerating onto an interstate where you will not be stopping soon it is better to accelerate with a higher load (tested on the same route, but not turning). The route we used worked well because sometimes we would get an unexpected Red Light, which would help us keep the variables there, just as they would be in real life. I didn't save the results, unfortunately, so once i get around to it again i'll save them and post them up here

[suspectnumber961]

I think I've improved my mpg by using a form of pulse and glide where I accelerate at ~32 TPS (automatic...16 TPS at idle)...then try to coast into stop signs or curves from maybe 45-50 mph to around 30-35 mph.

* Oval represents the 32 TPS range...around 2500 RPM. 35-36 TPS and 3000 RPM might do about as well..but would involve more trans power loss.

The two other marks.....the lower one represents cruise at 50 mph or so and the middle one is accelerating at around 2000-2100 rpm. Converted from actual LOD and RPM numbers.



I'm pretty sure that by doing this over a 1/2 mile distance I can maintain my avg mpg figure. What mpg I lose on acceleration...I regain on coasting.

But I can never approach the mpg I could see with highway cruising....33 mpg vs +40 mpg?

[postmac]

suspectnumber961 are you coasting down to 30 mph and then putting on brakes?

[roflwaffle]

Originally Posted by Sidakers

So my question is this..

Is it possible that My engine just operates better under a 60% load than a 40% load, or Is it that pulse and glide is just that important to good fuel economy, even in an automatic?

Thanks,
Sidney

Definitely. Depending on the BSFC map 40% versus 60% can be a difference of ~400+g/kWh@40% versus ~250+g/kWh@60%. Drive slowly, accelerate quickly. Depending on the automatic transmission this can be offset somewhat by it staying unlocked and greater losses through the torque converter, but on the other hand some automatics won't lock up unless certain they've reached some higher speed, so on those high load won't hurt much, and since tq converter efficiency tends to increase w/ speed it may help a bit versus slower acceleration.

It all depends on the specifics, so what car/year/trans do you have?

[suspectnumber961]

Originally Posted by postmac

suspectnumber961 are you coasting down to 30 mph and then putting on brakes?

At stop signs or lights...for some curves I can just roll on through them.