Throttle Position During Acceleration and its effect on FE

[SVOboy]

<p><small><font face="Verdana,Arial">You'll get the best efficiency
around town by accelerating with full throttle, and shifting up to
the next gear quickly, before engine RPMs rise too high.</font></small></p>
<p><font size="2" face="verdana,arial">There are
two reason why this works. One is that the higher the gear, the lower
the frictional losses. By getting to 5th quickly, the total number
of engine revolutions is reduced, with a corresponding reduction in
frictional losses. The second reason is pumping losses. This is the
work done to force air past a partially closed throttle plate. The
more closed the throttle is, the higher the losses. Accelerating quickly
reduces throttle losses, because the throttle is open. In addition,
engine speed is lower in fifth, so to generate the same amount of
power, you have a larger throttle opening (and hence lower throttle
losses) - this is another reason to get to higher gears as quickly
as possible.</font></p>
<p><font size="2" face="verdana,arial">The best fuel economy is gained
by a combination of the gas petal to the floor with shifts at as low
an rpm as possible to sustain your desired acceleration. This ensures
a fully open throttle during the entire acceleration event and gets
you out of the lower gears as quickly as possible, for lower frictional
losses. It also gives you even more electric motor assist, as the
acceleration takes longer. There is only one problem with this technique
- its virtually impossible to do. You have to shift like a race car
driver, except that the shifts are at low rpm instead of high rpm.
You wind up going through gears like a maniac. It can be a lot of
fun, but you have to be really involved with driving (no cell phones)
and the technique is extremely counterintuitive. The combination of
mashing the accelerator and shifting at 2500 or 3000 rpm just doesn't
feel right. However, if someone is willing to practice this, they
can get a nice boost in FE around town.</font></p>
<p><small><font face="Verdana,Arial">After accelerating this way in
1st &amp; 2nd gears, you'll often be up to your desired cruising speeds.
Now it is time to switch modes, using as little energy as possible
to maintain this speed. Typically you'll now want to be in fifth gear.
The <strong>1-2-5 shift pattern</strong> (or 1-2-4 as the case may
be) may seem strange at first. After all, why are those other gears
there if you shouldn't use them? The answer is to think of them as
passing gears or cruising at slower speed gears. In most cases, this
is the way I use the five gears:</font></small></p>
<blockquote>
<p><small><font face="Verdana,Arial"><strong>1:</strong> Accelerating
gear<br />
<strong>2:</strong> Accelerating gear<br />
<strong>3:</strong> Even slower cruising / quick passing gear<br />
<strong>4:</strong> Slow cruising / slower passing gear<br />
<strong>5:</strong> Cruising gear</font></small></p>
</blockquote>
<p><small><font face="Verdana,Arial">Once you're up to speed, in addition
to quickly getting into your cruising gear, you'll also want to begin
feathering the throttle. Since you should have done all your acceleration
before going into cruising gear, you now want to use as light a throttle
as possible that will still maintain your current speed. If you find
that you want to accelerate further, you may be better to downshift
for a quick blip of acceleration, and then shift back into your cruising
gear.

</font></small></p><p>&nbsp;</p><p>&nbsp;Does this sound right? Am I crazy? Slam it to 50 mpg in second then shift into 5th? Huh? What? It is from insightcentral so they know their stuff, but really? <br /></p>

[MetroMPG]

FYI, for what it's worth (not much), I used moderate load, low RPM acceleration for the majority of my most recent 75 MPG tank. (Metro95 - I need another acronym, please!)

That proves nothing about the effectiveness of the technique because it still needs a proper test. Maybe my tank would have been higher with glacial acceleration; maybe it would have been lower. All it tells me is it probably doesn't have a large negative impact.

Also, Phil on the MaxMPG list has confirmed he has a copy of the BMW acceleration study and he says it does NOT advocate WOT. He's going to get me a scan or a digital pic. I'll post it when it arrives.

[philmcneal]

Originally Posted by MetroMPG

Also, Phil on the MaxMPG list has confirmed he has a copy of the BMW acceleration study and he says it does NOT advocate WOT. He's going to get me a scan or a digital pic. I'll post it when it arrives.

awesome! i knew it sounded fishy when WOT = better mileage. Thats totally opposite logic.

[MetroMPG]

Phil Knox was kind enough to share the text of the BMW reference he remembered from Popular Science magazine, July 1981:

Good news for jackrabbits

Sudden starts waste gas, right? Wrong, according to a BMW study which shows that three-quarter-throttle acceleration is more efficient than a slow, gradual speedup. Reason: Your engine runs more efficiently at a heavy throttle position, and makes the most of the gas it burns. At about 2,000 RPM, you should shift gears and accelerate the same way, getting into high gear as quickly as possible. (You can accomplish the same thing with an automatic-transmission car by accelerating quickly, then letting up on the pedal to allow the transmission to shift.) Then let your engine loaf along at a steady speed near the 2,000-rpm level.

The problem with slow acceleration is twofold: At slow speeds, the engine is doing little more than idling: and it must run longer to get from point A to point B.

This advice runs counter to the readings you get from those computerized mpg meters, which will indicate poor fuel economy during fast acceleration ("Mpg Meter," Aug. '80). The reason for this is that most meters measure engine vacuum level, not fuel consumption. Their readings are accurate at cruising speeds but can be misleading during acceleration from a stop.

I think the explanation there is a little weak, but suspect that's the fault of the Pop Sci writer, not BMW.

Now that I have a bit more info, I'll track down the original study to see the methodology & details.

[SVOboy]

Indeed, it makes sense, it does, but I think there is a point where the length between stops balances out a bit. I mean, if you're stopping every half mile and you jackrabbit to speed you're going to lose all that to braking almost immediately. Since the benefit is in elongated coasting time and not gas used during acceleration you need to be able to use that extra distance you've gained yourself effectively to cancel out the loss from the added throttle.

[MetroMPG]

Agreed on your caveat. Regardless of rate of acceleration, you need to avoid turning fuel into brake dust.

[MetroMPG]

Bad news on finding the original BMW reference:

Thank you for your email.

I spoke to several colleagues who are working in the department which was responsible for this kind of study, but obviously the original does not exist anymore. Since the premises on engine development changed drastically since the oil crisis - and this was a long time ago - it was probably not deemed necessary to file every single study.

That's sad, but there's nothing else we can do. Also the study would be of not much help today, because it was based on the technologies of this time, which - especially regarding high-tech engine - have changed a lot.

We regret that we cannot give you another answer.

Yours sincerely,
BMW Mobile Tradition
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[JanGeo]

looking at the graph above it seems 20-30% power at around 2000 rpm is ideal for max efficiency - interesting because that is about 40mph in 5th gear on my xB where I get sometimes 50mpg . . . willl have to check the LOD reading at that speed.

[omgwtfbyobbq]

Word on the street is that during WOT the ECU switches to open loop and a richer mixture to maximize power. Not sure if it's true, but it's what I've always 100% 4real heard. Now the funny thing is, the best FE during acceleration will probably be right before you trigger open loop, so we have to ask, from a stop, how much throttle before open loop in each gear? It's probably different for each car depending on how that manufacturer wants the car to drive, i.e. what's throttle tip-in, what are the gear ratios at the gear we're in, what does the torque curve look like, etc... The only problem is, aside from testing, how can we determine this?

[JanGeo]

ScanGauge tells you if you are open loop or not as well as LOD (% of pwer at current RPM)