Instantaneous MPG readings for best fuel economy

[JimboK]

In researching ICE efficiency, some of our technical geniuses have defined efficency ranges that correlate with RPM. RPM too low, the engine "loafs," and you might as well glide (at <40 MPH) or warp stealth (at higher speeds). Too high, it burns more fuel than necessary.

To help develop a simple rule for the the common man (and woman) without added instrumentation, lately I've been watching the relationship between vehicle speed, RPM, and instantanous fuel consumption. Here's my initial draft of such a rule: Keep the MPG between half the vehicle speed and 1.2 times the vehicle speed. Based on my preliminary observations, I'm estimating that this results in engine speeds staying between 1400 and 2400 maybe 90% of the time or better.

Most of my observations have been in suburban and urban driving, so there could be differences at highway speeds.

I am interested in the observations and feedback of others with a ScanGauge, Can-view, or other engine monitoring device. Is the concept viable? Are your observations similar? Do the upper and lower ends need to be tweaked (but still keeping the math simple)? Does the rule work at highway speeds? Has this been suggested previously and I just haven't searched enough to find it?

Thanks in advance!

[efusco]

Interesting and potentially very helpful to those wishing to maximize w/o instrumentation.

I've a few suggestions for you to fine tune though.
1)See if you can narrow down the instantaneous readout specifically for "acceleration/pulsing" at speeds b/w 10mph and 40mph. IMO that's the most critical range esp. for those who use P&G.
2)Be sure to correlate the SOC of the battery....as SOC gets to or below 50% you get much more charging of the battery and less power to wheels. This may not affect the instant. FE read-out, but it may and deserves a closer look.
3)Once you've pinned this down a bit more closely, esp. if you can get it to hit that 1700-2300 'sweet spot' most of the time, you might even want to make a chart showing a target instant FE for a given speed in 5mph increments.

Good idea and good work!

[KV55]

Looks like a data logger would be useful. The SoC is going to make the results hard to repeat. Is the drive state of "deadbanding" the best for acceleration or should one take a little from the battery? and does MPG remain constant in this deadband as the speed increases?

This morning I used litres per hour and miles per litre alongside rpm and coolant temperature on the ScanGauge. Litres per hour was used as a measure of acceleration effort and miles per litre to fine tune the throttle when cruising (at around 50mph). These settings might be useful in comparing MPG and rpm. I am using litres because it gives me better resolution (and the US gallons are short measures).

[efusco]

<div class='quotetop'>QUOTE(KV55 @ Apr 30 2007, 07:53 AM) [snapback]432477[/snapback]</div>

Looks like a data logger would be useful. The SoC is going to make the results hard to repeat. Is the drive state of "deadbanding" the best for acceleration or should one take a little from the battery? and does MPG remain constant in this deadband as the speed increases? [/b]

Actually, you want a little charge going TO the battery during acceleration and that is one of the things that's accomplished during 'sweet spot' accelleration b/w 1700-2300rpm. This is the 'pulse' portion of pulse and glide. Recall that a small drain from the battery occurs during gliding and it's necessary to put some back during the pulse portion.
I long believed the mantra of 'dead banding' until Hobbit showed the sweet spot range and as my understanding of P&G slowly settled in.

[John in LB]

<div class='quotetop'>QUOTE(JimboK @ Apr 30 2007, 03:57 AM) [snapback]432458[/snapback]</div>

To help develop a simple rule for the the common man (and woman) without added instrumentation, lately I've been watching the relationship between vehicle speed, RPM, and instantanous fuel consumption. Here's my initial draft of such a rule: Keep the MPG between half the vehicle speed and 1.2 times the vehicle speed.
[/b]

As I am not going to multiply numbers in my head as i am accelerating from a red light... to even simplify further, could we state that if we kept the MPG figure approximately the same as MPH figure, we would be doing just fine?

That would be a great rule of thumb... very easy to follow and visualize.

[Highly ImPriused]

Thanks for posting this. I have a dumb question; why would accelerating with MPG > 1.2MPH be a problem? I don't see the need for an upper limit. Or is that just the best you can typically achieve?

Since you posted this I have noticed yesterday and today that when I accelerate from a stop, my MPG is usually somewhat less than 0.5MPH until I get up to speed, even worse going up hill obviously. I haven't played around with acceleration techniques too much, but I generally try to get up to speed pretty quickly and smoothly without jackrabbiting or anything. I thought that was the most efficient method. Could I not be pushing it enough? When forced to go slower (if someone is in front of me), I have noticed that MPG is still usually less than 0.5 MPH. Once I start maintaining speeds around 45 - 60 MPH, MPG usually equals or exceeds MPH unless I'm going uphill, but then I get much higher MPG on the downhill glide/warp stealth. I think I really need to get a ScanGauge to start monitoring RPMs.

[JimboK]

<div class='quotetop'>QUOTE(Highly ImPriused @ May 1 2007, 11:19 AM) [snapback]433231[/snapback]</div>

I have a dumb question; why would accelerating with MPG > 1.2MPH be a problem? I don't see the need for an upper limit. Or is that just the best you can typically achieve? [/b]

Not a dumb question at all. At the lowest range of RPM the ICE is operating inefficiently. It's better to let it cut off completely (through gliding or warp stealth) or give it a little more pedal to nudge it up into a more efficient range. (This is essentially what pulse and glide does: accelerate to a certain speed with RPM within its efficient range then back off into an ICE-off glide.) From my ScanGauge, the RPM seems to mostly reside in this low inefficient range when MPG > 1.2 x MPH.

For my simple mind I try to keep things on a "see Spot run" level. To that end, here's an analogy that helps me explain it. Whether it's scientifically valid, I'll let the experts decide. When your body moves on foot, the most efficient method is walking naturally. It's fast enough to get you where you're going but not too fast to tire prematurely (i.e., run out of fuel). But let's say you slow your pace almost to a shuffle: each pace covers, say, a foot or so. Calorie expenditure per step presumably drops off, but the dropoff in speed is likely greater. So you'd probably cover less distance per [x] amount of fuel burned. Make sense?

So for the ICE, the dropoff in power at these low ranges is probably greater than the dropoff in fuel consumption.

As for your acceleration, try moderating it a bit to keep the MPG higher than 0.5 x MPH. I think you'll find it adequate for most situations except moderate to steep uphills. This will also help me validate whether the rule is workable.

A ScanGauge is a very useful tool, and seeing the actual RPM is more precise. But I am hoping we can develop an easy-to-use tool for folks who choose not to buy a SG, assuming they can do some simple math quickly in their head. John raises a valid point that some are better or more comfortable with math than others.

John, your rule of course would be even simpler, though less consistently maintainable.

[Highly ImPriused]

I see now. Thanks Jim. I'm going to play around some more with acceleration from a stop on the way home today and will let you know if I have any more luck getting MPG above 0.5 x MPH.

FWIW, I don't see the math problem with this rule as comparing your MPG readout to half your speed barely qualifies to me. I guess with the 1.2 x MPH it could be more problematic for some. But I would think that not letting MPG get too much above MPH would be sufficient for most.

[John in LB]

<div class='quotetop'>QUOTE(Highly ImPriused @ May 1 2007, 09:10 AM) [snapback]433321[/snapback]</div>

I see now. Thanks Jim. I'm going to play around some more with acceleration from a stop on the way home today and will let you know if I have any more luck getting MPG above 0.5 x MPH.

FWIW, I don't see the math problem with this rule as comparing your MPG readout to half your speed barely qualifies to me. I guess with the 1.2 x MPH it could be more problematic for some. But I would think that not letting MPG get too much above MPH would be sufficient for most.
[/b]

Keeping the MPG equal to MPH is not rigid... just visually easy. In fact, even if not achievable, it would be just an easy objective to try to attain.

As a specific example, Highly ImPriused is now at 0.5 MPH... and you can see that he is trying to change / understand his acceleration behavior to move closer towards 1.0 MPH. Of course, if you can do better than 1.0 MPH then you might be better off... but it has diminishing returns and, for the general public, moving them towards 1.0 MPH would have a significant impact on MPG.

Another way to look at it is for the instruction to be: As you accelerate, try to keep the MPG number equal or close to the MPH number. If you go below half the MPH number, then you really need to improve.

[Highly ImPriused]

So I tried some different ways of accelerating from a stop on my commute home yesterday and back in this morning. I found that in order to get MPG > 0.5 x MPH, I had to accelerate pretty gingerly. Much slower than what I had previously been led to believe was the most efficient way of accelerating, based on other posts on here. I never floored it before, just gave it what I considered to be decent acceleration; nothing that would throw your head back or anything. I had to go much slower to achieve this goal; certainly not slow enough to use just battery power, but not a whole lot more than that. I wonder if this is just me for some reason or if others are finding this to be the case as well. BTW, acceleration to maintain speed or increase it a little bit when already moving hasn't been a problem. I'm easily able to get MPG in Jim's target zone in those situations on a flat.

Regarding the upper limit of 1.2 x MPH, a situation came up a few times that had me questioning it again. Say you're traveling about 40 MPH and need to maintain speed due to traffic on a slight uphill where gliding is not possible. I've found that in these situations I'm only using a little bit of ICE power and getting MPG > 1.2 MPH (maybe about 60 MPG at 40 MPH). I understand that technically this isn't a very efficient way to use the ICE. The only way to get out of it (since gliding isn't an option going uphill) is to accelerate a bit more and I find that I can indeed get MPG close to MPH while gaining a bit of speed. I don't have a problem with this, but isn't doing that ultimately less efficient if you're getting lower MPG? Say you have this situation for 1 mile prior to the next downhill where you can glide. To get to that point you are obviously going to use more gas over that stretch if you average 60 MPG vs. 40 MPG. I realize that we aren't talking about much gas here; I calculate it as a difference of about 0.0083 gallons (or just a little over 1 ounce) for the hypothetical 1 mile stretch. I think I'm probably using more battery power in getting the higher MPG and maintaining speed, but in my situation I very rarely see the SOC drop below 4 bars anyway (it usually resides between 5 and 7). I have no problem regenerating adequate energy due to the need for multiple stops on a hilly commute. So for my situation, it seems to me that maintaining MPG > 1.2 MPH in these limited situations is more efficient over all but doesn't really make that much a difference. Am I missing anything?