Instantaneous MPG readings for best fuel economy

<div class='quotetop'>QUOTE(ystasino @ Jul 9 2007, 04:14 PM) [snapback]475710[/snapback]</div>

Hmm, good question. I'll keep an eye on that. Even better would be to look at it through data logging, the option for which I have with my Can-View. But I'm having some problems with data capture. Once I get the problems sorted out then I can tell what those relationships are for sure.

Meanwhile, I was reading some old PC posts related to fuel economy, and I stumbled upon this. In particular, this caught my eye:

" ... this means that instantaneous MPG is less than MPH but more than half of MPH." [Emphasis added.]

Theorist, if you're reading this, I swear I didn't steal this from you.

 

I propose an update to the original rule. In short, for the average driver, don't worry about the low-RPM component of keeping iMPG below vehicle speed x 1.2 during ICE-on driving segments. First, as some have suggested here, it's a little more difficult to make that mental calculation on the fly. Second, recent reports from others suggest that sub-1400 RPM ranges likely don't cause a significant hit to fuel economy, and actually could help.

Meanwhile, the high-RPM component of the rule -- keeping iMPG above half the vehicle speed -- has been further validated. I address it in the study results on high speed driving methods I recently posted, and Hobbit has confirmed it as well.

I will add that Hobbit is not among the advocates of low-RPM methods, so depending on which approach you believe is best, you might choose still to follow the iMPG < (MPH x 1.2) rule. Or less precise, perhaps, but easy: simply keep iMPG less than vehicle speed during ICE-on.

 

Yet another update, or more accurately, more validation of the update I proposed in the previous post.

The subject of how much go-pedal to give during ICE-on conditions still is a matter of substantial discussion. I don't care much for subjective terms like "brisk" or "moderate" acceleration. What is moderate to one person may be either brisk or gentle to others. The car's limited factory instrumentation certainly doesn't help. So I proposed an objective measure here that anyone can apply without added instrumentation.

Low-RPM conditions are thought by many to be inefficient, and the theoretical power curves of the Prius seem to confirm this. That is what led to the iMPG < MPH*1.2 part of the original rule. But anecdotally, many hypermilers (me included) have reported good results with keeping RPM as low as 1200 or 1300. That pushes the iMPG well above the vehicle speed, perhaps up to x1.5. It also is gentle acceleration by anyone's definition, despite the subjectivity of the word "gentle."

I have done some preliminary testing comparing various acceleration rates during pulse and glide, and the results suggest rates in the range of 1300 are at least as efficient as those in higher ranges. The key word is preliminary, however; the test environment was less than ideal (traffic disrupted my glides on the two-lane rural road I was using), and I'd like to repeat the tests in a different environment.

Meanwhile, Hobbit recently has experimented using fuel injector timing as a surrogate indicator of engine load. The ICE really is inefficient when it's not running under load, and his view seems to be that as long as injector timing has reached a certain operational plateau, the ICE is loaded. Quoting him:

"Even a fairly gentle startoff from the line yields injector times of high sixes or 7.x at ridiculously low RPMs like 1250, as high torque is produced to help takeoff."
​

My own recent experience applying this and using injector timing as my load criterion (monitored in CAN-View) suggests that in ICE-on conditions at low speeds it's actually difficult for the ICE not to be under load. Especially from a dead start, after "goosing" the go-pedal just enough for the ICE to light, I show injector timing levels well above the "loaded" threshold even with these low RPM.

So to summarize, what this means to me in the context of this thread is that at low speeds, the iMPG can be well above MPH and the engine still is under load and, therefore, the car accelerates efficiently. Again, this seems to validate that as long as the ICE is on, the low-RPM component of the original rule of thumb isn't terribly important.