Terminal F/E; What's yours?!
 

After playing around with my VAFC for a little bit (fine tuning Vtec engagement) I decided to find out what my terminal fuel economy would be using a very simple method... Given that my car runs at 2700 Rpms during cruising speed, I tested the unloaded engine (vehicle stopped, trans in neutral) for fuel consumption in gph and netted approx 45mpg terminal F/E

70mph / 1.55gph = 45mpg

This would be the resultant mpg my Acura would make at a constant 70mpg, flat road, with no rolling resistance, air resistance, or drivetrain losses. Obviously this on its own does relatively no good, but it does allow for a realization of expectations, and will help guide you with the use of EOC, DWL, P&G, DFCO, aero mods, etc. in order to help achieve your goals

So my question is, what's yours?

Does this men your car used 1.55 gph at 2700 rpm, not moving, warmed up, with all accessories switched off? I think that's what you're saying.

If so, try this calculation. Turn the math around and divide gallons by miles. That will tell you how much fuel is being consumed per distance, JUST to keep the engine turning. That is, not considering any useful output from it.

Now compare that number to your actual usual gallons per mile at that cruising speed. For my car, I found a surprisingly high percentage of fuel was used just to spin the mechanicals.

Now, if only you could cover distance without rotating your engine quite so many times. You'd save all that fuel.

Voila. The reason why many of us are big fans of coasting, with the engine idling in neutral or off as appropriate. Every revolution of the engine eliminated is fuel saved. You're better off at 2/3 or 3/4 throttle part time and engine "on vacation" the rest of the time, than you would be driving in a reasonable gear at whatever throttle position would work at that speed.

Using that technique, your terminal mpg isn't terminal at all because you can cover distance on highly reduced fuel use or none at all.

unrealistic of real-world driving conditions.

2700 RPM while not moving is a significantly different situation compared to 2700 RPM while accelerating in any gear, 2700 RPM while holding a speed in any gear and 2700 RPM while engine braking in any gear.

the load placed upon the engine is different in all of those situations.

Yes, that is exactly what I did, netting a Terminal F/E of approx 45mpg at 70mph.


Yes, I understand all this, hence "Terminal."

My point in this thread is that by calculating your completely unloaded engine F/E at a given RPM, is that you CAN NOT exceed (nor by any chance meet) said MPG without the addition of other methods for extending fuel economy.


Was my original post really that hard to understand?

Interesting idea.

I think for me, a more usable figure would be to run it in gear, drive wheels off the ground, at "cruising speed", & see what THAT figure is. (we can actually get "miles covered" that way, too). I know that adds in the drivetrain drag, but how do we realistically reduce that? If you're going to say thin or synthetic oil, etc, etc, ok good. But (to me, again), maybe THAT's the way to test the synthetic oil, etc, etc. - wheels jacked up, running at cruising speed in gear. To me, that removes the aerodynamic and "real life" road variables (wind, grade, traffic, etc), but still leaves in the "real world" drivetrain & wheels turning.

What do you all think of that?

Or am I out in "left field"?

Not at all in left field!... That would be a great way to include the drivetrain losses in the calculation. Unfortunately though, running with the wheels off the ground creates a whole mess on its own... No matter how balanced your tires may be, they will not rotate at such a high rate of speed without excessive hop, as every forgiving tolerance becomes less loaded (bearings, CV's, etc.) If you've ever seen a HMMWV walk itself off of jack stands, you'll know how dangerous this can be!!! O.o

Utilizing a dyno would mitigate these dynamics, but unfortunately we don't all have one in our garage!

So you're cruising at 70? You could save quite a bit if you slowed that down.:turtle:

nbleak21's original post here shares an interesting idea. I and others have gone off on various (interesting?) tangents. That said, nbleak21 deserves credit for the idea.

Whoa! Yeah, "walking off the jackstands" certainly wouldn't be good - especially if you weren't prepared for it. :o

Hmmm... before I go out & do it, I guess I'd better come up with some "safetys"!!

Thanks! :thumbup:

Yessir! Ideally I would gain a few MPG dropping my speed, but most of my commute is interstate, and slow lane speeds avg 65-70mph travelling with the flow of traffic, so generally I find a semi to get some limited draft off of (I don't do the tailgate drafting!)