Driving like an idiot increased my MPG

[GasSavers_Red]

To the tune of 3 MPG give or take.

On the way home after class and a nice 4.something earthquake I felt like flogging the Jeep the 15 miles home. So I did everything FE wrong; floored it out of stop lights, red lined through the gears, on the highway set cruise control for 65 but maintaining 70-75 running around trucks. The way back home is all uphill too.

By the time I got home my trip average was 19.9 MPG instead of the usual 16.9, which is baffling to me. I was expecting something along the lines of 10-12 considering what I was doing. Any ideas why abusing the engine increased my FE? Its not carbon build up, I ran some Seafoam through it about a month ago(~1000 miles)

[The Toecutter]

Increasing your engine load usually increases its thermal efficiency, but it is also true that efficiency tends to drop with increasing rpm.

I'd need a BMEP vs rpm vs fuel consumption map of your engine to really get an idea of what is going on(assuming the engine is free of problems).

[Peakster]

I too sometimes find that it's better to have some get-up-and-go to get to the best MPG speed. Remember, Fuel efficiency is MPG over distance. If you floor the vehicle and get 5 mpg over 500 feet (and reach your cruising speed) rather than 30mpg over 4000 feet by gradually accelerating under boggingly-low RPM (and then reaching the same cruising speed), you'll get better FE.

This is actually one of the problems (or trends) I'm currently finding. With all this ice on the roads I can't get up to cruising speed for a very long time, whereas if it was dry outside I could accelerate to 50mph easy and set the cruise control right there after the shorter accelerating distance.

[omgwtfbyobbq]

Yup yup. Flooring your engine results in way better efficiency than babying it. Look at the average BSFC on the top line, compared to near the bottom.

If you baby it up to whatever cruising speed with a light throttle you'll be in the 500g/kwh range, but if you floor it up to the same speed, you'll be in the 300g/kwh range. So, give lots of throttle when accelerating, and short shift at ~3k rpm in each gear for the best mileage. The BSFC map for your engine won't look exactly the same, but it's a fairly safe guess that peak efficiency happens at ~2/3 throttle at ~2-3k rpm.

[caprice]

I have head of that before. I thought acording to phisics, accelerating at a gradual pace than rapidly takes less energy? Or not?
Also, running an I.C.E. at higher RPM is less effecient Because the fuel mixture can only burn so fast, at higher RPM it gets pushed into the exaust before it is completely burnt, wasting potental energy?

david

[The Toecutter]

omgwtfbyobbq, where are you getting these maps from? where'd you get the TDi map you posted months ago, for instance? I am in want of information for an article I'm writing on how to design high mileage cars with high performance from the ground up.

[cfg83]

caprice -

Originally Posted by caprice

I have head of that before. I thought acording to phisics, accelerating at a gradual pace than rapidly takes less energy? Or not?
Also, running an I.C.E. at higher RPM is less effecient Because the fuel mixture can only burn so fast, at higher RPM it gets pushed into the exaust before it is completely burnt, wasting potental energy?

david

I have gleaned that every engine has a sweet spot for efficiency, right? If the car is being run in that "zone", then maybe Red could be using his engine in an optimum manner.

I think that people have said here that the principle of accelerating slowly is correct, but in some cases, you can be driving so far outside the engine's sweetspot that it starts to work against you (i.e. lugging).

Does this make sense ?

CarloSW2

[omgwtfbyobbq]

Originally Posted by cfg83

I think that people have said here that the principle of accelerating slowly is correct, but in some cases, you can be driving so far outside the engine's sweetspot that it starts to work against you (i.e. lugging).

Does this make sense ?

CarloSW2

I don't think so. We want to accelerate fast up to our target cruising speed because if we accelerate fast the engine's operating more efficiently than if we accelerate slow. We also want to short shift because after ~3k rpm friction losses start to hurt our BSFC, but even then, it's still more efficient to floor/redline than it is to grandma it with a really light throttle. Incidentally, that's why someone who lives in a hilly area can get really good mileage pretty easily in a M/T car. Going down they can coast with the engine off, and going up they give it lots of throttle with the engine operating near peak efficiency, as opposed to driving at a steady speed with no change in elevation, since the engine will only be at some fraction of throttle and lower efficiency.

[Silveredwings]

Driving like an idiot increased my MPG

I'm surprised that doesn't seem to work for the masses...

[rh77]

A few questions and points:

~Did you completely floor it, meaning it was in open loop on acceleration?

If so, that is odd, but

~Once you get up to speed, it's much more efficient to maintain speed

I've found (even with the automatic) that short-shifting into "2" forces the 1-2 shift, then I can load it up to 3K with about 70-80% throttle and back off to have it shift into the 3rd, repeat (more with about 35-40 %), then 4th (have to modulate those gears -- full automatic at that point -- if I get too squirelly with it, it shifts back-and-forth, which is frustrating).

2 examples -- over 2 miles of operation on level ground, up to 60 mph:

Agressive acceleration over 0.5 miles for an average of 12 MPG. 1.5 miles are spend at a cruise of 30 mpg. ((0.5 x 12) + (1.5 x 30)) / 2 = 25.5 mpg avg.

Granny Acceleration over 1.0 mile, gaining 15 mpg, and 30 mpg cruise for 1-mile.
(15+30)/2 = 22.5 mpg avg. A 3.0 mpg decrease.

As you can see, in many cases it pays to get up to speed.

The only drawback is using all of that energy to get up to speed, only to stop at the next light -- that's when Granny Mode comes in handy. But...if you're getting on the highway and staying there for a while, get up to the target speed without delay takes advantage of cruising speeds early.

Also, TJs have a lot of torque for their utility, so you might be using a lot of fuel in the lower RPMs -- just a guess there.

RH77