Very frustrated (winter MPG)

[Nerys]

so thats why I don't see much of a drop then. my commute is so long that even though warm up is longer its a small fraction of my overall drive.

wow on the outback. For some reason I thought they got pretty good mpg. even my van does better than 8mpg in the city (12-14mpg) why do you think the outback gets such low mpg in the city?

[brucey]

I usually get better, the 8 MPG was because it was plowing through a foot of snow. It never left first gear.

[zjrog]

For my Jeep, I reset the computer last month and the last two fill ups have netted me 20+... Even with my daughter driving it. Better than the 17 though. So why don't I get better mileage when its warmer?

[SentraSE-R]

Nerys,

You aren't looking very hard, if you're not seeing retreads on your roads. Do a web search for truck retreads Pennsylvania, and you'll find plenty of dealers selling them.

[Superturnier]

Originally Posted by zjrog

For my Jeep, I reset the computer last month and the last two fill ups have netted me 20+... Even with my daughter driving it. Better than the 17 though. So why don't I get better mileage when its warmer?

zjrog, What brand is your efi computer? Is it maybe Renix?

http://www.pirate4x4.com/tech/billav...ion_manual.pdf

The reason why I'm curious, is that I have noticed that my winter car gets better mpg when weather is colder. My winter car is a 1994 Renault (made in France) and it has renix type efi.

I'm monitoring the mpg daily with the MPGuino and this week I had some interesting results:

Yesterday morning:
Trip from home to work, 13km (8miles) Temperature -2C (29F) Engine block heater used 3 hours before start. FE resulted 6.7L/100km (35mpg)

This morning:
Same route, same pre-heating, only the temperature was different: -13C (8.6F) And FE resulted 5.8L/100km (40mpg)!

After resetting the computer (battery disconnected) the FE was much worse on this trip: 10,5L/100km (22mpg). There must be some kind of adaptation in the ECU. The next day things went back to normal. First I thought the bad FE was because I didn't use the block heater, but the reason is more likely the computer reset which I did at the same time.

I re-tested the effect of the block heater and it seems that using block heater does not make so much difference in FE. I got 6,3L/100km without block heater

So, Do any of you have an idea, why I'm getting better FE results when it's very cold (not just cold)? I think it must be the code inside the efi computer, maybe it has different mapping for different temperatures. Then maybe others with the same brand of efi could get similar results?

[brucey]

If you've got a block heater, that will negate a lot of the short trip wear and tear/high gas consumption from the engine.

Also, I've just recorded my worse tank ever. 14.4 average for a tank. Bahhumbug.

[Superturnier]

I had an idea while sitting on a couch with a glass of red wine...

Could it be so that the mapping in the efi computer was designed so that in freezing temperatures the fuel mixture is allowed to go leaner, since the cold air lowers the burn temps inside the cylinder and with cooler burn there is not risk of NOx pollutions.
That leaner fuel mixture would explain why my FE seems to be better at freezing temps.

This would mess up your WAI theory a little bit, Sorry about it.
I have to make further investigations about this...

[tjts1]

I've never been a fan of the warm air theory but that's another story.

Speaking from my own personal experience using megasquirt, there is an add on piece of software called 'mega log viewer. You essentially tell the computer what your ideal air fuel ratio targets are all RPMs and loads are, drive the car for 30 minutes, compile the data on the laptop which generates a new VE (volumetric efficiency) table. You repeat the process a few times and the VE table becomes closer and closer to your ideal AFRs. The closer you get to the AFRs you specified, the smaller the O2 sensor corrections will have to be and therefore more accurate. Eventually with this process the law of deminishing return catches up to you and you know you VE table tuning is finished.
The point being, if I do all my VE table tuning at around around 60-70f and then I try to do another thing session at 45f, the computer will want to bring down my VE table about 1-2% in order to maintain the same AFRs. This isn't a problem when you're done tuning because the wide band O2 sensor can easily correct for this in real time. So yes I also believe the engine is more efficient at lower air temperature once the block has reached operating temp. It's a more efficient air pump. The engine has to do work in order to suck in a given volume of air. Colder air is denser so the engine does less work for a given ammount of O2. Also the efficiency of of the engine depends on the temperature difference before and after combustion. The bigger the temp difference the more efficient the combustion process is.

[Nerys]

Hmm No doubt you probably know more than I do but the last part about temperature difference is logically confusing to me.

a perfectly efficient engine will be ambient temperature. Ever erg of energy used to "heat" something is energy NOT USE to "move" something. ie wasted energy less efficient.

Can you clarify? its my understanding that heat = bad because heat is not whats moving the car so its wasted energy.

BUT I also know that a HOTTER energy is more efficient. at one time they were trying to make a ceramic engine so they could use HIGHER temperatures but it was too brittle.

so I am confused.

[Superturnier]

I corrected my previous post about the use of a block heater.

I re-tested my route without pre-heating and the FE was not significantly worse than with pre-heating.
But if you need to warm the engine at idle a few minutes before leaving, then it starts to make a difference.