Speed vs mpg in 6th gear wot

[nubbzcummins]

Did a few runs yesterday in 6th gear wot to observe instant mpg readouts. Started at 45 mph and went up to around 80-85 mph. This was done in manual/sport mode in my auto car to where it will stay in one gear without downshifting however I do not think the tc will lock up. Do not remember the specifics however mpg started around 16 mpg at 45 mph and was up near 20 mpg at 80-85 mph. I will do some more runs and write down the details. Any ideas on why mpg climbed with speed in this test? Something to do with bsfc of motor since speed and rpm was only thing changing? Btw very slow accel in 6th gear wot in this car!!!

[razor02097]

An engine has a "zone" where it is happy and efficient. When you stomp the throttle at low engine speed it is known as "lugging the engine" which is very bad.

What happens is the ECU greatly reduces power by increasing fuel and retarding the timing. It is less controlled because at WOT the ECU enters what is called a power enrichment mode. Typically this involves temporarily entering open loop mode taking the O2 sensors out of the fuel calculation equations. Power enrichment mode is not desirable when you have fuel economy in mind not only because you are using large amounts of fuel to travel the same distance but since the ECU is ignoring the O2 sensors the engine typically ventures into a rich burning state which can be bad for plugs, cat converters, etc.

You aren't really making usable power from the extra fuel but the ECU is making effort to control engine damage... Dumping mass amounts of fuel in and retarding the timing helps the engine lug along until it reaches a point where it is happy again. Once the engine is happy i.e. reaches an appropriate RPM it will add timing and begin to actually make power.

On a side note... 6th gear is an overdrive gear it really isn't meant as an acceleration gear it is meant to be economical.

[nubbzcummins]

Originally Posted by razor02097

An engine has a "zone" where it is happy and efficient. When you stomp the throttle at low engine speed it is known as "lugging the engine" which is very bad.

What happens is the ECU greatly reduces power by increasing fuel and retarding the timing. It is less controlled because at WOT the ECU enters what is called a power enrichment mode. Typically this involves temporarily entering open loop mode taking the O2 sensors out of the fuel calculation equations. Power enrichment mode is not desirable when you have fuel economy in mind not only because you are using large amounts of fuel to travel the same distance but since the ECU is ignoring the O2 sensors the engine typically ventures into a rich burning state which can be bad for plugs, cat converters, etc.

You aren't really making usable power from the extra fuel but the ECU is making effort to control engine damage... Dumping mass amounts of fuel in and retarding the timing helps the engine lug along until it reaches a point where it is happy again. Once the engine is happy i.e. reaches an appropriate RPM it will add timing and begin to actually make power.

On a side note... 6th gear is an overdrive gear it really isn't meant as an acceleration gear it is meant to be economical.

I did this test just to see what mpg would be in highest gear wot with increasing speed and rpm. Also checking in severial gears to see what would trigger open loop/fuel enrichment. I found that to go into open loop I had to be near wot, lower gears, higher rpm which I can not do in the higher gears due to the speeds that would be reached. Also noticed that in open loop/fuel enrichment conditions afr was in the 11.5-12 range. During this test it stayed in closed loop however afr does get in the lower 13's vs normal 14.7 with less throttle. I guess the cars computer can make the mix richer than 14.7 in closed loop or it is not telling me it is in open loop. Either way it was gaining mpg with speed and rpm which I guess is a sign of bsfc overcoming all of the drag factors?

[razor02097]

Take a look at the timing advance number. I would bet dollars to donuts that the computer is pulling out timing during the lower RPMs. Your ECU is trying to prevent detonation by pulling timing and dumping in the fuel hence the low afr.

Like I said your fuel economy went up because the ECU was finally able to start to add timing as the RPMs went up.

[nubbzcummins]

Originally Posted by razor02097

Take a look at the timing advance number. I would bet dollars to donuts that the computer is pulling out timing during the lower RPMs. Your ECU is trying to prevent detonation by pulling timing and dumping in the fuel hence the low afr.

Like I said your fuel economy went up because the ECU was finally able to start to add timing as the RPMs went up.

Ya I know about the timing and fuel dumping and what you mentioned in earlier post..... I did not realize it could dump the fuel in closed loop though so that was unexpected when I saw that. I thought this could only be done with a piggyback or standalone controller..... At first I was like cool, closed loop=14.7 afr and wot, then look at afr and was in the low 13's and was like??? I meant to look at the timing numbers and totally forgot to probably due to the afr and closed loop throwing me off! That is another reason I am going to do this again to see what timing numbers are doing. Also going to test in different gears to see if it has the same correlation and if so at what point mpg will begin to fall off.

[pete c]

there is a big difference between WOT and full load. A lower rpms, you can reach max load (a good thing) at a fairly low throttle opening. Going past that point is when things go to pot.

[nubbzcummins]

Originally Posted by pete c

there is a big difference between WOT and full load. A lower rpms, you can reach max load (a good thing) at a fairly low throttle opening. Going past that point is when things go to pot.

Ya I am going to look at what point I reach max load at certain speeds/rpm/gear as well as what conditions trigger it to richen the afr's and alter timing in closed loop. Once I find this out it will give me an idea on what conditions I need to avoid in hopes of improving mpg.

[nubbzcummins]

Ok here is what I have found:
With wot in 6th gear going from 50-70mph:

mpg is around 16mpg at 50mph and goes up aprox 1mpg/5mph from there.
absolute load starts at 89% and climbs slowly to around 91% by 70mph.
calc load is maxed out at around 95% and does not move.
timing starts around 10-11deg and slowly goes to 7-8 deg.
afrs stay in lower 13's.

When I am going down the road at same speeds as tested and ease into the throttle the calc load maxes out before absolute load does. Also afrs slowly creep richer with more throttle and both load values however dive faster once more throttle is applied after max calc load is achieved. The throttle point here depends on speed however I do not know what throttle position reading to go by. Per scan tool my car has around 4 tps readings that are all different numbers and unsure which is most dependable/accurate to go by? Also should different spark plug heat ranges and or octane rating have any effect on the timing being ran since it is indicating that it is in closed loop?