2008 AWD FEH, no NAV.

When I first start out in the morning (engine completely cold), the FEH turns on the engine, but apparently most of the propulsion is coming from the batteries. I say this because the Throttle Position Sensor (TPS) is set to a certain value - which varies, but is generally 16 - 18 - and does not change, and the RPM does not change. My energy meter goes to "assist". This is all normal for the FEH, and not new.

This morning I was watching the guages, and I realized that my SG 2 was indicating about 28 MPG, until I let off the accelerator, at which point the "assist" meter went towards charge, the TPS did not change, but the MPG went to about 35 or better. I didn't think to check the RPM guage.

Why would the MPG get better when I eased off the accelerator, if the primary propulsion was the electric motors? I would have expected the TPS to change to a lower number, indicating less gas was being used.

FWIW, my TPS is never lower than 11.

Any ideas?

 

There is a couple of statement in your post that need to be "corrected" and lets wait for the long time FEH owners and experts to post their reply.

 

Hopefully not wwest, since he is on my "ignore" list!

 

It's good he isn't a "long time FEH owner"

 

Sounds like you let off the accelerator right when the FEH was coming out of warm-up mode. It does this at a CAT temp of about 1250F on my 2005. Your IGN value probably went up and the rich bias closed loop mode was probably exited.

 

What you said is true. For the first ( 30 seconds? ) of a cold start, you are getting primarily electric propulsion. But not exclusively electric propulsion.

Letting off the pedal you went from slight gas powered drive to no gas powered drive is my guess. See if you can repeat your data three times or more and see if you can nail down the pattern.

I think the explanation will present itself.
Carl's explanation is also very good, and kind of what I'm trying to say.
The engine went from slight load, to no load. Fuel Trim probabably changed, and I gurantee RPM's changed, if only slightly.
HTH,
-John

 

I should have clarified that in my 2005 FEH, a sample size of one, virtually all of the motive force is being supplied by the electric motors during warm-up mode. This can be seen by the fact that the generator torque is countering the small amount of torque the ICE is producing while the motive torque is being supplied by the traction motor. Maybe in extremely cold starts this is different. For sure if you mash the accelerator in warm-up mode you can force the ICE to add torque but in all other cases its contribution is as close to zero as it can get during warm-up mode. Warm-up mode can be identified by an IGN reading around -10 or -11 and positive GTQ. LOOP may be open or closed but is only open on my FEH for a few seconds after initial start-up. When closed loop it is in a rich-biased mode during warm-up but that can only be seen with an oscilloscope or data logs that show a non-50% duty factor for the O2 sensor signal. The ICE RPM during warm-up mode depends on the temperature but for mild temps it is not much above 1000 in my FEH, but increases as warm-up mode is exited. A rough estimate of ICE torque can be displayed with a built-in xgauge like the horsepower xgauge, but LL no longer documents it due to the strange readings it sometimes produces and the fact that it is not all that accurate. At any rate, in warm-up mode the faster you accelerate (up to the point you force the ICE to contribute,) the higher your MPG reading will be, but the faster your SoC will drop.

 

FWIW, during warm-up, my 2005 is always at 1200-1300 RPM.
I know mine is done with warm-up when it drops to 1000 RPM.

( I also drive primarily between 3500 ft and 5000 ft elevation which may have everything to do with it. There are many variables to consider. )

In severe cold ( sub zero F ) the rpms are 1600-1700.

Once or twice in very, very cold, mine would stay ~2000 RPM.

We still don't have a good description of why that is.
But many people report this, so it is assumed to be 'normal'.

 

It was in the mid 50s this morning, and I checked it again. The RPMS were at about 1550 (I don't have RPM on my SG, maybe I'll try that next). The same thing happened; the TPS remained at 17 and the RPMs did not appear to change.

It is obvious when the FEH exits this "first start" mode - the RPMs and the TPS begin to fluctuate based on my accelerator pedal imputs.

 

Something else unusual happened this morning. I was at full warm up (having driven for 15 minutes), HWT at around 160, sitting at a stop with the engine off, waiting for a stop light. When I accelerated, the FEH went into the "early morning cold start" mode for about 15 seconds - the engine went to about 1550 RPMs and the TPS stayed at 17, regardless of how hard I used the pedal (within reason, of course). After about 15 seconds it reverted to normal operation.

I sure wish I could figure out what enabled this behavior - I could use it to accelerate to cruising speed on a regular basis. I think I may have stumbled on a programming quirk in the FEH computer module. Of course it may only occur at certain environmental levels - it was damp this AM due to rain, for example, and about 55 degrees.