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Discussion Starter #1
I can't seem to find a direct answer, but anyone know the explanation for the following two peculiarities about the ICE operation:

1) The 30-second "warm-up" idle when the engine comes on the first time. Is this just good practice overall for any ICE, but we have the luxury of a battery buffer? Or is it something unique to the Volt because the ICE is prepping to operate at WOT?

2) After a couple minutes -- maybe 3 minutes into CS or HOLD mode -- the ICE briefly "pauses" and all power comes from the battery for a few seconds. After that, the ICE resumes, but at a different RPM. Although this brief transition is usually seamless, on occasion (like merging onto a highway), this transition has momentarily reduced my expected available power. Again, not a big deal, but it made me pay attention to this cycle and wonder why.
 

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Yes the warmup is to allow the engine time to reach a decent warmup temperature.
Volts will disconnect the ice and shift to single-motor mode if full power is needed. There is a one or two second delay when this happens. If the ice is coming on due to depleted battery, it will often run faster at first to replenish the battery's buffer. You can somewhat mitigate the momentary hesitation during the "shift" by easing the throttle down in anticipation of a pass or full acceleration. Our Camry hybrid does the same thing. It is all normal.
 

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Not sure but warming up the ICE makes sense. In regards to WOT, according to GM the gen 1 ICE operates at, or near WOT all of the time. Supposedly this increases efficiency by reducing pumping losses.

I haven't noticed the behavior in item 2, but it's possible I just haven't noticed.
 

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The primary reason for the increased rpm during warmup is to quickly heat the catalytic converter and O2 sensors. (Cleaner emissions)

Engine coolant, on the other hand, takes a couple minutes to get up to normal operating temps.


The second part you mention about the ICE briefly shutting off ( cycling) while in HOLD mode is normal, ICE operation is a demand based. Once you set the HOLD level, the ICE will run to maintain that level. However, if let your foot off the accelerator, the car will be in regen mode putting excess energy back into the HV battery. This is especially true if you happen to be going down a slope or long down grade. When you get to the point where the you need to step on the pedal again to maintain speed, any excess battery voltage will be consumed before the ICE will run again.


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I think the behavior in item #2 (gas engine goes to idle and then resumes a few seconds later) will occur if you're on the highway or a constant near-highway speed. I've noticed this and I believe what the Voltec system is doing is switching operating modes from the ICE running in series during warm-up to the ICE running in parallel (direct connection to the wheels) to improve efficiency. I've definitely noticed this in my Volt. It makes sense because the ICE would have to match RPMs and reduce power to make the shift. And remember what you're seeing in the power display on the dash is power - not engine speed.

I usually wait until I'm up to speed on the highway before turning on the ICE so I don't experience that transition on highway onramps.
 

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Discussion Starter #6
The primary reason for the increased rpm during warmup is to quickly heat the catalytic converter and O2 sensors. (Cleaner emissions)

Engine coolant, on the other hand, takes a couple minutes to get up to normal operating temps.


The second part you mention about the ICE briefly shutting off ( cycling) while in HOLD mode is normal, ICE operation is a demand based. Once you set the HOLD level, the ICE will run to maintain that level. However, if let your foot off the accelerator, the car will be in regen mode putting excess energy back into the HV battery. This is especially true if you happen to be going down a slope or long down grade. When you get to the point where the you need to step on the pedal again to maintain speed, any excess battery voltage will be consumed before the ICE will run again.


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Thanks for that answer on #1, but I probably didn't explain #2 well enough. It is something probably only an obsessive kW watcher like me would notice. It's not the CS mode "cycling" series-hybrid behavior. Instead, it happens just within the first few minutes of ICE operation. It goes like this:

Hold Mode pressed (or CS mode reached)
00:00 to 00:30 ICE Idling, battery still propelling car
00:30 to 02:00 ICE WOT, seems to be mostly recharging battery, long green bar on battery side under power menu on dash
02:00 to 02:05 ICE "pause" -- Big yellow line on Battery Side--battery power only propelling vehicle, NO POWER from ICE
02:05 to .... ICE "resumes" at different RPM

While my timelines are just estimates, this is within the first few minutes of the ICE starting. It doesn't happen again during that drive cycle. I'm just curious about the rationale behind what is clearly a normal and intended behavior of the ICE (like the 30-second idle warmup). There is some reason the engineers wanted to pause the ICE, run the car off battery power for just a moment, and then resume. It doesn't seem to matter what else is going on--it seems "built in" to the ICE operation within its first few minutes of startup.

The only thing I can think of is that maybe a planetary gear is adjusting so the ICE can send some direct energy to the wheels if needed and the ICE must "pause" (requiring all battery power) for a moment to ensure there is no "surge" while that adjustment happens.

Just curious about it since I see the behavior every morning on my commute (I watch the kW power display too much!).
 

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For #1, the engine doesn't "prep for" WOT. It is always WOT.

For #2, there seems to be some confusion between time and power. If the system calls for full power (merging), a hesitation will be evident as it switches from CS to all EV for that short burst. The "higher RPM" is probably the recovery of the lower SOC after the call for full power. The timing "issue" may be the OP's misinterpretation of the power readings.

This all sounds normal to me. Not peculiar. Explaining a black box can lead to different interpretations based on what the human is concentrating on during the observations. And then running that data through the filters of past experience.

As far as I know, no one on this forum has directly reverse-engineered the millions of lines of code that roll up to Volt's operations.
 

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ICE can send some direct energy to the wheels if needed and the ICE must "pause" (requiring all battery power) for a moment to ensure there is no "surge" while that adjustment happens.

Just curious about it since I see the behavior every morning on my commute (I watch the kW power display too much!).
Volt is not a Prius. The main drive is always EV.
 

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Volt is not a Prius. The main drive is always EV.
Exactly. There are times when I floor it, the car takes off, then a few moments later the ice kicks in. Other times I'm driving along at a high speed and the ice shuts completely off. I could come to a stop light, stop, the ice is WOT for a few seconds, then abruptly stops. It's like it has a mind of its own, but it is easily explained as an on demand generator that reacts to the battery's needs which also kicks in for heat (erdtt, or when the battery is really cold and the ice needs to supply all the propulsion energy until the batteries warm up sufficiently). Don't try to understand it, just embrace the wonderful engineering and enjoy the ride.
 

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Volt is not a Prius. The main drive is always EV.
Not exactly.

What the OP is describing (as several others have already pointed out) is the Volt switching to 2-motor mode, in which the engine has a direct mechanical path to the wheels to allow for more-efficient operation during highway cruising. But in order to "shift" into this mode, certain clutches have to be engaged/disengaged and RPMs matched, so that's what's going on.

I linked to an article describing this below. (It's kind of an old, early article, so there may be newer posts with a more complete description of how this all works,but as far as I know it still holds up.)

(Edited...)
I believe* the sequence goes:
(1) clutch between MG-A and the engine is disengaged
(2) clutch between MG-A and the (stationary) ring gear is engaged
(3) clutch between ring gear and case is disengaged
(4) clutch between MG-A and the engine is re-engaged

(*If I got any of that wrong, someone please correct me. I'm not 100% sure of the exact sequence.)


http://gm-volt.com/2011/12/09/solving-the-volt’s-extended-range-combined-mode-power-split/

Don't try to understand it, just embrace the wonderful engineering and enjoy the ride.
As an engineer, I say: "Blasphemy!!! I must understand how everything works!!!" :p
 

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Discussion Starter #11
Volt is not a Prius. The main drive is always EV.
Ok, Ok, I know better than to say things less precise, I was just referring to Mode 4, the high-speed series-parallel hybrid mode, where the engine begins to directly drive the ring gear. I was thinking that the car might be preparing for this and it temporarily needs to discharge the battery heavily while it switches. I'll try to take a video.

I was just trying to learn more about the car and the reasons for some of the ICE behavior.
 

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#1 is 50 second warmup @ 1400 RPM then it will use engine as required (which could also mean shutting off vs revving up)

#2 is it switching in or out of two-motor mode (which as a side effect pauses the engine so that the clutch can engage or disengage)
 

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I've seen the direct/parallel drive mode mentioned a lot but never at what speed.
Presumably the efficiency becomes greater which triggers the switch, but is only speed (>100km/h ??) or are there other factors taken into account?
I could see the engine load being more important then speed, since it would integrate slope, temperature, tire pressure, passengers/weight, etc.
 

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Not exactly.

What the OP is describing (as several others have already pointed out) is the Volt switching to 2-motor mode, in which the engine has a direct mechanical path to the wheels to allow for more-efficient operation during highway cruising. But in order to "shift" into this mode, certain clutches have to be engaged/disengaged and RPMs matched, so that's what's going on.

I linked to an article describing this below. (It's kind of an old, early article, so there may be newer posts with a more complete description of how this all works,but as far as I know it still holds up.)

I believe* the sequence goes:
(1) MG-A slows the engine to (near?) 0 rpm
(2) clutch between MG-A and the (stationary) ring gear is engaged
(3) clutch between ring gear and case is disengaged
(4) MG-A spins engine back to appropriate RPM and engine restarts

(*If I got any of that wrong, someone please correct me. I'm not 100% sure of the exact sequence.)


http://gm-volt.com/2011/12/09/solving-the-volt’s-extended-range-combined-mode-power-split/



As an engineer, I say: "Blasphemy!!! I must understand how everything works!!!" :p
Close. Actually the car uses the throttle to hold the engine at a fast idle that's near the final target rpm while the shift is happening, so the first step is just the clutch disconnects and the throttle idles (which requires all motive power to briefly come from the battery, as the OP noted.)

The only important piece that I didn't see anyone mention is that one of the principle reasons for the 30 second warm-up cycle is to get the catalytic converter to operating temperature quickly for reduced emissions. It's also helpful for other things, of course.
 

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For #1, the engine doesn't "prep for" WOT. It is always WOT.
The engine is not "always WOT". I notice this posted two or three times here. The RPM varies depending on load. Sometimes it is at high RPM sometimes it is just barely more than an idle.
 

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The only important piece that I didn't see anyone mention is that one of the principle reasons for the 30 second warm-up cycle is to get the catalytic converter to operating temperature quickly for reduced emissions. It's also helpful for other things, of course.
You must not have read the first part of post # 4.

My understanding of the voltec system is that the ICE is not directly connected to the drivetrain like that of a Prius type system, It is just a generator.

Perhaps I need to review a system demonstration video again.

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Close. Actually the car uses the throttle to hold the engine at a fast idle that's near the final target rpm while the shift is happening, so the first step is just the clutch disconnects and the throttle idles (which requires all motive power to briefly come from the battery, as the OP noted.)
That does seem to make more sense.

Is this right...?

(1) clutch between MG-A and the engine is disengaged
(2) MG-A slows and clutch between MG-A and the (stationary) ring gear is engaged
(3) clutch between ring gear and case is disengaged and MG-A speeds back up
(4) clutch between MG-A and the engine is re-engaged
 

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You must not have read the first part of post # 4.

My understanding of the voltec system is that the ICE is not directly connected to the drivetrain like that of a Prius type system, It is just a generator.

Perhaps I need to review a system demonstration video again.

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That was GM's initial position, and for some reason they took a lot of flack for announcing later that the engine does drive the wheels at freeway speeds when the engine is on anyway. It improves economy by ~10%, and they announced it in detail with videos before the car went on sale, but still a sizeable crowd complained that GM lied to them, and made it sound like they would prefer worse gas mileage. Sometimes I don't understand people.

First generation Volts have four operating modes, two with the engine on. In steady state driving at over 36 mph with the engine on, the car will switch to power split mode, during which the engine drives the wheels mechanically in a fashion that's pretty similar to the Prius.
 

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That does seem to make more sense.

Is this right...?

(1) clutch between MG-A and the engine is disengaged
(2) MG-A slows and clutch between MG-A and the (stationary) ring gear is engaged
(3) clutch between ring gear and case is disengaged and MG-A speeds back up
(4) clutch between MG-A and the engine is re-engaged
That matches my understanding and what I've seen on OBDII instrumentation.
 

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That was GM's initial position, and for some reason they took a lot of flack for announcing later that the engine does drive the wheels at freeway speeds when the engine is on anyway. It improves economy by ~10%, and they announced it in detail with videos before the car went on sale, but still a sizeable crowd complained that GM lied to them, and made it sound like they would prefer worse gas mileage. Sometimes I don't understand people.

First generation Volts have four operating modes, two with the engine on. In steady state driving at over 36 mph with the engine on, the car will switch to power split mode, during which the engine drives the wheels mechanically in a fashion that's pretty similar to the Prius.
That's interesting. Thanks for the clarification. Now I will need to review a video tutorial on voltec again.


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