CS mode: ~2200 RPM, ~1500 RPM, then "CD mode"

[scottf200]

So last yesterday I drove to work (11.5*2) and to racquetball (19*2) on a couple charges but on the way home from racquetball ran out of battery (Charge Depleting). I was experimenting in cruise control driving 55 MPH in the right lane.

Observation with Power Flow display up:
Initially switching over to CS (Charge Sustaining) the RPM was around 2200 for say 2 minutes, then it went to 1500 RPM for say 4 minutes, and then it went to "CD" mode (ie. using the battery) for say 1 minute. I saw this happen a couple times. I didn't time it so times may be off but that is what it felt/recalls like.

At that point in my drive I only had 15 or so minutes to get home so my experiment was pretty short. I think the outside temp was around 40.

Note: RPM numbers came from my ScanGuage.

P.S. Love the Power Flow display for learning. On our recent long drive where we were in cruise control for miles and miles at 65 MPH it was very surprising initially to see the regen braking working on a slight decline ... then it became a duh moment.

[George S. Bower]

So last yesterday I drove to work (11.5*2) and to racquetball (19*2) on a couple charges but on the way home from racquetball ran out of battery (Charge Depleting). I was experimenting in cruise control driving 55 MPH in the right lane.

Observation with Power Flow display up:
Initially switching over to CS (Charge Sustaining) the RPM was around 2200 for say 2 minutes, then it went to 1500 RPM for say 4 minutes, and then it went to "CD" mode (ie. using the battery) for say 1 minute. I saw this happen a couple times. I didn't time it so times may be off but that is what it felt/recalls like.

At that point in my drive I only had 15 or so minutes to get home so my experiment was pretty short. I think the outside temp was around 40.

PS glad you are using scan gauge....not an MDI but I think as we go forward this will be where the action is in the learning curve on this car. I especially am eager to see what after market software/hacks come out. The possibilities are endless.

Note: RPM numbers came from my ScanGuage.

P.S. Love the Power Flow display for learning. On our recent long drive where we were in cruise control for miles and miles at 65 MPH it was very surprising initial to see the regen braking working on a slight decline ... then it became a duh moment.

Hi Scott,
Interesting observation. As we have speculated before GM has programmed the system to run the engine at lower RPM than where the SFC islands are the best. ie one would think that a higher load point would get better SFC out of the ICE than 2200 or 1500 RPMs. They probably do this for 2 reasons: 1) NHV and 2) betting that perhaps some regen will come into the picture and the juice would be "free".

There was a thread where this was discussed and Herm was pointing out that the Hyundai Sonata picks a higher load (for better SFC). Also we have discussed a mode that could be added to the software whereby the driver could select the higher load option. This might be useful if one knew that they were going to be on flat and level road for a while. Run the engine at a higher load and better SFC and fill the battery up to it's buffer zone max of 1.6 kwh, then shut the engine off for around 5 minutes.

[scottf200]

Quick note for us beginners: SFC = Specific Fuel Consumption (assuming per context and my internet searching)

http://encyclopedia.thefreedictionar...el+Consumption

Specific fuel consumption may refer to:
Brake specific fuel consumption, fuel efficiency within a shaft reciprocating engine
Thrust specific fuel consumption, fuel efficiency of an engine design with respect to thrust output

[George S. Bower]

Quick note for us beginners: SFC = Specific Fuel Consumption (assuming per context and my internet searching)

http://encyclopedia.thefreedictionar...el+Consumption

Thx Scott,
I lived my life by the words. Have you heard of "Larry Laws"?? He's in the book and is the official energy/ systems guy for the Volt. I was the same on a gas turbine truck engine program for many years. I reduced all the dyno data and compared it to the theoretical predictions of our off design models. You have heard of the theory of conservation of energy: "energy can not be created or destroyed". The ironic thing is (and I think Larry Laws would agree) a better way of putting it is: energy just disappears, and any energy conversion efficiency you may have down on paper will always be optimistically high when the data is finally analyzed.

[Rich Remund]

Hey scott, so what data can you expose with that scanner. I've been trying to determine what informtion is available, seems some data is hidden. For Example there must be a total gallons used fiqure which is used to calculate Lifetime MPG, and a total engine hours, and EV Miles. EV Miles are displayed on OnStar yet no where in the cars display.

[Ziv]

Scott, you just identified something that I had hoped would keep the Volts' CS mpg higer than it did. Back in 2009, GM released a graph that seemed to show that CS mode would work for 4 minutes, then it would return to CD mode for 2 minutes, repeat as needed. I thought that the Volt would get around 45 to 50 mpg due to the fact that though the engine never completely recharged the battery, it looked like it would operate efficiently enough to only need the ICE to run for 2/3 of the time after you hit the battery depletion point. It looks like you are showing that GM actually needs the ICE to run 6/7 of the time. So instead of the ICE being needed 66.6% of the time, it actually is on 85.7%, approximately.
I wonder if as the battery pack gets lighter and the car sheds pounds with the 2013 Volt, we might not see the ICE used less and the CS mpg improve noticeably. Obviously, the weight decrease will help more in town than on the highway, and your testing at 55 mph is about as selfless an act as I have seen on GM-volt.com! Grin

So last yesterday I drove to work (11.5*2) and to racquetball (19*2) on a couple charges but on the way home from racquetball ran out of battery (Charge Depleting). I was experimenting in cruise control driving 55 MPH in the right lane.

Observation with Power Flow display up:
Initially switching over to CS (Charge Sustaining) the RPM was around 2200 for say 2 minutes, then it went to 1500 RPM for say 4 minutes, and then it went to "CD" mode (ie. using the battery) for say 1 minute. I saw this happen a couple times. I didn't time it so times may be off but that is what it felt/recalls like.

At that point in my drive I only had 15 or so minutes to get home so my experiment was pretty short. I think the outside temp was around 40.

Note: RPM numbers came from my ScanGuage.

P.S. Love the Power Flow display for learning. On our recent long drive where we were in cruise control for miles and miles at 65 MPH it was very surprising initial to see the regen braking working on a slight decline ... then it became a duh moment.

[scottf200]

I'll use a stopwatch or the like in an future experiment and longer drive to get a better feel/stats. The above times were very very rough so deriving any percentages or stats from them is not really suggested for readers of this thread.

<snip> and your testing at 55 mph is about as selfless an act as I have seen on GM-volt.com! Grin

In general I'm a modest driving going 65 on the highway (and speed limit+5 in the burbs / traffic flow) ... but I thought I could be forgiven for both driving 55 in this fun car *and* for driving using gas (CS) all in the name of science.

More later folks ... they actually want me to work at work ... sheeze.

[doggydogworld]

This is interesting data, Scott. Please keep us updated as you learn more. Here's my guess as to what the Volt is doing:

1. At 25% SOC CS mode kicks in
2. ICE runs 2200 rpm and high load to bring SOC back up to 28%
3. ICE then attempts to match road load as long as gas pedal is depressed
4. The ICE also attempts to vary RPM according to road load so the ICE "sounds right"
5. If road load drops below some minimum level, however, ICE power stays above road load and excess tops up battery
6. When battery hits 30% ICE shuts down

Matching road load avoids cycling the battery unnecessarily and the associated generator-power electronics-battery-power electronics-motor losses (which are significant). Better to operate the ICE a few percent off optimal than to suffer those 25%-ish round-trip losses. But at very low power output the ICE is much more than a few percent off optimal, and it makes more sense to keep the ICE at a minimum power/efficiency level and feed the excess into the battery. If this continues for more than a couple minutes battery SOC eventually hits a trigger point which causes the ICE to shut off and the car to re-enter CD mode.

All of this is just my guess, and I completely made up the 25%/28%/30% thresholds for purposes of illustration. I have no idea what the actual thresholds might be.

[George S. Bower]

This is interesting data, Scott. Please keep us updated as you learn more. Here's my guess as to what the Volt is doing:

1. At 25% SOC CS mode kicks in
2. ICE runs 2200 rpm and high load to bring SOC back up to 28%
3. ICE then attempts to match road load as long as gas pedal is depressed
4. The ICE also attempts to vary RPM according to road load so the ICE "sounds right"
5. If road load drops below some minimum level, however, ICE power stays above road load and excess tops up battery
6. When battery hits 30% ICE shuts down

Matching road load avoids cycling the battery unnecessarily and the associated generator-power electronics-battery-power electronics-motor losses (which are significant). Better to operate the ICE a few percent off optimal than to suffer those 25%-ish round-trip losses. But at very low power output the ICE is much more than a few percent off optimal, and it makes more sense to keep the ICE at a minimum power/efficiency level and feed the excess into the battery. If this continues for more than a couple minutes battery SOC eventually hits a trigger point which causes the ICE to shut off and the car to re-enter CD mode.

All of this is just my guess, and I completely made up the 25%/28%/30% thresholds for purposes of illustration. I have no idea what the actual thresholds might be.

2. ICE runs 2200 rpm and high load to bring SOC back up to 28%

I agree w/ your analysis but this is not a "high load" condition. In some situations it might make sense to charge at a higher rate at a better SFC and store the full 1.6 kwh of juice.

[scottf200]

Via: http://www.motortrend.com/roadtests/...ne_graphs.html
Below graph:

We recorded vehicle speed (blue), engine speed (red), barometric pressure and intake-manifold pressure information off the Volt's OBDII port. As you can see, the electric generator manages to place sufficient load on the engine at all times (except during transitions) to run the engine at wide-open throttle (no manifold vacuum -- the green trace is at zero). Note also how the engine revs usually attempt to match acceleration in a way that people are accustomed to, though it need not do so.

Below graph:

We've computed the horsepower expended to produce the acceleration measured on the Volt in gas/electric and electric-only modes versus the Prius (which is operating in gas/elect mode for this full-power run). Even assuming minimal driveline loss, the combined output of the engine and electric motor likely tops Chevy's claimed 149-hp at 100 mph.