CS-mode and hwy driving?

[bonaire]

This is a CS-mode question that comes up when thinking about the recent GM articles about "summer electric road trips".

One thing I don't entirely understand about the Volt yet and want to before jumping in and buying one.

Say you're on a flat highway (ie. Iowa) driving at 55mph and viewing corn fields for hours on end. Let's say demand to push the car at this speed is 17 horsepower. If the ICE is running at say 1/2 speed or 3/4 speed, isn't it putting out more hp than 17? In CS-mode, we're maintaining the SOC but shouldn't we be able to be charging and increasing SOC (keeping in mind charging losses from crankshaft to battery charger circuits)? If ICE runs say 30hp while driving is drawing 17hp from the batteries, wouldn't the excess ICE output be "bankable" into the battery without requiring a specific Mountain Mode setting?

My thinking is if the CS-mode ICE output is higher than needed to turn the wheels, we should see an ever-increasing SOC so that after a nice flat moderate-speed drive, SOC should be nearing full. Or, does the ICE shut down after some time so as to be back on CD-mode for a while (basically a set-point range where if SOC rises above 40%, run on battery only and if below 30% power-on the ICE and go into CS-mode). In addition, after a climb if a 3-mile hill in CS-mode, going down the other side in L should put a few kWh into the battery as well and should turn off the ICE for a while, correct?

Just the efficiency mind in me thinking "every excess horsepower should be put into the battery".

[kickincanada]

My understanding is that on the highway even in CS mode it will kick in to electric mode for a few miles using up any built up S0C

[JohnK]

When in CS mode the ICE does not run at a constant speed. It does not respond quite the same as a conventional ICE, but it does run approximately at the speed needed for the immediate electrical load. There is a bit of a difference if running in mountain mode. In mountain mode the ICE is running at a speed such that additional power is being generated so that the battery can be charged up to the higher reserve charge point dictated by the anticipation of hill climbing.
So, under normal conditions in CS mode there will not be any excess power to be put into the battery. And yes, the ICE will at times shut down entirely even in CS mode (like when you come to a stop light).

[stresstool]

Or, does the ICE shut down after some time so as to be back on CD-mode

That is my experience with it. However, the SOC does not increase much. I don't know what the number is, it provides for a small hysteresis curve between on and off. I.e. engine runs for 2 or 3 minutes, then off for 2 or 3 minutes, etc.

[tfford]

Say you're on a flat highway (ie. Iowa) driving at 55mph and viewing corn fields for hours on end. Let's say demand to push the car at this speed is 17 horsepower. If the ICE is running at say 1/2 speed or 3/4 speed, isn't it putting out more hp than 17? In CS-mode, we're maintaining the SOC but shouldn't we be able to be charging and increasing SOC (keeping in mind charging losses from crankshaft to battery charger circuits)? If ICE runs say 30hp while driving is drawing 17hp from the batteries, wouldn't the excess ICE output be "bankable" into the battery without requiring a specific Mountain Mode setting?

I can confirm this for you as I've tested it numerous times. At 70MPH (and the ICE gets better mpg at 70 than 55, I've tested this numerous times as well) under CS mode simply maintaining the SOC I get about 39 - 41 mpg. At the same speed (and even a little less), actually charging the battery (Mountain mode), my fuel economy drops to about 25 mpg.

[gwmort]

If the car charged itself while running it would increase drastically the number of charge - discharge cycles over the life of the battery for a wider range of SOC and shorten battery life.

[jsmay311]

...the ICE gets better mpg at 70[mph] than 55...

I'd be willing to wager a hefty sum that this is not true.

[DavidV]

The ICE will change speed to match the electric demand of the motor. If there is excess power available even at the lowest ICE speed, then yes, that will be used to charge the battery, it is not thrown away. So if you run the car long enough in that state, the car will switch to battery, use up the charge that has built up, and then switch back to ICE. I've never actually seen this happen on the highway, but have seen it happen at 45mph a number of times. I think unless you drive on a long downhill, driving the car at highway speeds with the ICE does not produce any excess power, thus the great fuel efficiency. As has been pointed out many times, using the ICE to recharge the battery is far less efficient than use the ICE to power the car, because you are wasting power during the conversion/charge phase and then wasting again during the discharge/conversion phase. Thus if you are going to use mountain mode, you should do so before you drop below 40% of charge so the ICE is powering the car but not recharging the battery.

[tfford]

I'd be willing to wager a hefty sum that this is not true.

Get your checkbook out. Assuming the trip odometers and avg. fuel economy are anywhere near accurate I've watched the mileage maintain a steady ~39mpg at 70 and drop from there to around 35 driving 55mph. And over enough distance to matter.

If our instrumentation is not accurate, then every post to date regarding fuel economy, EV distance, etc. is worthless.

[AZ-Volt]

Would this seemly odd mileage difference be related to how the ICE is semi-direct engaged (outer ring gear locked) above 60-65MPH?