Author’s note: The work presented here was a collaboration between myself (George S ) and Walter Crowe (saghost on the forum). I wrote the excel program but I relied on Walter’s experience with his Volt and the Dashdaq data acquisition device to guide me in selecting the proper operating modes for the driving cycle we model here. (For a description of the Dashdaq device click here.)
Ahh, the beauty of a pure series extended range vehicle. The Karma runs that way and now Bob Lutz is extolling the virtues for his VIA extended range pure series truck.
What are the advantages of the pure series configuration?
But what MPG will we get in extended range mode? It should be quite good. Yes? We can run the ICE at its sweet spot.
An investigation into pure series mode was made using a new model of the Volt. The model was run in both normal Volt extended range mode (power split included of course) and in pure series mode with the ICE at its sweet spot in order to make a comparison.
Volt Model Discussion
The model was written in Microsoft Excel. Input to the model comes in the form of:
1) ICE map shown in figure 1 and 2.
2) Road load map as shown in figure 3.
The equations for the planetary gear set and the Volt’s 4 operating modes are also included. A solution is derived across the planetary gear set (discussed here). The model includes drive train mechanical losses as well as losses for the inverter and motors. (3%, 5% and 7% respectively).
The Highway driving cycle was used as shown in figure 4. In order to simplify the analysis, the HWY driving cycle was simplified as shown in figure 5 and broken into 25 segments. The HP to accelerate the vehicle is added to the basic road load (figure 3) so that for each segment we have an average speed and total road load into the Volt’s front wheels.
Using the speed and road load into the tire as input to the equations for the 4 operating modes, a solution is found for each segment of the driving cycle. Output from each segment comes as speed splits in the planetary gear set, ICE match point, fuel consumption and battery charge rate.
The battery state of charge (SOC) is tracked as we step thru the driving cycle. A charging window of 3.5 kwh-4.0 kwh was used (.5 kwh from the bottom of the 21.9-86% usable SOC battery range).
Selecting Operating Modes
This was the tricky part. One must know how the Volt thinks in order to pick an operating mode for each segment of the driving cycle. Initially I had the first lower speed (below 50 MPH) segments of the driving cycle run in series mode with the last half (above 50 MPH) primarily in power split (PS). However, consultation with Walter resulted in changing the operating modes to primarily power split. Operating at power split at these low speeds would come as a shock to most people but this is in fact how the Volt operates as observed by Dashdaq (given these low acceleration rates).
“The Volt is full of surprises. Initially, I thought power split only happened at freeway speeds but discovered with Dashdaq that, at any throttle setting that allows it (anything less than moderate-hard acceleration,), at any speed over 35 mph, if the engine is on the car will be in power split mode.”
Referring to tables 1 and 2, we can see the battery initial state of charge=3.5 kwh. Battery SOC increases until segment 12 when we hit the top of the charge window (4.0 kwh) where the Volt switches to EV mode for segments 12 and 13, then back to power split for the rest of the segments. Note at the end of the driving cycle the SOC is 3.94 kwh- ie we have excess kwh in the battery compared to where we started at 3.5 kwh so we must correct for this.
Also note from figure 6 the operating range on the SFC characteristic. We see that the Volt’s ICE always operates within 6% of min sfc (quite good IMO).
Pure Series Simulation
The driving cycle was then run in series mode with the ICE matched at minimum sfc (its sweet spot). At this condition the ICE puts out 33 hp. Most of the loads at the tire are usually less than 33 hp so the extra hp is used to charge the battery.As we step thru the driving cycle, we see charging rates as high as 20 kw.
Toward the end of the driving cycle we switch to EV mode to use the stored battery kwh’s. As you can see, the battery SOC does not come out to exactly 3.5 kwh at the end of the cycle so, as before, we must correct the predicted MPG for the difference.
How much have we benefited from running in Pure Series with the ICE at its sweet spot??
NOT AT ALL. In fact, we show a predicted MPG for pure series operation of 7% LESS than the base case Volt.
The analysis proves that the conversion losses when running in series mode swamp the minor benefit of running the ICE at its sweet spot. We show a 7% loss in MPG for running in pure series rather than in power split.
Is this a big surprise??
Not to me.
The Holy Grail does not exist.
That’s why the Volt has POWER SPLIT
This entry was posted on Friday, August 24th, 2012 at 5:55 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.