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.)
Introduction
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?
Simplicity and cost are first on the list.
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).
Quoting Walter:
“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.”
Results
Click on tables to expand for easier reading.
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.
Conclusion
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.
+3
Aug 24th, 2012 (6:19 am)You list the mechanical, inverter and motor losses as 3%, 5% and 7% respectively. What about the energy lost in the battery when electrical energy is converted to chemical energy during energy storage and the energy lost when chemical energy is converted back to electrical energy when the battery is supplying stored energy?
GM engineers claim the power split mode increases efficiency in the range of 10% to 15%. If you did not include battery energy conversion losses in your model it would explain the difference between your estimated 7% and GM’s 10% to 15%
+13
Aug 24th, 2012 (6:39 am)Thanks for the thorough analysis.
This is one of many reasons that the Volt/Ampera has won so many awards, including the Society of Automotive Engineers “Best Engineered Vehicle.”
GSP
+11
Aug 24th, 2012 (8:23 am)Thanks, you guys went to a lot of work. The GM engineers went to even more work to come up with this system in the first place.
Please GM, spread Voltec technology across many if not all of your product lines.
+2
Aug 24th, 2012 (8:32 am)“As we step thru the driving cycle, we see charging rates as high as 20 kw.”
————-
When I mash my brakes, I can get my Volt to say it’s regenerating 59kW.
Also, when I floor it, it will say it’s outputting 109kW.
+2
Aug 24th, 2012 (8:37 am)Very interesting work! I have to go back and re-read it to absorb more info, but one thing jumped out at me, in the 4th paragraph before the conclusion your paper states, ” we see charging rates as high as 20 kw. ”
How long can the Volt charge at 20 kw? Momentarily, or for an extended period of time? Since the Volt uses such a small percentage of its total capacity, it would seem to indicate that the Volt could be completely recharged in just over 30 minutes, or would it?
This really shows how much work went into building the code to operate the Volt as efficiently as possible.
On edit: How can the Volt recharge at such a high rate using regen when it’s max charge rate with a charger is so much lower? Or am I completely missing something here, which is quite possible.
+15
Aug 24th, 2012 (8:42 am)After 1 month of ownership, I hit 1,000 miles on my Volt this morning. Specifically, 874 EV miles and 129 gas miles (87% on electricity). I’ve used 3.5 gallons of fuel that came with the original tank when I bought the car. If I had not made a trip to Detroit one weekend, I would not have used any fuel.
This morning, I stopped at the gas station for coffee and saw that gas prices were at $4/gallon again. All I could do was hope others there would see my car and that, along with the prices, would help motivate them to make the switch to electric driving.
+5
Aug 24th, 2012 (8:50 am)“What are the advantages of the pure series configuration?
Simplicity and cost are first on the list.”
—————
Both of those reasons are good ones. I’ll add:
Freedom of design
Possibly better weight distribution
Possibly more seating room/trunk space
Less things to go wrong (KISS)
I agree that the Volt is more efficient using a powersplit setup, but I would like to see what engineering can come up with when they can design the range extender from a blank sheet of paper. A series setup also allows any type of range extender, it just needs to produce electricity.
For the advantages listed above, even if there is a slight efficiency loss in CS mode, if you only drive in CS mode 10% of the time, and out of that 10%, only 25% of that is using the power-slit mode, doesn’t it make it somewhat insignificant?
+1
Aug 24th, 2012 (9:05 am)Assuming we aren’t overlooking some major factor (suspect optimized engine tuning isn’t enough) a series vehicle isn’t going to be better unless the generator is a more efficient technology. For example, a fuel cell instead of an ICE. Maybe a diesel is enough to tip the balance?
+6
Aug 24th, 2012 (9:25 am)Jon Bereisa, who worked on all of GM’s electric vehicles, has said this about the MPG in CS Mode:
“When we modeled the Volt’s engine, theoretically we could have gotten to high 40s or low 50s mpg in gasoline mode. But we would have had to run it continuously at 3500-3800 rpm and just switch it on and off, and the noise and pleasability wouldn’t work. We had to drop it down, which got us to 37-38 mpg. But I think a lot of fuel economy still can be gained without major expenditures in tooling or engineering.”
http://green.autoblog.com/2011/11/03/where-does-erev-technology-go-from-here/
The BYD pure serial did exactly this and does get better MPG. It also apparently sounds like you have a guy with a jackhammer in the car. I don’t think this agrees with George’s conclusion.
Agree with kdawg that given how few miles people put on the car in CS Mode the MPG isn’t a significant factor.
Also George has my eyes glazed over!
Aug 24th, 2012 (9:31 am)Ziv,
The pure series simulation is a just a simulation not what the Volt really does (since it is not a pure series machine). (So it’s a what if.) I am not sure if the existing Volt would support that high a charge rate. We would need Walter or someone with Dashdaq (or other) monitoring device to advise on that. However I would think that it is not out of the realm. A 1C charging rate would be 16 kw and 20 is less than 1.5C so it’s possible.
+1
Aug 24th, 2012 (9:40 am)Yes I have noticed that EE’s always seem to prefer a pure series arrangement rather than a MECHANICAL one!
but all kidding aside, after I wrote this I sat back and thought. You know, 7% isn’t all that bad considering the other trade offs you list. Especially for vehicles that spend very little time in CS mode. So as you say , on a fleet basis pure series should be able to get some pretty good numbers. Just not as good as a system with power link.
+1
Aug 24th, 2012 (9:47 am)Could you help me out with an introductory paragraph explaining what a pure series extended range vehicle is, how it would differ from the existing Volt, and what the cost and other tradeoffs are, etc.
Aug 24th, 2012 (9:51 am)There are no losses for that in the model….but perhaps there should be. I vaguely remember discussion about this from the old forum days. NIMHY bats have a lot of losses in that regard but Li batts are pretty darn efficient, but how efficient I don’t know.
Do you have links or references that show what these losses are??
Aug 24th, 2012 (10:00 am)Volt doesn’t ‘think’ it just runs programs. Before we get to AI, it’s a whole ‘nother order of magnitude code-wise.
Aug 24th, 2012 (10:34 am)Wiki has a lot of good info. Basically a series hybrid has no mechanical connection between the internal combustion engine and the wheels. The torque at the wheels is only from a traction motor running on electricity.
Click —> http://en.wikipedia.org/wiki/Hybrid_vehicle_drivetrain#Series_hybrid
+2
Aug 24th, 2012 (10:38 am)— [pushes voice command button]
Driver: “Volt! Go to sport mode.”
Volt: “I’m sorry Dave, I can’t do that”
(an old photo I made)
Aug 24th, 2012 (10:59 am)DonC,
That’s why switching the range extender to the 2.0l Turbo makes total sense. It will produce equivalent or better torque output at a much lower RPM (to drive the generator, etc) at a much lower SFC. You won’t need the peak output from the 2.0L Turbo, just the small rpm range needed for Volt/ELR.
Look for this to improve the CS mode MPG by a significant amount.
+1
Aug 24th, 2012 (11:28 am)I would like to see a Serial Hybrid Only mode just for the ease of hacking the electric drivetrain into different vehicles.
Just take out the drive motors and gen set, put in a very light (or rugged 4X4) structure and you are good to go.
The split transmission is more efficient but it is also more expensive, complicated, harder to maintain and harder to re-purpose. I look forward to the future generations of the Karma and a Voltec hack that accomplishes the same goal.
If it is good enough for the world’s diesel electric trains, it is probably good enough for smaller transportation systems.
Aug 24th, 2012 (11:40 am)#5
I dunno, but it sure seems to. I leave home in the AM showing 43-44 miles AER. By the time I reach the freeway 10 miles away, 20 minutes later, and 800 feet lower, I am showing 50. And it seems real, because I drive another 15 miles on the freeway and arrive showing 29 miles AER. But of course the payback comes on the way home, LOL. I arrive there with something like 8 miles AER after going back up the hill.
+7
Aug 24th, 2012 (11:42 am)No Volt criticism ever drove me more crazy than in the early days when some self-proclaimed EV enthusiasts claimed the Volt was an inferior EV (or not one at all) simply because it was not a series hybrid in CS mode. Some of them even seemed to think this was some sort of cheap shortcut by GM, when it’s actually much more complicated and expensive to be parallel.
Not only did this show they knew nothing about EVs and hybrids, but that they completely missed the point for having EREVs/hybrids completely — once you’re burning gas, all that matters is mpg, period.
+1
Aug 24th, 2012 (11:43 am)#6
Yeah, I always have to smile when I go into a gas station after 2 or 3 months and realize that the price has gyrated again and I haven’t even noticed it, LOL. +1
Although we towed our 2 race cars 300+ miles to Laguna Seca this weekend with gas at $4/gal+, which was a REAL reality check!
+3
Aug 24th, 2012 (11:44 am)Thanks for a great weekend read. The conclusion confirms why GM designed the E-REV transmission to turn the wheels with the ICE for greater efficiency. I only hope that GM will continue to add battery power to extend the EV plug-in range so drivers will experience fewer ICE moments and more 100% EV drives.
+1
Aug 24th, 2012 (11:48 am)#2
Amen. +1
Why am I not surprised by this result? Remember how many people complained that the Volt wasn’t a “pure” series system? Whatever works is my philosophy.
+1
Aug 24th, 2012 (11:54 am)#9
I have to agree too. +1
I used to be really disappointed that it doesn’t do better in CS mode, but now I realize that I drive so little therein that you’re right and it is indeed insignificant.
That said, I would still endure some pretty significant increases in NVH to gain 50 mph bragging rights and shut up some “P-word” owners, LOL.
+5
Aug 24th, 2012 (12:17 pm)GM engineers … “…been there done that” …. I’m sure.
Chevy Volt … best engineered car on the road.
+5
Aug 24th, 2012 (12:18 pm)How shortsighted the Gop is on alternate energy production but refuse to see it. $$ from the oil firms drive its policy. Even countries like China see the potential of Solar, wind etc and provide incentives .. China will soon become the nation with largest fleet of EV, EVER etc to curb its oil demand. Gop in US drives its policy based trickle down economics and non-sense bs about other forms of energy. It will lie, distort the advantages of Alternate source of energy and work thru propaganda channels like Fox to spread the lies and thus affect the low IQ folks to swallow the lies hook, line and sinker. Amen
+1
Aug 24th, 2012 (1:17 pm)Adding to what Pat said, there are many that complain about the $7500. stimulus for The Volt and other EV’s. But All Vehicles that burn oil products are indirectly subsided. Annual Subsidies for Oil are over $40 Billion with another $6 Billion for Ethanol, not to mention almost $3 Trillion spent in Iraq and Afghanistan over the last 10 years. And then there are environmental, health, global warming, homeland security costs plus lives lost in two wars, and many more that can be tied to our use of petroleum products. You can easily do some math and conclude that the $7500. EV subsidy is a bargain.
Aug 24th, 2012 (1:29 pm)#16
Drat, beat me to it again! +1
Aug 24th, 2012 (1:30 pm)Simple questions:
1) How much more does the buyer of the Volt have to pay extra to have the “power split” technology in place of a pure series implementation?
2) How much gasoline, over the life of the car for the typical Volt driver, does the power split technology save?
Obviously what I’m getting at is: does the added cost of the power split technology pay for itself over the life of the car in energy savings?
I didn’t see any cost numbers, i.e. $$$, in George’s analysis, which is what is also needed to make the case for including in the Volt the power split technology.
Let’s take this a step further: suppose the added cost of the power split is $2000 (includes R&D, material, labor, warranty reserve, etc) and the savings to the average consumer is $3000 in gasoline over the life of the car. Why not strip out the power split to lower the street price of the car to enable selling more Volts? How many buyers would pay the extra $2K up front to POTENTIALLY save $3K over the life of the car?
Aug 24th, 2012 (1:36 pm)#26
And amen again! +1 and +100 if I could.
How you got a “-1″ on God’s honest truth is beyond me.
What I really love (NOT!) are the commercials for “clean coal”, natural gas, and oil on CNN. Beyond FUD comes direct disinformation. Terrifying IMHO.
When Anderson Cooper comes back on I just have to look at him and shake my head. That’s IF I haven’t already changed the channel so i don’t have to endure the BS.
Aug 24th, 2012 (1:37 pm)kdawg,
That is the VOLT in all but very rare circumstances in which the engineers determined that a direct connection is more efficient. Is this post quibbling with their engineering, which by the way is designed to maximize mileage?
Aug 24th, 2012 (1:37 pm)#27
And amen to that too! +1
+2
Aug 24th, 2012 (1:42 pm)DonC,
Doesn’t Jon Bereisa’s comment imply that the best BSFC for the Volt engine is at 3500-3800RPM instead of the ~2100-2200RPM implied in figure 1?
I have seen the figure 1 chart in the past in previous BSFC discussions but I’ve never seen that it was absolutely confirmed that the chart is for the Volt engine as opposed to some other engine in the same GM category. Has that confirmation been made? Perhaps Volt’s BSFC chart is different, with a sweet spot at 3500-3800RPM? Am I missing something here?
Aug 24th, 2012 (1:43 pm)#29
I would. As we have discussed endlessly here, it’s not about some sort of a rigid cost/benefit analysis, at least not in the sense of only gas savings.
It’s about that, plus all of the other things Captbently so rightly cites at #27. And just the joy of helping to push technology forward, not to mention looking COOL when you drive around in what Automobile Magazine rightly called “the most technologically advanced car in the world”. I would personally sacrifice a lot more to get the CS mileage up, but that’s just me.
Aug 24th, 2012 (1:45 pm)#31
I think it’s validating the engineering in the end, which seems pretty valuable as so many of us Volt wonks have been skeptical about it.
+1
Aug 24th, 2012 (1:50 pm)Not quibbling with the engineering really. The Volt is what it is based on the time & money GM had at the time. I’m OK with that. But now that they have more of both, I’d like to see a series-hybrid for the reasons I listed. I would like a complete redesign of the range-extender with only 1 objective in mind, creating electricity, not mechanically connecting through a planetary gear/clutch. It doesn’t have to be called the Volt, and it doesn’t even have to be an ICE range extender. Ideally, I would just drive a BEV, but we are not there yet, so the next step away from a BEV is a series hybrid. Who knows, if battery break through happens, maybe people could rip out their range-extenders in their series-hybrids and put in some new batteries. The traction motor doesn’t care where it gets the electricity from, and its not mechanically coupled to anything other than the wheels.
+5
Aug 24th, 2012 (2:13 pm)This “power split” does not COST, it SAVES. In order to run the car purely by electric power, both MG1 and MG2 are needed as drive motors (the power is blended as vehicle speed changes). In order to run the Volt in pure series mode there would have to be an MG3 added or else prevent MG1 from being used as a drive motor while it is being used as the generator – that would reduce performance and handling characteristics. GM did not increase complexity (mechanical) though the software probably got a bit more complex.
Just my opinion.
John
+1
Aug 24th, 2012 (2:59 pm)Good point JohnK. The cost of power split is only the cost of adding one clutch. Of course then there will be those that argue that we only need 1 traction motor. Tesla did it right?? However I would not be surprised if Tesla’s 1 traction motor costs more than the Volt’s 2 traction motors.
+1
Aug 24th, 2012 (3:19 pm)What makes even more sense is an Atkinson cycle. No turbo needed and all the extra costs involved. With Atkinson we have superior sfc at even lower rpms (down to 1000 rpm). If you notice in this analysis the Volt’s engine never goes below 1450 RPM and this has been verified by Walter on dashdaq. Why?? Because the sfc of the existing volt 1.4 ICE gets very bad at lower speeds. So that also tends to verify the sfc characteristic published in figures 1 and 2. And also is somewhat of an answer to DonC’s skepticism that the min sfc is really at 2200 rpm and not at 3500 rpm.
Atkinson can also can put out more power so the Volt be much less suseptable to “propulsion power reduced” on steep grades.
I hope to have an analysis of the effect of Atkinson done soon.
Aug 24th, 2012 (3:55 pm)Of course if you had a series-hybrid you could easily stick a traction motor in the rear and one in the front and have an AWD vehicle.
Aug 24th, 2012 (3:58 pm)Whatever happened to the Scuderi engine?
Aug 24th, 2012 (5:13 pm)The Volt trades power on a high voltage bus – there’s absolutely nothing stopping GM from doing this if/when they want to. For that matter, it’s exactly how Toyota’s Highlander Hybrid and RX450h work, with a second (third) 50kW electric motor driving the rear wheels.
Walter
+1
Aug 24th, 2012 (5:17 pm)It’s not clearly spelled out anywhere, but from the timing of the remarks and the history of the development effort, I believe that his remarks refer to a different engine than the one finally adopted – most likely the 3 cylinder turbo proposed in the concept car. That would explain the vast discrepancy between what he suggested and what we see on the car – without a VTEC style multi-profile cam, very few cars breathe better over 3k – certainly the current engine doesn’t.
Walter
Aug 24th, 2012 (5:24 pm)George S. Bower,
Hi George: Great post and analysis. Certainly enlightening. As VOLT developed, it seemed GM boldly made design priorities of no-compromise quality with performance over price. However and whatever it took– VOLT retains its relevance. History in the making.
At launch, the consensus here was VOLT had an 18 month window of EV market segment exclusivity. Only now we’re seeing a potential bevy of new RE-EV competition. My point here is VOLT’s outstanding engineering policy of attention to any detail sets RE-EV performance at a very high bar.
Maybe you saw: A new product announcement of an all electric driven supercharger; claiming a 7%+ efficiency increase. (No belts) — I could imagine a combination intake-exhaust all electric configuration possible. (That same announcement hailed a 48 V motor/generator development to reduce electric current levels significantly. I thought the 48 V concept was abandoned.)
+1
Aug 24th, 2012 (5:27 pm)This may sounds a little crazy, but I’m with JohnK on it. Under the specific circumstances, power-split was as cheap as a pure series car to develop, or possibly cheaper.
Remember, the Volt’s transmission housing, planetary set, differential gearing and MG A were all lifted directly from the front wheel drive two mode hybrid GM was developing then (supposed to be a 2009 introduction, cancelled very late – late enough that some news media folks got test drives, like these two:
http://www.examiner.com/article/road-test-2009-saturn-vue-2-mode-hybrid-car-review-full-measure
http://www.hybridcars.com/vehicle/saturn-vue-green-line-two-mode.html
To make Voltec work on their tight schedule, what they did is delete one of the planetary sets, repurpose the clutches, and upgrade MG B to a new design with twice the power.
Thus, they already had most of the pieces, and production tooling for them. To build a pure series car, they would have had to start from scratch, making new tooling as they went along. Thus, for GM in 2008, power split was likely cheaper and easier to develop than a pure series car due to design synergy. For another company, or at another time, this likely would not be the case.
Walter
Aug 24th, 2012 (5:28 pm)George –
Doesn’t the power split also include a planetary geer system over a pure series EREV?
By the way, what is the cost increase of building a car with the power split system over a pure series EREV?
Dave
Aug 24th, 2012 (5:31 pm)Walter,
Walter – post #45 – thank you for taking time to provide such a good answer that makes sense. Dave
+3
Aug 24th, 2012 (5:40 pm)Umm… No.
Lower RPMs don’t magically mean better fuel economy. It kinda seems that way for a typical car because in the normal driving situation, the engine is operating at part throttle, and if you sustain the same speed (power requirement) at lower RPMs, you have to open the throttle more – and a wider throttle is always more efficient (except for cars that go open loop and overfuel at wide open throttle.) The Volt always runs the engine with the throttle wide open, except for a warmup cycle at half throttle and momentary transitions at idle.
Engines are most efficient at their torque peak at wide open throttle (naturally aspirated – turbo and supercharged engines are more complex and have to be looked at case by case) – the better “breathing” (higher volumetric efficiency) tends to dominate all the other factors (ring leakage at low speeds, sliding friction at high speeds, etc.)
There’s no question the 2.0T is seriously overpowered for any conceivable Voltec application (except maybe a semi or delivery truck,) and so either the engine would be run inefficiently at low RPMs, or it would drive massive charging loads (running the engine efficiently and sucking most of the power into the battery,) with associated increased losses.
OTOH, using the engine block and heads from the 2.0T could make a fair amount of sense. The extra displacement would let you build a naturally aspirated atkinson cycle engine that had a similar (or slightly higher) output to today’s 1.4L, and combined with direct injection (which would allow a crazy compression ratio – about 16:1 (on the compression side it’d only be running about 12:1 – well within the range of good fuel cooled by GDI; the rest would be for the unequal expansion of Atkinson,)) could produce really good fuel economy.
Walter
Aug 24th, 2012 (5:46 pm)Maybe we weren’t quite clear enough in the introduction. It’s a simulation based on a math model developed by George, but all of the operating points he shows are within the data we’ve seen directly from the car, on the power split side. On the series side, it’s not quite as clear – but I see no reason to doubt the 20kW charging – after all, the Volt does regenerate up to 60kW during moderate braking. The current car does not generally choose to operate in that condition (higher charge rates mean more power lost as well as more noise for the driver,) but the battery can take it, and the car will operate the engine close to that speed a lot of the time when the road load justifies it.
So maybe the clearest answer for your question is: The hardware is certainly capable of it, but the programming doesn’t operate that way.
Walter
Aug 24th, 2012 (5:53 pm)Would they have gotten more mpg with an “Atkinson cycle engine”?
How bout Diesel?
Aug 24th, 2012 (5:54 pm)A modern diesel will may make a more efficient series car (if you don’t have to compromise too much for emissions) than the current power split car – but a power split car with that engine will be more efficient still. Basically, for any engine that delivers power mechanically, it’s more efficient to link to mechanically, as long as the gears lose less power than the electric loop (generator-inverter-battery-inverter-motor – though under some conditions the battery won’t be cycling the power.)
A fuel cell or other direct to electric source would of course be a different story – and there can be good reasons not to link mechanical engines (like Jaguar’s microturbines, where you’d have to have multiple reduction gear stacks to get from 70k down to ~4k, the gears would have to live in/near the turbine hot section, and the engine would really rather run at one constant RPM.)
Walter
+6
Aug 24th, 2012 (6:02 pm)I live in a densely populated area. Each morning at sun up I hear several drivers starting their cars and trucks for the work commute. The grinding of starter motors. Smell of coolant and exhaust. Engines coughing with a few unwilling to smooth out.
My Volt is nearing 20,000 miles now. My gasoline usage has dropped from 1 gallon PER DAY with the old Honda. To .6 gallon PER WEEK with the new Chevy. Very smooth and quiet. GM has done a very good job configuring the Volt.
+1
Aug 24th, 2012 (6:55 pm)Anytime you are running any ICE, and you want to focus on various efficiencies, average calculated engine load and average absolute engine load graphing is indispensable.
Pinpointing acceleration values in relationship to anything else so affected, and comparing the averages of the various stages of a test, as well as with dozens of other parameters allow for the refinements of statistical confidence levels. The internal software of the PCM reports these PIDS, and it is not really a difficult formulation to average them if your scanner does not already do this.
+2
Aug 24th, 2012 (7:28 pm)Here is the link for Fox idiot claiming that Nissan gets it …
http://www.greencarreports.com/news/1078724_fox-anchor-hates-volt-loves-infiniti-le-claims-it-was-his-idea
geez any can cab converted into induction charge..Cost $$ to do that as if plugging a ord is too much for him .. Alas he earns millions talking bs and has no time to plug in cord so likes the idea of wireless inductive charging …
He is Senior Vice President of Business News at Fox …What a joke this guy is … with his analysis can he dissect business plans or anything else?
Aug 24th, 2012 (8:22 pm)#55
That’s what I really love. All American, Republican, lapel flag wearing, GM hating, imported car lovers. Sometimes I swear think that this country is doomed.
Aug 24th, 2012 (8:51 pm)Pat,
Noel Park,
LOL.. I can hear Cavuto now… “Your marriage is safe. With inductive charging there will be no divorces due to someone forgetting to plug in”. What a joke. I wonder if Cavuto knows the EV1 (from a domestic car company) used inductive charging. Or that Plugless Power has been working on inductive charging for the Volt for some time.
I wonder when was the last time Cavuto pumped his own gas. Does he just pull up to a pump and expect “inductive fueling”?
Aug 24th, 2012 (9:09 pm)Why don’t we ever hear anything more about the Wave Disk Generator?
http://en.wikipedia.org/wiki/Wave_disk_engine
Jeff – I don’t know if this would be something worth investigating, or if there’s anyone at MSU that could answer questions.
This is the most recent news I could find after scanning the web a bit.
http://www.usnews.com/news/blogs/at-the-edge/2012/02/23/wave-disk-engine-could-be-new-hybrid-powerplant
Aug 25th, 2012 (12:07 am)Perhaps a steep and long downhill descent test run is in order. Under level or uphill driving, 20kw couldn’t be flowing into the battery for long unless the generator were producing the power for it. I don’t think there is a speed where the Volt’s engine does produce that kind of excess for any significant stretch of time.
Aug 25th, 2012 (1:54 am)I believe the modeling done in the “Comparison of Powertrain Configuration Options for Plug-in HEVs from a Fuel Economy Perspective” SAE paper posted a couple of weeks ago validates your analysis. I think their analysis indicated a 6% improvement for the Voltec over the Serial. Well done!
+1
Aug 25th, 2012 (4:14 am)I was quite busy yesterday but I finally got around to looking this up. You are correct that lithium ion has a higher charge-discharge efficiency than NiMH but the losses are significant. Here’s a link that says the “round trip” charge-discharge efficiency of lithium ion cells is in the range of 80% to 90%.
http://www.pluginhighway.ca/PHEV2007/proceedings/PluginHwy_PHEV2007_PaperReviewed_Valoen.pdf
This means the round trip charge-discharge energy loss is between 10% and 20%. This should not surprise anyone who is aware that a significant amount of heat is generated by charging or discharging any type of electro-chemical battery.
I hope this does not come across as criticism of the accuracy of your model because this additional energy loss only reinforces your overall conclusion that series hybrid mode is significantly less efficient than power split mode.
Obviously, the Volt design team did the math way before the decision was made to NOT use a pure series hybrid configuration. Their final design solution is quite elegant and perhaps it takes an engineer to fully appreciate this. The Volt design team deserves all the awards and accolades they have received.
Aug 25th, 2012 (4:40 am)This EE is an exception to that rule.
+2
Aug 25th, 2012 (4:49 am)I know exactly how you feel. The Volt has the most ingenious drivetrain I’ve seen in all my years of being a car nut.
+1
Aug 25th, 2012 (8:59 am)I totally agree with the fuel cell. The Volt’s power split may achieve better MPG with the ICE, but the ICE itself has much power loss as heat, and some mechanical loss. The fuel cell will have less heat loss and no mechanical loss. So in a pure series setup, the FC generator is better, especially al low and medium speeds where most of the Volt travel is needed. I understand that at higher speeds, the ICE does help.
Since I am mostly a city driver,I will not need the extra power at high speeds because the maximum highway speed here is 65 MPH and I hardly drive that fast. A pure BEV will be my best vehicle for over 95% of my driving needs. So if GM doesn’t bring the Volt, I can check the other American EVs. I will never buy an import EV but I will take one as a gift!
Raymond
Aug 25th, 2012 (9:28 am)smithjim1961,
Thx for the link. Figure 3 show 98% round trip efficiency. Am I looking at the proper figure.??
Aug 25th, 2012 (11:17 am)The only turbine example I can think of where the reduction gears are near the hot section is the AGT-1500 for the M1 tank. Think it’s more common for the gears to be located elsewhere. Then again, I don’t know about EVERY configuration out there.
+1
Aug 25th, 2012 (12:11 pm)“I wonder when was the last time Cavuto pumped his own gas. Does he just pull up to a pump and expect “inductive fueling”? ”
Hilarious ..:) many big honchos at the top especially in the media have no clue about lot of things …they live in their own world and cud not bother to check facts etc ..or even what the hell they are saying! Especially so at Fox .
Looks like Volt may break the sales record this month…more # of days plus from forums it looks like sales are picking up. Go Volt break the monthly record!
Aug 25th, 2012 (1:53 pm)A valid point – in helicopters we mostly take power out the front of the compressor stack. At that point, I was just looking for a case where it wasn’t worth the complexity to make a mechanical connection, and wrote without fully thinking it out.
Walter
Aug 25th, 2012 (3:30 pm)#60
True that. +1
Aug 25th, 2012 (3:33 pm)#63
How about the Spark? Assuming that they don’t get some sort of a setback from A123′s difficulties.
Aug 25th, 2012 (3:36 pm)FWIW, one our local dealers, WIN Chevrolet in Carson, CA, is advertising in today’s paper to lease a 2012 HOV eligible for $189/mo, 10K miles/year, $0.20/mile over 10K, “no security deposit”, whatever that means. I know squat about leasing, but that sounds like a heck of a deal compared to what I have been seeing here.
+1
Aug 25th, 2012 (4:25 pm)GM has been providing us with engine efficiency data in the forms of calculated engine load and absolute engine load (which you can simply subtract to find the percent of long term efficiency degradations). This has been used out here in the independent servicing market for quite some time, where the vast amount of reliable GM vehicles are the best for advanced training of high technologies, and how they normally (yet reliably and very slowly) wear.
For the most part, very few techs run these simple comparisons.
Even comparing the long term (very high mileage) degradations from the mid ’90′s of all OEM’s, GM “nets” the differences for you from 1% (inefficiency) when a vehicle is new, to 8% (inefficiency) when it is very old and systems have collectively become slightly less efficient, (which is normal and expected, of course).
So, GM has been BY A LIGHT YEAR, ABSOLUTELY the very best as far as scanning datum are concerned……
+1
Aug 25th, 2012 (4:30 pm)……….. FOR THE LAST 18 YEARS!!
Aug 25th, 2012 (5:31 pm)Before getting into the technical details I’d like to thank you for writing on this subject. I love learning new things and you’ve motivated me to learn more about the subject of battery charge-discharge efficiency.
Yes, figure 3 shows 98% efficiency but figure 3 is for constant current. The author of this paper is a making the point that when current is not constant things change. In the words of the author of the paper,
“…Average current was found to be insufficient for characterizing real life duty cycles. The performance of any battery is highly dependent on the duty cycle and knowledge of the power profile is crucial for battery and battery pack design…. It is also shown that the charge and discharge efficiencies decrease as the duty cycle eccentricity of the current profile is increasing. This holds for all three battery chemistries tested…”
Check out figure 5.
A series hybrid with ICE running at a constant output BUT the power demands from the traction motor are variable is probably an example of “duty cycle eccentricity”
Aug 25th, 2012 (10:01 pm)but why the change between the y axis units??.
Aug 26th, 2012 (12:18 am)thanks very much, great post !!
The questions is the 7% loss is due to current engine tuning . I wonder , the engine can be run on a sweet spot rpm and load characteristic to compensate on this and still keep the simplicity.
The all study looked to me like Toyota did the right thing with the HSD and what ford is now doing is also the right thing.
Aug 26th, 2012 (1:39 am)That’s a good question. I don’t know the answer but I’d like to find out more about it. I’m going to do more research on this later but in the mean time I’ll share some thoughts that are not rigorously scientific. I’m making an educated guess that real-world charge-discharge efficiency must be less than 98% because of the amount of heat generated in the battery. If there were not a lot of heat generated there would be no need for an expensive liquid cooling system.
+1
Aug 26th, 2012 (11:30 am)Pardon the OT, but another runaway acceleration has occurred with a 2011 KIA SUV.
The report on ABC this morning suggested “another accelerator pedal caught under the floor mat”.
The vehicle was said to have gone out of control for well over 30 minutes of driving on the freeway, and the poor driver, who is still alive, said that she “tried everything”. I would be fairly confident that she tried to move the floor mat or reposition it in that much time, and that the cause was not the floor mat.
In the interest of safety, I would like to have a reference here (and please excuse this as not being an unfounded plug), to my website if there is interest in ground fault indexed analysis which can be provided that would or would not define a potentially present root cause quickly.
Yes, I know we have been through this before with Prius, whereupon voters slammed my post with a negative thirteen votes, But I have been teaching this as an ASE Recertified L-1 Educator for four years now on live repair orders with extremely valid success. As well, it is listed on the Texas State Department of Public Safety website for years now also, so it would certainly not be there if there was any lack of credibility in my intellectual properties.
http://www.petit-tech.com
Aug 26th, 2012 (1:26 pm)Very good post… thank you!!
Aug 26th, 2012 (4:15 pm)Been busy prepping for a hurricane this weekend so haven’t had much time to post. Sure wish GM had offered V2H as an option. I, and I’m sure many others. would be happy to pay a premium for it. Hopefully George and Walter are still following this thread as I would like their feedback on the following.
Looking at Figure 2 & 3, I calculate the most efficient generator output to be at 24.6kw. This should translate to about 75mph (Figure 3) given the conditions and assumptions for the Figures. There should be an inflection point on the Volt Road load for this but that is about where the graph ends. Is this correct? Once you factor in other loads and assuming constant speed, level ground that translates to about 68-73mph for varying “other” loads. One thing the Volt doesn’t have that I have been asking for is a max efficiency cruise. Of course this may not be practical for hilly or congested driving conditions but most major highways miles are pretty flat. Once set, the Volt’s control software keeps output as close to max efficiency rate as possible. Buffer intermittent load changes with the battery but the net power flow with the battery would be minimal. For many people the resultant speed matches up nicely and I don’t think that was a coincidence on GM’s part.
BTW, nice work and thank you for the efforts putting this together. Your report and the comments of Volt drivers over time have solidified in my mind that cost is paramount to the choice of future engines for EREV. Weight would be next in ranking of importance. Of course their are packaging and NVH limitations and considerations that come next along with efficiency. Personally, I think we will end up with a small, light weight, low cost, and reasonably efficient rotary of some sort. The relatively low power and load consistency control match up well. I do not see a larger, heavier, more expensive option as a good one for the Volt even if it were slightly more efficient under some circumstances. It would make sense for higher power needs of a performance model, pick up, or SUV. In the meantime, I agree with Walter on a more literal interpretation of Atkinsons but not so sure about DI because of cost.
The conclusion of pure series being less efficient is generally accurate assuming the same components but there are benefits to pure series from a cost, implementation, and complexity perspective. I disagree with Walter’s comment about little added cost to implementing the EVT. It did add some gearing and clutches as others have noted but more significantly it added more testing and control software programming than would have been needed for a series configuration. I do think we will see viable pure series choices but for the Volt and, I agree, GM made the right choice and it will probably remain the right choice for a larger share of the market. For Tesla, had the continued on their REEV (predates EREV) path, I think pure series would have made more sense. Once the battery is big enough then the range extender can very small and effective.
Aug 26th, 2012 (4:54 pm)Thx for stopping in Koz. Hope all is OK w/ the hurricane.
Koz quote:
“Looking at Figure 2 & 3, I calculate the most efficient generator output to be at 24.6kw. This should translate to about 75mph (Figure 3) given the conditions and assumptions for the Figures. There should be an inflection point on the Volt Road load for this but that is about where the graph ends. Is this correct? Once you factor in other loads and assuming constant speed, level ground that translates to about 68-73mph for varying “other” loads.”
Yes I agree with those numbers. However it does not mean that one gets better MPG at 75 just because we are at min sfc on the engine map. Slower is better still from an MPG point of view.
It should be interesting to see what GM does with the range extender in Volt Gen 2. My money is on Atkinson cycle (not turbo). Just from a cost point of view. However I bet an Atkinson cycle will still be more expensive than the little 1.4 that we have now (which is a pretty darn good and cost effective choice).
The other thing about a 1.8 Atkinson is that, besides getting better SFC across a wide RPM band, it also has more power at the high RPM end which would make for a quieter engine and less time in “propulsion power reduced mode.”
+1
Aug 26th, 2012 (5:58 pm)She didn’t “try everything”. The only way to stop a runaway engine is to turn the ignition off! I feel sorry for all those foolish drivers who don’t turn off the engine and just coast to the curb, yet get into trouble and even cause a loss of life. I just hope that future ICE drivers learn better and remember that the ignition switch controls the life of the engine, not the accelerator, or, worse, the brake pedal!
Raymond
Aug 26th, 2012 (7:10 pm)Thank you for the concern. It’s been more of a scare than anything but these can sometimes bring significant power outages. It is mostly past and I still have power (fingers crossed).
Yes, I realize going slower will give better fuel economy but speed (time) versus economy is always a trade off for me. If our 2012′s had the power flow meter then I could drive close to max generator efficiency but it would be nice for the car to do it automatically. It would be much simpler for the driver and a better implementation.
Aug 27th, 2012 (9:34 am)You do understand that maximum engine efficiency in cruise doesn’t spell best fuel economy, right? It’s the best fuel economy *per unit of power* (in the industry, they use bsfc – brake specific fuel consumption to describe this.)
However, dragging a car rapidly through the air is inefficient, and becomes more inefficient the faster you drag it (this doesn’t change with better aerodynamics – a more streamlined car will have a lower drag value at any given speed, but the drag on any given car will increase with the square of the speed.)
This is where the “lowest speed you can sustain in the tallest gear” advice for normal cars comes in – low speeds with wide throttle openings lead to relatively good bsfcs for the engine, top gear gives you the least engine overhead/pumping per mile, and the lower speed is less drag.
In the Volt, the “most efficient speed” for cruise is likely quite low. Tesla decided that the roadster’s most efficient speed with climate control off was 17 mph, and driven electrically, the Volt’s is likely in the same ballpark. (With HVAC loads, the most efficient speed is faster – not that the car becomes more efficient at higher speeds than – it doesn’t – but that it becomes more inefficient at lower speeds, and so the less efficient higher speed is now the best you can do.)
If the Volt were a pure series design, that’d be the same answer for extended range mode, too. However, because the Volt does power split, I believe that the most efficient cruise speed for it in extended range is the lowest that it will stay in power split – somewhere in the 35-40 mph range. I believe that the savings from direct connection will be larger than the increased drag losses.
You’re entitled to your own opinion on costs, but I think you’re wrong. If someone had been developing a clean sheet design, you’d be right – there are certainly extra pieces and extra complexity – but using existing pieces with existing production tooling is a heck of a lot cheaper than building new tooling – enough so that I’m convinced that the amortized production cost is lower despite the extra pieces.
(Note that the gearing isn’t extra – with a clean sheet design you’d still need a way to gear the main motor down to the 7-8:1 range, and the single planetary gear the Volt has plus a transaxle differential is one of the more likely ways to do that – which is exactly what the Volt does in lower speeds or high throttles, at 7:1. The only difference is you’d have the ring gear bolted to the wall instead of clutched, and you wouldn’t have the clutch between MG A and the ring gear (or likely the clutch between the engine and MG A, since there’d be no reason to disconnect to two.) Also note that the Volt is more efficient *on electricity* than a series car would be at highway speeds – because it runs in two motor EV, which lets it slow the main drive motor and reduce the inevitable back EMF losses. GM says it’s worth 2-4% overall. Look at the city/highway MPGe for the Leaf and Volt to see the impact…)
Since we’re talking about clean sheet of paper engine designs, I’ll note in passing that there’s no fundamental reason you couldn’t make an Atkinson rotary. Changing the “valve timing” on a rotary is a complicated nightmare, but having it set permanently to a late intake closing isn’t hard. The challenge with a rotary is the awkward shape of the combustion chamber once you get past ~7:1 or so (very long, very flat, with lots of surface area which cools it rapidly.) Also gas blowing by the seals at low RPMs, but in an EREV that’s manageable (by never operating in that range.) The simplest solution might be a two rotor design, with one larger rotor serving to compress the air that gets combusted in the other rotor (and expand it back out – kinda a cross between a turbocharger and a supercharger.) Wankel himself developed a diesel rotary using this two stage approach in prototype form.
Walter
+1
Aug 27th, 2012 (9:55 am)Raymondjram,
I am sure that you are unaware that “hard switches” are now connected to “soft switches” IN THE IGNITION SWITCH AS WELL AS THE SHIFTER AS WELL AS THE “ON-OFF” BUTTON!!!
This should make you not just a little bit uneasy. It should scare the heck out of you when software is totally “locked in one setting, WIDE OPEN THROTTLE!!
It is not your job to know these things, but it is arrogant of many to dismiss and be closed minded that an outside agency has a proprietary system (which they must pay a nominal cost-of-doing business low cost to acquire), to be proactive for these conditions on the assembly line as well as at the dealership. (And I thank God every day for the presence of this site.)
First grade arithmetic delta formulations only require one thing to learn: an open mind.
I trust that the critical nature of my diagnostics process is beginning to become apparent at least to those in positions of technical responsibility. Although it is the good readers here whom are the ones who may know where to forward my posts, as I do not have access to that information.
Aug 28th, 2012 (4:07 pm)I came to this thread late, but just wanted to commend this terrific work. Hopefully this post, plus Smithjim’s comments showing battery losses will put pure serial’s disadvantage well above 10%, will kill the “pure serial is more efficient” line of thinking. At least among those who think.
I’d love to see a similar post to debunk the “pure serial is cheaper” meme. There is some good info in the comments, but power split is not only cheaper because GM had some gearbox parts waiting in the wings. Power split is cheaper even in a clean sheet design, because it gets double duty from the most expensive components in the drive train – the motor/generators and power electronics.
A pure serial needs a traction motor sized for road load plus a generator sized to match the engine. Power split can downsize the traction motor and let the generator pitch in when needed. Just to make up some example numbers, a pure serial with a 120 kW traction motor and 60 kW generator could be matched in performance by a power split with a 60 kW traction motor and 60 kW generator. The rule of thumb for lightweight auto motors used to be $50/kW, implying a $3000 cost disadvantage for pure serial. What’s the cost of a couple extra clutches attached to the planetary gearset (which you need any way for reduction)? $100? There’s no contest when it comes to cost – power split is cheaper.
Sep 12th, 2012 (7:57 pm)I know this comment is late…I hope someone sees it.
So, what are the implications of George & Walter’s findings for the “mountain mode” games that some Volt drivers have been playing?