The study compares real-world performance of a standard strong hybrid, a conversion plug-in hybrid, a dedicated plug-in hybrid capable of running the full EPA city cycle on electricity alone and an extended range EV like the Volt. The limited power output of the electric motors in current hybrids restricts the ability to run at higher speeds and acceleration levels. As a result, even with larger batteries, few of the PHEVs currently being developed will be able to go as far as people might think without running the engine. Read on after the jump for our discussion of the study findings.
[Source: General Motors]
ABG: Pete, what is your role at General Motors?
PS: I work in GM Powertrain and I'm Engineering Director for our hybrid systems and I've got special responsibilities on the front end of new hybrid and electric type systems. So simulation, analysis, controls development, electric motors that kind of thing.
ABG: So you're defining the hybrid systems before they get too far down the design process, defining what the requirements are?
PS: Future development is the GM-speak for that, but it's powertrain architecture development. I've worked on the Two-mode, the BAS hybrids and going all the way back to the EV1.
ABG: Let's start off by talking a bit about the background of what prompted this particular study and what your methodology was.
PS: We thought it would be good to take a look at the world of vehicles that plug in and examine the range of such vehicles because the topic has received so much attention lately. And it's really the basis of a couple of product initiatives at GM, we thought what is the most straightforward way to have a plug-in vehicle and what would we do if we wanted to take that to its logical extreme and through a process that we coined as electrification which is just increasingly adding electric power, electric tractive power and electric energy on board a vehicle towards the end of making the vehicles more efficient and ultimately replacing fossil fuels from the vehicle.
A real interesting area we saw in those vehicles that plug in, is in the way that the vehicles would be used we thought in the real world, might determine in fact the benefits or the merits of the various types. So we took a look at the easiest type, we thought, which we coined in the paper as a conversion plug-in vehicle which is to take a full hybrid vehicle that is capable of driving on electric power alone but albeit at limited power levels or limited speeds and simply add a battery and a capability to plug in and replenish the battery so not just with regen braking but also it takes some energy off the grid.
Then we went to the next step and said hey, what if we modify that vehicle substantially, modified its powertrain, added a much bigger battery and extended the electric driving so that maybe higher vehicle speeds could be reached or higher power could be achieved on the electric power alone and we coined that an urban capable hybrid. So we set the bar high enough to be able to run on the U.S. urban schedule on electric power alone so that would take us out to 50 or 60 kilowatts of electric power and we limited the battery to one that we thought could be carried in a largely unmodified full hybrid so we can make the battery pack bigger but not substantially bigger such that the chassis or the body structure would be compromised and then we went all the way.
We said let us just make an electric car that has the ability to do its full performance top speed, acceleration, etc on electric power alone and augment that with the means to recharge the battery only when that battery was depleted and we call that the extended-range EV. So let us take a look at how they would work in the real world so the methodology is let us go get real world data and it turns out that the southern California Association of Governments went out and they captured the actual driving on GPS data from over 600 drivers in various southern California areas and this gives us quarter second by quarter second resolution on their acceleration and speed throughout a driving day.
So this is not the urban schedule or the highway schedule. This is the real world of over 600 drivers in southern California in 2003. We use that as the basis for analysis to see the differences in those vehicles.
ABG: Your paper refers to the EPA city and highway cycles which are used for measuring emissions and fuel economy. There is also reference to the US06 test cycle which I think I have heard of before but can you give me a little information on how that one differs from the EPA cycles.
PS: I don't know the exact history of the US06 schedule but it is one of the schedules that is now used for the calculation of fuel economy labels, the new labeling procedure in '08 uses some US06 miles and it has also been a schedule that is part of what is used by the EPA to calculate the emissions on the vehicle so the US06 is an aggressive driving schedule. It was developed and thought to be in line with, let us say, a 98 percentile aggressive driver so it has harder accelerations, harder decelerations and higher top speeds and I believe this schedule touches just over 80 mph in portions of it.
So because it exists, it is already used as a basis for labeling and for regulatory purposes, we thought that it would be a good one to include as a point of comparison.
ABG: In the past when I was studying engineering, I spent some time in the lab with a vehicle on the dynamometer running through the EPA city and highway cycles as they were back in the late '80s and early '90s. I know from that experience that the way those cycles are set up includes extremely mild driving. Anybody who has seen how those cycles work would not be surprised that the vast majority of people never actually came close to meeting the EPA mileage sticker values. It sounds like the US06 cycle is a much more realistic cycle at least for the way American drivers typically drive.
PS: I think the data here from the SCAG would say that is a pretty high bar, the US06. You could eyeball this and I think we have calculated at one point if we looked at let's say average driving intensity and that might be measured by energy per mile driven. You could say that if you looked at the driving intensity to take a vehicle out of EPA urban, about 15 percent or so of these drivers drove at the urban schedule or milder than that with less intensity. If you looked at the highway that number might be about 25 or 30 percent. So you would say 70 percent of drivers in this population were driving more aggressively and if you set the bar at the US06, that number is probably around 95 percent so it is still a high percentile. It is still a high bar and actually maybe the median drivers somewhere between highway and US06 but I would daresay at my commute today, I was a US06 driver as were most everyone out on I-75 this morning.
ABG: For example on Figure 9 in the paper if you drew a line midway between the EPA highway and US06 lines it would probably capture 75 percent of the drivers and, as you said, the US06 is probably somewhere around 95 percent. It sounds like what you did was you take this data from this regional travel survey in California and you did some analysis with that, plotted it against the three different cycles. Was the intent to determine what would be required in terms of an electric powertrain in order to satisfy the needs of most drivers in the real world?
PS: That is correct. If you look at, let's say, what if we wanted to make a vehicle that was capable of driving the urban schedule on electric power alone, how much power would that take and then in the real world, what kind of merits would that achieve. Then we wanted to say what kind of merits would we get in the real world if we had an extended range EV and how would either of those compare to the merits of just simply converting a full hybrid to plug-in power.
ABG: Obviously, from an implementation point of view, the new PHEV Vue is a conversion from the Two-mode model. From a functional standpoint would you also classify that as an urban-capable plug-in or strictly as a conversion plug-in?
PS: I would categorize that as a conversion plug-in that, while its electric driving is expanded relative to the non-plug-in version of the two-mode Saturn Vue, it is not capable of driving the entire urban schedule on electric power alone. The urban schedule goes out to about 60 mph top speed. I think you will find when you go to modify full hybrid systems that speed can become problematic. The gearing in the hybrid is not necessarily enabling full driving capabilities and most of us not only would drive the urban schedule but as the data shows, even faster than that and more aggressively.
figures 5 and 6
The other thing that becomes problematic when trying to extend the electric capability on a conversion basis is to upgrade the electric power rating on the motors to achieve really high acceleration or capture more regen energy and we characterize those cycles a little bit in some of the figures in the draft paper. Figures 10 through 11 show that in the real world there is considerable energy available at higher levels of regen and there is energy required at higher levels of acceleration than what would normally be incorporated into a full hybrid.
ABG: So would you say that with the hybrids that are being developed right now, like the Two-Modes, that the limiting factor as far as the ability to recapture kinetic energy is actually in the motor as opposed to the ability of the battery to absorb the energy or the other way around.
PS: They are pretty well balanced on almost all our hybrids, the Two-Mode included. The motor's capability pretty well matches the batteries in that regard so both constraints roughly get hit at about the same time.
ABG: Figures 5 and 6 show duty cycles for the conversion and urban capable plug-in hybrids. For the Vue, can we expect to see a duty cycle that looks more like the conversion where the engine starts sooner and then cycles on and off or the urban PHEV where the vehicle runs in EV mode longer before starting the engine.
PS: It would be like the urban-capable PHEV. We are trying to provide as much electric driving as is sensible within the constraints of the plug in Vue.
ABG: Let's talk about the initial starts. It has long been known that most of the emissions that are produced by the engines for modern cars happens during the cold start and then as soon as the engines and the catalysts warm up, the level of emissions drops almost to zero. With hybrids, that are cycling on and off, particularly the way current ones do, how much real benefit are we actually seeing in terms of the emissions performance on those vehicles if the engines are cycling on and off? Is there anything that is being done, for example maybe to preheat the catalysts when the vehicle is running in auto stop mode?
PS: I am not aware of any production hybrids that use catalyst pre-heaters and all of them seem to use just the best practices from conventional vehicles to get the catalyst tightly coupled to the energy in the exhaust of the engine to get them active or light them off as fast as possible to get past the cold start emissions period. But I think you are touching on a real key element in one of the findings here. Because in the real world and we think that is represented by this RTS data from southern California that if you had a conversion hybrid or if you had even an urban capable one, that still the power levels and the speed levels that people drive are going to be pulling on the engine fairly frequently and on most trips. The engine will be on.
Even with the urban capable hybrid, most drivers are going beyond that and therefore that urban capability in the electric motor and maybe the hybrid hookup are still not going to be sufficient to prevent the engine from coming on and therefore those vehicles are going to experience a cold start. So from the paper, our finding here is the amount of cold starts or initial trip starts reduced from a conventional vehicle for instance through a conversion- or an urban-capable hybrid, is somewhere in the neighborhood of 10 or 15 percent. Only 10 or 15 percent of the time, if you had a plug-in and you went on a trip, you would actually prevent a cold start.
Whereas the E-REV, that one that has got full electric driving capability we are probably going to eliminate about 70 percent or so of those initial starts. Another factor to consider is that on most of these drives, by the time you actually saw an E-REV with an engine start, it was not likely to be anytime in the morning but rather it would be delayed to the afternoon and some atmospheric scientists think that the hydrocarbon emissions, the cold start emissions in the afternoon are less harmful than those same emission if they were experienced in the morning.
I think there is a huge benefit that you could attribute to an E-REV in this area and very little benefit to an urban-capable plug-in relative to just a conversion-plug-in and modest benefits for both relative to the full hybrid.
ABG: Are the extra emissions that come during start up largely be due to the catalyst being cold and not being able to react initially or is that due to actually having to use more fuel at the initial start up?
PS: It is the former. It is just the reactivity of the reaction. It is the level of reactivity there, so the hotter it is, the more reactive it is. People call it a cold start, I like to just call it initial start, exact temperature where that reactivity is good enough is going to vary on different vehicles and in different environments but to be sure, that first start is the start that creates the most emissions and subsequent starts create a lot less and we know it takes many minutes for the catalyst and engine to return back to a cold state. So once an initial start is experienced, any other vehicle and the driving conditions and the ambient conditions, it could be five minutes, it could be 25 minutes before another cold start would happen. The time is fairly long.
ABG: On any of the current or future hybrids do you know if there is any monitoring of the temperature of the catalyst that would then be fed back into the controls to perhaps in some cases just leave the engine running a little bit longer perhaps on a lighter load mode in combination with the electric drive to minimize the cold starts?
PS: I could not comment exactly on that right now Sam. I think all I could say is we see great architectural benefits to the extended range EV in that regard and then of course we saw huge benefits on fuel consumption.
ABG: In the case of the extended-range EV, would it be correct to say that what you would want to do when the battery state of charge drops is to start up the engine and then leave it running for an extended period of time until you restore the battery to a certain state of charge rather than cycling it on and off again in order to minimize the restarts and that sort of thing?
PS: Yes, you bring up an interesting point. In general, what we would do with an E-REV, let us say we take our Chevy Volt and we are going to drive to Chicago from here and by the time we drove around 40 miles, we would have brought the engine on and we are going to be burning gas. Your question is would we leave the engine on till another 60 miles or so and then now deplete again another 40. We think when someone is going on a trip of that length, it is the most efficient thing to do and the best thing to do for petroleum and keeping in mind emission is to cycle the engine as we would in a conventional full hybrid in what we call just charge-sustaining operation because we really do not know that the driver is going to Chicago and it is not more efficient to charge up the battery and if in fact somebody stopped after 60 miles and they could plug in again and we would like to get them the opportunity to replenish as much of that battery as possible off the grid.
RP: I think there is another benefit to is as well with what Pete said. You don't necessarily know that you are going to Chicago. If your daily commute is 41 miles and 42 miles, you want to make sure that they can draw as much energy off of the grid as possible because that is displaced petroleum.
ABG: In the definitions at the end of the paper you talk about an E-REV powertrain with 8kWh of usable electrical energy. Would that be referring to what is planned for the Volt, which is quoted as having a 16kWh battery where they are planning to use about 50 percent of that range?
PS: Yes, that is approximately correct and in the study, just to be clear we did not look exactly at the Volt. What we did is we set up a typical midsize vehicle, so we took the loss factors associated with the vehicle that has the size and the rolling resistance and aerodynamic drag of a brand new Malibu and then we fitted that vehicle with some various converted- and urban-capable plug-in hybrids and in a E-REV configuration, we handicapped those vehicles with whatever mass differences we thought they would see. When we fitted that Malibu-like vehicle with the E-REV powertrain, we fitted one that was approximately like what is in the Volt. Not exactly, but approximately, and your math there on the 8 kilowatt hours yielding of the Volt is consisting of what we said about the concept car and it is more or less in line with our direction for production.
ABG: All the data in here was done based on simulation using the data from the RTS study, correct? Have you validated some of this data against real world data, how confident are you in the results that you have got from your simulations?
PS: We are very confident. The simulation tools that we used are developed in house. Those tools have been used for the development of the Two-Mode hybrids and the BAS hybrids. So we have done calibration of those tools based on thousands and thousands of real world driving on a variety of hybrid systems and like I said, it is the basis of our own internal developments. We have also used these tools to evaluate competitive vehicles and we see that they are quite accurate. Our findings on the fuel consumption and the elimination of the initial trips starts for the E-REV are very solid. The elimination of about ¾ of the petroleum used by a full hybrid when you step from a full hybrid to an E-REV, I think that is a very solid finding. The finding that an urban-capable plug-in hybrid is still going to have the engine on, all but 6 percent of the drivers at the end of the day, we think that is solid. The conversion PHEV is 4 percent. These are virtually the same proposition when it comes to the real world on how often the engine is turned off and how much fuel or cold starts are eliminated.
We just do not see a difference between those vehicles in any practical sense. The big find, which frankly surprised us, is just how well the extended-range EV did, that the in the real world, about 64 percent of these drivers and you see from the data that some of those drivers are driving 280 miles. There is very little we could do about that. But in the real world, 64 percent of those drivers never had the engine come on during the day and none of them would have it come on in E-REV because they were driving 90 miles an hour or because they were going wide open throttle on the freeway entry ramp. So that piece is quite solid and that result we expect to hold up when we are able to produce E-REVs.
ABG: If this data holds up, I would say that the prospects for E-Flex type vehicles definitely look very good in terms of actually getting real world benefits in terms of both emissions and fuel consumption reductions.
PS: Really you think of it as an electric vehicle. Our experience on the EV1 taught us a number of things, but what stands out is that for a lot of the drivers, even though, and the data bears it out, even though most of them on most days just do not drive that far, they want to have the assurance to know and the peace of mind when driving that they have got 300 miles range when they set out on their day. Somehow that is money in the bank that provides them the ability to kind of withdraw from keeping track of how many watt hours do I have in that battery and how long was I plugged in last night and how cold is it today. They don't have to worry all the time and speaking as an EV1 driver myself for a period of time, that is a big thing that relieves that worry and so we do that when we came back to electric vehicles, we would have to do it in a way that is still practically gets almost all the benefits of carrying around a giant battery pack but still relieve the stress on the driver to have to keep track of their next charging opportunity and the quality of their last charge.
ABG: Definitely that range anxiety is a serious issue. I recently had the opportunity to test drive the Tesla Roadster and towards the end of that seeing, the gauge getting down into the lower regions and starting to wonder if we were actually going to be able to make back to our starting point before running out of juice.
PS: That is on a test drive and if you think about, I went to work but I am going to meet my family at a restaurant or I am going to go visit my brother. Change of plans midday, suddenly your life starts to revolve around that outlet and that charge opportunity. Really people have come to expect a certain amount of freedom that comes with 300 miles of range and we know that a successful electric vehicle is still going to have to deliver the range part of the equation.
ABG: Is there anything else you would like to add before we finish up?
PS: I am not sure how many folks appreciate the differences between an E-REV and the plug-in hybrid vehicles. We think that the paper goes a long way to illustrate the real world usage differences and I think also people and maybe they will come to appreciate it, the proposition that the development risk associated with taking a full hybrid and converting it. There is certainly work that has to be done to find an upgraded battery that can deliver repeatedly additional energy, maybe two or three times what was in the full hybrid and to put a charging system on it but we know that hobbyists and aftermarket folks have been doing that for the last several years and we want to be able to illustrate the level of risk that it takes to go do an E-REV which is after all an electric vehicle and requires much larger much more capable energy storage.
These large battery packs, the thermal system that goes with the battery packs and on the power delivery side, the electric motors, the power electronics and the thermal system that supports that. Then there is the entire vehicle integration. After all these big batteries require special accommodations on board the vehicle to get the chassis system right and the structural system right and to make the car behave in a way that you will say that is a good car for an E-REV. We want people to say, hey that is just a great car, period. I really just like driving that car and so we want to be able to separate the proposition of a converted hybrid from an E-REV and in folks' minds to say I like a plug-in. A plug-in does some things for me. It begins the electrification, it shifts some of the burden to the grid but maybe I aspire to the E-REV and I see that as being a huge next step. We're hoping that by publishing the paper and getting a chance to talk about it people also come to appreciate those differences.
ABG: Yes, I think a lot of people might be surprised at the performance limitations of a plug-in hybrid particularly a conversion plug-in hybrid. As you said there are the limitations of the electrical side of the power train. Just adding a bigger battery and plug-in capability is not necessarily enough to be able to make it behave as much like an electric vehicle as some people might think.
PS: Right, exactly.
ABG: If you got something that is designed for that purpose, that is another story, but then that adds a whole new level of cost and complexity beyond what some of the current conversions are doing.
PS: Right, so when we sell the plug-in Vue, we want to make sure our customers have their expectations adjusted correctly, that this car is going to drive as much as it can like an EV. There are real practical limitations to that and we think it's a great thing and expect that car is going to do well. At the same time that we want to also set expectations for how good an electric vehicle can be and return back to that whole level of electric performance we had EV1, but this time we are going to take care of the range anxiety.
ABG: One final thought as we move toward E-REVs and plug-in hybrids, is the current EPA test cycles and how the way they are done today very likely would not really reflect the true benefits of those vehicles in terms of reduced emissions and fuel consumption. Do you think that the study and some of the data you've got here and I am sure others are doing similar studies, will be be used in working with the EPA towards modifying those cycles in the future. To give them a more accurate reflection of the performance of those types of vehicles?
PS: Yes, and it is very astute. I was just kind of grinning here. Yes, some of this data is being used by us and folks that we work with on the SAE J1711 committee to calculate fuel economy labels for plug-in vehicles. Right now there is a methodology that is kind of grandfathered in from a long time ago when this was first anticipated, but not really well thought through. We are working to make sure that method reflects the likely regular usage of vehicles that plug-in to make sure that the label accurately reflecting what most drivers, whether it's a 50th percentile driver or a 90th percentile driver are doing. We are going to work through that in the committee with our colleagues from the industry and come up with a good answer that reflects a label that is a level assurance that that kind of mileage could be achieved by most drivers. It's exactly this kind of data that leads us to those calculations.
ABG: Thanks so much for your time today.