Volt Birth Watch 174: Enough With The Prius Comparisons!

Edward Niedermeyer
by Edward Niedermeyer

As we saw in the last VBW, the Volt’s range-extender still needs some software work. But efforts to to keep the gas engine from acting like a thrashing, disembodied dervish will have to balance the desire for smooth operation and maximum efficiency. And it’s looking like efficiency in charge sustaining (CS) mode won’t match the hybrid standard-setters. Volt chief powertrain engineer Alex Cattelan breaks the news gently to the true believers at GM-volt.com

You’ve got to understand that all of the decisions that we’ve made around this product are made because its an EV. That is the first and foremost thing that it needs to be. So because it is an EV some of the decisions that we’ve made around engine operation will be different than what Toyota makes in its parallel hybrid. For them they are always operating in hybrid mode so they need to optimize everything for engine operation.

In our case we’re optimizing everything for EV operation and the secondary is certainly going to be better than conventional vehicles, but were not necessarily totally optimizing the system for charge sustaining mode because we don’t want to compromise electric vehicle mode.

Cattelan goes on to describe the sophistication of the Volt’s constantly-updating efficiency software, sourced from the abortive Two-Mode hybrid system. And as the chart above shows, the plan for charge-sustaining mode is an interesting one. Essentially, it involves keeping the battery state of charge between 30 and 35 percent, once the 40 miles of (estimated) EV range is tapped. Which is a fine idea as long as the engine on-off improves. Otherwise, drivers might just find themselves nervously counting down the five percent charge range before the 1.4 liter range extender thrashes to life again. Hoping for an answer to that question, Gm-volt notes “I’ve driven the 2-mode and notice you can see the switched in mode of operation without feeling it in the car.” Cattelan’s response reveals the trade-off that’s in play:

Which is the goal, you don’t want you to feel it in the car, we don’t want the customer to know these transitions are taking place, but we need to be able to enable them for efficiency.

Later, when Cattelan has explained the efficiency benefits of having a range-extending engine that’s independent of the drive axle, Gm-volt pushes again on the charge sustaining-mode efficiency question, saying “It seems to me then you should make CS mode even more efficient then in a car where the engine always has to turn the axle?” Cattelan’s answer once again downplays the notion, saying

Right and it is more efficient than a conventional vehicle because they do have to have that engine coupled. Again were optimizing some of those efficiency point puts we are really doing is focusing on the optimization of the EV. There are trade offs because we absolutely consider this product an EV by nature.

It’s not a hybrid! We’re focusing on EV mode! More efficient than a “conventional vehicle” in CS mode! Which means, what, 35 MPG? As Paul Niedermeyer explained some 18 months ago, the Volt is going to have a hell of a time beating the Prius on a mass-market basis. Which is what happens when you come up with the marketing line (“40 miles without burning a drop of gasoline”) before you develop the car.

Edward Niedermeyer
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  • Rusted Source Rusted Source on Nov 26, 2009

    I think Pfizer is working on a pill for Range Anxiety.

  • T2 T2 on Nov 26, 2009
    don’t follow why you pick 2400rpm to 4200rpm. If you’re being specific about a known product combination then you ought to say so. It’s possible to design an efficient diesel-permanent magnet gen-set that operates at 500rpm if you wanted to. Ok , for what it's worth - why you pick 2400rpm to 4200rpm - I have been baselining my argument on quoted figures for the 3 cyl turbodiesel from the SMART Fortwo as follows : Max torque 74lbs-ft @ 2400 rpm dropping to 50lbs-ft @ 4200 rpm. A 75% speed increase from 2400 rpm is accompanied by a 32% torque decrease. It so happens that I have also taken the Smart vehicle for a test drive on back streets and concur with the published figures. Torque sure drops off quickly when accelerating in any specific gear !! And yes I don't deny It’s possible to design an efficient diesel-permanent magnet gen-set that operates at 500rpm if you wanted to. Sure if you specify a specific rpm you can select a generator to be at optimum efficiency at that particular fixed rpm when supplying maximum power. But that's not the case here. The genset for the Volt will be required to run over a whole range of power demands when the battery approaches a state of charge of about 30%. You want the engine to be running at differing speeds but always at max torque as often as possible, similar to the operation of the Prius, in order to attain a high mechanical efficiency for your engine. However a 32% torque drop off above 2400 rpm is not going to sit well with the generator. It will need be sized for 74lbs-ft to take advantage of the torque heavy diesel characteristic at 2400 rpm regardless of the fact that it will receive a lower 50lbs-ft at the system's 4200 rpm top speed. Such a generator will therefore be 50% heavier and more costly than the one that a gasoline engine would require with its simply constant 50lbs-ft at all speeds. Nothing stops you from using the cheaper 50lbs-ft generator for this diesel but then you would have to leave 24 lbs-ft on the table at the lower rpms and just make the diesel run about 50% faster than it would have done when used with the more appropriate 74lbs-ft generator. Obviously making the diesel engine run faster while restricting the torque is going to impact system efficiency. 'Nuff said. A Diesel here is not a good idea. You might be wondering why I have made it my business to research this. Well the reason is that I am in fact building a genset drive powertrain. To do that a couple of months ago I formed a mastermind group. I sold the idea to a group of aquaintances skilled in areas that I'm not. The result is that today I took delivery of our genset. The prime mover is a brand new utility 15Hp single cylinder air cooled gasoline engine. It is coupled by means of a V-belt to a 5.5Kw Yaskawa servo which serves as a generator. The whole assembly is mounted on an off-the-shelf, heavy duty, two wheel dolly so that we can move it around before it becomes resident in our target vehicle, an awd Chrysler minivan. We are now examining the use of a throttle body actuator from a fuel injected engine to operate the throttle linkage of our carbureted engine. The other half of the dynamic duo which makes up a GSD system is the inverter/ induction motor combo which will be driving the rear wheels after we've removed and severely shortened the rear shaft from the transfer case. Mounting the motor will be someone else's job too. My role at this end will be to spoof the inverter bus voltage and the incoming 3-phase to make it appear as if this vector drive was really connected to a genuine 460vac 60hz supply. In actuality its power rectifier section will be seeing anything but. The inverter control electronics will be getting a standard 115vac input courtesy of the 12vdc battery using an off-the-shelf 500watt 12vdc to 115vac converter. An earlier forensic examination was conducted to see how this 115vac input would be used internally. I discovered the designers had used a pair of off-the-shelf switchers - this time to step the 115vac back down for their low voltage circuits, LOL. The trick is to know what not to do. The likelihood of a successful design is increased when you can avoid blind alleys. That's why I come on to TTAC. I have the same theme usually. This is what you shouldn't do. And this is why you shouldn't be doing it. Unfortunately the drawbacks of both the Honda and Toyota systems are rarely discussed. Although the VOLT has been selected to be under the spotlight here, there is the feeling that all these designs are merely interim solutions while companies experiment with electric automotive traction. In my case I just decided to go ahead and build my version of the logical conclusion of these designs. When the 15Hp concept is up and running it won't be such a large stretch to switch to a liquid cooled, high rpm, fuel injected engine with direct shaft coupling to the generator. Right now I am using the correct technology which is simply the affordable technology that is available.
  • Daniel J Until we get a significant charging infrastructure and change times get under 10 minutes, yes
  • Mike I own 2 gm 6.2 vehicles. They are great. I do buy alot of gas. However, I would not want the same vehicles if they were v6's. Jusy my opinion. I believe that manufacturers need to offer engine options for the customer. The market will speak on what the consumer wants.For example, I dont see the issue with offering a silverado with 4cyl , 6 cyl, 5.3 v8, 6.2 v8, diesel options. The manufacturer will charge accordingly.
  • Mike What percentage of people who buy plug in hybrids stop charging them daily after a few months? Also, what portion of the phev sales are due to the fact that the incentives made them a cheaper lease than the gas only model? (Im thinking of the wrangler 4xe). I wish there was a way to dig into the numbers deeper.
  • CEastwood If it wasn't for the senior property tax freeze in NJ I might complain about this raising my property taxes since most of that tax goes to the schools . I'm not totally against EVs , but since I don't drive huge miles and like to maintain my own vehicles they are not practical especially since I keep a new vehicle long term and nobody has of yet run into the cost of replacing the battery on an EV .
  • Aquaticko Problem with PHEV is that, like EVs, they still require a behavioral change over ICE/HEV cars to be worth their expense and abate emissions (whichever is your goal). Studies in the past have shown that a lot of PHEV drivers don't regularly plug-in, meaning they're just less-efficient HEVs.I'm left to wonder how big a battery a regular HEV could have without needing to be a PHEV.