Why the Chevy Volt Should Drive Its Wheels with Gas

This opinion piece was contributed by Chris Ellis, CEO, HyKinesys.

The UK’s Telegraph caused a stir in June when it speculated that G.M. soon might turn the Chevy Volt into a hybrid by directly linking the engine to the wheels. HybridCars.com also reported that the 2011 model already has hardware, like a planetary gear set, which makes it possible for the Volt to work more like a hybrid than an electric car. Here’s the question that hasn’t been discussed: Why would G.M. ever want to use the engine to directly drive its wheels?

In a nutshell, the main reason is to make the car more efficient and economical when cruising at high speeds. It’s the same reason lock-up clutches were added to automatic transmissions decades ago—to improve efficiency at more than 50 mph in top gear.

How It Works Now

Let’s be clear. I’m not talking about when the Volt’s battery pack initially has more than 8 kilowatt-hours remaining of its total of 16 kilowatt-hours. As long as there’s enough juice in the battery, it operates just like a battery-only vehicle. The engine stays off until the battery drops below the trigger level. Only at that point does the engine start up and the Volt begins to operates as a charge-sustaining series hybrid. The control system doesn’t attempt to re-charge the pack much above the trigger level. Full re-charging must wait until the Volt is next plugged in.

When the engine is running, the principal flow of energy is from the gas tank via the engine and its generator to the electric motor connected to the differential and the front wheels. The battery is still involved, but only as an energy buffer during regenerative braking and strong acceleration.

How It Would Work

In most driving conditions, nothing would change—until the Volt’s (hypothetical at this point) direct drive mode would be called into action when the Volt reaches cruising speeds. At that point, the engine would be mechanically connected to the differential, bypassing the electrical pathway. The motor and battery pack will still be involved during strong acceleration and regenerative braking, but the engine will maintain cruising speed and provide mild acceleration by the most efficient means possible—that’s right, by putting power directly from the engine to the wheels.

When the engine is running, the Volt will operate as a series hybrid in the city but it will become a parallel hybrid above, say, 50 mph. Direct drive should be able to transfer energy from the engine at upwards of 95% efficiency at 70 mph.

Compare that level of efficiency with what happens with the current Volt at highway speeds. Although each element of the electrical pathway is relatively efficient, the losses start to multiply because there are four devices connected in series: the generator, two controllers and the electric motor.

Let’s assume the generator operates at 93% efficiency. It passes current to its controller, which is liquid-cooled because it also has losses. Let’s say this controller operates at 97% under these conditions. It passes current to the motor’s controller that also operates at 97% in passing current to the motor. Finally, the motor transforms electrical energy into mechanical energy at 92%.

Consequently the overall efficiency formula of the electrical pathway is something like this: 93% x 97% x 97% x 92% = 80.5%. Apologies for the theoretical math, but it’s important to understand how the losses multiply. You can see how a Volt with direct drive might need about 15% less fuel when traveling at 70 mph, compared to the current indirect drive.

A Bonus: Highway Acceleration

Adding the parallel option to the Volt not only promises reduced freeway fuel consumption but it also opens up the possibility of better acceleration. Early road tests suggest that the Volt’s initial acceleration is competitive, but there have been complaints that the Volt is sluggish over 50 mph, when the torque curve of the electric motor tails off.

With direct drive, the power of the engine will be added to that of the electric motor. In its simplest form, the additional power will be relatively low because the direct drive will be geared to deliver the highest possible mpg at around 70 mph. After all, the name of the game is efficiency and reduced use of petroleum, not more oomph on the highway or even “electricity at all costs.”

Making the Chevy Volt more of a plug-in parallel-series hybrid might be heresy to electric purists and Volt true believers. It also creates a slight challenge for G.M. marketers who have insisted that the Volt is a pure electric car (despite having a gas engine on board). But at the end of the day, G.M. executives and engineers hopefully will be guided by an ethos of maximum efficiency and good economics—and delivering the best possible ride using the least amount of energy.


  • calvin

    It always seemed more efficient to me to go directly from, either, battery->electric motor->drive shaft or gas->ICE->drive shaft, instead of gas->ICE->dynamo->battery->electric motor->drive shaft. Even with regenerative braking, there must be a huge loss in efficiency from the parasitic loss between each component. And yet you still see massive dump trucks and boats (and now even proposed eco-planes) that use gas to power a generator to power an electric motor.

    I have to assume that most of those applications are designed like that to generate greater low-end torque and a more constant power curve, so efficiency isn’t the overriding concern. But it just doesn’t make sense in a hybrid-electric designed for efficiency.

    Though, ideally, if we’re to burn fossil fuels, it should be done at an industrial power plant as even conventional ICE engines are incredibly inefficient in comparison.

  • Anonymous

    given the efficiency fundamentals between electric motor and ICE, it’s no coincidence volt is going down a similar path as prius unless volt decide to have even bigger electric motor is in the car.

    the main difference i see between the volt and prius (none plug in version)?

    - series vs parallel hybrid
    - a big price tag difference
    - unproven vs proven track record.

    gm might have trouble selling this puppy if *anything* goes wrong. that’s a real possibility with gm’s quality reputation

  • Indigo

    The Volt is certainly a more modern serial hybrid. Whereas most hybrids run on gasoline and electricity, the Volt runs on hype and taxpayer subsidies.

  • calvin

    Yes, GM should be partially nationalized like the BBC and NTT (Japanese telecom 1/3rd owned by government) if they’re to get subsidies from the government. Or at the very least have most of their board of directors replaced.

  • Sean Maslow

    This seems terribly complicated. One of the many attractive attributes of pure EVs is their simplicity – dozens of moving parts compared to thousands in an internal combustion engine (ICE). Adding electric drive to ICE drive and the software/hardware intricacies to know when and how to connect which to the drive-train means so much more is going to go wrong. K.I.S.S. (keep it simple stupid) should be the guiding principle here.

  • calvin

    That’s nice and all, but right now there are almost no plug-in stations for a pure EV, and current battery prices are too expensive for a pure EV to possess the range people want at an attractive price.

    A bicycle is far simpler than an EV, and it confers many health benefits too, but that simplicity does no good when you require a car.

  • ACAgal

    The more operating systems in a vehicle, the more complex, and the heavier the vehicle. The more complex, the more chances for problems. The heavier the vehicle, the more energy required to start and stop the vehicle….or so I’ve been told.

    I like the bicycle idea, but it wouldn’t get me across the CA desert, or the Sierra Nevada range, I lack the energy.

  • Happy

    Wait a minute…didn’t we already do that with the bailout? Ohhh…yeah, I forgot that was a gift, cause we’ll never get repaid!!!

  • Charles

    The biggest difference between the Volt and the Prius or the Prius plug in is that the Volt has a strong enough electric motor to power the vehicle at highway speeds without the ICE. To me that is very significant.

    I see the following as a progression of vehicles from ICE to pure EV:

    1) Pure ICE, example my 2004 Ford Focus Wagon (PZEV if anyone cares).
    2) Stop/start technology that some have misnamed as hybrids or micro hybrids (example Chevy Malibu hybrid).
    3) Weak hybrids like the Hondas which must use the ICE to move and almost any speed.
    4) Strong hybrids like the Toyotas and Fords that can at least leave a stop sign as a pure EV.
    5) Weak plug in hybrids like the soon to be Ford Escape and Toyota Prius plug ins that can act like a pure EV at low speeds or a very short range.
    6) Strong plug in hybrids like the Volt that have full or close to full power for a significant (>20 miles) distance and speed (>80 MPH).
    7) Pure EVs.

    Each has its market and each will improve over time. As you move through the list the price goes up if you keep the same abilities. At this time we only have 1-4 with any significant sales volume. The pure EVs that are available or announced do not have the same abilities as the ICE vehicles. Current and planed EVs are very range limited and recharge time limited. I hope in a few years my significant other and I can move from ICE and strong hybrid to a pure EV and plug in hybrid (strong plug in hybrid if GM can really build a good reliable Volt like micro van) family.

  • usbseawolf2000

    Prius is already parallel-series hybrid. It is geared to torque 72% Parallel and 28% Series. The power is split to both directions at all the times. The amount of power split all depends on how fast the generator spins (power = torque x rpm x constant). Sometimes it is Series dominant (up to 57%), other times it is Parallel dominant. It cannot be 100% Series or 100% Parallel but rather always a combination of both.

    How Prius balances between Parallel and Series is what it is called e-CVT. At low speed the characteristic of Series hybrid is used as “low gear” tapping into high torque electric motor. When it is cruising on the highway, Prius uses most of the torque and power directly from the gas engine (Parallel characteristic). It is done with amazingly simple planetary gear-set.

    If the Volt operates like this article suggest, it can be 100% Series or 100% Parallel. However, multiple clutches are needed.

  • TrasKY

    Please know the facts before you make a comment.

  • ChrisHF

    6.5) Like (6) but using an engine specifically designed for usage: low weight, high efficiency, inconsequential power curve.

    6.8) Like (6.5) but with a lithium battery. (This is probably when I’ll buy one)

  • drmp

    That is exactly what I said at facebook on my discussions to one of the GM drivetrain engineers. It looks like my words were taken very seriously. It is very interesting how similar my words and what is explained on this article. Check this facebook discussion: http://www.facebook.com/photo.php?pid=446417&id=100000207560898&fbid=147227385294179#!/topic.php?uid=244534202311&topic=18196

    I was using my wife’s name “Roselle Nieva Peralta”

    Mark

  • Lost Prius to wife

    Calvin: “A bicycle is far simpler than an EV, and it confers many health benefits too, but that simplicity does no good when you require a car.”

    Calvin, I agree with you. It is like buying a Prius to move a half ton of rock when one should use a truck designed for that kind of job. It comes down to the needs of the person. This is why I do not snub my nose at Hummers and large trucks; the person driving those vehicles may actually be using those vehicles for their designed usage every work day. I only feel sorry that there are not a whole lot of more economical choices for those kind vehicles.

    In the last article that hinted at the Volt’s possible “direct drive”, I indicated that if it would make economical and MPG sense to add a clutch to the Volt, so be it. From the sounds of it, it may be a smaller power split device than the Prius. It sounds like the Volt is going for a more powerful performance than the Prius at speeds above 50 mph while retaining gas economy similar to the Prius. This means that people that do a lot of highway driving and want the economy of a Prius with more performance than the Prius will have a choice. And this may be a bigger market than one would think. Even if the Volt will cost more than the Prius, it still will be more economical that a similar ICE driven vehicle over an extended period of time.

  • simon@syd

    A good point was made here – that ICE has certain intrinsic advantages over electric at cruising speed (say, cruising above 50mph). Maybe hybrids aren’t just a stopgap, as long as there is some oil around of course.

  • calvin

    @Lost Prius to wife:
    Yea, but to be fair, the new H2-style hummers aren’t really utility vehicles. And the number of SUVs on the road in general far outnumber the actual number of people who need, both, the capacity of a van + the towing power of a truck on a daily basis. And both trucks and SUVs continue to get bigger and bigger for no apparent reason. So I have to conclude that the majority of SUV/truck buyers are simply following the “bigger is better” mentality. That said, I agree that having more fuel-efficient trucks and SUVs can only be a good thing.

    @simon@syd:
    Most current EV motors only have decent torque below 4000-6000rpm, but that could change in the future. Also, if battery costs decrease, you could have more powerful electric motors that can cruise at higher speeds at lower RPMs. So there’s no reason why we couldn’t eventually phase out ICE engines in cars altogether. There are already high performance all-electric cars that have no problem reaching highway speeds in very little time. It’s just that they’re incredibly expensive ($100k+).

  • Achilles

    The arguments for and against direct drive, initially for the Prius, have been going on for years, see my 2005 EVWorld article quoted at:- http://www.greenhybrid.com/discuss/f13/hsd-variable-ratio-transmission-8235/ I resent the suggestion that I have plagiarised anyone. Only those who are late to this issue could possibly make such an accusation.

    The new news is that GM seems likely to fit direct drive to the Ampera, which is easier to fix than the Prius.

    The same old complexity and reliability arguments mounted here were trotted out against lockup clutches for torque converters. When was the last time you heard of a lockup failure?

    There seems to be an assumption by some that there is an inevitable progression towards all light vehicles becoming battery-only. However, most automakers still claim that fuel cell vehicles are the end game, and most FCV prototypes are fuel-only series hybrids. My guess is that the reality twenty years from now will be a mix of solutions, proportions varying by vehicle size and region. Paris will be full of little electric cars. Beijing will have a much higher proportion of LaCrosse sized plug-in hybrids. These will still use liquid fuels. Globally, we will see a variety of biofuel sources, plus Gas-To-Liquid, gasoline, diesel, etc. And Americans will still be allowed to choose for themselves what they drive. I hope.

    Here’s a long range forecast. The ‘inevitable’ end game, still a long way off, is a fusion-powered fuel-only hybrid, requiring re-fueling only once a year or so. Well, why not? Batteries will be like buggy whips!

  • Fuel Sending Unit

    Really your thought will be a great mainstream.This is a large why the chevy volt should drive its wheels with gas very good though i would like to light it at the wall of my facebook.

  • Charles

    Chris,

    My progression was meant as a progression of ICE to EV in degrees. I do not think there is one “end game” way to power a vehicle. I do think over the next 20 years pure ICE personal vehicles will decline to a small percentage of the market. I hope and wish for pure EVs to be the dominant vehicle over the next 30 years, but I do not think that will happen. To have almost all pure EVs on the road the battery technology needs to improve beyond what I think we can archive, ever. FCVs may well be a great solution, but it does have many problems. H2 is hard to hold and is a potent greenhouse gas when it escapes. Making H2 is very energy intensive. I admit I have not looked at the data, but I bet it takes more energy to get H2 ready as a fuel compared to even something as bad as corn.

    I hope none of us live to see the “end game” for fueling vehicles, because that means we have lived to see the end of civilization.

  • JohnK

    Perhaps the main reason the Volt uses a series hybrid design is patent licensing issues. It seems Toyota and Honda, investing early, hold patents on the relevant, workable, parallel hybrid technologies. Ford entered into a technology swap deal with Toyota, because their(Ford’s) in house developed system would have infringed on Toyota’s patent.

  • clydeS

    I think that the Volt is wonderful!…
    It will be the most fuel efficient car that will be availible now (October)

  • JMNsir

    Volt does not need Mechanical drive for higher speeds. I believe your assumption for your math is faulty, The Volt is just a refinement of technology that been around for decades. Diesel-Electric trains have been using diesel driven generator to pull trains. If there was efficiency gain at higher speeds with mechanical drive then trains would have incorporated the technology. No the next refinement of the Volt will be replacement of the ICE with Fuel Cell

  • JMNsir

    Volt does not need Mechanical drive for higher speeds. I believe your assumption for your math is faulty, The Volt is just a refinement of technology that been around for decades. Diesel-Electric trains have been using diesel driven generator to pull trains. If there was efficiency gain at higher speeds with mechanical drive then trains would have incorporated the technology. No the next refinement of the Volt will be replacement of the ICE with Fuel Cell

  • JMNsir

    Volt does not need Mechanical drive for higher speeds. I believe your assumption for your math is faulty, The Volt is just a refinement of technology that been around for decades. Diesel-Electric trains have been using diesel driven generator to pull trains. If there was efficiency gain at higher speeds with mechanical drive then trains would have incorporated the technology. No the next refinement of the Volt will be replacement of the ICE with Fuel Cell

  • Achilles

    Oh dear, I knew someone would mention series hybrid trains. I should have inserted the following para in the article, to head off the three misleading comments above.

    The reason the series configuration makes sense for some trains is the need to drive multiple axles from a single large diesel, to provide the necessary traction with steel on steel. The mechanical alternative would be much more expensive. For a train, fuel consumption is not the critical issue.

    Take a look at the DuoDrive taxi, which is a series hybrid plus direct drive. Why would they bother adding direct drive if it didn’t improve cruising fuel consumption? And why will GM add direct drive to the Volt?

  • disbsam333

    Haha nice one Indigo. I am guessing you drive a Japanese or German car. Fun fact, Japan taxes American-made cars at 100%. Good job hating on America douche. Btw, Toyota got a bailout before from Japan before WWII. Mitsubishi made airplanes that bombed Pearl Harbor, Germany made cars for Hitler, and etc… what did GM do? Build the Humvees, Tanks, and Airplanes to help keep our country in existence. Not to mention the BILLIONS, maybe even over 1T in taxes that GM paid over the past century. I think they deserved the bailout (that they paid back). Only thing left is equity that is worth more now than it was in 2007 (i.e. a profit for the taxpayers).

  • calvin

    Um, from what I’ve read, Japan doesn’t apply any tariffs on imported cars. They only apply a tax based on vehicle weight and engine size. On the other hand, the U.S. not only limits the number of car models that foreign companies can bring to the U.S., but we also apply a 2.5% tariff on imported cars and 25% tariff on imported pick-up trucks.

    Also, good job in making Americans look like ignorant and delusional jingoists.

    Ford and GM, like many other international businesses of the time, collaborated with the Nazis during the war. There was even a class action lawsuit filed against Ford and GM for the slave-labor they used in their German factories. So don’t think they helped the American war effort out of some selfless altruism–they were just interested in making money, and they profited greatly from doing business both with the Allies and the Axis governments.

    Mitsubishi was equally interested in profit as GM and Ford. They could care less what their planes were being used for. Even still, the attack on Pearl Harbor killed far fewer Americans (and even fewer civilians) than did GM and Ford’s disregard for vehicle safety prior to the 1970s. In fact, GM and Ford demonstrated that they would rather slander and extort consumer rights activists than fix safety problems in their cars. Oh, and repayment of the bailout? Maybe you should do a little more research on that one…

    In fact, while you’re at it, maybe you want to research into who invented the internal combustion engine that gave birth to companies like Ford and GM in the first place. And, hopefully neither you nor anyone you care about will ever be involved in a serious car accident, but should that happen, you can thank Volvo for improving occupant survival rates by 50% by inventing the 3-point seat-belt.

    Your xenophobia is even more ridiculous given that companies like Toyota and Honda make a ton of their cars in the U.S., helping to employ Americans while Ford and GM have been exporting jobs and investing the bailout money overseas. In fact, Honda manufactured more cars in the U.S. this year than in Japan.

    Blind nationalism makes no sense in today’s global economy. Every single American auto maker has used parts manufactured by Japanese or European auto makers, just as many Japanese models were designed in cooperation with American auto makers. This technology exchange benefits both sides, which is why successful multinational corporations do not care about national borders when it comes to business.