Yes, I know it has an electric motor, batteries and a petrol/generator set but how does it balance engine running versus charging versus driving along?

At the Detroit auto show GM gave some road impressions as here

http://www.autocar.co.uk/CarReviews/FirstD...lectric/246503/

and revealed the batteries are designed for 6,000 charging cycles. The engine is a 1.4 litre petrol engine delivering 75bhp so its state of tune is pretty low. No torque or rpm figures have ever been published as far as I know.GM have always said the engine does not drive the wheels but they do not say how its power is split between driving and charging.

When the batteries near exhaustion there are three possible strategies

a) the engine takes over driving the car as in a diesel electric train engine. The engine is loaded by the generator characteristics and may not run at constant speed as the generator is loaded up to accelerate and backed off on coasting.

b) The engine purely recharges the battery at a constant load and the battery acts as a buffer to cover load variations on hills etc.

c) some clever combination of the two

The advantage of b) is that the engine can be tuned for constant revs and load so it's BMEP can be very high, maybe approaching a diesel without all the cost. Given the low bhp/litre the engine may run at low revs/open throttle under generator load control. This would maximise petrol fuel consumption I would think.

The disadvantge of b) is that the battery range has to be curtailed to ensure that there is enough in reserve to do the load buffering up hills etc. so what you gain on petrol mpg you might lose on lower electric running and thus less cheap overnight charging.

Also if the 6,000 recharges is strictly true then strategy b) may lessen battery life ( in years) because some recharge cycles were during running and not at night.

I suspect here is a huge powertrian optimisation activity here which is maybe why GM is so tight lipped about EXACTLY how the Volt works. Do any of the engine guru's here have any clues as to how you trade off petrol enine efficiency versus battery reserves etc. in such a car? One thought that comes to mind is that you could use smart learning in the ECU's so track daily driving habits and thus adjsut the engine start up points so as to arive home each night with the emptiest safe battery charge.

Actually they have described exactly how it works. Once the battery is down to 30% SoC the engine switches on and runs at any of 5 setpoints, or switches off again, depending on the instantaneous road load, keeping the SoC between 30 and 35%. If the SoC exceeds 35% the engine switches off again.

I think this is less efccint than the prius' approach, but it'll be interesting to see in practice. incidentally bob lutz only got 28 miles out of full battery pack, I'm guessing the lawyers are writing disclaimers as we speak.

What was the target range on batteries only - 40 miles?

Yes, on the EPA urban cycle, which is infamous for being unrepresentative anyway. It is at least better than the European cycle.

Greg, thanks for the info. Is there any one source where more about the Volt's technology is revealed?

On the subject of range EV's are, as I understand it, very dependent on how much you floor the throttle and the range can go quite low,however I am not sure if this is a big problem in practical terms in the USA market. The reason I say that is because many Americans drive to a workplace with dedicated corporate parking , as well as having private off street parking at home. If employers are willing to wire the parking lot then the Volt , or any EV, can be given a 6 - 8 hour midday charge. Obvioulsy that requires some employer investment and employe billing and it is not off peak power, but it probably means the 40 miles can become 50 -60 miles wihout too much trouble ( a bit like the Chinses taking their ebike batteries into work for charging). Where there is no dedicated parking, at home or at work ( e.g much of Europe) then battery life may be a bigger problem.

Any closed circuit test regime you can devise will be significantly non-representative of a given real-world driving loop, just as any real-world driving loop will be non-representative of all other real-world driving loops. Your mileage may vary, as they say. "Improvements" to the cycle often reflect individual biases, hobby horses, etc. All test regimes are arbitrary, including ordinary use. First, define ordinary.

The Volt's range rating will not be established on the EPA 75 city cycle (not long enough for one thing) though it will be the basis for the EPA mileage rating, obviously. The shortfall in CD mode range Lutz described is mainly due to cold weather ambient temperature. If the Li-ion battery pack delivers 40 miles at 60 F, it can't deliver 40 miles at 30 F. Chemistry.

The Volt's control strategy is designed to maintain the battery pack at 25 to 85 percent SOC for max battery life/efficiency. At the outskirts of CD (charge depletion) mode when SOC falls to +/-25 percent, the gas engine kicks in and much of the generator output is apportioned to the generator, the rest to the battery, to maintain SOC at 30 to 35 percent. Running the SOC back up to 85 percent with the gas engine would defeat the Volt's purpose -- electrons at the wall socket being many times cheaper, in dollars and in carbon, than electrons from the gas pump, though I expect to see more user control as time goes on. They need to establish how much user flexibility is wanted or practical. The production car will have a "power" button, I understand, which increases performance in the BEV/CD mode at the expense of range.

GM's development program for the Volt is fairly accessible to the public as these things go. There are blogs, facebook pages, semi-official forums, etc. Chief engineer Andrew Farah tweets from time to time. Shouldn't be too difficult to scare up all the info via Google.

Rambling off-topic, had an interesting conversation with Wayne Cherry, retired GM vice president, not long ago. He has been working on a program with MIT for inductive charging at traffic lights and parking lots -- system embedded in pavement. Predates and not related to the Volt.

The engine is a GM family 0, it was designed by Opel and has been around since the mid nineties in various versions.
http://en.wikipedia.org/wiki/GM_Family_0_engine

In general it's more efficient to power the wheels directly rather than to go the extra round over the batteries, which give additional losses. So, the best way to charge the batteries on a hybrid when the engine is running is using regenerative braking. Of course, this isn't always enough to keep the state of charge of the battery, so additional power might be required from the engine driven generator.

The shorter range due to cold weather has little to do with the battery capacity, and more with accessory loads such as cabin heating and defrosters. The battery itself is insulated and heated in cold weather in order to avoid capacity loss.

The battery efficiency problem at low temperatures only apply when it is cold. As it is used or charged it warms up, and so the 'lost' charge reappears - the so-called fairy volt effect .

The EPA urban cycle is not representative of modern driving speeds. When you add in the use of all the ancillaries that the EPA down't use in its test I think a 25% shortfall is what you'd expect.

I guess we should ask why there is a battery pack heater "to avoid capacity loss" if temperature does not affect discharge capacity. And what the heater runs on.

Like I said: If the discharge capacity is good for 40 miles at 60F, it won't be 40 miles at 30F. It could be made something close but it won't be 40 miles. Temperature has a significant effect on battery performance. For example, if the vehicle is left unplugged overnight and the battery pack temperature falls to 32F or lower, the control system is programmed to start and run the vehicle on the gas engine until the battery pack is up to temp.


“The range can vary on any given day depending on temperature, terrain, driving conditions and so forth -- especially temperature... Many people don’t understand that. The distance you can go in an electric vehicle varies hugely with the outside temperature, including with the Volt...If on a standard day you see 40 miles with the Volt, on a cold day where it’s right around 32 degrees, you’re going to see 28 or 30 miles.” -GM Vice Chairman Bob Lutz, link to full story below.

http://www.annarbor.com/business-review/gm...+News+Business)

The furphy there is that you won't see 40 on a standard day, if it is only good for 40 on the EPA cycle, unless you drive in a very peculiar fashion. I tried to force a Fusion Hybrid to drive in EV mode by being lightfooted rather than using the button (I don't know where the button is) and failed dismally.

Couldn't do that it in the UK, cars are not permitted to have the engine running without the key in the ignition.

... you are required to leave the key in the ignition.


...JUST KIDDING. LOL I never meant to imply that the vehicle starts itself up in the middle of night. Does that seem sensible to you? I mean that when you depart in the morning the vehicle is programmed to start and run on the engine, rather than on the battery as it normally would, until the battery pack is up to temp.

Ahhhh, got you.

In some countries (especially in cold and hot climates) the rules are different where remote starting of cars is used to either heat up or cool down the cabin to more comfortable temperature before you get in it, but I know that would be illegal in the UK. I brought a car back from Japan and VOSA objected to my "Turbo Timer" which allowed the engine to keep running after the key was removed from the ignition.

Don't believe you. Many Toyotas use a remote key, there is no ignition key or sockert in the car. Prius, Avalon, for a start.

You've got me thinking. Before I could register my car for use on UK roads it had to be tested at a VOSA (Vehicle and Operator Services Agency) to see if it would pass a SVA (Single Vehicle Approval) test. It failed on several things first time around, including things like the standard of the wiring to the Sat Nav units presented a "chafe hazard"! They also failed it for the Turbo Timer and yes, I repeat it was because it was illegal for the engine to be running without a key in the ignition. What I didn't say was the reason they gave was there should be no possibility of the steering lock engaging when the engine was running.

I've never driven a Prius or an Avalon (don't think Toyota sell them in the UK anyway) but could it be that they don't have any built-in steering locks?

I don't have access to either today. I bet the steering column unlocks when the car unlocks which is when you get close enough to the car.