Holden Volt: the technical guff

Holden insists that its American-sourced Volt — still about 12 months away from a local launch — is not a hybrid vehicle as we've come to understand the term. If 'Hybrid' means Prius or Insight to the average person in the street, then the Volt certainly isn't such a vehicle. The petrol engine on board doesn't provide direct motive power to the drive wheels as is the case with the Toyota and the Honda.

But according to Wikipedia, the Volt is actually a plug-in series hybrid, whereas the Prius is both a parallel hybrid and a series hybrid in the one package. The 'Plug-in' phrase refers to the vehicle's ability to recharge its battery pack from a domestic power supply. 'Series' refers to the provision of a petrol-fired range-extending internal-combustion engine that operates infrequently, to recharge the battery pack during a journey that exceeds the range available from one full charge of the battery.

Leaving aside formal definitions, the Volt will certainly run for extended periods on electric power alone. In fact, Holden has made a lot of mileage from American variety show host Jay Leno's claim to have driven his Volt as far as 16,000km without resorting to petrol power to keep the batteries charged. On that basis, the Volt is very much an electric vehicle, although Leno's claim makes no allowance for the regular occasions the petrol engine will start of its own accord to ensure lubricants get a stir and petrol is consumed before it turns stale.

According to Holden's Director of Electrical Engineering, Paul Gibson, the Volt runs on premium unleaded, 36 litres of which are required to fill the car's tank to capacity. And obviously, if you're a Jay Leno type who would prefer to keep the battery charged from a domestic wall socket, the Volt shouldn't need to use the fuel at any stage to recharge the batteries. It's nice to know that it's available just in case though, reducing what Holden MD Mike Devereux described as "range anxiety" during his presentation to the Australian press in December.

One problem with keeping the Volt fully charged for each day's commuting and shopping sorties from AC/mains power is that gums and solids begin to accumulate within the unused petrol, leading to the possibility of harder starting if or when the engine is required. Gibson explained to the media that the Volt comes with a pressurised petrol tank to extend the life of the petrol, but the car will eventually burn it off, to be replaced by fresh petrol.

"[The Volt has] a pressurised gas tank, because we can't let anything get in and out of it, because the fuel will go stale very quickly. But after a year... [the engine] starts to burn the fuel off," explained Gibson.

Another compromise in an electric vehicle designed to operate on occasions with electrical energy supplied from a petrol engine is the need for the petrol engine to crank over once in a while to ensure moving parts remain lubricated.

"If you've got a petrol engine in the car, it needs to lubricate occasionally," said Gibson. "So what it does is, every 42 days, it will actually turn itself on, lubricate itself and turn itself off."

The petrol engine is not mechanically linked to the road wheels, except technically, through an automatic clutch that allows torque from the (running) petrol engine to drive an electric generator when the battery charge is depleted. The generator can also operate as a traction motor when extra motive power is required, but in that circumstance the petrol engine won't be running, since the generator can't produce electricity while it's also supplying drive to the wheels. Motive power is normally provided by a separate electric motor Holden refers to as the traction motor. The traction motor drives the sun gear in a planetary gear set, to the final drive.

By the nature of the planetary gear set, the traction motor in the Volt effectively operates through a continuously-variable transmission, reducing the motor's revs at higher road speeds, conserving battery charge and keeping the motor working in the rev range where it produces more torque. On that point, the traction motor develops 370Nm of torque between 2500 and 2800rpm, or 320Nm below 2500rpm. That's diesel-like, but without the NVH issues.

The way the Volt relies on the planetary gear set is markedly different from the same transmission technology used in the Prius. With three separate clutches the Volt seems technically more complex than the Toyota, but, as mentioned, the petrol engine never provides direct mechanical transfer of power to the road wheels.

Controlled by computer, the planetary gear set's ring gear is locked by one of the three clutches (C1) for low-speed operation. This ensures adequate acceleration around town from the 111kW/370Nm traction motor.

At higher road speeds, the ring gear is unlocked and a second clutch (C2) between the planetary gear set and the generator links both for added motive power — the generator becoming a traction motor as the driver demands more performance from the car. If the driver backs off momentarily, the generator returns to recovering kinetic energy through the planetary gear set for conversion to electric energy for storage in the lithium-ion battery. According to Gibson, the immediate change in energy flow is undetectable by the driver.

The third clutch (C3) is located between the petrol engine and generator — which produces 55kW of electrical power — linking the two for those times when the petrol engine does need to operate and recharge the battery, thus extending the Volt's range.

In GM nomenclature, the setup is the 'Voltec electric drive system'. An integral part of the system is the 1.4-litre inline four-cylinder petrol engine, which develops 63kW of power and runs out of revs at just 4800rpm. It's an engine calibrated specifically as a generator rather than as a motive power unit.

Gibson admits that the 1.4-litre engine was not the optimal choice for such an engine, but it was the nearest and best choice for the eco-friendly application in the hurry to develop the Volt and roll it out as an example of American ingenuity when GM was on its knees, seeking financial support from Washington in the depths of the GFC.

"The easiest thing is take a good engine we had, which is a 1.4, which we actually use in the Cruze, with a turbo here, and design that engine to suit its purpose. If I was in engineering and we had to do it all again, you'd be looking at other alternative engines out there, like gas turbines or a small diesel engine — or maybe an LPG-fired engine. That's a possibility moving forward, but at the moment it was to get the car out there and get the technology [showcased].

There are four operating modes for the Voltec system: Normal, Sport, Mountain and Low.

"This is what makes it different from other EVs out there," Gibson explained. "It's got different driving modes..."

In fact most of the Japanese-developed EVs feature different driving modes, but the Volt's setup offers an extra mode and the characteristics are different.

Normal operating mode requires no explanation; Sport mode doesn't provide more performance, just a faster response to driver input. The Low mode provides more regenerative braking — up to 0.3g of deceleration. Mountain mode is only used in North America, but is designed to reserve 35 per cent of the battery charge by running the petrol engine constantly. On certain long, steep grades above an altitude of 600 feet, the battery alone cannot provide enough electrical power for the Volt to complete the ascent.

Gibson says that there is no call for it in Australia, since we have no roads as demanding of the battery's electric charge. There's potential for the Mountain mode to be used in a different scenario, according to Gibson. In the UK, for example, a Volt driver could commute from beyond the outskirts of London in Mountain mode, with the petrol engine maintaining the battery reserve of 35 per cent charge. As the vehicle enters the city, the driver switches over to electric-only normal mode and the petrol engine shuts off, avoiding the impost of the city's congestion tax. In the event that Australian cities introduce such a tax — and Sydney and Melbourne are understood to be contemplating such a move — the same benefit would apply.

These are modes that can be selected by the driver — and not to be confused with the vehicle's automatic operating modes, which can change to suit the respective driving environments without intervention from the driver. These are, respectively, low-speed single-motor operation, high-speed two-motor operation, extended-range driving (low speed) and extended-range driving (high speed).

Key to the Voltec system is the modular battery pack, which is mounted in the transmission tunnel and between the rear-wheels, explaining its T shape.

"This is where GM has invested most of its money," says Gibson. "This battery is designed in-house. All the technology out there in the field — GM couldn't find anybody who could do what they wanted to do. So they took this in-house. We have one of the largest battery labs in the world... located in Warren in Detroit.

"It's a 16kW lithium-ion battery; there are 288 lithium cells in it and basically, it's a dry-cell battery, so it's got an anode and a cathode — and in between [those] it has a silicon plate [that] actually then enables it to transfer the ions from the anode to the cathode and back again, depending on how you're charging it."

The battery comes with an eight-year warranty and GM is confident it will prove durable and reliable in the field, for one particular reason — it can stay cool under pressure.

"Between each battery cell," explains Gibson, "there's a thin plate that actually has cooling channels through it. One of the problems out there with batteries is that as you charge it, you get hot spots through the batteries — that actually kills batteries. What GM has is these cooling plates all the way through the batteries and they have sensors at every single cell, so we can detect how hot a battery cell is getting and then we can adjust the temperature for that to make sure we maintain a clear charge all the way through."

Gibson says that GM is already working on second and third generations of battery technology and efficiency is expected to improve, while cost will inevitably come down. And a final advantage to the battery pack is its modular construction, which makes it expandable for larger-vehicle applications down the track.

So the Volt is likely the first of a number of extended-range vehicles to hit the roads — and we may even see some of them here. Just don't call them hybrids in front of Holden staff...

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