On the road in Jaguar’s Limo Green

3 November 2010

Jaguar XJ Limo Green

“It’s a myth that electric cars don’t need gears,” says Steve Nicholls, the man in charge of the Limo Green project at Jaguar. It’s no surprise to learn, therefore, that there are two ratios available in the very stylish Limo Green mule I’m sitting in – a range-extended electric car built around the rakish aluminium body of a Jaguar XJ saloon.

Like the better known Chevrolet Volt, Limo Green is an electric car with an engine on-board to generate electricity, so that the car can keep going beyond the 30-mile range provided by batteries alone.

Unlike the Volt, Limo Green really is a purely electrically driven car. The engine only ever supplies electricity, and the generator never lends a hand as a drive motor. There’s no indirect, planetary-gear connection between engine and wheels to argue about. “We sized our motor to drive the car on its own,” Nicholls says simply.

The second gear he mentioned – it’s technically an overdrive – is under the car’s computerised control rather than selectable from the driver’s seat. Modulating the motor’s torque so that upshifts are imperceptible has proven simple, according to Nicholls, but achieving seamless downshifts has been much trickier.

They’re not allowing me to drive this precious rolling laboratory myself and I can’t say I blame them. So from my cream-leather passenger seat – an ultra-lightweight chair built by Limo Green consortium partner Caparo – I try to spot the baulky downchange.

I have to report that I can’t detect shifts of any sort. Partly that’s because the electric components of the drivetrain are silk smooth and nearly silent, producing only a faint whine under acceleration. And partly it’s because the range-extender engine – a purpose-built Lotus 1.2-litre triple weighing just 89kg wet – is neither silent nor smooth once it has spun into life and started churning out fresh electricity. It creates a noticeable amount of noise and vibration in what is otherwise a pin-drop quiet car.

The smooth power delivery may also be because my chauffeur – a chap from Mira, another of the consortium’s members – is driving like Miss Daisy is in the back.

The man from Mira does helpfully point out that the engine is intrusive only because refinement is not yet part of the development brief. The green part of running this big car is the focus, the limo part is the easy bit.

CAD drawings of standard Jaguar XJ and Limo GreenUnder the bonnet, Jaguar’s normal engine has been turfed out in favour of the compact Lotus unit, which looks really very tiny nestling at the back of the vast engine bay. Not that there’s much space left going spare, as there are plenty of new bits and pieces to accommodate. Ahead of the engine is the cylindrical generator, flanked by two large boxes containing power electronics. These are required to modulate and convert the power flowing between the generator, the drive motor and the rear-mounted battery pack. The drive motor and the wee little gearbox sit behind the engine, in the transmission tunnel, with a conventional propshaft and differential delivering drive to the rear wheels.

The rest of the engine bay is a rat’s nest of coolant tubes and thick power cables.

The project’s goal is to manage with an engine that is as light, simple, and reliable as possible while delivering enough power to meet the overall needs of a car in the large executive class. Crucially, the small engine with its weedy output doesn’t cripple this limo’s performance. The battery plus generator can deliver very high power to the electric motor for short bursts of acceleration, and the battery can then be recharged by the engine later, during quieter periods. Zero to 62mph takes just 7.9 seconds. Not bad for a 1.7-tonne full-size saloon.

However, while the range-extender engine doesn’t cap performance it does limit the maximum cruising speed.

Lotus range-extender engine

In the current development car, the engine can deliver 35kW (47bhp), the battery can hold 8.7kWh, and the motor can deliver 145kW (195bhp). In other words, ignoring losses, the drive motor is capable of emptying the battery about four times as quickly as the engine-driven generator can top it up again.

It doesn’t take a genius to realise that the car will not be able to provide full performance all of the time. For the current set up, the tipping point comes at roughly 80mph on the flat, at which speed it takes about 35kW simply to overcome friction and air resistance. At this velocity, the engine’s generator can just about supply the motor’s demands without depleting the battery. Go faster and the engine can no longer keep up, and the motor must run partly on battery reserves, assuming there are any. Go slower and the generator has the upper hand and can send surplus power back to the battery.

All of which explains why the XJ can cruise at 80mph as long as there is fuel in the tank, but can only sustain its 112mph top speed for a maximum of three minutes. When those three minutes are up, the battery is flat and it’s back down to eighty whether you like it or not.

As Nicholls concedes, Limo Green won’t win any prizes as an autobahn blaster. He adds, though, that he doesn’t see unrestricted autobahns remaining in the picture for much longer, given prevailing political tendencies.

Jaguar Limo Green engine bay

Drag increases with the square of speed, so to sustain a slightly higher speed requires a much bigger engine. A 50kW engine, 40 per cent more powerful than the current unit, would lift the maximum cruise to 140km/h (87mph), or about 9 per cent quicker. But the more powerful engine would have to be bigger and heavier, which would blunt acceleration. Increases in motor size and battery capacity to keep acceleration brisk would quickly spiral into a vicious circle, rubbing out the “green” part of the equation.

On that topic, the design goal was sub-120g/km CO2 output. Under current test regimes, the car scores about 50g/km, neglecting the CO2 emitted charging the batteries from the mains. A truly representative emissions test for cars that both plug in and burn fossil fuels has yet to be established.

Impressive as the Limo Green demonstrator is, the project itself has now drawn to a close. The work carries on though, with a new consortium and a new project, called REEVolution. It will see Jaguar Land Rover working with electric car pioneers Nissan and Think on taking Limo Green to the next level, helped by £9.5 million in government funding.

Never mind the corny title, it seems REEVolution will be a project worth watching closely.

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