(Mon 07 Sep 09)

Lotus Engineering has developed a new engine especially for hybrid cars, and will be displaying it at the Frankfurt Show next week.

The Range Extender, as it's called, is a 1.2-litre petrol (or ethanol or methanol) unit which develops a maximum of 47bhp. That's only about two-thirds of what a conventional engine of that size would normally be expected to produce, but there's a good reason for that. The Range Extender does not rev beyond 3500rpm - which is about as far as most drivers will go in everyday driving - and other 1.2s have to go a long way past that to achieve the normal 70bhp or so.

It's also important to point out that 47bhp is what the engine can achieve on its own. The total output would be considerably higher, enhanced by the power of an electric motor. Power-to-weight ratio is also crucial here, and this has been a key consideration in the design of the Range Extender. At just 56kg it is remarkably light, and Lotus has achieved this figure by making the engine block, cylinder head and exhaust manifold out of a single aluminium casting.

Apart from the weight advantage, this method of construction also reduces the size of the unit and its cost of assembly, while improving durability and emissions. Servicing it may be another matter, though since more than 30 parts required in a conventional engine are no longer required in this one there will at least be less to service.

"Most series hybrid vehicles that are currently being developed will use adaptations of existing, conventional engines which are therefore compromised in the efficiency that they can achieve, designed as they are for a wide range of operating conditions," says Simon Wood, Technical Director of Lotus Engineering. "Designing the Lotus Range Extender purely for use in series hybrids has allowed us instead to develop an optimised engine that has high thermal efficiency, low fuel consumption, multi-fuel capability and a 47bhp peak output from a 1.2-litre, low-cost architecture over the precise operating range required by a series hybrid drivetrain."

Lotus intends to make this light, cheap, hybrid-specific engine available to any manufacturer which wants to buy it, but the first name on the list is Jaguar. The Range Extender has been created as part of a project known as "Limo-Green", a collaboration involving Lotus, Jaguar, MIRA and Caparo Vehicle Technologies and funded by the UK Technology Strategy Board.

Through this project, the first application of the Range Extender will be in, of all things, a super-lightweight, low-drag Jaguar XJ saloon. A prototype vehicle is expected to be up and running by the end of this year, and however ludicrous the idea of a 1.2-litre XJ may seem, preliminary figures from Lotus suggest that it will have a 0-62mph time of 7.9 seconds, which is only 1.5 seconds less than that of a current XJ 3.0 diesel.

Lotus believes that the car will have a relatively low top speed of 112mph, since no matter how light or sleek the car is there comes a point when such a small engine will be devoting all its energy to overcoming the aerodynamic drag of an XJ. On the positive side, the car is expected to have a CO2 rating of under 120g/km, which in turn suggests combined fuel economy of well over 50mpg (compared with 40.1mpg for the current diesel) and a range of more than 600 miles.

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