Lotus Engineering will break cover on a new series-hybrid powertrain at next week’s Frankfurt auto show it says gives auto makers a lower-cost alternative for hybrid vehicles.

The secret to its money-saving design is its low mass of just 123 lbs. (56 kg), which Lotus says enables its “Range Extender” powertrain to employ a smaller battery pack.

At the heart of the system is a 1.2L 3-cyl. engine that features aluminum monoblock construction, which integrates the cylinder block, cylinder head and exhaust manifold into a single casting. The result is lighter, smaller package that is easier, and less costly, to assemble, Lotus says.

The monoblock construction allows Lotus to eliminate the cylinder head gasket and some 16 other parts. Integrating the manifold trimmed another 18 parts from the engine.

The engine is used as a generator to power an electric motor directly or charge the vehicle’s battery, which also can drive the electric motor. The concept is similar to the one General Motors Co. will use for its upcoming Chevrolet Volt extended-range electric vehicle.

The 2-valve, single-overhead-cam port-injected engine can run on gasoline or alcohol-based fuels. The engine has been optimized between two power-generation points, Lotus says, producing 20 hp of electrical power at 1,500 rpm and 47 hp at 3,500 rpm via the integrated electrical generator.

Lotus says the Range Extender has been designed with production methodologies and uses parts from low-volume suppliers, so an auto maker potentially could bring the concept to market fairly quickly.

“Most series hybrid vehicles that are currently being developed will use adaptations of existing, conventional engines, which are therefore compromised in the efficiency they can achieve, designed as they are for a wide range of operating conditions,” says Simon Wood, technical director.

“Designing the Lotus Range Extender purely for use in series hybrids has allowed us instead to develop an optimized engine…(with) the precise operating range required by a series hybrid drivetrain.”

The engine was developed as part of the “Limo-Green” project funded by the U.K.’s Technology Strategy Board to produce a large sedan with an emissions rating of less than 120 g/km of carbon dioxide. Working with Lotus on the project was Jaguar Cars Ltd., MIRA Ltd. and Caparo Vehicle Technologies.