At the Frankfurt Motor Show, Lotus Engineering unveiled a small range-extender engine for series-type hybrid applications.
Incorporating some advanced design and manufacturing techniques, the three-cylinder, 1.2-L unit produces 35 kW (47 hp) at 3500 rpm via an integrated electrical generator. Weight has been kept low at 56 kg (123 lb).
Lotus Engineering Technical Director, Simon Wood, said: “Designing the Lotus Range Extender purely for use in series hybrids has allowed us to develop an optimized engine that has high thermal efficiency, low fuel consumption, multi-fuel capability and low-cost architecture.”
He added that the engine is designed specifically to fit the operating range demanded by a series-type hybrid, whereas most series hybrid vehicles currently being developed would use adaptations of existing, conventional engines. The operating efficiency of such ICEs is usually compromised because they have been designed for a wide range of operating conditions.
In a series hybrid the combustion engine is connected to an electricity generator to provide an efficient energy source to power the vehicle’s electric motor or charge its battery – which can also power the electric motor.
With strict weight targets to achieve, Lotus opted for an aluminum monoblock architecture, integrating the cylinder block, cylinder head and exhaust manifolds in a single casting. Advantages include less engine mass, and lower assembly costs with reduced parts count. The range extender has been designed to give improved packaging, emissions and durability.
The engine is optimized between two power generation points:1500 rpm, when it produces 15 kW (20 hp), and 3500 rpm which gives 35 kW (47 hp) via the integrated electrical generator.
A combustion system featuring two valves per cylinder and port fuel injection is used. The engine is designed and calibrated to handle gasoline or alcohol-based fuels.
The need to overcome the potentially high cost of series hybrid technology is essential for market success. Lotus believes that the efficiency of its new range extender also will offer benefits in terms of downsized (and less expensive) batteries. Lotus Engineering designed the engine using production methodologies with parts procured from low-volume potential suppliers.
The decision to use monoblock construction not only avoids the need for a cylinder head gasket but eliminates some seventeen components. The water jacket is also said to be better optimised.
Tse of an integrated exhaust manifold also reduces parts count by about eighteen, with associated inventory, production logistics and aftermarket costs. Obviating the need for a separate exhaust manifold also saves weight.
Further plusses are a reduction in emissions via a faster light-off of the engine’s close-coupled catalytic converter, with a reduction in heat loss between exhaust port and catalyst inlet, claims Lotus Engineering. The development engineers also focused on reduced NVH.
The Lotus range extender forms part of the U.K.’s “Limo Green” project funded by the Technology Strategy Board, a collaboration which also involves Jaguar, MIRA, and Caparo Technologies aimed at demonstrating a large, lightweight, premium sedan, producing CO2 emissions of less than 120 g/km.