The paper discusses the concept, specification and performance of a new, dedicated range extender engine for plug-in series hybrid vehicles conceived and designed by Lotus Engineering. This has been undertaken as part of a consortium project called Limo Green, part-funded by the UK government. The Lotus Range Extender engine has been conceived from the outset specifically as an engine for a plug-in series hybrid vehicle, therefore being free of some of the constraints placed on engines which have to mate to conventional, stepped mechanical transmissions. The paper starts by defining the philosophical difference between an engine for range extension and an engine for a full series hybrid vehicle, a distinction which is important with regard to how much power each type must produce. As part of this, the advantages of the sparkignition engine over the diesel are outlined. The rationale leading to the adoption of an in-line 3-cylinder configuration and specification is then discussed, followed by a description of the principal design characteristics of the engine; it also points out any significant differences from what might be expected were a 'conventional' engine to be designed. The use of 'monoblock' construction which integrates the cylinder block with the cylinder head and also with a water-cooled exhaust manifold is described. An indication of the likely parts reduction possible with this approach is made, together with a brief discussion of the pros and cons of the approach and which other engine types this architecture might suit. Engine performance on 95 RON unleaded gasoline is detailed, together with observations on why the engine has been additionally protected to operate on ethanol and methanol. The paper also includes a brief discussion of other engine concepts which might suit the purpose of range extension, but which (compared to the more conventional reciprocating spark-ignition engine embodied by this approach) will necessarily require greater development even if they may offer the potential for further improved efficiency, package volume and/or NVH characteristics. Copyright © 2010 Lotus Cars Ltd.