So, how exactly does...

So, how exactly does the new powerplant work? The fact that GM engineers have been working for the past 10 years to perfect the Homogenous Charge Compression Ignition technology suggests there is no easy answer.

At its heart is the idea that a petrol engine can be made to work like a diesel. So rather than use a spark plug to burn fuel – as in a traditional motor – it compresses petrol until it becomes hot enough to burn unaided, and therefore more efficiently. HCCI units also direct some exhaust gas back into the cylinder to heat the incoming petrol-air mix, so it ignites as it is compressed (and further heated) by the piston.

Described in that way, the concept sounds simple – yet managing it is anything but.

Up to around half throttle and 3,000rpm – typical revs in traffic – the engine works in HCCI mode more economically. Above that, when the driver presses hard on the throttle, the engine automatically goes back into conventional spark ignition, to provide full power.

Currently, the system is 15 per cent more efficient – so a 2.2-litre unit could deliver well over 40mpg combined, in addition to lower emissions.

But challenges remain. We were told GM isn’t yet satisfied with the system’s refinement – engineers want to smooth out the transition from HCCI to normal spark ignition modes.