Full credit to GM for being brave enough to allow me to try an engine that's as far as seven years into the future - although it sounded like a diesel from 27 years in the past. 

That's because the highly advanced new power unit is still in the early stages of development, and it had been installed in a current-generation Vectra to give us a glimpse of the motor GM hopes will radicalise internal combustion.

The powerplant in question goes by the distinctly unsexy soubriquet of Homogeneous Charge Compression Ignition, or HCCI for short, a technology that promises the possibility of combining the emissions cleanliness of petrol with the economy of diesel.

You can read about how it works by clicking here, but the gist is that by combining both spark and compression ignition it should combine fuel savings with cleaner running.

There's still some way to go. As installed in this Vectra, it produced the sort of knock last head by drivers of the Leyland Sherpa van. It also had a low rev hesitancy that made it easy to stall, and a stutter as the motor transitioned between compression and spark ignition. 

The technology will be improved and brought to market, though: Mercedes and Volkswagen are also actively pursuing it. And proof of GM's seriousness lies in the cylinder head of the new corporate 1.4-litre turbocharged petrol motor, which has been engineered to be compatible with HCCI ignition. I'm looking forward to experiencing the finished engine.