Conceived in Scotland almost 200 years ago, the Stirling [engine] is a marvel of thermo-dynamics that could help to replace the internal combustion engine – in theory it can turn any source of heat into electricity, in silence and with 100 per cent efficiency. But corporations including Phillips, Ford and Nasa have devoted decades of research, and millions of dollars, to developing the engine, and all retired defeated, having failed to find a way of turning the theoretical principles of the engine into a workable everyday application. Kamen, nevertheless, has spent the past 10 years and, he estimates, up to $40 million working on the problem.

Now he and his engineers have built and tested a range of Stirling engines suitable for mass production that can be run on anything from jet fuel to cow dung. The one in the boot of the small blue car is designed to extend its range and constantly recharge its batteries to make a new kind of hybrid vehicle: one fit for the roads of the 21st century. A Stirling-electric hybrid, Kamen tells me, can travel farther and more efficiently than conventional electric cars; it generates enough power to run energy-hungry devices such as heaters and defrosters that are essential for drivers who, unlike those he calls the ‘tofu heads’ of California, must cope with a cold climate; and even using petrol, the engine runs far cleaner than petrol-electric hybrids such as Toyota’s Prius.

However, Kamen confesses, his new creation isn’t quite finished yet: ‘The Stirling engine’s not hooked up. Which really pisses me off.’

But it could work?

‘It will work,’ he says. ‘Trust me.’