Hybrids
"Volvo argued that closed-cycle hybrids, such as Toyota's Prius, are a technological dead-end because the same economy benefits can be more easily obtained using high-efficiency diesel engines."
That is what all the manufacturers say, who can make diesels but can't make hybrids. Don't forget that you have to get at least 10% better economy in a diesel before the efficiency (or, here in the UK, the cost) become equivalent. And the Toyota Prius is STILL the cleanest car on sale in Europe if all emissions are taken into account.