Diesel engines are more fuel-efficient than petrol engines.
There; I've finally said something nobody will argue with.
But diesel, as a fuel, leaves much to be desired; it requires humungous compression to start the combustion process, which means the engines that burn it have to be that much sturdier (read heavy and expensive to build).
They're noisy and smelly, although I must admit I enjoy driving behind a mate of mine's Mercedes, which he runs exclusively on used frying oil from a local chicken takeaway. They like it too, 'cause the smell always makes me hungry.
Both vegetable and fossil-based diesel fuels produce some nasty by-products - including solid particles - and their calorific value is low by spark-ignition standards so, unless you turbo the hell out of them (more weight, more expense) they just can't cut it in the real world of passenger cars.
Spark-ignition fuels, especially vegetable-based fuels such as methanol and ethanol, burn way cleaner than diesel fuels, produce more power for a given engine size, while causing less internal stress to said engine, so they can be made lighter and cheaper.
But, and this is a big but, you need a lot more of them.
Their specific fuel consumption is higher, unacceptably so by today's eco-conscious standards.
The trouble is that you can't just up the compression ratio of a petrol engine until the heat caused by squashing the fuel/air mixture between the piston and the cylinder head initiates combustion - what you get is detonation, which is as destructive to engines as it is to tall buildings.
Or do you? Something has changed in the last few years. Up till now spark-ignition engines have been fed with a 14:1 mixture of air and atomised fuel particles, but with the advent of high-pressure piezo-electric fuel-injectors, it's now possible to spray petrol (or methanol or ethanol) directly into the combustion chamber in a sequence of carefully-regulated micro-squirts, starting with a tiny one, which, if it detonates, won't do any damage.
Then, when the rest of the fuel is sprayed into the cylinder, there is already a flame front in place and you shouldn't get any detonation.
It's a grand theory, but it's not a theory any more.
Harry Husted, the chief engineer at US technology company Delphi, has built a single-cylinder four-stroke compression-ignition engine that runs on petrol, using very sophisticated valve technology and multi-squirt fuel-injection timing.
And he's working on a multi-cylinder engine using the same hardware - which is not as simple as it sounds. But, he says, computer simulations show that the potential is astonishing.
He told Auto Express: “The results of initial simulation work show this technology could improve engine efficiency by up to 50 per cent.”
To put that into perspective, an 1800cc, four-cylinder compression-ignition petrol engine would make more power than a Toyota Prius, using less fuel and without the extra weight (and cost) of electric motors and batteries.
According to Husted: “A multi-cylinder engine is being developed and will be ready to test later this year. It's going to be a very torquey engine, with a similar feel to a diesel.”
Now that doesn't surprise anybody; it will have all the features we like about diesels, while running on clean-burning volatile fuels rather than oil-based diesel fuels.
Which means it won't need a particulate filter either, which should endear it to the car companies.
Husted reckons the new engine could be in production and in showrooms by 2020.