In combination, DI and CR make diesels quicker, cleaner, quieter and even more economical. Their secret, simply put, is to squirt a mist of fuel directly into each cylinder under electronic control from a common gallery or rail at very high pressure - up to 20,000 pounds per square inch. Imagine the weight of a small car on your little fingernail, and you have some idea of the force involved.
Dubbed HDI (for high-pressure direct injection), PSA's new turbodiesel now powers the Peugeot 306 (the best-selling diesel in Britain) and its in-house rival, the Citroen Xsara. As the owner of an older 306 D Turbo, once the yardstick by which rivals were judged, I can vouch for the superiority of HDI.
My car's outmoded engine averages 48mpg, so the 27 per cent improvement claimed for the HDI should give 60mpg. Realistic? Probably not. Peugeot quotes 54.3mpg for the mixed cycle. Even allowing for the misguided tax premium levied on derv, fuel savings of 40 per cent can be expected over petrol cars of comparable performance - meaning comparable torque, not power.
The 90bhp, 2.0-litre HDI (the 110bhp inter-cooled version is restricted to PSA's up-range models, more's the pity) is no more powerful than the 1.9. But it is decisively more flexible, and pulls hard from low revs where the 1.9 languished. While performance is up, emissions are down - black smoke is virtually eliminated. The Xsara 2.0 HDI emits less "greenhouse" carbon dioxide than the petrol Citroen Saxo Spree which, because of its sub-1.1-litre engine, costs pounds 55 less per year to tax. So much for "green" concessions.
So which is best, Peugeot's 306 D Turbo or Citroen's Xsara HDI? There's less difference in price, performance, economy and emissions, than in character.
With its stiffened suspension, all-disc brakes and well-shod alloy wheels, the three-door D Turbo issporty, steers precisely and corners with unusual tenacity. By a narrow margin, it's the better driver's car.
However, the HDI engine sounds and feels smoother in the impressively refined Xsara. The five-door Citroen also has the better cabin and a more comfortable driving position. I could happily live with either.
Models tested: Peugeot 306 D Turbo HDI; Citroen Xsara SX HDI
Prices: Peugeot pounds 14,595, Citroen pounds 14,650
Engine (both cars): 1997cc, four cylinders, eight valves, 90bhp at 4000rpm, 155lb ft of torque at 1900rpm
Transmission: front-wheel drive, five-speed manual.
Performance: max speed 112mph (both cars); 0-60mph 12.3 sec (Peugeot), 11.1sec (Citroen); Fuel consumption 54.3mpg mixed cycle (both cars)
Ford Focus TDdi Ghia 5-door, pounds 14,850. Cracking car. Engine not as refined as some, but very economical. Keenly priced and well equipped.
Renault Megane RSE 1.9dTi, pounds 15,400. Nippy, frugal and smart-looking car - hatchback, saloon and people carrier. Coupe not available with diesel.
VW Golf 1.9TDi 110, pounds 16,975. Pricey but good - the nearest thing to a diesel GTi. Impressive figures - 120mph and 57.6mpg. Lesser 90bhp diesel slower and thirstier. Not as refined at Citroen HDI.