Volkswagen Passat 1.9 TDI PD 115

IT'S NOT so much the car but the engine that is on test this week, as it points to a new chapter in diesel technology - and one that promises to lower resistance to diesel from drivers and politicians alike. Volkswagen's range of direct injection diesel engines needs little introduction as the benchmark by which diesel is judged: no fewer than half of Britain's top 20 cleanest cars are Volkswagen TDIs.

The 110bhp 1.9-litre unit in particular is notable in being one of the most thermally efficient in production and is also a class-leader on CO2 emissions with a rating of just 143g/km. It puts the Passat TDI 110 among the cleanest full-size upper-medium cars on the market, which will favour it when the new emissions-based company car tax laws come into force in 2002.

Direct injection is Volkswagen's view of the future of diesel rather than the common rail technology favoured by some other makers. But now, VW has stepped up the technology again with the introduction of a new version of the same engine producing an extra 5bhp and a massive rise in torque - or pulling power - from 173lb-ft to 210lb-ft at the same 1,900rpm, a 20% increase. To put it into perspective, that's more than the top-of-the-range petrol Passat 2.8 V6.

The secret to such an impressive output is in the fuel pressure at the cylinders. By raising it to some 2,050 bar (or kg/sq cm) by means of a pump/injector (the meaning of pumpe dnze), more fuel can be forced through a much smaller injector nozzle, and therefore with far greater accuracy. The biggest benefits are in economy and emissions but there's also a payoff in refinement: the PD is noticeably quieter than the standard 110 TDI. With common rail systems, the fuel pressure can be raised to only 1,350 bar or so, higher than with conventional diesel engines but much lower than the PD.

PD-engined cars - identified by red 'DI' lettering - will go on sale in the Passat in early September and in the Bora and Golf early next year. Initially, they will augment the existing TDIs at a premium we understand will be less than ú500, including (on the Passat) traction control, six-speed manual gearbox and ASR - anti-slip regulation. The Sport pack, which lowers the suspension by 20mm and generally firms things up, costs ú160. That makes the TDI PD 115 Sport with air conditioning tested here cost ú20,055 on-the-road. That's a hefty price to pay for an upper-medium diesel and the difficulty Volkswagen will have is persuading customers to spend the extra on what is visually an identical car to the stock 110.