Motoring: The diesel Holy Grail

WHY BUY a diesel? They use less fuel, so they cost less to run, and emit less carbon dioxide, the "greenhouse gas". Some hair-shirt-wearers talk of the languid, low-revving power delivery and the relaxed, aggression- free state of mind this encourages. In the end, though, few people buy a diesel for sensual pleasure. To most of us, diesels are noisy and smelly.

But maybe, if a diesel could be made even more economical, and less smelly into the bargain, the unappealing noise might be easier to bear. Most modern turbo-diesels are quite lively, after all.

So welcome, please, the diesel's Holy Grail, the common rail, and theCitroen Xantia HDi (High-pressure Direct injection) to which it is fitted. This is a new way of feeding a diesel engine its oily drink. It is the way of all future diesel engines, for it makes them clean and even more fuel- efficient without spoiling their pace. Quite the opposite: it makes a muscular turbo-diesel even more muscular. Gains all round, then.

Some words of explanation. Diesels work by compressing the air in the engine's cylinders about twice as much as in a petrol engine. This makes the air very hot, and at the crucial instant a high-pressure pump squirts fuel into the cylinder. Such is the heat, that the fuel ignites without the help of a spark plug. Diesel fuel has a higher calorific value, or energy content, than petrol, so you need less of it to produce similar motive force. That is the main reason why diesels are more economical.

Trouble is, up to now the fuel delivery has had to be handled by crude, mechanically pulsing pumps. Nothing electronic has existed that could cope with the huge forces involved in squirting fuel against the the rising compression pressure in the cylinders. But now we have the common rail, a shared reservoir of fuel under even more pressure, from which injectors, controlled by powerful new electronics, can draw precisely metered amounts of fuel as required. The benefits are manifold. There is never too much fuel going into the cylinders, so no sooty clouds come out. The extra-high injection pressure atomises the fuel more finely, so it burns more efficiently. And the electronics allow the injectors to give a little pre-squirt of fuel before the main spray, so combustion can happen more gradually and therefore less noisily.

Does it work? To find out, I drove the Xantia HDi back-to-back with a petrol-fuelled Xantia 1.8 16V. The 2.0-litre HDi engine delivers 110bhp and a spectacular 184lb/ft of torque, or pulling power; the petrol engine produces slightly more power (112bhp) but much less torque (114lb/ft). Driven to their limits, the cars are about equally as fast, but in normal driving the HDi is a much squirtier overtaker. My test drive of just under 200 miles took in motorways and busy main roads. Rain lashed the windscreen, traffic was slow, progress was gentle. So it was not too surprising when even the petrol Xantia returned an excellent 36.8mpg. But the HDi, the car with solid accelerative thrust from low engine revs, even in the sky-high, ultra-relaxed fifth gear, managed a remarkable 52.9mpg. Which proves the point rather well.

The new engine is acceptably quiet, too, partly because it is encapsulated in sound-deadening shrouds. It matches the old 1.9-litre TD engine here, a low-tech unit with quieter but less efficient indirect injection that continues for a while, and is much sweeter than the ousted 2.1TD. It responds more eagerly than these to the accelerator, too, although the HDi still can't match a petrol engine's crispness.

You pay more for a Xantia HDI than for a 1.8 petrol version, but the extra overtaking vigour makes it worthwhile if the economy and planet- friendly credentials aren't enough.This year's models have a built-in Trafficmaster Oracle congestion-warning system in most versions.

Citroen's claim to have the first UK-available common-rail passenger car is erroneous, though. That honour goes, if we bend the passenger-car definition a little, to the Isuzu Trooper off-roader, launched last May. And as I write this, the Xantia is joined by an HDi version of its PSA Group stablemate, the Peugeot 406. With Fiat and Renault poised to follow, common rail is truly on track.

SPECIFICATIONS

Make and models: Citroen Xantia HDi, from pounds 17,455 (LX saloon) to pounds 21,305 (Exclusive estate)

Engine: 1,997cc, four cylinders, common-rail direct-injection turbodiesel, 110bhp at 4,000rpm.

Transmission: five-speed gearbox, front-wheel drive.

Performance: 119mph, 0-60 in 10.8 seconds, 48-53mpg.

Rivals

Ford Mondeo 1.8 TD LX: pounds 16,020. Cheaper, but engine is crude and low- tech. Revisions due soon, but no common rail.

Peugeot 406 LX 2.0 HDi: pounds 17,050. Same engine as Citroen, lower price, better steering and handling.

Renault Laguna 1.9dTi RT: pounds 15,570. Middle-equipment version is lively and remarkable value, but not as clean.

Vauxhall Vectra 2.0 dti LS: pounds 16,745. Engine has 16 valves and is very efficient, rest is lacklustre. Revised Vectra is imminent.

Volkswagen Passat 1.9 TDi S: pounds 16,830. First direct-injection diesel not to assault eardrums. Pulls well, beautifully built car.