Yes, we have all read about diesel cars being better these days, but its not just about how much they have changed in such a short time, but how good they really are now. The diesel engine has expanded to fit most niches in the current car market but even more applications are on the way. Well-known for their sluggishness, noise and clouds of black smoke in early generations, modern diesels are truly a breed apart from their ancestors.

(i) NB - US gallons are much smaller! Eu gallon = approx 4.54 litres, US gallon approx 3.8 litres

Diesel engines in cars, traditionally known for their solid torque (pulling power) have pushed the limits to new highs with the benefit of high pressure direct injection and turbocharging - far higher than is produced by the frantic multi-valve petrol engines on the market today. This greater torque over a wider rev range means that the diesel driver is able to make reasonable progress without the need for 'stirring up' the engine through frequent use of the gearbox. Torque for A & B-road overtaking is readily available without the need to change down a gear (unless you're in a high 'overdrive' sixth top gear). Indeed, most modern diesels will quite happily accelerate smoothly in fifth gear from as low as a 15mph - rather like driving an automatic, and with the typical 50mpg (i) + economy of the diesel engine. Ultimately a very relaxing, very economical drive, especially now that modern diesels are so quiet at cruising speed.



Diesel's great torque is what helps 'white van man' to remain an unshakeable six feet from your back bumper in the motorway fast lane in his Transit, Sprinter etc - come long hill, come driving rain.



The Volkswagen Golf GTPDI - the first truly sporting diesel hatch

When the driver wishes to make more sporting use of his diesel however, things begin to become less clear-cut in comparison with petrol cars. Whilst in general diesels produce superior torque and hence overtaking ability, the more highly the engine is tuned, the greater the peakiness of the torque characteristics. That great torque in the highly tuned diesels, although up to 50% greater than a comparable petrol engine, can tail off much more rapidly either side of its peak, meaning that the rate of acceleration varies markedly according to where you are in the engine's rev range. Lower-tuned diesel engines have a flatter torque peak, whereas maximum torque in a petrol engine can actually be constant across a very wide rev range. In order to make best use of that great, highly-tuned, peaky, diesel torque, the driver would ideally need a close-ratio multi-speed gearbox - to best keep the engine as close to that narrow peak torque band as possible.

(ii) Pumpe-düse, or high pressure electronic unit injectors

The above is especially true of the higher-powered PD(ii) engines favoured by the Volkswagen Audi Group, which, admittedly, do give you the six-speed close ratio gearbox that you need to make the most of that phenomenal peak torque. The group has recently created the first truly sporting diesel hatches -witness the four wheel drive Golf GTPDI 150 and 150bhp Seat Leon Cupra (neither of which are on sale in the UK) - both offering brutal acceleration. The Passat is also available in high level 130bhp tune with six speed gearbox.



Thought: a sporty, responsive CVT constantly variable gearbox would be the ideal way to keep peaky diesels at their torque peak - but could it reliably handle such high torques? Over to you, Audi!


Golf diesel out-accelerates the Golf Gti!

Golf model
Power (BHp)
Torque (Nm)
0-60mph (secs)
30-50mph (4th gear)
50-70mph (top gear)
GTI Petrol 1994cc[iii]
150
210
8.6
6.4
9.2
GT PD Diesel 1896cc[iv]
115
294
9.7
5.6
7.2
[iii] Five speed manual gearbox
[iv] Six speed manual gearbox
Figures reproduced with the kind permission of Diesel Car magazine

(v) Now the diesel injection system of choice for Peugeot Citroen, who will make over one million HDI engines this year

Less highly tuned diesels offer a more constant power delivery and are more relaxing to drive (still with much greater torque than petrol engines). They are also able to get away with a five speed manual or four speed automatic because their torque characteristics are less peaky. Such linear torque delivery engines mostly use common rail injection systems(v) - and are generally quieter and smoother too. These are already in use in big V8 form in the Mercedes S Class and BMW 7 Series and available in sizes all the way down to 1.4 litres in the Peugeot 307 (and forthcoming Fiesta and 206). Peugeot have just announced a diesel grand touring coupe - the svelte 406 Coupe with 2.2HDI engine, including soot-eating particulate filter. Alfa Romeo's 5 cylinder 2.4 litre JTD common rail engine is another powerful sporting contender. There are, of course, also lower-tuned PD-engined Volkswagen Audi Group models (such as the 100bhp Passat and 90 and 110bhp Golf) that also turn in a very linear, refined and competitive performance.

(vi) The UK will tax company vehicles according to CO2 output (instead of annual mileage) from the 6th April 2002

So there we have it: diesels can be all things to all people. Big grand tourers - on their way, luxury vehicles - already here, motorway executives - already here, SUV / 4x4 family carriers - already here, superminis - currently emerging, brutal Gtis - just about to be realised on a more mass market level (especially when Ford sets its marketing muscle to work on the Mondeo, Focus and next Fiesta common rail diesels). And they all offer greater fuel economy and produce less CO2 than their petrol rivals, in addition to a satisfying drive. My money's on the UK company diesel car market exploding over the next couple of years....(vi)

By Mark Wilkinson


To view related research reports, please follow the links below:-

Diesel Engines & Parts to 2003

The 2000-2005 World Outlook for Automotive Diesel Prices

Global market for automotive turbochargers and superchargers: Forecasts to 2005


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