The Diesel Era
January 2002
12 years ago, I read an article in Autocar regarding the revolution of diesel engines. It foresaw the increasing popularity of diesel and the fast improvement of performance, refinement and emission. Autocar picked the contemporary BMW 325td engine as an example - the turbocharged 6-cylinder displaced 2.5 litres and produced 115 horsepower, matching the output of a low-end 3-series petrol. Today, we might not understand why that was regarded as a revolution, because the progress of diesel technology during these 12 years was even bigger. Now we are starting to compare the output of diesel and petrol engines of the SAME displacement, not just in terms of the usually-acclaimed torque but also horsepower. Ford Focus TDCi needs just 1.8 litres and four cylinders to produce the same 115hp as the old 325td. BMW itself is even more impressive - the new 320d generates 150hp out of 2 litres, making it more efficient than its own Valvetronic-and-Bi-Vanos 2-litre petrol. Volkswagen's pump injection even achieves 150hp from 1.9 litres, although it has drawback on turbo lag and noise. 

Common-rail injection technology is leading us into the Diesel Era, an era in which most family cars, executive sedans, MPVs, on/offroaders are powered by diesel. Right now, one-third of new cars sold in Europe are already diesel cars. If the trend is to continue, if not accelerate, 10 to 20 years later petrol engines will be reserved for sports cars only. We might even see a M-badged BMW diesel, just like what AMG is doing in Stuttgart. Undoubtedly, the trend is going to be accelerated by the introduction of "second-generation" common-rail diesel, which has been serving the aforementioned Focus TDCi and 320d. By increasing injection pressure from 1350 to 1600 bar, the new system enables finer fuel spray hence more efficient combustion. It also applies "multi-jet" to cool cylinder head, hence avoiding knock and reducing noise. It seems that under the competition of second-generation common-rail VW's pump injection technology will be eventually phased out. In fact, no one else goes the pump-injection route. If not Volkswagen group's substantial buying power, I'm afraid Bosch will no longer produce pump injection system for it. 

Admittedly, diesel engines will never match petrol for power efficiency, as measured by power-to-weight ratio. While a 150hp petrol 2-litre engine is normally aspirated, BMW 320d needs a variable geometry turbocharger, an intercooler and a sophisticated injection system. Therefore I said sports cars will still favor petrol engines for their weight and weight distribution advantage. Also because of their instantaneous response. However, the advantage is reducing quickly as common-rail diesel is gaining efficiency much faster than petrol. 12 years ago, a 2-litre 16V engine produced about 140hp, virtually unchanged until today (though gained a little bit more torque). During the same period, turbo diesel gained about 50% power. Now I am predicting within this decade we can see some good diesel engines breaking the 100hp/litre barrier. Can you believe ? 

Mark Wan

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