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Little Diesels

Lightweight cars with exceptional fuel economy from their small diesel engines

by Julian Edgar

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World’s Smallest Diesel

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The DaimlerChrysler Smart FourTwo features the world’s smallest direct injection diesel engine. The 800c turbo 3-cylinder develops 33kW at just 3800 rpm.

Over the European fuel test cycle, the FourTwo diesel engine gives fuel economy of 3.3 litres/100km and emits just 88 grams of CO2 per kilometre.

Fuel injection pressure is 1600 Bar and 7-hole injectors are used. A large amount of exhaust gas is recirculated – as much as 60 per cent of exhaust gases are cooled and then returned to the combustion chambers.

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As is the case with other current direct injection engines, the electronically-controlled diesel uses a pilot injection squirt that ignites and preheats the cylinder, reducing noise emissions.

A tiny turbo is fitted. The compressor has a diameter of only 33mm but it rotates at up to 280,000 rpm. A maximum boost pressure of 1.2 Bar is used. The small turbo and diesel configuration allows for low-rpm torque development – 85Nm is available at just 1500 rpm!

However, the 770kg car is abysmally slow – 100 km/h comes up in a fraction under 20 seconds and top speed is only 135 km/h.

Most Economical 5-Seater

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Volkswagen claims their Polo BlueMotion is the most economical 5-seater in Europe.

The 59kW 1.4 litre 3 cylinder develops maximum torque of 195Nm at 1800 rpm. As with other VW diesels, the BlueMotion doesn’t use common rail injection but instead pump nozzle injection that’s electronically controlled. A turbo with variable geometry turbine is used.

Over the standard Polo TDi, the BlueMotion features exhaust gas recirculation (EGR) with a more powerful cooler and electronically controlled flow valve. Taller gear ratios are used for gears 3, 4 and 5 in the 5-speed manual gearbox.

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Low rolling resistance 165/70 14 tyres are fitted and the Polo body feature aero tweaks to get its drag coefficient down to 0.30.

Fuel consumption is listed as 3.8 litres/100km, corresponding to a CO2 output of 99 grams/kilometre. Performance is also quite respectable – the Polo BlueMotion can accelerate to 100 km/h in 12.8 seconds and reach 176 km/h.

The Fun Bubble

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Not a tiny car like the original, nonetheless the new Fiat 500 can still claim to be amongst the most economical of cars in its class. It uses in one model a 16-valve, 1.3 litre (closer to 1200cc – it’s actually 1248cc) common rail diesel that develops 55kW at 4000 rpm and 145Nm at 1500 rpm.

A fixed geometry turbo is used in conjunction with a wastegate and intercooler.

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Fuel consumption of the 980kg car is 5.3 l/100 km in the urban cycle, 3.6 l/100 km out of town and 4.2 l/100 km in the combined cycle, and CO2 emissions are 111 g/km. The engine weighs only 130kg and is 50cm long and 65cm tall. As with the other cars covered here, a particulate trap is used in the catalysed exhaust and recirculated exhaust gas is cooled.

Performance includes a 0-100 km/h time of 12.5 seconds (measured two-up with an additional 20kg!) and a top speed of 165 km/h.

But where’s Japan?

With their love of big brawn, you might expect that the US is not going to have a super economical diesel-powered entry – but where is Japan in the development of tiny car diesels?

No where.

In anyone’s terms, Japanese engineers have long been masters of superbly built and engineered small engines. But diesels, while manufactured in large numbers for export cars (and export and domestic commercials) are almost invisible in Japanese car sales. Recent numbers show annual sales of a little over 2000 diesel cars in a market of just under 6 million!

So the jewel-like turbo 660cc and 1 litre cutting-edge diesels that you might expect to see being produced in Japan, powered in development by local demand, don’t exist.

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