In race for fuel economy, BMW set to put three-cylinder into 3 Series

The German marque is on the verge of a radical move to boost its green credentials.

One of BMW's most senior executives refused to rule out slotting the German company's upcoming three-cylinder engine into its next-generation 3 Series mainstay.

The fuel-saving three-cylinder motor has been developed largely for the 1 Series, but BMW's board member in charge of product development, Dr Klaus Draeger, insists the engine is strong enough to power the larger sedan and wagon as well.

"It's too early to say when we will make a decision about whether the 3 Series will get the three-cylinder engine, but customers would accept it for a car of that size," Draeger confirms.

Other BMW sources said Draeger's comments were code to confirm that 2012's front-drive version of the 1 Series would receive the three-cylinder engine and the 3 Series would launch with four and six-cylinder engines, but would get the three-cylinder as soon as people saw how good it was.

"It's nothing to do with the strength of the engine," Draeger says. "The three-cylinder is strong enough now and, with our kit-box idea, it should be thought that it will fit into the 3 Series.

"If, in time, people accept an engine like this for larger cars, then we will change. After all, a four-cylinder, for example, is now accepted in the Z4."

Draeger explains that BMW was turning to a three-cylinder engine for fuel economy, rather than to make a cheaper engine.

"It's not cheaper, necessarily, because all the technology is still there. It's lighter and more economical, but it's still got double VANOS [variable valve timing] and twin-scroll turbocharging and direct fuel injection and it can easily give 40kW [54hp] per cylinder.

"That is 120kW [160hp] or more from three cylinders and the way a three-cylinder likes to rev up and the way it sounds are in some ways better for a BMW driver than a four-cylinder.

"There are certain ways of vibrations that give a better performance from a three-cylinder than a four. There is less weight, too, and that helps handling."

Proving the idea of a three-cylinder 3 Series is serious, Draeger admits BMW's engine, gearbox and chassis engineers were already engaged in the philosophical discussion of whether to pull the shorter engine back into the chassis for better handling or leaving the engine on the four-cylinder's mounting points and spacing the gap to the gearbox.

But one of the main benefits will be in pulling the fuel economy down, says Draeger - something BMW is conscious of, especially as some of its new cars, like the 6 Series Coupe, have piled on the weight. "In terms of fuel economy [and emissions], we are talking about a target of 95 grams of CO2 and we have to reach this somehow.

"When you compare a 1.6L four with a 1.5L three, there is a 10 per cent advantage in terms of fuel consumption.

"The single-cylinder performance means a displacement of 500cc, but with a 1.6, you have 400cc. This is important because the ratio between performance and volume is 25 per cent lower, and the friction is a lot less, too."