The incredible shrinking motor

By Gavin Green

Cars, much like people, are getting bigger. They began to expand when safety regulations required beefier vehicles; customers, in turn, acquired a taste for larger models. The hulking SUV is the world’s fastest-growing breed of car; even medium-sized hatchbacks have inflated. The Volkswagen Golf is now more than half a metre longer than it was 40 years ago, and over 400kg heavier.

The growth spurt is damaging on many levels: it hurts performance, fuel economy and handling. A heavier car is invariably a less efficient car.

But while cars are bulking up, the engines that propel them are shrinking. Some cars that habitually used six-cylinder engines – many BMWs, for example – now use four cylinders. The xc90, Volvo’s biggest SUV, once relied on a mammoth eight cylinders for its top-end version, but now has stepped down to four without any diminution in speed.

The cubic capacity of engines is also falling. Ford, the British market leader, has had a 2.0-litre, four-cylinder engine in its everyman Mondeo for nearly 20 years; its latest models offer a tiny 1.0-litre, three-cylinder one with virtually the same power. Around 20% of all Fords sold in Europe contain this compact engine.

The imperative to reduce oil consumption and CO2 emissions has compelled car companies to shrink their engines, and new technical developments have made it possible. The most important one is turbocharging.

“Engine power isn’t dictated by size, it’s dictated by how much air you can get into the engine,” says Andrew Fraser, Ford’s chief engineer for petrol-engine development. “Air is power. In the old days, people turned to bigger engines to ingest more air. Now, we typically use turbochargers that boost power without increasing engine size.”

Turbocharging occurs when the car’s exhaust gases drive a turbine linked to a compressor that forces additional air into the engine’s cylinders. These turbochargers have been around since the 1970s but have improved enormously in recent years. They can help small engines deliver at least as much power as a big, naturally aspirated engine, while increasing fuel economy. Better materials mean they can spin much faster, act more responsively and operate at higher temperatures.

Volvo has been an especially enthusiastic advocate of smaller engines. Its recent decision to make four-cylinder engines only – an even smaller three-cylinder version is coming – was a bold move, especially when one of its biggest markets is America, where larger sizes remain popular.

“We never saw it as a significant risk,” says Peter Mertens, Volvo’s head of research and development. “We predicted the trend to downsizing and moved early. It’s becoming less of a marketing concern as time passes, as people get used to small engines that can be powerful.”

Smaller engines can also improve safety. There’s more space in which the energy-absorbing crash structures can work and a bulky engine is a potentially dangerous deadweight. And, says Thomas Ingenlath, the head of design at Volvo, dinkier engines allow for greater design freedom: “It gives us more opportunity to do great-looking cars.” Small engines allow for lower, more rakish bonnets and shorter front overhangs, and more space can be provided for passengers or luggage.

This trend might even eventually make cars smaller and lighter. There has been some progress here. Citroën’s new c4 Cactus, which comes with three- and four-cylinder engines only, is 200kg lighter than a Mini hatch but as roomy as the bigger Ford Focus.

Increased efficiency will also probably prolong the reign of the internal-combustion engine for the next 15 to 20 years as the dominant power source for passenger cars. It may be in the autumn of its long life, but there could be an Indian summer until electric motors take over.

ILLUSTRATION MARK OLIVER