Hamish McRae: A British invention gives Otto the elbow

Cars are inevitably a compromise. This was brought home to me the other day when we hired a Mercedes A-class. On paper, it is a genius of a design, cramming a mid-size interior into the length of a super-mini by dint of putting the people on top of the mechanics instead of among them. So there is a flat floor and if you have a crash, the engine evidently slides underneath the people instead of chopping off their knees. It is solidly made, quiet and well finished, as you would expect from Mercedes.

Only snag: it was horrid to drive. Or at least it was on the bumpy, twisty roads of Galloway, though it may be fine on the billiard-table surfaces of German suburbia, where it was designed. The problem is that the original design failed the Swedish elk test: it rolled over when the driver swerved as if to avoid an elk. So the Mercedes people had to redesign the suspension to give the car greater stability. They succeeded - elks all over Sweden are breathing sighs of relief - but at the cost of a very bumpy ride. A tall car is inherently less stable than a low one and suspension is a compromise between stability and comfort.

We accept these compromises as inevitable. They occur at every level of vehicle. A Ford Ka is lovely to drive and easy to park but is tiny inside for four. A Porsche 911 is exquisite but it is not really even a 2+2, as the back seats are strictly for children (lucky ones, too). The Citroen Picasso people carrier is hugely successful because it is roomy and cheap but it is profoundly unsatisfying to drive because of the sacrifices needed to give it space and keep the cost down.

Clever engineering can reduce the scale of the compromise but it cannot eliminate it. There is, however, one advance now taking place that is radically reducing the compromises in engine design. Nearly all engines, petrol and diesel, operate on the Otto four-stoke cycle, called after the German inventor Nikolaus Otto: induction, compression, ignition, exhaust. They can be tuned to give maximum top-end power, or bottom-end torque, or maximum economy. But there is always an element of compromise. If, however, you don't have to worry about either power or flexibility but just want economy, designers are largely released from such constraints. Indeed, they are released from the Otto cycle.

The new hybrids, such as the Toyota Prius, use an electric motor to provide additional overtaking power and starting-off torque. So the petrol engine can be tuned to give maximum economy because its main job is to provide steady power for cruising and topping up the battery.

The designers have found that an engine running on the Atkinson cycle is more efficient than on the Otto one. James Atkinson was a British engineer who invented a different type of crankcase design and different valve timing about the same time as Otto. It is the valve timing and an element of his crankcase design that both Toyota and Ford are using for the engines of their hybrids. (Put the names of the two engineers into www.google.com for stuff about the differences, which I find fascinating but which do get a bit anoraky.)

The result, at any rate, is a very sweet and extremely efficient engine. It does not have to produce much power or much torque so it can spin away quietly running at its most efficient pace. Hybrids seem likely to become the most important innovation in car design in the last half-century because they can combine efficiency with very low pollution. So here is one area of automotive design where there is a sudden leap forward - and where design is becoming much less of a compromise than ever before.

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