Road Test: Mercedes-Benz S Class S400L Hybrid SE Line 4dr Auto

On the face of it, a hybrid S-Class shouldn't exist. If anyone wants a slightly-more-economical-than-usual luxury car, they should buy a fractionally smaller (but still pretty ritzy in the overall scheme of things) luxury car like an E-Class. Or an S-Class diesel. Actually, if saving the world were really the priority, the vast expense of developing a car like the S-Class hybrid would be much better spent building tidal electricity generators, or just standing in the town square in Stuttgart and handing out free loft insulation.

But Mercedes is obliged to be part of the hybrid revolution. For a start, American luxury-car drivers don't think diesel is a proper fuel for anything smaller than a Peterbilt, whereas the mere existence of a hybrid in the range - albeit a vast and thirsty one - is seen by your average American as sufficient to turn the world's deserts into lush meadows overnight.

Never mind, this being Mercedes, the engineers have tackled the hybrid issue with comprehensive diligence. The company is developing two entirely separate hybrid systems. The first is this one, a comparatively mild system that sticks a disc-shaped electric motor in the place where the V6 engine's flywheel would be. Mercedes has another system in the works too, for its SUVs - a full-hybrid that entirely replaces the conventional transmission set-up.

The cost of developing this stuff means it makes sense only if the effort is shared. So the S400's mild hybrid system will also show up on a BMW 7-Series. By the same token, the full-hybrid system is the same as on some of GM's SUVs in the States, and on the forthcoming BMW X6 hybrid. 

Despite, or rather because of, the complex technology here, the driving experience is utterly... normal. I drove a prototype, by the way. The real thing will be introduced in the US later this spring, along with a mild range-wide face-lift. Merc calls it the S400 BlueHybrid, even though the engine is a 3.5-litre. Fair play, because it feels a bit more muscular than the S350, thanks to the extra torque that comes from the electric assistance, especially at low revs.

So you've got a car that wafts along with the sort of stately quiet a V6 petrol S-Class normally does. As with an S350, there's enough urge (slightly more in this case) that in normal traffic, or even at a good clip on A-roads, it never seems to be trying that hard. And then you commit to overtaking a bigger line of trucks than usual, or make a spirited attack on a series of uphill bends, and you're reminded thatan S-Class is a pretty hefty lumpof metal and leather; suddenly,the V6 starts to run out of answers and sounds pretty strained.

At full-throttle kickdown, the electric motor doesn't actually help much: it's optimised to complement the V6, not replicate it. So where a petrol engine is weak - under about 2,000rpm - the electric motor is useful. But at higher revs, the electric contribution fades. In numbers,it looks like this: the motor makes 118lb ft of torque at zero revs, but because its torque falls with revs, it contributes only 20bhp to the total by the time the petrol engine is in its stride. Still, all that low-down torque does make the car feel relaxed in traffic, the auto 'box making fewer shifts when you're travelling at suburban speeds.

The V6 cuts out as you cruise down to rest, leaving you in calm silence until you lift off the brake, ready to move away. At which point, the engine jumps back to life. Any anxiety about sitting there engine-less soon melts, because the restart is done by the big electric motor, rather than a normal starter motor, so it's much smoother and more confident. 

In the normal run of play, the battery gets its charge for free, from regenerative braking rather than the engine. That simplifies things in comparison with a full hybrid. But don't be fooled into thinking this is a simple engineering job - it's not.

For a start, the engine is heavily modified so as to be efficient in mid-revs, just as the electric motor runs out of puff. The power electronics monitor things so that if the battery is well-charged it throttles back the engine and calls on more effort from the motor, but, if the battery is low, then it forgets the electric motor and opens the V6's taps wider. You never notice the difference. 

The brakes are on a special variable linkage. When the battery is low on charge, the first section of pedal travel is given over to regenerative braking, and it's only if you push harder on the pedal that the wheel brakes are called up. But at full charge, the linkage alters so that you get friction braking as soon as you brush the pedal. It's canny, and as with the rest of the car, you don't actually feel this complicated bit of technology in operation. The pedal just feels like that on a normal S-Class.

The battery in question is a lithium ion job, one of the first of the kindin a production car. It's under the bonnet, so no sacrifice in cabin- or boot-space is called for. In all, the car gains just 75kg over a regular S350.

Bottom line, then: fuel savings. Significant, actually, at some 21 per cent in the European test cycle, compared with the regular V6 S350. But we won't be getting it in the UK, because the diesel is nearly that efficient already.

However, the beauty of this mechanically straightforward (if electrically complex) hybrid system is that it can be scaled for other Mercedes models, and even diesels. And actually, there is another reason to do hybrids. One thing about the future of the car is pretty much certain: electricity is going to be involved in one way or another - look at hybrids, fuel cells and battery cars.

The more a company knows about electricity storage and control, the better-placed it will be. And as the S-Class reveals, Benz knows a lot.