2010 Mercedes-Benz S400 BlueHybrid - Hybrid Operation - First Drive

OK, so that's how the power is dispensed. The battery collects its charge almost entirely free, from regenerative braking. The engine doesn't charge the battery in steady-speed running as it sometimes does with a Toyota or Lexus hybrid.

Lifting off the gas causes the graphic display in the center of the speedometer to show green arrows from the wheels to the transmission to the battery. That signifies that the electric motor is being switched to generate regeneration current. Press the brake pedal and at first the pads don't touch the discs. Instead, by increasing the generating effect of the motor, resistance builds up and the car is slowed by its rear wheels. Go beyond that and ask for serious braking and the friction brakes begin to act.

Fortunately you don't feel this as a multistage braking. Sure, there isn't a lot of pedal feel in the early stages, but there isn't in a normal S-Class, either. What counts is that retardation is predictable according to pedal travel, and strong.

The mixed braking effect is controlled by a new variable linkage between pedal and servo, which, under control of the hybrid electronics, is able to vary the point at which the disc brakes begin to act. For instance, when the battery is full, the friction brakes begin their job right at the top of the pedal travel.

So the control electronics have a crowded job description. There's more. As well as balancing the engine and motor torque for the most effective power delivery, they also have to manage the transitory states. It's no longer just a question of retarding the engine timing to smooth gearshifts, but also the electric motor's torque.

This is a tricky task if the car is coasting or under brakes, when the motor is in torque-reversed regen mode. And here you do occasionally feel a slight thump as the transmission downshifts at low speed.

Below 9 mph, the gas engine is switched off and the car coasts under torque-converter slip. The engine remains stopped until you lift off the brake pedal, when it instantly restarts under the power of the main hybrid motor.

The brand-new lithium-ion battery is barely bigger than a conventional 12V battery. It's scaleable to other cars, and a bigger version will soon be used for a test fleet of all-electric smarts. In the S-Class it lives under the hood, by the firewall on the passenger side. It's cooled by a special gel and its own mini radiator. The cells are the first pure-automotive li-ion cells, says Benz. Tesla uses commodity laptop batteries in their own thermo-regulated casing.