Luxury Autos: First Drive: 2010 Mercedes-Benz S400 BlueHybrid

It's probably best to call the new Mercedes-Benz S400 BlueHybrid a mild, rather than full, hybrid, because it can't propel itself on electrical power alone. In other words, the BlueHybrid is not powered by the two-mode system co-developed with GM for big SUVs, which will be an option for the M-Class next year. This is a smaller, cheaper system designed to be scaleable across all Mercedes car lines.

Despite its comparative simplicity, the S400 BlueHybrid's gasoline and CO2 savings are significant: some 21% in the European test cycle, compared with the regular V-6 S350. It's also quieter at rest and more muscular in real-world acceleration. There's very little penalty over the standard V-6 in terms of weight -- just 170 lb -- and none at all in passenger or luggage accommodation or payload.

Of course you can't buy a regular Euro-spec S350 in the U.S., so let's do the numbers against a V-8 S550. The BlueHybrid goes 47% farther on a gallon than the S550 on the Euro test cycle, but its performance figures aren't so far off -- about seven sec 0-to-60 instead of 5.4. That's why they called it S400: because it feels like a four-liter engine.

The BlueHybrid's electric motor effectively does duty as the gas engine's flywheel, turning at crank speed all the time. It offers 118 lb-ft of torque at zero revs, but because its torque falls with revs, it only contributes an extra 20 hp to the total by the time the gas engine is in its stride.

Compared with the standard V-6, this extra torque makes it feel slightly more like the V-8 at lower revs: a strong but lazy roll-on to the throttle without so many hectic downshifts.

The point, throughout this engineering job, was to make the hybrid feel like the regular car, except in the issue of tank range.

In fact, the V-6 has new cam timing and cylinder heads. It runs, like a Prius, on the Atkinson cycle. This delays inlet valve closing, reducing effective compression. It gives the engine great efficiency but over a fairly narrow rev range. In other parts of the rpm spectrum, the electric motor fills in the torque deficit. The transmission is reprogrammed to suit, aiming to keep the engine where it's happiest.

The electronics keep a constant eye on battery state of charge, and if it's near 100%, then the electric motor is deployed fully while the engine is throttled back. If you've just ascended a long steep hill and the battery is low, then more of the overall power will be coming from the engine.

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.

The new speedometer display also shows battery charge. I tried running down to the bottom of a steep, twisting hill, which charged it right up. I then turned and gunned the S-Class back up for a few miles, but the charge state didn't fall far or fast. Every time I touched the brake pedal for a corner, it would regenerate. So it's hard to envision a situation where it would be low. But if it were, the car would proceed happily, just with a little less performance than normal, until it was recuperated again.

The electric motor is just a couple inches thick, sandwiched between the engine and torque converter of the largely unaltered seven-speed transmission. But the transmission software is amended to get optimum shifts points and shift quality with the new powertrain.

The trans also gets its own electric oil pump for the times the engine isn't running. The air-conditioning has an electric pump, too, and the steering also becomes electrohydraulic. That way they all function when the engine is stopped.

At the moment, there's still an alternator and small 12V battery, but engineers say it's likely future iterations of the system will dispense with those two components entirely. They're really only backup: The 12V ancillaries mostly run from the lithium-ion high-voltage battery via an inverter.

Mercedes has built some 100 prototypes of the BlueHybrid, and they're still in durability testing. But the firm is confident the performance figures are final.

The car goes on sale in Germany in June 2009 and the U.S. in September. By then the whole S-Class range will have had a mild facelift.

Daimler chief Dieter Zetsche told me a few weeks back the car's fate had changed with high gas prices. "At first, I thought it was just a marketing tool, just so we could say we had a hybrid. Now it's hard to tell." Interest has been high. "But I'm not sure I want to sell too many, given the cost of it to us. Our intention is to ask the customer to participate in that cost."

In other words, it'll be more expensive than a regular V-6 S-Class in Europe, but still a loss-leader for Benz to ramp up the new technology. Beyond that, at this stage everyone at Mercedes has sealed his lips on pricing.

[source:MotorTrend]