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Combination of diesel engine and electric motor allows even lower fuel consumption

Stuttgart, Nov 15, 2005 The smart crosstown show car, presented at the IAA in 2005, certainly managed to attract attention. But it’s going to have to share its limelight: because now, smart has come up with another vehicle with hybrid drive. Indeed, the smart fortwo cdi hybrid boasts even lower fuel consumption than the (already very economical) smart crosstown – beating it by more than a litre per 100 kilometres.

The car’s combination of a diesel engine with an electric motor, with outputs of 30/20 kW respectively (in the smart crosstown the electric drive is combined with a petrol engine), means that the smart fortwo cdi hybrid consumes around just 2.9 litres of fuel per 100 kilometres under standard conditions – in other words, some 15 percent less than the (already very economical) smart fortwo cdi. At the same time, the smart fortwo cdi hybrid shows that a hybrid drive doesn’t just mean lower fuel consumption: it can provide a generous dose of additional driving pleasure, too.

Greater ride comfort with improved acceleration

The fortwo cdi hybrid’s acceleration statistics clearly indicate the additional driving pleasure that it provides. Because the car’s shift delays are bridged by the electric motor, it only needs 17.8 seconds to accelerate from zero to 100 km/h. That’s ten percent – or two whole seconds – better than with a conventional diesel drive. And in terms of everyday use, another figure is perhaps even more relevant: acceleration from 80 to 120 km/h. This, too, is ten percent better in the smart fortwo cdi hybrid than in the production car.

Gearshifting works in just the same way as for the production vehicle: in order to change between forward gears, the driver needs simply to move the gearshift lever forward or back. The clutch is opened and closed electrically, and a central control unit manages all of the shifting and clutch processes. That means that the smart fortwo cdi hybrid has no need for a clutch pedal – which is particularly convenient in traffic that’s constantly stopping and starting.

The diesel engine is switched off to reduce fuel consumption

But the electric drive doesn’t just help the driver during gearshifting: thanks to the car’s automatic start-stop system, the diesel motor is automatically switched off when the vehicle stops at, for example, a red light. And the vehicle doesn’t even need to be stationary – in overrun, too, and when driving at slow speeds, it makes a lot of sense for the combustion engine to be switched off temporarily, and for the electric motor to take over the drive.

As soon as acceleration is required again, the electric motor can deliver – when pressure is applied to the accelerator pedal – an output of up to 20 kilowatts, for short periods of time. It will continue to do so until the diesel engine is running again, and has built up sufficient torque. At the current state of the art, this takes around one second.

There is one exception: at speeds over 70 km/h, the diesel engine is not switched off. This is because the electric motor, by its very nature, delivers less drive torque at higher speeds. So the diesel engine continues idling, to ensure that it is ready for operation as soon as it is required. That way, if the driver spontaneously decides to speed up, the diesel engine will immediately be available to provide him with the acceleration he needs.

Using braking energy to recharge the battery

Of course, the smart fortwo cdi hybrid would be fully functional even if its battery were flat. In this respect it is the same as the smart fortwo cdi. But there’s no need to plug this two-seater into the mains: because during deceleration, the electric motor works as a generator, recharging the battery under the driver’s seat. It does this not just during braking, but also even if the driver simply takes his foot off the accelerator. This means the smart fortwo cdi hybrid is able to convert dynamic energy into electric energy. The resulting energy can then be saved in the vehicle’s battery, and used later for acceleration.

There are two exceptions to this: if the braking power demanded by the driver exceeds that of the generator (this is determined by the pressure applied by the driver to the brake pedal), then the hydraulic brake system will intervene, to slow the vehicle down some more. And once the battery is fully charged, braking is carried out by the hydraulic brake alone.