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Lots of driving pleasure, coupled with extremely low fuel consumption

Stuttgart, Nov 15, 2005 The smart show car crosstown, presented at the IAA 2005, goes to show once again that smart has the potential to offer environmentally-friendly drives not just today, but in the future as well. And it shows that it is possible to have the best of both worlds: low fuel consumption and a high degree of driving pleasure.

The smart crosstown show car thus highlights the smart brand’s commitment to the environment. The vehicle’s innovative concept is unique for a car of this class, yet it is the logical response to a growing demand among consumers: is considered absolutely standard among larger cars in the USA is increasingly becoming accepted in Europe, too.

The smart crosstown show car: electrically assisted, for greater economy and dynamism

Thanks to its combination of an electric motor (with an output of up to 23 kW) with the 45 kW/61 bhp petrol engine from the smart fortwo, the smart crosstown consumes considerably less fuel again - despite weighing 85 kilograms more. Even when compared with the extremely economical standard drive, it allows a reduction in fuel consumption of some 15 percent (4.7 litres of petrol per 100 kilometres) in the new European driving cycle (NEDC). Like the smart fortwo, the vehicle’s maximum speed is electronically limited to 135 km/h.

The smart crosstown furthermore offers even more ride comfort, and approximately ten percent better acceleration (the production car requires 15.5 seconds to accelerate from 0 to 100 km/h). At a red light, for example, the combustion engine is automatically shut off with the start-stop function. Yet starting up again at a moment’s notice is not a problem: while the petrol engine is starting and building up torque, the electric motor takes over the job of driving the vehicle. And that's not all: the electric motor also takes over when the tractive power is interrupted during gear changes.

Automatic start-stop system helps to save fuel

The idle phases of the combustion engine offer a fuel saving potential of between five and six percent. Therefore it stands to reason to switch the engine off when the car is not moving, in overrun and when driving slowly. A disadvantage of this method in conventional drive concepts has proved to be the inevitable pauses which occur when the engine is restarted. Even with the latest technological developments it takes around a second for a combustion engine to get running and build up sufficient drive torque.

The smart crosstown, on the other hand, immediately converts pressure to the accelerator pedal into adequate drive. Its electric motor supplies energy through the rear-wheel drive, and it can supply power of up to 23 kilowatts for a short time. By this time the combustion engine has started up, and can gradually take over the drive from its electric counterpart. At most, the driver perceives the transition from one drive unit to the other acoustically.

Electric drive during shifting breaks

The vehicle’s six-speed automated transmission gives the driver a great deal of added comfort. To change forward gears in the sequential mechanism the gearshift lever simply needs to be moved forwards or backwards. The clutch is also opened and closed electrically. This means that the smart crosstown – just like the smart fortwo – does not need a clutch pedal.

A central electronic control unit manages the shifting and clutch processes. Instead of a conventional automatic transmission and torque converter, the smart crosstown works with a manual transmission mechanism including friction-disc clutch. The advantages of this compared with a fully automatic transmission lie in the much higher efficiency, lower weight, more compact dimensions and lower production costs.

However, unlike an automatic transmission, this principle causes traction interruptions between opening and closing the clutch for gear shifting. In the smart crosstown these are bridged by the electric motor. For a few tenths of a second it supplies the propulsion power needed, enabling harmonious, steady acceleration over the whole gear spectrum. So the smart crosstown is not only more lively, it also offers added shifting comfort.

Brake power is converted to electrical energy

The partial recuperation of braking energy makes a major contribution to energy savings. During braking, the electric motor of the smart crosstown is operated as a generator. This enables dynamic energy to be converted to electrical energy and stored in the car’s battery – to be used later for acceleration.

The higher the braking power that the axle drive feeds into the generator, the higher the charge current for the battery. The generator is regulated roughly in proportion to the pedal pressure. The hydraulic brake system does not intervene and slow down the car some more until the braking power demanded by the driver exceeds that of the generator. The hydraulic brake only takes over the whole deceleration when the battery is fully charged. This process of recuperating braking energy enables a fuel saving of around five to six percent to be realized.

To achieve this result, the control unit causes the engine to be switched off and the clutch to be opened when the accelerator pedal position is zero. This strategy is pursued up to a driving speed of 70 km/h. At speeds above this the electronics merely open the clutch when the engine is idling. Because of its operating principle, the electric motor delivers less drive torque at higher speeds. This is why the combustion engine has to be ready for operation in order to be able to spontaneously comply with the driver’s request for acceleration.

Optimum operating conditions

The cooperation of the electric motor and the combustion engine also presents an opportunity to operate both the drive units at optimum effectiveness. The electric motor thus drives the car on its own at low speeds, for example in traffic-calmed zones. The battery must be sufficiently charged for this. The combustion engine produces a certain amount of power which is not required by the driver in the range of minimum specific consumption in order to charge the battery. This operating mode permits a fuel saving of up to five percent.

High operational reliability guarantees the parallel use of both driving engines. Unlike a serial concept, where the electric drive motor is fed by the combustion engine, the petrol engine of the smart crosstown is able to drive the vehicle even when the battery is empty. In this special case all of its functions correspond to those of the smart fortwo with a petrol engine. There is no regenerative braking through the generator in such a situation, but the engine’s drag torque (“engine braking effect”) is ready to decelerate.

Hybrid drive has everything under control

The only way that the close cooperation between the combustion engine and the electric motor manifests itself to the smart crosstown driver is in the improved comfort and lower running costs. The drive type is determined by the system itself – depending on the situation – in order to ensure the best possible performance. For this, the driver’s wishes, which are interpreted via the accelerator pedal, gearshift lever or brake pedal, are just as important as the engine speed, driving speed and driving dynamics, the road condition and topography, and the charge state and temperature of the electrical system.