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The 2008 Audi Q7 Hybrid?
Audi possesses a wealth of experience in the field of hybrid drives – the brand with the four rings began developing hybrid cars as early as 1989. Today, the development engineers' concentrated know-how can be seen in the Audi Q7 hybrid.
The concept of the Audi Q7 hybrid is a full hybrid vehicle in parallel configuration – a concept that Audi has been pursuing for many years. This principle, in which all major assemblies are linearly situated, has great benefits compared to a series configuration, such as the concepts used by the competition. There, the complex system of electric motor, separate alternator and planetary gears has drawbacks not only in terms of weight, but also in terms of energy loss. Functions such as "sailing" (coasting without power), which parallel hybrids are able to realise, are not possible with series hybrids. In certain situations the electric motor actually has to build up braking force to regulate the driving current.
Audi's hybrid drive uses a 3.6 litre FSI V6 with an output of 206 kW (280 bhp) and a peak torque of 375 Nm. An electric motor situated in the drive train adds up to 285 Nm of additional torque. It is a matter of honour for such a supremely powered Audi to be equipped with quattro four-wheel drive, which permanently ensures a maximum of driving dynamics and road safety. Power is transmitted via a six-speed tiptronic gearbox.
The driving performance of the Audi Q7 hybrid is equal to that of a sports car. From standstill, 100 km/h is reached in just 7.6 seconds. And the pulling power is even more impressive. In fifth gear, the large SUV powers its way from 60 to 120 km/h in just 11.9 seconds. In normal driving conditions, the Audi Q7 hybrid, weighing 2,480 kg unladen, consumes on average 9.8 litres of fuel for 100 kilometres. That is 23 percent less than the basic model. The efficiency of the Audi hybrid drive can hardly be demonstrated more convincingly.
Under the bonnet, the 3.6 litre six-cylinder engine is no different from a standard engine. It is equipped with the highly efficient FSI petrol direct injection system. This enables it to develop more output and dynamism than a conventional engine with manifold injection. The V6 is characterised by a beefy torque curve right up to the rated speed, agile response and economical fuel consumption.
A number of modifications in the periphery were necessary for the 3.6 FSI to be used on the Audi Q7 hybrid. The A/C compressor, the oil pumps for the power steering and automatic gearbox and the vacuum pump for the brake booster are all electrically driven. This makes their operation independent of the combustion engine. The new electrohydraulic power steering has the advantage that it needs almost 90 percent less fuel than a conventional system. The vehicle electrical system receives its 3 kW of power from the drive battery via a voltage transformer.
The electric motor, which generates 285 Nm and 34 kW, has a diameter of 330 millimetres and a length of 55 mm. It is located between the V6 and the torque converter of the automatic gearbox. This configuration ensures maximum economy when it comes to space, allowing the complete space in the passenger cell to be used without restriction.
The challenge of transition
A separating clutch links the three-phase synchronous motor with the engine. This makes it possible to drive the car with either of the power plants or with both together. Concentrated know-how can be found in its control system. Within very short periods of time, this has to switch two powerful power plants on, off and together without these operations being noticed. This was resolved by briefly increasing the torque of the electric motor and opening the torque converter lockup clutch.
Like the standard basic vehicle, the Audi Q7 hybrid also has space for three rows of seats and a large luggage compartment. The battery system that supplies energy to the electric motor has a power capacity of 38 kW and is located in the spare wheel recess at the rear of the vehicle.
The nickel metal hydride (NiMH) battery measures 347 x 633 x 291 millimetres and is made up of 240 cells. Each of these works with 1.2 volts, giving a total voltage of 288 volts. Fully charged, the battery is able to store 1.7 kWh of energy. A fan ventilates it with cooled air from the vehicle interior to make sure that it remains in the optimal temperature range.
Compared to earlier generations of hybrid vehicles, the electric motor and the battery are extremely lightweight. The battery weighs 69 kg and the hybrid module with the electric motor 40 kg.
Complex control electronics coordinate the two power plants. These primarily react to the current speed and the commands given by the driver, which the system receives via the accelerator and brake pedals or the tiptronic selector lever. Other important parameters are the charge state of the battery and information received from the wheel sensors, which supply the electronic stabilisation system (ESP) with information about brake operations and the current driving situation.
The electronics autonomously decide about the mutual interaction between the drive components, and implement the driver's commands in a perfect balance of sportiness and efficiency. A menu in the display of the Audi MMI (multi media interface) shows the current operating status.
Separate or combined forces
Fundamentally, three driving modes are possible. The FSI and the electric motor can each work on their own as the drive unit, or the vehicle can combine the power of the two for acceleration. In this mode, the petrol engine is responsible for basic operation, whereby it also needs to supply energy to the battery.
The electric motor on the other hand can be used for speeds up to 50 km/h – i.e. in city traffic – alone and almost inaudible. This can considerably reduce exhaust and noise emissions in built up areas.
The capacity of the battery allows the vehicle to be driven up to two kilometres on purely electrical power – coasting and braking feed additional energy into the system. When the capacity limit of the battery has been reached, the combustion engine is activated to recharge the battery.
The output of 206 kW / 280 bhp and the torque of 375 Nm that the 3.6-litre FSI engine develops are enough to accelerate the standard Audi Q7 from 0 to 100 km/h in 9.3 seconds. If the driver wants to accelerate even faster, the electronics also activate the electric motor to make use of the decisive advantage that the electric motor has: Its torque, unlike the combustion engine, is available as soon as it pulls away.
More fun accelerating
In practice, this torque boost provides a new dimension to acceleration, especially in the lower speed range. Over the first four metres, the Audi Q7 hybrid leaves its opposite number with the V6 petrol engine a full metre behind. In the sprint from nought to 100 km/h, the "boost" from the electric motor gives it a lead of 1.7 seconds.
And when it comes to pulling power, important when overtaking, the additional drive gives impressive assistance. The "boost" is of course automatic and cuts in as soon as the driver depresses the accelerator pedal beyond a certain, slight resistance. The necessary current comes from the battery in the rear.
But the electric motor can do more than just provide extra power. It is also able to feed kinetic energy back into the system and so recharge the battery when the vehicle is braked or coasting. During this operation, known as recuperation, the motor reverses its function in a flash to become an alternator.
Most energy is recuperated when the Audi Q7 hybrid is decelerated from low speed (under 60 km/h) with a rate of deceleration of about 3 m/s2 – a situation that is frequently encountered in city traffic.
The driver notices this no more than he notices another function that also helps to make the new drive system so efficient. If the Audi Q7 hybrid is coasting without the accelerator pedal being depressed, the combustion engine is switched off – this "sailing" is possible at speeds up to 120 km/h.
The six-cylinder engine is also switched off if the vehicle remains stationary for more than three seconds. To continue the journey, all the driver has to do is to remove his foot from the brake and to depress the accelerator pedal again. The engine resumes service immediately. The powerful electric motor will then play the part of the starter motor. This makes it possible to start particularly fast and yet quietly and comfortably.
The intelligent energy management system uses these strategies to achieve a high degree of efficiency in the drive system, making sure that the battery is not overloaded. The hybrid vehicle is able to demonstrate its potential for saving most effectively in city traffic. With an average mileage of 20,000 km per year, the Audi Q7 hybrid recuperates about 720 kWh of energy a year – approximately a sixth of what a four-person household uses.