Citroën at the 2007 Internationale

Automobil-Ausstellung in Frankfurt

The C4 Hybride HDi boasts low fuel consumption (3.4 l/km) and low CO₂ emissions (90 g/km). At the same time, it maintains the renowned driving pleasure of HDi engines, backed up by the specific advantages of the hybrid powertrain, such as all-electric mode at low speeds. 

This demonstration vehicle is based on cutting-edge technology already available in the Citroën range, including: the HDi engine, particulate filter (DPFS), Stop & Start and the six-speed electronic gearbox system. These technologies work together to reduce fuel consumption and pollutant emissions.

At the same time, the styling of the C4 is based on a clear architectural choice: limited height for optimised aerodynamics and fuel consumption. 

The C4 also has a wealth of other advantages, in terms of styling, technology content and road manners. Citroën has sold over 640,000 C4s worldwide since launch in November 2004, in a market segment accounting for a third of all vehicle sales. 

The C4 Hybride HDi is a medium-term solution that will enable Citroën to bring real progress in fuel consumption and CO₂ emissions and to make these benefits available to the greatest number in a European market where diesel vehicles have strong presence.

The C4 Hybride HDi combines a hybrid drive train and a HDi diesel engine to bring decisive progress in fuel consumption and CO₂ emissions. Petrol/hybrid technology does not significantly outperform HDi diesel technology in terms of fuel consumption and CO₂ emissions. 

The work carried out by PSA Peugeot Citroën has shown that a petrol hybrid car consumes as much as fuel as a car equipped with a conventional diesel engine with identical characteristics (such as weight, dimensions and Cd.A). At the same time, the petrol version cuts CO₂ emissions by just 10%. Yet petrol hybrids cost a great deal more than their diesel equivalents. 

High-tech (HDi) diesel engines have enjoyed growing success on the European market since the end of the 1990s. Today, diesel engines are fitted on one in two passenger vehicles on average, compared with one in four in 1998. In some countries, such as France, diesel vehicles accounted for 70% of sales in 2006. This continuous growth in sales shows that consumers are keen to buy low-consumption vehicles that make no trade-offs in driving pleasure and that are compatible with their purchasing power.

Also, with the additional benefits of the particulate filter (DPFS), diesel engines are extremely impressive environmental performers. Citroën has sold more than 500,000 vehicles equipped with the DPFS to date.

This being so, it is understandable that so few petrol hybrid vehicles have been sold in Europe, despite the tax advantages offered by some European Union member states.

Advantages of the HDi engine

The C4 is an all-round performer, at ease in the city, on the road and on the motorway. Combining a hybrid system with a HDi engine, it allies the outstanding fuel economy of the HDi diesel engine (consuming 25% less fuel than a petrol engine), used over its best operating range, with the advantages of an electric motor, suited mainly to city driving. 

In the city and on the open road, the hybrid system brings real gains in consumption (3.4 l/100 km over a combined cycle) and CO₂ emissions (90 g/km). A diesel hybrid vehicle outperforms an equivalent vehicle fitted with a petrol hybrid system by 25%, or one litre every 100 km over a combined cycle, for a significant increase in range.   

The intrinsic qualities of the HDi engine, mated to the high-performance six-speed electronic gearbox system, improve fuel consumption by 25% on the motorway.   

Furthermore, no sacrifices have been made in terms of driving pleasure, which remains a priority for motorists. The C4 Hybride HDi fulfils this requirement, while offering motorists additional driving pleasure in two areas: 

• first, in the city and at low speed, the combustion engine goes into stand-by, functioning entirely in electric mode (ZEV: Zero Emission Vehicle), for increased acoustic comfort and vibration control.
• second, the C4 Hybride HDi responds to driver input to provide far better acceleration than a combustion engine of equivalent power. The driver can thus gain as much as 23 kW of extra power generated by the electric motor, compared with the combustion engine alone.  

C4, an aerodynamic model of appropriate qualities

The C4 Hybride HDi is a medium-term solution that will enable Citroën to bring real progress in fuel consumption and CO₂ emissions and to make these benefits available to the greatest number. 

The C4 is the Citroën vehicle best suited to developing hybrid HDi technology. It is a family car, designed for a wide variety of uses (city, open road and motorway), which fulfils the key requirements of the European market. Citroën sold over 190,000 C4s in Western Europe in 2006 in the most popular market segment, accounting for more than 30% of total vehicle sales. 

The C4 is designed for outstanding aerodynamic performance, one of Citroën’s traditional strengths. It has an excellent drag coefficient, a mere 0.28 for the coupé version – the model chosen for the C4 Hybride HDi demonstration vehicle. This achievement contributes directly to reducing fuel consumption and CO₂ emissions. 

Lastly, the C4 has all the qualities needed to satisfy a wide range of customer expectations. 

The C4 is available as both a hatchback and a coupé. Each model has its own strong and distinctive styling, but they share the same generous space. The hatchback has rounded forms, whereas the coupé favours a sharper line. 

The C4 passenger compartment is designed to be both comfortable and relaxing. It combines understated high-tech interior styling – witnessed in the pure lines of the dashboard – with innovations that are both user-friendly and ergonomic.  

The multifunctional fixed-centred controls steering wheel brings the main comfort functions and driving aids within easy reach. It is associated with an innovative instrument cluster that displays information on a translucent strip in the centre of the dashboard. 

In addition to its impeccable road manners, the C4 offers a wealth of modern and efficient driving aids. These include Xenon dual-function directional headlamps, tyre pressure detector, and the lane departure warning system (LDWS), which, on a motorway or a fast road, warns the driver if the car crosses a continuous or dotted lane marking, if the indicator is not activated.

Concerning occupant protection, the C4 received five stars in EuroNCAP impact tests. The C4 is fitted with six airbags, and also boasts the world's first shape-optimised driver airbag, a feature made possible by the fixed central section of the steering wheel, which protects the driver's chest more effectively. The C4 obtained excellent results in pedestrian impact, with three stars out of a possible four, and in child protection, with four stars out of a possible five.

The C4 Hybride HDi relies extensively on the cutting-edge technology already used in the Citroën range, including: the HDi engine, particulate filter (DPFS), Stop & Start and the electronic gearbox system.

The Hybrid HDi also features an electric motor, a power inverter, a high-tension battery pack and a regenerative braking system.

These technologies work together to reduce fuel consumption and pollutant emissions.

HDi engine and DPFS 

The C4 Hybride HDi is equipped with a 1.6 HDi engine developing 66 kW EEC (92 bhp DIN) at 4,000 rpm. It supplies torque of 215 Nm at 1,750 rpm.

Citroën modified the engine to make it compatible with a hybrid drivetrain.

In particular, it has a dedicated control system, using operating instructions coordinated directly by the powertrain management unit. These instructions control engine stops and starts, while delivering the torque required by the driver.

To ensure outstanding emission performance, the engine has been fitted with a new-generation particulate filter (DPFS) that exceeds the Euro IV standard.

The trade-off between NOx emissions and fuel consumption has been improved, using the degree of freedom provided by the hybrid system. Citroën was thus able to use the capabilities of the electric motor to manage engine shutdown time and to use the combustion engine at its most efficient operating points.

Electronic gearbox system

The C4 Hybride HDi benefits from gains in fuel savings provided by the six-speed electronic gearbox system. This gearbox is already available on the C4, C4 Picasso and Grand C4 Picasso. 

The electronic gearbox system fitted is an automated version of a manual shift without a clutch pedal. The gearbox’s hydraulic control system shifts gears more quickly, thus enhancing driving pleasure. At the same time, it continuously manages the powertrain by selecting the best operating point in real time. The gearbox thus delivers ensures the best trade-off between consumption, acceleration, braking, driving pleasure and emission control. And it leaves the driver free to choose automatic or sequential mode for gear changes. 

Stop & Start system

Following the C2 and C3, the C4 Hybride HDi is equipped with the Stop & Start system which, in the hybrid powertrain, serves to restart the combustion engine. 

On C2 and C3, Stop & Start technology cuts the engine just before the vehicle comes to a standstill (traffic lights, traffic jams, etc.), and remains in standby while the driver keeps a foot on the brake pedal. It starts again automatically and instantaneously as soon as the driver takes his or her foot off the pedal to move forward again. These micro-hybrid vehicles cut fuel consumption and CO₂ emissions by around 10% over an urban cycle and 6% over a combined cycle. 

On the C4 Hybride HDi, the Stop & Start system boasts 40% more power than the first generation system, to restart the HDi 92 engine.

In addition, while the Stop & Start function on the C2 and C3 only kicks in when the vehicle stops and restarts, it can be activated at any time on the C4 Hybride HDi if the vehicle is travelling at less than 60 kph.

When the driver accelerates hard, the Stop & Start system immediately activates the combustion engine.

Electric motor and power inverter

The electric motor develops 16 kW of continuous power for 80 Nm of torque. It provides peak power of 23 kW for 130 Nm of torque to meet occasional demand from the driver. Citroën has made a design trade-off between size and performance to make the technology compatible for use in the city.  Power levels were set to ensure that the vehicle can run in electric mode for speeds of up to 50 kph, typical of city driving conditions. The hybrid powertrain control system selects this mode whenever conditions – in particular the battery charge level – permit. 

The electric motor uses synchronous permanent magnet technology, offering the best size-performance ratio currently available. Connected to the power inverter, it operates in a voltage range of between 210 and 380 volts. Given the limited space available, the electric motor and power inverter cannot be used with a conventional engine cooling system since its temperature is typically too high. Water cooling process is thus provided by a special radiator and a low-temperature circuit at 60°C.

The power inverter regulates the torque of the electric motor by controlling the current from the battery pack – in the same way as an injection pump adjusts the fuel supply from the tank when the driver accelerates. 

High-voltage battery pack

The high-voltage battery pack is fitted in the boot, where the spare tyre would usually be. This involved making a slight modification to the cut-out of the compartment.   The batteries do not reduce boot capacity.

The high-voltage battery pack consists of 240 Ni-MH (Nickel Metal Hydride) cells,  delivering 23 kW of power at a nominal voltage of 288 volts. The battery cells are cooled by special air intakes that recover air from the cabin, taking advantage of its temperature control.

The vehicle maintains the conventional 12V battery, which continues to handle its usual functions. 

Controlled regenerative braking

When the car decelerates or brakes, the controlled braking system optimises the transformation from kinetic energy to electric energy. This electric energy is stocked in the high-voltage battery pack.

It can then be used in electric mode or to boost the combustion engine during acceleration.

This process provides a "free" source of energy, thus further reducing fuel consumption.

Intuitive and easy to use

Turning the key in the ignition starts the car but not the combustion engine. When the driver accelerates, the electric motor alone can drive the vehicle forward, depending on the pressure placed on the pedal. The combustion engine kicks in when the driver accelerates more emphatically. 

The electronic gearbox system delivers the same level of efficiency as a manual gearbox, while supplying the automation necessary for the hybrid powertrain. The decision to shift gears and the actual change takes place automatically without input from the driver, unless he or she opts for sequential mode. 

All of these functions are managed transparently by the Power Train Management Unit (PTMU). 

The PTMU selects the best operating mode for low fuel consumption, depending on the driver's actions. This is normally achieved by using the electric motor on its own at low speeds and during deceleration, the combustion engine on its own on main roads and motorways, or both when greater acceleration power is required. 

Intelligent braking control

One of the advantages of the hybrid system is its ability to recover kinetic energy from the vehicle for later use. 

When the driver takes his or her foot off the accelerator pedal at a speed of under 60 kph, the combustion engine goes into stand-by and is disconnected. The electric engine then provides engine braking and recovers the kinetic energy from the vehicle.  The PTMU stores this energy in the batteries until they are fully charged.

When the driver brakes, the PTMU controls brakeforce distribution, using electric braking (regenerative) and hydraulic braking (dissipative), while giving priority to safety functions. It also optimises energy recovery to reduce fuel consumption.

The NaviDrive control screen

C4 Hybride HDi drivers receive real time data on powertrain operation via the large colour 16/9 screen of the NaviDrive telematics system. The screen provides clear and immediately understandable on energy transfers between the combustion engine, the electric motor, the battery pack and the wheels.

Additional information is also available, such as the battery charge levels or the power requested by the driver from the combustion engine or electric motor. 

This system makes the driver more aware of how the vehicle works, thus encouraging a more economical driving style. 

Special functions 

• All-electric mode: a ZEV (Zero Emission Vehicle) driving

The ZEV button on the central console lets the driver select an "extended" all-electric mode, optimising the normal operating range of the vehicle in electric mode. 

This allows motorists to drive further and faster than they normally would in hybrid mode, without activating the combustion engine. In this case, the combustion engine is used only for harder acceleration.

This function makes the C4 Hybride HDi a responsible urban citizen. It emits no pollution and no noise, while providing all the occupants with outstanding acoustic comfort.

All-electric mode can be deactivated automatically, when the high-voltage batteries lose their charge, or manually, by pressing a button.

• Sequential mode and dynamic mode

The driver selects sequential mode using the lever of the electronic gearbox system. He or she then shifts gears as and when necessary, according to his or her driving manoeuvres (for example, shifting down to overtake over a short distance).

In this case, gear changes are fast and the combustion engine is in continuous operation.

The “boost” function, which adds torque from the electric motor to the combustion engine, can be used at any time. 

For a more dynamic driving style, the driver can press the button marked “Dy” at the base of the gear lever. In this mode, pressing the accelerator pedal conveys the information necessary to make torque immediately available. At the same time, gear changes are as fast as the mechanics of the system will allow. In this case, gear changes are entirely automatic.

• C4 Hybride HDi: independent in all circumstances

If the high-voltage battery pack of the C4 Hybride HDi discharges – for example, after a long period without use, the driver can still start the car. This is not the case with other hybrid systems.

The Stop & Start system connected to the diesel engine can start from the 12V battery and generate power through the alternator. The vehicle then functions independently, but in “limited” combustion mode. In less than 10 minutes of driving, high-voltage battery pack will start recharging, at which point all vehicle functions are restored.

If a problem (such as an electrical fault) requires the high-voltage battery has to be disconnected, the C4 Hybride HDi can continue to run on the combustion engine alone. This so-called “back-up” operating mode gives the driver the time to find a garage or repair shop.

Citroën and useful technologies

As part of a global environmental protection strategy pursued over a number of years, Citroën has developed a range of efficient and affordable solutions adapted to a wide range of user needs. 

Over the years, the Marque has regularly put forward new ideas and concepts for the future in the shape of demonstration vehicles. They include the Berlingo Dynavolt electric prototype, the Xsara Dynalto, fitted with an alternator-starter, the Xsara Dynactive, a hybrid vehicle, and – most recently – the C4 Hybride HDi. 

• With Xsara Dynalto, unveiled at the 1998 Geneva Motor Show, Citroën presented a volume-production model that could cut fuel consumption by 10% (over a standard combined cycle), with a corresponding reduction in CO₂ emissions. These gains were achieved primarily by the "Stop and Go" system, which automatically switched the engine off and on again when the car stopped in a traffic jam or at a red light for example, or if it remained stationary for any length of time. This function was designed to eliminate noise pollution, particularly in the city. 

This innovation was subsequently brought to market through the C2 and C3 Stop & Start, an affordable first level of hybridisation. The Stop & Start system cuts consumption by 10% over an urban cycle, 6% over a combined cycle, and up to 15% in heavy traffic. 

• Presented at the 1999 Geneva Motor Show, Berlingo Dynavolt was a variant on the electric vehicle and the series hybrid. The idea was to top up electricity taken from the EDF grid and stored in vehicle batteries with extra power produced by an auxiliary generator. Through intelligent energy management, the electric motor could be partly powered by this auxiliary unit when driving outside busy urban areas. 
• Xsara Dynactive, presented at the 2000 Geneva Motor Show, explored new possibilities in parallel hybrid technology. The vehicle was equipped with a 55 kW (75 bhp) petrol engine and a 25 kW (34 bhp) electric motor, mated to an automatic gearbox. With this original architecture, the Xsara Dynactive was particularly impressive in ZEV mode in the city, where it had a range of around 20 km, using the gear ratios of an automatic gearbox.