You will find the figures mentioned in this article in the German issue of ATZ 09I2007 beginning on page 734.

Forschungsfahrzeug Efficient-C als Diesel-Vollhybrid

Research Car Efficient-C as a Diesel Full-Hybrid

Author:David Greenwood

One of the winning proposal vehicles at the “Ultra Low Carbon Car Challenge” was born by collaboration between Ricardo UK, PSA Peugeot Citroën and QinetiQ. Completed in mid-2006, the “Efficient-C” project resulted in a pro-duction-feasible diesel full-hybrid vehi-cle based on the family car Citroën Ber-lingo. Without compromise to passen-ger comfort and options, the research car achieves only 99 g/km CO2 emis-sions based on the NEDC.

1 Introduction

In 2003 the UK Government launched the “Ultra Low Carbon Car Challenge” (ULCCC), which sought proposals from industrial consortia to demonstrate the feasibility of an affordable, family sized ultra-low carbon car. One of the winning proposals selected for part-funding was a collaboration be-tween Ricardo UK, PSA Peugeot Citroën and QinetiQ. Completed in mid-2006, the Effi-cient-C project resulted in a production-fea-sible diesel full hybrid vehicle based on the Citroën Berlingo. Without compromise to passenger comfort and options, the Effi-cient-C demonstrator car emits only 99 g/km CO2 based on the NEDC and has more power than its equivalent diesel produc-tion model.

The Efficient-C project commenced in February 2004 with an initial feasibility study followed by an intensive vehicle devel-opment and test programme. The baseline vehicle chosen was a Citroën Berlingo mult-ispace – a vehicle slightly larger than typical

for a C-segment car, based on a light van platform and incorporating a high roofline. The selection of this vehicle was intention-ally made to demonstrate the effectiveness of the technologies to be deployed in a chal-lenging environment. The Berlingo’s power-train is however identical to smaller saloon models, opening the prospect of further ef-ficiency gains if the hybrid powertrain is transferred to a lighter and more aerody-namic vehicle (where its CO2 emissions would be even more impressive). To comply with the stringent fuel efficiency and CO2 emissions targets, all vehicle systems were subjected to intense scrutiny for possible en-ergy savings. A further important design imperative was to maximise commonality with non-hybrid variants, thereby reducing manufacturing costs.

2 Project Partner Responsibilities

The Efficient-C project comprised three partners: Ricardo UK was project leader and

ATZ 09I2007 Volume 1096

contributed programme management and hybrid vehicle systems integration expertise. It provided its own experience in hybrid-ve-hicle systems – including development of powertrain and vehicle electronics. Its own “rCube” rapid prototype tool was used as the basis of the supervisory vehicle control system. The company was also responsible for demonstrator vehicle build and the se-ries production cost-benefit analysis.

PSA Peugeot Citroën provided its exper-tise in vehicle architecture. The OEM also provided the base vehicle hardware, hybrid powertrain components and engineering support to the integration of hybrid tech-nologies, including powertrain, energy storage and associated control systems. The partner QinetiQ contributed expertise in the areas of energy storage, battery man-agement and high voltage wiring systems.

3 Powertrain Architecture

The overall architecture of the Efficient-C diesel hybrid powertrain is shown in Figure 1 and Figure 2. The 1.6-litre HDi engine of PSA Peugeot Citroën adopted for the demonstra-tor car was identical to that of the baseline product Citroën Berlingo Multispace. This is a highly efficient turbo-charged and inter-cooled diesel engine with high-pressure common-rail fuel injection and cylinder head and cylinder crankcase both made from aluminum. It delivers 69 kW (92 bhp), complies with Euro IV standards, and with the addition of a diesel particulate filter for this application, it has barely measurable particulate emission levels.

A 288 V brushless, permanent magnet DC electric machine, with a maximum power output of 23 kW and capable of 130 Nm of peak torque is put into the transmis-sion input. The core of the hybrid system, this machine enables electrical torque as-sist, efficient electrical power generation, regenerative braking and full electric vehi-cle (EV) operation at low speed. The ma-chine is fitted directly to a lengthened in-put shaft on the transmission so that it can also be used to synchronise shaft speeds during gear shifts.

An automated version of a standard five-speed manual transmission was chosen for the car, using electro-hydraulic clutch ac-tuation and electric gearshift actuation. This simple set-up is cost effective and also more energy efficient than other options such as a constantly variable transmission (CVT) or a conventional automatic trans-mission. The gear ratios and final drive were modified for the increased torque and

the control software automatically selects the most efficient gear ratio.

4 Energy Storage

Lithium-ion technology was selected for the battery since it has a higher power and energy density than currently available nickel metal hydride batteries. It is also more efficient in operation, retains better performance at low temperatures, and holds its charge for longer. A 288V lithium-ion battery, comprising 80 cells and ar-ranged in 8 modules, was sited together with a sophisticated battery management system to monitor cell voltage, state-of-charge and temperature under the vehi-cle’s luggage compartment, Figure 3 (actu-ally the area for the spare wheel). Battery cooling was achieved through an array of fans drawing air from the passenger com-partment. Integrated within the battery pack is a fail-safe battery disconnection sys-tem linked to the vehicle crash sensor to ensure immediate isolation of the high voltage system in the event of a collision.

In addition to the main electric ma-chine, a three-phase 12-V starter-alternator was incorporated. Based on a PSA Peugeot Citroën series production unit, the starter-alternator benefits from increased power rating to meet the starting torque require-ment of the diesel engine. The starter-alter-nator is used to start the engine smoothly and quickly during the transition from the electric to the hybrid operating mode, without interrupting the torque delivery to the wheels.

5 Supervisory Vehicle Control System

Given the sophisticated power manage-ment architecture, it is unsurprising that almost half the project time went into per-fecting an advanced supervisory vehicle control system. Based on the Ricardo “rCube” rapid prototype tool, Figure 4, the system co-ordinates the many vehicle sys-tems to meet driver demands whilst opti-mising fuel economy. More than 70 MB of bespoke control code were written for the project,. Ease of operation is enhanced by the provision of all required driver infor-mation on a large, centrally-mounted graphical interface.

The demonstrator car Efficient-C is fit-ted with a standard hydraulic braking sys-tem for reasons of safety, cost and time; and a 12-V vacuum pump provides braking assistance during all-electric operation. To

minimise energy loss through the conven-tional braking system, the vehicle reclaims braking energy by using the electric ma-chine to convert vehicle kinetic energy into electrical energy for storage in the battery. Power steering must also remain function-al during all-electric mode operation, and for this reason a 12-V electro-hydraulic sys-tem is provided and offers efficiency bene-fits by providing assistance only when re-quired.

The regenerative braking of the Effi-cient-C is one of the most important effi-ciency-saving features of the innovative powertrain design, enabling the partly re-cycling of the vehicle’s kinetic energy. When the driver lifts off the accelerator, the engine is shut down and de-clutched. Light regenerative braking is applied to replicate the effect of engine braking. Once the brake pedal is applied the amount of regenerative braking is in-creased, according to a position sensor on the brake pedal. Only when the electric machine – operating in generator mode – reaches saturation do the hydraulic brakes come into action. Under normal city driv-ing conditions, almost all braking is done electrically.

6 Cooling and Air Conditioning

A small electric pump maintains flow in the main coolant circuit for cabin-heating and for cooling the engine immediately af-ter it is switched off. To ensure the opti-mum efficiency and durability of all power electronic systems and the e-machine, a separate low temperature cooling system (at approximate 40 °C) is fitted. All system cooling requirements are coordinated by a central control unit, which also controls the main cooling-fan. Power for these func-tions is provided at 12 V by a DC/DC con-verter from the 288-V system – a more effi-cient method than direct generation at low voltage.

Passenger comfort in Efficient-C is en-sured by the provision of a 288 V electri-cally powered air conditioning system, which matches the performance of its conventional counterpart but also deliv-ers efficiency benefits by providing air conditioning only to the level required. Unlike an engine driven air conditioning compressor, this system is able to func-tion fully during electric operation. For the continuous operation of all electric functions, the provision for charging the battery from the diesel engine while sta-tionary is an essential feature. All addi-

ATZ 09I2007 Volume 109 7

COVER STORYAlternative Drives

tional components have been incorporat-ed without encroaching on the passenger or luggage space of the standard Berlingo, Figure 5.

The operating modes of Efficient-C can be divided in six points:1. Diesel engine only – the most efficient

mode for high speed cruise conditions.2. Generation – in which the motor ab-

sorbs torque from the engine and uses this to generate electrical to be stored in the battery for future use. The diesel en-gine is thereby pushed into its zone of optimum operation, see Figure 1.

3. Boost for acceleration – in which the motor draws power from the battery to deliver torque to wheels in addition to that provided by the diesel engine.

4. Electric drive – used for pull away and low-load operation. This zero emissions mode may also be selected by the driver for urban stop-start driving with a range of up to 10 km.

5. Regenerative braking – with the engine off, the vehicle’s kinetic energy is cap-tured by the motor and stored in the battery.

6. Stationary generation – allows to engine to charge the battery, permitting the long term stationary use of electrically-driven features such as air conditioning.

7 Driving the Efficient-C

To maximise efficiency, Efficient-C has six different driving modes (see box). At low load a zero emission, electric traction mode is selected with the vehicle run directly by the electric machine using only battery power. This mode of operation can be main-tained under light load cruising conditions at speeds of up to 50 mph. At higher loads but where demanded torque is still lower than the point of peak engine efficiency the electric machine acts as a generator; with the additional load pushing the diesel engine into its operating region of opti-mum efficiency, Figure 6. In addition, the hybrid powertrain can be used to provide a boost to performance over and above that of the baseline engine, with the electric machine drawing energy from the battery to deliver additional torque to the wheels.

A regenerative braking mode enables energy which would otherwise be lost through the brakes to heat to be captured and stored in the battery for subsequent use. Finally, provision is made for the using the diesel engine and electric machine to generate electrical power and hence charge the battery while the vehicle is stationary.

8 Superior Performance, Reduced CO2

The final figure for tail pipe CO2 emissions of the Efficient-C vehicle based on the NED cycle is 99 g/km. When the size of the vehi-cle is taken into account, this represents truly class-leading performance, which – if transferred to the compact car segment such as the Citroën C4 or the Peugeot 307 – would fall to around 90 g CO2 per km. Compared with the standard diesel driven Berlingo, this represents a 30 % improve-ment in efficiency, and under urban driv-ing conditions – in which reduced emis-sions are a greater imperative – the increase in efficiency rises to 45 %. Of the overall 30 % improvement, 6 % comes from reduced rolling resistance and slightly improved aerodynamics, 4 % is contributed by the en-gine’s stop-start system, 8 % by the optimi-sation of the engine’s operating regime, and 12 % comes from the storage and re-use of regenerative braking.

Not only is the Efficient-C a class-leading vehicle in terms of emissions but it actually outperforms the conventional model in its top speed (171 kph) and acceleration (from 0 to 100 kph in 13.4 s). The vehicle is fully viable for mass production but is estimated to have an incremental manufacturing cost of approximately 4400 Euro (3,000 GBP) above the standard model. PSA Peugeot Cit-roën estimates that this hybrid technology would be commercially feasible for mass market application at a price increment of around the half of this level. With advanced development into hybrid drive technology, particularly in the area of energy storage, Ricardo estimates that this could become market feasible in less than five years.

COVER STORY

ATZ 09I2007 Volume 1098

Alternative Drives