Beloved by politicians keen to show their green credentials, the Toyota Prius is the car other hybrids are judged against, but it may not be the most efficient around.

The Prius returns 65mpg and emits 104g/km CO2.

Two years ago West Sussex automotive design firm Ricardo led a team including UK technology consultancy QinetiQ and PSA Peugeot-Citroen to create Efficient-C, a full hybrid diesel demonstrator vehicle based on a 52mpg Citroen Berlingo Multispace, which emits 99g/km CO2 and runs at 75mpg. A lot was learned.

Overall, Ricardo’s chief engineer Dave Greenwood is pleased with the hybrid configuration chosen and the components selected to implement it. However, he would take a different approach to the battery and the gearbox.

Expensive chemistry
The original design used a 288V 2kW Li-ion battery. Its voltage is a trade-off between high voltage to reduce I2R losses everywhere and to cut the size of the motor, and a low voltage to cut cost by reducing the number of cells in the battery and associated per-cell temperature and voltage sensing.

The capacity is dictated by the specified range of the car in all-electric mode, and the cell chemistry is dictated by the compact nature and low losses of Li-ion technology.

According to Greenwood, the voltage and capacity are about right, but the chemistry is too expensive.

The incremental cost of the Efficient-C system, applied to Citroen’s C4, was calculated to be €3,000. Based on fuel prices and consumer spending power then, the consortium estimated a commercial system would have to be priced around €2,000.

“The battery is at least a third, and often two thirds of the incremental cost of adding hybrid operation to a car,” said Greenwood. “We have started a programme called Red-Lion, for reduced-cost lithium ion, with ­QinetiQ and some other partners.”

Red-Lion is looking at lithium iron chemistry as an alternative to the lithium cobalt and lithium manganese used in laptop and phone Li-ion cells.

“The battery chemistry has been developed by QinetiQ and gives the same performance as conventional Li-ion batteries, but is less costly,” said Greenwood. “At Ricardo we are taking these cells to an automotive specification, looking at internal cooling, cell mounting, and packaging.”

The need for a different gearbox is more subtle. Connecting the motor directly to the gearbox on the engine side allows it to drive the wheels through the gearbox.

“The motor is a straightforward 18kW [23kW peak] permanent magnet DC design,” said Greenwood.

If it had to drive straight to the wheels, without the five-speed gearbox in between, it would have to operate over a wider speed and torque range and would have to be larger.

Efficient-C has regenerative braking. To recover maximum energy during most regenerative braking events, this smaller motor has to use more than one of the five gears.

“You can capture a greater proportion of the braking energy, otherwise you would be limited by the maximum torque of the motor,” said Greenwood.

With the gearbox as it is, changing gear during braking is not a smooth process and has been judged unacceptable to occupants. Instead, an algorithm selects a single gear to be used throughout the braking event.

If two clutches were added between the gearbox and the wheels, one connected to the odd gears and one connected to the even gears, both clutches can be slipped briefly during a gear change. This would allow smooth multi-gear regenerative braking, with the added advantage that the extra clutches can also smooth normal engine-powered gear changes.

“Like the Audi DSG system, you get the efficiency of a manual gearbox with the smoothness of an automatic,” said Greenwood.
Ricardo is said to have developed a direct shift gearbox similar to that used in the Bugatti Veyron.

As it stands, the Efficient-C system was designed with thoughts of introducing it commercially in the aerodynamic Citroen C4 where it would return an estimated 90g/km CO2.

However, changes in world finance could mean PSA Peugeot Citroen has first to add hybrid operation to more expensive cars. We will find out in two years. “The group plans to reveal its first HDi hybrids by 2010,” said the car firm.