It seems that hybrid gasoline-electric vehicles (HEVs) may have a future after all and not be the short-term stopgap that many had prophesied. Toyota, in particular, is fully committed to the concept and has made public its aim of selling 300,000 hybrid cars a year by 2005. It has also agreed to supply arch-rival Nissan with its technology for use in the U.S. from 2006, with a sales target of 100,000 units by 2011. Honda is also active in hybrid technology, launching the Insight and then the Civic Hybrid while Ford has prophesied that the Big Three will introduce 250,000 HEVs to the U.S. market over the next five years. By then, it is not beyond the realms of fantasy that the U.S. market could be absorbing around 750,000 units a year, rising to around a million in 2010, figures that could be also be reflected in Europe and Asia, as well. By 2015, HEVs could have a 15% market penetration rising to more than 25% in 2020.
One of the drawbacks of hybrid vehicles at the moment is their cost. While Toyota has claimed that it is already making money on the Prius, the generally held view is that it is almost certainly not making a return on the investment in development. This will come as the volumes grow, but it will take many years.
It is against this background that UK-based automotive engineering company Ricardo and Valeo, a French-based tier-one supplier, have worked together to develop their own cost-effective hybrid technology. Called the I-MoGen–intelligent motor generator–it is a mild hybrid vehicle with 42-volt electronics, regenerative braking, integrated starter alternator (ISA), and a highly complex electronic control system. The challenge they set themselves was to use a mild hybridization systems approach to develop a lightweight, downsized high-speed diesel engine from a donor unit and optimize engine performance for hybrid operation, while matching the driving characteristics of a conventional 2.0-litre turbodiesel. At the same time, it had to achieve anticipated Euro 5 emissions. "Our biggest achievement is to have shown a 28% reduction in fuel consumption whilst retaining the same drivability as the original car," says Richard Gordon, I-MoGen program chief engineer at Ricardo. "While it's easy to reduce the fuel consumption of vehicles, it usually leads to a drop-off in how good they are to drive, how well they accelerate and how they feel on the road. We've beaten this."
The I-MoGen concept integrates five key systems into a three-door Opel Astra–a medium-sized (by European standards) family hatchback. They include: a 2.0-litre diesel engine that has been downsized to 1.2 liters; a 42-volt, 6 kW water-cooled flywheel-mounted integrated starter-alternator; 42 volt ancillaries including the water pump, fans and the heater; a 17-kg, 42-volt NiMH battery; and a control system. The Ricardo-developed 1.2-litre, four cylinder HSDI diesel engine is capable of delivering 74 kW. It has 20% less fuel consumption compared with a 2.0-litre conventional diesel engine and offers possible total weight savings of up to 30%, thereby compensating for the additional weight normally expected in mild hybridization. At 0.0125g/km particulate matter and 0.125g/km NOx, exhaust emissions in the vehicle–as demonstrated on the tested–are half of the future mandated light diesel duty Euro 4 limits due to come into force in 2005.
"We had to make a decision about whether to target low NOx or low soot engine-out," says Craig Goodfellow, the team leader at Ricardo on exhaust aftertreatment. "The opposite strategy of reducing NOx significantly with the aftertreatment would have meant either SCR (selective catalytic reduction) using urea as a reductant, or a lean NOx trap technique. SCR is well proven on heavy-duty engines but the temperature window is poorly suited to lighter vehicles; with lean NOx traps, there are questions about their durability, the ability to run the diesel engine rich, and fuel sulfur levels. We felt it the best overall strategy to control NOx in the engine and then tackle higher levels of particulates with a particulate filter. Thanks to 42-volt electrics, we were able to develop an electrically heated particulate filter that gave us a fuel consumption penalty of less than 0.5% compared to the lean NOx trap, where the penalty can be up to 4%."
Valeo's high-efficiency ISA–with its three key functions of start-stop, torque boost and regenerative braking–is the system that has enabled the downsized diesel engine to have a better performance than a much larger and heavier engine. With its main priority being that of achieving the best power-to-volume ratio at the lowest possible cost, the maximum torque available from the ISA varies from 45 Nm at 1,000 rpm to 30 Nm at 2,000 rpm (between 5 and 6 kW of mechanical power).
A custom-designed supervisory controller, developed using Ricardo's unique and proven VEMPS (Vehicle and Engine Management Prototyping System), ensures that all systems work optimally together. There are five control units on the vehicle: Bosch for the fuel injection; three Valeo controllers for the battery, ISA and the HVAC system; and the Ricardo VEMPS controller running the supervisory control strategies, as well as new engine-related functions such as thermal management and exhaust after treatment. The supervisory architecture allows the other controllers to do all the low-level work with the I/O; they talk in reasonably high-level language to the supervisory control that allowed Ricardo to develop hybrid strategies independent of the low-level work.
Central to I-MoGen's cooling system is Valeo's THEMIS, a system capable of managing and optimizing engine temperature according to driving conditions. With this electronically controlled system, engine warm-up time is reduced by up to 50% which means less engine wear and lower emissions while fuel consumption can be reduced by up to 5% at low engine speeds. THEMIS also enhances cabin comfort, providing heat even after the engine has been switched off.
The innovative 42-volt HVAC system, which only uses energy "on demand," has an electrical compressor and provides significant fuel economy benefits. Additionally, the system is equipped with a 42 volt 1.3 kW cabin air heater that helps to compensate for the low heat rejection of a high-efficiency, downsized diesel engine under warm-up conditions from a cold start.
"We have been able to show that extreme downsizing is possible and that we are able to assist clients to find the most appropriate downsizing approach for them," says Gordon. "This includes all the influences of refinement, turbo lag minimization and packaging improvements. The same principles can also be applied to gasoline engines as well."