Several alternative technologies have been proposed to replace the car's internal combustion engine; some companies are already marketing hybrid gasoline/electric hybrids, though these have their shortcomings. Jon Severn reports on a mild hybrid that makes no compromises, yet it drastically cuts fuel consumption and emissions.

Most of us, as consumers of the automotive industry's products, would like to see the next generation of cars have improved performance, driveability that is better or no worse than what we have now, more gadgets, lower fuel consumption, and a price that is lower. Largely on our behalf, the industry and legislators would also like future vehicles to generate reduced emissions, though there is an ongoing argument over where the balance should be between CO2, NOx and, for diesel-engined cars, particulates; Europe and Japan agree rather more closely with each other than with the USA.
Some commentators proclaim that the future lies with hydrogen-powered fuel cell cars, whether the hydrogen is generated onboard or stored in tanks; meanwhile, others prefer LPG (liquid petroleum gas) or CNG (compressed natural gas). Honda and Toyota have already committed to the hybrid gasoline-electric option, with their Insight and Prius models, respectively. But there is one underlying view that is gaining support from an ever-wider audience: achieving significantly lower emissions will be very difficult to achieve by simply making further incremental changes to the conventional gasoline and diesel internal combustion engine.
Looking at the hybrid option, which possibly gives the lowest CO2 emissions when the entire well-to-wheels process is taken into consideration (rather than simply tank-to-wheels), both the Honda Insight and Toyota Prius have shown that excellent fuel consumption figures can be obtained. But there is always a compromise. For the Insight, customers have to accept a two-seater coupe configuration with somewhat quirky styling to achieve low aerodynamic drag; the Toyota Prius also has styling that is not to everybody's taste due to the way large banks of batteries have to be packaged within an aerodynamically efficient vehicle while still leaving a reasonable space available for luggage. Honda's newer Civic hybrid uses the more conventional four-door Civic saloon design to package technology based on that developed for the Insight, but there is no getting away from the fact that a very small engine needs a large bank of batteries for performance and driveability. This leaves two difficulties: location of the batteries and cost.
An alternative that has been proposed, in particular by Ricardo, the automotive consultancy, is the mild hybrid. The basic principle is the engine is downsized and a relatively low capacity battery is used in conjunction with an electric motor to boost torque when required. In reality, engineering such a system is far more complex than that. Nevertheless, Ricardo has been working with Valeo, one of the leading Tier 1 suppliers, to develop the concept as far as a fully roadworthy, thoroughly tested demonstration vehicle (Fig. 1).
Known as Imogen (or i-MoGen, for Intelligent motor generator), the project has proved to be a huge success. It was decided early on to work on a C-segment car because this category of vehicle (which includes the VW Golf and Ford Focus) represents the largest single segment, one-third, of the 15 million-car automotive market in Europe. As a baseline the project team selected an Opel Astra 2.0-litre diesel model, partly because Ricardo had already developed a high-performance downsized diesel engine that could be readily installed, and partly because the car has electrically-assisted power steering - which suited the overall concept.
The engine used in the Imogen vehicle is a turbocharged 1.2-litre common-rail, direct injection diesel with high-pressure injection and a relatively low 17.5:1 compression ratio. Power output is 74kW (100PS) and the peak torque is 230Nm, both of which are identical to the figures for the 2.0-litre that it replaces. Interestingly, when the electrical boost is added, the top-gear acceleration for the Imogen car is better than that of the baseline car.

Integrated starter/alternator

Mounted on the flywheel is the Valeo-developed water-cooled integrated starter/alternator (ISA). Thanks to an additional crankshaft bearing, the air gap between the rotor and stator has been maintained at just 0.45mm, thereby helping to improve the machine performance. The overall length of the ISA is only 62mm, but because some of this length can be taken up in the bellhousing - which is usually wasted space - the additional length for the engine assembly is as little as 40mm.
Output from the ISA is 45Nm at 1000rpm, reducing to 30Nm at 2000rpm. When used to boost the engine torque, the ISA can generate 5-6kW of mechanical power, or around 8 per cent of the total power available. Although the Imogen vehicle used an integrated starter/alternator, a production vehicle could employ a belt-driven unit instead, depending on the overall vehicle specification.
An important point about the ISA is that it is used to generate electricity for storage during regenerative braking, with the engine management system controlling the inlet and exhaust valves so as to minimise internal losses and therefore allow the generator to reclaim as much of the vehicle's kinetic energy as possible.
Valeo has worked closely with Saft, which has provided the NiMH batteries that are located in the spare wheel well beneath the boot floor (Fig. 2). The two companies have also collaborated to develop an advanced battery management system that provides state-of-charge and state-of-health information that is required by the supervisory control system. In order to ensure the optimum long term performance from the batteries it is necessary to keep them within a temperature range of 20 to 25ûC, which requires the installation of a simple but controllable thermal management system. One of the aims when building the Imogen demonstrator was to avoid compromising the luggage space. Although this vehicle has the batteries installed in the spare wheel well, a production vehicle could easily have the batteries installed beneath the floor.
Running a 6 kW integrated starter/alternator at 12 volts is clearly unfeasible, so Valeo has moved to 42V in line with the likely future trend for automotive electrical systems.
Having a 42V circuit also meant that a fresh look could be taken at many of the ancillary systems. For example, the water pump and HVAC (heating, ventilation and air conditioning) system are electrically powered. In fact there are no belt-driven ancillaries at the front of the engine, which saves a considerable amount of underbonnet space (Fig. 3).
One of the benefits of a hybrid system is that the engine can be switched off when the vehicle comes to a halt, and the ISA can immediately restart it when first gear is selected. Not only does this improve the emissions performance, but it also helps minimise cabin noise. However, it also makes electrically-operated ancillary systems all the more important, such as the HVAC and thermal management.
Developed by Valeo, the thermal management intelligent system (Themis) makes a considerable contribution to the overall improvement in fuel economy. Whereas a conventional engine cooling system is designed to remove heat from an engine, the Themis system is actually called upon to retain heat in the engine at certain times. For example, during start-up from cold, the engine is required to reach its operating temperature as quickly as possible so that the oil achieves the correct viscosity and the frictional losses and engine wear are minimised. One way in which this is achieved is to operate valves that limit the coolant circulation flowpath and, therefore, the volume of water and its thermal inertia. Once the radiator is switched into the cooling circuit, twin variable-speed fans and a variable-speed water pump help to keep the engine at the optimum temperature. It is claimed the thermal management system has enabled the warm-up time to be halved and contributes 5 per cent to the overall reduction in fuel consumption.

A crucial area of the entire project was the supervisory control system, which was built around a Ricardo VEMPS system. This system is particularly innovative and the methodology is the subject of a patent application. One of the functions of the supervisory controller is to decide in which mode to use the motor/generator; depending on the battery charge, electrical load and other factors, the torque boost may be provided by either the engine or electrical system.
Controlling emissions is not simply a case of engine design and engine management; exhaust aftertreatment is also important. Carbon dioxide and NOx emissions are an issue for both gasoline and diesel engines, but diesels also produce soot particles that need to be dealt with. There is traditionally a trade-off between NOx and particulates and the Ricardo engineers decided to develop the engine for low NOx and then use exhaust aftertreatment for removing the particulates. Fortunately, with a reasonably large 42V electrical power supply available, the opportunity arose to use an innovative electrically heated diesel particulate filter (DPF).

FUEL CONSUMPTION PENALTY

Although the DPF gives a fuel consumption penalty of around 0.5 per cent, this compares extremely favourably with alternative technologies such as a lean NOx trap where the penalty can be up to 4 per cent. For the two minutes or so when regeneration in the PDF is required (which only occurs once every several hundred kilometres), there is initially a heavy demand for electrical power to initiate the reaction; when the soot starts to burn, the reaction is largely self-sustaining. However, the diesel inlet is also throttled to help raise the exhaust temperature to around 550ûC, so other combustion parameters have to be changed to ensure that the driver does not notice that the regeneration is taking place. All of this complex process is managed by the supervisory control system.
After all the development, the results are certainly impressive. With uncompromised performance, handling, driveability and accommodation, the Imogen car achieves fuel economy of 3.98litres/100km in the NEDC cycle (a 28 per cent improvement over the baseline vehicle) with tailpipe emissions that are half of the Euro 4 standards that will come into force in 2005. The car has now completed over 1000 test-drives in Europe, the USA, Japan and Korea with complete reliability of the hybrid systems. Drivers who are used to C-segment cars do not tend to notice any difference from driving a conventional ICE-engined car, the importance of which should not be underestimated.
If mild hybrid vehicles are to enter production it is most likely that they would be sold alongside conventional cars, with customers having the choice of either a standard or hybrid powertrain (Fig. 4). Because of the downsized engine and limited requirement for costly batteries, the additional cost for a mild hybrid would remain low - perhaps less than the cost of an aftermarket LPG conversion - and the reduced fuel consumption could enable the hybridisation to pay for itself easily within the typical period of ownership (of course, this will always depend on the usage pattern).
The future for mild hybridisation looks bright, perhaps brighter than that for fully hybridised vehicles. Following the success of the Imogen project, it appears that the automotive industry is now taking a great interest in no-compromise mild hybrids.