Bosch pursues diesel technology.

With the cost of fuel remaining at high levels around the world, the quest to find the most cost-effective solution on how to propel a vehicle remains as intense as ever. In Europe, the diesel engine is increasingly being seen as the answer to reach the desired levels of emissions, fuel efficiency and performance. "The popularity of modern diesel engines with direct injection continues to grow, thanks to their sporty performance--due to their high torque even at low speed--and their low fuel consumption," says Dr. Ulrich Dohle, president of Bosch's diesel systems division. "The low level of C[O.sub.2] emissions which accompanies this low fuel consumption means that diesel engines will continue to be an appropriate power source for modern cars into the future." To this end, Bosch is pursuing a variety of technical developments with the aim of further reducing the level of toxic emissions from diesels in order to make them even more environmentally friendly, while meeting increasingly stringent emission regulations. "Following on from the introduction of the Euro 4 emissions standard, the future Euro 5 standards are at the heart of Bosch's developments," says Dohle. "This new standard, which will come into force by 2010, represents an enormous challenge for the development engineers in relation to emissions of nitrogen oxides [NOx] and soot particulates."

The emission levels for the planned Euro 5 standards have not yet been fully decided by the European Union. However, the proposal is for the maximum permissible level of particulates to be set at one-fifth current levels. This represents an emission level of only 5 milligrams per 0.6 miles, instead of the present 25. "This target would make it necessary for all diesel cars to be fitted with particulate filters," says Dohle, adding the proposed regulations will require a rethinking of the "emissions margin," or safety cushion, would have to be increased to assure the vehicle will continue to meet the emissions standards throughout the life of the vehicle, while also assuring the emissions targets do not reduce the performance and fuel-economy benefits of diesel engines. "The emission of soot particulates, which is having a major impact on the current public debate about diesel engines, can be greatly reduced by the use of a diesel particulate filter [DPF], even though the emissions from diesel-powered passenger cars only make around a three percent contribution to urban pollution particulate levels. In addition to exhaust gas after treatment, Bosch's engineers are also working to lower raw emissions in order to produce the cleanest possible low-toxin combustion process."

Currently, Bosch is working on the fourth generation of common-rail systems which will use a hydraulically amplified diesel injector. This utilizes a special conversion piston which amplifies the pressure in the system and can produce injection pressures of up to 2,500 bar. "This new technology opens up the possibility of working with a significantly lower pressure in the common-rail system itself, with the required maximum pressure being generated in the injector," he says. "We are also aiming at a lowering of harmful emissions through a special geometry by means of which the fuel is injected into the combustion chamber with an increasing pressure, rather than abruptly. This produces an especially favorable injection rate. It improves the preparation of the fuel mixture and prevents the formation of soot and carbon deposits resulting from incomplete combustion. This system, like the current generation, also allows multiple injections." While it has been virtually sidelined by the almost universal adoption of common-rail, Bosch still perseveres with its unit injector system, the latest version permitting a maximum pressure of 2,200 bar and the injection process being able to be varied considerably by using two actuators instead of one. This will make the degree of flexibility in the injection rate comparable with that of the third and fourth generations of common-rail systems. "New, high-speed magnetic valves will be used," says Dohle. "One valve controls the generation and release of pressure, while the other one operates independently to open and close the needle--in a similar way to the control valve of a common-rail injector. That will make it possible to have five variable injections per cycle also in the unit injector system, contributing to low soot and nitrogen oxide emissions."

Bosch is now developing a new control unit, due to go into series production in 2006. "It will enable complex feedback effects between pressure waves in the storage rail and the amount of fuel injected, for example, to be detected and precisely compensated for," says Dohle. Using revolution counters, the system can analyze the injection parameters of each cylinder and adjust them if necessary. In contrast to a control system based on engine characteristics, a model-based approach makes it possible to have a simple control system, which can easily be transferred to a variety of engines and which benefits from the direct feedback from the engine. According to Dohle, the precision and resistance to mechanical wear can be optimized. One of the key components of a modern diesel system is the diesel particulate filter DPF which can trap more than 97% of the particulates. They also help counter one of the challenges of the diesel engine, which is that while the low emissions of nitrogen oxide are part of the engine's profile, they arise in just the opposite way to soot particulates. If the diesel engine is adjusted for low particulate emissions, more nitrogen oxide is produced. Conversely, a low nitrogen oxide target value produces higher particulate levels. The high efficiency of the DPF means that combustion can be optimized towards minimal nitrogen oxide emissions by internal engine adjustments. This reduces the effort required to remove nitrogen oxide from exhaust gases. "With the introduction of increasingly strict exhaust emission limits, it becomes more and more difficult to reduce raw emissions, especially for heavy and high-performance vehicles, or when attempting downsizing approaches," says Dohle. "Nitrogen oxide storage catalytic converters [NSC] or selective catalytic reduction [SCR] systems based on the 'AdBlue' reducing agent are being used to break down the nitrogen oxide. In NSC systems it is necessary to carry out a desulphurization process from time to time, since even so-called sulfur-free diesel fuel still contains a low residue of sulfur resulting from its production process." Bosch is developing special control strategies for the engine management unit to prevent the catalytic converter from being damaged during sulfur regeneration. "SCR systems don't have this disadvantage, but they do require 'AdBlue', which converts the nitrogen oxides into nitrogen and water," says Dohle. "Bosch is developing dosing units and control concepts for these SCR systems and has been supplying the so-called 'Denoxtronic' dosing unit for commercial vehicles since 2004. Second generation systems are being developed which will dispense with the pressurized air atomization commonly used to date. The denox systems from Bosch enable a particularly effective removal of nitrogen oxides with a conversion rate of up to 85 percent. There is thus the option of an NSC system or an SCR system, depending on the vehicle weight and its specific engine power rating."

By William Kimberley, Editor

Automotive Engineer, London, [e-mail]