Past and present developments in diesel technology

Diesel technology has improved substantially over the last 20 years. The main breakthrough in fuel economy was brought about by direct injection technology. Direct injection has improved energy efficiency by 15-20%. This has been achieved by

• improvements in injectors (with multiple orifices and two-stage injection) and
• higher injection pressures (as a result of improvements in injection pumps and electronic control)

Present R&D efforts are mainly aimed at further reductions of exhaust emissions. There is usually a target conflict between fuel economy and emission reduction. Therefore, the current focus is to improve emission control without giving away the achieved fuel economy rather than to improve energy efficiency.

Future diesel technology and fuel economy

The innovation dynamics of diesel technology has been very high (higher than in spark ignition technology) in the recent past. This dynamics is expected to continue at its high level. These are some of the innovations and research foci coming up in diesel technology:

• Injection system and electronic control: state-of-the-art direct injection offers potential for further development, especially concerning the temporal injection characteristics. In today's common rail injection systems, the fuel is not just injected once per cycle but split into several partial injections. The time and load scheme of these partial injections can be further improved if the operation speed of the injection valves is increased. For this purpose, Siemens VDO is currently developing piezoelectronic actuators to replace conventional solenoid valves. These actuators are much faster and allow for a sophisticated injection regime, e.g. two pre-injections with very small volumes of fuel followed by the main injection and two smaller post-injections. This improves mainly exhaust emissions but is also expected to yield slight gains in fuel economy. Piezoelectronic actuators exploit the behavior of piezoelectric crystals. If an electric charge is connected to such a crystal, the crystal lattice reacts within a few milliseconds by expanding. When discharged the material returns to its original size.
• Steam injection engines: Intensive research is currently focusing on the development of a innovative combined diesel and steam process. The concept is based on the injection of high temperature and high-pressure steam into the combustion chamber. By utilising the air and cylinder cooling and exhaust heat to generate steam, the method is expected to increase thermal efficiency and thus fuel economy while at the same time reducing NOx production. This requires fundamental research in spray combustion.