Influence of EGR on Combustion and Exhaust Emissions of Heavy Duty DI-Diesel Engines Equipped with Common-Rail Injection Systems

At the Internal Combustion Engines and Combustion Laboratory of the Swiss Federal Institute of Technology in Zurich we are currently developing low emission strategies for heavy duty diesel engines that engine manufacturers can implement to meet stringent emissions regulations. The technologies being studied include high-pressure fuel injection (with common-rail injection system), multiple injection strategies (with pilot or post injections), turbo charging, exhaust gas recirculation (cooled EGR), oxygenated fuels and the optimization of the air management system.

This paper focuses on the effects of exhaust gas recirculation (cooled EGR) in combination with very high injection pressure. Measurements were carried out on a heavy-duty diesel single-cylinder research engine equipped with a modern common rail fuel injection. The engine investigations were conducted in different operating points in the engine map covering wide speed and load ranges.

Results of the study indicate that:

• EGR increases the premixed combustion portion (though in a non-linear way) and reduces the maximum rate of heat release at high load respectively increases the maximum rate of heat release at low load. EGR increases the ignition delay and the combustion duration. The premixed portion has been derived from experimental data based on an algorithm that uses Viebe correlations.
• The NOx emissions decrease almost linearly with the EGR; extremely low NOx emissions levels (less than 1-2 g/kWh) can be achieved at EGR Rates up to 40%.
• A strong increase in the particulate emissions was measured with high degrees of EGR, but the effect can be significantly counteracted in a certain range by using very high injection pressures up to 1600 bar.
• BSFC tends to be higher for increasing EGR rates because of a less favorable scavenging and longer combustion duration; the thermodynamic process is nevertheless still of similar efficiency compared to the one without EGR. However, the influence of the exhaust enthalpy on the turbocharger, and thus on the combustion system, is not considered in this study.
• Noise indication is higher, especially at low engine load with higher injection pressures. With the use of pilot injection the increase of combustion noise can be diminished.
• EGR allows to fully take advantage of a high pressure Common Rail system and to operate the engine very efficiently and with low emission (near or below the EURO IV limits).

This investigation shows the potential of EGR combined with a flexible modern high-pressure injection system (common rail). There are no technical solutions for a full production engine considered in this paper. Although the results have been obtained on a single-cylinder research engine, it should be possible to obtain similar emissions and BSFC on a full production engine.