The Effects of Fuel Additives on Diesel Engine Emissions during Steady State and Transient Operation

Internal combustion engines have propelled society’s transportation and power needs for the last century. However, with the regulatory demand to reduce air pollution, internal combustion engines are a major focus to reduce the emissions from these engines. Compression ignition or diesel engines are a major contributor to NOx and PM pollution. However, the life of these engines is much longer than that of their spark-ignited counterparts, causing the fleet of diesel engines to consist of a significant number of old, higher polluting engines. Fuel additives are one method of reducing emissions and/or enhancing performance in these older diesel engines without the need for technology upgrades (new engines/aftertreatment). Although diesel fuel additives’ ability to reduce harmful emissions is well known in the literature, the mechanism as to how these additives work is not well understood. To explore the mechanism, three cetane improvers (2-EHN, DTBP, and ODA) were investigated on a 1992 DDC Series 60 engine and 2004 EGR-equipped Cummins ISM370 engine incorporating sensors for in-cylinder pressure measurement and analysis. The engines were tested on the heavy-duty FTP cycle and the steady state SET test. The cetane improvers, depending on the additive, treat rate, and base fuel (excluding the biodiesel blends), showed significant reduction in NOx (2.2-4.9%) on the 1992 DDC engine and no change or significant increase (1.3-1.4%) on the 2004 Cummins engine when exercised over the transient FTP cycle. In the SET tests, low loads produced a NOx decrease (up to 8%) and high loads a NOx increase (up to 1.8%) with cetane improvers on the 1992 DDC engine. The 2004 Cummins engine showed little NOx decrease (up to 1%) or a NOx increase (up to 6.1%) with cetane improvers compared to the base fuel on the SET test. The biodiesel blends showed a similar trend with the additized neat fuel with decreased NOx at low load and increased NOx at high load on the 1992 DDC engine, suggesting a cetane effect due to the high cetane number of biodiesel. The heat release parameters showed that the change in NOx was due to the change in maximum cylinder pressure, maximum cylinder gas temperature, premix fraction, and pressure at the start of combustion on the 1992 DDC engine. Overall, the fuel additives reduced the premix fraction of the heat release on the 1992 DDC engine at all loads and reduced the premix fraction at low load (25-50% load) on the 2004 Cummins engine. The 2004 Cummins engine had higher boost pressure, compression ratio, and manifold air temperature, which may have created the low premix fraction. A phenomenological combustion model was developed to provide local NOx formation characteristics. The combustion model and heat release correlations showed that reducing the ignition delay with cetane improvers shifted the global heat release rate and produced the NOx change on the 1992 DDC engine. The reduced ignition delay with cetane improvers created an earlier start of combustion that shifted the in-cylinder pressure and temperature, which resulted in the NOx increase at high load and NOx decrease at low load.