Abstract
Objective
The diesel-ignited natural gas engine is a promising approach for the next generation of alternative fuel converters as it combines low CO2 abatement cost with high efficiency. Diesel is the engine’s ignition source, once compression ignited the diesel provides ignition centers for the premixed natural gas. Diesel-ignited natural gas engines offer additional degrees of freedom in controlling the combustion in comparison to a conventional engine. The timing of the diesel injection is thereby of particular interest. As shown in previous publications, the diesel quantity that is required for achieving a certain combustion phasing is dependent on the timing of the diesel injection. There is a timing that leads to the smallest possible diesel quantity for a particular operating point. The objective of this study is to investigate the effect of this minimal diesel approach on engine performance and emissions. In addition, constraints are considered that are imposed by the diesel injector.
Methodology
The empirical investigations are carried out on a two liter dual fuel engine equipped with a port fuel injection system for natural gas as well as a diesel direct injection setup. Several operating points are investigated regarding the engine parameters load, speed and fuel-air equivalence ratio. The engine is equipped with in- cylinder pressure sensors allowing for cylinder individual feedback control. The control strategy aims to keep the combustion phasing constant at a specific value under all conditions and is the key to achieving good comparability between the measurements
Results
Diesel minimal control enables a stable combustion over a broad range of operating conditions. Under stoichiometric conditions, running the dual fuel engine at minimum diesel quantity is a promising compromise considering the individual pollutant species. However, with a lean combustion, the dependency of the emissions on diesel timing changes. While in-cylinder methane conversion features low dependency on diesel timing, nitrogen oxide emissions show a higher sensitivity on injection timing. Injection timings earlier than the ones required for the minimal diesel strategy might be beneficial in terms of emissions. Further investigations have to be carried out for the final submission. In case the desired diesel injection quantity is too small for the given diesel injector, the injection timing has to be adapted in order to avoid deviations in the combustion phasing. For this purpose, an extended version of the diesel minimal control strategy is presented that either advances or retards the diesel injection.
Limitation of this study
In this study mainly part-load operation points are considered since they are of particular interest in terms of emission abatement.
Contribution
The contribution of this paper lies in showing the effect of the minimum diesel timing control strategy on pollutant emissions and engine performance. This submission shows under what conditions a minimal diesel injection timing is beneficial for the operation of a dual fuel engine. All major pollutant species and engine parameters are considered for this objective.
Conclusions
Using diesel minimal control is a simple and stable method to obtain good overall performance with stoichiometric operated dual fuel engines.
Keywords : Natural gas diesel, dual fuel, diesel injection timing