Abstract
To comply with the NOx and particles limits of the Euro 4 emissions regulations with low fuel consumption, it has been necessary to use additional strategies with the classic injection timing retardation. One of the more important systems has been the EGR (Exhaust Gas Recirculation) circuit. This strategy result in lower combustion temperatures, and so, the use of EGR cause a decrease of the NOx emissions at the same time that increase soot emissions due to low concentrations of the O2 in the cylinders. Therefore, the quantity of the recirculated exhaust gases must be limited to ensure that the combustion chamber receives enough oxygen to support combustion of the injected fuel.
In this way, this effect is clearly more important during a transient process where the late response of the turbocharger produce less airflow in the engine than during a stationary process. Thereby, in order to avoid the particulate emissions during the transient process, it is necessary to limit both the mass fuel flow and the EGR flow.
The actual engines are controlled with an Electronic Control Unit (ECU). These ECUs dispose several maps to control the mass fuel injected into the cylinders. Normally, the implemented maps at ECU are the result of a lot of test over the engine in standard bench. At this moment, in those tests, the EGR percentages are not taken into account to implement the injection maps.
The objective of this article is to show a methodology to improve the injection maps during a load transient process in turbocharger engine with EGR during a load transient process.
The soot emissions during a transient process could be avoided with the limitation of the 1/AFR (mass fuel/air in the cylinder) under a target value. This target value can be fixed or can be depended on other parameters like function mode (crankshaft speed, intake pressure, %EGR, ). In this article, the relationship between AFR and the EGR percentage have been taken into account to obtain the correct injected mass of fuel.
This communication explains a methodology used in the Engine Department of the University Polytechnics of Valencia. To get an optimum fuel limitation map has been necessary to use both, result from the engine bench tests and result from a model named Wave Actions Model (WAM). This modelling program has been developed entirely at our Department and it has been employed and validated as an important tool in order to analyse many thermodynamics processes in internal combustion engines.
Using an exhaustive modelling of the engine during a transient process is possible to know the instantaneous fresh mass of air in the cylinder. With that information, the model can decide which quantity of mass fuel is injected to the cylinder. In this way, WAM controls the AFR in the cylinder. A new limitation map has been introduced in the ECU of the engine using, as a guide, the results from the model. The data obtained from the tests of the engine have been compared with the result from the model, assessing the accuracy of the predictions and generating correction criteria to adjust modelled criteria.
With a feedback between both result from the model and result from the test is possible to get several options in order to obtain the optimum fuel limitation map for each transient process.