In-Cycle Closed-Loop Combustion Controllability with Pilot-Main Injections

In-cycle closed-loop combustion control has been proved to reduce cycle-to-cycle variations on emissions and indicated thermal efficiency. In this paper, the in-cycle closed-loop combustion con-trollability achieved by a pilot-main fuel injection scheme is investigated. The controllability is studied by means of the maximum reachable indicated thermal efficiency (MRE). A combustion model is used for the heat release optimisation. The pilot model parameters were modified to simulate the dis-turbances of the pilot combustion. The MRE is the result of optimizing the heat release, constrained to a constant load, with a disturbed pilot injection and adjusting the main start of injection (SOI) and its duration. The nominal indicated thermal efficiency was optimized at the central operating condi-tions.
The results showed that the most influential variable in the indicated thermal efficiency was the dis-turbances in the pilot mass. 79% of the efficiency variability can be explained by the actual injected pilot mass. The second and third most significant variables were the variances in the combustion effi-ciency and either of both, the start of vaporisation or the start of combustion (depending on the vari-ables interaction). The disturbances in the pilot combustion resulted in a reduction down to -0.8%unit net indicated thermal efficiency compared to the nominal maximum indicated thermal efficiency.
The results confirmed that by adjusting the main SOI, the indicated thermal efficiency can be im-proved in 86% of the total cases with an average change about +0.1%unit in net indicated thermal effi-ciency. The maximum improvement was +1%unit net indicated thermal efficiency for pilot masses larg-er than the nominal point, with longer ignition delay and increased combustion efficiency. The pilot in-jection is observable 1CAD after its start of combustion, which limits the controllability of the main SOI. In 86.2% of the considered cases, the main SOI was controllable in-cycle. When the main SOI was constrained to the controllable window, the reduction in indicated thermal efficiency was negligi-ble compared to the maximum indicated thermal efficiency achievable by in-cycle closed-loop com-bustion control.