Investigation Into Advanced Architecture and Strategies For Turbocharged Compressed Natural Gas Heavy Duty SI-engine

CNG is at present retaining a growing interest as a factual alternative to traditional fuel for SI engine thanks to its high potentials in reducing the engine-out emissions. Increasing thrust into the exploitation of NG in the transport field is in fact produced by the even more stringent emission regulations which are being introduced into the worldwide scenario. Specific attention is also to be devoted to heavy duty engines given the high impact they retain due to the diesel oil exploitation and to the PM emissions, the latter issue assessing for the need to shift towards alternative fuels such as natural gas. A thorough control of the air-to-fuel ratio appears to be mandatory in spark ignition CNG engines in order to meet the even more stringent thresholds set by the current regulations. The accuracy of the air/fuel mixture highly depends on the injection system dynamic behavior and to its coupling to the engine fluid-dynamic. The amount of injected fuel should in fact be properly targeted by the ECU basing on the estimation of the induced air and accounting for the embedded closed-loop strategies. Still, these latter are normally derived from engine-base routines and totally ignore the injection system dynamics. Thus, a sound investigation into the mixing process can only be achieved provided that a proper analysis of the injection rail and of the injectors is carried out. The first part of the present work carries out a numerical investigation into the fluid dynamic behavior of a commercial CNG injection system by means of a 0D-1D code. The research has been focused on defining the set of parameters to be precisely reproduced in the 0D-1D simulation so as to match the injection system experimental behavior. Specific attention has been paid to the one component which significantly contributes to fully defining its dynamic response, i.e. the pressure reducing valve. The pressure reducer is made up of various elements that retain diverse weights on the valve behavior and should consequently be differently addressed to. A refined model of the pressure reducer has hence been proposed and the model has been calibrated, tested and run under various operating conditions so as to assess for the set-up validity. Comparisons have been carried out on steady state points as well as through out a vehicle driving cycle and the model capability to properly reproduce the real system characteristic has been investigated into. The proposed valve model has proved to consistently replicate the injection system response for different speed and load conditions. A few methodological indications concerning modeling aspects of a pressure regulator can be drawn from the present study. The model has been run in a predictive mode so as to inquiry into the response of the system to fast transient operations, both in terms of speed and load. The model outputs have highlighted mismatches between the ECU target mass and the actually injected one and have hinted at the need for dedicated and refined control strategies capable of preventing anomalies in the mixture formation and hence in the engine functioning. The second part of the present work aims at deeply investigating into the potentials of a heavy duty engine running on CNG and equipped with two different injection systems, an advanced SP one and a prototype MP one. The considered 7.8 liter engine was designed and produced to implement a Sigle-Point (SP) strategy and has hence been modified to run with a dedicated Multi-Point (MP) system so as to take advantage of its flexibility in terms of control strategies. More specifically, a thorough comparison between the experimental performances of the engine equipped with the two injection systems has been carried out at steady state as well as at transient operations. Better performances in terms of cycle-to-cycle variability were proved for the MP system despite poorer mixture homogeneity. Attention has also been paid to the different engine control strategies to be eventually adopted in compliance with the constraints set by the two different layouts. A 0D-1D model has also been built and validated on the experimental data set to be hence exploited for investigating into different strategies both for the SP and for the MP layout. An extensive simulation has been carried out on the effects of the injection phasing on the SP system performance referring to the engine power output and to the air-to-fuel ratio homogeneity amongst the cylinders. Finally, as far as the MP injection system is concerned, the innovative fire-skipping (DSF) or cylinder deactivation has been considered and deployed by means of the numerical model, assessing for an overall decrease in the fuel consumption of 12% at part load operations.