Analysis and compensation of fuel quantity variation in multipulse fuel injection
A single fuel injection pulse can be split into smaller pulses for optimizing the engine performance. In every injected pulse, the engine requires a specific quantity of fuel which is decided by the Engine Control Module (ECM). The quantity of fuel injected is critical to the performance of the engine. A technique to assess and compensate for fueling errors and variability in the injected fuel quantity estimation is of great interest. The current study is intended to identify the critical parameters that cause variability in the injected pulse quantity and devise suitable control strategies to control those parameters to enhance the accuracy of the fuel delivered in every shot during injection. The focus here is on the Cummins Inc. Heavy Duty XPI injector configured to run a small pilot and a large main pulse. The work involves using an injector model in the Gamma Technologies suite (GT Fuel). The model was validated for single pulse injection. Since multiple pulse injection has timing issues very different from the single pulse injection, this work involves generating a new critical parameter tree for multiple pulse injection. From the Critical Parameter Flow Down, body pressure dynamics were identified as the primary cause of the variability in fueling. There is a pressure wave set up inside the injector body due to the first injection pulse. When the second pulse is fired, the pressure in the injector body is fluctuating. The condition at which the second pulse is fired depends upon the spacing between pulses and the quantity injected in the first pulse. Since body pressure is critical, a simple model to estimate the body pressure trace across an injection event has been developed. The simple model results are compared with the results from GT Fuel. Good agreement is found between the simple model results and the GT Fuel Model. To pinpoint the focus of the fueling error, the injection rate shapes for the multipulse cases were analyzed in comparison with the single pulse case with matching conditions. The opening portion of the injection rate shape was identified as the major contributor to the fueling error. The start of the second pulse shifts owing to the preceding pulse and the opening slope of the rate shape changes as well. An approximation to quantify this error using rate shape parameters has been devised. This approximation captures about 70% of the total fueling error. To use this approximation, the rate shape parameters need to be estimated. Since these parameters depend upon the system dynamics, correlation with the body pressure parameters was sought. Regression models were used to estimate the dominant rate shape parameters influencing fueling. The predictor variables in the regression models are the body pressure parameters. The opening slope is estimated within an error of 5% while the shift in the start of injection is estimated within 15%. Thus, the body pressure is initially estimated, and using the parameters of the body pressure in the regression equations, the rate shape parameters are estimated. This results in the estimation of the fueling error. A simple technique to correct for the error in fueling is to alter the on-time of the injector. A scheme for correcting the on-time is also described. This scheme is specific for the injector and the information to correct for the on-time is obtained from the Fueling-On-time Curve drawn up for a particular injector. This estimation-correction scheme can reduce the fueling variation by about 40% on average. This thesis provides insight into the functioning and the underlying aspects of multipulse injection in solenoid operated diesel fuel injectors. A scheme to correct this error is described which is proposed to work satisfactorily in steady state engine operating conditions.
Meckl, Purdue University.
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