Efficiency improvement of a spark-ignition engine at full load conditions using exhaust gas recirculation and variable geometry turbocharger – Numerical study
Introduction
Future challenges in the internal combustion engines development consist of further reduction of fuel consumption and exhaust gas emissions including greenhouse gases, e.g. CO2. Compression ignition engines (CI) have shown significant improvement in fuel consumption over the last decades, while for satisfying exhaust gas emissions requirements they require complex and costly exhaust gas after treatment systems. On the other hand spark-ignition (SI) engines show slower progress on the fuel consumption side, but the exhaust gas emissions, excluding CO2, are very low. With growing concern regarding the influence of CO2 on the climate change [1] and by acknowledging the fact that the CO2 emission is proportional to fuel consumption, the efficiency of IC engines is becoming one of the primary development targets. Therefore, the amount of research in the field of SI engines has shown significant growth over the last couple of years.
There are numerous technologies that have been introduced to reduce fuel consumption such as variable valve timing [2], turbocharging [3] and direct injection [4], while the exhaust gas recirculation (EGR) [5] has been introduced to reduce the exhaust emissions of NOx. The configurations of cooled EGR systems can be divided into three main groups: low-pressure EGR (LP-EGR), high-pressure EGR (HP-EGR) and mixed-pressure EGR loop [6]. In [5] the authors showed that in a SI engine LP-EGR system allows wider EGR area in turbocharger region, better knocking suppression and higher exhaust gas temperature reduction. However, they also reported that due to low differential pressure and long EGR passage it is more complicated to obtain stable EGR control in LP-EGR compared to HP-EGR strategy. Regarding CI engines, in [7] it was shown that LP-EGR allows higher EGR mass flow rate in wider CI engine operating range compared to HP-EGR system.
Each of the above mentioned systems can be controlled to achieve specific operation which should help in reaching targeted efficiency. This means that the number of engine operating parameters is growing. Since each individual system influences the operation of other systems and of the engine, the complexity of the overall engine operation is increasing. Therefore, finding optimal operating parameters of the engine can be challenging. Also, since all systems interact with each other, predicting overall performance of the engine in the design phase of development is difficult. Therefore, there are efforts to use numerical tools for predicting engine performance of future engines and for finding the approximate operating conditions for each specific operating point [8]. The entire engine system is usually modeled with cycle simulation models (0D/1D), which if properly calibrated offer a good tradeoff between accuracy and calculation time. Such simulation models usually predict the average engine cycle [9] which means that the influence of operating parameters on the combustion stability cannot be captured. Combustion stability becomes very important for conditions with late spark timing and large EGR fractions which are more frequently used in modern, turbocharged SI engines [10].
This study aims to investigate the influence of various control parameters of a port fuel injection (PFI), turbocharged SI engine, equipped with a HP-EGR loop on overall performance, by using a cycle simulation model with the novel approach in simulating combustion that includes modeling of cycle-to-cycle variations (CCV) [11]. The emphasis of the study is to research the influence of HP-EGR on overall engine performance and to investigate the possibility of using EGR for lowering fuel consumption at full load conditions.
The methodology consists of creating a reduced cycle-simulation model of a cylinder that corresponds to a modern four valve chamber design. The cylinder and combustion model constants are calibrated by comparison of experimental and simulation results at three different engine speeds (2000, 3500 and 5000 rpm) at full load conditions. Once the model of the cylinder is calibrated the model is extended to feature all systems of a turbocharged SI engine equipped with HP-EGR loop. The performance analysis of this new engine design is made by the parametric investigation of the influence of spark timing, air to fuel ratio and EGR on engine performance results. Overall engine performance is obtained by excluding the operating conditions that are not allowed based on the results of knock occurrence, exhaust gas temperature, excessive combustion instability and turbocharger limits (surge operation).
Section snippets
Cycle-simulation model
The cycle-simulation model of the engine that will be described later is made in AVL BOOST. This model is a 1D/0D simulation model because the flow through the intake and exhaust pipes is calculated using the 1D finite volume method, while the calculation of the in-cylinder state is based on 0D thermodynamic approach. In this work, combustion process and knock occurrence were calculated using several newly developed sub-models. Although these sub-models were described and validated in
Methods
The base cylinder geometry for this study is taken from the 4-cylinder naturally aspirated SI engine with typical modern layout, equipped with a 4-valve cylinder head and a total displacement of 1.4 L. In order to calibrate and validate the cycle simulation model created in AVL BOOST, three operating points at full load conditions with engine speeds of 2000, 3500 and 5000 rpm were selected from the experimental data. In the selected operating points the spark timing was in the range from 7 °CA to
Simulation results and discussion
In the method section it was described that the parametric studies were made at three different engine speeds. Therefore, performance results of each engine speed are analyzed separately. In the following figures the performance results at KLSA are shown while other limits are studied separately. The dashed parts of the curves and empty markers represent the restricted area of operation by one of the limiting factors (turbine inlet temperature or CoV IMEP). In the diagrams for engine speed of
Conclusions
The paper described here shows a novel platform for studying the individual effects on overall engine performance. The platform consist of a cycle simulation model that uses physical models of engine flow and combustion, a newly developed model for engine knock and novel approach for modeling of CCV. The novelty of the approach is the calculation method in which engine performance is obtained in the same manner as it is obtained on the test bench, i.e. on cycle to cycle basis. This novel
Acknowledgements
The numerical study was done within the ExDAM Project “Experimentally Supported Development of Advanced Numerical Combustion Models for Internal Combustion Engines” funded by Croatian Science Foundation and AVL-AST Ltd. company. This help is gratefully appreciated.
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