Accurate real time engine models are an essential step to allow the development of control algorithms in parallel to the development of engine hardware using hardware in the loop application. A physics-based model of the engine high-pressure air path and combustion chamber is presented. The model has been parameterised using data from a small set or carefully selected operating conditions for a 2.0L Diesel engine. The model has subsequently been validated over the complete engine operating map with and without EGR. A high level of fit was achieved with R 2 value above 0.94 for mean effective pressure and above 0.99 for air flow rate. Model run-time was then reduced for real-time application by using forward differencing; single precision floating point numbers; and by only calculating in-cylinder prediction for a single cylinder. A further 25% improvement in run time was achieved by improving sub-models, including the strategic use of 1D/2D look-up tables with optimised resolution. The model exceeds the performance of similar models in the literature achieving 0.5°CA resolution at 4000rev/min. This simulation step size 2 still yields good accuracy compared to 0.1°CA resolution and has been validated against experimental results from an NEDC drive cycle. The real-time model will allow the development of control strategies before the engine hardware is available, meaning more time can be spent ensuring the engine can meet performance and emissions requirements over it full operating range.
To meet the increasingly stringent emissions standards, Diesel engines need to include more active technologies with their associated control systems. Hardware-in-the-Loop (HiL) approaches are becoming popular when the engine system is represented as a real-time capable model to allow development of the controller hardware and software without the need for the real engine system. This paper focusses on the engine model required in such approaches. A number of semi-physical, zero-dimensional combustion modelling techniques are enhanced and combined into a complete model, these include — ignition delay, pre-mixed and diffusion combustion and wall impingement. In addition, a fuel injection model was used to provide fuel injection rate from solenoid energizing signals. The model was parameterized using a small set of experimental data from an engine dynamometer test facility and validated against a complete data set covering the full engine speed and torque range. The model was shown to characterize Rate of Heat Release (RoHR) well over the engine speed and load range. Critically the wall impingement model improved R2 value for maximum RoHR from 0.89 to 0.96. This reflected in the model’s ability to match both pilot and main combustion phasing, and peak heat release rates derived from measured data. The model predicted indicated mean effective pressure and maximum pressure with R2 values of 0.99 across the engine map. The worst prediction was for the angle of maximum pressure which had an R2 of 0.74. The results demonstrate the predictive ability of the model, with only a small set of empirical data for training — this is a key advantage over conventional methods. The fuel injection model yielded good results for predicted injection quantity (R2 = 0.99), and enables the use of the RoHR model without the need for measured rate of injection.
Measuring and analyzing combustion is a critical part of the development of high efficiency and low emitting engines. Faced with changes in legislation such as real driving emissions (RDE) and the fundamental change in the role of the combustion engine with the introduction of hybrid-electric powertrains, it is essential that combustion analysis can be conducted accurately across the full range of operating conditions. In this work, the sensitivity of five key combustion metrics is investigated with respect to eight necessary assumptions used for single zone diesel combustion analysis. The sensitivity was evaluated over the complete operating range of the engine using a combination of experimental and modeling techniques. This provides a holistic understanding of combustion measurement accuracy. For several metrics, it was found that the sensitivity at the mid-speed/load condition was not representative of sensitivity across the full operating range, in particular at low speeds and loads. Peak heat release rate and indicated mean effective pressure (IMEP) were found to be most sensitive to the determination of top dead center (TDC) and the assumption of in-cylinder gas properties. An error of 0.5 deg in the location of TDC would cause on average a 4.2% error in peak heat release rate. The ratio of specific heats had a strong impact on peak heat release with an error of 8% for using the assumption of a constant value. A novel method for determining TDC was proposed which combined a filling and emptying simulation with measured data obtained experimentally from an advanced engine test rig with external boosting system. This approach improved the robustness of the prediction of TDC which will allow engineers to measure accurate combustion data in operating conditions representative of in-service applications.
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