Search

Modern internal combustion engines show complex architectures in order to improve their performance in terms of brake torque and fuel consumption. Concerning naturally-aspirated engines, an optimization of the intake port geometry, together with the selection of a proper valve timing, allow to improve the cylinder filling and hence the performance. The identification of an optimal calibration strategy at test bench usually requires long and expensive experimental activities. Numerical tools can help to support engine calibration, especially in the early design phases. In the present work, a 12-cylinder naturally aspirated spark ignition engine is investigated. The engine is experimentally tested under full and part load operations. Main performance parameters, in-cylinder pressure cycles and raw pollutant emissions are measured. The engine is schematized in a one-dimensional model (GT-Power™), where “user routines” are employed to simulate turbulence, combustion, knock and pollutant production. 1D model is validated against the experimental data, denoting a good accuracy. A calibration procedure is implemented by an external optimizer, coupled with the 1D engine model, with the aim of minimizing the fuel consumption. The procedure decision parameters are intake and exhaust valve timings, and combustion phasing. Proper constraints are posed for residual gas fraction and knock intensity. The optimal calibration strategies have been recognized for two operating conditions, where the engine most frequently works along an RDE driving cycles. Main drivers for engine efficiency are intake de-throttling at part load, thanks to the internal EGR caused by a Miller-Atkinson valve strategy, and cylinder filling maximization at high load. A ‘virtual’ calibration of the considered engine, employing the developed automatic procedure, is identified on completely theoretical basis. The proposed methodology shows the capability to drive and support the experimental engine calibration and presents the potential to be very helpful in reducing the related costs and time-to-market.

The modern internal combustion engines show complex architectures in order to improve their performance in terms of brake torque and fuel consumption. Among the different solutions, a compression ratio (CR) increase represents a well assessed path to achieve the above result. However, CR has to be limited in order to comply with the mechanical and thermal engine safety and to avoid knocking combustion. In the present work, a 10-cylinder naturally aspirated spark ignition engine is investigated to evaluate the effects of an increased CR on the performance. In a preliminary stage, the engine is experimentally tested under full load operation for a base CR of 12.6. The main performance parameters and the in-cylinder pressure cycles are measured. The engine is schematized in a one-dimensional model (GT-Power™), where “user routines” are implemented to simulate the turbulence, combustion, knock and heat transfer phenomena. The 1D model is validated against experimental data at full load, denoting a good accuracy. The model is then used to estimate the engine performance variations passing from the base CR up to an increased CR value of 13.3. The results underline a reduced improvement of the engine performance for the higher CR configuration, mainly deriving from a higher thermodynamic efficiency. The proposed methodology shows the capability to predict the effects of a partial engine re-design on a completely theoretical basis and presents the potential to be very helpful in reducing the related experimental costs and time-to-market.

The results of the experimental analyses, described in Part 1, are here employed to build up an innovative numerical approach for the 1D modeling of combustion, cycle-by-cycle variations and knock of a high performance 12-cylinder spark-ignition engine. The whole engine is schematized in detail in a 1D framework simulation. Proper “in-house developed” sub-models are used to describe the combustion process, turbulence phenomenon, cycle-by-cycle variations (CCV) and knock occurrence. In a first stage, the engine model is validated in terms of overall performance parameter and ensemble averaged pressure cycles, for various full and part load operating points and spark timings. Then, the correlation regarding the maximum in-cylinder pressure distribution developed in Part 1 is here applied to predict representative faster-then-average and slower-than-average cycles, miming the effects of the experimentally observed CCV. A proper knock index is introduced and evaluated with reference to the above faster-than-average cycle. An automatic procedure is implemented to identify the Knock Limited Spark Advance (KLSA), based on the same threshold level utilized in the experimental knock analysis of Part 1. The numerical and experimental KLSA presents an excellent agreement, denoting the accuracy of the proposed combustion and knock modeling.

In this paper, a high performance V12 spark-ignition engine is experimentally investigated at test-bench in order to fully characterize its behavior in terms of both average parameters, cycle-by-cycle variations and knock tendency, for different operating conditions. In particular, for each considered operating point, a spark advance sweep is actuated, starting from a knock-free calibration, up to intense knock operation. Sequences of 300 consecutive pressure cycles are measured for each cylinder, together with the main overall engine performance, including fuel flow, torque, and fuel consumption. Acquired data are statistically analyzed to derive the distributions of main indicated parameters, in order to find proper correlations with ensemble-averaged quantities. In particular, the Coefficient of Variation (CoV) of IMEP and of the in-cylinder peak pressure (pmax) are correlated to the average combustion phasing and duration (MFB50 and Δθb), with a good coefficient of determination. In addition, a high-pass-filtering technique is used to derive the cycle-by-cycle scattering of the Maximum Amplitude of Pressure Oscillation (MAPO) index. A similar statistical analysis is carried out to derive the log-normal distributions of the MAPO index and a methodology to asses a proper knock threshold is applied to identify the presence of knocking combustion. The above data represent the prerequisites for the implementation and validation of an advanced 1D model, described in Part 2, taking into account cycle-by-cycle variations, and finally aiming to identify the knock-limited spark advance on a completely theoretical basis.