The further thermal efficiency improvement of marine natural gas engine is constrained by a knocking phenomenon that commonly occurs in gas-fueled spark-ignited engines. It plays an important role to investigate how the knocking occurs and how to predict it based on the engine simulation model. In this paper, a two-zone model is developed to provide the prediction of knocking performance and NO emission, which is verified by engine test bed data from a transformed marine natural gas spark ignition (SI) engine. Cylindrical division theory is used to describe the shape of the two zones to decrease the computational cost, as well as a basic mechanism for NO concentration calculation. In order to solve the volume balance, three boundary parameters are introduced to determine the initial condition and mass flow between the two zones. Furthermore, boundary parameters' variation and knocking factor (compression ratio and advanced ignition angle) will be discussed under different working conditions. Result shows that the two-zone model has sufficient accuracy in predicting engine performance, NO emission and knocking performance. Both the increasing compression ratio and advanced ignition angle have a promoting effect on knocking probability, knocking timing and knocking intensity. The knocking phenomenon can be avoided in the targeted natural gas SI engine by constraining the compression ratio smaller than 14 and advanced ignition angle later than 30 • before top dead center (BTDC).Energies 2018, 11, 561 2 of 23 Two broad categories of experimental based methods are used to detect the knocking phenomenon in a certain natural gas engine: the former one is based on direct measurement, like an intensified charge coupled detector (ICCD) camera and Laser-induced Fluorescence (LIF) imaging [8,9]; other methods [10,11] are based on indirect measurement such as in-cylinder pressure analysis, cylinder block vibration, exhaust gas temperature, etc. On the other hand, simulation models enable engineers to explore the details comprehensively during the design period in order to determine the best case, saving research time and development cost. In general, numerical simulation of the natural gas engine working process is classified as follows: the mean value model, zero-dimensional model, quasi-dimensional model and multi-dimensional model [12]. The mean value model is not primarily intended for engine development, but it is efficient for integrated system research, thus it is usually based on a large amount of engine test data and has scarcely no ability to predict [13,14]. For knocking prediction simulation models, the main objective is to characterize the end-gas temperature, which has a direct effect on knocking occurring. It is difficult for the zero-dimensional model to achieve this goal since the in-cylinder temperature and species concentrations are assumed to be uniform throughout the cylinder [15]. The multi-dimensional simulation model (or Computational Fluid Dynamics model, CFD) provides most details of in-cylinder para...

As the gaseous fuels interchangeability, which requires that the two gaseous fuels must be nearly identical in terms of their combustion characteristics and result in a similar engine performance, is important for internal combustion engines operation in cases of the fuel composition variation or the main fuel supply failure. In such cases, simulation tools of sufficient accuracy can be effectively employed in fuel interchangeability studies as well as for predicting the engine performance and emissions. In this study, a zero-dimensional diesel engine model is extended for simulating multi-fuel engines by considering the thermodynamic properties of the employed fuels. The model is verified against experimental data and subsequently employed to investigate the performance and knocking resistance of an SI engine operating with interchanged gaseous fuels mixtures. The derived results demonstrate that the Wobbe Index estimation is not sufficient for the characterisation of the engine performance and therefore simulation must be used for the accurate engine performance prediction with fuels interchangeability. The addition of either carbon dioxide or nitrogen results in reducing the knocking probability and retarding the knocking onset crank angle. It is inferred that the carbon dioxide addition is more effective than the nitrogen addition and concluded that the proposed model for multi-fuel engines provides results of sufficient accuracy to investigate the fuel interchangeability influence on the engine performance and knocking resistance.

Both spark ignition (SI) natural gas engines and compression ignition (CI) dual fuel (DF) engines suffer from knocking when the unburnt mixture ignites spontaneously prior to the flame front arrival. In this study, a parametric investigation is performed on the knocking performance of these two engine types by using the GT-Power software. An SI natural gas engine and a DF engine are modelled by employing a two-zone zero-dimensional combustion model, which uses Wiebe function to determine the combustion rate and provides adequate prediction of the unburnt zone temperature, which is crucial for the knocking prediction. The developed models are validated against experimentally measured parameters and are subsequently used for performing parametric investigations. The derived results are analysed to quantify the effect of the compression ratio, air-fuel equivalence ratio and ignition timing on both engines as well as the effect of pilot fuel energy proportion on the DF engine. The results demonstrate that the compression ratio of the investigated SI and DF engines must be limited to 11 and 16.5, respectively, for avoiding knocking occurrence. The ignition timing for the SI and the DF engines must be controlled after −38°CA and 3°CA, respectively. A higher pilot fuel energy proportion between 5% and 15% results in increasing the knocking tendency and intensity for the DF Engine at high loads. This study results in better insights on the impacts of the investigated engine design and operating settings for natural gas (NG)-fuelled engines, thus it can provide useful support for obtaining the optimal settings targeting a desired combustion behaviour and engine performance while attenuating the knocking tendency.