اعتبار سنجی و تجزیه و تحلیل حساسیت از دو مدل موتور دیزلی منطقه ای برای پیش بینی احتراق و تولید گازهای گلخانه ای

The present two zone model of a direct injection (DI) Diesel engine divides the cylinder contents into a non-burning zone of air and another homogeneous zone in which fuel is continuously supplied from the injector and burned with entrained air from the air zone. The growth of the fuel spray zone, which comprises a number of fuel-air conical jets equal to the injector nozzle holes, is carefully modelled by incorporating jet mixing, thus determining the amount of oxygen available for combustion. The mass, energy and state equations are applied in each of the two zones to yield local temperatures and cylinder pressure histories. The concentration of the various constituents in the exhaust gases are calculated by adopting a chemical equilibrium scheme for the C–H–O system of the 11 species considered, together with chemical rate equations for the calculation of nitric oxide (NO). A model for evaluation of the soot formation and oxidation rates is included. The theoretical results from the relevant computer program are compared very favourably with the measurements from an experimental investigation conducted on a fully automated test bed, standard “Hydra”, DI Diesel engine installed at the authors’ laboratory. In-cylinder pressure and temperature histories, nitric oxide concentration and soot density are among the interesting quantities tested for various loads and injection timings. As revealed, the model is sensitive to the selection of the constants of the fuel preparation and reaction sub-models, so that a relevant sensitivity analysis is undertaken. This leads to a better understanding of the physical mechanisms governed by these constants and also paves the way for construction of a reliable and relatively simple multi-zone model, which incorporates in each zone (packet) the philosophy of the present two zone model.

One of the major polluting contributors to our environment today is the internal combustion engine, either in the form of spark ignition (Otto) or Diesel versions. In parallel to this serious environmental threat, the main source of fuel for these engines, crude oil, is being depleted at high rates, so that the development of less polluting and more efficient engines is today of extreme importance for engine manufacturers [1] and [2]. Also, to this end, the fact of the increasing threat posed by the rivals of the internal combustion engine, for smaller size engines, such as the electric motors, the hybrid engines, the fuel cells and the like [1] and [3] corroborates the importance. Experimental work aimed at fuel economy and low pollutants emissions from Diesel and Otto engines includes successive changes of each of the many parameters involved, which is very demanding in terms of money and time. Today, the development of powerful digital computers leads to the obvious alternative of simulation of the engine performance by a mathematical model. In these models, the effects of various design and operation changes can be estimated in a fast and non-expensive way, provided that the main mechanisms are recognized and correctly modelled [4] and [5]. The process of combustion in a Diesel engine is inherently very complex due to its transient and heterogeneous character, controlled mainly by turbulent mixing of fuel and air in the fuel jets issuing from the nozzle holes. High speed photography studies and in-cylinder sampling techniques have revealed some interesting features of combustion [2]. The first attempts to simulate the Diesel engine cycle substituted the “internal combustion” by “external heat addition”. Apparent heat release rates were empirically correlated to fuel injection rates and eventually used in a thermodynamic cycle calculation to obtain the cylinder pressure in a uniform mixture [6]. Models based on droplet evaporation and combustion, while still in a mono-zone mixture, can only partially take into account the heterogeneous character of Diesel combustion [7]. The need for accurate predictions of exhaust emissions pollutants forced the researchers to attempt developing two zone combustion models [8], [9], [10] and [11]. Eventually, some multi-zone combustion models have appeared, carrying the expected drawbacks of the first attempts, where the detailed analysis of fuel-air distribution permits calculation of the exhaust gas composition with reasonable accuracy [12], [13], [14] and [15]. However, this happens under the rising computing time cost when compared to lower zones Diesel combustion models. At this point, it is mentioned that multi-dimensional models have proved useful in examining problems characterized by the need for detailed spatial information and complex interactions of many phenomena simultaneously [16] and [17]. However, these are limited by the relative inadequacy of sub-models for turbulence, combustion chemistry and by computer size and cost of operation to crude approximations to the real flow and combustion problems. Therefore, it is felt that a reasonable choice seems to be a two zone model, which includes the effects of changes in engine design and operation on the details of the combustion process through a phenomenological model where the geometric details are fairly well approximated by detailed modelling of the various mechanisms involved [18], [19] and [20]. This is going to have the advantage of relative simplicity and very reasonable computer time cost. Thus, the object of the present work concerns a comprehensive two zone model, applied to a direct injection (DI) Diesel engine, similar in broad outline to others, but with several differences that one must expect from an independent research source. The model contains upgraded jet mixing, heat transfer and chemistry sub-models, using as simply as possible the numerical analysis treatment of the governing differential and algebraic equations, thus leading to good solution convergence with reduced computer time cost. Detailed description of this model and the companion computer program structure have been reported in a recent publication, with validation against experimental data [21]. Extending that work further, the present paper, after exposing a rather short description of the model, as a first step, verifies its validity by using data from a vast experimental investigation. This data is taken at the authors’ laboratory on a fully automated test bed, four stroke, water cooled, standard “Hydra”, direct injection, high speed, Diesel engine. Plots of pressure, temperatures in the two zones, nitric oxide (NO) concentration, soot density, efficiency and other interesting quantities are presented as a function of crank angle, for various loads and injection timings, providing insight into the physical mechanisms governing the combustion and pollutants formation. After gaining confidence in the predictive capabilities of the model, the second step follows with an extensive investigation of the sensitivity of the model to variation of the constants used in the fuel preparation and reaction sub-models, which are proved critical to the model predictions. For this purpose, the coincident experimental and predicted points are used as baseline values around which changes to these constants are effected. As a feedback, this leads to a better understanding of the physical mechanisms governed by these constants, explaining the behaviour of the combustion and pollutants formation for various fuels and conditions used, as reported in the literature. At the same time, this analysis paves the way for the construction of a reliable and relatively simple multi-zone model, which incorporates in each zone (packet) the philosophy of the present two zone model, while it may also be useful for the construction of a combustion model during transient engine operation [22].

A comprehensive, two zone, thermodynamic model of direct injection Diesel engine cycles is studied, with emphasis laid on including most of the processes taking place inside the cylinder and describing them by correct modelling of the relevant physical and chemical laws. The model is validated against the performance and emissions data results from an extended experimental investigation, which is conducted on a fully automated test bed, Ricardo–Hydra standard, DI Diesel engine, located at the authors’ laboratory. The two zone model proposed proves to be very effective in predicting the engine performance and exhaust emissions, specifically concerning the effect of the two major operating parameters of load and injection timing. The same values of the set of calibration constants of the various sub-models were used throughout for all conditions examined, performance- and emission-wise, following a judicious multi-parametric analysis for their optimum choice. After gaining confidence in the predictive capabilities of the model, an extensive investigation followed of the sensitivity of the model to variations in the values of the constants used in the fuel preparation and reaction sub-models, which are proved critical to model predictions. For this purpose, the coinciding experimental and predicted points were used as baseline values around which changes to these constants were effected. This was used to explain, for example, the behaviour of combustion and pollutants formation for fuels of various properties and conditions of preparation used. As a feedback, this led to a better understanding of the physical mechanisms governed by these constants. At the same time, this analysis paved the way for construction of a reliable and relatively simple multi-zone model, which incorporates in each zone (packet) the philosophy of the present two zone model, which is to be reported in a later communication. Furthermore, it can be proved useful for the construction of a combustion model during transient engine operation.