Abstract
In the recent years there is a steadily growing interest in fuelling engines with various gaseous fuels. This will necessitate further research and development projects concerning adaptation of contemporary engines to gas fuels. Due to existing resources and their price, natural gas will play a major role in fuelling systems, which in medium size and high power engines can be burnt in dual fuel or spark ignition systems. The dual fuel systems are especially attractive, because despite of gas fuelling the engine, they can still preserve a majority of positive features of diesel engines. However, dual fuel engines are also a source of many problems connected with adjustment of the gas-air mixture, maintaining the proper parameters of the combustible mixture at various loads, selection of proper quantity of initial dose and timing of its injection. Impact of gas concentration on ignition delay and loss of thermal efficiency at partial engine loads is the other fundamental problem present in dual fuel engines. To better understand the combustion process of both fuels in engines adapted to dual fuelling, further testing, beyond what has been already accomplished, should be performed regarding these problems. This further work may be based on the proposed model.
The paper presents a two-phase theoretical model describing combustion course in dual fuel engine. It was assumed in the theoretical model that the processes of combustion of diesel oil and gas are different. Beginning of combustion is the same for the both fuels. Combustion time and rate are different for liquid and gaseous fuels. It has been assumed, that the progress of combustion process is described by the Viebe function with various exponents mp and mg for the pilot dose and gas respectively. The parameters of the working medium are the sum of thermal effects of simultaneous combustion of the both fuels.
Calculation of the composition of the medium is based on the chemical equations of combustion, where combustion progress is predetermined by transient values of the Viebe function. Calculation of the parameters of the working medium on beginning of combustion are based on the pressure read from real indicator diagram taken from engine tests or calculated from the model. Due to the above, the model can be used in the development of a complete course of dual fuel engine operation or for a comparison of the course recorded during bench tests with a course calculated based on the theoretical model.
Development of a complete cycle of dual fuel operation enables further investigation of the impact of combustion of various gaseous fuels on engine parameters for different adjustments and components of the gas-air mixture. Accuracy of the calculations depends on accuracy of applied boundary conditions, which should be based on dual fuel engine tests.
Comparison of calculation results with combustion courses obtained experimentally enables utilization of a large quantity of data, obtained from the experiments, in the theoretical model and can reduce the calculations to the combustion phase. Comparison analysis with parameters obtained from indicator diagrams enables making assumptions on combustion course of the gas alone, especially in the range of lean gas-air mixtures significantly different from lower limit of ignition. This facilitates determination of possible adjustments to the composition of the combustible mixture as well as a proper selection of doses of Diesel oil, which initiates gas ignition.
Comparison of test results with results of the calculations shows that the theoretical model quite accurately simulates the real course of combustion in the engine. However, it shows smaller maximal heat release rates in limits of 7-9% for liquid fuel and up to 18% for gas. Smaller heat release rates calculated in the model cause the pressure of the medium, as anticipated by the theoretical model, to be smaller in the limits of 4-5.8% as compared to pressures recorded during tests.
Results of performed calculations and comparison with results obtained from engine bench tests prove the usefulness of the model for investigation of the impact of engine adjustment and gas-air mixture composition on such parameters of the dual fuel engine as: level of maximal combustion pressures, temperature of the medium, effective output power and thermal efficiency. Results obtained from the calculations, based on the presented model, should facilitate research connected with adaptation of Diesel engines to dual fuel feeding.
Quantity of data incorporated in this model is significantly smaller from the requirements of complicated theoretical models. This facilitates practical utilization of the model. Input data taken from the test of dual fuel engines shall improve accuracy of forecasts based on the model. Moreover, results of the calculations can be transferred on engines similar to the tested engine.