| Environmental problem and energy crisis are two major issues facing the world today. Energy conservation and pollution reduction is the eternal subject and responsibility of the scientific research workers. After entering the21st century, China’s economy is developing rapidly, ranking first in the world with a double-digit or so GDP growth each year. However, the main pattern of China’s economic development nowadays relies on the highly energy-consuming industries such as building, metallurgy, transport, to drive the development of other industries, which leads to serious environmental pollution and tremendous energy consumption. Large-area of haze and fog weather and over-dependence on imported oil become two main concerns of Chinese government. In order to reduce emissions and improve energy diversification, it is the inevitable choice for automobiles to adopt alternative fuel. And natural gas, with the advantages of slight emission and availability, becomes the main alternative fuel for vehicles. Compared with compressed natural gas(CNG), liquefied natural gas(LNG) has higher energy density and longer mileage, which makes it the preferred fuel for gas engine which applied in heavy trucks. Focused on WP10NGE40mono-fuel, turbocharged inter-cooled lean-burn natural gas engine, systematic numerical simulation and experimental research on the fuel supply system, intake mixing system, combustion system, engine performance of LNG engine as well as the combined operation of gas heater and LNG engine was carried out, and heavy vehicle lean-burn LNG engine with good performance was developed.In this paper, six models of both straight mixer and cross mixer with different structure parameters were designed. The mixing process of natural gas and air was simulated, and the influence of parameters such as the diameter and location of mixing holes on the mixing uniformity was analyzed. The calculation results show that Cross mixer is more beneficial to the uniform mixing of natural gas and air with the advantage of increasing disturbance of mixing process and distribution scatter degree of mixing holes. Mixture concentration in the middle of the manifold is slightly higher when adopting the cross mixer, which is beneficial to subsequent mixing and combustion. Based on the comprehensive evaluation on various parameters, the mixer of WP10NGE40was selected optimally. One-dimensional model of thermodynamic process and three-dimensional model of flow process in the intake manifold were established for the engine.1D/3D coupled transient simulation technology was adopted to study the effects of intake manifold with rectangular and trapezoidal cross-section on intake distribution evenness. The simulation results indicate that when choosing the manifold with trapezoidal cross-section for WP10NGE40engine, better distribution evenness of intake charge can be obtained. Four kinds of combustion chamber with different structure were designed and the mesh of intake port and combustion chamber for moving calculation was established. Then a transient simulation for the process of intake, compression, ignition, and flame propagation was carried out with standard CFM-2A model which was selected based on the condition of excess air coefficient of WP10NGE40engine in the range of0.6-1.7. The effect of squeeze flow area of combustion chamber, combustion chamber recess fillet, compression clearance and combustion chamber recess depth on the gas flow movement in the cylinder, ignition, flame development and propagation was studied. The simulation results reveal that decrease of compression clearance and appropriate increase of squeeze flow area and combustion chamber recess depth is good for the development and fast spreading of the flame center within the combustion chamber. The gas flow movement and combustion process in combustion chamber of four schemes was analyzed and the combustion duration was compared. The structural parameters of combustion chamber of WP10NGE40engine were finally determined.Considering the flowing out difficulty in the use of LNG resulting from the low filling pressure of domestic LNG refueling station and the insufficient vaporization pressure which can’t meet the working demand of the inter-cooled engine with single-point mixing system, a new kind of LNG fuel tank with self-pressurized system was put forward. What’s more, a new kind of fuel supply system of nature gas engine was designed. It has a double-level regulation:one is the control for heat transfer intensity of LNG vaporizer and the other is temperature regulation of nature gas after vaporization. A series of experiments on fuel supply system were done, including experiments of heat preservation property and reliability and contrast test between the influence of LNG tank with and without self-pressurized system on gas supply capacity. According to the results, the gas supply is stable and the engine operates smoothly after the system is applied, the application requirements are satisfied.In this paper, the operation parameters influencing the power performance and economy of engine, including plug gap, ignition advance angle, injection pressure, were investigated. The results show that, the gas consumption rate of the engine reaches the lowest at1000r·min-1was fuel air ratios of0.77; At the speed of1400r·min-1and fuel air ratios of0.76, the gas consumption rate of the engine reaches the lowest; When the plug gap is0.4mm, the power performance, economy, reliability of the engine are balanced; At the speed of800r·min-1to1200r·min-1, the decrease of injection pressure causes the increasing of gas consumption rate with the increasing extent smaller than1%; At the speed of1200r·min-1to1800r·min-1, the increase of gas consumption rate rises with the improvement of rotary speed, further more gas consumption rate increases by6.8%at the speed of1800r·min-1. When the injection pressure rises from5.5bar to6bar, the fuel economy of WP10NGE40engine is improved. Meanwhile the injection pulse is shortened and the life of nozzle is prolonged.Basing on turbocharged intercooled natural gas engine equipping with inlet manifold single-point mixing system, HC and NOx emission of external characteristic and load characteristic were studied. The results demonstrates that mixture scavenging phenomenon exists in turbocharged engine with inlet manifold single-point mixing system, and the percentage of CH4emission accounts for more than95%of THC emission under external characteristic of operation; under external characteristic,HC emission at medium speed and high speed is more than low speed and decreases with the reduction of ignition advance angle, however, at the speed of1400r·min-1(large torque speed), ignition advance angle has the greatest impact on HC emission; With the engine load rate increasing from0to100%, HC emission shows an overall upward trend. At the speed of2000r·min-1, the percentage of CH4emission in THC emission increases with load rate rises. Under external characteristic, NOx emission of lean-burn natural gas engine at low speed is higher compared with high speed, but NOx emission would increase when equipping with catalytic converter in the condition of external characteristic; At the speed of1000r·min-1and1400r·min-1, NOx emission firstly increases and then decreases with increase of load rate. Under medium load, the effects of ignition advance angle is obvious at the speed of1000r·min-1and1400r·min-1. at50%load rate, NOx emission decreases obviously after equipped with catalytic converter; At the speed of2000r·min-1, NOx emission would be low at the entire range of load rate, while after equipped with catalytic converter, the overall level of NOx emission would increase.To resolve the problems of difficult cold starting, insufficient gas supply, low supply temperature and heating that arise when LNG engine used by heavy trucks running in severe cold regions, the combined operation system was designed, and the bench test and cold region test were carried out. It can been seen from the results that the combined operation system runs steadily when the gas-fuel heater and the LNG engine share the same fuel supply system, and the heater starts easily as well as its combustion is stable during the experiment; The combined operating of heater and engine improves the cold start of LNG engine obviously. In cold region, the cold starting of LNG engine can be effectively improved by the combined operating of gas heater and engine.In conclusion, the WP10NGE40engine developed in this paper achieves ideal performance of dynamics, fuel economy, emission and cold starting. Through the above studies, key techniques of LNG engine development were mastered, independent development platform for LNG engine were established, and theoretical basis and operation experience of experiments were gained for continued research. The research results are conductive to advancing the core competence of domestic LNG engine and automobile product, reducing automotive industry’s addiction to oil and improving energy structure. |
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