Study On FAI Injection Two-Stroke DISI Engine

Two-stroke engine for motorcycles is reaching an important appointment for its survival due to its high emission level and poor fuel economy. Electronically controlled in-cylinder direct fuel injection system is becoming the key solution to solve these emission and fuel consumption problems.A simple, reliable and cost effective FAI (Free Armature Injection) system is applied to a 98cc two-stroke engine in this dissertation. The combustion system for DISI is developed; a prototype engine is designed and then equipped on an AX-100 prototype motorcycle; and preliminary road tests are performed.New gas exchange parts, fuel injection system and combustion chamber is developed or redesigned, based on the original carebureted engine. A great number of dynamometer experiments are carried out to investigate the effects of structure characteristics of the injector, configuration of combustion chamber, design of the bypass valve and fuel injection phasing on the performance, emission, fuel consumption of the engine during light load operating conditions. After a final optimization and other practical considerations, the control MAPs for the two-stroke DISI prototype engine are obtained. The torque performance, fuel consumption and emission economy of the engine is tested and compared with those of the original carbureted engine.The charge stratification of the two-stroke DISI engine is studied by multidimensional numerical simulation, using the CFD software FIRE with moving mesh technology. The effects of combustion chamber design, spray position and orientation, injection timing and SMD of injected droplets on the charge stratification are investigated theoretically. The results show that highly fine fuel atomization and the combined optimization of chamber structure and injector position characteristics are required to realize better charge stratification. According to the results, better charge stratification is achieved when the injector is mounted on the cylinder head with 15o to the central vertical axis, and the injection timing is set to be BTDC 95o; two designed chamber(hemispheric shape and tapered shape) are both applicable, while the hemispheric chamber achieves better charge stratification.Results of dynamometer tests demonstrate that, the method of intermitted injection can effectively reduce the HC emission and the fuel consumption during idling operation; injectors with high pressure and small lift can provide better fuel atomization, resulting in less HC emission and fuel consumption during light load operations. Bigger section area of the bypass can reduce the amount of residual gases and the pumping loss, which in consequence improves emission and fuel consumption, but it can also cause the torque output difficult to adjust, resulting in unstable idlingspeed. With smaller bypass section, the inclined-tapered chamber is more appropriate to obtain better emission and fuel consumption, while the hemispheric chamber is more appropriate with a bigger bypass section. During heavy load operations, the injection timing should be advanced as much as possible, given the limit that the fuel injected can not directly pass out of the exhause port, to realize homogenous combustion to provide higher torque output and reduce the HC emission and fuel consumption to a large extent.Preliminary road tests after the prototype engine was equipped on the AX-100 motorcyle show that, the developed two-stroke DISI motorcycle achieves good drivability, smoothly acceleration, easy start, and significant reduction of HC emission compared to the original carbureted engine. However, the idling speed is a little high, and the stability of idling requires further improvement.In a conclusion, the FAI system can completely remove the fuel short-circuiting, and greatly improve the performance during heavy load operating conditions. During light load operations, the fuel consumption and HC emission are reduced by adopting charge stratification, but still need further development. Within the full operating range, the fuel consumption is reduced by 20-30%. The prototype motorcycle shows a good performance during transient operations.