Method and device for controlling diesel engine

A diesel engine with control device, comprising:

• an engine unit having a geometric compression ratio that is set to 15:1 or less, and that is configured to cause compression self ignition of fuel supplied into a cylinder;
• a fuel injection valve arranged facing to an inside of the cylinder and for directly injecting the fuel into the cylinder;
• an injection control module for controlling an injection mode of the fuel into the cylinder through the fuel injection valve;
• and
• an EGR ratio control system for adjusting an O2 concentration within the cylinder by adjusting an EGR ratio associated with an adjustment of an EGR gas volume introduced into the cylinder;
• wherein the EGR ratio control system adjusts the EGR ratio according to a load of the engine unit so that the O2 concentration within the cylinder decreases gradually with an increase in the load of the engine unit up to a predetermined load of the engine unit, and so that the O2 concentration increases gradually above the predetermined load of the engine unit with an increase in the load of the engine unit past the predetermined load;
• wherein the injection control module sets to a premix combustion mode where a fuel injection is terminated before a top dead center of a compression stroke in an operating range of a low load including the predetermined load where the O2 concentration within the cylinder is lowest and, after that, the fuel ignites and combusts, while the injection control module sets to a diffusion combustion mode where the fuel injection is executed in parallel to the ignition and combustion of the fuel in an operating range where the load of the engine unit is higher than that of the operating range in the premix combustion mode and the O2 concentration within the cylinder is relatively high;
• wherein the injection control module executes in the diffusion combustion mode, a main injection for injecting the fuel to perform main combustion where diffusion combustion is mainly caused, and two or more pre-stage injections for injecting the fuel at a timing before the main injection to perform pre-stage combustion before the main combustion;
• wherein the injection control module controls an injection mode of the pre-stage injections and an injection mode of the main injection so that a heat release rate due to the main combustion starts increasing after a heat release rate due to the pre-stage combustion reaches a peak and then begins to decrease;
• and
• wherein an operating range of a relatively low load, including the predetermined load where the O2 concentration within the cylinder is lowest for an adjustment control of the EGR ratio, is set to the premix combustion mode, and the operating range of a higher load side than the premix combustion mode is set to the diffusion combustion mode.