The white smoke (white vapor cloud) and blue smoke (blue smoke) that occur during the cold start-up and warm-up process of diesel engines mainly consist of unburned fuel particles with a high H/C ratio. White smoke is formed when the particle diameter is around 1.3 micrometers, usually occurring during cold start-up and idle conditions, and should disappear after improving the start-up performance and during warm-up (30 seconds).
Blue smoke particles are smaller, with a diameter of less than 0.4 micrometers, and usually occur when the diesel engine has not been fully preheated or is at low temperature and low load. It should disappear after the engine reaches normal speed.
White smoke and blue smoke have no essential difference, except for the difference in particle size, which causes a difference in coloration under light. The blue color of blue smoke is caused by the refraction of blue light on these particles.
The causes of white smoke and blue smoke are as follows: First, temperature plays a decisive role, with white smoke forming below 250°C and blue smoke forming between 250°C and the ignition temperature of diesel. Second, poor ignition conditions cause it, as since white smoke and blue smoke are mainly unburned fuel droplets, poor fuel-air mixture and poor ignition conditions should be the most important factors in their formation.
During cold start-up, warm-up, idle and low-load operation, the following situations can all generate white smoke and blue smoke:
1. The mixture formed after fuel injection is outside the combustion limit in concentration, or the local concentration is outside the combustion limit, and cannot reach the ignition state.
2. During cold start-up in a low-temperature environment, the mixture in contact with the cold wall does not reach the ignition temperature.
3. Some fuels, although reaching the ignition temperature after forming the mixture, have late fuel injection, and the fuel stays in the combustion chamber for too short a time to be completely burned before being discharged from the engine.
4. In the already ignited area, due to deteriorating working conditions, the flame is extinguished.
5. Poor atomization of the tail injection, and the volume of the fuel injector pressure chamber is too large.
To control the emission of white smoke and blue smoke, the following aspects should be noted:
1. The fuel enrichment during cold start-up should be moderate, with the concentration within the combustion limit. The mechanism of cold start-up phenomena is divided into two stages: the first is the ignition process, and the second is the self-sustaining process. The fuel enrichment determines the self-sustaining process, that is, whether the engine can continue to run after ignition depends on whether the ignition energy of the start-up fuel can overcome the resistance of the engine to make it run.
However, if the start-up fuel is too much, the concentration will be outside the combustion limit, and since it cannot fully reach the ignition state, some unburned fuel will be discharged from the engine, forming white smoke or blue smoke.
2. Strictly control poor atomization of the tail injection and achieve rapid return of fuel. In one cycle of fuel supply, the fuel supply consists of pre-fuel supply, geometric fuel supply and post-fuel supply. Since the post-fuel supply is generated during the continuous decrease in pressure, the atomization is very poor. Therefore, the larger the proportion of post-fuel supply, the greater the poor atomization of the tail injection, and the greater the HC (unburned fuel) emissions.
3. Accurate fuel injection timing, that is, the fuel injection timing error between each cylinder and each injection should be within ±0.5 degrees (°CA).
4. The fuel spray from the nozzle should be uniform, the volume of the fuel injector pressure chamber should be small, and the fuel cut-off should be sharp.
5. The engine cylinder pressure, clearance volume, and combustion chamber surface area must meet the design requirements.
6. During idle warm-up operation, the fuel supply uniformity of each cylinder's fuel injector should be controlled within ±7%.