Experimental Study On Iso-octane HCCI Combustion Characteristics By The Use Of A Rapid Compression Machine

The homogenous charge compression ignition (HCCI) is a new combustion concept, which incorporates the best features and discards the disadvantages of both traditional spark ignition (SI) gasoline engine and compression ignition (CI) diesel engine. On one hand, HCCI has not only high thermal efficiency, but also only forms extra low brake specific emission of NOx. On the other hand, the air/fuel ratio is very high which can improve fuel economy.Experimental study on HCCI combustion fueled with Iso-octane was carried in this paper. A new device called rapid compression machine was used in the study to solve the problem of the simulation and control on whole combustion conditions on the real engine. The device is made up of the supply system, drive system, combustion system, control and data gathering system, and so on. As a result, several steps must be executed before the study proceeded on this device. First, the air-fuel mixture confect system should be warm-up to a certain temperature. After this step, the Iso-octane will be injected into an air-fuel mixer in liquid state with a certain volume which is decided by the air-fuel ratios we needed. Through a venturi tube equipped in the air-fuel mixer the Iso-octane will be pressed into the blending chamber by the intake air mass. Subsequently, the air-fuel mixture will be preheated in the blending chamber. At last, the air-fuel mixture will be transported into the combustion chamber and kindled by the compression of the piston. Accordingly, the combustion date will be recorded by the computer.According to the content of the study, our work is divided into two parts.(1) because of the requirement of the device and HCCI combustion, we demarcated the combustion conditions such as mixture temperature,initial pressure and excess air ratio.Temperature field of blending chamber was measured with more than 20 temperature sensors. It was described with axial and radial direction temperaturedistributing trend. According to the experiment, blending chamber center temperature is almost equal to the average temperature either the blending chamber is heated at intermittent time or the blending chamber is controlled at a steady temperature. Temperature filed of combustion cylinder is test with preheated air going into the cylinder to make the condition similar to the combustion experiment. With the datum, a graph about the relationship between combustion cylinder`s average temperature and the temperature which was detected by the cylinder sensor was got. We can get combustion cylinder`s average temperature from the graph. From the experiment, we conclude that blending chamber`s temperature does little to combustion cylinder temperature filed. So we define the cylinder temperature as air/fuel mixture temperature.Filling the air/fuel mixture from blending chamber to combustion cylinder, the relationship of blending chamber pressure, combustion cylinder pressure and filling time can be got. After discharging air/fuel mixture from combustion cylinder freely, we know the relationship between combustion cylinder pressure and discharging time. So we can speculate the cylinder initial pressure according to discharging time.Furthermore, we modify the formula of excess air ratio calculating and make a program about the formula which makes it easy to calculate excess air ratio.(2) According to combustion analysis, we can make certain the effect of the combustion conditions to the Iso-octane HCCI combustion .In this part, the air/fuel mixture temperature, the pressure in the cylinder and the air/fuel ratios will be observed as the combustion parameters and the effect of these parameters to Iso-octane HCCI combustion will be studied as well. So, the kindling time, the heat release ratios, the pressure increasing ratios and the combustion temperature during the Iso-octane combustion process will be researched to evaluate the combustion characteristic.Through the study on the effect of the combustion conditions to the Iso-octane HCCI combustion, the kindling time, the heat release ratios, the pressure increasing ratios and the temperature during the Iso-octane combustion process have beenevaluated. Under the condition of compression ratio equaled to29, we have drawn a conclusion that when the mixture temperature increased, the combustion became more rapidly. So, an advanced kindling time, a shorter combustion duration, a greater in-cylinder peak pressure and a higher pressure increasing ratio appeared during the combustion process.Moreover, an experiment wan conducted with different air-fuel ratio mixture under a same temperature. Nine kind of mixture with excess air ratios from 2 to 9 was used in this experiment. According to the test result, a slight increase of the combustion beginning time appeared by the use of thicker air-fuel mixture, but it`s influence is very slight.