Abstract
Keywords: Gasoline engine, knock, HCCI, temperature measurement, reaction, CARS, LIF, IR absorption
Figure 1 shows photos of three HCCI combustion cycles obtained directly by high-speed photography(1). The photos clearly show the autoignition process and that combustion reactions induced by autoignition take place very rapidly throughout the entire combustion chamber in all three cycles. The combustion duration is only about 13 crank angle (CA) degrees, with a compression ratio of 18.6:1 and an air-fuel ratio of 36.5:1. This combustion process does not require spark ignition. Engine combustion controlled by human drivers is generally the power source of cars on the market today. Knowledge of combustion has been obtained and analyzed through many kinds of tests, measurements and visualizations in an effort to optimize thermal efficiency and reduce exhaust emissions.
Figure 2 illustrates the schlieren method of visualizing autoignition or knocking behavior(2). This method facilitates visualization of quick reactions and was first used to obtain new information about the autoignition process in 1978. In this setup, a transparent quartz window is attached to the cylinder head of a two-valve engine. This method is called laser shadowgraphy.
Figure 3 shows visualized knocking cycles(3) obtained with the system in Fig. 2. The crescent shape is for a combustion chamber fitted with transparent quartz windows and using the piston crown as a mirror. The spark plug is located on the left side of each figure, and the details are shown in the lower portion of the figure. The lower row shows quasi-color figures, with the black portions shown in red. It is clear that autoignition occurred in the unburned area and that the place where it first appeared touched the cylinder liner. This is very strange because the temperature of the cylinder liner surface is lower than the unburned gas temperature. This reason for this phenomenon is explained in Fig. 4. This figure shows the ignition delay time (vertical axis) as a function of temperature (horizontal axis) for two kinds of fuel.
It is clear that normal heptane displays simple combustion behavior. As the temperature increases, the ignition delay becomes shorter. In contrast, iso-octane exhibits a unique characteristic. From about 680 to 750K, the ignition delay becomes shorter as the temperature decreases.(4) This characteristic is indicative of the negative temperature coefficient (NTC) effect. This means that the temperature of a portion of the air-fuel mixture is in the NTC region and that the reactions begin under a lower temperature regime. This phenomenon explains the knocking cycles in Fig. 3. The reason for the occurrence of autoignition under a lower temperature regime like that near the cylinder wall is explained by this effect.