Fuel injection during negative valve overlap period was used to realize diesel homogeneous charge compression ignition (HCCI) combustion. In order to control the combustion, $CO_2$, in-cylinder injection was used to simulate external EGR. Effects of $CO_2$ injection parameters (injection timing, quantity, pressure) on HCCI combustion and emission characteristics were investigated. Experimental results revealed that $CO_2$ in-cylinder injection can control the start of combustion and effectively reduce $NO_x$ emission. Either advancing $CO_2$ injection timing or increasing $CO_2$ injection quantity can reduce peak cylinder pressure and mean gas temperature, delay the starts of low temperature reaction (LTR) and high temperature reaction (HTR), and lower pressure rise rate; $NO_x$ emission was reduced, while smoke, HC, and CO emissions increased. Since the combustion phase was improved, the indicated thermal efficiency was also improved. Injection pressure determines the amount of disturbance introduced into the cylinder. Generally, with the same injection quantity, higher injection pressure results in higher momentum flux and total momentum. Larger momentum flux and momentum has a stronger disturbance to air-fuel mixture, resulting in a more homogeneous mixture; therefore, larger injection pressure leads to lower $NO_x$ and smoke emissions.