Explain the function of a 'sac volume' in a diesel fuel injector.

The sac volume in a diesel fuel injector is a small cavity located at the very tip of the injector nozzle, just below the spray holes. Its primary function is to minimize dribbling and improve the overall atomization of the fuel during and after injection. After the main fuel injection event, there is still a small amount of fuel remaining in the injector body and especially in the area around the needle valve. Without a sac volume, this remaining fuel would tend to dribble out of the injector nozzles due to residual pressure and capillary action once the injection pressure drops. This dribbling can cause incomplete combustion, increased smoke, and carbon deposits on the injector tip and cylinder. The sac volume provides a space for this residual fuel to collect after the injection event. Because the sac volume is relatively small and its contents are exposed to the high temperatures inside the combustion chamber, the fuel trapped within quickly vaporizes and combusts during the next combustion cycle, minimizing dribbling and unburnt fuel. This leads to cleaner combustion, reduced emissions, and improved engine efficiency. The size and design of the sac volume are critical and must be carefully optimized for the specific engine and fuel injection system to achieve the desired performance.