Cummins’ XPI Fuel System was a significant breakthrough in heavy-duty fuel systems.
Dr. Julius Perr (page 22) conceived the basic design, which he originally called the “Flexible and Efficient Fuel System.”
At the time, modern heavy-duty engines were fueled by “unit injectors” similar to the designs that originated with Clessie Cummins – individual fuel injectors for each engine cylinder where combustion takes place.
Alternative “common rail” fuel systems, which were being used on some smaller, high-speed diesels, were more flexible because they were powered independently by a separate fuel pump with electronically controlled injection.
But they were inefficient for heavy-duty applications, where good fuel economy is a must, and their injection pressure capability was too low for future emission controls.
Cummins created a design that retained common rail flexibility but made it much more efficient and at the same time greatly increased the maximum injection pressure. The increase was so far above competitors’ capabilities it was named XPI, short for eXtreme high Pressure Injection.
XPI allows for a small amount of fuel to be injected into the combustion chamber slightly before the main injection to reduce noise and prepare the chamber for lower emissions. A small injection after the main injection can optimize aftertreatment temperatures.