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Cummins STC – Step Timing Control In 1986 Cummins introduced Step Timing Control to the PT fuel system. These systems provided the following advantages: y Reduced start-up and cold weather white smoke y Improved idle characteristics y Improved light load fuel economy y Reduced injector tip/cup carboning The STC system was used primarily to reduce N0x emissions by allowing an advanced injection timing during light load and idle operation and retarding the timing at loads above ~25% of rated torque. This was accomplished by the installation of a collapsible hydraulic tappet in the top of the injector. At light loads the tappet filled with oil and caused a timing advance by causing the plunger to travel downwards sooner. When under load, the tappet did not have oil supplied by the control valve and timing was consequently retarded. During the advance mode the injector plunger seated firmly in the injector cup and the oil was pushed out of the tappet.

¾Oil is supplied to the STC injector under the correct conditions by the STC control valve. ¾The control valve sends oil to the injector until fuel pressure overcomes spring tension within the valve, which causes oil flow to be cut-off.

¾During Normal timing – no oil flow to injector tappet ¾During advance timing – rail pressure is low and oil can flow through spool valve to injector tappets. Note an external adjustment screw to control shift points. ¾Inner base circle adjustment are performed on STC injectors