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Mount Vernon, WA | Here you go, straight from Cummins (sorry, pictures will not copy):
Step Timing Control, commonly referred to as STC, controls the engine timing in an effort to minimize white smoke at cold engine start-up.
Refer to Step Timing Control Familiarization, Bulletin 3387380, for addition information on STC.
STC has two stages of injection timing. The engine operates in the ADVANCED mode of injection timing during starting and light engine load conditions and at NORMAL timing during medium to high engine load conditions.
STC offers many advantages. During ADVANCED injection timing, it:
•Improves cold weather idling characteristics
•Reduces cold weather white smoke
•Improves light load fuel economy
•Reduces injector carboning.
During NORMAL injection timing, STC:
•Controls cylinder pressures
•Reduces nitrogen oxide emissions.
The STC system consists primarily of:
•STC injectors
•STC oil control valve
•STC plumbing and check valve.
These components control injection timing based on fuel pump rail pressure (engine load).
In the injector, injection timing is controlled by the STC hydraulic tappet. The tappet has an inner piston (plunger) and an outer piston (sleeve). These tappet components work together with the injector plunger to control injection timing.
In NORMAL timing, no oil is in the tappet. As the cam follower starts up the camshaft injection ramp, the injector rocker lever begins to force the inner piston downward. Because no oil is in the tappet, the inner piston must make direct contact with the outer piston before the injector plunger can begin its downward travel.
In ADVANCED timing, the tappet is filled with oil and the injector is metering fuel. As the cam follower starts up the camshaft injection ramp, the injector lever begins to force the inner piston downward. Since the oil between the pistons forms a solid link, the downward pressure is immediately transmitted to the outer piston and the injector plunger begins its downward travel earlier than it does in NORMAL timing. This causes the fuel to be injected earlier.
Engine oil flows from the STC oil control valve through the oil manifold to the tappets. Whenever the oil pressure in the oil manifold exceeds 70 kPa [10 psi], it moves the tappet inlet check ball from its seat and fills the cavity between the inner and outer pistons.
During the injection cycle, the oil is held inside the tappet by the inlet check ball and the load cell check ball. When the rocker lever forces the inner piston downward, the solid link of oil causes the injector plunger to contact the fuel earlier; therefore, the injection timing is in the ADVANCED mode. At the end of the injection cycle, injection force increased the oil pressure in the tappet and holds the injector plunger firmly in the cup.
This increased pressure moves the load cell check ball from its seat. The oil drains past the load cell check ball and through the drain holes in the injector adapter and returns to the oil pan through drain passages in the cylinder head and block. Meanwhile, with continued cam lift, the inner piston makes mechanical contact with the outer piston and maintains injector plunger seating force.
The STC control valve uses fuel pressure and spring force to control the position of an AFC style plunger. The position of the plunger dictates whether the oil passage to the hydraulic tappets is open or closed. Fuel pressure acts on the piston end of the plunger.
During ADVANCED timing (low fuel pressure), the spring opposes the fuel pressure and holds the plunger in the open position. Pressurized lube oil flows to the tappets and initiates ADVANCED engine timing. As fuel pressure increases, the spring holds the plunger in the open position until the fuel pressure rises above the certified switching pressure.
At this certified level, the higher fuel pressure overcomes the spring. This action shifts the plunger and closes the oil passage. The oil supply to the tappets is interrupted and the engine begins to operate in the NORMAL timing mode.
The control valve supplies oil to the STC rocker housing connection through the STC valve oil outlet line (1).
A check valve in the elbow fitting (2) prevents the oil from draining back into the engine when it is shut off. This prevents any delay of oil to the tappets during cold starts.
An internal oil manifold connects the oil supply to each STC injector in the rocker housing.
Fuel pressure to the STC valve is provided by a hose (1) between the fuel inlet passages in the cylinder head and the STC valve. The internal spring cavity of the valve is vented to the engine crankcase by the crankcase vent line (2) in order to allow the plunger to cycle freely.
The oil control valve is calibrated to a specific flow and pressure using a fuel pump test stand. Tampering with the valve or plumbing will result in the loss of both fuel economy and engine durability. Correct valve operation is necessary to maintain acceptable cylinder pressures and white smoke levels and to assure optimum fuel economy.
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cummins stc system (step timing control)
