In the combustion process, diesel fuel takes a certain amount of time to ignite and burn. As the engine runs faster, the burn time remains the same, and much of the burning takes place after TDC (top dead center). This is called ignition lag and almost always results in lowered performance. To offset this ignition lag, fuel must be injected sometime before TDC to give good performance at rated speed. However, with this fixed advance of injection, engine performance is optimum at rated speed only. Engines that vary speeds over a wide range, that is, automotive vehicles, need injection timed correctly at all speeds. This is the function of the timing device.
The Bosch automatic timing device is used on inline camshaft driven pumps (Figure 19-46), and is classified
as a flyweight-operated device. Mounted at the front of the injection pump on the camshaft, the timing device is connected to the driving gear of the engine (Figure 19-46). Through the action of centrifugal force, the flyweights swing outward with increasing speed. Rollers mounted on the flyweights push against the cam plate (Figure 19-47), which is connected to the pump camshaft. This causes the camshaft to rotate a maximum of 8°, providing proper timing in relation to engine speed.
Stop pins limit the maximum amount of advance that can be obtained from any timing advance assembly. As with any automatic timing device, should the unit become worn or damage, fuel injection timing will not be con trolled correctly. Result in poor engine performance and possible engine damage. Generally, if the timing advance unit were to stick in the full-advance position, the start of fuel injection would be too early at an idle speed, and severe combustion knock would result, together with a tendency for white smoke to appear at the exhaust stack. On the other hand, a timing advance unit that will not advance past the idle setting would result in late injection and the engine would be very sluggish, together with black smoke appearing at the exhaust stack though incomplete combustion.
