In this article, I will discuss the Automatic Injection Timer in detail.
Automatic Injection Timer–
During the injection process, the nozzle is opened by a pressure valve which is propagated through the fuel injection tuning at sound velocity. While the necessary time is constant and independent of engine speed, the injection ag in crank degrees increases with speed.
Similar conditions apply to the ignition process. The delay period from the beginning of injection to the initiation of ignition is approximately constant in time and independent of engine speed. For this reason, the ignition lag in crank degrees also increases with speed.
Since the most favorable combustion and optimum performance of a diesel engine are obtained only at a determined piston position referring to the degrees of crank angle, it is recommended to advance the beginning of the delivery of the injection pump with increasing speed. This is achieved by an Automatic Injection Timer.
The function of Automatic Injection Timer-
During operation, the piston rotates the pump shaft up to 8 degrees with respect to the engine shaft, so that the beginning of the injection is advanced by the same value. The Automatic Injection Timer is generally recommended for engines with a wide speed range(Motor Vehicle Engines) and longer fuel-injection tubings. There are many designs of Automatic Injection Timers, however, their principle of operation is essentially the same.
The function of the automatic injection timer depends on the engine speed since centrifugal force is used to affect the advancement of injection. The main parts of the automatic injection timer are the coupling flange, housing, driving flange, bearing cover, two flyweights, and two helical springs.
The coupling flange is mounted on the camshaft by a round nut and washer. Both flyweights are hinged on the pivot pins, and seated on the coupling flange. The Pivot pins act at the same time as supports for the helical springs. The driving flange with its two coupling dogs and the bearing cover is positively welded together. The inside face of the driving pins protrudes through two holes in the bearing cover and serves as rests for the two holes of the bearing cover and serves as a rest for the two helical springs. The driving members thus transmit the driving torque from the engine to the injection pump camshaft through the flywheels which are positively connected to the driving members through the tension of the springs. The flyweights have a curved surface, the curvature of which varies according to the required degrees of advance.
A helical spring of a certain predetermined initial tension and stiffness is inserted between each pivot pin and driving pin. The initial tension can be inserted shims under the spring whereby both the speed range within which the timing device will operate and the angle of advancement can be changed. The housing is screwed on the coupling flange. Gaskets are installed between the housing and the coupling flange, housing, and driving flange, and between the driving flange and the sealing edge of the coupling flange. These gaskets ensure proper sealing of the timing device.
Working principle of Automatic Injection Timer-
|Working principle of Automatic Injection Timer
With increasing speed, the centrifugal force causes the flyweights to move outward. Thereby the curved surfaces of the flyweights slide along the driving pin B of the driving flange. The latter is positively connected to the engine drive shaft. The driving and coupling flanges are interconnected only by the two helical springs that can be rotated about each other to the extent of degrees of the angle of advance.
The centrifugal force acts on the flyweights against the spring tension but in the direction of rotating. By this, it exerts a pull on the pivot pin ‘A’ on the coupling flange in such a way that when the speed increases, the coupling flange and with it the camshaft, advances with the driving flange by a maximum angle of 10 degrees. Thereby, the commencement of delivery in each injection pump element will be advanced accordingly.
The curvature in the flywheel is so designed that at low speeds i.e. at low centrifugal force, the travel of flyweight per one degree of advance is relatively long and it becomes gradually shorter with increasing speed. This arrangement provides for sufficient advancing torque at low speeds. At higher speeds i.e. with a high centrifugal force, shorter flyweight travel is sufficient to advance the camshaft, since the value of the centrifugal force increases with the square of speed.
The construction and characteristics of EP/SCZ and EP/SF types injection timers are the same as those of EP/SA type. They are, however, driven by gears fixed to the timers instead of coupling dogs.
Maintainance of Automatic Injection Timer-
EP/SA type Automatic Injection Timer should be inspected every 5,000 km for grease or oil leakage(grease or oil traces on adjustment parts).
If leakage is observed, it must be removed and repaired in a fully equipped service station, For example, a MICO service station.
After 20,000 to 30,000 km, the injection timer must be lubricated in a special service station.
EP/SCZ and EP/SF types of injection timers are maintenance-free and are constantly lubricated by engine oil in the gear case.