Diesel Engine Starting System | Part 2

The job of any starter motor assembly is to take a stored energy (electric, air, or hydraulic) and convert it into mechanical rotation to crank the engine fast enough to begin the engine’s ignition sequence. The most common type of starting system uses electrical energy; however, compressed air and hydraulic energy can be used as well. The following are the main comparable components of the three main types of starting systems:

We’ll first examine the different electric starter motor designs, next discuss air and hydraulic starter motors, and then look at the control circuit for starters. An electric starter will take stored electrical energy from a battery (or sometimes a capacitor) and convert it into torque at the starter’s pinion gear. The pinion then engages with the ring gear that is part of the engine’s flywheel and turns the flywheel that rotates the engine’s crankshaft. ● See Figure 2 for a cutaway of a starter and its main parts.

There are two main types of electric starter motor assemblies: Direct drive (pinion is driven directly by the armature): A direct drive electric starter has a motor that is designed to generate high torque at low speed and operate at high speed with low torque (the motor will sometimes exceed 5000 rpm) for a short length of time. It will use a solenoid actuated shift lever to push out the pinion to engage it with the ring gear before or just as the armature (rotating shaft in the motor assembly) starts turning. Gear reduction (higher speed motor output to a gear reduction and then to pinion): A gear reduction starter (planetary or pinion reduction) is designed to use a smaller higher speed electric motor to produce higher cranking torque with the same or less electrical power consumed. The heaviest and bulkiest part of a direct drive starter is the motor so by reducing motor size and weight the engineers have saved space and weight. Some direct drive starters are twice the weight as a comparable output gear reduction starter. Although this isn’t a big concern for a large machine, you will be thankful for the lighter weight whenever it comes time to change the starter. Gear reduction starter motor assemblies can have their motor offset from the output shaft or use planetary gears and have the motor shaft in-line with the output shaft. Direct Drive Starter Components ■ Starter housing: Center section that holds the pole shoes and field coils in place. ■ Nose piece: The drive end of the starter where the pinion gear is located. Holds the shift lever in place and supports the armature shaft with a bushing. ■ End cap: Opposite end of the starter from the nose piece. Supports brush holder assembly and the other armature shaft bushing. ■ Armature: The rotating part of the motor that has several windings that have each of their ends loop to a commutator bar. It will have splines to drive the starter drive. ■ Brushes: Contact the commutator bars and transfer electrical current to the armature. ■ Brush holders: Spring loaded to keep the brushes in contact with the armature. ■ Field coils: Heavy copper windings that create a strong magnetic field when current flows through them. ■ Pole shoes: Iron cores for the field coils that help to increase magnetism. ■ Solenoid: Has two windings (pull-in and hold-in) that get energized by the starter control circuit and magnetically move a plunger. The plunger is connected to a heavy contact disc that is a switch. The switch will send current from the battery terminal to the field coils. The plunger could also be connected to a shift lever that will move the pinion. ■ Pinion gear: The starter output that engages with the flywheel and cranks the engine. ■ Overrunning clutch: Drives the pinion from the armature shaft but will not allow the armature to be driven by the ring gear. ■ Shift lever: Used to push the pinion out to engage with the ring gear. (To Be Continued: For More Parts Visit Our Blog)