LO 8-11 Identify and describe the purpose, function, and types of diesel engine flywheels

1. Flywheels

Flywheels

Flywheels are like large plates bolted to the rear of the engine, and they serve several purposes. Their primary purpose is to provide some inertia between power impulses to keep the engine rotating. Storing energy in the heavy flywheel produced during each power impulse and releasing it when the engine speed slows down helps to smooth out engine speed. A toothed ring gear, fastened to the circumference of the flywheel, provides a drive point for the starting motor pinion gear. To crank the engine, the starter will engage the engine through the flywheel ring gear. Starter ring gears are usually friction fitted; this means they are first heated and placed over the circumference of the flywheel before they contract. Using an electric welding machine, a bead of filler material is applied to the flywheel and ring gear in several places to assure adhesion to the flywheel when ring gears are replaced in the field.

Behind the flywheel is the rear main bearing seal (also called the rear seal ). The rear seal prevents crankcase oil from leaking into the flywheel housing. Turning tools are often temporarily attached to the flywheel to rotate the engine when performing service procedures Figure 71.

A flywheel also provides a mounting surface for a clutch in the case of a manual transmission and a torque converter for automatic transmissions. The heavier mass of the torque converter requires only a thinner and lighter flywheel (called a flexplate ) to connect to the engine Figure 72. Flywheel shape and construction can vary depending on the type of clutch used and whether torsional damping capabilities are built into the flywheel.

On the flywheels of many engines are scribed markings, letters or numbers that are used as reference points to adjust valves or install fuel injection components. For this reason, the flywheel must be precisely lined up with the correct engine position corresponding to markings on the flywheel. Flywheel bolt holes may be offset to ensure that the flywheel is installed in the correct position. A dowel pin may also be used to properly locate a flywheel to align the centerline of the flywheel with the crankshaft Figure 73.

FIGURE 08-71: The turning tool attached to the bell housing of this Detroit DD15 is used to rotate the engine during service procedures.

FIGURE 08-72: A thinner, lighter flywheel, called a flexplate , is used to connect the torque converter of an automatic transmission to the engine.

FIGURE 08-73: This flywheel has a dowel pin to verify that the flywheel is installed in the correct position.

1.1. Dual Mass Flywheels

Dual Mass Flywheels

Torsional vibration damping is important to drivetrain durability and reduction of noise, vibration, and harshness. A number of MD engines incorporate specialized torsional vibration damping springs into the flywheel and are called dual mass flywheels (DMFs). When connected to manual transmissions, DMFs enable the use of lighter transmissions and drivelines by minimizing torque spikes produced by torsional vibration Figure 74. Transmissions shift easier, and transmission gear rattle is virtually eliminated with DMFs.

DMFs are made from two separate sections that should be disassembled and inspected during clutch servicing. The clutch surface and ring gear are each separate halves of the flywheel. DMFs are further recognized by a series of springs located around the flywheel circumference on the engine side of the flywheel. One set of bolts (near the center hub) connects the flywheel to the crankshaft and another set of bolts holds both halves of the flywheel together Figure 75.

FIGURE 08-74: Dual mass flywheels minimize the transfer of torsional vibration to transmissions and drivelines.

FIGURE 08-75: A dual mass flywheel is constructed of two major parts. This rear section contains damping springs used to absorb torsional vibration. A flat steel clutch plate (removed) bolts to the front surface section of the flywheel.

Power is transmitted to the flywheel clutch mounting surface through a series of springs located around the circumference of the inner plate, which attaches to the crankshaft. These springs act like the damping springs of a clutch disc to reduce torsional vibration. Between the inner and outer sections of the flywheel, a friction ring permits the inner and outer parts of the flywheel to slip. This friction ring will slip only under high torque input, which prevents transmission damage from excessively high torque inputs that are caused by loading the vehicle beyond its gross vehicle weight (GVW) capacity. The friction ring is made of a material similar to brake lining and can wear out if excessive torque loads are continuously applied. If this occurs, there is a loss of torque transfer to the powertrain and the flywheel requires replacement.

1.2. Reinstalling a Flywheel Housing

Reinstalling a Flywheel Housing

A flywheel housing and engine crankshaft must share a common centerline to prevent flywheel runout between it and the transmission input shaft. This is a particularly important check that must be made whenever a flywheel housing is transferred from one engine block to another, such as during engine replacement. If the reinstalled housing centerline does not match the centerline of the crankshaft, even if alignment dowels are used, significant problems can develop. First, the rear main seal may leak. Second, on manual transmissions, the input shaft bearing and the seal will quickly fail because the input shaft is operating at an angle. Vibrations produced by the nonaligned input shaft will travel up the main shaft in direct gear and cause gear jump-out in a non-synchronized transmission. Clutch plate splines and the input shaft will wear rapidly. With automatic transmissions, the transmission input seal will also leak. The starter motor may even be excessively noisy.

To reinstall a flywheel housing, follow the guidelines in Skill Drill 8-1.

1.3. SKILL DRILL 8-1  Reinstalling a Flywheel Housing