diesel fuel systems

Mechanical Governer

Friday, August 15, 2008

Diesel Fuel Systems Mechanical Governors This Meeting Guide is the third in a series dealing with the basic diesel engine fuel system and components. It is about the diesel governor. Fig. 01 Each Caterpillar diesel engine is equipped with a governor. Why? Diesel engines can accelerate-increase speed-at the rate of more than 2000 revolutions per second. Yes, PER SECOND. Without a governor a diesel engine can quickly destroy itself. Fig. 02 GOVERNORS Never operate a diesel engine without a governor controlling it. If you were to move the fuel rack of a diesel engine to the full “ON” position without a load and with the governor not connected, the engine speed might climb and exceed safe operating limits before you could shut it down. One second...two seconds...before you knew what was happening, the engine may have been seriously damaged by overspeeding. This warning - never operate a diesel engine without a governor controlling it - is concerned with one of the purposes of governors: to prevent engine overspeeding. Governors also keep the engine at the desired speed and increase or decrease engine power output to meet load changes. WARNING Fig. 03 This presentation introduces and explains the mechanical governor. The mechanical governor is the simplest of the various types of governors and is basic to their operation. Besides the mechanical governor, Caterpillar engines use: servomechanical governors, hydraulic governors and electronic governors. These governors will be discussed in future presentations. MECHANICAL Fig. 04 This tractor is equipped with a mechanical governor. We can see the governor control lever, the control linkage, the governor and the fuel injection pump housing. Fig. 05. This is a closeup of the governor, mounted on the rear of the fuel injection pump housing. Let’s look at the construction and operation of the mechanical governor using schematic illustrations. Fig. 06 Diesel engine mechanical governors consist of two basic mechanisms: the speed measuring mechanism and the fuel changing mechanism. Fig. 07 The speed measuring mechanism senses engine speed changes, and the . . . . Fig. 08 . . . fuel changing mechanism increases or decreases the amount of fuel supplied the engine to correct these changes. Let’s look at each basic mechanism separately and learn how it operates. Fig. 09 The speed measuring mechanism is simple, has few moving parts and measures engine speed accurately. The main parts are: 1) gear drive from the engine, 2) flyweights, and 3) spring. Fig. 10 The flyweights and “L” shaped ballarms which pivot are mounted on the governor drive. Fig. 11 The flyweights are rotated by the engine. Fig. 12 As the flyweights rotate, they exert a centrifugal force outward. The flyweights move outward pivoting the ballarms upward. The amount of outward force depends on the speed of rotation. Centrifugal force is the basic operating principle of the speed measuring mechanism. Now, what is centrifugal force? Fig. 13 If we tie a ball on a string . . . . Fig. 14 . . . . . and swing it around and around . . . Fig. 15 faster and faster, an outward force-centrifugal force- is exerted on the ball. This centrifugal force swings the ball outward and upward until the ball is nearly straight out. And, we can see that the faster we swing it, the greater the pull on the string and the farther outward it swings. Fig. 16 This force - centrifugal force - is the basic principle used in the speed measuring operation of the diesel engine governor. Keep centrifugal force in mind as we discuss the other parts of the speed measuring mechanism. Remember, the greater the engine speed, the greater the centrifugal force and, therefore, the greater the movement of the flyweights and ballarms. Fig. 17 We need to control this centrifugal force, so we have the governor spring. The spring acts against the force of the rotating flyweights and tends to oppose them. The force exerted by the spring depends on the governor control setting. Fig. 18 A lever connected to the governor control pushes on or compresses the spring. The spring force opposes the flyweights to regulate the desired engine speed setting. The governor control, shown here as a simple push-pull knob, may be a hand operated control lever or a foot operated accelerator pedal. Fig. 19 As long as the spring force equals the flyweight centrifugal force, engine speed remains constant. Fig. 20 The speed measuring mechanism, then, senses and measures engine speed changes. The fuel changing mechanism links the speed measuring mechanism with the fuel injection pumps to control engine. Fig. 21 The fuel changing mechanism consists of the: 1) connecting linkage, 2) rack and 3) the fuel injection pump. Fig. 22 Flyweight movement - outward in this example - due to engine speed changes, are transferred through the simple linkage to the rack and, therefore, to the fuel injection pump plunger. Fig. 23 When the engine load increases - as when a dozer digs in - the speed decreases. The flyweight force decreases, and the spring moves the linkage and rack to increase the fuel to the engine. The increase fuel position is held until the engine speed returns to the desired setting, and the flyweight force again balances the spring force. Fig. 24 When the engine load decreases, the speed increases. The flyweight force increases, overcoming the spring force, moving the rack to decrease fuel to the engine. The decrease fuel position is held until engine speed returns to the governor control setting, and the spring force again balances the flyweight force. Fig. 25 In summary, the basic governor consists of the: drive gears, flyweights, spring, and control lever of the speed measuring mechanism, and the connecting linkage, rack and fuel injection pump of the fuel changing mechanism. Fig. 26 The rack which meshes with the injection pump plunger gear segments extends from the injection pump housing into the governor. The rack and fuel injection pumps are parts of the fuel injection pump housing assembly. Fig. 27 As you recall, Meeting Guide 43, Fuel Systems: Part 2, explained fuel injection pump operation and how the fuel injected into each cylinder is increased or decreased. Fig. 28 In this cutaway governor and fuel injection pump housing, we see that the rack extends into the governor. Rack movement controls the amount of fuel injected in each cylinder. Let’s look at a closer view of our cutaway governor. Fig. 29 In this cutaway section of our housing, see the flyweights, spring, spring seat and thrust bearing. The thrust bearing (not previously mentioned) is an anti-friction bearing between the flyweight ballarms which rotate and the spring seat which, of course, does not rotate. Fig. 30 The governor is driven by the lower gear bolted to the fuel injection pump camshaft. The control lever has been removed from its shaft in the governor housing and set in place to show how it is positioned. Fig. 31 Looking closer, we can see (from right to left) the drive gear , flyweights , spring, spring seats, control lever and the collar and bolt which connects to the rack. The purpose of the collar is explained later. Fig. 32 This governor cross section illustrates: (1) lever, (2) spring seat, (3) spring, (4) spring seat and thrust bearing and (5) flyweight assembly. The arrows indicate drive gear rotation and rack movement. Fig. 33 Two adjusting screws limit the travel of the governor control lever between LOW IDLE position and the HIGH IDLE position. The low idle stop and high idle stop are simply minimum and maximum engine rpm settings with no load on the engine. Fig. 34 The high and low idle adjusting screws are located under the cover on the governor. Fig. 35 Notice that the holes in the cover are shaped to lock the screws and prevent them from turning after they are adjusted. Fig. 36 The operators control is positioned at the desired governor setting: low idle, high idle or fuel off. Fig. 37 When the lever in the governor is in the LOW IDLE position, a spring loaded plunger in the lever assembly contacts the low idle stop of the adjusting screw. Fig. 38 When the lever in the governor is in the HIGH IDLE position, the lever contacts the high idle adjusting screw. Fig. 39 To shut the engine down, the governor control is moved full forward - past . . . . Fig. 40 . . . the low idle stop. It is necessary to force the plunger over the shoulder on the low idle screw . . . Fig. 41 . . .to move the rack to the FUEL OFF position. Fig. 42 Looking, again, at the governor cross section see (1) the high idle adjusting screw and (2) the low idle adjusting screw. The lever is against the HIGH IDLE screw. The low idle and high idle screws, then limit minimum and maximum engine rpm with no load on the engine. What limits engine power output when the engine is fully loaded? Fig. 43 A collar and stop bar limit rack travel and, therefore, the power output. The collar is secured by a bolt connecting the rack linkage. The stop bar is mounted in the governor housing. With the rack moved to the FULL LOAD position, the collar just contacts the stop bar. Fig. 44 When our engine is operating with the governor at high idle (1) and picks up a load, the speed decreases, flyweight centrifugal force lessens, and the spring moves the rack to give the engine more fuel increasing power. The collar (2) and stop bar (3) limit the distance the spring can move the rack. As the collar contacts the stop bar, full load position is reached. This limits the fuel delivered to the engine so as not to exceed design limitations. Fig. 45 Returning to the governor cross section, note the location of the: (1) collar, (2) stop bar, (3) bolt and (4) rack. Like other diesel engine components, the governor must be lubricated for long life. Let’s look at a governor lubrication system schematic. Fig. 46 The governor is lubricated by the engine lubricating system. Oil from the diesel engine oil manifold is directed to the governor drive bearing. All other governor parts are lubricated by splash. The oil drains from the governor, through the fuel injection pump housing, back to the engine crankcase. Fig. 47 In summary, we have discussed the mechanical governor’s primary components and principle of operation. Remember a governor has two basic mechanisms: the speed measuring mechanism and the fuel changing mechanism. Fig. 48 In our cross section we located the lever, spring, spring seats, flyweights, thrust bearing, drive gears and rack. We also discussed the high and low idle settings and the full load stop. At the beginning of this lesson we warned: NEVER OPERATE A DIESEL ENGINE WITHOUT A GOVERNOR CONTROLLING IT. Why are governors so important to a diesel engine? Fig. 49 Note: The instructor should make clear we are not saying gasoline engines never have a governor. Some gasoline engines use a governor for the same reasons as a diesel: to control engine speed and to regulate engine power output. First, gasoline engines are self-limiting. Engine speed is controlled by a butterfly valve in the intake manifold which limits the air supply Limiting the amount of air taken in for combustion, limits engine speed. Fig. 50 Diesel engines, however, are not self-limiting. Engine air intake is not limited, and the cylinders always have more air than is needed to support combustion. The amount of fuel injected into the cylinders controls engine speed. Fig. 51 And, as the fuel is injected directly into the cylinders rather than into the air intake manifold, engine response is immediate. This, resulting greater power stroke, adds up to very rapid acceleration. As we said earlier, diesel engines can accelerate at a rate of more than 2000 revolutions per second. Because of this rapid acceleration, manual control is difficult, if not impossible. Fig. 52 NEVER OPERATE A DIESEL ENGINE WITHOUT A GOVERNOR CONTROLLING IT. Fig. 53 At this point, we have built up the basic diesel mechanical governor. This governor works fine on engines whose engine speed is held fairly constant and the governor is controlled by hand. However, on other engines, the force needed to compress the governor spring or to move the rack -just operating the governor - could be very tiring to the operator. Fig. 54 With the servo-mechanical governor, the work operation of compressing the governor spring is done with engine oil pressure. Fig. 55 With the hydraulic governor, the work operation of moving the fuel injection pump rack is done with engine oil pressure. These governors are discussed in . . . . Fig. 56 . . . . Meeting Guide 60, “Servo Mechanical Governors.” Fig. 57