Engine Speed Governing

During operation, the load on the engine of an automobile frequently varies, depending on the ambient conditions (lay of the ground, soil condition, etc.). These load variations cause the engine speed to change accordingly, provided the position of the throttle valve or fuel control rack remains unchanged.

When the load on the engine is decreased, the engine speed may rise beyond safe limits, causing accelerated wear of the engine working parts and increased fuel consumption. A device for automatically controlling the speed of an engine by regulating the intake or injection of fuel, so that the engine speed is maintained at the desired level under all conditions of loading, is termed the governor.

Governor control may be effected by centrifugal (mechanical), hydraulic, pneumatic or combined (pneumatic-centrifugal) means. Speed governors may be classed as speed-limiting, constant-speed or all-speed. Speed-limiting and constant-speed governors are used on automobile engines and on auxiliary engines employed for starting tractor diesel engines. An automobile governor is in essence a maximum-speed governor.

The automobile maximum-speed governor is of pneumatic-centrifugal type. It comprises two mechanisms: centrifugal governor sender and diaphragm-operated governor actuator. The centrifugal governor sender includes a rotor whose shaft receives the rotation from the engine camshaft. The sender unit is mounted on the timing gear cover. The rotor houses the valve which is pulled away from its seat by a spring.

The governor actuator includes a diaphragm which is connected by a rod to one end of the bell-crank. The other end of the bell-crank is connected with the governor spring. The bell-crank is fixed to the throttle spindle. The throttle control mechanism incorporates a special claw coupling which enables the governor actuator to close and open the throttle valve irrespective of the position of the accelerator pedal. The spaces above and below the diaphragm are interconnected through the governor sender by pipes. On the other hand, the space above the diaphragm communicates with the carburetor chamber below the throttle valve and that below the diaphragm, with the space below the choke valve, via passages.

So long as the engine speed remains below the preset maximum (3200 rpm), the centrifugal force of the valve is insufficient to overcome the tension of the spring, and so the valve stays open. As a result, the space above and below the diaphragm communicate with each other through the governor sender. When the maximum speed is attained, the centrifugal force of the valve overcomes the resistance offered by the spring, and the valve closes ott to its seat, thus breaking communication between the spaces above and below the actuator diaphragm. This action causes the depression communicated from the carburetor chamber to the space above the diaphragm, via the lower passage, to grow higher. If the engine speed increases further, this depression becomes sufficiently high for the diaphragm to deflect upward against the governor spring force and close the throttle valve cy a certain amount, thus reducing the engine speed.

The main component parts of the governor are a housing, a shaft with a drive disk, a spring, and ball-shaped weights (balls) (Fig. 9)

The Governor housing is held to the engine crankcase through the medium of an adaptor plate. The governor shaft notates together with the drive disk whose slots accommodate balls that are sandwiched between the thrust washer and the conical shaped movable disk.

The movable disk is pressed against the balls by the bell-crank loaded by a spring. Rigidly fixed on the pivot pin of the bell-crank is the external governor lever which is connected with the throttle rod. When the engine is not operating, the spring forces the movable disk to the extreme left-hand position and the throttle nod, to the extreme right-hand position. With the throttle rod in this position, the throttle valve is fully open. During engine operation, the centrifugal forces of the rotating balls cause the balls to move outward against the spring force and displace the movable disk to the right and the throttle rod to the left, thus closing the throttle valve.

With a ready load on the engine, an equilibrium exists between the centrifugal force of the rotating balls and the governor spring force. If the load on the engine is increased, the engine speed, hence the governor shaft speed, starts dropping. In this case, the centrifugal force of the balls decreases, and the governor spring, acting through the bell-crank, moves the throttle rod to the right to open the throttle valve. As a result, the engine speed rises until its nominal value is reached. As the engine load is reduced, the engine speed rises accordingly. The centrifugal force of the rotating balls then increases and, overcoming the governor spring force, moves the throttle rod to the left to close the throttle valve. This causes the engine speed to drop down to the nominal value.

The desired engine speed is set by varying the preload on the governor spring by means of adjusting the screw. This is done at the factory, after which the screw is locked in place by a lock nut and sealed.

Fig. 9 Constant-speed governor.
1-throttle rod; 2-external lever; 3-spring; 4-adjusting screw; 5-bell-crank; 6-housing; 7-pivot pin; 8-movabie disc; 9-governor shaft; 10-drive disc; 11-ball; 12-thrust washer.

The Valve Mechanism of an Automobile Engine

The exhaust valves of some auto mobile engines are made to rotate positively by a special mechanism during engine operation. This mechanism, known as the valve rotator, consists of a stationary housing with five ramplike grooves along its circumference that contain five steel balls loaded by return spring. Freely placed on top of the balls is a flexible washer (conespring) against which rests valve spring through the intermediary of a seating collar (Fig. 9).

Fig. 9 Valve mechanism of an automobile engine
a) exhaust valve completewith valve rotator; b) valve rotator; c) valve rotator position with closed valve; d) valve rotator positoin with open valve
1-stopper; 2-hollow valve stem; 3-valve spring; 4-valve rotator; 5-valve guide; 6-insert valve seat; 7-valve face; 8-snap ring; 9-seating collar; 10-flexible washer (coneSpring); 11-valve rotator housing; 12-steel ball; 13-return spring.

As the valve is opened, spring is compressed and its increasing load causes the flexible washer to flatten out and force balls 12 down their ramps against the resistance offered by their return springs. As the balls roll down, they turn through some angle the flexible washer, seating collar 9, and valve spring together with the valve. When the valve is closed, the valve spring load decreases, the flexible washer deflects to acquire its initial conical shape and, abutting against a shoulder in housing, releases the balls which are then forced by their coil springs to return to their starting position.

In automobile engines, seating face of the exhaust valves is hard-faced with a heat-resistant alloy. Valve stem is made hollow and the cavity is filled 50 to 60% full with metallic sodium. The cavity is sealed off by stopper welded to the valve head. The metallic sodium liquefies, but does not vaporize at valve operating temperature and is capable of conducting heat much better than the valve material itself. Hence, the shaking of the liquid sodium due to the motion of the valve increases the heat transfer from the valve head to the stem and thence to the cooled region of valve guide, thus reducing the valve head temperature.

The intake valves heat less during engine operation, since they are cooled by the incoming air or fuel-air mixture on the intake stroke. To lessen the leakage (pulling) of oil into the cylinder through the clearance between the intake valve stem and its guide, some engines use an umbrella-type oil deflector cup of oil-resistant rubber placed on the valve stem. This extends over the valve guide and is held fast under the valve spring seating collar.

Valve guide supports the valve stem and guides its movement so that the valve face remains perfectly concentric with the valve seat and fits it without any skewing. Replaceable, or insert, valve guides are fabricated from cast iron or a cermet material and pressed in the cylinder head.

The intake and exhaust valve seats in many engines are formed by intake and exhaust valve seats in many engines are formed by insert rings of the heat-resistant cast iron that are pressed in the cylinder head. This facilitates their renewal during repair.

The valve spring provides the force necessary to close the valve and hold it tightly against its seat.

Valve springs may have their coils spaced either evenly or variably. Variably spaced valve springs, also known as the progressive rate springs, lessen the tendency toward surging as a result of resonance effects where such spring are used, the close-wound end (with less coil spacing) must be installed to-ward the valve had.

Some engines use two springs on each valve, which reduces the size of the springs, improves their reliability, and makes their operating conditions less arduous.

The motion-transmitting components of the valve mechanism serve to transmit motion from the camshaft to the valves. These include valve lifters, push rods, rocker arms with adjusting screws, and rocker-arm shaft with pedestals and locating springs.

Valve lifters, or tappets are made of steel. The tappets, also known as cam followers, may be either of sliding or of pivoting type. The sliding type may have a plain cylindrical or a mush-room-like form. Plain cylindrical tappets and mushroom-type tap-pets are provided with spherical seating to effect engagement with the pushrods. The bottom contact faces of these tappets may be either flat or slightly convex. To ensure a uniform distribution of wear over the face, the tappets are in duced to rotate as they reciprocate during engine operation. With flat-bottomed tappets, rotation is achieved by offsetting the tappet barrel axis relative to the cam axis, while in the case of spherical bottom tappets, this effected by making use of cams that are slightly tapered across their width, as a result of which the contact point between the tappet face and the cam is offset relative to the tappet axis.

Push rod serves to transmit force from the valve lifter to the rocker arm. Push rod may be fabricated from a solid or hollow steel bar. Push-rod ends are made in the form of either a ball or a socket and are thoroughly ground and heat treated. The lower end of the pushrod is ball-shaped. It engages the spherical socket in the valve lifter barrel. The upper end may-be either ball-or socket-shaped and it mates with the spherical socket or the convex ball on the end of the adjusting screw in the rocker arm.

The pivoting roller-type cam followers take the form of a hinged lever arm. On one end of the follower arm, there is a boss with a bore into which is pressed brone bushing to receive a hollow shaft supported in the cylinder –block-and-crankcase unit. The other end of the cam follower carries roller that rotates in a needle bearing and contacts the cam. Immediately above the roller, the follower is provided with an insert spherical seating that is press-fitted into a bore in the follower arm and engages the ball end of the push-rod.