DIESEL ENGINE SPEED, FUEL CONTROLS , AND PROTECTION

6 Jun

Understanding how diesel  engines are controlled  and the types of  protective instrumentation available is important for a complete understanding of the operation of a diesel engine.

E ngine C ontrol

The control of a diesel engine is accomplished through several components: the camshaft, the fuel injector, and the governor.  The camshaft provides the timing needed to properly inject the fuel, the fuel injector provides  the component that meters and injects the fuel, and the governor regulates the amount of fuel that the injector is to inject. Together, these three major components ensure that the engine runs at the desired speed.

F uel Injectors

Each cylinder has a fuel injector designed to meter and inject fuel into the cylinder at the proper instant.  To accomplish this function, the injectors are actuated by the engine’s camshaft.  The camshaft provides the timing and pumping action used by the injector to inject the fuel.  The injectors meter the amount of fuel injected into the cylinder on each stroke.  The amount of fuel to be injected by each injector is set by a mechanical linkage called the fuel rack.  The fuel rack position is controlled by the engine’s governor.   The governor determines the amount of fuel required to maintain the desired engine speed and adjusts the amount to be injected by adjusting the position of the fuel rack.

Each injector operates in the following manner.  As illustrated in Figure 26, fuel under pressure enters the injector through the injector’s filter cap and filter element.  From the filter element the fuel travels down into the supply chamber (that area between the plunger bushing and the spill deflector).  The plunger operates up and down in the bushing, the bore of which is open to the fuel supply in the supply chamber by two funnel-shaped ports in the plunger bushing.

The motion of the injector rocker arm (not shown) is transmitted to the plunger by the injector follower which bears  against the follower spring. As  the plunger moves  downward under pressure of the injector rocker arm, a portion of the fuel trapped under the plunger is displaced into the supply chamber through the lower port until the port is closed off by the lower end of the plunger.  The fuel trapped below the plunger is then forced up through the central bore of the plunger and back out the upper port until the upper port is closed off by the downward motion of the plunger.  With the upper and lower ports both closed off, the remaining fuel under the plunger is subjected to an increase in pressure by the downward motion of the plunger.

When sufficient pressure has built up, the injector valve is lifted off its seat and the fuel is forced through small orifices in the spray tip and atomized into the combustion chamber.   A check valve, mounted in the spray tip, prevents air in the combustion chamber from flowing back into the fuel injector. The plunger is then returned back to its original position by the injector follower spring.

On the return upward movement of the plunger, the high pressure cylinder within the bushing is again filled with fresh fuel oil through the ports.   The constant circulation of fresh, cool fuel through the injector renews the fuel supply in the chamber and helps cool the injector.  The fuel flow also effectively removes all traces of air that might otherwise accumulate in the system.

The fuel injector outlet opening, through which the excess fuel returns to the fuel return manifold and then back to the fuel tank, is adjacent to the inlet opening and contains a filter element exactly the same as the one on the fuel inlet side.

In addition to the reciprocating motion of the plunger, the plunger can be rotated during operation around its axis by the gear which meshes with the fuel rack.  For metering the fuel, an upper helix and a lower helix are machined in the lower part of the plunger.  The relation of the helices to the two ports in the injector bushing changes with the rotation of the plunger.

Changing the position of the helices, by rotating the plunger, retards or advances the closing of the ports and the beginning and ending of the injection period. At the same time, it increases or decreases the amount of fuel injected into the cylinder.  Figure 27 illustrates the various plunger positions from NO LOAD to FULL LOAD.  With the control rack pulled all the way (no injection), the upper port is not closed by the helix until after the lower port is uncovered. Consequently, with the rack in this  position, all of the fuel is  forced back into the supply chamber and no injection of fuel takes place.  With the control rack pushed all the way in (full injection), the upper port is closed shortly after the lower port has been covered, thus producing a maximum effective stroke and maximum fuel injection.  From this no-injection position to the full-injection position (full rack movement), the contour of the upper helix advances the closing of the ports and the beginning of injection.

G overnor

Diesel engine speed is controlled solely by the amount of fuel injected into the engine by the injectors.   Because a diesel engine is not self-speed-limiting, it requires not only a means of changing engine speed (throttle control) but also a means of maintaining the desired speed.  The governor provides the engine with the feedback mechanism to change speed as needed and to maintain a speed once reached.

A governor is essentially a speed-sensitive device, designed to maintain a constant engine speed regardless of load variation.   Since all governors used on diesel engines control engine speed through the regulation of the quantity of fuel delivered to the cylinders, these governors may be classified as speed-regulating governors.  As with the engines themselves there are many types and variations  of  governors.  In  this  module, only  the common mechanical-hydraulic type governor will be reviewed.

The major function of the governor is determined by the application of the engine.  In an engine that is required to come up and run at only a single speed regardless of load, the governor is called a constant-speed type governor.  If the engine is manually controlled, or controlled by an outside device with engine speed being controlled over a range, the governor is called a variable- speed type governor.    If the engine governor is designed to keep the engine speed above a minimum and below a maximum, then the governor is a speed-limiting type.  The last category of governor is the load limiting type.  This type of governor limits fuel to ensure that the engine is not loaded above a specified limit.  Note that many governors act to perform several of these functions simultaneously.

O peration of a G overnor

The following is an explanation of the operation of a constant speed, hydraulically compensated governor using the Woodward brand governor as an example. The principles involved are common in any mechanical and hydraulic governor.

The Woodward speed governor operates the diesel engine fuel racks to ensure a constant engine speed is maintained at any load.   The governor is a mechanical-hydraulic type governor and receives its supply of oil from the engine lubricating system.  This means that a loss of lube oil pressure will cut off the supply of oil to the governor and cause the governor to shut down the engine.  This provides the engine with a built-in shutdown device to protect the engine in the event of loss of lubricating oil pressure.

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