G333C ENGINE Caterpillar

Systems Operation


Gas, Air Induction, And Exhaust Systems


In addition to components illustrated in the above diagram, some installations have a safety shut-off-valve attachment in the gas supply line. The valve is electrically operated from the ignition system and can also be manually operated to shut off the engine. Manual resetting is required after shutdown to start the engine.

Turbocharger (Engines so Equipped)

The turbocharger is mounted on the exhaust outlet of the engine exhaust manifold. The energy that normally would be lost from the exhaust gases is used to drive the turbocharger.

As the engine starts, the flow of exhaust gases from the exhaust manifold is directed through inlet (12) to the turbine wheel (6). The turbine wheel and compressor impeller (1) are mounted on a common shaft. The exhaust gases pass over the turbine wheel forcing it and the impeller to rotate.

Incoming air, from the air cleaner, flows through inlet (7) to the center of the impeller. The rotating impeller compresses the air and forces it into the carburetor.

The turbocharger bearings are pressure-lubricated by engine oil. The oil enters through port (4) and is directed through passages to lubricate the thrust baring (3), sleeves (9 and 11) and the journal bearings (10). Oil leaves the turbocharger through a port in the bottom of the center section and is returned to the engine pump.

1-Compressor impeller. 2-Compressor housing. 3-Thrust bearing. 4-Lubrication inlet port. 5-Turbine housing. 6-Turbine wheel. 7-Air inlet. 8-Exhaust outlet. 9-Sleeve. 10-Shaft journal bearings. 11-Sleeve. 12-Exhaust inlet.

Differential Pressure Regulator (Engines Equipped With Turbocharger)

The differential pressure regulator mounted on the turbine housing, serves as an exhaust gas bypass valve. It limits the volume of exhaust gases to the turbine wheel by bypassing a part of the exhaust gases. The exhaust bypass valve is actuated directly by a pressure differential between the atmosphere and turbocharger compressor outlet pressure to the carburetor.

One side of the diaphragm in the regulator senses atmospheric pressure through a breather in the top of the regulator. The other side of the diaphragm senses air pressure from the turbocharger compressor outlet side through a control line. When outlet pressure to the carburetor reaches the proper value, the force of the air pressure upon the diaphragm overcomes the force of the spring and atmospheric pressure. This force unseats the bypass valve, allowing exhaust gases to bypass the turbine wheel.

The bypass passage is located inside the turbine housing. Under steady load conditions, the valve will assume a fixed position, allowing just enough exhaust gas to the turbine wheel to maintain the proper air pressure to the carburetor.

DIFFERENTIAL PRESSURE REGULATOR (Engines Equipped with Turbocharger)


The amount of air/fuel mixture admitted to the cylinders determines the power output of gas engines. This amount of mixture is controlled by the throttle plate (14) in the carburetor.

Air enters the carburetor through the air horn (6) and fills the outer chamber (8). Air enters the inner chamber (10) (mixing chamber) by forcing diaphragm (1) away from ring (7). Fuel enters the carburetor through the fuel inlet (11) and flows past the power mixture adjustment (13) to the center of the carburetor and into the fuel outlet tube. A fuel valve (4) is mounted on the diaphragm.

With the engine stopped, a spring holds the diaphragm against the ring and the fuel valve closed. No air or fuel can flow to the inner chamber. As the engine is started, the vacuum in the cylinders, created by the intake stroke of the pistons, creates a low pressure situation in the inner chamber. This low pressure is sensed above the diaphragm. The higher inlet pressure acting on the outer portion of the diaphragm overcomes the spring force and moves the diaphragm up. This also moves the fuel valve up and allows air and fuel to flow into the inner chamber. A small volume of air is also metered into the inner chamber through idle adjustment opening (12).

When the engine is operating at a constant RPM, the diaphragm remains in one position. This position is determined by the pressure differential between the incoming gas and air. The pressure differential to the carburetor can be varied by adjusting the line pressure regulator.

1-Diaphragm. 2-Sensing holes. 3-Spring. 4-Fuel valve. 5-Chamber. 6-Air horn. 7-Ring. 8-Outer chamber. 9-Fuel outlet tube. 10-Inner chamber. 11-Fuel inlet. 12-Idle adjustment opening. 13-Power mixture adjustment. 14-Throttle plate. 15-Balance line connection.

Line Pressure Regulator

A line pressure regulator is required on turbocharged engines. Also, naturally aspirated engines with a gas supply pressure exceeding the desired inlet pressure differential between the gas and air must use a line pressure regulator.

The regulator is adjusted by turning the adjusting screw (2).

1-Spring side chamber. 2-Adjusting screw. 3-Spring. 4-Outlet. 5-Valve disc. 6-Main orifice. 7-Main diaphragm. 8-Lever side chamber. 9-Lever. 10-Pin. 11-Valve stem. 12-Inlet.

Gas flows through the inlet (12), main orifice (6), past the valve disc (5), and through the outlet (4). The chamber (8) on the lever side of diaphragm (7) senses outlet pressure.

As gas pressure in chamber (8) increases to a value higher than the force of the diaphragm spring (3) and air pressure in the spring side chamber (1) (atmosphere on naturally aspirated engines; turbocharger boost on turbocharged engines), the diaphragm is forced against the spring. This pivots the lever (9) at pin (10) and causes the valve stem (11) to move the valve disc to close the inlet orifice. As the diaphragm moves in this manner, air is expelled from the spring side chamber. This is accomplished quickly enough to prevent any lag in the main diaphragm movement due to air compression.

With the inlet orifice closed, gas is pulled from the lever side chamber through the outlet. This reduces the pressure in the chamber and becomes less than pressure in the spring side chamber. Spring and air pressure in the spring side chamber forces the diaphragm toward the lever. This pivots the lever and opens the valve disc, allowing additional gas flow to the carburetor.

When the pressures on either side of the diaphragm are equal, the regulator supplies gas to the carburetor at a constant pressure differential above the incoming air.

Balance Line

The balance line is used only on turbocharged engines. It maintains the correct pressure differential between the line pressure regulator and carburetor inlet.

As the load on the engine changes, turbocharger boost pressure changes in the inlet manifold. The balance line communicates this change in pressure to the spring side of the diaphragm in the line pressure regulator. This pressure change acts to assist the regulator diaphragm to position the line regulator valve to correct the gas pressure to the carburetor. Thus, the correct pressure differential between the line pressure regulator and carburetor inlet is maintained.


Governor Operation

When the engine is operating, the balance between the centrifugal force of revolving weights (4) and the force of spring (5) controls the movement of valve (12). The valve directs pressure oil to either side of positioning piston (13). Depending on the position of the valve (12), piston (13) will move the shaft to increase or decrease fuel to the engine to compensate for load variation.

Pressurized lubrication oil enters passage (15) in the governor cylinder. The oil encircles sleeve (14) within the cylinder. Oil is then directed through a passage in piston (13) where it contacts valve (12).

When engine load increases, engine RPM decreases and revolving weights (4) slow down. The weights move toward each other and allow governor spring (5) to move valve (12) forward. As valve (12) moves, an oil passage around valve (12) opens to pressure oil. Oil then flows through passage (7) and fills the chamber behind piston (13). The pressure forces the piston and shaft forward increasing the amount of fuel to the engine. Engine RPM increases until the revolving weights rotate fast enough to balance the force of the governor spring.

When engine load decreases, engine RPM increases, revolving wights (4) speed-up, and the toes on the weights move valve (12) rearward, to allow the oil behind piston (13) to flow through a drain passage opened at the rear of the piston. At the same time, the pressure oil between sleeve (14) and piston (13) forces the piston and shaft to rear to decrease the amount of fuel to the engine. Engine RPM decreases until the revolving weights balance the force of the governor spring.

1-Collar. 2-Rear cover. 3-Seat. 4-Weight. 5-Governor spring. 6-Thrust bearing. 7-Oil passage. 8-Drive gear (weight assembly). 9-Cylinder. 10-Bolt. 11-Spring seat. 12-Valve. 13-Piston. 14-Sleeve. 15-Oil passage. 16-Governor drive housing. 17-Shaft. The governor valve is shown in the position when the force of the weights and the force of the spring are balanced.

Oil from the engine lubricating system lubricates the governor weight bearing. The various other parts are splash lubricated. Oil from the governor drains into the governor drive and timing gear housings.

Lubrication System

Lubrication system components and oil passages (naturally aspirated engine shown). 1-Oil passage through rocker arm shaft. 2-Engine oil cooler. 3-Oil filter base (includes bypass valve). 4-Oil cooler bypass valve. 5-Timing gears (in front compartment). 6-Oil manifold (in cylinder block assembly). 7-Oil pump outlet. 8-Oil pan (sump). 9-Oil filter bypass valve.

The lubrication system consists of a sump (oil pan), oil pump, oil cooler and oil filter. The cylinder block contains an oil manifold and oil passages to direct the oil to the various parts.

The pump draws oil from the sump and forces the oil through the oil cooler, oil filter, and into the oil manifold. Oil flows through connecting passages to the external and internal engine parts. A regulating valve in the pump body controls the maximum pressure of the oil from the pump.

When the engine is started, the lubricating oil in the oil pan is cold (cool). This cool viscous oil does not flow readily through the system. This cool oil forces bypass valves, in the oil cooler to open, and allows an unrestricted oil flow through the engine.

As the temperature of the oil increases, the viscosity and pressure of the oil decreases, and the bypass valves close. Now, only filtered oil is delivered to the engine parts.

A dirty or clogged oil filter element will not prevent lubricating oil from being delivered to the engine parts. The oil filter bypass valve will open, allowing oil to bypass the element.

The oil manifold directs lubricant to the main bearing supply passages, timing gear bearings, to a passage leading through the cylinder head to the valve rocker arm shaft, and the rocker arms and valves.

Oil spray orifices in the engine block, near the crankshaft main bearings, spray oil on the underside of the pistons. This cools the pistons and provides lubricant for the piston pins, cylinder walls and piston rings.

The connecting rod bearings receive oil through drilled passages in the crankshaft, between the main bearing journals and connecting rod journals.

Oil draining from the valve rocker arms lubricates the valves, push rods and lifters. The camshaft cams, camshaft intermediate and rear bearings are splash lubricated. The timing gear bearings and camshaft front bearing are pressure lubricated. Oil is supplied to the bearings through passages in the cylinder block.

External lines deliver lubricating oil to the turbocharger, on engines so equipped.

When the engine is warm, and running at rated speed, the oil pressure gauge should register in the "operating range". A lower pressure reading is normal at idling speeds.

Cooling System

1-Radiator. 2-Bypass line. 3-Temperature regulator. 4-Cylinder head. 5-Water cooled exhaust manifold. 6-Crankcase oil cooler. 7-Radiator outlet pipe. 8-Water pump. 9-Cylinder block.

1-Expansion tank. 2-Bypass line. 3-Temperature regulator. 4-Water cooled exhaust manifold. 5-Cylinder head. 6-Expansion tank outlet line. 7-Cylinder block. 8-Oil cooler bonnet (flow divided to exhaust manifold and cylinder block). 9-Water pump. 10-Crankcase oil cooler. 11-Cooling tower.

Ignition System

The ignition system consists of a low tension magneto, ignition transformers, low and high tension leads, spark plugs, and engine instrument panel.

The magneto generates primary voltage and delivers it through low tension leads to ignition transformers. The transformers boost the voltage induced by the magneto and delivers this electrical surge through high tension leads to the spark plugs. The spark plugs ignite the air/fuel mixture in the cylinder.

The ground terminal on the magneto is connected to the magnetic switch on the instrument panel. A manual stop switch, oil pressure gauge, and a water temperature gauge are also connected to the magnetic switch.

In the event of normal stopping, high water temperature or low oil pressure, the magnetic switch is energized and interrupts the magneto output, thus stopping the engine. On solid state attachment systems a different magnetic switch is required to carry the higher output of the magneto.

Before cranking a cold engine, the magnetic switch and oil pressure gauge reset buttons (3) and (4) should be pushed in. This overrides grounding the magneto as is usual in the case of low oil pressure. Normal oil pressure releases the lockout and the gauge switch is ready to signal a shutdown in case of low oil pressure.

When the water temperature gauge switch is operating properly, a hot engine cannot be started until the engine has cooled. Holding in the magnetic switch reset button will override the oil pressure and water temperature gauge switches.

The safety-shut-off gas supply line valve attachment is also operated by the instrument panel upon shut down.

1-Stop switch. 2-Oil pressure gauge. 3-Magnetic switch reset button. 4-Oil pressure gauge reset button. 5-Water temperature gauge.


The magneto is a self-contained alternating current generator that produces the electric source (and interrupts it at the right time) necessary for spark-ignited engines. The basic components of a magneto are transformer, rotor, contact breaker, distributor and condenser.

The transformer has a heavy wire primary coil. One end of the primary coil is grounded to the transformer core. The other end of the primary coil is connected to the contact breaker.

The rotor is a permanent magnet. When the rotor turns, the north pole of the rotor magnet comes under the core of the transformer. Magnetic flux flows from the magnet north to the magnet south by passing through the laminated metal core of the transformer. The turning rotor induces more flux (in the core) until the magnet north is completely under the core, and flux is at its greatest strength. As the rotor continues to turn, the magnet north moves out from under the core and flux diminishes. The rotor continues turning and the magnet north comes under the opposite side of the transformer core. Now the magnetic flux must change its direction through the transformer core. Flux builds up and diminishes in the opposite direction. This change in flux direction is repeated with each rotor revolution. Magnetic flux passing through the core surrounds the coil wires of the transformer and creates electricity in the wires.

An interruption in the alternating current cycle occurs when the primary coil voltage build up is high. At this time, the cam opens the contact points and the current cycle is interrupted. The magnetic flux (around the primary coil wires) suddenly collapses. This rapid collapse of flux creates a peak voltage.

At peak voltage, the distributor disc contact is in a position to complete the circuit through the brush and spring assembly in the distributor block and through low tension leads to the ignition transformers. This transforms the primary 250 volts (primary coil voltage) into a high voltage (up to 35,000 volts) needed to fire the spark plugs.

The condenser prevents damaging arcs from jumping across the contact breaker as it opens. The condenser collects the rush of electrical energy that would normally discharge across the contact as it opens. When the contact breaker opens wider, the electrical energy absorbed by the condenser discharges back into the primary coil, thus adding to the voltage build-up.


When the RPM of the rotor increases, the spark of the spark plug electrodes intensifies. An impulse coupling is used to increase rotor RPM momentarily when the engine is being cranked. The impulse coupling will disengage when the engine starts.

Ignition Transformers

The ignition transformers step up the magneto voltage to the high voltage required to fire the spark plugs. For good operation, the terminals must be clean and tight. The transformer terminal (either + or -) to be grounded is connected by a wire to a grounded screw on the magneto.


Do not pierce the insulation on the high voltage wire to the spark plug. A hole in the insulation will cause misfiring.

Spark Plugs And Adapters

Natural gas engine spark plugs use a dual ground electrode. This allows the spark plug to run longer before it needs to be regapped or replaced.

The boot covers the high tension lead and the terminal portion of the spark plug. This prevents water, dirt and other foreign material from getting into the spark plug adapter.

Starting And Charging Systems


The alternator is belt driven from the crankshaft pulley. It is a three-phase self-rectifying charging unit with three main functional parts: A rotating magnetic field (rotor) which produces flux; a stationary armature (stator) in which alternating current is induced; and stationary rectifying diodes that change alternating current to direct current.

The alternator field current is passed through brushes. The field current is in the order of 2 to 3 amperes. The rectifying diodes will pass current from the alternator to the battery or load, but will not pass current from the battery to the alternator.


The separate transistorized voltage regulator is an electronic switching device. It senses the voltage appearing at the oil pressure switch and supplies the necessary field current to maintain the required system voltage. The voltage regulator has two basic "circuits". The "load circuit" conducts positive potential from the regulator input lead, through a diode and transistor, to the regulator output lead, providing the circuit to the rotor (field) winding. The "control circuit" consists of a voltage sensitive zener diode, driver transistor and a voltage divider network. The "control circuit" directs the transistor in the "load circuit" to turn off and on at a rate that will provide the required charging voltage.


Never operate the alternator without the battery in the circuit. Making or breaking alternator connection with a heavy load on the circuit can result in regulator damage.

Starting Motor

The starting motor is a device used to rotate the flywheel of an engine fast enough to start the engine. A solenoid is used with the starting motor. The action of the solenoid engages the pinion with the ring gear on the engine flywheel when the solenoid is energized. The pinion always engages before the electric contacts in the solenoid close the circuit between the battery and the starting motor.

An overrunning clutch prevents the starting motor from being overspeeded. Releasing the starter switch disengages the pinion from the ring gear of the flywheel. The starting motor is grounded to the engine.

Starter Solenoid

A solenoid is a magnetic switch that utilizes low current to close a high current circuit. The solenoid has an electromagnet with a movable core. There are contacts on the end of the core. The contacts are held open by a spring that pushes the core away from the magnetic center of the coil. Low current will energize the coil and form a magnetic field. The magnetic field draws the core to the center of the coil and the contacts close.

Wiring Diagrams


IGNITION SYSTEM DIAGRAM (Equipped with Point Gap Magneto)

Back to top
The names Caterpillar, John Deere, JD, JCB, Hyundai or any other original equipment manufacturers are registered trademarks of the respective original equipment manufacturers. All names, descriptions, numbers and symbols are used for reference purposes only.
CH-Part.com is in no way associated with any of the manufacturers we have listed. All manufacturer's names and descriptions are for reference only.