3126E Truck Engines Caterpillar

Illustration 1	g00787037
Airflow schematic (1) Air line (2) Aftercooler core (3) Air inlet elbow (4) Exhaust outlet from turbocharger (5) Turbine side of turbocharger (6) Compressor side of turbocharger (7) Air cleaner

The components of the air inlet and exhaust system control the quality of the air that is available for combustion. These components also control the amount of the air that is available for combustion. The components of the air inlet and exhaust system are listed below:

• Air cleaner

• Turbocharger

• Aftercooler

• Cylinder head

• Valves and valve system components

• Piston and cylinder

• Exhaust manifold

Inlet air is pulled through the air cleaner. The inlet air is then compressed and heated by the compressor wheel of turbocharger (6) to about 150°C (300°F). The inlet air is then pushed through air-to-air aftercooler core (2) and the inlet air is moved to air inlet elbow (3). The temperature of the inlet air at air inlet elbow (3) is about 43°C (110°F). Cooling of the inlet air increases the combustion efficiency. Increased combustion efficiency helps to lower fuel consumption. Also, increased combustion efficiency helps to increase horsepower output.

Aftercooler core (2) is a separate cooler core. Aftercooler core (2) is installed in front of the core (standard) of the engine radiator on the machine. Air that is ambient temperature is moved across the aftercooler core by the engine fan. This cools the turbocharged inlet air.

From aftercooler core (2), the air is forced into the cylinder head in order to fill the inlet ports. Air flow from the inlet port into the cylinder is controlled by the inlet valves.

Illustration 2	g00805952
Air inlet and exhaust system (2) Aftercooler core (4) Exhaust outlet (5) Turbine side of turbocharger (6) Compressor side of turbocharger (8) Exhaust valve (9) Inlet valve (10) Air inlet (11) Exhaust manifold

There are two inlet valves and one exhaust valve for each cylinder. Inlet valves open when the piston moves down on the inlet stroke. When the inlet valves open, cooled compressed air from the inlet port is pulled into the cylinder. The inlet valves close and the piston begins to move up on the compression stroke. The air in the cylinder is compressed. When the piston is near the top of the compression stroke, fuel is injected into the cylinder. The fuel mixes with the air and combustion starts. During the power stroke, the combustion force pushes the piston downward. After the power stroke is complete, the piston moves upward. This upward movement is the exhaust stroke. During the exhaust stroke, the exhaust valve opens, and the exhaust gases are pushed through the exhaust port into the exhaust manifold. After the piston completes the exhaust stroke, the exhaust valve closes and the cycle starts again. The complete cycle consists of four stages:

• Inlet stroke

• Compression stroke

• Power stroke

• Exhaust stroke

Exhaust gases from exhaust manifold (11) enter the turbine side of turbocharger (5) in order to turn the turbine wheel. The turbine wheel is connected to a shaft which drives the compressor wheel. Exhaust gases from the turbocharger pass through the exhaust outlet pipe, the muffler and the exhaust stack.

Turbocharger

Illustration 3	g00294193
Turbocharger (1) Air inlet (2) Compressor housing (4) Compressor wheel (5) Oil inlet port (6) Bearing (7) Turbine housing (8) Turbine wheel (9) Exhaust outlet (10) Oil outlet port (11) Exhaust inlet

The turbocharger is installed on the center section of the exhaust manifold. All the exhaust gases from the engine go through the turbocharger. The compressor side of the turbocharger is connected to the aftercooler by a pipe.

The exhaust gases go into turbine housing (7) through exhaust inlet (11). The exhaust gases then push the blades of turbine wheel (8). The turbine wheel is connected by a shaft to compressor wheel (3) .

Clean air from the air cleaners is pulled through compressor housing air inlet (1) by the rotation of compressor wheel (3). The action of the compressor wheel blades causes a compression of the inlet air. This compression gives the engine more power by allowing the engine to burn more air and more fuel during combustion.

When the load on the engine increases, more fuel is injected into the cylinders. The combustion of this additional fuel produces more exhaust gases. The additional exhaust gases cause the turbine and the compressor wheels of the turbocharger to turn faster. As the compressor wheel turns faster, more air is forced into the cylinders. The increased flow of air gives the engine more power by allowing the engine to burn the additional fuel with greater efficiency.

Illustration 4	g00294194
Turbocharger with wastegate (12) Actuating lever (13) Canister (14) Line (boost pressure)

When the engine is operating under conditions of low boost, a spring pushes on a diaphragm in canister (13). This action moves actuating lever (12) in order to close the valve of the wastegate. Closing the valve of the wastegate allows the turbocharger to operate at maximum performance.

As the boost pressure through line (14) increases against the diaphragm in canister (13), the valve of the wastegate is opened. When the valve of the wastegate is opened, the rpm of the turbocharger is limited by bypassing a portion of the exhaust gases. The exhaust gases are routed through the wastegate which bypasses the turbine wheel of the turbocharger.

Note: The turbocharger with a wastegate is preset at the factory and no adjustment can be made.

Bearings (4) and (6) for the turbocharger use engine oil under pressure for lubrication. The oil comes in through oil inlet port (5). The oil then goes through passages in the center section in order to lubricate the bearings. Oil from the turbocharger goes out through oil outlet port (10) in the bottom of the center section. The oil then goes back to the engine lubrication system.

Valve System Components

Illustration 5	g00294195
Valve system components (1) Rocker arms (2) Bridge (3) Spring (4) Pushrods (5) Exhaust valve (6) Inlet valves (7) Lifter (8) Camshaft lobe

The valve system components control the flow of inlet air into the cylinders during engine operation. The valve system components also control the flow of exhaust gases out of the cylinders during engine operation.

The crankshaft gear drives the camshaft gear through an idler gear. The camshaft must be timed to the crankshaft in order to get the correct relation between the piston movement and the valve movement.

The camshaft has two camshaft lobes for each cylinder. The lobes operate the inlet and exhaust valves. As the camshaft turns, lobes on the camshaft cause lifters (7) to move pushrods (4) up and down. Upward movement of the pushrods against rocker arms (1) results in downward movement (opening) of valves (5) and (6) .

Each cylinder has two inlet valves and one exhaust valve. Valve springs (3) close the valves when the lifters move down.

Air Inlet Heater

The engines are equipped with an electric heater that is located behind the air inlet elbow. The electric heater has two functions:

• Aid in starting

• Aid in white smoke cleanup during start-up

Under the proper conditions, the ECM turns on the electric heater. The following conditions are evaluated prior to activating the electric heater:

• Jacket water coolant temperature

• Inlet manifold air temperature

• Ignition switch position

• Duration of time

The system is capable of delivering heat for 30 seconds prior to start-up and during cranking of the engine. After the engine has started, the system is capable of delivering heat constantly for 7 minutes, or the system can cycle the heat for 13 minutes. During the heating cycle, the heat is on for ten seconds and the heat is off for ten seconds.

If the air inlet heater malfunctions, the engine will still start and the engine will still run. There may be a concern regarding the amount of white smoke that is present. Also, there may be a concern regarding the need for an alternative starting aid.

System Components

The system of the air inlet heater consists of the following basic components:

• Relay of the air inlet heater

• Heater element

• Coolant temperature sensor

• Inlet air temperature sensor

• ECM

• Indicator lamp

Illustration 6	g00863561
Location of components (1) Inlet air temperature sensor (2) Ground strap (heater to engine) (3) Air inlet heater (4) Coolant temperature sensor (5) ECM

The air inlet heater relay turns the 12 V heater ON and OFF in response to signals from the ECM (5) .

The air inlet heater (3) is a component of the air inlet cover. The heater element has a ground strap (2) that must be connected to the engine.

There are three conditions that would cause the air inlet heater to be activated:

• Powerup and Mode of Preheat

Regardless of temperature, the heater and the lamp of the heater should come on for two seconds when the ECM is first powered (lamp check). When the sum of the coolant temperature plus the inlet manifold air temperature is less than 25°C (109°F), the ECM will turn on the heater and the lamp for 30 seconds. This is a cycle of preheat.

The ECM will then turn off the heater and the lamp. When the operator attempts to start the engine prior to the completion of preheat, the ECM proceeds into the mode of cranking for heater control.

• Mode of cranking

During engine cranking, when the sum of the coolant temperature plus the inlet manifold air temperature is less than 25°C (109°F), the ECM will turn on the heater. The heater will remain on during engine cranking. If the engine fails to start, the ECM reverts to preheat. Reverting to preheat will activate the heater for another 30 seconds.

• Running of the engine

After the engine has started, the same combination of inlet manifold air temperature and coolant temperature will determine the state of the heater. The cycle has two strategies.

The two strategies are continuous and intermittent. During the continuous strategy, the heater will remain on for a maximum of 7 minutes after starting. If the same conditions exist, the ECM will activate the intermittent strategy. During the intermittent strategy, the heater is cycled for a maximum of 13 minutes. During this cycle, the heater is turned on for 10 seconds and the heater is turned off for 10 seconds. After the 13 minute time limit, the heater is shut off.

When one of the temperature sensors fails, the system will operate in the following manner:

• Coolant temperature sensor

When the coolant temperature sensor has an open circuit or a short circuit, the coolant temperature sensor has failed. During this condition, the heater will be activated when the inlet manifold air temperature is less than 10°C (50°F).

• Inlet air temperature sensor

When the inlet air temperature sensor has an open circuit or a short circuit, the inlet air temperature sensor has failed. During this condition, the heater will be activated when the coolant temperature is less than 40°C (104°F).

Under the proper condition, the heater will be reactivated. When the sum of the coolant temperature and the inlet manifold air temperature has dropped below 25°C (109°F), the heater will be reactivated. This condition could exist after a warm engine has cooled and the operator attempts to start the engine.

When the sum of the coolant temperature and the inlet manifold air temperature does not attain 35°C (127°F), the heater will be activated. The heater can be activated no longer than 20 minutes (maximum). The ECM will turn off the heater after the 20 minute time limit.

For additional information on the air inlet heater, refer to Troubleshooting, "Air Inlet Heater Circuit - Test".

Illustration 7	g00786279
Schematic of air inlet heater (typical example) (1) Battery (2) Fuse panel (3) Inlet heater lamp (4) ECM (5) Engine ground (6) Air inlet heater (7) Air inlet heater relay

Illustration 8	g00801132
Flow Chart Of Controller Of Air Inlet Heater