C7.1 Industrial Engine Air Inlet and Exhaust System Caterpillar

Air Inlet and Exhaust System
1.1. Turbochargers
2.1. Crankcase Breather
3.1. Valve System Components

Illustration 1g02349356
Air inlet and exhaust system
(1) Aftercooler core
(2) Air filter
(3) Air control valve for the aftertreatment regeneration device
(4) Aftertreatment regeneration device
(5) Diesel particulate filter and diesel oxidation catalyst
(6) Low-pressure turbocharger
(7) High-pressure turbocharger
(8) Wastegate actuator
(9) Exhaust cooler (NRS)
(10) Exhaust gas valve (NRS)
(11) Inlet gas throttle valve
(12) Wastegate regulator

The components of the air inlet and exhaust system control the quality of air and the amount of air that is available for combustion. The air inlet and exhaust system consists of the following components:

  • Air cleaner

  • Exhaust gas cooler (NRS)

  • Exhaust gas valve (NRS)

  • Turbochargers

  • Aftercooler

  • Inlet manifold

  • Cylinder head, injectors, and glow plugs

  • Valves and valve system components

  • Piston and cylinder

  • Exhaust manifold

  • Aftertreatment regeneration device

  • Diesel oxidation catalyst

  • Diesel particulate filter

  • Inlet gas throttle valve

Air is drawn in through the air cleaner into the air inlet of the low-pressure turbocharger by the low-pressure turbocharger compressor wheel. The air is compressed to a pressure of about 150 kPa (22 psi) and heated to about 120° C (248° F). From the low-pressure turbocharger, the air passes to the high-pressure turbocharger. The air is compressed to a pressure of about 325 kPa (47 psi) and heated to about 240° C (464° F) before the air is forced to the aftercooler. As the air flows through the aftercooler, the temperature of the compressed air lowers to about 55° C (131° F). Cooling of the inlet air assists the combustion efficiency of the engine. Increased combustion efficiency helps achieve the following benefits:

  • Lower fuel consumption

  • Increased horsepower output

  • Reduced NOx emission

  • Reduced particulate emission

From the aftercooler, the air flows to the exhaust gas valve (NRS). A mixture of air and exhaust gas is then forced into the inlet manifold. Air flow from the inlet manifold to the cylinders is controlled by inlet valves. There are two inlet valves and two exhaust valves for each cylinder. The inlet valves open when the piston moves down on the intake stroke. When the inlet valves open, cooled compressed air from the inlet port is forced into the cylinder. The complete cycle consists of four strokes:

  • Inlet

  • Compression

  • Power

  • Exhaust

On the compression stroke, the piston moves back up the cylinder and the inlet valves close. The cool compressed air is compressed further. This additional compression generates more heat.

Note: If the cold starting system is operating, the glow plugs will also heat the air in the cylinder.

Just before the piston reaches the top center (TC) position, the ECM operates the electronic unit injector. Fuel is injected into the cylinder. The air/fuel mixture ignites. The ignition of the gases initiates the power stroke. Both the inlet and the exhaust valves are closed and the expanding gases force the piston downward toward the bottom center (BC) position.

From the BC position, the piston moves upward. The piston moving forward initiates the exhaust stroke. The exhaust valves open. The exhaust gases are forced through the open exhaust valves into the exhaust manifold.

Illustration 2g02297554
Typical example

The NOx Reduction System (NRS) operates with the transfer of the hot exhaust gas from the exhaust manifold to the exhaust cooler. The hot exhaust gas is cooled in the exhaust cooler (9). The now cooled exhaust gas passes through the assembly of the exhaust gas valve to an electronic controlled valve (10). The electronically controlled valve is electronically actuated.

The reed valves that are located in the exhaust gas valve (NRS) have two main functions. The first function is to prevent the reverse flow of charge air from the inlet side of the engine to the exhaust side of the engine. The second function of the reed valve is to obtain exhaust gas when the peak exhaust pressure is above the average inlet pressure.

As the electronically controlled valve (10) starts to open the flow of cooled exhaust gas from the exhaust cooler (9) mixes with the air flow from the charge air intercooler. The mixing of the cooled exhaust gas and the air flow from the charge air aftercooler reduces the oxygen content of the gas mixture. This results in a lower combustion temperature, so decreases the production of NOx.

As the demand for more cooled exhaust gas increases the electronically controlled valve opens further. The further opening of the valve increases the flow of cooled exhaust gas from the exhaust cooler. As the demand for cooled exhaust gas decreases, the electronically controlled valve closes. This decreases the flow of cooled exhaust gas from the exhaust cooler.

The electronically controlled exhaust gas valve and the inlet gas throttle valve (11) for the NOx Reduction System (NRS) are controlled by the ECM. In some instances, the engine will need to use the electronically controlled exhaust gas valve (10) and the inlet gas throttle valve (11) for the NOx Reduction System (NRS) in order to generate the required flow of exhaust gas. The inlet gas throttle valve for the NOx Reduction System (NRS) works by reducing the pressure in the inlet manifold in order to draw through extra exhaust gas.

Exhaust gases from the exhaust manifold enter the inlet of the high-pressure turbocharger in order to turn the high-pressure turbocharger turbine wheel. The turbine wheel is connected to a shaft that rotates. The exhaust gases travel from the high-pressure turbocharger through the duct on the turbine side into the turbine inlet of the low-pressure turbocharger in order to power the low-pressure turbocharger. The exhaust gases pass from the low-pressure turbocharger through the following components: exhaust outlet, Aftertreatment Regeneration Device (ARD), Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF) and exhaust pipe.


Illustration 3g00302786
Typical example of a cross section of a turbocharger
(1) Air intake
(2) Compressor housing
(3) Compressor wheel
(4) Bearing
(5) Oil inlet port
(6) Bearing
(7) Turbine housing
(8) Turbine wheel
(9) Exhaust outlet
(10) Oil outlet port
(11) Exhaust inlet

The high-pressure turbocharger is mounted on the outlet of the exhaust manifold. The low-pressure turbocharger is mounted on the side of the cylinder block. The exhaust gas from the exhaust manifold enters the exhaust inlet (11) and passes through the turbine housing (7) of the turbocharger. Energy from the exhaust gas causes the turbine wheel (8) to rotate. The turbine wheel is connected by a shaft to the compressor wheel (3).

As the turbine wheel rotates, the compressor wheel is rotated. The rotation of the compressor wheel causes the intake air to be pressurized through the compressor housing (2) of the turbocharger.

Illustration 4g02299033
Typical example
(12) Actuating lever
(13) Wastegate actuator
(14) Line (boost pressure)

Illustration 5g02299034
Typical example
(15) Wastegate regulator

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, air is compressed to a higher pressure and more air is forced into the cylinders. The increased flow of air into the cylinders allows the fuel to be burnt with greater efficiency. This produces more power.

A wastegate is installed on the turbine housing of the turbocharger. The wastegate is a valve that allows exhaust gas to bypass the turbine wheel of the turbocharger. The operation of the wastegate is dependent on the pressurized air (boost pressure) from the turbocharger compressor. The boost pressure acts on a diaphragm that is spring loaded in the wastegate actuator which varies the amount of exhaust gas that flows into the turbine.

The wastegate regulator (15) is controlled by the engine Electronic Control Module (ECM). The ECM uses inputs from a number of engine sensors to determine the optimum boost pressure. This will achieve the best exhaust emissions and fuel consumption at any given engine operating condition. The ECM controls the wastegate regulator, that regulates the boost pressure to the wastegate actuator.

When higher boost pressure is needed for the engine performance, a signal is sent from the ECM to the wastegate regulator. The wastegate regulator reduces the pressure in the air inlet pipe (14) that acts upon the diaphragm within the wastegate actuator (13).

The spring within the wastegate actuator (13) forces the wastegate valve that is within the turbine housing to close via the actuating rod and lever. When the wastegate valve is closed, more exhaust gas is able to pass over the turbine wheel. This results in an increase in turbocharger speed and boost pressure generation.

When lower boost pressure is needed for the engine performance, a signal is sent from the ECM to the wastegate regulator. This causes high pressure in the air inlet pipe (14) to act on the diaphragm within the wastegate actuator (13). The actuating rod (12) acts upon the actuating lever to open the valve in the wastegate. When the valve in the wastegate is opened, more exhaust gas from the engine is able to bypass the turbine wheel. The exhaust gases bypass the turbine wheel results in a decrease in the speed of the turbocharger.

The shaft that connects the turbine to the compressor wheel rotates in bearings (4) and (6). The bearings require oil under pressure for lubrication and cooling. The oil that flows to the lubricating oil inlet port (5) passes through the center of the turbocharger which retains the bearings. The oil exits the turbocharger from the lubricating oil outlet port (10) and returns to the oil pan.

Crankcase Breather

The engine crankcase breather is a filtered system. The crankcase breather system consists of two main elements, a primary separator in the valve mechanism cover and a filtered canister that is mounted on the cylinder head. The gases exit the engine through the valve mechanism cover. The gases then pass through the primary separator. The primary separator removes most of the liquid oil from the gas. The liquid oil is then returned to the engine.

The gas then passes through the filter element before exiting to atmosphere in an open breather system or back to the induction system in a closed breather system via the breather vent pipe.

Any liquid oil that is captured by the filter drains from the bottom of the canister. The liquid oil is returned by the drain pipe that runs from the bottom of the canister back to the crankcase. A valve connects the drain pipe to the crankcase. This valve prevents the bypass of gas past the filter and oil from passing up the drain pipe.

A pressure relief valve is located in the rear of the canister in the integral mounting bracket. Under normal operation of the engine, this valve will not operate. If part of the system becomes blocked the valve will open at a pressure of 8.5 kPa (1.2 psi). The open valve will allow gas to bypass the filter and the pipes for venting.

The filter element can be accessed from either the top of the canister by removing the top cap or from the bottom of the canister by removing the bottom cap. Refer to Operation and Maintenance Manual, "Engine Crankcase Breather Element - Replace" for the correct procedure.


The crankcase breather gases are part of the engines measured emissions output. Any tampering with the breather system could invalidate the engines emissions compliance.

Valve System Components

Illustration 6g01924293
Valve system components
(1) Bridge
(2) Rocker arm
(3) Pushrod
(4) Hydraulic lifter
(5) Camshaft
(6) Spring
(7) Valve

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 (5) must be timed to the crankshaft in order to get the correct relation between the piston movement and the valve movement.

The camshaft (5) has two camshaft lobes for each cylinder. The lobes operate either a pair of inlet valves or a pair of exhaust valves. As the camshaft turns, lobes on the camshaft cause the lifter (4) to move the pushrod (3) up and down.

The lifter (4) incorporates a hydraulic lash adjuster which removes valve lash from the valve mechanism. The lifter (4) uses engine lubricating oil to compensate for wear of system components so that no service adjustment of valve lash is needed.

The engine lubricating oil enters the lifter (4) through a non-return valve. The engine lubricating oil increases the length of the lifter (4) until all valve lash is removed. If the engine is stationary for a prolonged period the valve springs will cause the lifter (4) to shorten so that when the engine is started engine valve lash is present for the first few seconds.

After cranking restores oil pressure the lifter (4) increases in length and removes the valve lash. When load is removed from a lifter (4) during service work by the removal of the rocker shaft the lifter (4) increases in length to the maximum extent. Refer to Systems Operation, Testing and Adjusting, "Position the Valve Mechanism Before Maintenance Procedures" for the correct procedure.

During reassembly of the rocker shaft the engine must be put into a safe position to avoid engine damage. After load is imposed on the lifters by reassembling the rocker assembly, the engine must be left in safe position for a safe period until the lifters have reduced to the correct length. Refer to Disassembly and Assembly, "Rocker Shaft and Pushrod - Install" for the correct procedure.

Upward movement of the pushrod against rocker arm (2) results in a downward movement that acts on the valve bridge (1). This action opens a pair of valves (7) which compresses the valve springs (6). When the camshaft (5) has rotated to the peak of the lobe, the valves are fully open. When the camshaft (5) rotates further, the two valve springs (6) under compression start to expand. The valve stems are under tension of the springs. The stems are pushed upward in order to maintain contact with the valve bridge (1). The continued rotation of the camshaft causes the rocker arm (2), the pushrods (3) and the lifters (4) to move downward until the lifter reaches the bottom of the lobe. The valves (7) are now closed. The cycle is repeated for all the valves on each cylinder.

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