Turbocharged engines pull outside air through an air cleaner into the air intake of the turbocharger. The suction is caused by the turbocharger compressor wheel. Then, the turbocharger compressor wheel compresses the air. The air flows through the inlet manifold which directs an even distribution of the air to each engine cylinder. Air is pulled into the engine cylinder during the intake stroke of the piston. Then, the air is mixed with fuel from the fuel injection nozzles.
Each piston makes four strokes:
- Intake
Air is drawn into the cylinder through the open inlet valve. Fuel is sprayed into the engine by the fuel injection nozzle.
- Compression
The mixture of air and fuel is compressed in the cylinder in order to heat the mixture to the temperature of combustion.
- Power
The mixture of air and fuel ignites at the top of the compression stroke. The expansion of gases from the combustion forces the piston downward. This force creates the power of the engine.
- Exhaust
The piston moves upward in order to force the gases of combustion from the cylinder through the open exhaust valve.
The sequence of the strokes by all of the pistons in all of the engine cylinders provides constant air flow from the air inlet system during the engine operation.
A turbocharger increases the temperature and the density of the air that is sent to the engine cylinder. This condition causes a lower temperature of ignition to develop earlier in the compression stroke. The compression stroke is also timed in a more accurate way with the fuel injection. Surplus air lowers the temperature of combustion. This surplus air also provides internal cooling.
A turbocharger improves the following aspects of engine performance:
- Power output is increased.
- Engine torque is increased.
- Engine efficiency is increased.
| |
| Illustration 1 | g00302786 |
|
Components of a turbocharger (typical example) (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 | |
A turbocharger is installed between the exhaust and intake manifolds. The turbocharger is driven by exhaust gases which flow through the exhaust inlet (11). The energy of the exhaust gas turns the turbine wheel (8). Then, the exhaust gas flows out of the turbine housing (7) through the exhaust outlet (9).
Turbine wheel (8) and compressor wheel (3) are installed on the same shaft. Therefore, turbine wheel (8) and compressor wheel (3) rotate at the same rpm. The compressor wheel is enclosed by compressor housing (2). The compressor wheel compresses the intake air. The intake air flows into the engine cylinders through the inlet valves of the cylinders.
The oil from the main gallery of the cylinder block flows through the oil inlet port (5) in order to lubricate the turbocharger bearings (4) and (6). The pressurized oil passes through the bearing housing of the turbocharger. The oil is returned through the oil outlet port (10) to the oil pan.
The turbocharger has a wastegate. The wastegate is controlled by the boost pressure. This allows some of the exhaust gases to bypass the turbine wheel at higher engine speeds. The wastegate is a type of flapper valve that automatically opens at a preset level of boost pressure in order to allow exhaust gas to flow around the turbine. The wastegate allows the design of the turbocharger to be more effective at lower engine speeds.
NOx Reduction System (NRS) - Turbocharged Engines
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. The now cooled exhaust gas passes through the assembly of the exhaust gas valve to an electronic controlled valve. The electronically controlled valve is electronically actuated. The valve is controlled by the Electronic Control Module (ECM).
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.
The electronically controlled valve opens to allow the flow of cooled exhaust gas from the exhaust cooler to mix with the air flow from the inlet. The mixing of the cooled exhaust gas and the air flow from the inlet reduces the oxygen content of the gas mixture. This results in a lower combustion temperature, so decreases the production of NOx.
In some instances, the engine will need to use the electronically controlled exhaust gas valve for the NOx Reduction System (NRS) in order to generate the required flow of exhaust gas.
| |
| Illustration 2 | g00905459 |
|
Cross section of the inlet and exhaust valves in the cylinder head (typical example) | |
| |
| Illustration 3 | g00905464 |
|
Cylinder head and valves (typical example) (1) Collets (2) Valve spring retainer (3) Valve spring (4) Valve seal (5) Valve guide (6) Cylinder head (7) Cylinder head gasket (8) Pushrod (9) Lifter (10) Exhaust valve (11) Inlet valve | |
The valves and the rocker shaft assembly control the flow of air into the cylinders and out of the cylinders during engine operation. The cylinder head assembly has two valves for each cylinder. Each valve has one valve spring (3). The ports for inlet valve (11) and exhaust valve (10) are on the left side of the cylinder head.
The valve moves along a steel valve guide (5). The valve guides can be replaced.
The inlet valve and the exhaust valve are opened and closed by the rotation and movement of the following components:
- Crankshaft
- Idler gear
- Camshaft
- Valve lifters
- Pushrods
- Rocker arms
- Valve springs
The camshaft gear is driven by the idler gear. The camshaft gear, the idler gear, and the crankshaft gear are timed together. When the camshaft turns, the valve lifters are moved up and down. The pushrods move the rocker arms. The rocker arms make the inlet valves and the exhaust valves open and close. This is in sequence with the firing order of the engine. The valve springs force the valves back to the closed position.
| NOTICE |
|---|
|
The crankcase breather gases are part of the engines measured emissions output. Any tampering with the breather system could invalidate the engines emissions compliance. |
The engine has a low-pressure closed circuit breather system installed.
The valve mechanism cover contains a closed breather assembly. The breather is sealed from the outside air by a diaphragm. The gases in the valve cover, which are caused by blowby, pass from the crankcase to the next component depending on the type of aspiration system the engine has.
On the naturally aspirated engine, the crankcase gas is recirculated internally directly into the inlet manifold. The inlet connection is a straight out connection. The inlet connection contains the inlet temperature sensor and the air supply for the air compressor.
On the turbocharged engine, the crankcase gas is routed through an external pipe from the valve mechanism cover to the Oil Mist Separator (OMS). This uses a filter element to separate the oil vapor from the blow-by gases. The gas exits the filter. The gas is then piped into the air inlet hose. The gas then flows into the turbocharger before entering the engine again through the inlet manifold.