Exhaust Gas Recirculation: Description and Operation

Linear EGR Valve:

EGR Components:

EGR Valve Control Schematic:

PURPOSE
The Exhaust Gas Recirculation (EGR) system is used in automotive engines to decrease the emission levels of Oxides of Nitrogen (NOx). NOx defines a group of chemical compounds containing nitrogen and varying amounts of oxygen that can have harmful environmental effects in large quantities.

NOx forms during the combustion process in amounts that is dependent on the concentration of oxygen in the combustion chamber and the duration that the combustion process temperatures exceed 1500°F. Decreased NOx levels are accomplished by reducing the peak combustion temperatures through dilution of the incoming fuel/air charge with exhaust gas. When combusted, exhaust gas (largely nonreactive carbon dioxide and water vapor) acts to absorb a portion of the combustion energy, resulting in lower temperatures throughout the combustion process and yielding lower amounts of NOx.

Desired amounts of EGR depend upon the geometry of the combustion chamber and the operating condition of the engine. Extensive laboratory and vehicle tests are used to determine optimal EGR rates for all operating conditions. Too little EGR can yield high NOx, while too much EGR can disrupt combustion events. This engine uses a computer controlled valve to precisely regulate the amount of exhaust gases delivered to the engine for all operating conditions.

OPERATION
As illustrated, exhaust gases are routed to the engine through a corrugated Semi- flexible feed pipe (EGR valve pipe) which connects the crossover exhaust pipe to the crossover waterpump housing.

In the crossover waterpump housing, exhaust gases are precisely metered by the PCM controlled EGR valve, then cooled by engine coolant and finally routed to the front and rear cylinder heads. A potential drawback with EGR is that for certain driving schedules, deposits can accumulate when hot exhaust gases are cooled. The Premium V system uses the crossover waterpump housing as a cross-flow heat exchanger to cool exhaust gases in large easily cleaned passages to virtually eliminate any concern with deposit accumulation during the service life of the engine. This is done by having the cooling passage reduce EGR gasses below their deposit forming temperature prior to routing these gasses into cylinder distribution channels.

In each cylinder bank, exhaust gases travel under the intake manifold along in irregularly shaped sandwich passage made up of the aluminum alloy cylinder head and a non-metallic distribution plate. Engine vacuum acts to draw exhaust gases through outlets in the distribution plate where mixing with the incoming fuel/air charges for each cylinder occurs. Although the openings look small, the EGR valve pintle is the flow limiter in this system.

LINEAR EGR CONTROL
To regulate EGR flow to the engine, the PCM controls the solenoid to directly vary the pintle position relative to the closed valve position. The EGR valve contains a potentiometer type position sensor that provides a voltage proportional to pintle position. Pintle position is used by the PCM for "Closed Loop" control of valve pintle position to follow commanded position, for diagnostics, and to correct fuel and spark for EGR.

The PCM learns the pintle position sensor voltage offset at closed valve position, and uses the pintle position sensor's voltage/travel slope to control up to the wide open pintle position, corresponding to 6.25 mm (0.24 in.) of valve opening. Similar to Throttle Position (TP) sensor reading, the pintle position is expressed as being 0% at closed valve position, and 100% at PCM controlled wide open, corresponding to 6.25 mm (0.24 in.) of travel. PCM controls pulse width modulation duty cycle to the solenoid to make the "actual pintle position" match the PCM commanded "desired pintle position." Valve opening requires increase of duty cycle; valve closing requires duty cycle reduction. Variations in the sensors that the PCM monitors requires that the PCM uses closed loop control to minimize pintle position error.

RESULT OF INCORRECT EGR SYSTEM OPERATION
Too much EGR flow tends to weaken combustion, causing the engine to run roughly or stop. With too much EGR flow at idle, cruise, or cold operation, any of the following conditions may occur:

- Engine stops after cold start.
- Vehicle surge.
- Engine stops at idle after deceleration or idles rough.

If the EGR valve should stay open due to a stuck valve, the engine may not run.
Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:

- Spark knock (detonation), especially on light acceleration.
- Emission test failure.