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Exhaust Gas Recirculation (EGR) Systems
Home Autos & Trucks
By: Indranil Chitre Email Article
Word Count: 1074 Digg it | Del.icio.us it | Google it | StumbleUpon it

  

What Is EGR System?
Due to high temperatures inside the combustion chamber, often in excess of 2,500 degrees F, Nitrogen (N2) and Oxygen (O2) combine to form oxides of Nitrogen (NOx). NOx is a harmful gas which in the presence of sunlight reacts with hydrocarbons in the atmosphere to form Ozone (O3).
Exhaust gases are generated by the combustion chamber once the air-fuel mixture is burnt during the Power Stroke. These exhaust gases are "thrown out" of the combustion chamber during the Exhaust Stroke. The exhaust gases, including NOx, pass through the Exhaust Valve, the Exhaust Passages and eventually to the atmosphere through the Exhaust System.
The EGR system routes some of the exhaust gases (6 – 10%) through the EGR valve, back in to the Intake System, and eventually inside the combustion chamber. The exhaust gases are inert i.e. they do not burn in the combustion chamber; also, the exhaust gases take up some of the space which would otherwise be taken by the intake air. This reduces the amount of intake air available for the combustion process during each Power Stroke; which in turn reduces the peak combustion temperatures. This results in lower NOx emissions.
An ancillary benefit to using the EGR system is that it reduces Detonation, by allowing advance Timing.
Key components within the EGR system:
a) Exhaust gas passages
b) EGR valve
c) Sensors – position, feedback
d) EGR passages
In Inline engines, the exhaust passages are connected to the intake manifold through an external tube. In V-type engines, there are cast passages within the exhaust and intake manifolds.
Overview of Essay
1) Types of EGR valves
a. Back Pressure
i. Positive pressure
ii. Negative pressure (Vacuum)
b. Computer Controlled
i. 1-Solenoid
ii. 2-Solenoids
iii. 3-Solenoids
iv. Linear EGR
2) Types of Position Sensors
a. EGR Valve Position Sensor (EVP)
b. Pressure Feedback EGR Sensor (PFE)
3) OBDII Monitoring Strategies
a. O2 Sensor
b. Delta Pressure Feedback EGR Sensor (DPFE)
c. MAP Sensor
4) Diagnosis
a. Vacuum Pump & Gauge
b. Universal DTCs

Types of EGR Valves – Backpressure

Reference Schematic above – when the valve is open, exhaust gases (2) pass through to the intake manifold (3). Note, in some cases the exhaust gases mix with the intake air before the throttle body. The EGR valve can be either positive backpressure i.e. the pressure from the exhaust gases "pushes" the valve open; or it can be negative backpressure i.e. vacuum applied through the vacuum port (4) "pulls" the valve open.

Types of EGR Valves – Computer Controlled

In these types of EGRs, the PCM controls the on/off for the vacuum source to the EGR (1-Solenoid). In 2-Solenoids, the second solenoid controls the amount of vacuum to be applied.
3-Solenoid design – there are 3 holes, small-medium-large, each solenoid controls the opening and closing of the respective hole. This results in 7 different flow rates, in addition to complete shut-off.
Linear EGR design uses a pulse-width modulated solenoid to precisely control the exhaust gas flow rate.

Types of Position Sensors

Most, if not all, OBDII systems have a feedback loop to the actual EGR valve position.
EGR Valve Position Sensor (EVP) – sits on top of the EGR valve. This sensor is simply a potentiometer that relays a signal to the PCM depending on the open/close position of the EGR valve.
Pressure Feedback EGR Sensor (PFE) – sits on the exhaust side of the EGR valve. If the EGR valve is open; the pressure on the exhaust side will be higher than if the valve were closed. The PFE converts this pressure data in to a voltage signal, and transmits it to the PCM.

OBDII Monitoring Strategies
All OBDII systems perform a diagnostic test to ensure proper functioning of the EGR valve. All 3 outlined methods/sensors use changes in local dynamics, whether Oxygen levels or Pressure, to signal to the PCM proper function of the EGR valve.
O2 Sensor – When the EGR valve is open, Oxygen levels detected by the O2 sensor should drop.
DPFE Sensor – A metered orifice is placed on the intake side of the EGR valve. If the EGR valve is open, the pressure at this orifice should be lower compared to when the EGR valve is closed. The reason is that the orifice is exposed to some of the vacuum when the EGR valve is open. DPFE sensor measures this pressure difference and relays it to the PCM.
MAP Sensor – If the EGR valve is open, exhaust gases flow to the intake manifold. This changes the vacuum levels and fuel trim of the charge in the intake manifold. MAP Sensor "notices" this change, and if the change falls within acceptable, pre-calculated levels, gives a "thumbs-up" to the EGR functioning.

Diagnosis
There are a few things that could go wrong with the EGR system. If the EGR valve is stuck closed, or exhaust gas flow is restricted i.e. rendering the EGR system non-functioning, NOx levels increase and Detonation could occur. If the EGR valve is stuck open, rough idle and stalling conditions could occur in addition to engine power issues at low speed.
EGR system functioning can be checked using a vacuum pump and gauge combination. California Smog Check program requires EGR functional test to be performed on TSI – 1995 and older vehicles. Basically, 2 items are looked at a) whether the EGR valve is able to hold vacuum and b) whether the engine stalls on application of vacuum, as it should. If the vehicle is unable to hold vacuum – the EGR valve or the EGR passages have a problem. If the engine does not stall – the exhaust passages have a problem.
The PCM itself sets universal DTCs if it notices a problem with the EGR system. P0400/P0401/P0402.

References:
a) Level 1 Inspector Training, STI, Chapter 28 – Exhaust Gas Recirculation (EGR) Systems
b) Wikipedia
c) Various links as noted in the body of the essay

Owners and operators of various smog test stations across southern California www.starsmogcenter.com

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