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Understanding Exhaust Gas Recirculation Systems
By Henry Guzman
Exhaust gas recirculation (EGR) systems were introduced in the early '70s to reduce an exhaust emission that was not being cleaned by the other smog controls. Oxides of nitrogen (NOx) are formed when temperatures in the combustion chamber get too hot. At 2500 degrees Fahrenheit or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HCs) and the presence of sunlight, produces an ugly haze in our skies known commonly as smog.
How to reduce NOx NOx formation can be reduced by:
* Enriching the air fuel (A/F) mixture to reduce combustion temperatures. However, this increases HC and carbon monoxide (CO) emissions.
* Lowering the compression ratio and retarding ignition timing; but this leads to reduced performance and fuel economy.
* Recirculating some exhaust gases.
How EGR systems work The EGR valve recirculates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation.
The design challenge The EGR system of today must precisely control the flow of recirculated exhaust. Too much flow will retard engine performance and cause a hesitation on acceleration. Too little flow will increase NOx and cause engine ping. A well-designed system will actually increase engine performance and economy. Why? As the combustion chamber temperature is reduced, engine detonation potential is also reduced. This factor enabled the software engineers to write a more aggressive timing advance curve into the spark timing program. If the EGR valve is not flowing, onboard diagnostics (OBD) systems will set a code and the power control module (PCM) will use a backup timing curve that has less advance to prevent engine ping. Less timing advance means less performance and economy. Do your customer a favor and fix those EGR codes that you may have previously deemed as unimportant.
Evolution of the EGR systems The first EGR valves appeared in 1973 on GM cars. Bolted to the intake manifold next to the carburetor, it has ports to the intake and exhaust manifolds. It has a diaphragm that pulls open a valve stem, which allows exhaust to enter the intake manifold when ported vacuum is applied to it. Ported vacuum increases with throttle opening. A thermal vacuum switch prevents vacuum from reaching the EGR during cold engine starts. This system had many problems. It would often open too soon or too much, which caused a hesitation on acceleration as massive amounts of recirculated exhaust hit the combustion chamber. Many people simply disconnected it when it began to cause problems because they did not understand its importance or design. By 1975, if you unplugged an EGR valve, you'd have a driveability complaint of engine ping. Manufacturers and technicians of that era experimented with vacuum orifice restrictors and vacuum delay valves to try to find a happy medium between clean air and performance.
Closed loop systems By 1981, closed loop computer controls were in place. EGR flow was now more carefully controlled with dual diaphragm and back-pressure EGR valves. Modulating the vacuum to the EGR valve's pull, open diaphragm controlled the flow of recirculated ex- haust. Called by various names such as amplifiers, transducers and modulators, both remote and integral vacuum modulated devices were used. The flow of vacuum was further controlled by solenoids that blocked the vacuum ports until certain criteria were met such as engine temperature, rpm and manifold absolute pressure (MAP).
As the manufacturers began to use these complex schemes with vacuum amplifiers, delay valves and solenoids, they added a lot of "spaghetti" to the engine compartment. Plastic vacuum connections would break and rot with age and were not very reliable. Vacuum diagrams were invented and became essential to the smog driveability technicians of the day. As these systems evolved, they had fewer parts and less vacuum tubing. This was achieved by the use of pulse width modulated EGR solenoids. The PCM controlled EGR flow through the use of these solenoids to modulate vacuum to the EGR valve instead of just turning it on or off periodically.
What is pulse width modulation? Let's take a moment to discuss how computers think so we can better understand this common form of PCM control. Computers are binary. The machine language they operate in consists of only two variables: on or off, true or false, high or low. That's the only way a PCM can think. As a result, computer controlled outputs are always on or off, high (system voltage) or low (ground). Therefore, a computer output is always a square wave, or an on-off step when viewed on a lab scope. The high portion of the waveform will usually be battery voltage or PCM voltage of approximately 5 volts, with a few exceptions where the PCM operates at a different voltage.
Once the PCM receives its inputs, such as rpm, throttle angle, coolant temperature and MAP, it then calculates a response based on the software program that is embedded into it. Next, it makes its decision and sends a command in the form of a pulse width modulated signal to turn the EGR solenoid on and off rapidly. The EGR solenoid has two vacuum nipples. One side gets either manifold or ported engine vacuum. The other nipple goes to the EGR valve. Its default position is to block vacuum to the EGR valve. A vent is incorporated to bleed off vacuum when the solenoid is being pulsed. Vacuum flows to the EGR in rapid on-off pulses as the solenoid is commanded by the PCM.
OBD I systems With each succeeding year, the EGR designs became more refined. The California Air Resources Board (CARB) liked GM and Chrysler's onboard diagnostic systems. In 1988, CARB required that all cars sold in California be equipped with an onboard diagnostic system and a "check engine" light to notify the driver of emission system failure. By this time, all manufacturers had to have an EGR system that was capable of alerting the driver if it was not working. OBD I diagnostics and trouble codes were added in to flag opens, shorts and sticking solenoids.
OBD II EGR systems OBD II requires that the EGR system be monitored for abnormally low or high flow rate malfunctions. The EGR is considered malfunctioning when an EGR component fails or a fault in the flow rate results in the vehicle exceeding the Federal Test Procedure (FTP) by 1.5 times. FTP is the government-mandated drive cycle smog test that all new cars must pass and adhere to.
The diagnostic executive, also called the diagnostic task manager by Chrysler, controls the EGR monitor. The executive is an OBD II software agent given the task of managing all the onboard monitors and the scan tool interface. The executive coordinates the sequencing and actuation of all the monitor's test routines. There are eight main monitors whose sole function is to directly monitor and test the components assigned to them to ensure they meet FTP standards for life. These monitors are:
* Catalyst monitor
* EGR monitor
* EVAP monitor
* Fuel system monitor
* Misfire monitor
* Oxygen monitor
* Oxygen heater monitor
* Secondary air injection monitor
A closer look at the EGR monitor Monitor tests are both intrusive and non-intrusive. An example of an intrusive test is when the EGR monitor cycles the EGR valve during a condition when it normally would be closed. In some cases, the customer may feel an intrusive test as a slight miss.
The method of testing used by the EGR monitor varies according to the manufacturer, but there are three main types.
One method includes looking for a change in manifold pressure as the EGR valve is actuated on and off.
A second method involves cycling the EGR valve and looking for a change in short-term fuel trim. When the EGR valve is opened, it displaces some of the air fuel mixture. When the EGR valve is closed, more oxygen enters the combustion chamber, which then leans the mixture somewhat. The O2 sensor will respond with a lean signal to the PCM, which in turn increases pulse width. This is called short-term fuel trim compensation. The EGR monitor looks to see that all these things are occurring as they should. It repeats the tests and averages the results. Before the EGR monitor can begin its testing, it must first receive clearance from the diagnostic executive. The executive ensures that there are no conflicting conditions that would invalidate the EGR monitor's tests. For example, if the car had a lazy O2 sensor, fuel trim compensation to the EGR opening and closing would be inaccurate. Therefore, there are many safeguards built into OBD II to prevent this type of occurrence from happening. OBD II also has rationality checks. In other words, it uses deductive logic and constantly compares its inputs against each other to make sure all are in sync with one another. After the EGR monitor gets the OK to run its tests, it uses strict enabling criteria to ensure accurate testing such as:
* Engine temperature more than 170 F.
* Ambient air temperature more than 20 F.
* Engine run time more than three minutes since 170 F.
* Engine speed 2248-2688 (auto. trans.), 1952-2400 (manual trans.).
* Manifold absolute pressure from 5-20 hg.
* Short Term Adaptive Fuel Trim is adjusting pulse width by less than +7 percent and more than -8 percent.
* TP sensor from 0.6 to 1.8 volts.
* Vehicle speed sensor more than 40 mph.
The above is used for illustrative purposes only. Refer to your manual or CD-ROM information system for specifics to the car you are working on.
The third type of EGR monitoring design includes monitoring an EGR position sensor and a back-pressure sensor. Some Fords use a differential pressure feedback sensor that reads exhaust back-pressure upstream and downstream of the EGR valve to determine its flow rate and operation.
While OBD I systems would usually flag an inoperative EGR system, OBD II systems are given the task of determining the correct amount of EGR flow to keep the car running clean.
Next month, we will get into diagnosis, testing and repair techniques for all the different types of EGR systems. I will also cover pattern failures of all types, including mechanical problems such as plugged EGR passages that can cause rpm specific misfire concerns.
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline and diesel engines.
EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a gasoline engine, this inert exhaust displaces the amount of combustible matter in the cylinder. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.
Because NOx formation progresses much faster at high temperatures, EGR reduces the amount of NOx the combustion generates. NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature.
* 1 EGR in spark-ignited engines
* 2 EGR implementations
* 3 References
* 4 External links
 EGR in spark-ignited engines
The exhaust gas, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature.
In a typical automotive spark-ignited (SI) engine, 5 to 15 percent of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the requirement of the mixture to sustain a contiguous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and NOx emissions. A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms:
* Reduced throttling losses. The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
* Reduced heat rejection. Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
* Reduced chemical dissociation. The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC, rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is minor compared to the first two.
It also decreases the efficiency of gasoline engines via at least one more mechanism:
* Reduced specific heat ratio. A lean intake charge has a higher specific heat ratio than an EGR mixture. A reduction of specific heat ratio reduces the amount of energy that can be extracted by the piston.
EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle. The EGR valve also cools the exhaust valves and makes them last far longer (a very important benefit under light cruise conditions)
 EGR implementations
Usually, an engine recirculates exhaust gas by piping it from the exhaust manifold to the inlet manifold. This design is called external EGR. A control valve (EGR Valve) within the circuit regulates and times the gas flow. Some engine designs perform EGR by trapping exhaust gas within the cylinder by not fully expelling it during the exhaust stroke, which is called internal EGR. A form of internal EGR is used in the rotary Atkinson cycle engine.
EGR can also be implemented by using a variable geometry turbocharger (VGT) which uses variable inlet guide vanes to build sufficient backpressure in the exhaust manifold. For EGR to flow, a pressure difference is required across the intake and exhaust manifold and this is created by the VGT.
Another method that has been experimented with, is using a throttle in a turbocharged diesel engine to decrease the intake pressure, thereby initiating EGR flow.
Early (1970s) EGR systems were unsophisticated, utilizing manifold vacuum as the only input to an on/off EGR valve; reduced performance and/or drivability were common side effects. Slightly later (mid 1970s to carbureted 1980s) systems included a coolant temperature sensor which didn't enable the EGR system until the engine had achieved normal operating temperature (presumably off the choke valve and therefore less likely to block the EGR passages with carbon buildups, and a lot less likely to stall due to a cold engine). Many added systems like "EGR timers" to disable EGR for a few seconds after a full-throttle acceleration. Vacuum reservoirs and "vacuum amplifiers" were sometimes used, adding to the maze of vacuum hoses under the hood. All vacuum-operated systems, especially the EGR due to vacuum lines necessarily close to the hot exhaust manifold, were highly prone to vacuum leaks caused by cracked hoses; a condition that plagued early 1970s EGR-equipped cars with bizarre reliability problems (stalling when warm or cold, stalling or misfiring under partial throttle, etc.). Hoses in these vehicles would be checked by doing a vacuum leak test or pressure smoke test, with a professional smoke generator. When testing, smoke escapes from the hose being tested or the vacuum test gauge indicates a particular hose is leaking.
Modern systems utilizing electronic engine control computers, multiple control inputs, and servo-driven EGR valves typically improve performance/efficiency with no impact on drivability.
In the past, a fair number of car owners disconnected their EGR systems in an attempt for better performance and some still do. The belief is either EGR reduces power output, causes a build-up in the intake manifold, or believe that the environmental impact of EGR outweighs the Nitrous Oxide emission reductions. Disconnecting an EGR system is usually as simple as unplugging an electrically operated valve or inserting a ball bearing into the vacuum line in a vacuum-operated EGR valve. In most modern engines, disabling the EGR system will cause the computer to display a check engine light. In most cases, a disabled EGR system will cause the car to fail an emissions test.
* Heywood, John B., "Internal Combustion Engine Fundamentals," McGraw Hill, 1988.
* van Basshuysen, Richard, and Schäfer, Fred, "Internal Combustion Engine Handbook," SAE International, 2004.
* "Bosch Automotive Handbook," 3rd Edition, Robert Bosch GmbH, 1993.
 External links
* Lecture notes on improving fuel efficiency that discusses the effects of specific heat ratio, University of Washington
* Diesel cycle calculator that can be used to show the effect of specific heat ratio, Georgia State University HyperPhysics
* Understanding exhaust gas recirculation systems (part 1) (part 2), Henry Guzman
* A Chrysler Imperial fan club describes different EGR control mechanisms
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