In Internal Combustion Engines, Exhaust Gas Recirculation (egr) Is A

In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in most 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 increases the amount of matter in the cylinder, which means the energy of combustion raises the temperature of the matter less, and the combustion generates the same pressure against the piston at a lower temperature. 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. 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 an SI engine 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. EGR in diesel engines In modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, where there is otherwise a very large amount of excess air) in controlling NOx emissions. Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines (see above). However, exhaust gas (largely carbon dioxide and water vapor) has a higher specific heat than air, and so it still serves to lower peak combustion temperatures. There are trade offs however. Adding EGR

to a diesel reduces the specific heat ratio of the combustion gases in the power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that corresponds to an increase in EGR. Particulate matter (mainly carbon) that is not burned in the power stroke is wasted energy. Stricter regulations on particulate matter(PM) call for further emission controls to be introduced to compensate for the PM emissions introduced by EGR. The most common is particulate filters in the exhaust system that result in reduced fuel efficiency[citation needed]. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures and available oxygen these filters need to function properly to burn off soot, automakers have had to consider injecting fuel and air directly into the exhaust system to keep these filters from plugging up. EGR deletion EGR deletion in the Diesel is considered justified by a wide range of people, including the environmentally conscious. Although deleting the EGR system results in increased Nitric oxide. Hydrocarbon, Particulate, Carbon monoxide and Carbon dioxide are drastically reduced. Further adding to benefits of EGR deletion, is the increase in fuel economy which can be over 25%. Reduced fuel consumption has environmental benefits that extend beyond the vehicle itself. End gas recirculated back into the cylinder adds wear inducing contaminants and increase engine oil acidity. This can result in a poorly, inefficient running engine. The increased level of soot also has negative effects on Diesel particulate filters. This increase in soot creates a whole subset of problems and scenarios that can negatively impact the immediate environment.

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 used 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.

Other methods that have been experimented with are using a throttle in a turbocharged diesel engine to decrease the intake pressure to initiate 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 in close proximity 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, stalling when cold, stalling or misfiring under partial throttle, etc.). Hoses in these vehicles should be checked by passing an unlit blowtorch over them: when the engine speeds up, the vacuum leak has been found. 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 NOx 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 vacuumoperated EGR valve. In most modern engines, disabling the EGR system will cause the computer to display a check engine light. In almost all cases, a disabled EGR system will cause the car to fail an emissions test, and may cause the EGR passages in the cylinder head and intake manifold to become blocked with carbon deposits, necessitating extensive engine disassembly for cleaning.

Purpose The EGR system is used to lower NOx (oxides of nitrogen) emissions caused by high combustion temperature and excessive oxygen. Adding exhaust gases back into the intake, displaces oxygen and decreases combustion temperatures. A pipe from the RH exhaust manifold feeds exhaust gas to a port at the back of the intake manifold. An internal passage in the intake manifold feeds over to where the EGR valve mounts (lower, round hole). The EGR valve mounted on the back of the intake manifold is used to meter small amounts of exhaust gas

(via upper, square-ish hole) back into the intake and on to the combustion chambers . Flow diagram

Operation Vacuum is used to operate the EGR valve. Only a small amount of exhaust gas is allowed to pass through the valve. Too much exhaust gas can hinder combustion. The valve is usually open when the engine is warm and above idle speed. Scan tools or programs will usually show when the valve is commanded open by the PCM.

EGR Control Vacuum to the EGR valve is controlled by a solenoid valve that is pulse width modulated by the PCM. This modulation of ON and OFF many times per second controls the amount of time vacuum is applied to the EGR valve. The PCM uses RPM and info from the following sensors to regulate the valve: Engine Coolant Temperature (ECT) sensor  Intake Air Temperature (IAT) sensor  Throttle Position Sensor (TPS)  Manifold Absolute Pressure (MAP) sensor  Park/Neutral Position (PNP) switch  Vehicle Speed Sensor (VSS) For testing purposes, grounding the DLC output/field service enable terminal (1994-up), with the key ON and the engine not running, will operate the solenoid and allow vacuum to pass to the EGR valve.

Negative Backpressure EGR Valve The 4th Gen F-body uses a negative backpressure EGR valve. The amount of exhaust gas is varied, depending on the amount

of manifold vacuum and exhaust backpressure. This is why it is typical to get an EGR diagnostic code when the exhaust system is altered. Adding headers or removing the catalytic converter can create changes in backpressure. OBD-II has higher sensitivity to this and will "throw a code" more often than OBDI will. The diaphragm on the EGR valve has an internal vacuum bleed hole which is held closed by a small spring when there is no exhaust backpressure. The PCM driven EGR solenoid controls vacuum to the valve. Engine vacuum opens the EGR valve against the pressure of a large spring. When vacuum combines with negative exhaust pressure, the vacuum bleed hole opens and the EGR valve closes.

EGR Valve Identification

 Negative backpressure EGR valves will have a "N" stamped on the top side of the valve after the part number.  Positive backpressure EGR valves will have a "P" stamped on the top side of the valve after the part number.  Port EGR valves have no identification stamped after the part number. If you have to replace a valve, compare the stampings to be sure you have the right one.

Results of incorrect operation Too much EGR flow will dilute the a/f mixture and make the engine run rough or stall. Excess flow weakens combustion and may result in the following conditions:  Engine stops after cold start  Engine stops at idle after deceleration  Vehicle surges during cruise  Rough idle

Too little or no EGR flow can allow combustion temps to get too high during acceleration and load conditions. This could cause:  Spark knock (detonation)  Engine overheating  Emission test failure

Functional Checking With engine idling, opening the EGR valve should cause the engine to run rough or die. On the forward side of the valve there are openings where you can get your finger or thumb in to press the diaphragm toward the back (opening the valve). If there is no change in engine rpm, the passages in the manifold may be clogged. This does not appear to happen very often. If you cannot get in there to push on the diaphragm, you can use a hand vacuum pump (like a Mityvac) connected to the EGR valve to open it. The valve should also hold vacuum, which would prove that the diaphragm is not leaking. You can check that the solenoid is getting adequate vacuum by unplugging the vacuum supply hose at the solenoid and putting a vacuum gauge on it. There should be at least 7" Hg of vacuum at 2000 rpm. If not, make sure the hose has no leaks and check the vacuum at the manifold fitting. The following will test whether vacuum will pass to the EGR valve when the solenoid is operated: To check the solenoid, remove the vacuum harness, rotate it and reinstall so that only the EGR valve side is connected to the solenoid. Unplug the vacuum hose at the EGR valve and install a vacuum gauge in it's place. Install a hand held vacuum pump (ex. Mityvac) to the manifold side of the EGR solenoid. Jumper pins 5 and 6 of the DLC and turn ignition to ON (don't start).

This will put the PCM in field service mode and energize the solenoid. Apply 10" Hg of vacuum with the pump and watch the gauge on the EGR valve side of the solenoid. It should read the same vacuum that you are applying. If not, you should check the hose from the solenoid to EGR valve for leaks or your solenoid could be bad. If your vacuum reads like it should, turn the key OFF. Vacuum at the gauge at the EGR valve end should bleed off (the pump gauge may/may not bleed off-not a problem). If you did not see the same vacuum at the gauge as on the pump, connect the pump to the EGR valve side of the harness. Apply vacuum and observe the gauge. The gauge should read the same as the pump gauge. If it does, your solenoid or hose connection is bad. The EGR valve can be removed and checked for excessive deposits that might hinder operation. Any particles that are dislodged should be removed, so they do not get into the engine or clog up the EGR valve. You can use a wire brush or wheel to clean the surfaces of the valve and manifold. If there are deposits in the orifices, you can use a screwdriver to remove them. Fastener specifications can be found in this table.

Keep or Remove? As previously mentioned, the EGR system can help control combustion and engine temperatures, reducing the chance of detonation. It does not make the engine run hotter because it is adding hot exhaust gases. The PCM will retard spark timing when enough detonation (spark knock) is detected. Therefore, it would seemingly be considered wise to allow the EGR system to

work and try to prevent detonation from even happening in the first place. This is beneficial for the high compression LT1. Because most of it is at the back of the engine, it does not take up much room and can hardly be seen. Removing it to "clean up the engine bay" hardly seems worth it. It does not operate at WOT (Wide Open Throttle), so there is no real performance enhancement for removing it, either. Some have speculated that the EGR pipe's proximity to the back of the intake manifold seal contributes to the infamous intake manifold leak. Excess heat there certainly does not help matters, but the pipe can be re-bent in some cases to increase the distance. Some heat wrap could also be used, but has potential as a fire hazard if not kept maintained. If you do wish or need to remove it, both the pipe and EGR valve ports can be blocked off with plates. GM p/n 10054880 (known as the LT4 block-off plate) can be used to block off the ports where the EGR valve is removed. If you want to block the EGR pipe entry, you will have to get that from another source (there are several on the internet or you can make it yourself). Note: LT4 engines did not require the EGR system due to the cam design used. OBD-II cars usually do not like removal of the EGR system and will result in trouble codes. PCM re-programming to disable it's detection will take care of it. I have also heard of a couple other more elaborate ways to trick the PCM into thinking it is working.