Technological Evaluation of Gas Recirculation of IC EnginesSeyed Alavi Panther ID: 2630064 Date: 08/04/2010 Advisor: Dr. Cao

Table of ContentsINTRODUCTION EGR IN SPARK-IGNITED ENGINES EGR IN DIESEL ENGINES EGR DELETION EGR IMPLEMENTATIONS OPERATION EGR CONTROL NEGATIVE BACKPRESSURE EGR VALVE EGR VALVE IDENTIFICATION RESULTS OF INCORRECT OPERATION FUNCTIONAL CHECKING KEEP OR REMOVE? REFERENCES 3 5 7 11 11 15 16 17 19 20 20 23 25

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IntroductionIn 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 recirculation 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.

Figure 1- EGR Configuration

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Exhaust Gas Recirculation (EGR) systems were introduced in the early 70s and have long been of interest to engine designers, researchers, and regulators. EGR was originally considered as a method to alter combustion and suppress knock in spark ignition engines. Considerable interest in EGR for gasoline engines developed shortly after 1955 when Haagen-Smit successfully demonstrated the dependency of smog on combustion-generated hydrocarbons and oxides of nitrogen. Five years later, Kopa and other researchers, demonstrated that EGR could in fact lower the concentration of NOx in the exhaust gas. Recently, EGR has emerged as a necessary means to meet the United States Environmental Protection Agency (EPA) nitric oxide (NOx) regulations for heavy-duty diesel engines. EPA started imposing air emission regulations on heavy- duty engines in 1985, to take effect in 1991, and then more stringent regulations in 1994. However, these initial regulations could be met with optimized combustion strategies, and improved combustion chamber design. EGR became a necessary component on heavy-duty diesel engines with the implementation of the 2004 regulations (accelerated to 2002 for six major manufacturers affected by the Consent Decree) where NOx release is restricted to 2.5 g/bhp-h. Nevertheless, introducing EGR effectively into the combustion chamber of a multi-cylinder engine remains a considerable challenge. 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.

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External EGR, using piping to route the exhaust gas to the intake system where it is inducted into the succeeding cycles, has emerged as the preferred current approach. However the high efficiency of a state-of-the- art turbocharger often establishes conditions where the intake manifold pressure is higher than the exhaust manifold pressure. Consequently, an auxiliary device, such as the Variable Geometry Turbine (VGT) is needed to increase the backpressure above the intake manifold pressure and allow flow in the proper direction.

Figure 2- EGR flow chart

EGR in spark-ignited enginesThe 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

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can cause misfires and partial burns. Although EGR does measurably slow combustion, advancing spark timing can largely compensate this for. 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:

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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.

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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.

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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:

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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.

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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 enginesIn modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of re-circulated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flame front; 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 a particulate filter in the exhaust system that result in reduced fuel efficiency. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures

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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.

Diesel engines have inherently high thermal efficiencies, resulting from their high compression ratio and fuel lean operation. The high compression ratio produces the high temperatures required to achieve auto-ignition, and the resulting high expansion ratio makes the engine discharge less thermal energy in the exhaust. The extra oxygen in the cylinders is necessary to facilitate complete combustion and to compensate for non-homogeneity in the fuel distribution. However, high flame temperatures predominate because locally stoichiometric air fuel ratios prevail in such heterogeneous combustion processes. Consequently, Diesel engine combustion generates large amounts of NOx because of the high flame temperature in the presence of abundant oxygen and nitrogen.

Diesel engines are lean burn systems when overall air fuel ratios are considered, commonly with an air excess ratio k 1 4 1:5 1.8 on full loads and higher k values as load reduces. During idling, for instance, the air to fuel ratio of a modern Diesel engine can be 10-fold higher than that of stoichiometric engines (k >10). However, diffusion controlled Diesel combustion is predominately stoichiometric burn, in a microscopic sense, because the flames are prone to localize at approximately stoichiometric regions within the overall fuel lean but heterogeneous mixture. The prevailing flame temperature can be estimated with adiabatic stoichiometric flame temperature calculations. For a given engine speed, it is obvious that the NOx generation rate is closely related to the fueling rate, the engine load level. On a power generation basis,

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therefore, the de- crease in overall mixture strength will not drastically reduce the specific rate of NOx generation.

Unlike Diesel engines, homogeneously charged engines, such as spark-ignited gasoline engines or other gaseous fuel engines can actually use k control to reduce NOx effectively. To a homogeneous charge, the weakening in mixture strength can effectively reduce the flame temperature and propagation speed. An excessively fuel lean mixture, k >1:2 1.4 (depending on the type of fuel), could produce substantially lowered NOx emissions. The trend in NOx reduction enhances with further weakening of the cylinder charge until sustainable flame propagation be- comes unreliable and unburned combustibles intolerable. When an extremely lean mixture is used, for instance when k 1:8, a homogeneous charge compression ignition

(HCCI) concept could be applied, where the engine operation improves fuel economy through nearly instantaneous combustion that normally produces very low NOx and PM emissions simultaneously. Although the concept is highly promising, to date, a viable model of an HCCI (homogeneous charge compression ignition)engine has yet to be fully developed.

Figure 3- Diesel EGR flow chart

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Considering the prevailing stoichiometric burning of Diesel engines, it would be more efficient to lower the specific heat capacity ratio of the working fluid in order to lower the flame temperature. The introduction of CO2 into the engine intake, which can be achieved by recycling a fraction of the exhaust gas into the engine intake as shown in Fig. 1, can increase the specific heat capacity effectively. Concurrently, the EGR dilutes the O2 concentration of the working fluid. Thus, NOx generation can be drastically lowered, which is the primary reason for Diesel EGR. However, diffusion controlled Diesel combustion is also associated with fuel rich pockets that are always struggling to find oxygen at the late stages of combustion, especially when the engine operates on high loads. The application of EGR worsens the scenario that increases the difficulties to burn smoke free. In contrary, homogeneous charge engines produce little PM as long as the charge is not fuel rich, largely irrespective of EGR applications. For stoichiometric or lean burn SI engines, the flame sweeps over a homogeneously distributed fuel that does not lack access to oxygen, even when EGR is applied.

Figure 4-EGR valve in diesel engine

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EGR deletionEGR 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 re-circulated 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 implementationsUsually, an engine re-circulates 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.

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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

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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 vacuum-operated 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.

Figure 5- engine assembly

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PurposeThe 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 (Fig 4) 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.

Figure 6-EGR pipe

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OperationVacuum 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. The Exhaust Gas Recirculation (EGR) Valve is an integral part of a vehicle s emission control system or EGR System. It controls the engine s emission of nitrous oxides by reducing combustion

temperature. Nitrous oxide, also called laughing gas for its euphoric effects, isFigure 7-EGR valve

formed when the fuel is burning at over 2,500 degrees Fahrenheit. At this combustion temperature, nitrogen in the air mix with other gases to form this gas which is capable of altering a person s bone marrow structure in only 3-4 hours of exposure to it. When released into the atmosphere, nitrous oxide reacts with oxygen and becomes nitrogen dioxide. The latter in turn becomes smog when it comes into contact with hydrocarbons. The EGR valve first appeared in automobiles in 1972 to counter this phenomenon. EGR valves basically do this by sending some of the exhaust gas through the intake manifold back into the cylinders. Because exhaust gas most often doesn t burn, it stays and takes up space in the combustion chamber and lowers the temperature there. Older vehicles used to have mechanical engine EGR valves, but the newly manufactured ones have electronic EGR valves.

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However, the function of EGR valves remains the same, whether they are mechanical or electronic. An EGR system usually doesn t require regular maintenance, but it should nevertheless be checked so as to ensure that you are always complying with emission standards. A malfunctioning EGR valve could also damage your engine if not repaired right away.

EGR ControlVacuum 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.

Figure 8-EGR Solenoid valve

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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 ValveThe 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 OBD-I will.

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Figure 9-Backpressure EGR valve

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.

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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.

Figure 10 EGR valve identification

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Results of incorrect operationToo 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 CheckingWith 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.

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Figure 11- EGR valve diaphragm and openings

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

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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 its 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.

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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.

Figure 12-EGR valve service

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

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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.

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Referencesy He ywood, J oh n. Inte rnal Combust i on E ngi ne Fundame nt al s. 1. McGr a w-Hi l l Sci en c e/ E n gin eer in g/ Ma th , 1988. 930. Pr in t. Zh en g, Min g. "Di esel en gin e exh a ust ga s r eci r cul a t i on a r evi e w on a dva n ced an d n ovel c on cept s. " E ne rgy Conv e rsi on and Manageme nt . (2003): 19. Prin t .

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Ra ja n , K. "Th e E ffe ct of E xh a ust Ga s Reci r cul a t ion (E GR) on th e Per for m an ce an d E m i ssi on Ch ara ct er i st i cs of Di es el E n gin e wi t h Sun fl ower Oi l Meth yl E st er. " Int e rnat i onal J ournal of Che mi c al E ngi nee ri ng Re se arc h. (2009): 20. Pr in t

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