Fuel in any engine is burnt with air. Air is a mixture of gases and it contains approximately 78% nitrogen and 21% oxygen. Some of the oxygen is used to burn the fuel during the combustion process and the rest is supposed to just pass through unreacted. But when the peak temperatures are high enough for long periods of time, the nitrogen and oxygen combine to form a class of compounds called nitrogen oxides, collectively referred to as NOx. These compounds are one of the chief constituents of smog, which have an adverse effect on ecological systems. They also contribute to the formation of acid rain.
NOx emissions can be reduced by lowering the cylinder temperatures. This can be done by three ways 1) Enriching the air fuel mixture 2) Lowering the compression ratio and retarding ignition timing 3) reducing the amount of Oxygen in the cylinder that inhibits the combustion process. The first two methods reduce the efficiency of combustion and so the best way is to reduce the amount of Oxygen. This is done by recirculating some exhaust gas and mixing it into the engine inlet air. This process is known as Exhaust Gas Recirculation (EGR).
The goal of Exhaust Gas Recirculation (EGR) is to reduce the amount of NOx produced. The EGR valve recirculates gases 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.
All internal combustion engines generate power by creating explosions using fuel and air. These explosions occur inside the engine's cylinders and push the pistons down, which turns the crankshaft. Some of the power thus produced is used to prepare the cylinders for the next explosion by forcing the exhaust gases out of the cylinder, drawing in air (or fuel-air mixture in non-diesel engines), and compressing the air or fuel-air mixture before the fuel is ignited.
Fig 1. Working of four stroke engine.
There are several differences between diesel engines and non-diesel engines. Non-diesel engines combine a fuel mist with air before the mixture is taken into the cylinder, while diesel engines inject fuel into the cylinder after the air is taken in and compressed. Non-diesel engines use a spark plug to ignite the fuel-air mixture, while diesel engines use the heat created by compressing the air in the cylinder to ignite the fuel, which is injected into the hot air after compression. In order to create the high temperatures needed to ignite diesel fuel, diesel engines have much higher compression ratios than
gasoline engines. Because diesel fuel is made of larger molecules than gasoline, burning diesel fuel produces more energy than burning the same volume of gasoline. The higher compression ratio in a diesel engine and the higher energy content of diesel fuel allow diesel engines to be more efficient than gasoline engines.
1.1. Formation of Nitrogen Oxides (NOx)
The same factors that cause diesel engines to run more efficiently than gasoline engines also cause them to run at a higher temperature. This leads to a pollution problem, the creation of nitrogen oxides (NOx). You see, fuel in any engine is burned with extra air, which helps eliminate unburned fuel from the exhaust. This air is approximately 79% nitrogen and 21% oxygen.
When air is compressed inside the cylinder of the diesel engine, the temperature of the air is increased enough to ignite diesel fuel after it is ignited in the cylinder. When the diesel fuel ignites, the temperature of the air increases to more than 1500F and the air expands pushing the piston down and rotating the crankshaft.
Fig 2. NOx formation zone.
Generally the higher the temperature, the more efficient is the engine
1. Good Performance
2. Good Economy
Some of the oxygen is used to burn the fuel, but the extra is supposed to just pass through the engine unreacted. The nitrogen, since it does not participate in the
combustion reaction, also passes unchanged through the engine. When the peak temperatures are high enough for long periods of time, the nitrogen and oxygen in the air combines to form new compounds, primarily NO and NO2. These are normally collectively referred to as NOx.
1.2. Problems of NOx
Nitrogen oxides are one of the main pollutants emitted by vehicle engines. Once they enter into the atmosphere, they are spread over a large area by the wind. When it rains, water then combines with the nitrogen oxides to form acid rain. This has been known to damage buildings and have an adverse effect on ecological systems.
Too much NOx in the atmosphere also contributes to the production of SMOG. When the sunrays hit these pollutants SMOG is formed. NOx also causes breathing illness to the human lungs.
1.3. EPA Emission Standards
Since 1977, NOx emissions from diesel engines have been regulated by the EPA
(Environmental Protection Agency). In October 2002, new NOx standards required the
diesel engine industry to introduce additional technology to meet the new standards
The EPA has regulated heavy duty diesel engines since the 1970s. The following chart shows the trend to ever-lower emissions. Understanding the details of the chart is not of interest to most truckers. Even though the emissions standards become increasingly more difficult to meet, the diesel engine industry has always been able to continue to improve engine durability, reliability, performance, and fuel economy. A quick look at the bottom right hand side of the chart also shows that emissions from diesel engines built in 2007 and beyond will approach zero.
Fig 3. EPA Heavy Duty Engine Emission Standards
1.4. How can NOx be reduced
Since higher cylinder temperatures cause NOx, NOx can be reduced by lowering cylinder temperatures. Charge air coolers are already commonly used for this reason.
Reduced cylinder temperatures can be achieved in three ways.
Â¢ Enriching the air fuel (A/F) mixture.
Â¢ Lowering the compression ratio and retarding ignition timing.
Â¢ Reducing the amount of Oxygen in the cylinder
Enriching the air fuel (A/F) mixture to reduce combustion temperatures. However, this increases HC and carbon monoxide (CO) emissions. Also Lowering the compression ratio and Retarded Ignition Timing make the combustion process start at a less than the optimum point and reduces the efficiency of combustion.
Fig 4. NOx reduction by lowering the temperature
These techniques lowers the cylinder temperature, reducing NOx, but it also reduces fuel economy and performance, and creates excess soot, which results in more frequent oil changes. So, the best way is to limit the amount of Oxygen in the cylinder. Reduced oxygen results in lower cylinder temperatures. This is done by circulating some exhaust gas and mixing it into the engine inlet air. This process is known as Exhaust Gas Recirculation.
2. EXHAUST GAS RECIRCULATION
Exhaust Gas Recirculation is an efficient method to reduce NOx emissions from the engine. It works by recirculating a quantity of exhaust gas back to the engine cylinders. Intermixing the recirculated gas with incoming air reduces the amount of available O2 to the combustion and lowers the peak temperature of combustion. Recirculation is usually achieved by piping a route from the exhaust manifold to the intake manifold. A control valve within the circuit regulates and times the gas flow.
2.1. Uses of Exhaust Gas Recirculation
First, exhaust gas recirculation reduces the concentration of oxygen in the fuel-air mixture. By replacing some of the oxygen-rich inlet air with relatively oxygen-poor exhaust gas, there is less oxygen available for the combustion reaction to proceed. Since the rate of a reaction is always dependent to some degree on the concentration of its reactants in the pre- reaction mix, the NOx-producing reactions proceed more slowly, which means that less NOx is formed.
In addition, since there is less oxygen available, the engine must be adjusted to inject less fuel before each power stroke. Since we are now burning less fuel, there is less heat available to heat the fluids taking place in the reaction. The combustion reaction therefore occurs at lower temperature. Since the temperature is lower, and since the rate of the NOx-forming reaction is lower at lower temperatures, less NOx is formed.
2.2. Basic Parts Of EGR
There are 3 basic parts of EGR
Â¢ EGR Valve
Â¢ EGR Cooler
Â¢ EGR Transfer Pipe
Typical Four Stroke Diesel Engine with Basic Parts of EGR
When EGR is required engine electronic controls open the EGR valve. The exhaust gas then flows through the pipe to the cooler. The exhaust gases are cooled by water from the truck cooling system. The cooled exhaust gas then flow through the EGR transfer pipe to the intake manifold.
2.3. EGR Operating Conditions
There are three operating conditions. The EGR flow should match the conditions
1. High EGR flow is necessary during cruising and midrange acceleration
2. Low EGR flow is needed during low speed and light load.
3. No EGR flow should occur during conditions when EGR flow could adversely affect the engine operating efficiency or vehicle drivability. ie, during engine warm up, idle, wide open throttle, etc.
2.4. EGR Impact on ECS
The ECM (Electronic Control Machine) considers the EGR system as an integral part of the entire ECS. Therefore the ECM is capable of neutralizing the negative aspects of EGR by programming additional spark advance and decreased fuel injection duration during periods of high EGR flow. By integrating the fuel and spark control with the EGR metering system, engine performance and the fuel economy can actually be enhanced when the EGR system is functioning as designed.
2.5. EGR Theory of Operation
The purpose of the EGR system is to precisely regulate the flow under different operating conditions. The precise amount of exhaust gas must be metered into the intake manifold and it varies significantly as the engine load changes. By integrating the fuel and spark control with the EGR metering system, engine performance and the fuel economy can be enhanced. For this an ECM (Electronic Control Machine) is used to regulate the EGR flow. When EGR is required ECM opens the EGR valve.The ECM is capable of neutralizing the negative aspects of EGR by programming additional spark advance and decreased fuel injection duration during periods EGR flowThe exhaust gas then flows through the pipe to the cooler. The exhaust gases are cooled by water from the vehicleâ„¢s cooling system. The cooled exhaust gas then flow through the EGR transfer pipe to the intake manifold.
Fig 7. Relationship between EGR Ratio and Load
4. EGR LIMITS
This is based on an experiment conducted. The research objective is to develop fundamental information about the relationship between EGR parameters and diesel combustion instability and particulate formulation so that options can be explored for maximizing the practical EGR limit, thereby further reducing nitrogen oxide emissions while minimizing particulate formation. A wide range of instrumentation was used to
acquire time-averaged emissions and particulate data as well as time-resolved combustion, emissions, and particulate data. The results of this investigation give insight into the effect of EGR level on the development of gaseous emissions as well as mechanisms responsible for increased particle density and size in the exhaust. A sharp increase in hydrocarbon emissions and particle size and density was observed at higher EGR conditions while only slight changes were observed in conventional combustion parameters such as heat release and work. Analysis of the time-resolved data is ongoing.
The objective of this work is to characterize the effect of EGR on the development of combustion instability and particulate formation so that options can be explored for maximizing the practical EGR limit. We are specifically interested in the dynamic details of the combustion transition with EGR and how the transition might be altered by appropriate high-speed adjustments to the engine. In the long run, we conjecture that it may be possible to alter the effective EGR limit (and thus NOx performance) by using advanced engine control strategies.
Experiments were performed on a 1.9 liter, four-cylinder Volkswagen turbo-charged direct injection engine under steady state, low load conditions. Engine speed was maintained constant at 1200 rpm using an absorbing dynamometer and fuel flow was set to obtain 30% full load at the 0% EGR condition. A system was devised to vary EGR by
manually deflecting the EGR diverter valve. The precise EGR level was monitored by comparing NOx concentrations in the exhaust and intake. NOx concentrations were used because of the high accuracy of the analyzers at low concentrations found in the intake over a wide range of EGR levels.
4.1. Combustion Characterization with HC and NOx Emissions
Steady state measurements were made of CO, CO2, HC, NOx, and O2 concentrations in the raw engine-out exhaust using Rosemount and California Analytical analyzers. Crank angle resolved measurements were also made of HC concentration in the exhaust using a Fast Flame Ionization Detector. The HC sampling probe was located in the exhaust manifold and the data were recorded.
Fig 8. Trade-off between HC and NOx concentration as a function of EGR Level
Time-averaged HC and NOx concentrations in the raw engine-out exhaust are shown in the Figure versus EGR level. This figure shows NOx concentration decreasing and HC increasing with increasing EGR as would be expected. Note the sudden increase in HC and leveling-off in NOx at approximately 45% EGR, where there appears to be a significant shift in combustion chemistry. This major transition is in sharp contrast to the slight changes observed in the integrated pressure parameters, HR and IMEP. Because of the suddenness of the emissions change at 45% EGR, it is clear that dynamic engine behavior at or above this operating point will be highly nonlinear. Thus it is imperative that any control strategies being considered should be able accommodate such behavior.
4.2. Combustion Characterization with PM
Our measurements have identified significant changes in PM emissions with EGR level as was expected. Similar to the gaseous emissions (e.g., HC and NOx), there was a sharp increase in PM at a critical EGR level. This critical level corresponding to a sharp increase in PM was observed in mass concentration, particle size, and particle density.
a) Mass Concentration
A Tapered Element Oscillating Microbalance (TEOM) was used to measure particulate mass concentration and total mass accumulation as a function of time. A sample of diluted exhaust is pulled through a 12 mm filter to the end of a tapered quartz element. The frequency of the element changes with mass accumulation. The instrument has approximately 3 sec resolution on mass concentration.
Particle mass concentration and total mass accumulation were measured on dilute exhaust using the TEOM. Mass accumulation rates were calculated based on over 100 mass data points and are shown in the figure as a function of EGR level. Mass accumulation rates begin to increase significantly at 30% EGR and continue to increase rapidly until the maximum EGR level. The intersection of the particulate mass and NOx curves represents a region where the engine out particulate mass and NOx concentration are minimized for this engine condition.
Fig 9. Relation of PM Accumulation Rate and NOx emission with EGR.
b) Particle Size
A Scanning Mobility Particle Sizer (SMPS) was used to measure the steady state size distribution of the particulates in the exhaust stream. The particles are neutralized and then sorted based on their electrical mobility diameter. The range of the SMPS was set at 11 nm â€œ 505 nm.
Particle sizing was performed on dilute exhaust using the SMPS. Number concentration vs. particle diameter is shown in the figure for several EGR levels. Two aspects of the data stand out. The first is the increasing number concentration with level of EGR. The second is the increasing particle size. Note that the particle size at the peak concentration increases by a factor of approximately two between 30% and 53% EGR.
Fig 10. Time-averaged size distributions as measured by the SMPS.
The likely mechanism for particle growth is the reintroduction of particle nuclei into the cylinder during EGR. The recirculating exhaust particles serve as sites for further condensation and accumulation leading to larger particles. A significant fraction of the measured size distribution appears larger than the 500 nm upper bound of the SMPS for the highest EGR rates. This is significant because these particles contain much of the exhaust particulate mass.
The frequency plot in the figure illustrates the disappearance of small particles and the growth of much larger particles. The divergence between the curves for particles > 100 nm and particles 60-100 nm increases significantly at 30% EGR and continues to increase. The figure does appear to show that the smallest particles are contributing to the growth of the largest ones. The increase in larger particles is less steep than the increase in particle mass in the figure.
Fig 11. Frequency of occurrence of particle size classes as a function of EGR.
4.3. NOx reduction effect of EGR
Fig. 12 shows the typical NOx reduction effect of EGR at the mid-speed range of the test engine.Under all load conditions, the amount of NOx decreases as the EGR rate increases. The graph also shows that the NOx reduction curves with the 0 % EGR point as the origin slope downward at different angles according to the load; the higher the load, the steeper the angle. In other words, the NOx reduction effect at the same EGR rate
increases as the engine load becomes higher.
Fig.12. Relationship between EGR rate and NOx
It is generally known that there are two reasons to reduce NOx by EGR. The first of them is the reduction of combustion temperature. The addition of exhaust gases to the intake air increases the amount of combustion- accompanying gases (mainly CO2), which in turn increases the heat capacity and lowers the combustion temperature. The second effect is the reduction of oxygen concentration in the intake air, which restrains the generation of NOx. Fig. 13 shows the NOx emission test results as a function of the concentration of oxygen in the intake air/EGR gas mixture. This graph shows that the NOx reduction rate depends mostly on oxygen concentration, and not on the engine load or EGR rate.
Fig 2 Relationship between oxygen concentration and NOx reduction
Fig.13 shows the results of NOx emission tests conducted while varying both the engine operating conditions and EGR rate, in which the test results shown in Fig. 13 are merged. As in Fig.13, almost all the data are on or in a single curve, indicating that there is a strong correlation between the oxygen concentration and NOx reduction rate. The reason for this is thought to be as follows: In Fig.12, the NOx reduction rate under a certainload is different from that under another load even when the EGR rate remains the same because the difference in load causes a difference in the amount of combustion-accompanying gases and oxygen concentration in EGR gas, which in turn changes the oxygen concentration in the intake gas (mixture of intake air and EGR gas).
5. INTERNAL EGR
When a fraction of the combustion products is still present in the cylinder at the moment that the exhaust valves close, the mixture at the beginning of the next engine cycle will consist of air and fuel, as well as combustion products. These products are called internal EGR (in contrast to external EGR, which means that exhaust gases are recycled to the intake system, after which they mix with the air and fuel.) The fraction of internal EGR that is present in the cylinder at the beginning of the compression stroke is mainly dependent on the timing of the intake and exhaust valves.
The valve timing of traditional engines, such as the Diesel and Otto engines, is such that the fraction of exhaust gases (or residuals) at the start of the cycle is as small as possible. Traditional engines have Residual Gas Fractions (RGF) in the range 5-15 mass%.
6. TECHNICAL ISSUES
6.1. Combustion Contamination
Exhaust gas from any combustion process may have certain contaminants, including acid forming compounds, unburned and partially burned hydrocarbons, air pollutants, and liquid water. These contaminants can be successfully reintroduced into the combustion chamber but may lead, over time, to serious combustion degradation and instability, and shorter component life. Such effects need to be fully understood and documented, and appropriate improvements made to the combustion process to protect the customerâ„¢s investment and maintain true long-term emissions compliance. This activity would be a key element of any major engine manufacturerâ„¢s development process.
6.2. Control System Stability
Control systems for modern engines have been developed over two decades and involve integrated strategies to adjust air/fuel ratio, ignition timing, and air flow rates to maintain emissions control at varying loads, speeds, and fuel conditions. These systems are at the heart of successful engine operation today and are vital to satisfactory long term operation. Adding EGR into the combustion process introduces further complexity that must be carefully integrated into the entire engine control system approach for successful operation over a wide range of conditions. For instance, if fuel quality changes over time, the air/fuel ratio, ignition timing, air system rates, and the EGR rate must be adjusted accordingly to keep the combustion system stable and emissions in compliance. On the other hand, if the engineâ„¢s load changes rapidly from part load to full load and back to part load, the EGR system dynamics must be included in the overall control strategy response to make sure the engine operates smoothly during this transition.
6.3. Materials and Durability
EGR systems may decrease long-term life of the components affected, including the EGR coolers and control valves, the pistons and cylinder heads, exhaust manifolds and sensors, as well as the post engine catalyst. Operating a few hundred hours per year may not lead to any significant materials degradation in the overall lifespan of an engine. However, continuous duty applications at 8500 hours per year may cause near term emissions noncompliance and longer term materials breakdown, shorter component life, and even unexpected, catastrophic engine failures. To minimize or eliminate the potentially negative impacts of EGR on engine components, compatible components and designs must be used that often require thousands of hours of lab and field test operation for validation. Although both expensive and time consuming, such efforts are a necessary part of proving any new combustion design including EGR systems. Therefore, major engine manufacturers worldwide need to plan for and execute these tests in order to develop the materials needed for successful EGR applications.
6.4. Liquid Dropout
During exhaust gas recirculation, the gasses must be cooled with an external cooler before being reintroduced into the cool inlet manifold of an engine. The cooling process for the EGR may result in liquids being formed in the return lines, depending on temperatures and local humidity, much as liquids are formed in the tailpipe of an automobile at certain conditions. This liquid dropout could be a continuous stream that needs to be carefully understood and managed with the needs of the local environment in mind. While there may be ways to reintroduce this liquid into the combustion process, doing so may create further problems with combustion and lead to other emissions complications and instability. As such, managing liquid dropout needs careful study and development in an integrated development program.
Thus, as seen that using Exhaust Gas Recirculation Technique in engines, the emissions are vary much controlled due to lesser amounts of NOx entering the atmosphere. Thus the emission levels to be maintained are attained by the engines. As seen, Exhaust Gas Recirculation is a very simple method. It has proven to be very useful and it is being modified further to attain better standards. This method is very reliable in terms of fuel consumption and highly reliable. Thus EGR is the most effective method for reducing the nitrous oxide emissions from the engine exhaust. Many of the four wheeler manufacturers used this technique like Ford Company, Benz Motors etc to improve the engine performance and reduce the amount of pollutants in the exhaust of the engine.
EXHAUST GAS RECIRCULATION IN INTERNAL COMBUSTION ENGINES
Oxygen is required for fuel to be burnt in an engine. This is usually supplied by taking in air from the atmosphere. the air that surrounds us also contains a large quantity of nitrogen.
Normal working temperatures of IC engines are 1500Ã‚Â°F.
This high temperatures found within the engine cause nitrogen to react with any unused oxygen to form nitrogen oxides [NOx].
Nitrogen oxides are one of the most toxic pollutants emitted by vehicle engines.
Nitrogen oxides combine with water in the atmosphere and result in acid rain.
The most effective means of tackling nitrogen oxides is to use Exhaust Gas Recirculation [EGR].
This technique directs some of the exhaust gases back into the intake of the engine. These gases have already been used by the engine and no longer contain much oxygen. By mixing the exhaust gases with fresh air, the amount of oxygen entering the engine is reduced. Since there is less oxygen to react with, fewer nitrogen oxides are formed. This can greatly reduce the amount of nitrogen oxides that a vehicle releases into the atmosphere.
2. Methods for reduction of NOx formation
The figure shown below depicts the variation of cycle temperatures in an IC engine
The triangular portion of the graph which is shaded shows the NOx formation region.
The best way of reducing NOx is to reduce the cycle peak temperatures by introducing an inert gas into the cylinder.
The wisest choice would be the exhaust gas itself because, it has practically no role in the combustion of the air and the fuel inside the cylinder beyond which the formation of NOx starts.
We can use exhaust gas for reducing the formation of the NOx in the engine, by re -circulating a certain percentage of the exhaust gas.
3. The Basic EGR System
The figure shown below shows the pictorial representation of a basic EGR system.
There is a routing from the exhaust manifold to the intake manifold through a pipe. The quantity of exhaust admitted into the engine is controlled by a valve known as the EGR valve.
Modern EGR system would contain a number of components like the EGR valve, EGR cooler, turbocharger etc.
4. Classifications of EGR
Various EGR systems are classified mainly into three groups.
The figure shown below shows the various types of EGR in
4.1 Based on the method of trapping the exhaust gas
EGR is classified mainly into two.
1.) Internal EGR
2.) External EGR
4.1.1 Internal EGR
This type of EGR differs from the other EGR owing to the fact that it has no external routing through which the exhaust gas is routed from the exhaust manifold to the intake manifold.
For all other types of EGR, we use an external piping to stream the exhaust into the cylinder of the engine.
The internal EGR is more disadvantageous when compared to external EGR. Internal EGR is inefficient and reduces fuel economy.
4.1.2 External EGR
This type of EGR is the most common type of EGR. This, unlike internal EGR, has an external routing through which the required amount of exhaust gas is re - circulated into the engine.
This has many advantages over the internal EGR that, in this system, the exhaust routed from the engine can be cooled externally with additional equipments like the EGR cooler or even, the exhaust can be filtered to avoid particulate matter (PM) entering the cylinder, which can cause over wear of the moving parts in side the engine.
4.2 Based on the path of exhaust gas
Based on the path of exhaust gas, the EGR is classified into two, basically.
1.) Short path EGR
2.) Long path EGR
4.2.1 Short path EGR
In this kind of EGR system, a portion of the exhaust is routed into the engine to the downstream of the compressor of the turbocharger, which is normally at a pressure well below the exhaust pressure.
4.2.2 Long path EGR
The exhaust formed in the cylinder is routed into the turbine of the turbocharger at first to run the compressor turbine of the same. A portion of the exhaust is taken to the EGR cooler to cool the exhaust and then its flow is regulated by the EGR valve which has links to the engine management system.
4.3 Based on the temperature of the exhaust gas admitted
1.) Hot EGR
2.) Cooled EGR
4.3.1 Hot EGR
This kind is not that popular since it increases the temperature of the intake mixture , the Hot EGR is the type of EGR in which the exhaust gas is re-circulated as such.
It reduces the life of the engine.
4.3.2 Cooled EGR
Here, the exhaust routed from the engine is cooled to a desired level before admitting it into the engine. This is more popular than Hot EGR due to the reduced risk of knocking and formation of hot-spots inside the cylinder of the engine.
5. Working of EGR
1.) When EGR is required, the engineâ„¢s electronic controls open the EGR valve.
2.) The exhaust gases flow through the EGR transfer pipe to the EGR cooler.
3)The exhaust gases are cooled by water from the cooling system.
4) The cooled exhaust gases then flow through the EGR transfer pipe into the
7.1 Effects of EGR on NOx emissions
Compared to the other two conditions, the initial emission level and the rate of NOx reduction are much greater at 1200 RPM - mid load.
7.2 Effect of EGR on Other Emissions
7.3 Engine System Performance and Efficiency
The two main causes for decreasing brake thermal efficiency are attributed to decreased combustion work (i.e. indicated work) and increased pumping work (assuming that friction remained constant). The decreased combustion work is the consequence of combustion degradation due to lower combustion temperatures and changes in A/F ratio.