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Effect of Exhaust Gas Recirculation temperature on the emission and performance of a single cylinder naturally aspirated stationary diesel engine.

Introduction

Diesel engines are widely used as major power sources of men and material transportation because of their durability and high energy efficiency. However, diesel engines exhibit an affinity for high nitrogen oxides (NOx) which results severe environment pollution. Due to adverse health effects of such pollutants and increasingly stringent emission standards that exist worldwide require the simultaneous reduction of NOx and PM emissions from direct-injection diesel engines. Consistent efforts have been made to improve diesel exhaust emissions, principally for NOx. Unfortunately, this is a difficult goal because most strategies to reduce either NOx or PM emission cause fuel penalty and increased HC emissions.

NOx Formation

NOx are the various possible oxides of nitrogen, NO, N[O.sub.2], N[O.sub.3] and N[O.sub.4], of which N[O.sub.2] is the main emission of concern. NOx are formed from elements present in air, the nitrogen [N.sub.2] that makes up about 79% of our atmosphere, and the [O.sub.2], which makes up the bulk of the rest. It arises when these two elements are made hot together. So all combustion processes create NOx. The longer and the hotter the combustion processes the more NOx that are formed. At 1600 degrees Centigrade or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HC's) and the presence of sunlight, produces an ugly haze in our skies known commonly as smog (at ground level). The process is mainly governed by Zeldovich mechanism. Since higher combustion temperatures can lead to greater thermal efficiencies some trade-offs between high efficiencies or low NOx are possible.

EGR Tchnique

Exhaust gas recirculation (EGR) technique involves NOx abatement during the combustion process. EGR systems were introduced in the early '70s to reduce an exhaust emission that was not being cleaned by the other smog controls. The EGR routes known percentage of engine exhaust back 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. Too much flow will retard engine performance and cause a hesitation on acceleration. Too little flow will increase NOx and cause engine ping.

Methodology of Experiment

Test Engine

The test engine is a Kirloskar AV1[R] engine with data acquisition systems (DAS) for recording engine data measurements. Modifications were made to cool the exhaust gas being recirculated using two heat exchangers. An orifice meter was used to determine the flow rate of recirculated exhaust gas. Percentage of exhaust gas was taken as ratio of amount of exhaust gas recirculated to total engine exhaust gas. AVL[R]smoke meter was used to measure the smoke level and INDUS[R]5 gas analyzer for [O.sub.2], CO, C[O.sub.2], HC and NOx measurements. Specification of the test engine is mentioned in table 1.

[FIGURE 1 OMITTED]

Experimentation Procedure

Test was conducted in three steps. First the engine was run without EGR for load from 0.5kW to 2.5 kW in steps of 0.5kW. Exhaust gas was then made to pass through only first heat exchanger. A part of the cooled exhaust gas is recirculated in percentage of total exhaust gas in steps of 10% up to 50 % (further increases led to rough running of the engine). The same procedure was carried with exhaust gas through two heat exchangers which is referred to as COLD EGR and finally the exhaust gas from the engine was directly passed to mix with fresh air at intake manifold, referred to as HOT EGR. A rotameter, specially designed for diesel engine exhaust gas was used to measure the flow rate of the exhaust and the same is verified using an orifice meter. A settling tank was used to minimize vibrations of the gas and a filter with wire mesh filled with glass wool arrests soot partials entering in to the engine. Results of the experiment are compared for different cooling of the EGR in the following section.

Results and Discussion

The most relevant components of the EGR (exhaust gas) are C[O.sub.2] and [H.sub.2]O. The displacement of inlet charge with C[O.sub.2] and [H.sub.2]O alters the combustion process in several ways. There is no physical inlet air throttling for Diesel engines (i.e. without a throttle). This enables engine admit as much air as it is practicable to trap at a given engine running condition. Thus, the application of EGR involves displacement of some of the inlet air by EGR. A consequence of this air displacement, there is a reduction in the air available for combustion (Fig. 2). A decrease of 41 to 59% from light load to full load was recorded for 50% EGR in the Oxygen concentrations (Fig.3).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Presence of burnt gases during combustion increases mixing time and causes longer burn duration. Reduction in Oxygen concentration within the engine cylinders that is available for combustion also reduces. The consequence of above two is lowered flame temperatures. This is an outcome of dilution effect. Fig.4 shows the decrease in exhaust gas temperature as percentage of EGR is increased. Exhaust gas temperature reduced in the range of 6 to 15% from low to full load at 50% EGR for HOT EGR mode. This reduction is marginally less for cooled EGR case.

[FIGURE 4 OMITTED]

The increased heat capacity of an EGR laced mixture associated with higher specific heat capacities of both C[O.sub.2] and [H.sub.2]O in comparison to that of Oxygen also results in lowered flame temperatures. Higher mass of C[O.sub.2], a nonreactive gas with high specific heat participates in the combustion absorbing heat energy released due to combustion. The result is a reduced flame temperature. This is referred to as Thermal Effect. At 40% EGR variations in C[O.sub.2] from low to full load are presented Fig.5. for three different cases under consideration. This thermal effect of EGR substantially curbs NOx at formation level.

[FIGURE 5 OMITTED]

As expected NOx emissions drastically reduce with increased EGR percentage (Fig.6) predominantly due to Dilution Effect and partly due to Thermal Effect. Increase in C[O.sub.2] concentrations with increased percentage of EGR is low as compared to decrease in [O.sub.2] concentrations. Thermal mechanism largely depends on C[O.sub.2] concentrations and this variation is less than 10% between Hot and Cold EGR from 10 to 50% EGR. The reduction in [O.sub.2] varies from 24% to 46% between Hot and Cold EGR. This difference is clear from Fig.7.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Hot EGR displaces more air compared to cooled EGR (Fig.2). On the other hand fuel flow increases as load increases at constant speed. Reduced Oxygen mass and increased amount of fuel reduces (AFR) Air Fuel Ratio (Fig.7). As reductions in the air mass inducted are more for HOT EGR, AFR for HOT EGR is comparatively low. This reduction in air-fuel ratio affects exhaust emissions, thermal and volumetric efficiencies substantially. Combustion quality deteriorates due to reduced air availability and extended burning duration resulting in to increased HC and smoke emissions (Fig.8).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Engine torque and BP slightly reduces as percentage EGR increase for all cases of EGR cooling. This results in to a reduction in Thermal Efficiency. Volumetric efficiency reduces due to reduced air availability caused by thermal throttling. Reductions in NOx emissions are remarkable at all loads and a reduction of 64% at 10% EGR and 94% at 50% was recorded for Hot EGR case. Reduction in N[O.sub.X] slightly decreased when recirculated EGR temperature is decreased. These values are plotted in Fig.9 and 10.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Reference

[1] Tsunemoto, H. and Ishitani, H. "The Role of Oxygen in Intake and Exhaust on NO Emission, Smoke and BMEP of a Diesel Engine with EGR System." SAE Paper 800030, 1980.

[2] Ropke, S., Schweimer, G.W. and Strauss, T.S. "NOx Formation in Diesel Engines for Various Fuels and Intake Gases." SAE Paper 950213, 1995.

[3] Hirao, O. "Exhaust Clarification of Automobile Engines. Fuel * Combustion * Catalyst" Editorial Committee for the Achievements of the Grant in aid for Special Project Research, "The Fundamental Research on Exhaust Clarification of Automobile Engines" p. 109, 1980.

[4] 4 Nord K., Haupt, D., (2005) "Reducing the Emission of Particles from a Diesel Engine by Adding an Oxygenate to the Fuel", Submitted to Environmental Science and Technology.

[5] Ladommatos, N., Abdelhalim, S. M., Zhao, H., and Hu, Z., "The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions, Part 4: Effects of Carbon Dioxide and Water Vapour", SAE Paper No. 971660 (1997)

[6] Ropke, S., Schweimer, G. W., and Strauss, T. S., " NOx Formation in Diesel Engines for Various Fuels and Intake Gases", SAE Paper No. 950213 (1995).

[7] Ladommatos, N., Abdelhalim, S. M., Zhao, H., and Hu, Z., "The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions, Part 1: Effect of Reducing Inlet Charge Oxygen", SAE Paper No. 961165 (1996).

[8] Ladommatos, N., Abdelhalim, S. M., Zhao, H., and Hu, Z., "The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions, Part 2 Effects of Carbon Dioxide", SAE Paper No. 961167 (1996).:

[9] Heywood, John B., Internal Combustion-Engine Fundamentals, McGrawHill, New York, 1988

[10] Ladommatos, N., Abdelhalim, S. M., Zhao, H., and Hu, Z., "The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions, Part 3: Effects of Water Vapour", SAE Paper No. 971659 (1997).

[11] Ladommatos, N., Abdelhalim, S. M., and Zhao, H., "The Effects of Carbon Dioxide in EGR on Diesel Engine Emissions", IMechE Paper No. C517/028/96 (1996).

[12] Zelenka P, Aufinger H, ReczekW, CartellieriW1998 Cooled EGR--A technology for future efficient HD diesels. SAE 980190

[13] Egnell, R. "Combustion Diagnostics by Means of Multizone Heat Release Analysis and NO Calculation". SAE Paper 981424.

[14] Gurumoorthy S Hebbar, Dr. Anant Krishna Bhat, "Effect of Exhaust Gas Recirculation Temperature on the Emission and Performance of a Single Cylinder Naturally Aspirated Stationary Diesel Engine", International Journal of Applied Engineering Research.

Gurumoorthy S Hebbar (1), * and Anant Krishna Bhat (2), **

(1) Mechanical Engineering Department, Shridevi Institute of Engineering & Technology, Tumkur 572 106, Karnataka, INDIA

* E-mail: gshebbar@rediffmail.com

(2) Mechanical Engineering Department, Gogte Institute of Technology, Belguam 590008 Karnataka, INDIA

** E-mail: akbhat2006@ rediffmail.com
Table 1: Test Engine Specification.

Type                            4-Stroke, Single Cylinder Diesel
                                Engine (Water Cooled)
Make                            Kirloskar AV-1.
Loading                         Electrical, Resistive Air Heaters
Rated Power                     3.7KW, 1500 RPM
Bore & Stroke                   85mm x 110mm
Cylinder Capacity               624.19 cc
Compression Ratio               16.5 : 1
Pressure Transducer             Piezo Sensor, Range: 2000 PSI.
Starting                        Auto Start.
Orifice Diameter for air flow   15mm
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Author:Hebbar, Gurumoorthy S.; Bhat, Anant Krishna
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Nov 1, 2009
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