Effect of Exhaust Gas Recirculation (EGR) on Performance and Emission of a Compression Ignition Engine with Staged Combustion (Insertion of Unburned Hydrocarbon)

The usage of EGR adopted diesel engine increasing day by day worldwide to reduce the NOx emissions. The EGR adopted engines can reduce NOx Considerably but it adversely improves the emissions of UHC nearly 40 to 50 %. We can avoid this effect by Reutilizat ion of UHC. In this paper the experimental investigation has been carried out on EGR adopted direct injection compression ignition engine with insertion of unburned hydro carbon rich exhaust. The results were presented and compared with all EGR levels. The result were evidenced that can reduce 20 to 25 % of UHC by this method.


Introduction
Better fuel econo my and h igh er po wer with lo wer maintenance cost has in creased the popularity of d iesel engine vehicles. Diesel engines are used for bulk movement of goods, powering stationary/ mob ile equip ment , and to generate electricity more economically than any other device in this size range. In most of the global car markets, record diesel car sales have been observed in recent years [1]. The exhorting anticipation of addit ional improvements in diesel fuel and diesel vehicle sales in future have fo rced diesel engine manufacturers to upgrade the technology in terms of power, fuel econo my and emissions. Diesel emissions are catego rized as carcinog en ic [2]. The st ringent emiss ion leg is lat ions are co mpelling eng ine manu factu rers to develop technologies to combat exhaust emissions. To meet these emission regulations with co mpetitive fuel economy, exhaust gas after-treat ment and optimized co mbustion are necessary. However, it is still unresolved which concept will succeed considering production and economic feasibility [3].
Dies el eng in es are v ery po pu lar po wer p lants fo r decentralized power production in rural areas all over the world as well as for powering the farm equip ment due to their fuel econo my , ease o f maintenance and robustness. Diesel engines are assumed as a good alternative to gasoline engines because they produce lower amount of emissions [4]. On the other hand, higher emissions of oxides of nitrogen (NO x ) and particulate matter (PM) have been noticed as major problems. Although, major constituents of diesel exhaust include carbon dioxide (CO 2 ), water vapor (H 2 O), nitrogen (N 2 ), and o xygen (O 2 ); carbon mono xide (CO), hydrocarbons (HC), o xides of nitrogen (NO x ), and particulate matter (PM) are present in s maller but environmentally significant quantities. In modern diesel engines, first four species normally consist of mo re than 99% exhaust, while last four (the harmfu l pollutants) account for less than 1% exhaust [5]. NO x co mprise of n itric o xide (NO) and nitrogen dioxide (NO 2 ) and both are considered to be deleterious to humans as well as environmental health. NO 2 is considered to be more to xic than NO. It affects human health directly and is precursor to ozone formation, wh ich is mainly responsible for s mog format ion. The ratio of NO2 and NO in diesel engine exhaust is quitesmall, but NO gets quickly o xid ized in the environment, forming NO 2 [6]. Since diesel engine main ly emits NO hence attention has been given to reduce the NO formation [7].

Nox Formation Mechanism
NO is formed inside the co mbustion chamber in post-flame co mbustion process in the high temperature region. The NO format ion and decomposition inside the combustion chamber can be described by extended Zeldovich Mechanism [8] The sensitivity of NO format ion rate to temperature and oxygen concentration is evident from this equation. Hence in order to reduce the NOx format ion inside the combustion chamber, the temperature and oxygen concentration in the combustion chamber need to be reduced. Even though, certain cetane improving addit ives are capable of reducing NOx, the amount of reduction is reported to be inadequate. Moreover, most of these additives are expensive. Retarded injection is an effect ive method employed in diesel engines for NO x control. However, this method leads to increased fuel consumption, reduced power, increased HC emissions and smoke. Water in jection is another method for NO x control however this method enhances corrosion of vital engine components.
In addition, it adds to the weight of the engine system because of requirement of a water storage tank. It is also difficult to retain water at a desired temperature during cold climate [9].

Exhaust Gas Recirculation
Exhaust Gas Recirculation is an effective method for NOx control. The exhaust gases mainly consist of carbon dioxide, nitrogen etc. and the mixture has higher specific heat compared to at mospheric air. Re-circulated exhaust gas displaces fresh air entering the combustion chamber with carbon dio xide and water vapor present in engine exhaust. As a consequence of this air displacement, lower amount of oxygen in the intake mixture is available for co mbustion. Reduced o xygen availab le for co mbustion lowers the effective air-fuel rat io. This effective reduction in air-fuel ratio affects exhaust emissions substantially. In addit ion, mixing of exhaust gases with intake air increases specific heat of intake mixture, wh ich results in the reduction of flame temperature. Thus combination of lower o xygen quantity in the intake air and reduced flame temperature reduces rate of NO x format ion reactions [10,11]. The EGR (%) is defined as the mass percent of the recirculated exhaust (MEGR) in the total intake mixture (M i).
[ 2 ] ℎ − [ 2 ] The engines using EGR emit lower quantity of exhaust gases compared to non-EGR engines because part of the exhaust gas is re-circulated [13]. Thus even if the concentration of to xic substances in the exhaust gas remains unchanged, the total quantity of emission of toxic substances reduce for the same volu metric concentration. Diesel engines operating at low loads and generally tolerate a h igher EGR ratio because re-circulating exhaust gases contain high concentration of o xygen and low concentration of carbon dio xide and water vapors. However at higher loads, the oxygen in exhaust gas becomes scarce and the inert constituents start dominating along with increased exhaust temperature. Thus, as load increases, diesel engines tend to generate more smoke because of reduced availability of oxygen [5]. Wagner et al. tried to achieve lower emission of NOx and soot using highly diluted intake mixture. At very high EGR rate (around 44%), PM emission decreased sharply with a continuous drop in NOx emission but this high EGR rate significantly affect the fuel economy [14]. Sasaki et al. conducted experiments using EGR on direct injection gasoline engine and reported that an appropriate volume of EGR improves fuel econo my and HC emissions. This phenomenon was presumably due to the intake temperature increase by EGR, wh ich improved the flame propagation in the relatively lean region of the air-fuel mixture, which is non-uniformly distributed [15]. Kusaka et al. also found that at low loads, EGR co mbined with intake heating can favorably reduce THC emission with imp rovement in thermal efficiency [16]. EGR was also used in a direct injection spark ignition engine as an effect ive way for improving fuel economy [17,18]. Das et al. used EGR to reduce NOx emissions in hydrogen -supplemented SI engine without any undesirable co mbustion phenomena [19]. Sato et al. performed experiments using methanol in d irect injection co mpression ignition engine and found that combustion performance beco mes inferio r under light load conditions because temperature in combustion chamber fell due to very high latent heat of methanol, thus hampering formation of co mbustible air-fuel mixture [20]. Selim et al. operated the diesel engine in dual fuel mode with natural gas and found inferior performance and emissions at low loads because lean mixtures formed at low loads were hard to ignite and had slow burn ing characteristics. EGR was found to be a method of improving engine performance and emissions of such engines [21]. Ho wever, application of EGR also leads to penalties. In case of d iesel engines, these penalties include higher specific fuel consumption and particulate matter emissions. Effectively, a trade-off between NOx and soot is observed with the use of EGR [22][23][24][25][26][27]. The reduction in flame temperature reduces the rate of soot oxidation/re-burning. As a result, in EGR system, mo re soot is formed during co mbustion and it remains un-oxidized and eventually appears in the exhaust [10]. The rise in smo ke (soot) level of engine exhaust due to EGR affects the engine performance in various ways. Increased soot level causes considerable increase in the carbon deposits and wear of the various vital engine parts such as cylinder liner, piston rings, valve train and bearings. Wear of the materials also increase due to chemical reactions taking place on the surface (adsorption, corrosion) or due to abrasion of material or rupture of anti-wear film by soot. The application of EGR also adversely affects the lubricating oil quality and engine durability [28][29][30][31][32][33]. Gautam et al. experimentally proved that soot interacts with oil addit ives reducing its anti-wear properties possibly by abrasive wear mechanis m. Increased wear due to EGR is because of presence of soot in lubricating oil [29]. Studies on valve-train wear in p resence of soot were performed by Nagai et al. As the EGR rate was varied fro m 0 to 17% to 25%, the wear of cam noses and rocker arm t ips was found to increase significantly [30]. If the exhaust gas is re-circulated direct ly to the intake, it results in increased intake charge temperature i.e. hot EGR. An increase in in let charge temperature always results in shorter ignition delay and may imp rove thermal efficiency [34]. If the exhaust gas is cooled before recirculat ion to combustion chamber, then it is called cooled EGR. Cooling of EGR increases the charge density therefore improves volumetric efficiency of the engine. Also, it provides additional benefits by lowering NOx emissions to a greater extent. However, condensation of moisture present in the exhaust increases corrosion in combustion chamber. Plee et al. reported that major influence on NOx emission is due to change in temperature rather than oxygen availability [35]. In the present study, EGR was imp lemented to see the effect of EGR on wear of piston rings. The piston rings are most vital parts between piston and cylinder. The engine was operated for 96 h in normal running conditions and the wear of the piston rings and deposits on vital engine parts were assessed. The engine was again operated for 96 h with EGR and similar observations were made for piston ring wear and deposits. Since, EGR results in more soot formation, which in-turn affects the lubricating oil by thickening the oil and increases the wear debris in lubricating oil. Increased soot level and wear debris in lubricating oil may adversely affect the piston rings because piston rings are used to scrap off the excess lubricating oil fro m the cylinder liner and return it to o il sump. Engine performance and carbon deposits on injector tip, cylinder head and piston crown were also investigated. Various emission legislations were tabulated in Table 1

Experimental Setup and Methodology
A two-cylinder constant speed diesel engine generator set was chosen to study the effect of EGR on the performance and emissions, carbon deposits, and wear of diesel engine components. The specifications of engine are given in section 2.1. The engine is coupled with an AC generator and the current generated is used by a resistive load bank, thus in-turn loading the engine. The generator is calibrated and all losses in the generator such as copper losses, armature current losses and friction and windage losses (unaccounted losses) are accounted for and taken into consideration wh ile analysing the data. For recirculat ion of the exhaust gas, appropriate plumbing was done. No insulation on the pipe line was provided therefore allowing the re-circulated exhaust gases to partially cool down. the Exhaust gas drawn fro m the 1 st and 3 rd cylinder were partially allo wed inside the 2 nd cylinder with the help of separate EGR control valve and the fuel supplied to the 2 nd cylinder was controlled by separate flow valve and corresponding fuel rate also considered for calculat ion The schematic d iagram of the engine setup is shown in Fig. 1. The quantity of EGR can be regulated by a control valve installed in the EGR loop. An air bo x was provided in EGR loop to dampen the fluctuations of the pulsating exhaust. An orifice was installed in the EGR loop to measure the flow rate of re-circulated exhaust gas. To measure the intake air flow rate orifice meter was installed. Su itable instrumentation for measurement of temperatures at several locations was done. Fuel consumption measurement was done using a gravitational manometer. Exhaust gas emission measurements were done by raw exhaust gas emission analyser (Make: Horiba, Japan, Model: EXSA-1500). Oxygen, CO, NO x and CO 2 were measured by manual Orsat apparatus . The exhaust gas opacity was measured by a smoke opacity meter (Make: A VL, Austria, Model: 437). To achieve the objectives of the study, engine was run under normal operating condition and at different EGR rates. The data for HC, NO x , CO, s moke opacity, exhaust gas temperature, and fuel consumption were recorded. Then, engine performance and emission patterns were co mpared. Optimu m EGR rate was found on the basis of performance and emissions of the engine. Then, the engine was run with and without EGR and also with staged combustion for total 6 h in each phase separately using a fixed test cycle shown in Table 2.

Results and Discussion
The engine was run on different loads at 1500 rp m with different EGR rates (fro m 0% to 25%) to investigate the effect of EGR on engine performance and emissions. The performance and emission data was analysed and presented graphically for thermal efficiency, BSFC, exhaust gas temperature, HC, CO, NOx emission, and smoke opacity.

Engine Performance Analysis
The trends of thermal efficiency are shown in Fig. 2. Thermal efficiency is found to have slightly increased with EGR at lower engine loads. The possible reason may be re-burning of hydrocarbons that enter the combustion chamber with the re-circu lated exhaust gas. At part loads, exhaust gas has less CO 2 and fairly h igh amount of O 2 . Also, partly-cooled EGR acts like a pre-heater of the intake mixtu re. When this exhaust gas is re-circulated in the cylinder, the unburned HC in exhaust gas burns because of sufficient O 2 availab le in co mbustion chamber and reasonably high intake temperatures. In staged combustion excess of unburned hydrocarbons utilized with reduced fuelling rates at higher engine loads, the thermal efficiency remains unaffected by EGR. At h igher loads, exhaust gas has higher amount of CO 2 , wh ich reduces maximu m temperature in co mbustion chamber along with o xygen availab ility therefore re-burning of HC is not significant.  Fig. 3 represents comparison of BSFC for all datasets using EGR with baseline data. BSFC is lower at lo wer loads for engine operated with EGR co mpared to without EGR. However, at higher engine loads, BSFC with EGR is almost similar to that of without EGR. At higher loads, amount of fuel supplied to the cylinder is increased at higher rate and oxygen availab le for co mbustion gets reduced. Thus, air fuel ratio is changed and this increases the BSFC. In staged combustion the fuel was burned with Exhaust hence it further reduces the BSFC.
The exhaust gas temperatures are shown in Fig. 4. It has been observed that with increase in load, exhaust gas temperature also increases. When the engine is operated with partly-cooled EGR, the temperature of exhaust gas is generally lower than temperature of exhaust gas at normal operating condition. Exhaust gas temperature decreases with increase in EGR rate. The reasons for temperature reduction are relatively lower availability of o xygen for co mbustion and higher specific heat of intake air mixture as exp lained earlier. In staged combustion the recirculated exhaust was entered in the 2 nd cylinder, moreover it further reduces the temperature of exhaust.    5 represents the variation o f exhaust gas temperature at the entry to the inlet manifold for different EGR flow rates. These graphs show that exhaust gas temperature at entry to inlet man ifo ld is not very high because exhaust gases are partly cooled before mixing with fresh air at at mospheric temperature. At part loads, this temperature at various EGR rates is close to atmospheric temperature. However, when load and EGR rates are increased, the exhaust temperature at the entry to the inlet man ifold becomes higher than atmospheric temperature and therefore EGR acts as a pre-heater to fresh intake air but in case of staged combustion the inlet man ifold temperature were observed higher compared to EGR because of the reason of the raw exhaust drawn fro m the cylinder was directly introduced in to the 2 nd cylinder   6 represents volumetric efficiency for different EGR rates. It can be seen that as the EGR rate is increased, volumetric efficiency decreases. The intake air mass flow reduces because of EGR imp lementation and this means the volumetric efficiency drops. Ghazikhanietal et al. also found that volumetric efficiency drops when EGR rate is increased [36].

Engine Emission Analysis
Effect of EGR on unburned hydrocarbon (HC) and carbon mono xide (CO) are shown in Figs. 7 and 8, respectively. These graphs show that HC and CO emissions increase with increasing EGR. Lower excess oxygen concentration results in rich air-fuel mixtures at different locations inside the combustion chamber. This heterogeneous mixture does not combust completely and results in higher hydrocarbons, and carbon mono xide emissions. At part loads, lean mixtures are harder to ignite because of heterogeneous mixture and produce higher amount of HC and CO. Fig. 9 shows the main benefit of EGR in reducing NOx emissions from diesel engine. The degree of reduction in NOx at higher loads is higher. The reasons for reduction in NO x emissions using EGR in diesel engines are reduced oxygen concentration and decreased flame temperatures in the combustible mixture. At the part load, O 2 is available in sufficient quantity but at high loads, O 2 reduces drastically, therefore NOx is reduced mo re at higher loads compared to part loads.   The smoke opacity of the exhaust gas is measured to quantify the particulate matter present in the exhaust gas. The smoke opacity is shown in Fig. 10. Higher smo ke opacity of the exhaust is observed when the engine is operated with EGR co mpared to without EGR. The variation in the s moke opacity level at h igh loads was higher co mpared to that at lower loads. EGR reduces availab ility of o xygen for combustion of fuel, wh ich results in relatively inco mplete combustion and increased format ion of part iculate matter. EGR system is significantly more than that of engine operated without EGR. The higher carbon deposits in the EGR system seem to be because of higher soot format ion. These pictures support the results obtained by smoke opacity (Fig. 10).

Conclusions
In the present research, experimental investigations were conducted to study the effect of EGR with and without staged combustion on performance and emissions of a diesel engine. EGR displaces o xygen in the intake air by exhaust gas re-circu lated to the co mbustion chamber. Reduced oxygen and lower flame temperatures affect performance and emissions of diesel engine in different ways. Thermal efficiency is slightly increased and BSFC is decreased at lower loads with EGR co mpared to without EGR. But at higher loads, thermal efficiency and BSFC are almost similar with EGR than without EGR. Exhaust gas temperature is decreased with EGR. Hydrocarbons, carbon monoxide, and smoke opacity are increased with EGR, but NOx emission decreases significantly. It can be observed that 15% EGR rate is found to be effective to reduce NOx emission substantially without deteriorating engine performance in terms o f thermal efficiency, SFC, and emissions. At lower loads, EGR reduces NOx without deteriorating performance and emissions at higher levels. At higher loads, increased rate of EGR reduces NOx to a great extent but deteriorates performance and emissions. Thus, it can be concluded that higher rate of EGR can be applied at lower loads. EGR can be applied to d iesel engine without sacrificing its efficiency and fuel economy and NOx reduction can thus be achieved.