Emissions of Diesel Engine Using Exhaust Gas Recirculation by Molecular Diffusion

Emissions of Diesel Engine Using Exhaust Gas Recirculation by Molecular Diffusion ADEL A. ABDEL-RAHMAN Mechanical Engineering Department Alexandria University, Alexandria 21544, Egypt E-mail: adel.abdel-rahman@alexu.edu.eg MOHAMED N. SAEED Mechanical Engineering Department Alexandria University, Alexandria 21544, Egypt E-mail: mnsaeed75@yahoo.com AMR F. SHARAFELDIN Naval Forces Alexandria, Egypt E-mail: amrfouad@hotmail.com Abstract:- The present work investigates experimentally a technique for reducing particles from exhaust gas recirculation (EGR). The method relies on the principle of Brownian diffusion of particles and molecules..diffusion coefficients for molecules such as CO 2, H 2 O and O 2 are several orders of magnitude higher than for typical exhaust particles. Therefore, a clean air stream of an engine intake air, passing next to an exhaust stream, portioned by a perforated tube, should become enriched with high concentration exhaust constituents such as CO 2, H 2 O,NOX, HC and loses oxygen molecules via molecular diffusion while exhaust particulate diffusion to clean air stream will be insignificant. To explore this phenomenon, a particle-free EGR exchanger (PF EGR E) was constructed. The exchanger is a shell-and-tube type made of copper with perforated tube section. A portion of the exhaust gas is recirculated to the engine intake through the EGR exchanger. Exhaust gas is the tube fluid and intake air is the shell fluid. Volume flow rates for both exhaust and intake air are controlled to be equal at different flow rates. The experiment was carried out for four engine speeds: 900, 1200, 1500 and 1800 rpm. For each speed the tests were performed at no-load, half-load and full-load conditions. Different EGR percentages were applied on each load condition ranging from 0% EGR to 20% EGR. The experimental setup for the proposed experiments was developed on a 4-cylinder, direct injection, water-cooled, compression ignition engine. Concentrations of NO X, HC, CO 2, O 2, CO and PM were recorded upstream and downstream of the (PF- EGR-E). The results showed that up to 88% particulate matter reduction can be achieved using this technique.. Keywords: EGR Particle-free EGR - Diesel Engines NO x emissions Exhaust particles - Pollution 1 Introduction The removal of NO x from diesel exhaust gases is a field of active research, in order to meet the challenges posed by the increasingly stringent environmental emissions legislations. Gas temperatures vary greatly between different applications of diesel engines. On one hand, the more or less steady exhaust gas temperatures of some large diesel engines make NO x reduction possible. On the other hand, the temperature fluctuations in smaller and non-stationary operated diesel engines cause NO x reduction to be much more difficult. Exhaust gas recirculation (EGR) is an approach that is used for reducing NO x emissions from engines. A portion of the exhaust gas is recirculated to the intake air system of an engine to enrich the intake air with inert constituents such as CO 2 and H 2 O molecules, reducing the level of oxygen entering the combustion process. This process results in a decrease in the peak and average temperature inside the combustion chamber of an engine, and thus reduces NO x emissions. However, due to the presence of soot ISBN: 978-1-61804-265-1 91

in the exhaust products, intake air with (EGR) containing particulate matter leads to several intake air system fouling. Returning exhaust products to the diesel engine combustion chamber accelerates the degradation of the lubricant engine oil [2]. The soot contaminates the lubricant and changes the chemical properties resulting in the lubricant ceasing to perform its functions [3]. George et al. [4] studied the interactions between soot and oil additives in order to develop high performance diesel engine oils for engines equipped with EGR. EGR not only helps to reduce NO x emissions but also contributes to achieve lower HC. In addition, no adverse effects were noted, especially at lowto-medium loads [5]. EGR can be advantageously combined with other emissions reducing measures such as flexible injection timing control, the use of high (EGR) rates accompanied by increased boost pressure, catalytic converters, use of nitrogen-enriched air (NEA) and recently using EGR with molecular diffusion [1]. Improvement and further development of engine management and control systems and exhaust gas aftertreatment for a reduction of nitrogen oxides and particulates is becoming important and their application in internal combustion engines is discussed in [6]. D.T. Hountalas et al [7] regarded that, one efficient method to control NOx is the use of rather high exhaust gas recirculation (EGR) rates accompanied by increased boost pressure to avoid the impact on soot emissions. Diesel particulate filter (DPF) is regarded as a useful technology to reduce particulate matter from exhaust gas of a diesel engine [8]. Nitrogen oxides emissions in diesel engine exhaust can be reduced by the use of nitrogen-enriched air (NEA) for combustion [9]. Many researches have been done to find new from EGR gases. EGR by molecular diffusion is one of these techniques. Verification of this technique on a real stationary diesel engine and studying its performance on engine emission reduction is the aim of the present work. It was shown that it is possible to achieve molecular equilibrium between a dilute exhaust stream flowing in parallel and the same flow rate with a clean air stream, while particle equilibrium was still far from being achieved. Fifty percent molecular transfer was achieved while particle transfer from the dilute exhaust to the clean air stream was less than 10% [1]. 2 Manufacturing of EGR Element Diffusion is a process of mass transport that involves the movement of one atomic species into another. In its simplest form, it occurs by random atomic jumps from one position to another and takes place in gaseous, liquid, and solid states for all classes of materials. Since diffusion involves atomic jumps, it becomes more rapid at higher temperatures because of the higher thermal energy of the atoms. The diffusion rate also depends on the openness of the structure, beginning generally more rapid for less densely packed structures. Consequently diffusion is most rapid in the gaseous state and least rapid in the solid state [10]. The particle-free exhaust gas recirculation element (PF-EGR-E) was designed and built as an experimental apparatus that can provide PF-EGR for diesel engines. The first step included the design of the PF-EGR-E with help of equations to predict particle and molecular diffusion. The second step was the manufacture of PF-EGR-E using materials of economic cost to realize design conditions. The third step included conducting four series of experiments at engine speed 900 rpm,1200 rpm, 1500 rpm and 1800 rpm. at different loads and EGR percentages to investigate the performance of the system. The preliminary design concept of the PF- EGR-E is shown in fig.1, where it consists of two concentric tubes. The inner perforated tube is for the flow of exhaust gas, and the outer tube was for the flow of engine intake air. EGR gas Exhaust gases out Shell tube Perforated tube Clean air Exhaust gases in Fig. 1: A sketch of a particle free EGR element Maintaining a high percentage of open area while preventing mechanical mixing of the two flow streams is extremely difficult to satisfy simultaneously. To increase the flow resistance in order to minimize cross flow or mechanical mixing due to pressure difference between the two streams, a small number of holes with small diameter is required. Generally, the mass of the cross flow between the exhaust gas and intake air depends on the pressure difference ( p), density (ρ), hole area (A h ), and a discharge coefficient (C d ) that ISBN: 978-1-61804-265-1 92

depends on the hole geometry and size. The proportionality of the mass flow across the perforated tube is essentially estimated in equation (1) m (1) cross α n Cd ρ Ah p Where m cross is the mass flow rate through the hole and n is the number of holes in the system. Reducing the total open area to prevent cross flow reduces the exposure for molecular diffusion significantly. As result the preliminary design approach considered here is to maximize the total open area while minimizing cross flow by maintaining the pressure difference between the two streams near zero [1]. 3 Experimental Set-up and Procedure The experimental setup is shown schematically in Fig.2. A 4-cylinder diesel engine is used in the present investigation. It is a water-cooled, direct-injection, four-stroke, compression-ignition engine coupled with a hydraulic dynamometer of the Froude type. Specifications of the engine are given in table 1. Part of the exhaust gas is to be recirculated and put back to the combustion chamber along with the intake air. The quantity of this EGR is to be measured and controlled accurately, hence a by-pass for the exhaust gas is provided along with the manually controlled EGR valve. The exhaust gas comes out of the engine during the exhaust stroke at high pressure, pulsating in nature. In order to make the the volumetric flow rate measurements of the recirculating gas possible, pressure pulses are to be removed. For this purpose, a pressure pulsation tank is used and installed in the EGR route to absorb the exhaust gas pulsations. Flexible expansion joints are fitted at inlet and outlet of pressure pulsation tank to reduce vibration at high flow rates. A particle free exhaust gas element provides the required area for molecular diffusion between exhaust gas and clean air streams. EGR stream enters the particle free exhaust gas recirculation element (PF-EGR-E) as a tube fluid. An electric driven air blower is used to drive clean air stream, through a ball valve and an orifice meter to control the clean air percentage, inside the particle free exhaust gas recirculation element as shell fluid. A differential pressure gauge is installed between the final exhaust gas and clean air streams. Combined exhaust and clean air stream (EGR gas) comes out of particle free exhaust gas recirculation element and goes to the engine intake through a stop check valve. Temperature measurements were routinely taken at seven locations, namely: engine exhaust, engine oil, engine intake, inlet and exit of engine coolant, inlet and exit of cooling water. A thermocouple of the J-type (ironconstantan) for exhaust gas, and thermocouples of the T-type (copperconstantan) for other temperatures were calibrated, and securely installed in the respective locations. The thermocouples were connected to a temperature compensated digital readout through a nine-channel selector. The mass flow rate of intake air was measured using a calibrated orifice meter. A Fox Thermal flow meter was used to measure exhaust gas flow rate. The device uses a constant-temperature differential (DT) technology to measure the mass flow rate of air and gases, where the electrical power required to maintain a constant temperature differential is directly proportional to the gas mass flow rate. The microprocessor then linearizes this signal to deliver a linear 4 20 ma signal and the monitor shows the flow rate value in liters per minute. The device is installed in series in the system with its own pipe. Exhaust gas samples have been analyzed by particulate meter and exhaust gas analyzer up stream and downstream of particle free exhaust gas recirculation element. Particulate matter (PM) was measured using contamination detector device (model A.E.L.MK3). The level of contamination (PM) is measured by using the principle of light transmission through a Millipore filter. Two Millipore filters are used in series. The first filter traps the particulate matter, whereas the second filter is subjected to clean exhaust gas. Thus, the difference between light transmissions through the two filters depends only on the amount of particulate matter. By measuring the difference between the amount of light transmitted through the contaminated and clean membrane, it is possible to establish the level of particulate matter (PM). The exhaust gases were analyzed by an exhaust gas analyzer (Multigas- model 488). The analyzer detects CO, CO 2, O 2, uhc, and NO x. ISBN: 978-1-61804-265-1 93

Fresh air Diesel engine Intake air Orifice meter Exhaust Recirculated exhaust gas Air blower p 0 Particle free EGR exchanger Pressure-pulsation tank Flow meter Exhaust Table 1: Test engine specifications Fig. 2: A schematic sketch of experimental set-up Type Bore (mm) Stroke (mm) 4 st., DI, DE 91.4 127.0 Compression ratio 17.4 No.of cylinders 4 Displacement (cm 3 ) 3330 Max power Max Torque Fuel 48 kw @ 2300 rpm 220 N m @ 1350 rpm Diesel fuel In the present study, four main experiments were conducted, for four engine speeds; 900 rpm, 1200 rpm, 1500 rpm, and 1800 rpm. Each experiment consisted of three runs; no load, half load, and full load conditions. For each run, different EGR percentages were applied, ranging from 0% to 20% with a 5% incremental step. 3.1 Particle-free exhaust gas recirculation exchanger (PF-EGR-E) The fabrication of a particle free exhaust gas recirculation (PF-EGR) exchanger was performed after the design analysis. A photograph is shown in Fig.3. It is a shell and tube heat exchanger with perforated tubes of round holes to provide the necessary molecular exchange between exhaust gas stream and clean air stream. Exchanger dimensions are 0.45 m diameter, 0.85m overall length, 0.25m perforated tube section length, overall number of tubes are 290, of 0.011m external diameter and 0.009 m tube internal diameter. Tubes are made of copper, each contains 52 round holes, each hole is 0.006 mm diameter. The total open area for each tube is about 46%. Five thin supporting rods were welded at both ends to provide a support of perforated tube assembly. A portion of perforated tube is shown in Fig.4. ISBN: 978-1-61804-265-1 94

Fig. 3: Particle-free EGR exchanger Fig. 4: Perforated tube 4 Results and discussion Measurements of the emissions upstream and downstream of the (PF-EGR-E) were made with the objective of checking the system performance. Figures (5) and (6) show the relation between particulate matter and NO x reduction ratios against EGR% s respectively. Each figure shows the relation for 3 different load conditions (0-, half- and full-load conditions) and for 4 engine speeds of 1800 rpm, 1500 rpm, 1200 rpm and 900 rpm (figures 4a, 4b, 4c and 4d). Figure (7) shows the effect of EGR% on the brake specific fuel consumption (bsfc) versus EGR percentage for half load and full load conditions and for an engine speed of 1800 rpm. Figure (5) shows the typical PM reduction effect of EGR rate for a speed range of 900rpm to 1800rpm. Under all load conditions, the amount of PM decreses as the rate of EGR increases. It is shown from the figure that for relatively low EGR rate (5 & 10 EGR%) the present technique results in a reduction of PM, ranging from 44% at 900rpm to 72% at 1500rpm. For high EGR rate (15 & 20%), reduction of PM registered higher values; 64% to 88% at 1800rpm. This is likely due to the lower residence time (high speed) in the perforated section of the tubes. Figure 6 is similar to figure 5, but for the NO x reduction instead of PM reduction. The figure generally shows that the transfer rate of NOx increases with the decrease of each of the engine speed and the EGR rate. For low EGR rate ( 5% and 10%) the present technique results in NO x transfer rate ranging from 22% at 1800 rpm to 43% at 900 rpm. For high EGR rate (15% and 20%), the technique shows a NO x transfer rate from 14% at 1800 rpm to 31% at 900 rpm. NOx transfer from exhaust gas stream to clean air stream for low EGR percentages (5%, 10%) reached up to 43% due to the relatively higher residence time (900rpm speed) in the perforated section of tubes. The transfer percentage of NO x when applying high EGR percentages (15%, 20%) Reached as low as 14%, due to the relatively higher residence ISBN: 978-1-61804-265-1 95

Fig. 5: Downstream to upstream PM ratio vs. EGR% for different loads and speeds time (900 rpm speed). Figure 7 displays the effect of the EGR rate on the brake specific fuel consumption (bsfc) for a speed of 1800 rpm, and for half-load and full-load conditions. For both load conditions, it is seen that the bsfc decreases with the increase of the EGR rate. At half-load, bsfc decreases from 427.1 g/kw h for 0 % EGR, all the way down to 414.1 g/kw h at 20% EGR. At full-load conditions, the bsfc shows the same decreasing trend; 289.5 g/kw h for 0 % EGR, down to 279.1 g/kw h at 20% EGR. It is to be noted here that at full- load conditions, the bsfc registers values much lower than those at half-loadd, while the fuel consumption rate is indeed higher for full-load conditions. This is due to the high brake power of the engine at full-load, as one may anticipate. The improvement in fuel consumption with increasing EGR is more likely due to the following two factors: firstly, reduced heat loss to cylinder walls due to the reduction in peak temperature inside combustion chamber; secondly, reduction in the degree of dissociation in the high temperature burned gases due to the oxygen level reduction inside combustion chamber. ISBN: 978-1-61804-265-1 96

. Fig. 6: Downstream to upstream NO x ratio vs. EGR% for different loads and speeds 5 Conclusion Particulate matter (PM): The best performance is achieved at a speed of 1800 rpm and 20% EGR, where between 12 % to 19% PM was transferred to the clean air stream. At a speed of 1500 rpm and 15 % EGR, between 22% to 24% PM was transferred to the clean air stream. Nitric oxides (NO x ) transfer rate to the clean air stream increases with the decrease in the engine speed and applied EGR%, ranging from 14 % at 1800 rpm and 20 % EGR to 43% at 900 rpm and 5% EGR. Carbon dioxide (CO2) transfer rate to the clean air stream increases with the decrease in engine speed and applied EGR%, ranging from 27 % at 900 rpm 5 % EGR to 22.5% at 1800 rpm and 20% EGR. Different materials of the tubes and improved surface finish of the tube internal surface are two points that deserve a further study. ISBN: 978-1-61804-265-1 97

[10] J. P. Schaffer, A. Saxena, D. Stephen, H. Thomas and B. Steren, The Science and Design of Engineering Materials, Warner 2 nd edition, McGraw-Hill, 2002. [11] J. B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988. Fig.7: Brake specific fuel consumption for different EGR% References: [1] I. A. Khalek, Particle free exhaust gas recirculation by molecular diffusion: Proof of concept, SAE paper No. 0301769, 2003. [2] S. Aldajah, O. O. Ajayi and I. L. Goldblatt, Effect of exhaust gas recirculation (EGR) contamination of diesel engine oil on wear, Int. J. of Wear Research, Vol. 263, pp. 93-98, 2007. [3] S. George, S. Balla, and M. Gautam, Effect of diesel soot on lubricant oil viscosity, Tribology International, Vol. 40, pp 809-818, 2007. [4] S. George, S. Balla, and M. Gautam, Effect of diesel soot contaminated oil on engine wear, Int.J. of Wear Research, Vol.262, pp 1113-1122, 2007. [5] A. A. Abdel-Rahman, On the emissions from internal-combustion engines: a review, Int. J. of Energy Research, Vol. 22, pp. 483-513, 1998. [6] W. Knecht, Diesel engine development in view of reduced emission standards, Int. J. of Energy Research, Vol. 33, pp. 264-271, 2008. [7] D. T. Hountalas, G. C. Mavropoulos and K. B. Binder, Effect of EGR temperature for various EGR rates on heavy duty diesel engine performance and emissions, Int. J. of Energy Research, Vol. 33, pp. 272-283, 2008. [8] M. Schjbal, M. Marek and P. Koci, Modeling of diesel filters for particulates removal, Int. Chemical Engineering J., Vol. 154, pp. 219-230, 2009. [9] M. Ajhar, M. Follmann and C. Matthias, Membranes producing nitrogen-enriched combustion air in diesel engine, Int. J. of Membrane Science, Vol. 323, pp. 105-112, 2008. ISBN: 978-1-61804-265-1 98