Fuel 88 (2009) 1357–1364
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Experimental study on diesel engine nitrogen oxide reduction running with jojoba methyl ester by exhaust gas recirculation H.E. Saleh * Mechanical Power Department, Faculty of Engineering, Mattaria, Helwan University, P.O. Box 11718, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 27 May 2008 Received in revised form 16 January 2009 Accepted 25 January 2009 Available online 14 February 2009 Keywords: Diesel engine Jojoba methyl ester Nitrogenous oxides Exhaust gas recirculation
a b s t r a c t Jojoba methyl ester (JME) has been used as a renewable fuel in numerous studies evaluating its potential use in diesel engines. These studies showed that this fuel is a very good gas oil substitute but an increase in the nitrogenous oxides emissions was observed at all operating conditions. The aim of this study mainly was to quantify the efficiency of exhaust gas recirculation (EGR) when using JME fuel in a fully instrumented, two-cylinder, naturally aspirated, four-stroke direct injection diesel engine. The tests were made in two sections. Firstly, the measured performance and exhaust emissions of the diesel engine operating with diesel fuel and JME are determined and compared. Secondly, tests were performed at two speeds and loads to investigate the EGR effect on engine performance and exhaust emissions including nitrogenous oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (HC) and exhaust gas temperatures. Also, effect of cooled EGR with high ratio at full load on engine performance and emissions was examined. The results showed that EGR is an effective technique for reducing NOx emissions with JME fuel especially in light duty diesel engines. A better trade-off between HC, CO and NOx emissions can be attained within a limited EGR rate of 5–15% with very little economy penalty. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The interest to renewable energy sources for energy production is not new. Several studies have been conducted to qualify various oil and their blends from plants and vegetables as alternative renewable energy sources. This renewable source of fuel may also help in reducing the net production of CO2 from combustion sources and our dependence on fossil fuels. Often the vegetable oils investigated for their suitability as biodiesel are those which occur abundantly in the country of testing. Therefore, soybean oil is of primary interest as biodiesel source in the United States while many European countries are concerned with rapeseed oil, and countries with tropical climate prefer to utilize coconut oil, hazelnut or palm oil [1–3]. Other vegetable oils, including sunflower, rubber, etc., have also been investigated. Furthermore, other sources of biodiesel studied include animal fats, salmon oil and or waste cooking oils [4–7]. In the search for a viable vegetable oil as fuel for Egypt, jojoba ranks high as the vast area of the Egyptian deserts can be used for production the seeds are used to produce the jojoba fuel [8–11]. Many studies have been working on jojoba as a promising vegetable oil fuel for diesel engines for many years [8–11]. During this time, many tests have been performed on neat jojoba and jojoba
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methyl ester (JME). These studies showed that this fuel is a very good gas oil substitute and offered the same product guarantees for JME as for gas oil due to the fact that the physiochemical properties of JME are close to those of gas oil. An increase in the emissions of nitrogenous oxides (NOx) at all operating conditions have been observed by [12]. The objective of this work was to quantify the efficiency of exhaust gas recirculation (EGR) when using JME as a renewable fuel. In this paper, all experiments described here were performed on a direct injection diesel engine in first to compare diesel fuel and JME fuel in terms of engine performance and exhaust emissions at various speeds under full load and the second to investigate the effect of various EGR rates with JME fuel for NOx reduction. Also, effect of cooled EGR with high ratio at full load on engine performance and emissions was examined. EGR effects on engine performance, engine emissions, exhaust gas temperature, combustion quality and fuel economy for both high and low load engine operating conditions at two speeds were investigated. 2. Jojoba seed oil extraction and biodiesel production One of the several renewable sources, and yet not widely known, jojoba oil, appears to be promising with scope for cultivation in the relatively hot weather. The chemical structure of jojoba oil is similar to sperm oil which has been used as a constituent in many lubricating oil formulations [13]. The studies [8–12] showed that the viscosity of jojoba raw oil is high and that lead to blockage
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H.E. Saleh / Fuel 88 (2009) 1357–1364
of fuel lines, filters, high nozzle valve opening pressures and poor atomization [14–16], thus warrants treatment of oil before it becomes a viable engine fuel. To solve the problems associated with the high viscosity of jojoba raw oil, Kader and co-workers [17] synthesized the jojoba methyl ester in the laboratory and showed that methyl ester formation was 60–65% complete at respective molar ratios of methanol/jojoba oil 4.6:1. The alkaline catalyst used was (NaOH) and was added with a percentage of 1% which proved to produce maximum yield. At 60 °C, 65% JME was produced in 2 h. In addition, these studies concentrated on measuring the ignition delay period of JME and JME-gas oil blends at different conditions in shock tube. An optimum method for JME fuel production was developed by [8] on the grounds of production economy and fuel properties. Radwan et al. [8] measured the burning velocity of JME at different conditions in constant volume bomb. It is found that JME liquid fuel exhibited lower burning velocities than isooctane. An experimental investigation has been carried out to examine for the first time the performance and combustion noise of an indirect injection diesel engine running with JME, and its blends with gas oil [9]. A Ricardo E6 compression swirl diesel engine was fully instrumented for the measurement of combustion pressure and its rise rate and other operating parameters. Test parameters included the percentage of JME in the blend, engine speed, load, injection timing and engine compression ratio. Results showed that the new fuel derived from jojoba is generally comparable and good replacement to gas oil in diesel engine at most engine operating conditions. With the same test rig, JME was investigated as a pilot fuel as a way to improve the performance of dual fuel engine running on natural gas or liquefied petroleum gas at part load [10,11]. Results showed that using the JME fuel with its improved properties has improved the dual fuel engine performance, reduced the combustion noise and extended knocking limits. Also, an experimental evaluation of using blends of jojoba oil with gas oil as compared to gas oil has been conducted by Bawady co-workers [18]. Heat flux mapping and metal temperature distribution was carried out using a single cylinder, naturally aspirated, indirect injection four-stroke diesel engine [12]. Results at variable loads and speeds were taken with JME and were compared with those obtained with gas oil. It was found that the heat flux level and gas face metal temperature in the cylinder liner and head with JME were higher than those with gas oil. Also, an increase in the emissions of nitrogenous oxides (NOx) at all operating conditions have been observed.
where it is inducted into the succeeding cycles and it is used in this study. External EGR has emerged as the preferred type of EGR for heavy-duty diesel engines and that due to external EGR can be cooled and that improves fuel economy and engine performance. EGR reduces NOx because EGR gases contain much lower concentrations of oxygen as compared to ambient air. As a result, EGR gas slows down the rate of combustion as there is less oxygen in the cylinder to combust with the fuel. In addition to having low levels of oxygen, EGR gas also has a higher heat capacity than ambient air. This aids in slowing down combustion because more heat can be absorbed in an air-fuel mixture that contains EGR gases. While EGR is effective in reducing NOx, it also has adverse effects on the engine efficiency and may cause pollution of lubricating oil and corrosion of inlet manifold and moving parts as exhaust gas contains a lot of particulate matter. Some simple algorithms for adjusting EGR with engine power have been explored in the past to minimize its impact on the engine thermal efficiency [22]. In this paper diesel particular filter adopted for diesel particulate reduction to supply nearly particle-free exhaust gas.
3. Mechanism of NOx formation and EGR technique
EGR rate ð%Þ ¼
Oxides of nitrogen are formed when nitrogen and oxygen (from air) react at the extremely high temperatures reached during combustion. In the direct injection diesel engines, the fuel sprayed into the cylinder forms tiny droplets. Oxygen interacts at the boundary surface between the air and fuel. This boundary surface is where combustion takes place. Due to burning of the fuel droplets, the localized temperature in the vicinity of the fuel droplets exceeds the limit at which NOx are formed. In general, high amounts of oxygen and high temperatures will result in high levels of NOx formation and these conditions are always present in the combustion chamber of diesel engines. Since diesel engines always run at a lean air-fuel mixture and at high compression ratios [19]. Recently, exhaust gas recirculation, has received attention as a potential solution. Research work results showed that EGR is one of the most effective methods used in modern engines for reducing NOx emissions [20,21]. There are two types of EGR; internal and external. Internal EGR uses variable valve timings or other devices to retain a certain fraction of exhaust from a preceding cycle. External EGR uses piping to route the exhaust gas to the intake system,
The flow rate of air into the engine was measured using a laminar flow element. The laminar flow element was a honeycomb-like structure, which provides for a certain pressure drop across itself for a certain volumetric flow rate of air. This pressure drop was measured using an inclined manometer. The flow rate of EGR into the engine was measured with a sharp edge orifice mounted on a pipe connected to the pressure tank. An inclined manometer was used to measure the pressure drop across the orifice. The fuel consumption is measured by a fuel meter. The air temperature, exhaust gas temperature were measured using type K thermocouples and the CO, HC and NOx levels were obtained with a testo 350 gas analyzer was used to measure engine exhaust pollutant emissions. Pressure gauges were fitted to measure the pressure of EGR, inlet mixer and fresh intake air. A full set of readings was taken for each data point recorded thus the EGR rate can be calculated based on the above data. One of the great benefits of this engine is that the block was machined to allow for instrumented pressure measurements inside the combustion chamber. The cylinder pressure measurements
4. Experimental equipment The experiments were carried out on a two-cylinder, water cooled, four-stroke, direct injection diesel engine. It was necessary to make some of modifications in the engine since the original engine had not EGR. It was necessary to connect the exhaust manifold with the air intake manifold, with an EGR valve at this connection. The experimental set up is shown schematically in Fig. 1 and comprises a hydraulic dynamometer, a pressure tank, a diesel particulate bag filter, a heat exchanger, an EGR valve, a liquid and gas fuel metering systems, and an exhaust gases analysis system. The engine specifications are given in Table 1. It was necessary to connect the exhaust manifold with the air intake manifold, with a pressure tank, bag filter, a heat exchanger and an EGR valve at this connection. The bag filter is used for particulate reduction and supply of clean gas for EGR. The pressure tank is used for reduction of exhaust pressure pulse and the heat exchanger is used as an exhaust cooler for cooling exhaust gas by water flow. An EGR valve was installed in this connection that enabled manual to control the flow rate of EGR to the intake manifold to attain various EGR ratios. The EGR mass rate is the ratio between recirculated exhaust mass flow and the total mass flow allowed to pass into the engine
mEGR 100 mAir þ mFuel
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H.E. Saleh / Fuel 88 (2009) 1357–1364
Fig. 1. Schematic of the experimental test rig.
will make it possible to evaluate coefficient of variation of indicated mean effective pressure (imep). The coefficient of variation (COV) can be evaluated from the indicated mean effective pressure calculation to establish the quality of the combustion at various EGR rate. An in-cylinder piezoelectric pressure transducer captured the in-cylinder pressure trace. This signal was fed to dual mode charge amplifier, the output voltage signal was sent to the NIDAQmx data acquisition card. Data was collected on a computer using a LabVIEW program. The number of cycles chosen forms an acceptable limit for generating statistically meaningful results for all parameters examined. It has been shown by [23] that a greater number of cycles than 400 cycles form a safe limit for a statistical analysis. In this study a 1000 successive cycles have been taken. The coefficient of variation can be calculated as follows:
imepmean ¼
rimep
n 1X imepi n i¼1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i ðimepi imepmean Þ ¼ n1
Table 1 Specification of the engine used. Model
112M-Helwan
Type Cooling Number of cylinder Diameters of cylinder (mm) Stroke length (mm) Capacity (cm3) Compression ratio Rated speed (rpm) Rated power (HP)
4-Stroke direct injection Water cooling 2 112 115 2266 16.4 1500 26
COVimep ¼
rimep imepmean
where n is the number of cycles, imepmean is the mean indicated mean effective pressure and rimep is the standard deviation of the mean effective pressure. 5. Experimental results and discussion The experimental results are given in two sections. The first section compares diesel fuel and JME fuel in terms of engine performance and exhaust emissions at various speeds under full load. The second section investigates the effect of various EGR rate with JME fuel on the engine emissions, exhaust gas temperatures and engine fuel economy and combustion quality under various conditions. The tests and data collection were performed at two different engine speeds and loads. The engine evaluation at 1200 rpm – 25% load to represent urban speed/load conditions. Thousand six hundred revolutions per minute – 25% load, this is a low load condition for highway operation. Thousand two hundred revolutions per minute – full load and 1600 rpm – full load to represent heavy load operation. Table 2 shows the JME fuel with its improved
Table 2 The properties of the JME fuel versus diesel fuel [12]. Properties
JME
Diesel
Density @ 15 °C Kinematic viscosity @ 40 °C (cSt) Flash point Calorific value (kj/kg) Cetane number Water content % by mass Carbon residue % by mass Ash content % by mass
0.866 19.2 61 47,380 63.5 0.5 0.5 0.002
0.820 1.6–7.0 Min. 55 44,300 46 0.1 0.1 0.01
H.E. Saleh / Fuel 88 (2009) 1357–1364
Table 3 Average uncertainties of some measured and calculated parameters.
800
1000
Parameters
Uncertainty (%)
Temperature Speed Mass flow rate of fuel Mass flow rate of air Mass flow rate of EGR Nitric oxides Unburned hydrocarbon Carbon monoxide Engine torque
0.7 1.5 2.8 2.5 2.2 5 3.5 3.5 5.5
800
NO x (ppm)
5.1.1. Engine performance The variation of engine speed on the brake thermal efficiency (gth ) and the brake specific fuel consumption (bsfc) with JME and diesel fuel at full load is shown in Fig. 2. The experimental results of both JME and diesel fuel indicate that the brake thermal efficiency variations were in harmony with the speed variations as the engine speed increases the thermal efficiency increases for both JME and diesel fuel. At low speeds, the engine power and brake thermal efficiency with JME are slightly higher than the diesel fuel. At high speed, the engine output and efficiency with JME were higher than that with diesel fuel. The bsfc with JME is lower than that with diesel fuel by 8.2%, 9.8% at N = 1200 rpm and N = 1600 rpm, respectively. The main reason for the higher power and efficiency or on the other hand, lower brake specific fuel consumption may be explained by the higher calorific value of JME [11,15]. Furthermore, JME as an oxygenated fuel has a beneficial effect on combustion, especially in fuel rich zones as indicated by [25,26]. The author [25] found that the power and the engine efficiency increased slightly with indirect diesel engine running on the tobacco methyl ester-diesel blends.
400
22
240 1200
1400
1600
1800
1800
200 2000
JME fuel
280
1000
1600
5.1.3. CO emissions The variation of carbon monoxide (CO) emissions versus engine speed at full load is shown in Fig. 4. The reduction in engine speed produced higher CO in the exhaust and that due to the increasing of the amount of fuel per stroke and decreasing in swirl intensity
2500
Full load
24
1400
5.1.2. NOx emissions The nitrogenous oxides (NOx) concentration versus engine speed is shown in Fig. 3. The results show that as the speed decreases, NOx increases for both diesel fuel and JME and that due to the reduction in engine speed at full load increases the engine volumetric efficiency, the maximum rate at which fuel is discharged from the injector nozzle and the quantity of fuel injected per stroke [27]. Increasing in the amount of fuel injected leads to higher cylinder temperatures which result in higher NOx concentration. It would be expected that the fuels with the highest in-cylinder temperature levels would have the highest NOx [19]. Since JME fuel was higher combustion chamber temperature as indicated from measured exhaust gas temperature, as shown in Fig. 3. The NOx concentration with JME fuel is higher than that with diesel fuel and that is may be due to the higher calorific value of JME. As shown in Fig. 3, at N = 1200 rpm, the increase of approximately 16% in the NOx emissions with JME than that with diesel fuel and about 14% at N = 1600 rpm.
360
320
26
1200
Fig. 3. Variation of NOx emissions and exhaust gas temperature with engine speed.
3000
30 28
1000
Engine speed ( N ) rpm
2000
Engine speed (rpm) Fig. 2. Relation between the gth and bsfc with the engine speed.
CO (ppm)
bsfc-Diesel
0
Full load
400
bsfc (g/ kw.hr)
Brake thermal efficiency (%)
η th -Diesel
32
Egt-Diesel
400
5.1. Comparison of performance and emissions with JME and diesel fuel
bscf-JME
NO x -Diesel
600
properties versus diesel fuel [11]. Three runs of tests were performed under identical conditions to check for the repeatability of all results and the experimental error is evaluated according to Ref. [24]. The details of the estimated average uncertainties of some measured at a typical operating condition are given in Table 3.
η th -JME
Egt-JME
600
200
34
NO x -JME
Exhaust gas temperature ( o C)
1360
Diesel fuel
2000
Full load 1500
1000 1000
1200
1400
1600
1800
Engine speed (rpm) Fig. 4. Variation of CO emissions with engine speed.
2000
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H.E. Saleh / Fuel 88 (2009) 1357–1364
sel fuel blends and that due to the poor spray characteristics of coconut oil, poor mixing, and consequently poor combustion because of coconut oil has a viscosity of about eight times as much as that of diesel fuel. The author [28] has been investigated the effect of coconut oil-diesel blends using a single cylinder, direct injection diesel engine. At high speed, CO concentration with JME is lower than that with diesel fuel that may be due to the beneficial effect of JME as an oxygenated fuel as mentioned before.
240 210
JME fuel Diesel fuel
HC (ppm)
180 150 120
Full load
90 60 30 0
1000
1200
1400
1600
1800
2000
Engine speed (rpm) Fig. 5. Variation of HC emissions with engine speed.
within the cylinder which causes poorer mixing since there is insufficient oxygen to convert all carbon in the fuel to carbon dioxide [27]. As shown in Fig 7, at low speed, CO concentration with JME is slightly higher than the diesel fuel and that due to the higher viscosity of JME which causes poorer atomization and distribution of the JME fuel–air. Also it has been shown by [28] that CO emissions increased when the coconut oil amount increased in the die-
600
1000
NOx (ppm)
800
NOx, 25% load
NOx, Full load
bsfc, 25% load
bsfc, Full load
500 400
600 300 400
Jojoba methyl ester 200
200
100
N = 1200 rpm 0
0
10
bsfc (g/kw.h)
a
20
30
40
50
5.1.4. HC emissions The variation of unburned hydrocarbons (HC) emissions versus engine speed at full load is shown in Fig. 5. The reduction in engine speed produced higher HC concentration due to poorer mixing, since the fuel–air pocket can be mixed to stoichiometric proportions and then diluted by more air before autoignition occurs in that element [27]. As a result there are contours for lean equivalence ratios and when autoignition occurs there is fuel and air mixed locally to proportions less than the lean flammability limit. Thus this local fuel mixture does not burn and will increase the HC concentration. Also, the percentage of heat loss to the coolant is increasing with decreasing the engine speed and that leads to higher HC concentration in the exhaust gases. As shown in Fig. 5, at low speeds, HC concentration with JME is higher than the diesel fuel and there is no appreciable difference between HC concentration with JME and diesel fuel at high speed. 5.2. Experimental study on diesel engine with JME using EGR technique In the above section have been observed a decrease in the emissions of CO and HC in the almost of the operating conditions but an increase in the NOx emissions in all operating conditions and the overall engine performance with JME was equivalent to that of the conventional diesel fuel. Thus, the aim in this section is to reduce NOx emissions using JME as fuel. The measurements of an experimental investigation of the effect of various EGR rates on NOx reduction with JME at different engine parameters are presented and discussed in the present section. Also, CO and HC emissions, exhaust gas temperatures, combustion quality and fuel economy are obtained for wide range of various EGR rates. 5.2.1. Effect of EGR on NOx emissions and fuel consumption with JME Fig. 6 shows influence of the various EGR rate on NOx emissions and fuel consumption when the diesel engine was operated with JME at 1200 rpm – 25% load, 1200 rpm – 100% load, 1600 rpm – 25% load and 1600 rpm – 100% load. It is shown that, EGR rates
0
EGR rate (%)
NOx, Full load
bsfc, Full load 500
NO x (ppm)
400 600 300 400
Jojoba methyl ester
8
N = 1200 rpm, Full load N = 1600 rpm, 25% load
7
N = 1600 rpm, Full load
accepted drivability limit
6 5 4
Jojoba methyl ester 3 2
200
100
N = 1600 rpm 0
200
COV of imep (%)
800
NOx, 25% load
bsfc, 25% load
N = 1200 rpm, 25% load
9
bsfc (g /kw.h)
b
10
600
1000
0
10
1
20
30
40
50
0
EGR rate (%) Fig. 6. Variation of NOx and bsfc with EGR rate at different operating conditions.
0
0
10
20
30
40
50
EGR rate (%) Fig. 7. Effect of EGR rate on COV of imep at different conditions.
H.E. Saleh / Fuel 88 (2009) 1357–1364
5.2.3. Effect of EGR on HC and CO emissions with JME It is important to measure the quantity of HC and CO emissions in the exhaust gases when the diesel engine was operated with JME because it is an indicator of the completeness of the combustion. So, the effect of EGR rate on HC emissions can be seen on Fig. 8 at different conditions. It is shown that the HC concentration can be maintained when increasing EGR rate reaches 15% under different engine conditions. Unfortunately, HC emissions increase rapidly with greater levels of EGR rate and this mean that the combustion quality worsens with the increase of EGR rate. Also, it can be observed that HC emissions at 25% load are higher than that at full load since the lighter load reduce turbulence and fuel mixing to too lean an equivalence ratio. This has a negative impact on the combustion efficiency and therefore leads to higher HC emissions. Also, Fig. 8 shows engine-out CO emissions versus EGR rate for speed/load cases as before. As previously, data points have been shown that the trend of CO emissions is similar to HC emissions change under the different working conditions. Increasing EGR rate above 15%, the CO emissions increased rapidly. The CO production
a
6000
800
5000
CO, 25% load
CO, Full load
HC, 25% load
HC, Full load
700
500
3000
400 300
2000
Jojoba methyl ester
1000
0
10
200 100
N = 1200 rpm 0
HC (ppm)
600 4000
20
30
40
0 50
EGR rate (%)
b
6000
800
5000
CO, 25% load
CO, Full load
HC, 25% load
HC, Full load
700 600
4000 N = 1600 rpm
500 400
3000
300
2000
HC (ppm)
5.2.2. Effect of EGR on combustion quality with JME The effect of EGR rate on NOx emissions and bsfc has been discussed in the previous section. One interesting side effect of using various levels of EGR rate is the coefficient of variation of indicated mean effective pressure. The COV of imep value gives a quantitative value to the degree of change in the combustion completeness with increasing the levels of EGR rate. In this study a 1000 successive cycles have been taken to ensure stationary values from the statistical analyses. For good drivability, the COV of imep should be no greater than 5% [30]. Fig. 7 shows the result of this evaluation under the different operating conditions. As expected, the COV of imep is increased continuously with increasing EGR rate at all the operating conditions. COV of imep ranged from 1.0% to 1.4% for all data sets when EGR rate of 5–40% at 25% load. Beyond 40% of EGR rate, COV of imep reaches 2%, and that supports the results of the previous sections where the combustion of mixture deteriorates and the cyclic dispersion increases. Also, at full load with EGR rate less than 12%, COV of imep ranged from 1.0% to 1.2%. Generally, COV of imep ranged from 1.0% to 2% for all various conditions
thus each point on the graph is a stable combustion point, defined as a point having a COV of imep less than 5%, which is well within accepted drivability limit. On the other hand, all COV of imep are below 5% which means that the diesel engine with JME will run stable with very little fluctuation in output work for EGR rate less than 40% at part load and 12% at full load.
CO (ppm)
produced a corresponding decrease in NOx emissions. As stated before; this is caused mainly by exhaust gas dilution of fresh air which causes decrease of oxygen concentration of in-cylinder combustible mixture and decrease of in-cylinder maximum temperature. To optimize NOx emissions reduction, EGR rate should be increased gradually by an EGR valve from 0% to the maximum allowable value according to speed/load conditions. As shown in Fig. 6, the bsfc is decreased or the fuel economy is raised with increasing EGR rate up to 5% at 25% load. Very important to note this increase in fuel economy and reduction in NOx emissions with EGR is obtained. This is due to increase of engine thermal efficiency because of unburnt hydrocarbons contained in the EGR would possibly re-burn in the mixture, leading to lower unburnt fuel in the exhaust [29]. Furthermore, lower combustion temperatures with EGR reduced heat loss to the walls. At 25% load, the maximum reduction in NOx emissions with minimum in fuel economy is obtained by increasing EGR rate to 15–20%. It has also been shown by [20] that at low loads, the CO2 and H2O concentrations in the exhaust gas are low, since EGR routes exhaust gas from preceding engine combustion cycles into the combustion chamber for succeeding combustion cycles. Because of this, the heat capacity of the cylinder charge decreased. So, large EGR rate (25–40%) can be used for high reduction efficiency of NOx (50–55%) without obvious increase of bsfc (less than 5%). Increasing of EGR rate above 40%, the combustion of mixture deteriorates. Since EGR in a diesel engine displaces a unit of fresh air with an equal unit of burned exhaust products, it not only alters A/F ratio, but causes a dilution effect. By reducing the oxygen concentration, the mixing time between the direct-injected fuel and the fresh oxygen increases and reduce the burn rate once diffusion combustion starts, therefore, makes stable combustion more difficult [20,27] to achieve and which indicates a speed and power output decrease and bsfc increases up to 17%. At high load, the heat capacity increases as the concentrations of CO2 and H2O are substantially higher. Both these molecules have higher than air heat capacities and with H2O being much higher than CO2 at typical combustion temperatures [27]. With the higher heat capacity of the mixture, more energy is required to pre-heat the incoming mixture, thus lowering the flame temperature and deterioration in diffusion combustion. Furthermore, at high load, the temperature of the mixture of EGR and fresh air increase, the cylinder trapped mass decrease and that has a detrimental effect on the volumetric efficiency [20]. So, it is difficult to employ EGR rate larger than 12% as shown in Fig. 6 and this may result in an excessive increase in bsfc up to 11%. At that level of EGR rate, the reduction in NOx was 33%.
CO (ppm)
1362
200 1000 0
Jojoba methyl ester 0
10
20
30
40
100 0 50
EGR rate (%) Fig. 8. Variation of CO and HC emissions with EGR rate at different operating conditions.
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H.E. Saleh / Fuel 88 (2009) 1357–1364
5.2.4. Effect of EGR on exhaust gas temperature with JME The effect of EGR rate on the exhaust gas temperatures under the different operating conditions is presented in Fig. 9. As expected, the exhaust gas temperatures were reduced continuously with increasing EGR rate. As stated before; increasing EGR rate augments the amount of inert gas in the combustion chamber, lowering the burned gas temperature and therefore lowering the NOx emissions. Since the composition of the EGR of diesel engine varies with load. At 25% load for both N = 1200 rpm and N = 1600 rpm, there is a little of CO2 and H2O concentrations in exhaust gas, so large EGR 35–40% can be used for significant reduction in exhaust gas temperature about 42–48% compared with the baseline case (without EGR) as shown in Fig. 9. At high load the heat capacity increases as the concentrations of CO2 and H2O are substantially higher so low EGR rate 14–17% can be used for the same reduction in exhaust gas temperature.
a
5.2.5. Effect of cooled EGR on engine performance and emissions at full load To increase the engine efficiency or in other words, decrease the bsfc at full load with EGR rate above 12%, an EGR cooler was added for cooling the hot exhaust gases in the EGR system to reduce the peak EGR temperatures in an attempt to regain the volumetric efficiency lost by operating with EGR rate. The effect of using cooled EGR on the volumetric efficiency and bsfc is depicted in Fig. 10 and on HC, CO and NOx emissions in Fig. 11 for an engine speed of 1600 rpm at full load with EGR rate of 16%. It is clear that the bsfc has decreased about 8% when using cooled EGR as compared to hot EGR and that may be due to improve the volumetric efficiency [20] as shown in Fig. 10. Since the temperature of the mixture of EGR and fresh air increased from a baseline level (without EGR) of 43–110 °C at 16% of hot EGR and 70 °C with using cooled EGR. Furthermore, by using the EGR cooler, the H2O with the higher heat capacity of the mixture is removed from exhaust gases and that improve the combustion efficiency and consequently, the engine efficiency [20]. Also it is shown from the Fig. 11 that using cooled EGR has a positive effect on reducing the combustion temperature which leads to a decrease in NOx emissions by 10%, HC emissions by 24% and also reduces CO emissions by 15%. It has been shown by Ref. [31] that using hot EGR in a diesel engine causes the combustion temperature to increase, as well as causing the combustion gases to spent longer periods at these higher tem-
500
baseline, tair = 43 0 C
100 25% load
with cooler, t mixture = 70 0 C
80
Full load
ηv (%)
Exhaust gas temperature ( o C)
400
300 N = 1200 rpm 200
100
40
200
20
100
0
10
20
30
40
0
η v
50
bsfc
Fig. 10. Effect of cooled EGR on gv and bsfc with EGR rate = 16%.
500
700
Full load 400
baseline, tair = 43 0 C
25% load
600 500
300
N = 1600 rpm
Emissions (ppm)
Exhaust gas temperature ( o C)
300
Jojoba methyl ester
EGR rate (%)
b
400
N=1600 rpm
Jojoba methyl ester
60
0 0
500
without cooler, t mixture = 110 0 C
Full load
bsfc (g/ kw.hr)
is most likely due to an incomplete combustion caused by the diluted mixture. Trade-off between HC, CO and NOx concentrations with increasing EGR rate because of reduction of HC and CO needs high temperature and complete combustion, which requires good fuel–air mixing and enough oxygen. Therefore, EGR can bring satisfactory results for NOx reduction, but at the same time HC and CO increase. From the emissions data presented above, a better tradeoff between HC, CO and NOx emissions can be attained within a limited EGR rate of 5–15%.
200
100
with cooler, tmixture = 70 0 C Full load
400
N =1600 rpm
Jojoba methyl ester
300 200
Jojoba methyl ester 0
without cooler, tmixture = 110 0 C
100 0
10
20
30
40
50
EGR rate (%) Fig. 9. Variation of exhaust gas temperature with EGR rate at different operating conditions.
0
NO x
CO × (10 )
HC
Fig. 11. Effect of cooled EGR on NOx, CO and HC emissions with EGR rate = 16%.
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peratures, which leads to an increase in the formation of both NOx and particulate emissions. 6. Conclusions In this paper, an experimental investigation of the effect of exhaust gas recirculation on exhaust emissions and performance in diesel engine operating with JME. Tests were made at two speeds and loads. Engine performance, exhaust emissions, exhaust gas temperatures and combustion quality were investigated. From the study carried out the following conclusions may be drawn: 1. JME is a good gas oil substitute as the brake power output with JME was higher than that with gas oil and NOx was higher with JME than those with gas oil. 2. The NOx emissions decreased with increasing the EGR rate. Also, at all engine speeds and loads, the JME produced a higher CO and HC emissions when the EGR rate is increased. 3. NOx emissions with minimum loss in fuel economy are obtained by increasing EGR rate to 15% at low load. 4. Using high levels of EGR (in excess of 40%) at low load has the beneficial effect of lowering the flame temperature, which leads to low NOx emissions of 50% with increase in bsfc of 5%. 5. From the COV of imep, the combustion was affected with levels of more than 15% of EGR for all operating conditions. 6. The optimum EGR level is 5–15% for all engine speeds and loads and that may be favorable in a trade-off between HC, CO and NOx emissions with little economy penalty. 7. When the exhaust gas temperature has decreased by using cooled EGR at full load, the engine economy significantly increased with decreased in the exhaust emissions.
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