Combustion characteristics and engine emissions of a diesel engine fueled with diesel and treated waste cooking oil blends

Combustion characteristics and engine emissions of a diesel engine fueled with diesel and treated waste cooking oil blends

Chemical Engineering Journal 172 (2011) 129–136 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevie...

1MB Sizes 1 Downloads 12 Views

Chemical Engineering Journal 172 (2011) 129–136

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Combustion characteristics and engine emissions of a diesel engine fueled with diesel and treated waste cooking oil blends A. Abu-Jrai a,∗ , Jehad A. Yamin b , Ala’a H. Al-Muhtaseb c , Muhanned A. Hararah a a b c

Department of Environmental Engineering, Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an P.O. Box 20, Jordan Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Jordan, Amman, Jordan Department of Chemical Engineering, Faculty of Engineering, Al-Hussein Bin Talal University, Ma’an P.O. Box 20, Jordan

a r t i c l e

i n f o

Article history: Received 19 April 2011 Received in revised form 18 May 2011 Accepted 19 May 2011 Keywords: Waste cooking oil Biodiesel EGR NOx CO2 HC

a b s t r a c t In this study, waste cooking oil from restaurants was used to produce a renewable and sustainable biodiesel through transesterification process. The Treated Waste Cooking Oil (TWCO) fuel produced has shown very promising chemical and physical properties; most notably; cetane number (∼49) and sulphur content (8 mg/kg). The combustion of conventional diesel and Treated Waste Cooking Oil (TWCO)–diesel blend (50/50 by volume; shown as TD50) was examined at different engine conditions. The combustion of TD50 resulted in a considerable reduction in the smoke opacity and unburnt hydrocarbons associated with an increase in the CO2 and NOx emissions due to unintentional advance of fuel injection timing, caused by the higher bulk modulus of TD50 fuel. Results indicated an increase in brake specific fuel consumption with simultaneous reduction in the engine thermal efficiency compared to conventional diesel. The lower smoke opacity in the case of TD50 fuel assists NOx reductions by exploiting the higher EGR tolerance of TD50 fuel. Thus, a more encouraging NOx -smoke tradeoff may be selected to reduce both NOx and smoke. The effect of EGR in NOx reduction was more noticeable with TD50 than conventional diesel fuel. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Diesel engines offer higher efficiency, better fuel economy and lower emission of green house gases than conventional gasoline engines [1]. However, swooning oil resources, global warming, environmental pollution and above all meeting the stringent requirements of the upcoming automotive emissions regulations, have resulted in a worldwide interest in exploring environmentally friendly renewable energy resources and their efficient utilization [2]. Biodiesel is one of the most promising alternative fuels to meet the above problems [3]. It is derived from the transesterification of fats and oils, and has similar properties to that of diesel produced from crude oil and can be used directly to run existing diesel engines or as a mixture with crude oil diesel. The main advantages of using biodiesel is that it is renewable, biodegradable, non toxic, can be used without modifying existing engines [4], and produces less harmful gas emissions such as sulphur oxide [5]. Chemically, biodiesel is a mixture of methyl esters with long-chain fatty acids and is typically made from nontoxic, biological resources such as vegetable oils [6,7], animal fats [8,9], or even used cooking oils [10].

∗ Corresponding author. E-mail address: ahmad [email protected] (A. Abu-Jrai). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.05.078

The storage and disposal of used cooking oils from restaurants and domestic use are both troublesome and costly. Most of the used cooking oil is poured into the sewer system of the cities. This practice contributes to the pollution of rivers, lakes, seas and underground water, which is very harmful for environment and human health [11]. It has been shown that the waste oils can be used in conventional diesel engines and can be transformed to an environmentally friendly fuel with lower emissions for HC and PM compared to diesel fuel [12–14]. The utilization of this alternative fuel may also provide considerable economic benefits in meeting the energy needs of industry. The price of feedstock oil is one of the most significant factors affecting the economic feasibility of biodiesel manufacture. Used cooking oil is basically a waste product and for that reason it is cheaper than unused vegetable oil. Zhang et al. [15], found that the transesterification process using waste cooking oil was more economically feasible because its lower raw material cost was quite enough to compensate for the higher initial investment. Regarding exhaust emissions, the use of biodiesel results in lower emissions of unburned hydrocarbons, carbon monoxide, smoke and particulate matter with some increase in emissions of NOx [16–18]. To date, exhaust gas recirculation (EGR) is still the most viable technique that can reduce dramatically NOx emissions in internal combustion engines, where a fraction of the exhaust gases are recycled through a control valve from the exhaust to the engine

130

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136

Table 1 Engine details.

2.2. Engine instrumentation

Name of the engine

Tempest

Force arm Bore and stroke Connecting rod length Swept volume Compression ratio Orifice area Injection timing Maximum power

0.4 m 72.25 mm × 88.2 mm 156.0 mm 1447 cm3 22:1 0.00181 m2 15◦ CA (deg crank angle) BTDC 22 kW at 3000 rpm

intake system [19,20]. Ladommatos et al. [21,22] have shown that the lower oxygen available for combustion, associated with EGR, resulted in prolonged ignition delay periods that shifted the whole combustion process further towards the expansion stroke. This resulted in the combustion gases spending shorter periods at high temperatures, leading to lower thermal NOx formation as well as lower rate of soot oxidation. The increase of the ignition delay period would have been expected to increase the amount of fuel being burned during the pre-mixed burning period; however, the reduction in oxygen availability associated with the application of EGR reduced the rate at which the fuel burnt in the pre-mixed phase. Moreover, the shift of the combustion process towards the expansion stroke resulted in earlier quenching of the flame (shorter combustion duration), which yielded higher levels of products of incomplete combustion in the exhaust and increased fuel consumption [23]. Operating of the EGR in the diesel engine by using jojoba methyl ester fuel to quantify the efficiency of the EGR was investigated by [24]. 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. Pradeep and sharma [25] conducted experiments to study the effect of EGR for NOx control in a compression ignition engine fuelled with bio-diesel from jatropha oil. The results indicated higher nitric oxide (NOx ) emissions when a single cylinder diesel engine was fuelled with JBD without EGR. NOx emissions were reduced when the engine was operated under EGR levels of 5–25%. However, EGR level was optimized as 15% based on adequate reduction in NOx emissions, minimum possible smoke, CO, HC emissions and reasonable brake thermal efficiency. To the best of our knowledge, comprehensive studies on the combustion characteristics and engine emissions of a diesel engine fueled with diesel and treated waste cooking oil blends are very rare in the literature. Therefore, the aim of this work was to perform a comprehensive and comparative study of conventional diesel and Treated Waste Cooking Oil (TWCO)–diesel blend (50/50 by volume; shown as TD50) in terms of the combustion characteristics (rate of heat release, brake specific fuel consumption, and engine thermal efficiency) and engine exhaust gas emissions (nitrogen oxides, smoke, carbon dioxide, and unburnt hydrocarbon). 2. Experimental setup and experiments 2.1. Test engine A four cylinder Tempest Engine coupled with dynamometer was used for data collection. The engine is a water-cooled engine, naturally aspirated, four stroke, and direct injection (DI) diesel engine. The main engine specifications are given in Table 1. The exhaust gas was recycled from the engine exhaust to the inlet (external EGR) and the volumetric flow rate of the EGR calculated according to the reduction in the air volumetric flow rate. An electric dynamometer with a motor and a load cell was used to load and motor the engine.

The engine is a completely self-contained test bed incorporating a swinging field DC dynamometer. The dynamometer which is capable of absorbing 22 kW (30 hp) is supplied in standard form for absorbing power only. Service includes oil/water heat exchanger for the main cooling system and a water/oil heat exchanger for the oil cooling system. The engine has manually adjustable ignition timing control with indicator, and one cylinder is provided with a transducer tapping point (10 mm metric thread). The orifice air meter uses a cylindrical reservoir or damping chamber to minimize the effect of pulsations and produce a steadier flow through the orifice inserted in one of its end walls. The other end wall of the chamber consists of a resilient synthetic rubber diaphragm secured at its edges between rings of marine plywood sealed to the cylinder. The engine speed was adjusted by revolution speedometer, and then a suitable load (weighting pieces) to balance the engine and minimize the vibration of the engine was adjusted. To measure the exhaust gas emissions, the sensor of the gas analyzer was positioned into the outlet of the exhaust. The values of voltage and ampere were taken by armature, also a multi-point thermometer was used to measure the values of inlet water, outlet water and exhaust temperatures. Fuel measurements were performed by simple gravity pipette type gauge, with two bulbs calibrated for 50 and 100 cm3 respectively. Flow rate of diesel was measured by a stopwatch (time required to consume 50 ml of fuel). The test rig included other standard engine test rig instrumentation such as to allow monitoring of flows (liquid and gaseous fuels, intake air and EGR), temperatures (oil, air, inlet manifold and exhaust) and pressures (gauges mounted at relevant points). Normal engine test bed safety features were also included. Atmospheric conditions (humidity, temperature, pressure) were monitored during the tests. A KISTLER 7613C pressure transducer, mounted flush at the cylinder head and connected via a charge amplifier to a data acquisition board was used to record the cylinder pressure. The crankshaft position was measured using a digital shaft encoder. Data acquisition and analysis were carried out using LabVIEW-based software. Output from the analysis of consecutive engine cycles included peak engine cylinder pressure, indicated mean effective pressure (IMEP), percentage coefficient of variation (% COV) of IMEP, average values and percentage COV of peak pressures, average crank angle for ignition delay.

2.3. Exhaust gas monitoring A KANE-AUTO SW3 gas analyzer with external water trap was used to measure exhaust emissions. Gas analysis included measurement of carbon dioxide, carbon monoxide (NIDR – non dispersive infrared), unburned hydrocarbons – hexane equivalent (FID – flame ionization detector) – oxygen (electrochemical method), and NOx (chemiluminescence) emissions. Smoke opacity was measured using a Bosch smoke meter, giving smoke emissions in terms of Bosch smoke numbers (BSN).

2.4. Tested fuels The fuels used were Treated Waste Cooking Oil (TWCO) and conventional diesel. The fuel properties for both fuels are given in Table 2. TWCO contains a number of different methyl esters (CH3OCOR) and the oxygen content of the TWCO molecule is 12 wt.%.

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136 Table 2 Fuel properties.

131

3. Results and discussion

Fuel analysis

Conventional diesel

Treated waste cooking oil

Cetane number Density at 15 ◦ C (kg m−3 ) Viscosity at 25 ◦ C (N/m s) LCV (kJ/kg) Sulphur content (mg/kg) Chemical formula Molecular weight C (wt.%) H (wt.%) O (wt.%)

46 838.4 5.2 45263 57 C12 H23 167 86.2 13.8 –

48.7 862.6 4.95 40360 8 C17 H31 O2 267 76.4 11.6 12

2.5. Experimental procedure The first step in this work was to prepare the biodiesel, the waste oil used in this work was provided from the university restaurant. For the complete production of biodiesel through transesterification the final mixture contained around 20% by volume methanol and around 100 g of sodium hydroxide (NaOH). Due to the long use of the waste oil, it was found that the oil contains a lot of dissolved materials, so the initial step was the filtration. After that and since the oil in most cases has small traces of water and the removal of these traces is crucial, the water traces have been removed through a heating process. Before adding the methanol to the oil, the sodium hydroxide – which is the catalyst – should be reacted with the methanol to conceive sodium methoxide. While the oil is still warm after the heating process, the methoxide has been added to the oil and steering under constant temperature of 50 ◦ C for at least one hour has carried out. The combustion of Conventional Diesel and Treated Waste Cooking Oil (TWCO)–Diesel blend (50/50 by volume; shown as TD50) was examined at 1500 rpm engine speed with three different engine loads 25%, 50%, and 75% of maximum load (these loads represents low, medium, and high load respectively). One percentage of EGR (50%) was used. The inlet charge was kept as much as possible at the same temperature (in the range of 25–30 ◦ C) when using EGR, so that the effects of the inlet charge temperature on the ignition delay and combustion process could be eliminated.

3.1. Combustion analysis The heat release rates at low, medium, and high load for the two tested fuels are shown in Figs. 1–3 respectively. It is clear that the use of TD50 affected the start of combustion compared to diesel fuel in all loads (Figs. 1–3). The use of TD50 fuel affected differently the fuel injection system timing compared to diesel, because of the different densities and bulk modulus of compressibility of the fuels [26,27]. Conventional diesel has lower density and is more compressible than TD50 fuel, so the pressure in the fuel injection system can develop slower, and pressure waves can propagate later for the same nominal pump timing. As a result, the injection of TD50 fuel starts earlier with higher pressure and rate (advance injection timing), and at the same degree crank angle the mass of diesel injected is lower than the corresponding mass of TD50 fuel. On the other hand, the relatively higher TWCO fuel cetane number (CN ∼49) compared to conventional diesel (CN = 46) leads to shorter ignition delay. For the all examined engine loads (25%, 50%, and 75%), the total combustion duration of TD50 was increased compared to diesel fuel, and the combustion was shifted to an earlier stage. Furthermore, the diffusion combustion duration was more pronounced for medium and high loads (50% and 75%). One of the reasons for decreasing NOx emission with the use of EGR is shown in Fig. 4 where a decrease in cylinder pressure is experienced when 50% EGR was added when the engine is operating with conventional diesel and TD50 fuels. The reduction in the in-cylinder pressure due to 50% EGR addition is increasing with engine load and this is mainly due to different EGR compositions and mainly CO2 and H2 O. With increasing engine load the concentration of CO2 and H2 O in the EGR is also increasing with simultaneously reduction in O2 concentration [21,22]. The use EGR resulted in the increase of the ignition delay and the overall combustion shifted to a later stage while the combustion duration was reduced. Both the cylinder pressure and the amount of fuel burnt in the premixed combustion phase were reduced. The shorter combustion duration is possibly related to the earlier flame quenching, which is associated with the yield of incomplete combustion products, increased smoke, fuel consumption and reduced NOx . Although the increased ignition delay period can be

Fig. 1. Rate of heat release (ROHR) from the combustion of Diesel and TD50. Engine condition 1500 rpm speed and 25% load.

132

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136

Fig. 2. Rate of heat release (ROHR) from the combustion of Diesel and TD50. Engine condition 1500 rpm speed and 50% load.

Fig. 3. Rate of heat release (ROHR) from the combustion of Diesel and TD50. Engine condition 1500 rpm speed and 75% load.

Fig. 4. Cylinder pressure and rate of heat release (ROHR) from the combustion of TD50 with and without 50% EGR. Engine condition 1500 rpm speed and 25% load.

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136

133

Fig. 5. Engine efficiency: (a) and Brake specific fuel consumption (BSFC); (b) for Diesel and TD50.

expected to result in more fuel burnt in the premixed combustion phase, the reduction of oxygen availability resulted in the reduction of the amount of the fuel burnt in the premixed combustion phase. Shorter premixed combustion with more pronounced diffusion combustion in parallel with the lower peak cylinder pressures (Fig. 4) and temperatures explains the lower NOx formation rates and higher smoke associated with the use EGR [22]. The effects of fuel type and EGR addition on the engine efficiency and brake specific fuel consumption for the three engine conditions of 25%, 50%, and 75% are shown in Fig. 5. There is deterioration in brake specific fuel consumption, measured in kg/kW h, for TD50 and as compared to conventional diesel. This is expected since TD50 fuel has oxygen content in its structure and has a lower calorific value on a gravimetric basis than conventional diesel [16]. And hence, conventional diesel gives higher thermal efficiency than TD50. Even though, advancing the injection timing – as in the TD50 case – can lead to improved engine thermal efficiency [28], but there is still two important factors dictate engine thermal efficiency; oxygen content and the calorific value of the fuel. As described in before the EGR addition increases the ignition delay, shift the combustion later in the expansion stroke, reduces the in-cylinder pressure which leads to reduced cycle work. To compensate for the loss of work (power) more fuel is provided to produce the desired work and this leads to more fuel

consumption. Furthermore the combustion duration was reduced due to early flame quenching which also lead to increased fuel consumption. 3.2. Exhaust gas emissions NOx emissions and Smoke opacity: Advancing the injection timing (Figs. 1–3) in the case of TD50 and hence, higher peak cylinder pressures and temperatures explains the higher NOx formation rates and lower smoke associated with the use of TD50 compared to conventional diesel (Fig. 6). The increase in NOx emissions when using TD50 fuel is at least partly attributable to an unintentional advance of fuel injection timing, caused by the higher bulk modulus of biodiesel fuels [26,27]. The higher bulk modulus of compressibility for TD50, which corresponds to a higher speed of sound in the fuel, causes a more rapid transferal of the pressure wave from the fuel pump to the injector nozzle [29], and so, advancing the injection timing, which lead to higher NOx emissions and lower smoke opacity numbers (Fig. 6). On the other hand, the atomic carbon-to-hydrogen ratio (C/H) of diesel fuels has been correlated to NOx emissions [30]. Miyamoto [30] reported that NOx emissions decreased with decreasing C/H ratio of diesel fuel. They noted that as C/H decreases, so does adiabatic flame temperature, as well as the tendency to produce prompt NOx . Despite this, they attributed the differences in NOx emis-

134

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136

Fig. 6. Nitrogen oxides (NOx ): (a) and Bosch smoke number (BSN); (b) for Diesel and TD50.

sions to differences in heat release rate. This justification could give us another reason for higher NOx emissions for TD50 which has C/H = ∼0.55 while conventional diesel has C/H = ∼0.52. For the TD50 fuelled engine the NOx emissions were increased by 37%, 29%, and 22%, but the smoke was reduced by 42%, 31%, and 30% compared with those associated with conventional diesel for the three testing engine load conditions; low, medium, and high loads respectively. EGR provided additional NOx reduction without an extensive penalty in smoke (Fig. 7). The use of 50% EGR with conventional diesel fuel resulted in a 33%, 39%, and 46% reduction in the NOx emissions for 25%, 50%, and 75% loads respectively, while the smoke opacity number increased by 58%, 55.2%, and 50%. Moreover, the use of 50% EGR with TD50 fuel resulted in a 40%, 47.5%, and 55% reduction in the NOx emissions for 25%, 50%, and 75% loads respectively, while the smoke opacity number increased by 42.8%, 55%, and 50%. It is clear that the effect of EGR increases as the load increases (Fig. 7) and this is mainly due to different EGR compositions and mainly CO2 and H2 O. Where with increasing engine load the concentration of CO2 and H2 O in the EGR is also increasing with simultaneously reduction in O2 concentration. Besides, the effect of EGR in NOx reduction is more noticeable with TD50 than conventional diesel fuel which could be elucidated to the higher percents of CO2 exists in the exhaust gas for TD50 fuel as it will be discussed later.

3.3. Carbon dioxide and unburnt hydrocarbon emissions Combustion of a hydrocarbon fuel should produce only carbon dioxide and water (H2 O). The relative proportion of these two depends on the carbon-to-hydrogen ratio in the fuel [30,31], thus, an engine’s CO2 emissions can be reduced by reducing the fuel’s carbon content per unit energy. For the fuels used in this work; TD50 has relatively higher (C/H) atomic ratio which could justify the higher CO2 emissions emitted compared to conventional diesel (Fig. 8). In addition, advancing the combustion for TD50 (Figs. 1–3) and hence, shifting the combustion process to earlier stages with higher cylinder pressures and temperatures reduces the possibilities of early flame quenching towards the end of the combustion, and increases the possibilities of complete combustion, consequently, increases the CO2 emissions and decreases the unburnt HC emissions (Fig. 8). It is noted from Fig. 8 that CO2 emissions increase nearly linearly as the load increases due to higher cylinder pressures and temperatures which leads to more complete and efficient combustion. For unburnt hydrocarbon (HC) emissions – which consist of fuel that is completely unburned or only partially burned – the trend with the load is different; where the maximum unburnt HC emissions occurred at 25% load (low load) which could be elucidated to the longer ignition delay period due to mixing of fuel to leaner than the lean combustion limit. For diesel engines an oxidation

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136

Fig. 7. Effect of 50% EGR on nitrogen oxides (NOx ) reduction and smoke increase for Diesel and TD50 fuels.

Fig. 8. Carbon dioxide (CO2 ): (a) and Hydrocarbon emissions (HC); (b) for Diesel and TD50.

135

136

A. Abu-Jrai et al. / Chemical Engineering Journal 172 (2011) 129–136

catalyst in the exhaust can further decrease unburnt hydrocarbon emissions. This process is aided by the excess air in the exhaust gas [32]. 4. Conclusions This work shows the feasibility of using Treated Waste Cooking Oil–diesel blend as diesel-replacement fuel. The main conclusions are: • Biodiesel fuel produced from used cooking oil through transesterification has shown very promising chemical and physical properties; most notably; cetane number (∼49) and sulphur content (8 mg/kg), whereas conventional diesel has cetane number (46) and sulphur content (57 mg/kg). • The combustion of TD50 in an unmodified diesel engine (nonoptimized injection timing) resulted in a considerable reduction in the smoke opacity and unburnt hydrocarbons but with an increase in the NOx emissions due to unintentional advance of fuel injection timing, caused by the higher bulk modulus of TD50 fuel. • Using TD50 has worsen the brake specific fuel consumption compared to conventional diesel, and that is attributable to the oxygen content in TD50 structure and the lower calorific value on a gravimetric basis than conventional diesel. • As expected, EGR reduced the NOx emissions and increased the smoke emissions for all tested fuels and conditions. However, the lower smoke opacity in the case of TD50 fuel assists NOx reductions by exploiting the higher EGR tolerance of TD50 fuel. Thus, a more encouraging NOx -smoke tradeoff may be selected to reduce both NOx and smoke. Acknowledgements Authors gratefully acknowledge the financial support of the Scientific Research Committee at Al-Hussein Bin Talal University and for the technical support provided by the Department of Mechanical Engineering at the University of Jordan. Appendix A. The engine brake power P (kW), brake specific fuel consumption BSFC (g/kW h), and engine efficiency (%) were calculated through Eqs. (1)–(3) respectively. P(kW) = 2N(rev/s)T (N m) × 10−3 BSFC(g/kW h) = (%) =

˙ Fuel (g/h) m P(kW)

P(kW) ˙ Fuel (kg/s) × LCVFuel (kJ/kg) m

(1) (2) (3)

˙ (g/h) where N is number of revolutions; T is the torque (N m); m and LCV (kJ/kg) is the lower calorific value of the fuel. References [1] P. Rounce, A. Tsolakis, P. Leung, A.P.E. York, A comparison of diesel and biodiesel emissions using dimethyl carbonate as an oxygenated additive, Energy Fuels 24 (2010) 4812–4819. [2] M.E. Tat, Cetane number effect on the energetic and exergetic efficiency of a diesel engine fuelled with biodiesel, Fuel Process. Technol. (2011), doi:10.1016/j.fuproc.2011.02.006.

[3] M.A. Fazal, A.S.M.A. Haseeb, H.H. Masjuki, Biodiesel feasibility study: an evaluation of material compatibility; performance; emission and engine durability, Renew Sust. Energy Rev. 15 (2011) 1314–1324. [4] A. Tsolakis, A. Megaritis, Exhaust gas assisted reforming of rapeseed methyl ester for reduced exhaust emissions of CI engines, Biomass Bioenergy 27 (2004) 493–505. [5] Z. Helwani, M.R. Othman, N. Aziz, W.J.N. Fernando, J. Kim, Technologies for production of biodiesel focusing on green catalytic techniques: a review, Fuel Process. Technol. 90 (2009) 1502–1514. [6] A.N. Danisman, Microwave assisted transesterification of rapeseed oil, Fuel 87 (2008) 1781–1788. [7] H.J. Berchmans, S. Hirata, Biodiesel production from crude Jatropha curcas L. seed oil with a high content of free fatty acids, Bioresour. Technol. 99 (2008) 1716–1721. [8] J.W. Goodrum, D.P. Geller, T.T. Adams, Rheological characterization of animal fats and their mixtures with #2 fuel oil, Biomass Bioenergy 24 (2003) 249–256. [9] F.F.P. Santos, J.Q. Malveira, M.G.A. Cruz, F.A.N. Fernandes, Production of biodiesel by ultrasound assisted esterification of Oreochromis niloticus oil, Fuel 89 (2010) 275–279. [10] T. Issariyakul, M.G. Kulkarni, L.C. Meher, A.K. Dalai, N.N. Bakhshi, Biodiesel production from mixtures of canola oil and used cooking oil, Chem. Eng. J. 140 (2008) 77–85. [11] K. Hamasaki, E. Kinoshita, H. Tajima, K. Takasaki, D. Morita, Combustion characteristics of diesel engines with waste vegetable oil methyl ester, in: The fifth International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines (COMODIA), 2001, pp. 410–416. [12] S. Pehan, M. Jerman, M. Kegl, B. Kegl, Biodiesel influence on tribology characteristics of a diesel engine, Fuel 88 (2009) 970–979. [13] R. Dinkov, G. Hristov, D. Stratiev, V. Aldayri, Effect of commercially available antioxidants over biodiesel/diesel blends stability, Fuel 88 (2009) 732–737. [14] M. Monteiro, A. Ambrozin, L. Li, A. Ferreira, Determination of biodiesel blend levels in different diesel samples, Fuel 88 (2009) 691–696. [15] Y. Zhang, M.A. Dubeˇı, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: economic assessment and sensitivity analysis, Bioresourc. Technol. 90 (2003) 229–240. [16] J. Szybist, S. Kirby, A. Boehman, NOx emissions of alternative diesel fuels: a comparative analysis of biodiesel and FT diesel, Energy Fuels 19 (2005) 1484–1492. [17] P. Karra, M. Veltman, S. Kong, Characteristics of engine emissions using biodiesel blends in low-temperature combustion regimes, Energy Fuels 22 (2008) 3763–3770. [18] Y. Lin, Y. Wu, C. Chang, Combustion characteristics of waste-oil produced biodiesel/diesel fuel blends, Fuel 86 (2007) 1772–1780. [19] A. Abu-Jrai, A. Tsolakis, K. Theinnoi, A. Megaritis, S.E. Golunski, Diesel exhaustgas reforming for H2 addition to an aftertreatment unit, Chem. Eng. J. 141 (2008) 290–297. [20] A. Abu-Jrai, J. Rodríguez-Fernández, A. Tsolakis, A. Megaritis, K. Theinnoi, R.F. Cracknell, R.H. Clark, Performance, combustion and emissions of a diesel engine operated with reformed EGR. Comparison of diesel and GTL fuelling, Fuel 88 (2009) 1031–1041. [21] N. Ladommatos, S. Abdelhalim, H. Zhao, Z. Hu, The effects of carbon dioxide in exhaust gas recirculation on diesel engine emissions, Proc. Inst. Mech. Eng., Part D: J. Automobile Eng. 212 (1998) 25–42. [22] N. Ladommatos, S. Abdelhalim, H. Zhao, Control of oxides of nitrogen from diesel engines using diluents while minimising the impact on particulate pollutants, Appl. Thermal Eng. 18 (1998) 963–980. [23] D. Hountalas, G. Mavropoulos, K. Binder, Effect of exhaust gas recirculation (EGR) temperature for various EGR rates on heavy duty DI diesel engine performance and emissions, Energy 33 (2008) 272–283. [24] H. Saleh, Experimental study on diesel engine nitrogen oxide reduction running with jojoba methyl ester by exhaust gas recirculation, Fuel 88 (2009) 1357–1364. [25] V. Pradeep, R. Sharma, Use of hot EGR for NOx control in a compression ignition engine fuelled with bio-diesel from Jatropha oil, Renew. Energy 32 (2007) 1136–1154. [26] A. Boehman, D. Morris, J. Szybist, E. Esen, The impact of the bulk modulus of diesel fuels on fuel injection timing, Energy Fuels 18 (2004) 1877–1882. [27] J.P. Szybist, A.L. Boehman, Behavior of a diesel injection system with biodiesel fuel, SAE Technical Paper No. 2003-01-1039, 2003. [28] J. Heywood, Internal Combustion Engine Fundamentals, Mc Graw-Hill, New York, 1988, ISBN 0-07-100199-8. [29] C. Stan, Direct Injection Systems for Spark-Ignition and Compression-Ignition Engines, Society of Automotive Engineers, Warrendale, PA, 1999, p. 228. [30] N. Miyamoto, H. Ogawa, M. Shibuya, K. Arai, O. Esmilaire, Influence of the Molecular Structure of Hydrocarbon Fuels on Diesel Exhaust Emissions, SAE Technical Paper No. 940676, 1994. [31] R. Stone, Introduction to Internal Combustion Engines, 3rd ed., Macmilan, London, 1999, ISBN 0-333r-r74013-0. [32] P. Eastwood, Critical Topics in Exhaust Gas after Treatment, Research Studies Press Ltd., 2000, ISBN 0-863380r-r242-7.