Combustion and emissions of compression ignition in a direct injection diesel engine fueled with pentanol

Combustion and emissions of compression ignition in a direct injection diesel engine fueled with pentanol

Energy xxx (2014) 1e7 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Combustion and emissions of...

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Energy xxx (2014) 1e7

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Combustion and emissions of compression ignition in a direct injection diesel engine fueled with pentanol Li Li, Jianxin Wang, Zhi Wang*, Haoye Liu State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2014 Received in revised form 27 November 2014 Accepted 5 December 2014 Available online xxx

Pentanol is a new generation bio-fuel that could help relieve the energy crisis and environmental problems. The objective of this study is to reveal the combustion and emission characteristics of pentanol in a single-cylinder direct-injection diesel engine. In the present investigation, experimental data for pentanol in conventional diesel combustion mode were presented. The emissions, combustion characteristics and thermal efficiency for pentanol and diesel fuel were obtained under the same operating conditions. Results show that NOx and soot emissions decrease significantly for pentanol with comparable efficiencies under single injection strategy without EGR (exhaust gas recirculation). Especially, the lowest NOx emission is 0.23 g/kW h while the soot is negligible for pentanol at 1600 rpm and 0.6 MPa IMEP (Indicated Mean Effective Pressure). Pentanol fuel offers obvious characteristics to achieve a smoother heat release rate with reduced peak pressure-rise rate in contrast to the diesel fuel. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Pentanol Compression ignition Emissions Heat release rate

1. Introduction Renewable fuels are being embraced throughout the world as a source of alternatives to fossil fuels due to their potential of improving energy security and reducing pollutant emissions for transportation vehicles. In addition, the biofuels could also balance the greenhouse gas emissions which are considered as the main factor for global warming. In the past decades, an extensive amount of studies on the short chain alcohols, especially methanol and ethanol, have been conducted, including those on performances and emission characteristics in diesel engines [1e3]. However, in recent years, a strong interest in higher alcohols, containing four or more carbons, as a renewable bio-based resource has emerged because of their favorable physical and thermodynamic properties [4]. Thus, higher alcohols can be used as fuel additives to improve the low temperature fluidity of palm oil [5]. Further, higher alcohols have the potential to overcome the drawback of lower alcohols due to the higher cetane number and better miscibility in diesel fuel [6]. Butanol is a higher alcohol with a 4 carbon structure and it has been widely investigated as a fuel or fuel additive recently [7]. Butanol can be used safely in diesel engines with favorable exhaust

* Corresponding author. E-mail address: [email protected] (Z. Wang).

emissions [8]. Valentino et al. [9] found that a noticeable decrease in soot emissions was achieved because of the longer ignition delay of butanol blends. Moreover, Yao et al. [10] concluded that the butanol addition can reduce the regular emissions without an adverse influence on brake specific fuel consumption and NOx emissions. Pentanol is an attractive next-generation bio-fuels with 5carbon structure, which can be produced from renewable feedstock [11,12]. Compared to the more frequently investigated short chain alcohols, pentanol has the advantage of having higher energy density, higher heating value, higher viscosity, lower hygroscopicity and lower volatility. These properties provide better compatibility with conventional diesel engines and existing fuel distribution infrastructure. However, significantly less work on pentanol have been reported in compression ignition engines. It is imperative to understand the basic combustion properties of pentanol in the modern CI (compression ignition) engines. Yang et al. [13] studied the fundamental combustion characteristics of iso-pentanol in HCCI (homogeneous charge compression ignition) engines, in which the intake charge of iso-pentanol was fully premixed. The results indicated that iso-pentanol had higher HCCI reactions than gasoline, and showed high ITHR (intermediate-temperature heat release), which was important for extending HCCI to high-load operation without knock. Javier Campos-Fernandez et al. [14] investigated the power and fuel economy performance of diesel/pentanol fuel blends (in a range

http://dx.doi.org/10.1016/j.energy.2014.12.013 0360-5442/© 2014 Elsevier Ltd. All rights reserved.

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between 10% and 25% volume pentanol) on a direct-injection Perkins diesel engine, and the results showed that a slight power reduction and an improvement on brake thermal efficiency was achieved with low volume ratio of pentanol addition. Wei et al. [15] evaluated the effects of different pentanolediesel blends (10, 20 and 30% by volume) in a naturally-aspirated direct injection diesel engine. Engine tests showed that the n-pentanol addition could significantly reduce both the mass concentration and number concentration of particulate matter, while NOx increased slightly. Furthermore, fundamental experiments and detailed kinetic modeling studies for pentanol/iso-pentanol combustion had also attractived many researchers' attention recently. Tsujimura et al. [16] developed a detailed chemical kinetic model for isopentanol and used to simulate HCCI combustion. Dayma et al. [17] measured the concentration of stable species in a JSR (jet stirred reactor) over a range of equivalence ratios and temperatures at 10 atm, and proposed a detailed chemical kinetic model of isopentanol. More recently, a detailed reaction mechanism of iso-pentanol including a wide range of temperature, pressures and equivalence ratios was developed by Sarathy et al. [18], and validated against the previous and new experimental data. Their results show that 1-pentanol is more reactive than iso-pentanol. Heufer et al. [19] reported a detailed kinetic model for n-pentanol based on modeling rules of C4-alcohols. The proposed model shows good agreement with the ignition delay time data [20], species concentration data of JSR and laminar flame velocity [21]. All these studies revealed the beneficial effects of the pentanol as a diesel blend. Growing attention is being converged on the oxygenated pentanol as an alternative fuel or additives for fossil fuels, and more experimental data was needed urgently to obtain an in-depth understanding for the influence of the pentanol parameters on engine performance. In particular, there is no experimental investigation on effects of neat pentanol in direct injection compression ignition diesel. The aim of the present study is to evaluate the emission performance, fuel economy and combustion characteristic of neat pentanol in a modern diesel engine. The combustion type of pentanol is a typical spray-diffusion combustion, and the results are compared with the baseline diesel data. 2. Experimental setup and test procedure 2.1. Test engine The experiments were performed in a single cylinder, fourstroke, water-cooled, direct injection diesel engine, which is retrofitted from a Euro 4 light-duty four-cylinder engine by deactivating cylinders 2e4. The engine with a common rail injection system was connected to an electric dynamometer, which was capable of producing 110 kW and was rated at a maximum speed of 4000 rpm. The engine was controlled by an open ECU (electronic control unit), which could control injection parameters flexibly, such as injection pressure, number of injection events and injection timing. In the experiment, lubricating oil and cooling water were maintained at 85  C to ensure that the engine was at best condition. Table 1 gives the main engine specifications and Fig. 1 shows a schematic of the experimental set-up. 2.2. Test fuel Table 2 lists the main properties of diesel and pentanol used in the study, and the properties of butanol are also provided in Table 2 for comparison. The commercial 0# diesel with the cetane number of 56.5 is used as the baseline fuel. The pentanol used in this study was neat fuel which was obtained from Tianjing Fucheng chemical reagent factory (purity > 98.5%). Compared to diesel fuel, pentanol has a

Table 1 Engine specifications. Compression ratio Bore (mm) Stroke (mm) Connecting rod length (mm) Number of valves Displacement (L) Injector Injection system Intake valve open ( CA BTDC) Intake valve close ( CA ABDC) Exhaust valve open ( CA BBDC) Exhaust valve close ( CA ATDC)

16.7 83.1 92 145.8 4 0.5 7 holes,0.19 mm diameter Common rail 24 50 86 16

lower CN (cetane number) and is less prone to auto-ignition. However, the characteristics of relatively long ignition delay may be beneficial for improving local mixing and forming more premixed combustion. Kalghatgi et al. [22e24] found that the autoignition resistance of the fuel is the most important property to achieve low emissions for PPCI (partially premixed compression ignition) mode. The gasoline-like fuel were studied subsequently by Manente et al. [25], Shi et al. [26], Zhang et al. [27], and Yang et al. [28] under different operating conditions for CI engine. Similar to the gasolinelike fuel, pentanol is more resistant to auto-ignition than diesel fuel. Meanwhile, due to lower viscosity and lower boiling point, pentanol has better evaporation property than diesel, which could improve atomization efficiency. When compared to diesel, the incylinder combustion temperature of pentanol is lower due to its higher heat of evaporation, resulting in less NO-formation [29]. Pentanol is an oxygenated fuel, thereby providing the potential to reduce soot emissions in diesel engines. Two fuels have similar density. Pentanol has a greater similarity to diesel fuels in physiochemical properties and better water tolerance than C1eC4 alcohols. 2.3. Test facilities and methods Cylinder pressure was measured with an AVL GH14P transducer and recorded with the data acquisition system (AVL Indimodul 621) at a resolution of 0.5 deg CA. Heat release and other combustion analysis parameters were calculated from the averaged cylinder pressure of 200 consecutive cycles. Fuel consumption was measured by an FCM-D digital fuel meter with a resolution of 0.1 g. The AVL 439 opacimeter was employed to measure soot in the exhaust gas. The gaseous emissions, including NOx, CO (carbon monoxide), CO2 and THC (total hydrocarbon), were measured by the AVL CEB-II exhaust gas analyzer. The entire engine-out emissions were averaged over 60s under steady state conditions. Table 3 reports the accuracy of the main acquired data. The experiments were conducted by sweeping the injection timing at constant engine speed of 1600 rpm and constant load of IMEP (Indicated Mean Effective Pressure) 0.6 MPa while no EGR was used. All test data was acquired with constant intake pressure of 1.2 bar, and constant IP (injection pressure) of 80 MPa for both fuels. For the diesel, single and double injection (a pilot injection and a main injection) strategies are both implemented in this investigation. For the double injection strategies, 10% of total fuel amount was delivered during pilot injection, the dwell between pilot and main injection was fixed at 16 CA while the main injection timing varies. The pentanol was also tested under the same condition with single injection strategy. After each fuel test, the previous fuel was flushed out from the fuel lines and injection system and the engine was running with the new fuel for at least 10 min before the next test. Considering the LHV (lower heating value) difference between pentanol and diesel, the fuel consumption for pentanol was scaled

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Fig. 1. Experimental setup.

with the same LHV of diesel for a homogeneous comparision, as the following equation:

. 0 mpentanol ¼ mpentanol  LHVpentanol LHVdiesel

(1)

" UR ¼

vR Ux vx1 1

2

 þ

vR Ux vx2 2

2

 þ…þ

vR Ux vxn n

2 #1=2 (2)

0

where mpentanol is corrected fuel consumption which converts the actual fuel consumption to a value based on the same LHV with diesel, mpentanol is actual measured fuel consumption, LHVpentanol and LHVdiesel are LHV of pentanol and diesel fuel respectively. Then the ISFC (indicated specific fuel consumption) of pentanol was corrected values in this paper. Tests were performed to explore the effect of a new fuel on emissions and performance at a fixed low-middle engine operating condition while the injection timing swept. 2.4. Error analysis An error analysis for the engine measurements and calculations, such as IMEP, ISFC etc. was carried out based on root mean square method [31e33]. The uncertainty in calculated variables was determined using formula (2).

Table 2 Properties of fuels tested.

Viscosity @20  C (mm2/s) Low heating value (MJ/Kg) Oxygen content (% weight) Auto-ignition temperature ( C) A/F stoichiometric Latent heating @25  C (kJ/kg) Boiling point ( C)

Cetane number Density @20  C (kg/m3)

0#Diesel

Pentanol

Butanol

4.127 42.68 0 200e220 14.3 270 T10 ¼ 223 T50 ¼ 266 T90 ¼ 311 56.5 830.4

2.89 35.06 18.15 300 11.76 308 138

2.63 33.1 21.6 343 11.21 581.4 117.7

20 815

12 808

*Data have been taken from Refs. [13e20].

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L. Li et al. / Energy xxx (2014) 1e7 Table 3 Uncertainty of measured and calculated parameters. Parameters

Uncertainty (%)

Pressure Engine speed ISFC IMEP

0.5 0.5 1.1 0.6

where uncertainty is being measured in R and xn are independent variables with measured uncertainties,andUx1,Ux2, Uxn are error limits of measured parameters. The details of the uncertainties were shown in Table 3. 3. Results and discussion 3.1. Emissions In this study, two fuels with various injection strategies were investigated. The first discussion is relevant to comparison between the diesel fuel with pilot-main injection strategies and the pentanol with single injection strategies. Secondly, results obtained for the diesel fuel with single injection strategies were compared to the pentanol. Fig. 2 illustrates the results of exhaust emissions for the diesel double injection and the pentanol single injection under different injection timing with error bars. The x axis in all figures is in the same scale for the start of injection (SOI) of single injection strategy and double injection strategy. It is clear that the NOx emissions for pentanol in CI mode were significantly lower than that of diesel combustion with all the varying injection timing tested. Moreover, very low soot emissions were attained at the

same condition. Experiments showed that low emissions with more than 90% decreases of NOx emissions and 50% decreases of soot could be achieved for pentanol. This phenomenon demonstrates the advantage of the pentanol on exhaust emissions which achieves the simultaneous reduction of NOx and soot effectively. While maintaining low soot emissions, NOx with levels of below 0.5 g/kW h can be easily attained by the pentanol without using EGR, which is impossible for the diesel fuel. The results reported in the literature have shown that the lowcetane fuel may be beneficial for achieving low NOx and soot emissions in contrast to conventional diesel fuel [25,26]. In current investigation, the pentanol of 20 CN is more resistant to autoignition and more volatile than diesel base fuels. The level of mixedness increases and the homogeneity between the local mixture strength and the global mixture strength increases as the ignition delay increases. This helps to attain diluted premixed combustion and relatively low-temperature combustion when the global mixture strength was lean. Moreover, the latent heating of pentanol is higher than that of diesel, which reduces the in-cylinder peak temperature. As a result, the NOx emission decreases drastically for the pentanol regardless of the injection timing, and NOx is 0.23 g/kW h as the lowest point shown for the pentanol in Fig. 2(a). Few studies that ultra-low NOx emissions and soot can be attained under conventional CI operating mode without using EGR have been reported in diesel engine. Under the test conditions, the soot emissions are much lower for all fuels e Light absorption coefficient was less than 0.027 for the diesel fuel and less than 0.015 for the pentanol fuel. However, the longer ignition delay of the pentanol fuel results in much lower soot emissions while the injection timing sweeps as shown in Fig 2(b). Moreover, almost every O atom in the pentanol fuel bond to carbon

Fig. 2. Comparison of emissions performance of the pentanol single injection mode and diesel double injection mode as injection timing sweeping at 1600 rpm, 0.6 Mpa IMEP. NO EGR is used.

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Fig. 3. Emissions performance of the diesel single injection mode as injection timing sweeping at 1600 rpm, 0.6 Mpa IMEP. NO EGR is used.

Fig. 3 shows NOx, soot, CO, THC as a function of main injection timing in the diesel single injection case with error bars. Under the test condition, the main injection timing of diesel sweeps from 1 CA BTDC (before top dead center) to 3 CA BTDC. When the diesel is injected earlier than 3 CA BTDC, the maximum pressure rise rate becomes unacceptably high; when the injection timing is retarded later than 1 CA BTDC, combustion becomes increasingly unstable (COV > 5%). Thus, there is a narrow injection timing window for the diesel using single injection strategy for stable combustion. As expected, the NOx and soot emissions of the diesel with single injection strategy are much higher than that of the pentanol, and the CO and THC emissions of diesel are slightly lower than that of pentanol. Different from the diesel combustion with single injection, the pentanol with single injection overcomes the tradeeoff relationship between NOx and soot in conventional CI mode. Thus a simultaneous reduction of NOx and soot can be achieved easily by fueling with the pentanol without EGR and complex combustion control strategies. The higher THC and CO can be after treated by using diesel oxidizing catalyst.

atom and the bond is not subsequently broken, As a result, this carbon atom cannot contribute to soot formation. So the oxygenated characteristics of the pentanol fuel are beneficial for the suppression of soot production because of the reduced soot precursor concentration and enhanced soot oxidation process [30]. Fig 2 (c) and (d) show the indicated specific THC and CO emissions with injection timing sweeping. Not surprisingly, the pentanol under single injection strategy generates more THC and CO than the diesel with double injection strategy. The increased CO and THC emissions for pentanol were mainly due to its ignition and evaporation characteristics of lower cetane number fuel. Firstly, pentanol has longer ignition delay because of its lower cetane number, so more pentanol fuel will vaporize before ignition. Secondly, the higher heat of evaporation will lead to a leaner combustion zone and quenching effect, which result in the reduction in post oxidation caused by lower combustion temperature. All these factors favor the CO and THC production of pentanol. On the other hand, the presence of oxygen molecule in the pentanol fuel will have a positive effect on CO oxidation, but the ignition characteristic of pentanol plays a dominate role in the increasing trend of CO and THC emissions in this work. In general, as alternative or addition of diesel, the most favorable result from the pentanol application is that it exhibits desirable changes in Soot/NOx trade-off curve.

Fig. 4 depicts the in-cylinder pressure and HRR (heat release rate) profiles of the pentanol (single injection) and the diesel fuel (single

Fig. 4. Comparisons of HRR, in-cylinder pressure of the Pentanol single injection mode and diesel single & double injection mode at 1600 rpm, 0.6 Mpa IMEP. NO EGR is used. (CA50 ¼ 14).

Fig. 5. Comparisons of PRR, in-cylinder temperature of the Pentanol single injection mode and diesel single & double injection mode at 1600 rpm, 0.6 Mpa IMEP. NO EGR is used. (CA50 ¼ 14).

3.2. Combustion characteristics

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release) phase, compared to the diesel with a weak ITHR. It is convinced that the ITHR helps to maintain the combustion stability by avoiding engine knock at the test running conditions. Meanwhile, the heat release analysis demonstrated that the pentanol fuel presents more diluted premixed combustion than the diesel at the same operating condition, which explained the reason why soot and NOx emissions decrease significantly. As shown in Figs. 4e6, in double injection mode with a fixed injection dwell of 16  CA CAD, the diesel fuel exhibits two-stage heat release, which is known as an effective way to alleviate maximum pressure rise rates and HRR. Furthermore, the pilot-main injection case exhibits the shortest ignition delay implying improved air utilization and reduced local fuel-rich regions. So the soot emission is lower than that of single injection case for diesel as shown in Fig. 2(b) and 3(a). 3.3. Fuel economy Fig. 6. Comparisons of ID (ignition delay), CD (combustion duration) of the Pentanol single injection mode and diesel single & double injection mode at 1600 rpm, 0.6 Mpa IMEP. NO EGR is used. (CA50 ¼ 14).

and pilot-main injection), while CA50 (50 percent burn point) is fixed at 14 CAD (crank angle degree) after TDC (top dead center). It can be found that the maximum heat release rate declines with the pentanol compared to the diesel under single injection strategy, which exhibits a very similar feature to that of diesel under double injection strategy. The different chemical constituents between the pentanol and the diesel fuel have different effects on the ignition timing and HRR characteristics. It can be seen that the ignition delay for the pentanol is obviously longer than that of the diesel fuel at the same CA50 in Fig 6, so that more mixing takes place before combustion as mentioned before. In single injection mode, the HRR of diesel shows the characteristics of single peak at the test point. The combination of higher CN and quicker fuel-air mixing lead to a fast and intense single-stage combustion event without obvious diffusion combustion phase. Different from the diesel fuel, the pentanol has a combustion feature with relatively slow and progressive heat release in stages, which results in a moderate HRR and lower maximum pressure rise rate as shown in Fig. 5. The results of HRR shows that in addition to its distinct physiochemical properties compared with diesel, pentanol has relatively strong heat release at intermediate temperature prior to hot ignition, and hence it is called ITHR [34]. As shown in Figs. 4 and 5, for the given CA50, the pentanol with stronger ITHR have a higher temperature rise rate before HTHR (high-temperature heat-

The ITE (indicated thermal efficiency) and ISFC profiles of the pentanol (single injection) and the diesel fuel (pilot-main injection) with error bars under different injection timing are compared in Fig 7. As discussed above, all the ITE and ISFCs are normalized based on diesel LHV. For diesel fuel tests with double injection strategy, two reasons leading to its poor fuel efficiency is clearly observed: one is the larger negative compression work because of the premature ignition before TDC, and another is the longer combustion duration in comparison of the pentanol of single injection mode. By contrast, the oxygenated pentanol fuel presents higher fuel efficiency than that of the diesel. The main reason is that the leaner combustion atmosphere and lower heat transfer loss because of the lower in-cylinder peak temperature. In addition, the center of the heat release rate profile closes to the compression TDC; hence a higher indicated thermal efficiency and lower ISFC can be attained for the pentanol fuel. Meanwhile, the decrease in ITE of the pentanol is milder when injection timing is retarded, which indicates a more robust combustion than the diesel regardless of single or double strategies. Results of the indicated efficiency and ISFC profiles of the diesel fuel (single injection) with error bars under different injection timing are reported in Fig 8. It can be seen that the single injection mode for diesel attains a lower ITE and higher ISFC in contrast to the pentanol single injection mode and the diesel double injection mode, Fig 8 also displays a rapid deterioration for the ISFC with retarded main injection timing. In this case, the high PRR (pressure rise rate) and COV (coefficient of variation) are the two factors limiting the injection timing window, so the optimal combustion phase and fuel economy cannot be obtained.

Fig. 7. Comparison of fuel economy of the pentanol single injection mode and diesel double injection mode as injection timing sweeping at 1600 rpm, 0.6 MPa IMEP. NO EGR is used.

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[5]

[6]

[7]

[8]

[9]

[10] Fig. 8. Fuel economy of the diesel single injection mode as injection timing sweeping at 1600 rpm, 0.6 Mpa IMEP. NO EGR is used.

[11] [12]

4. Conclusions

[13] [14]

Pentanol is a prospective second-generation biofuel. An experimental investigation on the combustion and emission characteristics of pentanol compression ignition was conducted in a single cylinder direct injection engine. Experimental data of the neat pentanol fuels were compared with the baseline diesel using different injection strategies. The influence on combustion and emission performance was analyzed. Some conclusions can be drawn as follows: 1. For the pentanol with single injection strategy, ultralow emissions of nitrogen oxides (NOx) and smoke can be attained without EGR under the conditions of 1600 rpm, 0.6 MPa IMEP, while maintaining high thermal efficiency. 2. For direct injection compression ignition, the pentanol overcomes the trade-off relationship between NOx and soot in conventional diesel engine combustion because of longer ignition delay. 3. Compared to the diesel, higher indicated thermal efficiency and lower ISFC could be attained for Pentanol as injection timing is rerated. 4. The pentanol presents a more stable premixed combustion leading to moderate, sequential heat release, and increased knock resistance compared to the diesel fuel. 5. Pentanol operated diesel engine resulted in higher HC and CO emission because of the reduction in post-oxidation, which could be solved by after-treatment using an oxidizing catalyst.

[15]

[16] [17]

[18]

[19]

[20]

[21]

[22]

[23]

[24] [25]

[26]

Acknowledgments

[27]

This work was sponsored by the Ministry of Science and Technology of China through the China-Singapore Project 2012DFG61960.

[28]

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Please cite this article in press as: Li L, et al., Combustion and emissions of compression ignition in a direct injection diesel engine fueled with pentanol, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.12.013