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diesel blends
Energy xxx (2015) 1e8
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Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends Haoye Liu a, Zhi Wang a, *, Jianxin Wang a, Xin He a, Yanyan Zheng b, Qiang Tang b, Jinfu Wang b a b
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China Beijing Key Laboratory of Green Reaction Engineering and Technology, 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 27 September 2014 Received in revised form 4 May 2015 Accepted 12 May 2015 Available online xxx
PODEn (Polyoxymethylene dimethyl ethers) have high oxygen content, cetane number and good solubility in diesel fuel. In this work, pure diesel and PODE3-4/diesel blends with 10e20% PODE3-4 by volume were tested in a light-duty direct injection diesel engine without any modifications on the engine fuel supply system. Engine performance, combustion and emission characteristics were compared at various loads. The results show that PODE3-4/diesel blends can improve engine efficiency and reduce engine-out emissions significantly, especially soot emissions. It is proved that PODE3-4, of which the mass production has been achieved recently, is a promising blending component for diesel fuel. © 2015 Elsevier Ltd. All rights reserved.
Keywords: PODEn Diesel engine Combustion and emissions Efficiency
1. Introduction Diesel engines, due to the high thermal efficiency and power performance, are widely used in vehicles and engineering machinery. However, with the increasingly strict emission regulation, it's a big challenge for conventional diesel engines to reduce NOx and PM emissions simultaneously to achieve the emission target. According to the 4(equivalence ratio)eT (temperature) diagram from Refs. [1], with long carbon chains, high aromatics contents and oxygen-free compositions, the diesel fuel has a large soot formation peninsular, leaving little region for clean combustion. To overcome the trade-off between the PM and NOx emissions, many new combustion concepts were proposed in the past decades. Kohima et al. [2] used late injection combined with high EGR (exhaust gas recirculation) and high swirl to extend ignition delay and reduce combustion temperature. This new concept, called MK (Modulated Kinetics) mode, can reduce NOx and smoke simultaneously. Akihama et al. [3] used large amounts of cooled EGR to reduce combustion temperature and the combustion occurred in
* Corresponding author. E-mail address: [email protected] (Z. Wang).
the lower temperature side of the soot formation peninsula in the 4eT map. Besides the two concepts mentioned above, other distinctive combustion concepts such as UNIBUS (Uniform Bulky Combustion System) [4], MULDIC (Multiple Stage Diesel Combustion) [5], etc, have been proposed to achieve clean combustion. If fuel contains oxygen, the soot formation peninsula shrinks due to the reduction in gas-phase species such as acetylene and PAHs. And the soot formation tendency decreases with increasing fuel oxygen content [6]. With oxygenated fuels, soot formation can be suppressed even though combustion occurs in fuel-rich regions. As a result, combustion process can be more easily organized to realize simultaneous PM and NOx emissions reduction. Moreover, facing the tight supply of crude oil, many countries have been searching for new alternative fuel sources. Oxygenated fuels are promising alternative fuels due to their availability and lower cost. The information released by WEC (World Energy Council) [7] indicates that the ratio of the Diesel þ Jet demand to gasoline demand will increase from 1.5 now to 3.8 in 2040. Conventional diesel engine, mostly used in commercial transportation sector, will go through more serious fuel supply crisis. Therefore, even putting the emissions aside, alternative oxygenated fuels caught more and more attentions being used as diesel additives or diesel fuel substitutes.
http://dx.doi.org/10.1016/j.energy.2015.05.088 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088
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H. Liu et al. / Energy xxx (2015) 1e8
UNIBUS MULDIC MK FAMEs DMM ECU COVs A/F ATDC HC ISFC T P20
Abbreviations PODEn WEC DME EGR SOI ITE IMEP CA HRR 4 P10
polyoxymethylene dimethyl ethers World Energy Council dimethyl ether exhaust gas recirculation Start of Main Injection indicated thermal efficiency indicated mean effective pressure crank angle heat release rate equivalence ratio 10% PODE3-4/Diesel blends
Among various oxygenates, alcohols, esters and ethers are the three major categories that might serve as engine fuels. Alcohol fuels are the compounds made of hydrocarbyl and hydroxide radicals. Methanol and ethanol are two kinds commonly used in internal combustion engines. However, due to the high research octane number, volatile and diesel-insolubility, methanol and ethanol are usually used as gasoline substitutes. Wang et al. [8,9] investigated alcohols-gasoline and gasoline-alcohols dual-fuel spark ignition combustion and found that both approaches can improve engine efficiency and knock suppression. Compared with low-carbon alcohols, high-carbon alcohols have higher cetane number, viscosity, provide better compatibility with conventional diesel engines. Li et al. [10] investigated pentanol in a diesel engine and found that pentanol reduces NOx and soot emissions with comparable efficiency with diesel fuel, but the cetane number of pentanol is around 20, which cause significantly high CO and HC emissions. Esters are often produced through the dehydration of acids and alcohol. The esters used in internal combustion engines are mostly complex FAMEs (Fatty Acid Methyl Esters). Due to the long carbon chain, FAMEs have high cetane number and good ignitability for compression ignition engines. Quite a lot researches have been conducted on FAMEs [11]. However, there is only one ester group (eCOOR) in the molecule of FAMEs, leading to oxygen content around 10%. This makes it unobvious if FAMEs are used to control soot emissions, because soot reduction is highly related to the oxygen content [12]. Ether fuels have a general formula of R-OR0 , here R and R0 stand for hydrocarbyl and O for oxygen atom. Ether fuels usually have high cetane number though the molecule chains are not long. The oxygen content of ether fuels is much higher than that of FAMEs, because there are usually more than one CH2O unit in a short-chain ether molecule. Soot emissions decrease as fuel oxygen content increases, and the molecule structure of ethers implies that ethers are more efficient in reducing soot emissions than other structures [13,14]. As another component of PM, SOF decreases as fuel cetane number increases [15]. In terms of combustion, ether fuels are far better than other oxygenated fuels. As the simplest ether, DME (dimethyl ether) can be synthesized from methanol and has a cetane number of 55. It has been widely studied and achieved low-soot combustion [16,17]. However, DME
Uniform Bulky Combustion System Multiple Stage Diesel Combustion Modulated Kinetics fatty acid methyl esters dimethoxymethane electronic control unit coefficient of variations air/fuel crank angle after top dead center hydrocarbon indicated specific fuel consumption temperature 20% PODE3-4/Diesel blends
is a gaseous fuel at standard condition and some disadvantages were found when added into diesel, such as, increased fuel vapor pressure, reduced viscosity and phase separation at lower temperatures [18]. To improve this, DMM (dimethoxymethane), another common liquid ether fuel, was studied [19]. However, the cetane number of DMM is only 29 and experimental results show that DMM and diesel blends are prone to cause vapor lock [20]. Further extension of molecule chain is needed to ensure the ether fuels have similar lower vapor pressure while maintaining high cetane number and oxygen content. PODEn (Polyoxymethylene dimethyl ethers) stand for the mixtures of ethers with the formula of CH3O(CH2O)nCH3. Compared with DME, PODEn have the similar oxygen content but lower vapor pressure and higher cetane number, and are usually in liquid form. The main properties of PODEn are listed in Table 1. Among PODEn, PODE2 does not fulfill the security criterion due to its low flash point [21], and if n is higher than 5, melting points will be too high, and there will be precipitate in diesel/PODEn blend fuel at low temperatures. Therefore, only PODE3e5 mixtures are ideal diesel fuel additives without compromising ignitability. However, for a long time, PODEn can only be produced in small quantity in laboratories until recently. Wang and Zheng et al. [22] developed an industrial process for commercial production of PODEn in 10 kt/a year capacity scale, as shown in Fig. 1. Methylal is synthesized from formaldehyde solution and methanol over heterogeneous acid catalyst. Methylal then reacts with paraformaldehyde forming PODEn components. The desired PODEn components to be blended with diesel are PODE3-5 and the ratio of the three components can be changed by adjusting reaction conditions. Unconverted methylal and undesired shortchain PODE2 compound are fed back to the reactor. Undesired long-chain PODEn (n > 5) compounds are recycled as solid fuels. Only a few studies of using PODEn as diesel fuels have been € rn et al. [23] studied the effects of conducted before. In 2011, Bjo PODEn on PM, PN and soot emissions in a diesel engine. The results show that PODEn can reduce these emissions significantly. In 2012e2013, Leonardo et al. [24,25] studied PODE3-5 supplied by Ente Nazionale Idrocarburi on diesel engines, and the mass distribution of oligomers in the PODE3-5 product was the following: 36%
Table 1 Properties of PODEn in comparison with DME and DMM. Molecule
Molecular weight [g/mol]
Density at 25 C [g/cm3]
Melting point [ C]
Boiling point [ C]
Cetane number
Oxygen content [wt%]
Lower heating value [kJ/kg]
DME DMM PODE2 PODE3 PODE4 PODE5 PODE6
46 76 106 136 166 196 226
0.67 0.86 0.96 1.02 1.06 1.1 1.13
138 105 65 41 7 18.5 58
25 42 105 156 202 242 280
55 29 63 78 90 100 104
34.8 42.1 45.3 47.1 48.2 49 49.6
27333.48 22444.74 20323.21 19137.65 18380.60 17855.31 17469.47
Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088
H. Liu et al. / Energy xxx (2015) 1e8
3
Fig. 1. Industrial process of commercial production of PODEn.
of n ¼ 3; 37% of n ¼ 4 and 27% of n ¼ 5. In 2012, 12.5%, 50% PODE3-5 blends and pure PODE3-5 were investigated using a Euro 4 production diesel engine. The results show that 12.5% PODE3-5 blends can reduce PM emissions by about 40%; high PODE3-5/diesel blend ratio enables simultaneous optimization of NOx, PM and noise. However, the injection system exhibits a different dynamic response when fed on pure PODE3-5 or 50% PODE3-5 blends. In 2013, 10% and neat PODE3-5 were tested on a Euro 2 Diesel engine. The results showed that neat PODE3-5 can reduce PM to 7 mg/km without DFP at the expense of increased HC, CO, NOx and CO2 emissions, but the high formaldehyde emissions and finer particle size became critical issues. 10% PODE3-5 blend can reduce 20% PM emissions with only slight increase in formaldehyde emissions. In 2014, Li et al. [26] blended 10% PODEn in diesel fuel and tested the blend fuel in diesel engine: 10% blend fuel didn't reduce the engine power compared to pure diesel; smoke reduction were claimed without specific data; an interesting result in the solubility test showed that the cloud points of PODE3-8/diesel blend and PODE3-5/ diesel blend were 7 C and 11 C, respectively. In order to use PODEn in low-temperature environment, PODE3 or PODE3-4 is preferred. From the above, using high blending level PODEn/diesel fuel and pure PODEn could cause problems to the engine hardware, and the PODEn sources are also bottlenecks even the production has increased recently. At present, a more practical application of PODEn is being blended with diesel at relatively low ratio. Previous researches focused on the effects of high ratio PODEn/diesel
blending fuels. For low blending ratio, the only result was the soot emissions of around 10% blend fuel. In this study, the author investigated the effects of 10/20% PODEn blend fuel on the combustion and emission characteristics at various loads. The combustion and emissions of the baseline operation of diesel engine fueled with diesel were also studied for direct comparison. 2. Test fuels Beijing market diesel was used as the base fuel for PODEn blends. The PODEn was synthesized and separated by Yuhuang Company, which has a mass distribution of PODE2:PODE3:PODE4 ¼ 2.553%:88.9%:8.48%. The content of PODE2 was too small to have adverse effect on security risk. Reaction conditions were set to avoid producing PODE5, PODEn mixture were mostly PODE3-4, which has a low cloud point and can work at low temperature conditions as mentioned above. In this study, PODE3-4 was blended with the market diesel at blending ratios of 10/90 (P10) and 20/80 (P20) by volume, and no solubility issue was observed during the experiments. Table 2 shows the properties of diesel and PODE3-4 used in this study. The viscosity of PODE3-4 is only 1.05 and the lower heating value is less than half of diesel. So it's hard to burn pure PODE3-4 in diesel engine without redesigning the injection system. Since PODE3-4 was blended with diesel at low ratio, no modification in the fuel injection system was needed. The cetane number of PODE3-4 is even higher than diesel, indicating better ignitability than diesel.
Table 2 Properties of diesel fuel and PODE3-4. Item
Diesel
Chemical formula
C16eC23
Density [g/cc] Cetane number Lower heating value [MJ/kg] Viscosity [mm2/s] Aromatics [%(m/m)] C [%(m/m)] H [%(m/m)] O [%(m/m)] S [ppm]
0.830 56.5 42.68 4.13 (at 20 C) 28.7 86.45 13.49 0.05 4.3
a
PODE3-4
1.019 78.6a 19.05 1.05 (at 25 C) 0 43.97 8.78 46.98 0
Estimated by Ref. [27].
Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088
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H. Liu et al. / Energy xxx (2015) 1e8
compression ignition engine. The engine specifications are listed in Table 3. The engine was equipped with a Delphi seven-hole injector with cone angle of 156 . Turbocharger was removed from the engine and an external air compressor was used to supply intake air. The engine was controlled by the ECU (electronic control unit) which is transformed to a research module, and the injection parameters such as injection pressure, number of injections and injection timing were set by PC flexibly. Fig. 3 illustrates the schematic of the engine testing system. In-cylinder pressure was measured with a pressure transducer (AVL GH14P) and combustion pressure data were recorded with the resolution of 0.2 CA. Combustion analysis was performed based on the average cylinder pressure of 250 consecutive cycles. HRR (Heat Release Rate) was calculated using the following equation:
100
Vaporized (%)
80
Diesel P10 P20
60 40 20 0 140 160 180 200 220 240 260 280 300 320 340 Temp (°C) Fig. 2. Distillation range characteristics of fuels.
dQ g dV 1 dp ¼ p þ V dq g 1 dq g 1 dq
Table 3 Engine specifications. Compression ratio Displacement Bore Stroke Connecting rod length Swirl ratio Injection pressure
16.7 0.5 L 83.1 mm 92 mm 145.8 mm 1.7 40e180 MPa
Fig. 2 shows the distillation curves of the diesel, P10 and P20. Adding PODE3-4 into diesel fuel extends the distillation range to lower temperature, because PODE3 and PODE4 have lower boiling points of 156 C and 202 C, respectively. Therefore, the volatility of PODE3-4/diesel blend fuel is higher than the diesel fuel, with the volatility increases with increasing PODE3-4 blending level.
(1)
where, g is the specific heat ratio, V is the instantaneous cylinder volume, and p is the in-cylinder pressure. CA10, CA50, and CA90 are defined as the crank angles at which 10%, 50%, and 90% fuel mass fraction was consumed, respectively. Ignition delay and combustion duration are defined as the crank angle interval between SOI (start of main injection) and CA10 and the crank angle interval between CA90 and CA10, respectively. Gaseous emissions including CO, HC and NOx were measured using an AVL CEBII pollutants analyzer, while soot emissions were measured by AVL 439 Opacimeter. The ITE (indicated thermal efficiency) was calculated according to the indicated work and the measured fuel flow rate:
hi ¼ Wi
. mf Huf
(2)
3. Experimental setup and method 3.1. Engine and test system The experiments were performed using a single-cylinder research engine retrofitted from a four-cylinder common-rail
where, Wi is the indicated work, mf is the fuel consumed per cycle, and Huf is the fuel lower heating value. The combustion efficiency was calculated according to the heating value of the CO and HC emissions, and assuming the heating value of HC is equal to the fuel.
Fig. 3. Engine testing system.
Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088
H. Liu et al. / Energy xxx (2015) 1e8
5
4. Results and discussion
Table 4 Operating conditions.
Fig. 4 shows the cylinder pressure and HRR at four testing loads for all three fuels. Due to the pilot injection, there is a small heat release before the main heat release. The pattern of heat release and cylinder pressure for the three fuels are similar at 2 bar, 4 bar, 6 bar indicated mean effective pressure (IMEP). Small differences are observed at 8 bar IMEP: the ratio of the premixed combustion as well as the peak value of the main heat release of P20 is higher than those of P10 and diesel. The reasons will be discussed later. Fig. 5 shows the combustion characteristics at different loads. At 2 bar, 4 bar and 6 bar IMEP, the ignition delay of diesel fuel is slightly longer than those of P10 and P20, P10 and P20 have the similar ignition delay. However, at 8 bar IMEP, the ignition delay increases obviously with increasing percentage of PODE3-4. This can be explained by the fact that since the fuel energy in the pilot injection is less than 7% of the total fuel energy, the ignition delay can be regarded as the interval between SOI and the start of main heat release. On one hand, the injection duration of the pilot injection are kept the same, the accumulated pilot heat release amount decreases as the blending ratio increases due to the lower energy content of PODE3-4, which causes lower temperature in the combustion chamber before the main injection, and retards the combustion phasing; on the other hand, higher PODE3-4 ratio increases cetane number of the blend fuels, which tends to advance the heat release. At low loads (less than 6 bar), the amount of pilot injection and the difference of the accumulated pilot heat release is small, there is no significant temperature difference in the chamber before the main injection. Cetane number dominates the ignition delay of main heat release, P20 exhibits the shortest ignition delay. At 8 bar IMEP, the larger pilot quantity enhances the difference of
1 15.0 65 16 4
. mf Huf
(3)
where, mCO and mHC are the mass of the CO and HC, HuCO and HuHC are the lower heating value of CO and HC.
3.2. Operating conditions
50
200
50
40
160
40
20
120
Diesel P10 P20
200 150
30 Diesel P10 P20
3
30
HRR(kJ/m deg) Cylinder pressure(bar)
Cylinder pressure(bar)
Table 4 lists the engine operating conditions. Four loads were tested with the engine speed controlled at 1600 rpm. The engine was running at nature aspiration mode without EGR. The A/F (air/ fuel) ratio decreased as the load was getting higher. The injection strategy was pilot injection plus main injection with pilot ratio less than 7% and injection pressure varied with load. This injection strategy was based on the strategy of the original engine. For all three fuels, the pilot injection has the same injection duration. The main injection duration was adjusted to achieve the same engine load. Here, A/F ratio stands for the mass ratio of intake air to the injected fuel in one cycle.
80
10
40
0
20
100
10
50
0
0 0
10
20
(b)
150 Diesel P10 P20
100
10
50
0 0 0
10
20
30
20
40 30 20
150 Diesel P10 P20
100 50
10
0
0 -10
30
200
50 Cylinder pressure(bar)
40
20
10
(a) 200
30
0
Crank Angle (deg)
3
Cylinder pressure(bar)
30
Crank Angle (deg)
50
-10 -10
0 -10 -10
HRR(kJ/m deg)
-10 -10
3
hc ¼ 1 ðmCO HuCO þ mHC HuHC Þ
1 23.5 60 15 3
8
HRR(kJ/m deg)
1 1 58.0 36.5 40 45 14 14 0 3 Less than 7%
4.1. Combustion characteristic
3
Diesel/P10/P20 Pilot plus main injection 2 4 6
HRR(kJ/m deg)
Fuel Injection strategy Indicated Mean Effective Pressure (IMEP) (bar) Intake pressure (bar) A/F ratio Injection pressure (MPa) Pilot injection timing (ATDC) Main injection timing (ATDC) Pilot ratio
0
10
Crank Angle (deg)
Crank Angle (deg)
(c)
(d)
20
30
Fig. 4. Cylinder pressure and HRR at (a) 1600 rpm, 2 bar (b) 1600 rpm, 4 bar (c) 1600 rpm, 6 bar (d) 1600 rpm, 8 bar for pure diesel, 10% PODE3-4 blend fuel (P10), and 20% PODE3-4 blend fuel (P20).
Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088
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H. Liu et al. / Energy xxx (2015) 1e8
Diesel P10 P20 12 11 10 40 35 30 25 20
duration (deg)
45
COV (%)
2.5 2.0
4.2. Emission characteristics
1.5 1.0 1
2
3
4
5
6
7
8
9
IMEP (bar) Fig. 5. Combustion characteristic parameters.
the accumulated pilot heat release amount. Blending PODE3-4 causes lower chamber temperature before main injection and increases ignition delay. Overall, the difference in the heat release pattern observed at 8 bar IMEP can be explained as follow: at 8 bar IMEP, due to the longer ignition delay and better fuel volatility, more well-premixed zones is formed during the ignition delay for P20, which enhances the ratio of premixed combustion as well as the peak HRR observed in Fig. 4d. Combustion duration is influenced by fuel physical and chemical properties. In Fig. 5, the combustion duration of PODE3-4/diesel blend fuel is shorter than that of diesel fuel. This is attributed to the high volatility and high ignitability of PODE3-4. The high volatility and the high ignitability increase the mixing rate and the chemical reaction rate [27], respectively, which both reduce combustion duration. Since the combustion duration increases as load increases, the difference in combustion duration is more remarkable at high load. The combustion duration of P20 is about 8 CA shorter than that of diesel at 8 bar IMEP, which implies higher thermal
Fig. 6 shows soot and NOx emissions at various loads. With increasing blend ratio of PODE3-4, soot emissions decrease as expected. And the effect is significant because soot emissions remain low even at high load with A/F ratio around 15. The reasons are mainly attributed to its ability to narrow the soot formation peninsula in the 4eT map. The high volatility and the lower aromatic content also helps to decrease soot emissions [28,29]. For P20, the oxygen content for the blend fuel is only 10%. At this oxygen content level, other oxygenated fuels can only reduce smoke by 30e50% [30e32]. However, more than 90% reduction in soot emission was observed at 8 bar IMEP, indicating that PODE3-4 with only CeO bond in its molecule is very efficient in suppressing soot formation during the combustion process. The detailed reason is not clear because the interactions between PODE3-4 and diesel during combustion at chemical reaction level are very limited. NOx emissions of P20 are slightly higher when load is lower than 6 bar IMEP and an obvious increase occurs at 8 bar IMEP. High temperature and oxygen-rich are the two main factors of NOx formation [33]. For blend fuels, the intramolecular oxygen and high volatility can both increase the oxygen-rich zones. The effects of PODE3-4 are not apparent at low loads, because oxygen is sufficient due to the high A/F ratio, combustion temperature is the dominant factors. At 8 bar IMEP, the A/F ratio is low, the effect of PODE3-4 on NOx emissions becomes apparent. Besides, P20 has higher release
25
2
CO (g/kW.h)
Diesel P10 P20
1 0
4
3
Diesel P10 P20
20 15 10 5
4
0 NOx (g/kW.h)
Opacity(1/m)
3
3 2 1
2 2
4 6 IMEP (bar)
8
Fig. 6. Soot and NOx emissions versus engine loads.
HC (g/kW.h)
delay (deg)
13
efficiency. COVs (Coefficient of variations) of IMEP are all lower than 3%, indicating all three fuels exhibit stable combustion at the tested loads. In general, blending PODE3-4 into diesel fuel doesn't significantly change the combustion process. In fact, although different main injection amount was used to achieve the same engine load for different fuels, thanks to the high density of PODE3-4 and the low blend ratio, the main injection duration for P20 is increased by only 10e15 ms (470e780 ms for the entire main injection duration at various loads) to compensate for the lower energy content of PODE3-4. So, no significant power loss is expected if P10 and P20 are used directly in existing engines designed for the conventional diesel. Besides, PODE3-4 blend fuels can shorten the combustion duration because of the higher volatility and cetane number. These effects are also closely related to the engine emission characteristics, which are further analyzed in the next section.
0 2
4 6 IMEP (bar)
8
Fig. 7. CO and HC emissions versus engine loads.
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ITE(%)
H. Liu et al. / Energy xxx (2015) 1e8
7
50
4.3. Fuel economic
45
Fig. 8 shows the ITE and combustion efficiency at different loads. The ITE increases slightly as PODE3-4 blend ratio increases because of the higher combustion efficiency and the shorter combustion duration as discussed above. Fig. 9 shows the ISFC (Indicated Specific Fuel Consumption) at different loads, the ISFC of PODE3-4 blend fuel is slightly higher than that of diesel fuel due to the lower energy content of PODE3-4.
40
Combusion efficiency (%)
35 100
30
99 98 Diesel P10 P20
97 96 95
2
4 6 IMEP (bar)
8
Fig. 8. ITE and combustion efficiency versus engine loads.
rate due to the longer ignition delay as analyzed above, the combustion temperature could be higher for P20, which benefits NOx formation. Overall, P20 increases NOx emissions by 16%. Fig. 7 shows CO and HC emissions. When engine load is lower than 6 bar IMEP, CO emissions are similar for all three fuels. CO emissions decrease as load increases. This is because combustion temperature is low at low loads, CO emissions are mainly from low temperature zones [34]. Increasing load increases higher combustion temperature and causes low CO emissions. At 8 bar IMEP, oxygen is not sufficient, CO mainly forms in the lean-oxygen regions during the diffusion combustion. The intramolecular oxygen and high volatility of PODE3-4 blend fuel can decrease the lean-oxygen regions as well as CO emissions during the diffusion combustion. The CO emissions of P20 are 90% lower than those of diesel fuel at 8 bar IMEP. Generally, HC emissions are mostly due to the following two reasons: (a) under-mixing zones where oxygen is not sufficient for fuel oxidation, (b) over-mixing zones where the A/F ratio is beyond the lean burn limit. The high volatility of PODE3-4 blend fuel can reduce under-mixing zones and increase over-mixing zones, respectively. However, the high ignitability of PODE3-4 promotes the combustion in over-mixing zones [35]. Overall, the final HC emissions of PODE3-4 blend fuels are slightly lower than those of diesel fuel.
5. Conclusion PODEn with high oxygen content and cetane number is a promising additive for diesel fuel. In this work, diesel and PODE3-4/ diesel blends with 10% PODE3-4 and 20% PODE3-4 by volume were tested in a light-duty direct injection diesel engine. Cylinderpressure, heat release, emissions, and fuel economy were tested at various loads. 1. In general, the combustion process doesn't change significantly by adding PODE3-4 into diesel fuel at all test points. PODE3-4 increases ignition delay at high load when pilot injection duration was kept constant. Combustion duration is shortened by PODE3-4 addition. 2. Due to the low PODE3-4 blend ratio and the high density of PODE3-4, only slightly increasing the injection duration is needed to compensate for the lower energy content of PODE3-4. The results prove that low percentages of PODE3-4 can partially be substituted to diesel fuel without any modifications in diesel engines. 3. At high load, adding PODE3-4 increases ignition delay. At all test points, adding PODE3-4 reduces combustion duration especially at high load due to the high volatility and cetane number. 4. Adding PODE3-4 in diesel fuel, soot emissions reduce significantly. Soot-free combustion can be achieved even at near stoichiometric conditions. Meanwhile, NOx emissions increase slightly. CO and HC emissions reduce with increasing PODE3-4 blending ratio. CO can be reduced by 90% compared to pure diesel when using P20 at 8 bar IMEP. 5. PODE3-4/diesel blends can improve fuel economy. By adding PODE3-4 into diesel fuel, ITE increases at all test points, but fuel consumption in g/kW.h also increases slightly due to the lower energy content of PODE3-4. Acknowledgment This work is sponsored by the Project of the National Key Basic Research Plan (Chinese “973” Plan) under Grant No.2013CB228404. The authors also wish to thank Yuhuang Company for the supply of PODEn.
280 260
References
ISFC (g/kW.h)
240 220 200 Diesel P10 P20
180 160 140 1
2
3
4
5
6
7
IMEP (bar) Fig. 9. ISFC versus engine loads.
8
9
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[22] Zheng YY, Tang Q, Wang TF, Liao YH, Wang JF. Synthesis of a green diesel fuel additive over cation resins. Chem Eng Technol 2013;36:1951e6. €rn L, Dieter R, Christian P, Reinhard L, Eberhard J. Oxymethylene ethers as [23] Bjo diesel fuel additives of the future. MTZ 2011;72:34e8. [24] Pellegrini L, Marchionna M, Patrini R, Beatrice C, GIacom N, Guido C, et al. Combustion behaviour and emission performance of neat and blended polyoxymethylene dimethyl ethers in a light-duty diesel engine. SAE Paper 201201-1053. 2012. [25] Pellegrini L, Marchionna M, Patrini R, Florio S. Emission performance of neat and blended polyoxymethylene dimethyl ethers in an old light-duty diesel car. SAE Paper 2013-01-1035. 2013. [26] Li XY, Yu HB, Sun YM, Wang HB, Guo T, Sui YL, et al. Synthesis and application of polyoxymethylene dimethyl ethers. Appl Mech Mater 2014;448e453: 2969e73. €m H. Partially pre-mixed auto-ignition of gaso[27] Kalghatgi G, Risberg P, Ångstro line to attain low smoke and low NOx at high load in a compression ignition engine and comparison with a diesel fuel. SAE Paper 2007-01-0006. 2007. [28] Liu HF, Zheng ZQ, Yue L, Kong LC, Yao MF. Effects of fuel volatility on combustion and emissions over a wide range of EGR rates in a diesel engine. SAE Paper 2014-01-2659. 2014. [29] Dumitrescu CE, Polinowski C, Fisher BT, Cheng AS, Lilik GK, Mueller CJ. An experimental study of diesel-fuel property effects on mixing-controlled combustion in a heavy-duty optical CI engine. SAE Paper 2014-01-1260. 2014. [30] Kawano D, Senda J, Kawakami K, Shimada A, Fujimoto H. Fuel design concept for low emission in engine systems, 2nd report: analysis of combustion characteristics for the mixed fuels. SAE Paper 2001-01-0202. 2001. [31] Hajime I, Goto Y, Odaka M, Kazakov A, Foster DE. Comparison of numerical results and experimental data on emission production processes in a diesel engine. SAE Paper 2001-01-0656. 2001. [32] Nikolic D, Wakimoto K, Takahashi S, Iida N. Effect of nozzle diameter and EGR ratio on the flame temperature and soot formation for various fuels. SAE Paper 2001-01-1939. 2001. [33] Heywood JB. Internal combustion engine fundamentals. New York: McGrawHill Book Company; 1988. [34] Kook S, Bae C, Miles PC, Choi D, Pickett LM. The influence of charge dilution and injection timing on low-temperature diesel combustion and emissions. SAE paper 2005e01-3837. 2005. [35] Petersen BR, Ekoto IW, Miles PC. An investigation into the effects of fuel properties and engine load on UHC and CO emissions from a light-duty optical diesel engine operating in a partially premixed combustion regime. SAE paper 2010-01-1470. 2010.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends Haoye Liu a, Zhi Wang a, *, Jianxin Wang a, Xin He a, Yanyan Zheng b, Qiang Tang b, Jinfu Wang b a b
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China Beijing Key Laboratory of Green Reaction Engineering and Technology, 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 27 September 2014 Received in revised form 4 May 2015 Accepted 12 May 2015 Available online xxx
PODEn (Polyoxymethylene dimethyl ethers) have high oxygen content, cetane number and good solubility in diesel fuel. In this work, pure diesel and PODE3-4/diesel blends with 10e20% PODE3-4 by volume were tested in a light-duty direct injection diesel engine without any modifications on the engine fuel supply system. Engine performance, combustion and emission characteristics were compared at various loads. The results show that PODE3-4/diesel blends can improve engine efficiency and reduce engine-out emissions significantly, especially soot emissions. It is proved that PODE3-4, of which the mass production has been achieved recently, is a promising blending component for diesel fuel. © 2015 Elsevier Ltd. All rights reserved.
Keywords: PODEn Diesel engine Combustion and emissions Efficiency
1. Introduction Diesel engines, due to the high thermal efficiency and power performance, are widely used in vehicles and engineering machinery. However, with the increasingly strict emission regulation, it's a big challenge for conventional diesel engines to reduce NOx and PM emissions simultaneously to achieve the emission target. According to the 4(equivalence ratio)eT (temperature) diagram from Refs. [1], with long carbon chains, high aromatics contents and oxygen-free compositions, the diesel fuel has a large soot formation peninsular, leaving little region for clean combustion. To overcome the trade-off between the PM and NOx emissions, many new combustion concepts were proposed in the past decades. Kohima et al. [2] used late injection combined with high EGR (exhaust gas recirculation) and high swirl to extend ignition delay and reduce combustion temperature. This new concept, called MK (Modulated Kinetics) mode, can reduce NOx and smoke simultaneously. Akihama et al. [3] used large amounts of cooled EGR to reduce combustion temperature and the combustion occurred in
* Corresponding author. E-mail address: [email protected] (Z. Wang).
the lower temperature side of the soot formation peninsula in the 4eT map. Besides the two concepts mentioned above, other distinctive combustion concepts such as UNIBUS (Uniform Bulky Combustion System) [4], MULDIC (Multiple Stage Diesel Combustion) [5], etc, have been proposed to achieve clean combustion. If fuel contains oxygen, the soot formation peninsula shrinks due to the reduction in gas-phase species such as acetylene and PAHs. And the soot formation tendency decreases with increasing fuel oxygen content [6]. With oxygenated fuels, soot formation can be suppressed even though combustion occurs in fuel-rich regions. As a result, combustion process can be more easily organized to realize simultaneous PM and NOx emissions reduction. Moreover, facing the tight supply of crude oil, many countries have been searching for new alternative fuel sources. Oxygenated fuels are promising alternative fuels due to their availability and lower cost. The information released by WEC (World Energy Council) [7] indicates that the ratio of the Diesel þ Jet demand to gasoline demand will increase from 1.5 now to 3.8 in 2040. Conventional diesel engine, mostly used in commercial transportation sector, will go through more serious fuel supply crisis. Therefore, even putting the emissions aside, alternative oxygenated fuels caught more and more attentions being used as diesel additives or diesel fuel substitutes.
http://dx.doi.org/10.1016/j.energy.2015.05.088 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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UNIBUS MULDIC MK FAMEs DMM ECU COVs A/F ATDC HC ISFC T P20
Abbreviations PODEn WEC DME EGR SOI ITE IMEP CA HRR 4 P10
polyoxymethylene dimethyl ethers World Energy Council dimethyl ether exhaust gas recirculation Start of Main Injection indicated thermal efficiency indicated mean effective pressure crank angle heat release rate equivalence ratio 10% PODE3-4/Diesel blends
Among various oxygenates, alcohols, esters and ethers are the three major categories that might serve as engine fuels. Alcohol fuels are the compounds made of hydrocarbyl and hydroxide radicals. Methanol and ethanol are two kinds commonly used in internal combustion engines. However, due to the high research octane number, volatile and diesel-insolubility, methanol and ethanol are usually used as gasoline substitutes. Wang et al. [8,9] investigated alcohols-gasoline and gasoline-alcohols dual-fuel spark ignition combustion and found that both approaches can improve engine efficiency and knock suppression. Compared with low-carbon alcohols, high-carbon alcohols have higher cetane number, viscosity, provide better compatibility with conventional diesel engines. Li et al. [10] investigated pentanol in a diesel engine and found that pentanol reduces NOx and soot emissions with comparable efficiency with diesel fuel, but the cetane number of pentanol is around 20, which cause significantly high CO and HC emissions. Esters are often produced through the dehydration of acids and alcohol. The esters used in internal combustion engines are mostly complex FAMEs (Fatty Acid Methyl Esters). Due to the long carbon chain, FAMEs have high cetane number and good ignitability for compression ignition engines. Quite a lot researches have been conducted on FAMEs [11]. However, there is only one ester group (eCOOR) in the molecule of FAMEs, leading to oxygen content around 10%. This makes it unobvious if FAMEs are used to control soot emissions, because soot reduction is highly related to the oxygen content [12]. Ether fuels have a general formula of R-OR0 , here R and R0 stand for hydrocarbyl and O for oxygen atom. Ether fuels usually have high cetane number though the molecule chains are not long. The oxygen content of ether fuels is much higher than that of FAMEs, because there are usually more than one CH2O unit in a short-chain ether molecule. Soot emissions decrease as fuel oxygen content increases, and the molecule structure of ethers implies that ethers are more efficient in reducing soot emissions than other structures [13,14]. As another component of PM, SOF decreases as fuel cetane number increases [15]. In terms of combustion, ether fuels are far better than other oxygenated fuels. As the simplest ether, DME (dimethyl ether) can be synthesized from methanol and has a cetane number of 55. It has been widely studied and achieved low-soot combustion [16,17]. However, DME
Uniform Bulky Combustion System Multiple Stage Diesel Combustion Modulated Kinetics fatty acid methyl esters dimethoxymethane electronic control unit coefficient of variations air/fuel crank angle after top dead center hydrocarbon indicated specific fuel consumption temperature 20% PODE3-4/Diesel blends
is a gaseous fuel at standard condition and some disadvantages were found when added into diesel, such as, increased fuel vapor pressure, reduced viscosity and phase separation at lower temperatures [18]. To improve this, DMM (dimethoxymethane), another common liquid ether fuel, was studied [19]. However, the cetane number of DMM is only 29 and experimental results show that DMM and diesel blends are prone to cause vapor lock [20]. Further extension of molecule chain is needed to ensure the ether fuels have similar lower vapor pressure while maintaining high cetane number and oxygen content. PODEn (Polyoxymethylene dimethyl ethers) stand for the mixtures of ethers with the formula of CH3O(CH2O)nCH3. Compared with DME, PODEn have the similar oxygen content but lower vapor pressure and higher cetane number, and are usually in liquid form. The main properties of PODEn are listed in Table 1. Among PODEn, PODE2 does not fulfill the security criterion due to its low flash point [21], and if n is higher than 5, melting points will be too high, and there will be precipitate in diesel/PODEn blend fuel at low temperatures. Therefore, only PODE3e5 mixtures are ideal diesel fuel additives without compromising ignitability. However, for a long time, PODEn can only be produced in small quantity in laboratories until recently. Wang and Zheng et al. [22] developed an industrial process for commercial production of PODEn in 10 kt/a year capacity scale, as shown in Fig. 1. Methylal is synthesized from formaldehyde solution and methanol over heterogeneous acid catalyst. Methylal then reacts with paraformaldehyde forming PODEn components. The desired PODEn components to be blended with diesel are PODE3-5 and the ratio of the three components can be changed by adjusting reaction conditions. Unconverted methylal and undesired shortchain PODE2 compound are fed back to the reactor. Undesired long-chain PODEn (n > 5) compounds are recycled as solid fuels. Only a few studies of using PODEn as diesel fuels have been € rn et al. [23] studied the effects of conducted before. In 2011, Bjo PODEn on PM, PN and soot emissions in a diesel engine. The results show that PODEn can reduce these emissions significantly. In 2012e2013, Leonardo et al. [24,25] studied PODE3-5 supplied by Ente Nazionale Idrocarburi on diesel engines, and the mass distribution of oligomers in the PODE3-5 product was the following: 36%
Table 1 Properties of PODEn in comparison with DME and DMM. Molecule
Molecular weight [g/mol]
Density at 25 C [g/cm3]
Melting point [ C]
Boiling point [ C]
Cetane number
Oxygen content [wt%]
Lower heating value [kJ/kg]
DME DMM PODE2 PODE3 PODE4 PODE5 PODE6
46 76 106 136 166 196 226
0.67 0.86 0.96 1.02 1.06 1.1 1.13
138 105 65 41 7 18.5 58
25 42 105 156 202 242 280
55 29 63 78 90 100 104
34.8 42.1 45.3 47.1 48.2 49 49.6
27333.48 22444.74 20323.21 19137.65 18380.60 17855.31 17469.47
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H. Liu et al. / Energy xxx (2015) 1e8
3
Fig. 1. Industrial process of commercial production of PODEn.
of n ¼ 3; 37% of n ¼ 4 and 27% of n ¼ 5. In 2012, 12.5%, 50% PODE3-5 blends and pure PODE3-5 were investigated using a Euro 4 production diesel engine. The results show that 12.5% PODE3-5 blends can reduce PM emissions by about 40%; high PODE3-5/diesel blend ratio enables simultaneous optimization of NOx, PM and noise. However, the injection system exhibits a different dynamic response when fed on pure PODE3-5 or 50% PODE3-5 blends. In 2013, 10% and neat PODE3-5 were tested on a Euro 2 Diesel engine. The results showed that neat PODE3-5 can reduce PM to 7 mg/km without DFP at the expense of increased HC, CO, NOx and CO2 emissions, but the high formaldehyde emissions and finer particle size became critical issues. 10% PODE3-5 blend can reduce 20% PM emissions with only slight increase in formaldehyde emissions. In 2014, Li et al. [26] blended 10% PODEn in diesel fuel and tested the blend fuel in diesel engine: 10% blend fuel didn't reduce the engine power compared to pure diesel; smoke reduction were claimed without specific data; an interesting result in the solubility test showed that the cloud points of PODE3-8/diesel blend and PODE3-5/ diesel blend were 7 C and 11 C, respectively. In order to use PODEn in low-temperature environment, PODE3 or PODE3-4 is preferred. From the above, using high blending level PODEn/diesel fuel and pure PODEn could cause problems to the engine hardware, and the PODEn sources are also bottlenecks even the production has increased recently. At present, a more practical application of PODEn is being blended with diesel at relatively low ratio. Previous researches focused on the effects of high ratio PODEn/diesel
blending fuels. For low blending ratio, the only result was the soot emissions of around 10% blend fuel. In this study, the author investigated the effects of 10/20% PODEn blend fuel on the combustion and emission characteristics at various loads. The combustion and emissions of the baseline operation of diesel engine fueled with diesel were also studied for direct comparison. 2. Test fuels Beijing market diesel was used as the base fuel for PODEn blends. The PODEn was synthesized and separated by Yuhuang Company, which has a mass distribution of PODE2:PODE3:PODE4 ¼ 2.553%:88.9%:8.48%. The content of PODE2 was too small to have adverse effect on security risk. Reaction conditions were set to avoid producing PODE5, PODEn mixture were mostly PODE3-4, which has a low cloud point and can work at low temperature conditions as mentioned above. In this study, PODE3-4 was blended with the market diesel at blending ratios of 10/90 (P10) and 20/80 (P20) by volume, and no solubility issue was observed during the experiments. Table 2 shows the properties of diesel and PODE3-4 used in this study. The viscosity of PODE3-4 is only 1.05 and the lower heating value is less than half of diesel. So it's hard to burn pure PODE3-4 in diesel engine without redesigning the injection system. Since PODE3-4 was blended with diesel at low ratio, no modification in the fuel injection system was needed. The cetane number of PODE3-4 is even higher than diesel, indicating better ignitability than diesel.
Table 2 Properties of diesel fuel and PODE3-4. Item
Diesel
Chemical formula
C16eC23
Density [g/cc] Cetane number Lower heating value [MJ/kg] Viscosity [mm2/s] Aromatics [%(m/m)] C [%(m/m)] H [%(m/m)] O [%(m/m)] S [ppm]
0.830 56.5 42.68 4.13 (at 20 C) 28.7 86.45 13.49 0.05 4.3
a
PODE3-4
1.019 78.6a 19.05 1.05 (at 25 C) 0 43.97 8.78 46.98 0
Estimated by Ref. [27].
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compression ignition engine. The engine specifications are listed in Table 3. The engine was equipped with a Delphi seven-hole injector with cone angle of 156 . Turbocharger was removed from the engine and an external air compressor was used to supply intake air. The engine was controlled by the ECU (electronic control unit) which is transformed to a research module, and the injection parameters such as injection pressure, number of injections and injection timing were set by PC flexibly. Fig. 3 illustrates the schematic of the engine testing system. In-cylinder pressure was measured with a pressure transducer (AVL GH14P) and combustion pressure data were recorded with the resolution of 0.2 CA. Combustion analysis was performed based on the average cylinder pressure of 250 consecutive cycles. HRR (Heat Release Rate) was calculated using the following equation:
100
Vaporized (%)
80
Diesel P10 P20
60 40 20 0 140 160 180 200 220 240 260 280 300 320 340 Temp (°C) Fig. 2. Distillation range characteristics of fuels.
dQ g dV 1 dp ¼ p þ V dq g 1 dq g 1 dq
Table 3 Engine specifications. Compression ratio Displacement Bore Stroke Connecting rod length Swirl ratio Injection pressure
16.7 0.5 L 83.1 mm 92 mm 145.8 mm 1.7 40e180 MPa
Fig. 2 shows the distillation curves of the diesel, P10 and P20. Adding PODE3-4 into diesel fuel extends the distillation range to lower temperature, because PODE3 and PODE4 have lower boiling points of 156 C and 202 C, respectively. Therefore, the volatility of PODE3-4/diesel blend fuel is higher than the diesel fuel, with the volatility increases with increasing PODE3-4 blending level.
(1)
where, g is the specific heat ratio, V is the instantaneous cylinder volume, and p is the in-cylinder pressure. CA10, CA50, and CA90 are defined as the crank angles at which 10%, 50%, and 90% fuel mass fraction was consumed, respectively. Ignition delay and combustion duration are defined as the crank angle interval between SOI (start of main injection) and CA10 and the crank angle interval between CA90 and CA10, respectively. Gaseous emissions including CO, HC and NOx were measured using an AVL CEBII pollutants analyzer, while soot emissions were measured by AVL 439 Opacimeter. The ITE (indicated thermal efficiency) was calculated according to the indicated work and the measured fuel flow rate:
hi ¼ Wi
. mf Huf
(2)
3. Experimental setup and method 3.1. Engine and test system The experiments were performed using a single-cylinder research engine retrofitted from a four-cylinder common-rail
where, Wi is the indicated work, mf is the fuel consumed per cycle, and Huf is the fuel lower heating value. The combustion efficiency was calculated according to the heating value of the CO and HC emissions, and assuming the heating value of HC is equal to the fuel.
Fig. 3. Engine testing system.
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H. Liu et al. / Energy xxx (2015) 1e8
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4. Results and discussion
Table 4 Operating conditions.
Fig. 4 shows the cylinder pressure and HRR at four testing loads for all three fuels. Due to the pilot injection, there is a small heat release before the main heat release. The pattern of heat release and cylinder pressure for the three fuels are similar at 2 bar, 4 bar, 6 bar indicated mean effective pressure (IMEP). Small differences are observed at 8 bar IMEP: the ratio of the premixed combustion as well as the peak value of the main heat release of P20 is higher than those of P10 and diesel. The reasons will be discussed later. Fig. 5 shows the combustion characteristics at different loads. At 2 bar, 4 bar and 6 bar IMEP, the ignition delay of diesel fuel is slightly longer than those of P10 and P20, P10 and P20 have the similar ignition delay. However, at 8 bar IMEP, the ignition delay increases obviously with increasing percentage of PODE3-4. This can be explained by the fact that since the fuel energy in the pilot injection is less than 7% of the total fuel energy, the ignition delay can be regarded as the interval between SOI and the start of main heat release. On one hand, the injection duration of the pilot injection are kept the same, the accumulated pilot heat release amount decreases as the blending ratio increases due to the lower energy content of PODE3-4, which causes lower temperature in the combustion chamber before the main injection, and retards the combustion phasing; on the other hand, higher PODE3-4 ratio increases cetane number of the blend fuels, which tends to advance the heat release. At low loads (less than 6 bar), the amount of pilot injection and the difference of the accumulated pilot heat release is small, there is no significant temperature difference in the chamber before the main injection. Cetane number dominates the ignition delay of main heat release, P20 exhibits the shortest ignition delay. At 8 bar IMEP, the larger pilot quantity enhances the difference of
1 15.0 65 16 4
. mf Huf
(3)
where, mCO and mHC are the mass of the CO and HC, HuCO and HuHC are the lower heating value of CO and HC.
3.2. Operating conditions
50
200
50
40
160
40
20
120
Diesel P10 P20
200 150
30 Diesel P10 P20
3
30
HRR(kJ/m deg) Cylinder pressure(bar)
Cylinder pressure(bar)
Table 4 lists the engine operating conditions. Four loads were tested with the engine speed controlled at 1600 rpm. The engine was running at nature aspiration mode without EGR. The A/F (air/ fuel) ratio decreased as the load was getting higher. The injection strategy was pilot injection plus main injection with pilot ratio less than 7% and injection pressure varied with load. This injection strategy was based on the strategy of the original engine. For all three fuels, the pilot injection has the same injection duration. The main injection duration was adjusted to achieve the same engine load. Here, A/F ratio stands for the mass ratio of intake air to the injected fuel in one cycle.
80
10
40
0
20
100
10
50
0
0 0
10
20
(b)
150 Diesel P10 P20
100
10
50
0 0 0
10
20
30
20
40 30 20
150 Diesel P10 P20
100 50
10
0
0 -10
30
200
50 Cylinder pressure(bar)
40
20
10
(a) 200
30
0
Crank Angle (deg)
3
Cylinder pressure(bar)
30
Crank Angle (deg)
50
-10 -10
0 -10 -10
HRR(kJ/m deg)
-10 -10
3
hc ¼ 1 ðmCO HuCO þ mHC HuHC Þ
1 23.5 60 15 3
8
HRR(kJ/m deg)
1 1 58.0 36.5 40 45 14 14 0 3 Less than 7%
4.1. Combustion characteristic
3
Diesel/P10/P20 Pilot plus main injection 2 4 6
HRR(kJ/m deg)
Fuel Injection strategy Indicated Mean Effective Pressure (IMEP) (bar) Intake pressure (bar) A/F ratio Injection pressure (MPa) Pilot injection timing (ATDC) Main injection timing (ATDC) Pilot ratio
0
10
Crank Angle (deg)
Crank Angle (deg)
(c)
(d)
20
30
Fig. 4. Cylinder pressure and HRR at (a) 1600 rpm, 2 bar (b) 1600 rpm, 4 bar (c) 1600 rpm, 6 bar (d) 1600 rpm, 8 bar for pure diesel, 10% PODE3-4 blend fuel (P10), and 20% PODE3-4 blend fuel (P20).
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Diesel P10 P20 12 11 10 40 35 30 25 20
duration (deg)
45
COV (%)
2.5 2.0
4.2. Emission characteristics
1.5 1.0 1
2
3
4
5
6
7
8
9
IMEP (bar) Fig. 5. Combustion characteristic parameters.
the accumulated pilot heat release amount. Blending PODE3-4 causes lower chamber temperature before main injection and increases ignition delay. Overall, the difference in the heat release pattern observed at 8 bar IMEP can be explained as follow: at 8 bar IMEP, due to the longer ignition delay and better fuel volatility, more well-premixed zones is formed during the ignition delay for P20, which enhances the ratio of premixed combustion as well as the peak HRR observed in Fig. 4d. Combustion duration is influenced by fuel physical and chemical properties. In Fig. 5, the combustion duration of PODE3-4/diesel blend fuel is shorter than that of diesel fuel. This is attributed to the high volatility and high ignitability of PODE3-4. The high volatility and the high ignitability increase the mixing rate and the chemical reaction rate [27], respectively, which both reduce combustion duration. Since the combustion duration increases as load increases, the difference in combustion duration is more remarkable at high load. The combustion duration of P20 is about 8 CA shorter than that of diesel at 8 bar IMEP, which implies higher thermal
Fig. 6 shows soot and NOx emissions at various loads. With increasing blend ratio of PODE3-4, soot emissions decrease as expected. And the effect is significant because soot emissions remain low even at high load with A/F ratio around 15. The reasons are mainly attributed to its ability to narrow the soot formation peninsula in the 4eT map. The high volatility and the lower aromatic content also helps to decrease soot emissions [28,29]. For P20, the oxygen content for the blend fuel is only 10%. At this oxygen content level, other oxygenated fuels can only reduce smoke by 30e50% [30e32]. However, more than 90% reduction in soot emission was observed at 8 bar IMEP, indicating that PODE3-4 with only CeO bond in its molecule is very efficient in suppressing soot formation during the combustion process. The detailed reason is not clear because the interactions between PODE3-4 and diesel during combustion at chemical reaction level are very limited. NOx emissions of P20 are slightly higher when load is lower than 6 bar IMEP and an obvious increase occurs at 8 bar IMEP. High temperature and oxygen-rich are the two main factors of NOx formation [33]. For blend fuels, the intramolecular oxygen and high volatility can both increase the oxygen-rich zones. The effects of PODE3-4 are not apparent at low loads, because oxygen is sufficient due to the high A/F ratio, combustion temperature is the dominant factors. At 8 bar IMEP, the A/F ratio is low, the effect of PODE3-4 on NOx emissions becomes apparent. Besides, P20 has higher release
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Fig. 6. Soot and NOx emissions versus engine loads.
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efficiency. COVs (Coefficient of variations) of IMEP are all lower than 3%, indicating all three fuels exhibit stable combustion at the tested loads. In general, blending PODE3-4 into diesel fuel doesn't significantly change the combustion process. In fact, although different main injection amount was used to achieve the same engine load for different fuels, thanks to the high density of PODE3-4 and the low blend ratio, the main injection duration for P20 is increased by only 10e15 ms (470e780 ms for the entire main injection duration at various loads) to compensate for the lower energy content of PODE3-4. So, no significant power loss is expected if P10 and P20 are used directly in existing engines designed for the conventional diesel. Besides, PODE3-4 blend fuels can shorten the combustion duration because of the higher volatility and cetane number. These effects are also closely related to the engine emission characteristics, which are further analyzed in the next section.
0 2
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Fig. 7. CO and HC emissions versus engine loads.
Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088
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Fig. 8 shows the ITE and combustion efficiency at different loads. The ITE increases slightly as PODE3-4 blend ratio increases because of the higher combustion efficiency and the shorter combustion duration as discussed above. Fig. 9 shows the ISFC (Indicated Specific Fuel Consumption) at different loads, the ISFC of PODE3-4 blend fuel is slightly higher than that of diesel fuel due to the lower energy content of PODE3-4.
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Fig. 8. ITE and combustion efficiency versus engine loads.
rate due to the longer ignition delay as analyzed above, the combustion temperature could be higher for P20, which benefits NOx formation. Overall, P20 increases NOx emissions by 16%. Fig. 7 shows CO and HC emissions. When engine load is lower than 6 bar IMEP, CO emissions are similar for all three fuels. CO emissions decrease as load increases. This is because combustion temperature is low at low loads, CO emissions are mainly from low temperature zones [34]. Increasing load increases higher combustion temperature and causes low CO emissions. At 8 bar IMEP, oxygen is not sufficient, CO mainly forms in the lean-oxygen regions during the diffusion combustion. The intramolecular oxygen and high volatility of PODE3-4 blend fuel can decrease the lean-oxygen regions as well as CO emissions during the diffusion combustion. The CO emissions of P20 are 90% lower than those of diesel fuel at 8 bar IMEP. Generally, HC emissions are mostly due to the following two reasons: (a) under-mixing zones where oxygen is not sufficient for fuel oxidation, (b) over-mixing zones where the A/F ratio is beyond the lean burn limit. The high volatility of PODE3-4 blend fuel can reduce under-mixing zones and increase over-mixing zones, respectively. However, the high ignitability of PODE3-4 promotes the combustion in over-mixing zones [35]. Overall, the final HC emissions of PODE3-4 blend fuels are slightly lower than those of diesel fuel.
5. Conclusion PODEn with high oxygen content and cetane number is a promising additive for diesel fuel. In this work, diesel and PODE3-4/ diesel blends with 10% PODE3-4 and 20% PODE3-4 by volume were tested in a light-duty direct injection diesel engine. Cylinderpressure, heat release, emissions, and fuel economy were tested at various loads. 1. In general, the combustion process doesn't change significantly by adding PODE3-4 into diesel fuel at all test points. PODE3-4 increases ignition delay at high load when pilot injection duration was kept constant. Combustion duration is shortened by PODE3-4 addition. 2. Due to the low PODE3-4 blend ratio and the high density of PODE3-4, only slightly increasing the injection duration is needed to compensate for the lower energy content of PODE3-4. The results prove that low percentages of PODE3-4 can partially be substituted to diesel fuel without any modifications in diesel engines. 3. At high load, adding PODE3-4 increases ignition delay. At all test points, adding PODE3-4 reduces combustion duration especially at high load due to the high volatility and cetane number. 4. Adding PODE3-4 in diesel fuel, soot emissions reduce significantly. Soot-free combustion can be achieved even at near stoichiometric conditions. Meanwhile, NOx emissions increase slightly. CO and HC emissions reduce with increasing PODE3-4 blending ratio. CO can be reduced by 90% compared to pure diesel when using P20 at 8 bar IMEP. 5. PODE3-4/diesel blends can improve fuel economy. By adding PODE3-4 into diesel fuel, ITE increases at all test points, but fuel consumption in g/kW.h also increases slightly due to the lower energy content of PODE3-4. Acknowledgment This work is sponsored by the Project of the National Key Basic Research Plan (Chinese “973” Plan) under Grant No.2013CB228404. The authors also wish to thank Yuhuang Company for the supply of PODEn.
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References
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IMEP (bar) Fig. 9. ISFC versus engine loads.
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Please cite this article in press as: Liu H, et al., Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.088