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ScienceDirect Energy Procedia 75 (2015) 2337 – 2344

The 7th International Conference on Applied Energy – ICAE2015

Performance, combustion and emission characteristics of a diesel engine fueled with polyoxymethylene dimethyl ethers (PODE3-4)/ diesel blends Zhi Wang*, Haoye Liu, Jun Zhang, Jianxin Wang, Shijin Shuai State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China

Abstract Polyoxymethylene dimethyl ethers (PODEn) have high oxygen content, cetane number and solubility in diesel fuel. In this work, pure diesel and PODE3-4/diesel blends with 10-20% PODE3-4 by volume were tested in a light-duty (LD) 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 improve engine efficiency and reduce engine-out emissions significantly, especially soot emissions. The ESC test cycle in a six-cylinder heavy-duty (HD) production diesel engine fuelled with pure diesel and 20% PODE3-4 was also conducted and similar results were achieved. It is proved that PODE3-4, of which the mass production has been achieved recently, is a promising blending component for diesel fuel. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

© 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE

Peer-review under responsibility of Applied Energy Innovation Institute

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 ĳ(equivalence ratio)- T (temperature) diagram from [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 [2-5]. 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 * Corresponding author. Tel.: +86-10-62772515; fax: +86-10-62772515. E-mail address: [email protected] (Z. Wang).

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.479

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occurs in fuel-rich regions. As a result, combustion process can be more easily organized to realize simultaneous PM and NOx emissions reduction. Polyoxymethylene dimethyl ethers (PODEn) 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. Among PODEn, PODE2 does not fulfill the security criterion due to its low flash point [7], 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 PODE3–5 mixtures are ideal diesel fuel additives. Due to the lack of sufficient PODEn supply before [8-10], only a few studies of using PODEn as diesel fuels have been conducted before. Björn Lumpp et al. [11] studied the effects of PODEn on PM, PN and soot emissions in a diesel engine. The results show that PODEn can reduce these emissions significantly. Leonardo Pellegrini et al. [12] studied PODE3-5 on diesel engines. 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 NOxˈPM and noise. However, the injection system exhibits a different dynamic response when fed on pure PODE3-5 or 50% PODE3-5 blends. 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 performance of 10/20% PODEn blend fuel (P10, P20) in light-duty (LD) and 20% PODEn blend fuel (P20) in heavy-duty (HD) engines. 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 using the technique provided by [8-10], which has a mass distribution of PODE2:PODE3:PODE4 = 2.553%:88.9%:8.48%. 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 1 shows the properties of diesel and PODE3-4 used in this study. Table 1. Properties of diesel fuel and PODE3-4 Item

Diesel

PODE3-4

Chemical formula

C16-C23

Density [g/cc]

0.830

1.019

Cetane number

56.5

71.3*

Lower heating value [MJ/kg]

42.68

19.05

Viscosity [mm2/s]

4.13 (at 20 oC)

1.05 (at 25 oC)

Aromatics [%(m/m)]

28.7

0

S [ppm]

4.3

0

*estimated by [14]


3. Experimental setup and method 3.1 Single-cylinder LD diesel engine test The first experiments were performed using a single-cylinder LD engine retrofitted from a four-cylinder LD common-rail compression ignition engine. The compression ratio is 16.7 and the displacement is 0.5 L. The engine was equipped with a Delphi seven-hole injector with cone angle of 156o. Turbocharger was removed from the engine and an external air compressor was used to supply intake air. Gaseous emissions including CO, HC and NOx were measured using an AVL CEBII pollutants analyzer, while soot emissions were measured by AVL 439 Opacimeter. Four loads (2 bar, 4 bar, 6 bar, 8 bar IMEP) were tested at 1600 rpm. The engine was running at nature aspiration mode without EGR. The injection strategy was pilot injection plus main injection with pilot ratio less than 7% and injection pressure varied with load 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. 3.2 Six-cylinder HD diesel engine ESC test cycle The second experiments were ESC test using a six-cylinder HD production diesel engine. The compression ratio is 18.0 and the displacement is 7.14 L. The engine was designed with aftertreatment system to meet Euro IV emission regulation. The engine exhaust was sampled from upstream of the aftertreatment for emission test. The gaseous emissions were measured by AVL FTIR (Fourier Transform Infrared Spectroscopy). PM was measured according to the Chinese standard GB 17691-2005, which was based on European standards. A partial flow sampling system provided by AVL SPC 472 was used for the PM measurement, which was equivalent to the traditional full-flow constant volume sampler (CVS) and met ECE-R 49 and EPA CFR 1065. The steady-state engine conditions in the ESC test cycle is shown in Fig. 1. In the ESC test cycle, the engine speeds at A, B, C points were 1403 r/min, 1734 r/min, 2066 r/min, respectively. The engine mode labels indicated the engine conditions, for instance, A25 meant that the engine was run under the A speed (1403 r/min) at 25% of full load. 1200

Engine mode

Torque (Nm)

1000 800 600 400 200 0

700

1403 1743 2066 Engine speed (rpm) Figure 1. Engine conditions in ESC test cycle.

4. Experimental Results and Discussions 4.1 Performance in the single-cylinder research engine 4.1.1 Combustion characteristic

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50

40

160

40

30

120

Diesel P10 P20

80

10

40

0

150

30 Diesel P10 P20

20

100

10

50

0

0 0

10

20

(b)

Diesel P10 P20

100

10

50

0 0 10 Crank Angle (deg)

20

30

40 30 20

150 Diesel P10 P20

100 50

10

0

0 -10

30

200

50

p_avg (bar)

p_avg (bar)

150

0

20

Crank Angle (deg)

40

20

10

(a) 200

30

0

Crank Angle (deg)

50

-10 -10

0 -10 -10

30

dQ_avg (kJ/m3deg)

-10 -10

dQ_avg (kJ/m3deg)

20

200

dQ_avg (kJ/m3deg)

200

p_avg (bar)

50

dQ_avg (kJ/m3deg)

p_avg (bar)

Fig. 2 shows the cylinder pressure and heat release rate 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 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 are higher than those of P10 and diesel. The combustion characteristics results show that: the combustion duration of PODE3-4/diesel blend fuel is slightly shorter than that of diesel fuel; coefficient of variations (COVs) of IMEP are all lower than 3, indicating all three fuels exhibit stable combustion at the tested loads.

0

10

20

30

Crank Angle (deg)

(c) (d) Figure 2. Cylinder pressure and heat release rate 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)

4.1.2 Emission characteristics Fig. 3a 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 when A/F is about 15, which is very close to stoichiometric. The reasons are mainly attributed to its ability to narrow the soot formation peninsula in the ĳ- T map as mentioned in [5]. The high volatility promotes air-fuel mixing, which also helps reducing soot formation [15]. In addition, the lower aromatic content also helps to decrease soot emissions [17]. 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 30-50% [17-19]. In this study, however, more than 90% reduction in soot emission was observed at 8 bar IMEP, indicating that PODE3-4 with only C-O bond in its molecule is very efficient in suppressing soot formation during the combustion process. However, 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 increase slightly with increasing blend ratio of PODE3-4.

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Fig. 3b 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. At 8 bar IMEP, oxygen is not sufficient, CO mainly forms in the lean-oxygen regions in the diffusion combustion [20]. The intramolecular oxygen and high volatility of PODE3-4 blend fuel can both decrease the lean-oxygen regions during the diffusion combustion, therefore, CO emissions decrease significantly as PODE3-4 content increases and the CO emissions of P20 are 90% lower than those of diesel fuel. The high volatility of PODE3-4 blend fuel can reduce under-mixing zones and increase over-mixing zones. However, the high ignitability of PODE3-4 promotes the combustion in over-mixing zones [21]. Overall, the final HC emissions of PODE3-4 blend fuels are slightly lower than those of diesel fuel. 25

1 0

15 10

4

3

Diesel P10 P20

20

5 4

0

3 2 1

HC (g/kW.h)

2

CO (g/kW.h)

Diesel P10 P20

NOx (g/kW.h)

Soot (1/m)

3

2

0 4 6 8 IMEP (bar) (a) (b) Figure 3. Emissions versus engine loads (a) soot and NOx emissions (b) CO and HC emissions

2

4 6 IMEP (bar)

8

2

4.1.3 Fuel economy Fig. 4 shows the ITE and combustion efficiency at different loads. The indicated thermal efficiency increases slightly as PODE3-4 blend ratio increases because of the higher combustion efficiency and the shorter combustion duration as discussed above. However, the Indicated Specific Fuel Consumption (ISFC) of PODE3-4 blend fuel is slightly higher than that of diesel fuel, which is due to the lower energy content of PODE3-4. 50 Combusion efficiency (%)

45 ITE(%)

100

Diesel P10 P20

40

35

30 2

4

6

8

99 98 97

Diesel P10 P20

96 95 2

4

6

IMEP (bar) IMEP (bar) Figure 4. Indicated thermal efficiency and combustion efficiency versus engine loads

4.2 ESC test cycle results in the 6-cylinder HD diesel engine

8

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HC (ppm)

CO (ppm)

The ESC test cycle emission results in the 6-cylinder HD diesel engine are shown from Fig. 5 to Fig. 7. The results is similar to that of LD diesel engine. At each operating condition in ESC test cycle, P20 gets lower CO and HC emissions and higher NOx emissions than those of diesel fuel. The weighted average results is shown in Table 2. By adding 20% PODE3-4, the CO, HC and PM emissions during the ESC test cycle are reduced by 14.7%, 33.3% and 36.2%, respectively; NOx emissions increase by 12.9%. 80 70 60 50 40 30 20 10 0

100 90 80 70 60 50 40 30 20 10 0

Diesel P20

Idle A25A50A75A100B25 B50 B75B100C25 C50 C75C100 Engine mode Figure 5. CO emissions in ESC test cycle

Diesel P20

Idle A25A50A75A100B25 B50 B75B100C25 C50 C75C100 Engine mode Figure 6. HC emissions in ESC test cycle

1200 Diesel P20

NOx (ppm)

1000 800 600 400 200 0

Idle A25A50A75A100B25 B50 B75B100C25 C50 C75C100 Engine mode Figure 7. NOx emissions in ESC test cycle

Table 2. Weighted average results in ESC test cycle CO

HC

NOx

PM

g/(kW•h)

g/(kW•h)

g/(kW•h)

g/(kW•h)

P20

0.262

0.030

6.637

0.037

Diesel

0.307

0.045

5.881

0.058

Item

Zhi Wang et al. / Energy Procedia 75 (2015) 2337 – 2344 Change %

-14.7

-33.3

12.9

-36.2

BTE (%)

Brake thermal efficiency (BTE) is shown in Fig. 8, in most operating conditions, the BTE of P20 is slightly higher than that of diesel fuel, indicating that adding PODE can also improve fuel economy in HD diesel engine. 50 45 40 35 30 25 20 15 10 5 0

Diesel P20

Idle A25A50A75A100B25 B50 B75B100C25 C50 C75C100 Engine mode Figure 8. BTE in ESC test cycle

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 LD direct injection diesel engine. Cylinder-pressure, heat release, emissions, and fuel economy were tested at various loads. The ESC test cycle in a six-cylinder HD production diesel engine fueled with pure diesel and 20% PODE3-4 was also tested. 1. P10 and P20 have similar cylinder-pressure, heat release with diesel fuel. Combustion duration is shortened by PODE3-4 addition. 2. Adding PODE3-4 in diesel fuel, soot emissions reduce significantly. Soot-free combustion can be achieved even at near stoichiometric conditions by using P20. Meanwhile, NOx emissions increase slightly. 3. 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. 4. The thermal efficiency improves by adding PODE3-4 into diesel fuel. The fuel consumption in g/kW.h increases slightly due to the lower energy content of PODEn. 5. In the ESC test cycle in a six-cylinder HD production diesel engine, adding 20% PODE3-4 can reduce HC, CO and PM emissions by 14.7%, 33.3% and 36.2%, respectively and increase NOx emissions by 12.9%. In most operating conditions, the BTE of P20 is slightly higher than that of diesel fuel. Acknowledgements This work was sponsored by the National Key Basic Research Plan (Chinese ‘973’ Plan), under Grant No. 2013CB228404. References [1]. [2]. [3].

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Biography Dr. Zhi Wang is an associate professor in the State Key Laboratory of Automotive Safety and Energy, Tsinghua University, China. He has 20+ years’ research and development experience ranging from GDI & HCCI combustion, emission control and alternative fuels with 40+ technical papers published in international journals and conferences.