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Investigation on Combustion and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel/Biodiesel/PODE Blends Hao Chen, Jingjing He, and Haining Hua Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01898 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017
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Investigation on Combustion and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel/Biodiesel/PODE Blends Hao Chen1, *, Jingjing He1, Haining Hua1 1
School of Automobile, Chang’an University, Xi’an 710064, PR China
*Corresponding author: Tel: +86 29 82334471, E-mail:
[email protected] Abstract: PODE (polyoxymethylene dimethyl ethers) is an excellent blend for diesel due to its high cetane number, high oxygen content and low viscosity. Combustion and emission characteristics are investigated based on experimental tests on a turbocharged, in-line 6-cylinder, common rail diesel engine. Results show that combustion starts earlier with PODE blending at low and partial loads. With pilot and main injection, the peak combustion pressures and peak heat release rates of Diesel/Biodiesel/PODE blend fuels increase due to large amounts of reactive radicals formed in the pilot heat release stage. With an increase in load, a slight decrease in both peak combustion pressure and peak heat release rate is observed. At low loads, the CA10s, CA50s and CA90s of Diesel/Biodiesel/PODE blend fuels advance and both rapid combustion and late combustion phases shorten. At medium loads and high loads, CA10s advance, CA50s remain unchanged and CA90s clearly advance. Rapid combustion phases increase very little, while the late combustion phases clearly shorten. Combustion durations shorten in each engine operation. It can be concluded that the addition of PODE is helpful for more concentrated heat release. As a result, combustion temperatures increase. The NOX emissions of blend fuels arise with PODE at 1400 and 2000 r/min, while soot emissions obviously decrease. In particular,
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Diesel/Biodiesel/PODE blends reduce soot emissions significantly at medium and high loads and reduce the number concentrations of ultrafine particles at low and partial loads. Keywords: Common rail diesel engine; Polyoxymethylene dimethyl ethers; Combustion; Nitrogen oxide; Soot; Ultrafine particles.
ABBREVIATIONS PM = particulate matter CN = cetane numbers PODE = Polyoxymethylene dimethyl ethers UFPs = ultrafine particles BMEP = brake mean effective pressure CA = crank angle HRR = heat release rate NGMD = number geometric mean diameter VGMD = volume geometric mean diameter D100 = 50% diesel and 50% biodiesel by vol. D95P5 = 95% D100 and 5% PODE by vol. D90P10 = 90% D100 and 10% PODE by vol. D85P15 = 85% D100 and 15% PODE by vol. LHV = lower heating value PCP = peak combustion pressure
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PHRR = peak heat release rate CA10 = crank angle at which 10% of the heat is released CA50 = crank angle at which 50% of the heat is released CA90 = crank angle at which 90% of the heat is released SOC = start of combustion TDC = top dead center NC = number concentration VC = volume concentration NCMP = nucleation mode AKMP = the Aitken mode ACMP = accumulation mode
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1.
Introduction Diesel engines are widely used in commercial vehicles, agricultural machineries
and engineering equipments. The EU has adopted strict new restrictions on pollutant emissions from diesel and petrol cars, limiting particulate matter (PM) which has posed the most serious health and environmental problems. The Euro 6 emission standard restrained PM to 0.01 g/kWh for heavy duty diesel engines. Concern about the increasingly strict emission regulations, out-cylinder emission control technologies have been applied on vehicles. DPF (Diesel particulate filter)1 and POC (particulate oxidation catalyst)2 can be used for PM limitation. The mixing of oxygenated additives with diesel oil supplies the oxygen required to form CO2 instead of carbon-rich particles and this in turn considerably reduces PM emissions3, such as biodiesel, alcohols and ethers. Liotta, F. and Montalvo, D confirmed that particulate emissions are directly related to the concentration of oxygen in the fuel and the particulate reductions are accompanied by small increases in NOx emissions.4 Biodiesel, as an alternative fuel of diesel, is renewable, biodegradable, oxygenated and has very similar properties to those of diesel fuel. Soot emissions show clear reductions when fueled with biodiesel or its blends with diesel.5-7 Biodiesel reduces the weighted particle mass concentration, total number concentration and the weighted geometric mean diameter of the particles.8 The interactions of fatty acid chains present in the biodiesel with the oxygen makes the fuel unstable, leading to poor oxidation stability.9,10 Due to high viscosity and low
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volatility, biodiesel provides inferior atomization and evaporation behavior compared to diesel, which results in poor homogeneity of the mixture. Moreover, high pour points and freezing points lead to a poor low temperature fluidity, which impedes the large-scale application of biodiesel. Short formation time and poor homogeneity of the air-fuel mixture inside the diesel engine produce the localised fuel-rich regions, which inevitably produce soot or PM emissions. Improving the fuel volatility is an effective method for promoting an oil and gas mixture. Decreasing fuel viscosity can improve the quality of the oil atomization. Based on the two factors, low-carbon alcohols are generally mixed with diesel or biodiesel. Compared to biodiesel, low carbon chain alcohols have a higher oxygen content, such as 50% of methanol and 34.78% of ethanol, which is helpful for PM reduction of diesel engines.11-13 J. Zaglinskis et al.11 studied the combustion and emission characteristics of diesel-biodiesel-methanol fuels and reported that a methanol additive contributed to a 13-45% reduction of soot concentration in the whole engine load range. However they are strong polar solvents and don’t mix well with diesel. Cosolvents or surfactants are needed to form stable, uniform and transparent diesel/alcohol micro-emulsion blends.14-17 Another problem is that low carbon alcohols are characterized by low cetane numbers (CN) and poor burning performances. A duel-fuel engine system is a possibility for the application of low carbon alcohols in diesel engines, and low soot emissions are achieved.18-20 However, cost is inevitably increased by the added fuel supply and injection systems. Ethers with a high oxygen content can effectively reduce soot/smoke/PM
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emissions in diesel engines. Blending diglyme and butyl-diglyme in diesel fuel lowers the smoke and unburned hydrocarbon emissions.21 Smoke emission is significantly reduced with the use of the diethyl ether/diesel fuel blends with respect to that of diesel fuel.22 Blending 30% methylal in diesel fuel reduces PM emissions by 35% based on modal averaged test results from the Cummins B5.9 diesel engine.23 The addition of tripropylene-glycol monomethyl ether effectively eliminates engine-out smoke emissions by curtailing soot formation and/or increasing soot oxidation during and after the end of fuel injection compared to pure diesel.24 The oxygen content of the fuel affected smoke emissions more than the increase in injection pressure.24 Effects on combustion and emissions of a bio-derivable glycerol-based ethers mixture, usable in a compression ignition engine, were investigated. At medium-high load conditions, there was a maximum decrease of about 70% in terms of PM emissions compared to a slight increase of NOx.25 Polyoxymethylene
dimethyl
ethers
(PODE),
with
the
structure
CH3-O-(CH2O)n-CH3 and with no C-C bond, are promising blend fuels for diesel due to low viscosities and pour points, high oxygen contents and high CNs.26 PODE can also be soluble with diesel at any proportion. Among the PODE compounds, PODE3-5 has the best properties amongst diesel additives. PODE can be synthesized by the reaction of a compound (e.g, paraformaldehyde, trioxane or formaldehyde) providing the group of CH2O as a chain segment with a compound (e.g., dimethoxymethane, dimethyl ether or methanol) providing methyl or methoxyl groups that can seal the ends of a chain. The reactions are catalyzed by acids.26,27
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PODE were synthesized by the condensation of methanol and trioxymethylene over the catalysts of several molecular sieves like HY, HZSM-5, Hβ, and HMCM-22; and with HMCM-22 as the catalyst, the formation of long chain PODE products was further enhanced with a 29.39% yield of PODE3-8.28 Y.Y. Zheng et al. carried out a theoretical analysis of the molecular size distribution of PODE synthesized from dimethoxymethane (DMM) and paraformaldehyde (PF) based on a sequential reaction mechanism. The molecular size distribution model followed the Schulz-Flory distribution, and showed a good prediction ability at different reaction temperatures (T) and DMM/CH2O mole ratios (M), which verified the sequential reaction mechanism during the formation of PODE.29 Using the molecular size distribution model and response surface methodology, at optimum operating conditions for T=105ºC and M=1.1, the conversion of formaldehyde had a high value of 92.4%, and the fraction of PODE3-5 in the PODE mixture was 33.2 wt%, while the fractions of PODE>5 and PODE2 were 9.4 wt% and 24.3 wt%, respectively.29 Few
studies
have
investigated
engine
performances
fueled
with
Diesel/Biodiesel/PODE blends. D.H. Li et al.30 investigated the effect of PODE addition on the spray and atomization of diesel spray by using a high pressure common rail system, and the results showed that as PODE was added into diesel, the number of droplets tended to be uniformly distributed around the 12-20 µm range. All of these characteristics of the spray indicated that PODE addition improved the atomization of diesel spray.30 The effects of diesel/PODE blends on the combustion
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and emission characteristics, with a 15% and 25% volume PODE, were experimentally investigated in a heavy duty diesel engine. It clearly showed that blending PODE can dramatically reduce the CO and soot emissions, especially at high load conditions with a high EGR, and decrease the HC and NOX emissions.31 Especially at low and medium loads, NOX emissions obviously decreased when the EGR ratio was zero.31 H.Y. Liu et al.32 proposed that diesel engines were able to work normally when fueled by a blend fuel ratio with a lower than 30% PODE3-4. Adding PODE3-4 improved engine efficiency and reduced soot, HC and CO emissions, while NOx emissions increased. A high oxygen content in PODE was also prone to produce higher NOx emissions.33 However, paper (31) published in 2016 and (32) in 2017 exhibited contradictory rules for NOx emissions; secondly, increased NOx emissions in paper (32) were attributed to extended ignition delays by blending PODE, which contradicts the high cetane number of PODE; lastly, the influence of NOx emissions derived from oxygenated fuels are very complicated and multi-layered. Injection timing, adiabatic flame temperature, radiation heat transfer, and ignition delay are the parameters which most strongly influence the observed differences in NOx emissions of alternative fuels.34 In a common rail diesel engine, injection timing can be easily controlled and its variation, caused by viscosity and the bulk modulus of alternative fuel, can be ignored. When the injection timing remains the same, ignition delay is mainly determined by the cetane number, which has a significant relationship with molecular structure factors, including fuel-bound oxygen, unsaturation, and the number and position of double bonds. Fuel-bound
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oxygen, aromatics, and double bonds were the lower-order parameters affecting adiabatic flame temperature.34 Many studies correlated the degree of saturation to NOx emissions; highly saturated fuel molecules produced lower amounts of NOx emissions.35-37 Multiple bonds tended to increase flame temperature and thus the NOx emissions.34 As an oxygenated fuel, biodiesel combustion was reported to have higher reaction temperatures than petroleum diesel combustion.38,39 The biodiesel NOx increase was not quantitatively determined by a change in a single fuel property, but rather was the result of a number of coupled mechanisms, whose effects may tend to reinforce or cancel one another under different conditions, depending on specific combustion and fuel characteristics.40 Nevertheless, charge-gas mixtures being closer to a stoichiometric mixture at ignition and being in the standing premixed auto ignition zone near the flame lift-off length, appeared to be key factors in helping to explain the biodiesel NOx increase under all conditions. These differences were expected to lead to higher local and average in-cylinder temperatures, lower radioactive heat losses, and a shorter, more-advanced combustion event, all of which would be expected to increase thermal NOx emissions.40 Diffusion flame temperature was likely to have a strong influence on NOx emissions, owing to the dominance of thermal NO formation and the close proximity of the diffusion flame to the NO formation zone.41 Accordingly, the average in-cylinder temperature in the diffusion combustion phase was used to investigate the NOx emissions of Diesel/Biodiesel/PODE blend fuels. Nowdays, emission studies for diesel engines have concentrated on ultrafine
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particles (UFPs). The world has experienced severe haze-fog pollution resulting from the dense accumulation of fine aerosol particles.42,43 Haze exists as a mixture of a variety of components varied in particle from 0.003 to 100µm.44 The Health Effects Institute confirmed the adverse health impact of UFPs.45 UFPs can penetrate into the blood-stream and even cause genetic damage.18 However, UFPs dervied from vehicles make up a very small mass concentration of particulate matter, while the number concentration ratio is very large. The number concentration of UFPs has a more significant effect on pathogenicity than the mass concentration.46,47 For diesel engines, the major part of the diesel PM mass consists of fine particles (Diameter (D)
