Oxygenate screening on a heavy-duty diesel engine and emission characteristics of highly oxygenated oxymethylene ether fuel ${\mathrm{OME}}_1$

Highlights

• Evaluation of 14 oxygenate substances on diesel engine.

• Main influences on soot reduction are O/C and H/C ratio.

• Dimethoxymethane was found most effective on particle reduction.

• Neat dimethoxymethane showed no soot-NO_x trade-off.

• Methane emissions were observed in stoichiometric diesel combustion.

Abstract

Oxygenated fuel components are known to reduce soot emissions in diesel engines significantly while having little effect on ${\mathrm{NO}}_x$ emissions. Several compounds were mixed with diesel fuel and tested for their emission characteristics on a 1.8 l heavy duty diesel engine. The ${\mathrm{C}}_1$-oxygenate dimethoxy methane (${\mathrm{OME}}_1$) which contains no C–C bonds in its molecular structure was found to have the best effect on the reduction of soot and particle number emissions. ${\mathrm{OME}}_1$ belongs to the group of oxymethylene ethers (${\mathrm{OME}}_n$) and has the molecular structure CH₃–O–(CH₂–O)_n–CH₃ while $n=1$.

For further investigations, a pure ${\mathrm{OME}}_1$-fuel with cetane number 48 was used which contained ${\mathrm{OME}}_1$ and additives to enhance viscosity, lubricity and cetane number. Engine testing including aftertreatment with a Pt coated oxidation catalyst (DOC) proved the soot-${\mathrm{NO}}_x$ trade-off to vanish completely even at stoichiometric operation. CO and most unburned fuel emissions were efficiently reduced by the DOC. The emission of formaldehyde and methane was measured using FTIR spectrometry. No CH₂O emissions could be detected. Near stoichiometric conditions, a growing output of methane was observed, which was not converted in the DOC. This can be explained with the high share of methyl groups, which react to methane with other radicals.

As ${\mathrm{OME}}_1$ shows low cetane number (CN), high volatility and weak viscosity and lubricity, the characterization of the less volatile ${\mathrm{OME}}_n$ with $n=3\dots 5$ and CN > 90 is recommended. These are expected to show similar emission characteristics and might have a smaller potential of methane formation.

Introduction

The use of oxygenates as fuels for CI engines or additives to conventional diesel fuel has been subject to many investigations, e.g. [1], [2]. This can be seen as an approach to improve engine out emissions not only by applying in-cylinder measures or exhaust aftertreatment technologies, but also by changing the chemical composition and structure of the fuel [3]. Oxygen present in the fuel showed to lower soot and particle emissions significantly with a strong correlation to oxygen mass content, while having little effect on ${\mathrm{NO}}_x$ emissions [4], [5]. However, absolute values for the change of soot and ${\mathrm{NO}}_x$ emissions compared to a reference fuel vary a lot in literature, according to engine type or operating points used in the investigations. Changes of ${\mathrm{NO}}_x$ emissions due to engine calibration effects can be of the same magnitude as changes due to fuel effects [6]. Besides oxygen content, an influence of molecular structure on emission behavior of the fuel has also been discussed. Ethers for instance may have a lower sooting tendency than alcohols at high loads [7].

Dimethoxy methane (${\mathrm{OME}}_1$) showed completely soot free combustion in flame experiments [8]. A reaction and soot formation mechanism for some oxygenates was used to predict that dimethylether (DME) and ${\mathrm{OME}}_1$ have lower sooting tendency than conventional fuel in diesel combustion [9].

The chemical structure of ${\mathrm{OME}}_1$ is similar to dimethyl ether (DME), as it is a ${\mathrm{C}}_1$ fuel without any C–C bonds and belongs to the group of ethers. The ${\mathrm{C}}_1$-fuels methanol and DME are the easiest sustainable fuels and can be produced from CO₂ and renewable H₂ or from biomass. As methanol is toxic and DME is gaseous, their transformation to oxymethylene ethers (${\mathrm{OME}}_n$: CH₃–O–(CH₂–O)_n–CH₃) allows easier handling and storage especially in mobile applications [10], [11]. ${\mathrm{OME}}_n$ are oxygenated ${\mathrm{C}}_1$-oligomers and are liquid under ambient conditions. ${\mathrm{OME}}_n$ can be mixed with diesel fuel at any ratio and are characterized by a low pourpoint, good material compatibility, and are toxicologically safe. The most simple compound of the ${\mathrm{OME}}_n$ family is ${\mathrm{OME}}_1$ (CH₃–O–CH₂–O–CH₃) which differs from DME only by one (–O–CH₂–)-unit.

${\mathrm{OME}}_1$ has a very low boiling point (42 °C) and low cetane number (CN 30 [12], CN 38 [13]) compared to conventional diesel fuel (boiling range 180–360 °C, CN 51), while ${\mathrm{OME}}_n$ with n = 3, 4, 5 have more diesel-like properties and show excellent cetane numbers. However, these substances are currently only available in small quantities, a newly planned production site in China has a scheduled start of production (10,000 t/a) in the first quarter of 2015 [14]. Samples of ${\mathrm{OME}}_n$ (n = 3, 4, 5) were tested on diesel engines in [15], [16], [17] exclusively. Due to their limited availability, testing results either refer to mixtures with diesel [15] or were only collected in one single operating point [16]. [17] presents results on a Euro-2 level passenger car engine where PM could be reduced by 77%. Until ${\mathrm{OME}}_n$ (n = 3, 4, 5) will be available on a large scale, ${\mathrm{OME}}_1$ can be seen as a model substance for the group of oxymethylene ethers.

Fig. 1 shows literature results of engine testing with oxymethylene ethers (${\mathrm{OME}}_n$). Tests with a 30% blend of ${\mathrm{OME}}_1$ in diesel (ca. 11% oxygen) showed a PM reduction of only 35% [18], [19]. Experiments with a 2% and 4% blend of ${\mathrm{OME}}_1$ in diesel (below 2% oxygen) even reduced soot by 46% and 57%, respectively [20], [21]. Similar results are reported in [22], [23]. Engine smoke was reduced by up to 80% with mixtures of 0–50% ${\mathrm{OME}}_1$ in diesel [12]. Neat ${\mathrm{OME}}_1$ was tested in a DI diesel engine with a three-way-catalyst in [24]. High EGR rates were applied to reduce ${\mathrm{NO}}_x$ emissions while the combustion was demonstrated to be smoke-free.

The current paper has two objectives. First, the soot reduction potential of ${\mathrm{OME}}_1$ and several other ether and ester oxygenates is to be compared on a 1.8 l single cylinder research engine. The goal is to determine if ${\mathrm{OME}}_1$ is more effective in reducing soot emissions than other oxygenates when tested on the same engine under equal operating conditions with a dedicated soot measurement technique (AVL Microsoot Sensor). To eliminate the influence of oxygen content, oxygenates are mixed with diesel to reach identical oxygen content. In a second step, ${\mathrm{OME}}_1$ is tested pure without any diesel addition with an oxydation catalyst [13]. To refine the results of smoke reduction achieved in [24], soot emission and particle number are measured. The emissions of unburned hydrocarbons are examined more closely using a FTIR analyser with a focus on methane and formaldehyde. Formaldehyde structures are present in all ${\mathrm{OME}}_n$ molecules and could be emitted after combustion. Methane contributes to the greenhouse effect about 25 times more effectively than CO₂ and could be formed from the methyl groups present in ${\mathrm{OME}}_n$ in the flame, as predicted in [25].