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Alternative Fuels for Transportation
6.4 DME Applications
6.4.1 internal Combustion engine Fuel
The first diesel engine results presented for DME were in relation to its use
as an ignition improver in methanol (Brook et al. 1984; Murayama et al. 1992),
which was under consideration as a method to reduce particulate emissions
in order to meet the strict particulate emission standards that had been proposed for city busses in the United States for 1991. While methanol can burn
without producing soot emissions, its low cetane number makes it difficult
to ignite in diesel engines. In order to improve the ignition properties of
methanol, nitro-akyl additives were used as ignition improvers. As these are
expensive, and produce exhaust particulate matter in diesel engines, DME
was thought of as an alternative. However, as Figure 6.9 indicates, so much
DME was needed that it would have to be present in larger amounts that
the methanol.
At the time of these studies, DME’s low self ignition temperature was
known, but based on the historically expensive small scale production processes for making high purity DME, it was basically regarded as a rather
expensive chemical. Therefore, it was thought most economically sound to
use DME in limited quantities as an additive in order to minimize cost, and
engine studies were not extended to pure DME operation.
Attention to DME as an alternative fuel by itself (“neat” fuel) first came
about as a result of introductory studies in diesel engines (Fleisch et al. 1995;
Kapus and Ofner 1995; Sorenson and Mikkelsen 1995). At the same time, it
was shown that DME can be produced in large scale at an attractive price
(Hansen et al. 1995). Due to its low ignition temperature, DME is well suited to
diesel engine operation, and unsuited to operation in a spark ignition engine,
where it would knock extensively. Without any modification to the diesel
combustion chamber, engines operating on DME were found to operate at
the same efficiency as on diesel fuel, while exhibiting smoke-free operation.
Because of the lack of smoke, it was possible to operate the engines under
conditions where emissions of oxides of nitrogen, NO x , were quite low. These
conditions include retarded injection timing, extended injection duration and
high rates of exhaust gas recirculation (EGR). Diesel engines have been operated on DME with EGR amounts of up to 70%, with no smoke emissions.
The reason that DME does not produce smoke in diesel engines is apparently not due to the structure of the fuel itself, but to the presence of the
oxygen in the fuel. This is supported by Dec (1997), where it is found that
particulate matter in a diesel flame is formed in the products of rich combustion of the fuel inside the burning spray, and not in the primary reaction of
the fuel. This is also supported by Figure 6.10, which shows relative carbon
formation in diesel engines as a function of fuel oxygen content for a variety
of fuels from a series of studies (Beatrice et al. 1996; Cheng and Dibble 1999;
Alternative Fuels for Transportation
6.4 DME Applications
6.4.1 internal Combustion engine Fuel
The first diesel engine results presented for DME were in relation to its use
as an ignition improver in methanol (Brook et al. 1984; Murayama et al. 1992),
which was under consideration as a method to reduce particulate emissions
in order to meet the strict particulate emission standards that had been proposed for city busses in the United States for 1991. While methanol can burn
without producing soot emissions, its low cetane number makes it difficult
to ignite in diesel engines. In order to improve the ignition properties of
methanol, nitro-akyl additives were used as ignition improvers. As these are
expensive, and produce exhaust particulate matter in diesel engines, DME
was thought of as an alternative. However, as Figure 6.9 indicates, so much
DME was needed that it would have to be present in larger amounts that
the methanol.
At the time of these studies, DME’s low self ignition temperature was
known, but based on the historically expensive small scale production processes for making high purity DME, it was basically regarded as a rather
expensive chemical. Therefore, it was thought most economically sound to
use DME in limited quantities as an additive in order to minimize cost, and
engine studies were not extended to pure DME operation.
Attention to DME as an alternative fuel by itself (“neat” fuel) first came
about as a result of introductory studies in diesel engines (Fleisch et al. 1995;
Kapus and Ofner 1995; Sorenson and Mikkelsen 1995). At the same time, it
was shown that DME can be produced in large scale at an attractive price
(Hansen et al. 1995). Due to its low ignition temperature, DME is well suited to
diesel engine operation, and unsuited to operation in a spark ignition engine,
where it would knock extensively. Without any modification to the diesel
combustion chamber, engines operating on DME were found to operate at
the same efficiency as on diesel fuel, while exhibiting smoke-free operation.
Because of the lack of smoke, it was possible to operate the engines under
conditions where emissions of oxides of nitrogen, NO x , were quite low. These
conditions include retarded injection timing, extended injection duration and
high rates of exhaust gas recirculation (EGR). Diesel engines have been operated on DME with EGR amounts of up to 70%, with no smoke emissions.
The reason that DME does not produce smoke in diesel engines is apparently not due to the structure of the fuel itself, but to the presence of the
oxygen in the fuel. This is supported by Dec (1997), where it is found that
particulate matter in a diesel flame is formed in the products of rich combustion of the fuel inside the burning spray, and not in the primary reaction of
the fuel. This is also supported by Figure 6.10, which shows relative carbon
formation in diesel engines as a function of fuel oxygen content for a variety
of fuels from a series of studies (Beatrice et al. 1996; Cheng and Dibble 1999;