192
Alternative Fuels for Transportation
relative motion can break down. This can give rise to contact between the
two surfaces, increasing friction, and most importantly causing wear that
can lead to the failure of engine parts.
This is important in diesel engine fuel systems. The fuel must be injected
into the engine under high pressure (up to 2000 atm in modern diesel engines
using diesel fuel, though DME operates satisfactorily with much lower pressures), and this is done by pressing the fuel through a small restriction using
a piston in a sealed cylinder of a pump. Tolerances required between the
piston and the cylinder are very small (on the order of microns) and wear
problems are important. Modern diesel fuel has had aromatic compounds
and sulfur reduced in order to combat emissions, with the results that the
wear tendencies in injection systems are increased. For modern low sulfur
diesel fuel, this has been solved using the addition of “lubricity improvers”
to the fuel. These are compounds that apparently attach themselves to surfaces and help prevent the surfaces from contacting each other.
Figure 6.14 shows the results of wear tests in the author’s laboratory with
DME using different lubricity improving additives. The diesel oil level refers
to an acceptable rate of wear on the standard wear test for commercial diesel
fuels using the similar high frequency reciprocating rig (HFRR) test procedure. The test uses a reciprocating ball pressed on a plate submersed in fuel
and operated for a fixed duration. It is the test used to certify the suitability of
diesel fuel for current commercial engines. A modified version of the test has
been developed in order to handle DME under its vapor pressure (Sivebeak
and Sorenson 2000) and the results shown in Figure 6.14 are for this test. At
the left hand side of the curve with no additive, it can be seen that the wear
with DME is significantly higher than that of diesel fuel. The test results show
that it is possible to improve the lubricity of DME to give a similar result to
0
100
200
300
400
500
600
700
1
1 0
100
1000
10000 100000 1000000
Wear scar diameter - (µm)
Diesel oil acceptance criterion
Rapeseed oil methyl ester
Castor oil
Diesel fuel lubricity additive
Additive amount – mass ppm
Figure 6.14
The wear of a reciprocating ball on plate test with DME with various lubricity improving
additives.
Alternative Fuels for Transportation
relative motion can break down. This can give rise to contact between the
two surfaces, increasing friction, and most importantly causing wear that
can lead to the failure of engine parts.
This is important in diesel engine fuel systems. The fuel must be injected
into the engine under high pressure (up to 2000 atm in modern diesel engines
using diesel fuel, though DME operates satisfactorily with much lower pressures), and this is done by pressing the fuel through a small restriction using
a piston in a sealed cylinder of a pump. Tolerances required between the
piston and the cylinder are very small (on the order of microns) and wear
problems are important. Modern diesel fuel has had aromatic compounds
and sulfur reduced in order to combat emissions, with the results that the
wear tendencies in injection systems are increased. For modern low sulfur
diesel fuel, this has been solved using the addition of “lubricity improvers”
to the fuel. These are compounds that apparently attach themselves to surfaces and help prevent the surfaces from contacting each other.
Figure 6.14 shows the results of wear tests in the author’s laboratory with
DME using different lubricity improving additives. The diesel oil level refers
to an acceptable rate of wear on the standard wear test for commercial diesel
fuels using the similar high frequency reciprocating rig (HFRR) test procedure. The test uses a reciprocating ball pressed on a plate submersed in fuel
and operated for a fixed duration. It is the test used to certify the suitability of
diesel fuel for current commercial engines. A modified version of the test has
been developed in order to handle DME under its vapor pressure (Sivebeak
and Sorenson 2000) and the results shown in Figure 6.14 are for this test. At
the left hand side of the curve with no additive, it can be seen that the wear
with DME is significantly higher than that of diesel fuel. The test results show
that it is possible to improve the lubricity of DME to give a similar result to
0
100
200
300
400
500
600
700
1
1 0
100
1000
10000 100000 1000000
Wear scar diameter - (µm)
Diesel oil acceptance criterion
Rapeseed oil methyl ester
Castor oil
Diesel fuel lubricity additive
Additive amount – mass ppm
Figure 6.14
The wear of a reciprocating ball on plate test with DME with various lubricity improving
additives.