Oxygenated Fuels in Compression Ignition Engines
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C OVER STORY Fuel s
Oxygenated Fuels in
Compression Ignition Engines
Internal combustion engine technology can also be applied in order to meet future greenhouse
gas and pollutant emission targets, if the fuel characteristics are included as free parameters within
the optimization of the powertrain. Therefore, a BMWi-funded consortium investigated oxygenated
low-carbon fuels for the application in compression ignition engines, in order to study their combustion
and pollutant formation properties on the engine and within a vehicle application.
© Ford
26
MOTIVATION powered by synthetic fuels from renew
able sources are promising in addition
The reduction and avoidance of Green to disruptive concepts such as battery
house Gases (GHG) has become one of electric vehicles.
the most important technology drivers
for industrial applications worldwide
REQUIREMENTS FOR
to limit global warming. In particular
SUSTAINABLE SYNTHETIC FUELS
the transport sector, which is one of the
major contributors to GHG emissions, Renewable synthetic fuels have become
is in the focus of political and social an important subject to powertrain
debates. Additional efforts are required research in recent years. However,
in order to reduce CO2 concentration selection the most promising fuel can
in the atmosphere and thus limit glo didates for future vehicle applications
bal warming. Evolutionary approaches requires specific criteria to bundle
based on internal combustion engines research and development resources.
Those are:
–– Actual CO2 reduction: Synthetic fuels
AUTHORS must show CO2 reduction on the vehi
cle usage for short-term introduction,
as CO2 legislation is currently tank-
to-wheel-based. Applications of syn
thetic fuels should therefore not
exceed the CO2 emission of current
fossil fuels (diesel as benchmark).
Dr.-Ing. Werner Willems
–– Reduction of pollutants: New fuels
is Technical Spezialist Sustainable should not only avoid CO2 emission,
Fuels Research at the Ford but also pollutants.
Research and Innovation Center –– Availability and costs: CO2 avoidance
in Aachen (Germany).
is a problem which requires immidiate
action. The development of standards
for new fuels and the introduction into
type approval regulations require time
and effort. Therefore, standardized
fuels, which are available globally and
at reasonable costs, should be favored.
Dipl.-Ing. Marcel Pannwitz
is Development Engineer in the
Hence, dimethyl ether (DME, CH3-O-CH3)
division Thermodynamics & and oxymethylene ether (OME1, CH3-O-
Powertrain Concepts of CH2-O-CH3) were identified as suitable
IAV GmbH in Berlin (Germany). and promising fuels for applications in
diesel engines. Thus, they were selected
for the investigations.
DME AS AN ALTERNATIVE FUEL FOR
COMPRESSION IGNITION ENGINES
Marius Zubel, M. Sc. DME was investigated from fundamen
is Research Assistant at the
tal high-pressure chamber tests over
Chair of Internal Combustion
Engines of the RWTH Aachen single/multi-cylinder engine tests up to
University in Aachen (Germany). vehicle demonstration in order to assess
its potential as diesel fuel replacement
within the framework of the project.
In the following, the physical and chemi
cal properties of DME as the most pro
mising methyl ether will be discussed.
Dr.-Ing. Jost Weber
PHYSICAL AND CHEMICAL
is Department Head diesel
System at the Denso Automotive PROPERTIES OF DME
Deutschland GmbH in the
Aachen Engineering Center It should be noted that due to the
in Wegberg (Germany). high oxygen content (34.8 % m/m)
MTZ worldwide 03|2020 27
C OVER STORY Fuel s
and the absence of C-C bonds for DME, – Unit EN590 diesel DME
an almost soot-free combustion can be
Boiling point °C 180–350 -24.8
achieved. Consequently, higher Exhaust
Gas Recirculation rates (EGR) can be Cetane number - 51–54 55–60
utilized in order to reduce NOx emis Density (15 °C) kg/m³ 830 671
sion simultaneously. TABLE 1 shows
Oxygen content % m/m
FIGURE 1 Optical investigations of the mixture preparation in the high-pressure chamber [2] (© RWTH Aachen University)
The act uation duration of the injector to the stronger cooling effect of the fuels and nozzle configurations. The
was ten = 1000 µs and the rail pressure evaporating jet. start of actuation and duration of the
was set to 1000 bar. Diesel fuel shows a injector as well as the EGR rate were
significantly longer liquid jet than DME adapted depending on the fuel and
SINGLE-CYLINDER
when comparing the LPL using the ref nozzle size in order to keep the center
ENGINE INVESTIGATIONS
erence nozzle (PC-HPC-D). This can be of heat release constant. Only a single
related to the lower boiling point and For the experimental combustion main injection was applied for the tests
vapor pressure of DME. Here, the vapor invest igations, a single-cylinder on the single-cylinder engine, in con
pressure is very close to the ambient engine was derived from the corre trast to the standard mapping of the
pressure in the chamber. The phase sponding multi-cylinder engine. It multi-cylinder engine. FIGURE 2 shows
change of DME from liquid to gaseous was equipped with the adapted injec the comparison between the diesel refer
occurs almost immediately, resulting tion system components. Characteris ence measurement and DME with two
in a shortened LPL. A significant influ tic load points based on mapping data different nozzle hole diameters. The die
ence on the LPL at DME was observed from the series diesel engine were sel baseline was measured with the ref
for the nozzle with the larger hole dia selected. In the following, an operat erence nozzle (PC-D) of the series con
meter (PC-HPC-2.5). While the penetra ing point at medium load for diesel figuration. The adapted nozzles for com
tion depth of the liquid diesel jet showed and DME will be analyzed in detail. pensation of the physical properties and
only a small increase, significant differ The engine boundary conditions injection pressure (PC-1.8 and PC-2.5)
ences could be observed for DME due were not changed for the different were investigated for DME. The EGR rate
Diesel (PC-D) DME (PC-1.8) DME (PC-2.5)
4 2,0 8 12 FIGURE 2 EGR variation
ISCO [g/kWh]
∆p [bar/°CA]
(n = 1750 rpm; IMEP = 8.6 bar)
ISHC [g/kWh]
3 1,5 6 10
FSN [-]
of diesel baseline (PC-D),
2 1,0 4 8 DME with scaled nozzle
(PC-1.8 und PC-2.5)
1 0,5 2 6 (© RWTH Aachen University)
0 0,0 0 4
45 2,5 65 550
BD5-90 [°CA]
43 2,0 50 500
Texh [°C]
ηi [%]
λ spindt
41 1,5 35 450
39 1,0 20 400
37 0,5 5 350
0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4
ISNOx [g/kWh] ISNOx [g/kWh] ISNOx [g/kWh] ISNOx [g/kWh]
MTZ worldwide 03|2020 29
C OVER STORY Fuel s
Engine specification Unit Ford 1.5-l I4 88 kW TDCi
Compression ignition,
Combustion system –
four-stroke
Engine displacement cm³ 1499
Rated power kW at rpm 88 at 3600
Maximum torque Nm at rpm 270 at 1500–2500
Compression ratio – 16.0
Stroke mm 88.3
Bore mm 73.5
Boosting system – VTG (single-stage)
Exhaust gas recirculation
– High-pressure, cooled
system
TABLE 3 Specification of the test engine (© Ford)
was adjusted to achieve different specific in case of high-temperature pyrolysis [3]. a lower pressure rise. This behavior
NOx emission levels (ISNOX). The specific HC (ISHC) and CO emis can be explained by the shorter burn
No soot emission corresponding to sions (ISCO) are of similar magnitude ing duration (BD5-90). However, the
the Filter Smoke Number (FSN) could be for all three cases. The HC emission of smaller nozzle (PC-1.8) showed higher
detected even at minimal NOx emission DME is lower compared to diesel fuel EGR tolerances compared to the larger
level for both nozzle configurations with only at low EGR rates. However, the one (PC-2.5). Thus, it appears more
DME, FIGURE 2. This behavior is due to maximum pressure rise rate shows dif advantageous for usage. Another big
the fast mixture formation, the high oxy ferences between the three configura difference between DME and diesel lies
gen content and the missing C-C bonds tions. The measurements with DME in the difference regarding indicated effi
[1]. The missing C-C bonds lead to the for show higher values despite the higher ciency (ηi), FIGURE 2. DME has a higher
mation of CO instead of soot precursors cetane number, which usually leads to efficiency relative to diesel, due to the
FIGURE 3 Required modifications of the engine hardware (© IAV)
30shorter burning duration and lower ing duration and the resulting lower smaller than 350 bar did not result in
exhaust gas temperature (Texh). exhaust gas temperature of DME rela further benefits. A similar trend could
tive to diesel fuel. Therefore, the opti be observed for the variation of the cen
mization had to be performed at a ter of heat release, as an advanced timing
MULTI-CYLINDER ENGINE INVES
relatively high specific NOx emission helped to increase the efficiency at simi
TIGATIONS FOR THE PASSENGER
(2.0 g/kWh) in order to ensure enough lar CO emission level. The variation of
CAR DME APPLICATION
flexibility regarding the VTG position. the air-to-fuel ratio showed that lean
A Ford 1.5-l diesel engine was used for A rail pressure reduction to 350 bar combustion should take place at least
the engine tests on the dyno and within with simultaneous adjustment of start at λ = 1.5. Stoichiometric combustion
the vehicle. The technical specifications of injection showed advantages in comparable to gasoline engines could
of the multi-cylinder engine are shown terms of CO and efficiency. The indi not be achieved, due to the decrease in
in TABLE 3. The engine hardware had to cated high-pressure cycle efficiency efficiency and the significantly increased
be adapted to the DME injection compo decreased with reduced rail pressure CO emission. The latter was well above
nents, FIGURE 3. The greatest challenge as expected, but the effective engine effi the value of modern DI gasoline engines.
was the integration of the high-pressure ciency increased. This can be explained Additional exhaust gas components were
pump. The pump was lubricated by the on the one hand by reduced friction measured with a particle counter and a
engine oil due to the lack of lubricity of losses due to lower drive power demand Fourier Transform Infrared Spectrometer
DME. The belt drive had to be adapted of the high pressure pump. On the other (FTIR). The CH4 emission downstream
due to the different phasing position of hand, the combustion duration increased of the diesel oxidation catalyst was not
the pump. Moreover, new sensors and in comparison to the base measurement affected by the variation of rail pressure
actuators for controlling the high-pres with 720 bar. Consequently, the exhaust and center of heat release. However, it
sure fuel system had to be integrated enthalpy increased enabling a higher increased with decreasing air-to-fuel
into the engine control system to enable turbocharger efficiency. Thus, the gas ratio and found its maximum of about
engine operation under steady-state and exchange losses decreased. A further 140 ppm at λ = 1.1. The further reduc
transient conditions. The engine control reduction in rail pressure to values tion of the air-fuel ratio to λ = 1.0 led to a
system, except for the injection parame
ters, remained in the serial engine con
trol unit whereas a separate second
Diesel DME n = 2000 rpm, BMEP = 7.0 bar, NOx = 2.0 g/kWh = constant, soot = 0.0 FSN
unit controlled the DME injection sys
tem. The communication between both Rail pressure variation αh50Variation λ Variation
Base rail pressure: 720 bar Base α Base λ: 1.74
control units is ensured by a specific h50
: 18 ° CA
CAN interface. λ: 1.74 λ: 1.74 λ: varied
αh50 : 18 ° CA αh50 : varied αh50 : 14 ° CA
The measurements on the multi-
Rail pressure: varied Rail pressure: 350 bar Rail pressure: 350 bar
cylinder engine were conducted at
ten part load and three full load operat 4 4 4
raw emission
CO [g/kWh]
ing points. A representative operating
point (n = 2000 rpm, BMEP = 7 bar) 2 2 2
will be discussed for DME operation to
show exemplarily the approach for cali 0 0 0
45 45 45
bration optimization, FIGURE 4. Varia
tions of the rail pressure, the center of
ηe [%]
35 35 35
heat release and the combustion air-to-
fuel ratio at constant NOx emission have 25 25 25
been conducted, while roughly main 105 105 105
raw emission
taining the diesel reference calibration
PN [#/cm3]
and boundary conditions. The tests 104 104 104
with DME have been done without the
use of a pilot injection in contrast to the 103 103 103
tailpipe emission tailpipe emission
base measurement with diesel fuel, as 200 200 200
CH4 [ppm]
the single-cylinder engine test results
100 100 100
showed promising trends from an
engine noise point of view by applying
0 0 0
low rail pressure in combination with
40 40 40
high EGR rates, when using the smaller
NH3 [ppm]
nozzle hole diameter. 20 20 20
The aim of the individual variations
was to achieve a maximum possible 0 0 0
efficiency with lowest CO emission. 200 350 500 650 800 5 10 15 20 25 1.0 1.5 2.0 2.5 3.0
Rail pressure [bar] αh50 [° CA] λ
The turbocharger was one of the limit
ing factors because of the shorter burn FIGURE 4 Optimization for n = 2000 rpm, BMEP = 7.0 bar at constant NOx emission (© IAV)
MTZ worldwide 03|2020 31C OVER STORY Fuel s
DME optimized Diesel n = 2000 rpm, BMEP = 7.0 bar
4 8 20 20
FSN [-]
2
dp/dα [bar/°CA]
6 15 15
CO [g/kWh]
CO2 [%]
0/2 4 10 10
HC [g/kWh]
1 2 5 5
0 0 0 0
50 2.5 65 550
45 2.0 50 500
BD5-90 [°CA]
Texh [°C]
ηe [%]
40 1.5 35 450
λ
35 1.0 20 400
30 0.5 5 350
0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4
NOx [g/kWh] NOx [g/kWh] NOx [g/kWh] NOx [g/kWh]
FIGURE 5 EGR-variation after optimization for n = 2000 rpm, BMEP = 7.0 bar (© IAV)
reduction of CH4 to an emission level modern DI gasoline engines but tended relative to diesel fuel. The CO emission
near the detection limit. Here, the to be of same magnitude or even higher was still high. The map-wide calibra
exhaust gas temperature increased at other operating points, FIGURE 4. tion based on the optimization of the
substantially (approximately 550 °C) The final EGR variation was carried steady-state operating points, which
so that the threshold value for CH4 con out based on the findings of the optimi implies the use of a deNOx system
version in the catalyst was exceeded. zation, FIGURE 5. The effective engine and a diesel particulate filter, is shown
However, this threshold value could not efficiency with optimized calibration in FIGURE 6.
be reached at low loads, due to the over was almost the same as that of the die A demonstrator vehicle based on a
all lower exhaust gas temperature level. sel-powered counterpart. DME offered Ford Mondeo with adapted engine hard
The raw emission of the Particulate an advantage in terms of CO2 emission ware and tank system was developed
Number (PN) remained well below of due to the more favorable H/C-ratio for the final prove of concept. It exhib
NOx [g/kWh] FSN [-] HC [g/kWh] CO [g/kWh]
24 Series
full load
18 curve with
5.0
BMEP [bar]
diesel fuel
3.5
12 1.5
0.0 0.5
1.0
6 0.5 1
1.5.0
15 0
10
4.0 2 25
0
ηe [%] λ dp/dα [bar/°CA] Texh [°C]
24
700
18
2
BMEP [bar]
1.
600
4
3
5
12
1. 1.3
500
34
4
40 5
1.
6
400
6 32 1.6
1.8 300
25 0 2.0 200 FIGURE 6 DME
2
0 engine maps (© IAV)
0
0
0
0
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
00
50
50
50
50
10
15
20
25
10
25
15
20
10
15
20
25
10
15
20
25
Engine speed [rpm] Engine speed [rpm] Engine speed [rpm] Engine speed [rpm]
32FIGURE 7 Emission improve-
ment of DME relative to
diesel in WLTC (© IAV)
ited the findings from the multi-cylinder as sustainable fuels for diesel engine
engine investigations. applications in passenger cars and com
The emission potential of DME com
pared to diesel fuel in WLTC is shown
mercial vehicles in the “XME-diesel”
project. DME in particular proved to be
THANKS
in FIGURE 7 and evaluated with regard a promising candidate as a substitute for The authors thank the German Federal Ministry of
to raw emissions of particulates and fossil diesel fuel, considering different Economics and Energy for their support of the
NOx as well as for CO2 emission down requirements for future sustainable fuel project and TÜV Rheinland, especially Dr. Bernhard
stream of the exhaust aftertreatment solutions for combustion engines (TtW Koonen, and the Forschungsvereinigung Verbren-
system. The soot/NOx trade-off of the CO2 and emission reduction, available nungskraftmaschinen (FVV) for their organizational
classic diesel combustion was no lon standards, global availability and low support. In addition, the authors the authors would
ger present for DME combustion. The costs). The performance of DME from like to thank their partners from the Technical
particulate mass could be brought to basic injection chamber measurements University of Munich, Dr. Martin Härtl, Kai Gaukel,
zero emission level, while NOx emis to single-cylinder and multi-cylinder M. Sc., and Dominik Pelerin, M. Sc., who covered
sion relative to diesel operation could tests to a demonstrator vehicles (Ford the heavy-duty applications in the project as well as
also be reduced significantly (-33%). Mondeo) were analyzed and demon Oberon Fuels and Prins-Westport, who supported
The CO2 emission level remained strated within the project. the project as well.
almost the same. A low pollutant emis
sion and nearly CO2-neutral operation REFERENCES
could be achieved by providing the fuel [1] Westbrook, C. K.; Pitz, W. J.; Curran, H. J.:
Chemical kinetic modeling study of the effects
from regenerative sources (E-DME) and of oxygenated hydrocarbons on soot emissions
a well-to-wheel consideration. from diesel engines. In: The journal of physical
chemistry A (2006), 110 (21), pp. 6912–6922
[2] Ottenwaelder, T.; Pischinger, S.: Effects of
CONCLUSION Biofuels on the Mixture Formation and Ignition
Process in diesel-Like Jets. SAE Technical Paper
As part of the technical program „Neue 2017-01-2332, 2017
Fahrzeug- und Systemtechnologien“ [3] Fischer, S. L.; Dryer, F. L.; Curran, H. J.:
The reaction kinetics of dimethyl ether. In:
funded by the German Federal Ministry
High-temperature pyrolysis and oxidation
of Economics and Energy, methyl ether in flow reactors. In: International Journal of
fuels (DME/OME1) were investigated Chemical Kinetics (2000), 32, pp. 713-740
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