The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
High NOx Reduction Achieved at Low Load Using Very
Advanced Valve Timing with Hydro-Treated Vegetable Oil
Matteo Imperato1*, Aki Tilli2, Teemu Sarjovaara3, Martti Larmi4
1
Aalto University School of Science and Technology
Puumiehenkuja 5 A
Espoo
Finland
matteo.imperato@aaltouniversity.fi
2
Aalto University School of Science and Technology
Puumiehenkuja 5 A
Espoo
Finland
aki.tilli@aaltouniversity.fi
3
Aalto University School of Science and Technology
Puumiehenkuja 5 A
Espoo
Finland
teemu.sarjovaara@aaltouniversity.fi
4
Aalto University School of Science and Technology
Puumiehenkuja 5 A
Espoo
Finland
martti.larmi@aaltouniversity.fi
* corresponding author
ABSTRACT
The objective of this paper is to analyze the performance of a large-bore single-cylinder
medium speed compression-ignition (CI) engine running with hydro-treated vegetable
oil. This fuel has a paraffinic chemical structure and high Cetane number (CN). These
features permit to achieve more complete and cleaner combustion, in many engine
operation points. The main benefits are thus lower emission compared to diesel fuel and
low soot values. The facility used in this study is a research engine, whose main
parameters that affect the performances are fully adjustable. The boundary conditions
upstream and downstream the engine are freely controlled by a separated supply air plant
and by a throttle valve, located at the end of the exhaust pipe. The injection system is
common-rail: rail pressure, injection timing and duration are completely adjustable. The
gas exchange valve system consists of electro-hydraulic actuators, used for controlling
the intake and exhaust valve timing. Using the flexibility of the engine parameters,
several configurations have been tested to realize different in-cylinder conditions before
the combustion. In fact, the in-cylinder compression temperature and the exhaust fraction
have been changed by modifying the valve timing and the boundary conditions. Instead,
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the in-cylinder compression pressure and the injection parameters have been kept
unchanged. Two are the loads presented in this paper and they are at medium and at low
load: the strategy has been the same for both the loads. The results are promising and
show the benefits of hydrotreated vegetable oil (HVO) compared to diesel fuel. In fact, it
has been possible to reduce nitrogen oxides (NOx) emission up to 50% running with
HVO and opportunely tuned valve timing. However a certain strategy does not bring the
same outcomes trend for every tested load and the trade-off between the results has to be
drawn out.
Keywords: NOx reduction, biofuel, diesel, CI engine, clean combustion, Miller, gas
exchange valve, adjustable timing
1. Introduction
The emission limits for diesel engines for ships are becoming stricter and stricter. In 2016
a new regulation, which reduces NOx emission by 80% [1], will become effective. The
development of new solutions and techniques for reducing emissions is therefore
mandatory in order to fulfill the stringent law demands.
In this concern the combined use of advanced techniques for emission reduction and
alternative fuels, like HVO, in compression-ignition (CI) engines can give very promising
results and lower emission outcomes, compared to the standard values achieved with
diesel.
In this study runs have been carried out with diesel EN590 and a type of HVO, called
NExBTL, and produced by Neste Renewable Fuels Oy. HVO has high CN, paraffinic
chemical structure and it has no sulfur or aromatic content [2].
The possibility to run CI engines with HVO has been previously studied. Kuronen et al.
[3] evaluated the possibility to use HVO as fuel in heavy duty engines. Also Aatola et al.
[4] compared the performance of HVO and diesel on heavy duty engine applications and
found out that running with HVO results in lower exhaust emissions, regulated and
unregulated, and smoke values. In addition, a study by Pflaum et al. [5] presented
experimental results achieved with different blends of HVO and diesel fuel; the results
showed that HVO can reduce both NOx and soot at the same time due to its lack of
complex chemical aromatics.
In this paper several techniques for emission reduction are presented at high load. The
main target was to reduce the NOx formation by combining the advantages of HVO
chemical structure and physical properties with advanced techniques that can reduce the
in-cylinder combustion temperature or increase the charge dilution with exhaust gases.
A useful way to lower NOx is using the Miller cycle [6], which consists of reducing the
effective compression ratio by changing the gas exchange valve timing. This means that
the in-cylinder compression temperature of the charge drops and also the combustion
process may happen at lower temperature. The Miller technique had already been tested
with EVE [7] and the results showed that a high reduction of pollutants can be achieved
only by advancing the intake valve closing (IVC).
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In addition to the Miller cycle also high quantity of the exhaust gas residuals in the
combustion chamber has been tested. A certain amount of inert gases in the combustion
chamber subtracts some heat produced during the combustion process and obstacles the
zones at high temperature, reducing the NOx formation [8]. Since EVE has not any gas
recirculation external system, the dilution of the charge has been achieved by shortening
the gas exchange scavenging period, operating both on intake and exhaust valves, and
opening briefly the exhaust valves also around the bottom dead centre (BDC) at the end
of the intake stroke: the so called internal exhaust gas recirculation (IEGR).
The combined use of Miller cycle and IEGR has been previously tested in diesel engines.
Edwards et al. [9] studied the combined effect of the Miller cycle and the IEGR on heavy
duty truck engines finding acceptable levels of in-cylinder temperature. Millo et al. [10]
analyzed different strategy of IEGR and found a NOx reduction by 13% and a remarkable
improved fuel economy. Osada et al. [11] showed that the exhaust gas recirculation
(EGR) can bring to higher fuel economy and reduced NOx, but an excessive EGR rate
may increase the soot values in the exhaust gases. No similar studies have been carried
out so far in large-bore naval engine like EVE.
2.
Experimental
2.1
Research engine
The facility used is a single-cylinder medium-speed CI research engine for ship
applications. The Extreme Value Engine (EVE) is in fact capable to operate in several
running conditions, due to the high flexibility of the gas exchange valves, fuel and
ancillaries systems [12]. The EVE has electro-hydraulic valve actuators (EHVA) [13]
instead of traditional camshaft mechanism. This system permits to have high degree of
freedom in the gas exchange phase. In fact it is possible to modify the opening and
closing timing of the gas exchange valves, their maximum valve lift as well as their
opening and closing lift slope. The valve actuators are controlled by pressurized oil (250
bar) that is the same oil used in the lubrication system. The EVE is connected to an
electric motor, which also allows running in motored mode. The fuel injection system is
common rail type with adjustable rail pressure, injection timing and duration. The engine
boundary conditions, such as intake air pressure and temperature as well as exhaust
pressure, are controllable by an air supply system and a backpressure throttle valve; this
allows running operating points with an infinite configuration of turbochargers. Besides
the quantities mentioned so far, all pressures and temperatures of the ancillary systems
(LT water, HT water and lubrication oil) are remotely adjustable.
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Figure 1: The EVE engine and its ancillaries
2.2
Measurement equipment
The pressure inside the combustion chamber has been measured via a pressure sensor
Kistler 6061B connected to a charge amplifier Kistler 5011. The pressure sensor is
installed in the cylinder head, very close to the combustion chamber to minimize any
error. In addition to this, a Kistler 7001 pressure sensor in an automatic switching adapter
Kistler 741A is installed close to the previous. The automatic switching adapter is used to
isolate the pressure sensor from the cylinder pressure during high pressure stage. The
influence of thermal shock caused by combustion can thus be avoided and an accurate
measurement of lower pressures achieved.
2.3
Experimental procedure
All the test-points are steady-state and the engine operation has been constant. When the
set-up is changed, some time is needed for the results stabilization. When the main
outcomes e.g. fuel flow, engine power, NOx value are reasonably steady, the engine runs
few minutes longer before the measurement is taken. At the beginning of every testing
day, a reference test is taken to compare the values with the ones achieved previously and
check that all the devices work consistently. With this procedure, a very good
repeatability is achieved and the results are very reliable.
2.4
Fuel composition
The fuel used in this study has been a type of HVO, called NExBTL by Neste Renewable
Fuels Oy. HVO production is based on hydrotreatment of triglycerides such as vegetable
oils and animal fats, whereas with traditional biodiesel (FAME) the process is based on
triglyceride esterification with alcohol. The end product is a clean paraffinic diesel fuel
similar to fuels produced by biomass, coal or natural gas gasification and Fischer-Tropsch
–synthesis. Hence, the process combines traditional biodiesel feedstock and the high
quality end product of synthetic diesel [2, 3].
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Regarding end-use properties, hydrotreated oils show superior quality compared with
conventional diesel fuel and especially conventional biodiesel. Regardless of the
feedstock, all paraffinic diesel fuels or components have many benefits. Firstly, they can
be used as it is or as blends in existing diesel engines and infrastructure and they enable
the development of engines with improved engine efficiency. Secondly, the fuels are
sulfur-free, with low aromatic content and free of materials compatibility or storage life
problems associated with FAME. Moreover, they are more biodegradable, less toxic and
not as harmful to aquatic organisms as conventional diesel [14, 15].
With no aromatics or sulfur, consisting of paraffins, the fuels burn very cleanly and as
such diminish emissions and especially soot compared to standard diesel. The high CN
means better ignition and complete combustion in difficult conditions. This reduces
particulate matters (PM), hydrocarbons (HC) and NOx emissions and allows higher EGR
rates to diminish NOx emissions [3-5].
Typical properties of HVO, FAME, synthetic diesel and typical EN 590 standardfulfilling diesel are reported in Table 1. The density of HVO is lower than that of diesel
and FAME, and the CN is much higher. Viscosities and distillation ranges of standard
diesel and FAME are slightly lower than with HVO on the average. Synthetic diesel
chemistry and thus properties are very similar to HVO.
Table 1: Typical properties of HVO compared with FAME, synthetic diesel and average European
EN 590-standard fuel. [14]
Fuel
HVO
Density at +15°C (kg/m3)
Viscosity at +40°C (mm2/s)
Cetane number
Distillation range (°C)
Cloud point (°C)
Heating value (MJ/kg)
Heating value (MJ/l)
Aromatics content (wt-%)
3.
FischerTropsch
770 - 785
3.2- 4.5
73 - 81
180-360
0 ... -25
43
34
0
775 - 785
3.0 - 3.5
80 - 99
180-320
-5 ... -25
44
34.5
0
FAME (RME)
Typical
885
4.5
51
350-370
-5
37.5
33
0
EN 590
Average
835
3.5
53
180-360
-5
43
36
30
Test program
Two loads have been run to see the influence of the strategy used along the load: in
particular 70 kW (medium load) and 25 kW (low load) as output power have been
chosen. The engine speed has been the same at every run point and it has been 900 rpm.
3.1
Valve timing
An extensive test program has been carried out. The main studied object has been the gas
exchange valve timing.
The starting valve timing has been chosen with low Miller rate and wide positive
scavenging period. With this timing the comparison between diesel and HVO has been
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performed; further tests have been run only with HVO. From the reference configuration
two advanced Miller rate have been tested by closing earlier the intake valve. They are
represented in Fig. 2.
18
15
valve lift (mm)
12
INT - Mil15 Sc60
9
EXH - Mil15 Sc60
INT - Mil50 Sc60
6
INT - Mil70 Sc60
3
0
120
180
240
300
360
CAD
420
480
540
600
Figure 2: the advanced Miller timing
With both the advanced timings (Mil50 and Mil70) different scavenging phases have
been tested, in order to create different mixture condition before the fuel injection. Beside
the positive scavenging (Sc 60), also zero scavenging (Sc 0) and negative scavenging (Sc
-30) have been tested. Advancing the intake valve closing (IVC) brings to lower
maximum lifts: this may require very high charge air pressure values and quick response
of the valve system. To avoid these effects, the negative scavenging is performed so that
the intake valve lift is the same of the one with zero scavenging and the exhaust valve is
closed much earlier during the exhaust stroke. The tested timings with Mil70 are shown is
Fig. 3.
18
INT - Mil70 Sc60
15
INT - Mil70 Sc0
INT - Mil70 Sc-30
EXH - Mil70 Sc60
valve lift (mm)
12
EXH - Mil70 Sc0
EXH - Mil70 Sc-30
9
6
3
0
120
180
240
300
360
420
CAD
Figure 3: the three tested scavenging periods
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3.2
Strategy
The strategy used has been such as the in-cylinder pressure at the end of the compression
stroke has been kept constant with every tested valve timing. The fuel injection
parameters are the same in all the runs; only the fuel dosage is different along the engine
load. In Tab. 2 the values are reported.
Table 2: the fuel injection parameters
Parameter
Rail pressure
Start of Injection (SOI)
Injection dose (at 70 kW)
Injection dose (at 25 kW)
Unit
bar
CAD ATDC
mg/cycle
mg/cycle
Value
1400
-10.5
670
365
The boundary conditions, such as intake and exhaust pressure have been drawn out from
a 1-D simulation model, which has a mathematical turbocharger, whose action is to
transfer part of the exhaust gas energy to the inlet side along a wished efficiency. In this
work the turbocharger overall efficiency has been set as 0.75.
4.
Results
The main test results are presented in this section. The in-cylinder conditions, considered
important in this study, can not be taken from the measurement system: an estimation
could be possible but it would give a high rate of uncertainty. For this reason part of the
results reported are drawn out from the simulation model.
4.1
Simulation results
The in-cylinder compression temperature and the exhaust gas fraction in the charge have
been retrieved by the simulations. Figure 4 shows that the values are not so different with
the load: the simulated in-cylinder compression temperature in the reference case is 920
degK for both the load and the exhaust fraction is negligible. The mere Miller usage
lowers as expected the in-cylinder compression temperature by 100 degK about.
Shortening the scavenging phase brings to increase of both in-cylinder temperature and
exhaust gas fraction. The latter does not exceed 5% in any case but the mixture
temperature considerably increases: in the case Mil50 Sc-30 at 70 kW load this value is
the same as the reference.
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6
920
5
900
4
880
860
3
70 kW - in-cylinder compression temperature
840
25 kW - in-cylinder compression temperature
70 kW - exhaust gas fraction
820
2
25 kW - exhaust gas fraction
800
exhaust gas fraction (%-mass)
simulated in-cylinder compresion temperature (degK)
940
1
780
760
0
Ref Diesel
Ref
Sc 60
Mil15 Sc60
Sc 0
Sc -30
Sc 60
Mil50
Sc 0
Sc -30
Mil70
Figure 4: the simulation results
4.2
Test results
The attention of the test results has been focused on the fuel consumption and the
emission outcomes. Most of the results are reported along the indicated power, since EVE
has very high mechanical losses due to its construction and figures along the output
power may provide unrealistic values.
Figure 5 presents the measured fuel consumption and NOx emission. One can see that the
only fuel conversion brings advantages both in fuel economy and in the NOx production:
in fact comparing the two cases with reference timing both ISFC and ISNOx have
become lower running with HVO. Advancing the IVC to 50 CAD BBDC, the NOx have
dropped in both the loads, especially at 70 kW load; on the other hand the fuel
consumption increases by 3-4 g/kWh. Using narrower scavenging has a benefit effect,
since the NOx reduction has been by 40-50% compared with the reference value run with
diesel. Also the fuel consumption has been lower, although the most advantageous
configuration has been negative scavenging (Mil50 Sc-30) at the medium load and zero
scavenging (Mil50 Sc0) at low load. Increasing the Miller timing (Mil 70) the outcomes
are very different with different scavenging periods. Positive scavenging (Mil70 Sc60) –
i.e. the mere application of the Miller technique- has resulted in very high NOx value at
25 kW load, whilst at 70 kW load there has not been further reduction compared with
other Miller timing (Mil50 Sc60). Reducing the scavenging period helps very much in the
NOx reduction: with negative scavenging (Mil70 Sc-30) the lowest NOx outcomes have
been measured in both the loads. The fuel consumption has risen at both the loads as the
scavenging has become narrower, and the increase has been 3-4 g/kWh compared with
the reference case.
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70 kW - ISFC
10
245
25 kW - ISFC
9
70 kW - ISNOx
235
25 kW - ISNOx
8
225
215
6
205
5
195
4
ISFC (g/kWh)
ISNOx (g/kWh)
7
185
3
175
2
165
1
0
155
Ref Diesel
Ref
Sc 60
Sc 0
Mil15 Sc60
Sc -30
Sc 60
Mil50
Sc 0
Sc -30
Mil70
Figure 5: NOx emission and fuel consumption
Figure 6 reports the NOx and the soot measured results. One can see that the soot values
have been almost negligible in many tested configuration. The use of HVO reduces the
soot and (Mil15 Sc60) and also with very extreme Miller rate the registered FSN is close
to 0. The highest soot values have been measured at 25 kW load when very negative
scavenging is applied (Mil50 Sc-30 and Mil70 Sc-30), but those results (FSN=0.4) have
not been such high as create visible effect from the exhaust chimney.
70 kW - FSN
10
0.8
25 kW - FSN
9
70 kW - ISNOx
0.7
25 kW - ISNOx
8
0.6
0.5
6
5
0.4
4
FSN
ISNOx (g/kWh)
7
0.3
3
0.2
2
0.1
1
0
0
Ref Diesel
Ref
Sc 60
Mil15 Sc60
Sc 0
Sc -30
Sc 60
Mil50
Sc 0
Sc -30
Mil70
Figure 6: NOx emission and soot values
5. Conclusions
An extensive study with HVO has been carried with EVE, a single-cylinder large-bore
medium-speed diesel engine used for research purposes. Two loads, one medium (70
kW) and one low load (25 kW) and different valve timings, modifying IVC and
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scavenging periods, have been tested. A starting configuration close to a standard timing
has been chosen as reference.
HVO has been compared with diesel EN590 to see the influence on the performance
outcomes. The use of HVO brings good advantages in terms of fuel consumption and of
NOx. The ISFC reduction is up to 7% and the NOx reduction up to 10%.
Further reduction has been achieved using advanced Miller rate (Mil 50). However, with
the most advanced Miller rate (Mil 70) there has not been any further benefit in terms of
NOx: at low load, the value has been very high, even greater than the reference
configuration.
The advanced Miller timing has given good results combined with negative scavenging
period (Mil50 Sc-30 and Mil70 Sc-30): the lowest NOx has been 3.5 g/kWh, obtained at
25 kW load with Mil70 Sc-30; this value is 50% lower than the reference with diesel and
almost 1/3 of the NOx figure achieved with positive scavenging (Mil70 Sc60).
The fuel consumption has slightly risen with advanced Miller and negative scavenging
(Mil70 Sc-30): the increase of ISFC is by 3-4% at medium load and slightly lower at low
load.
One of the main benefits of HVO is that it is possible to run with different conditions of
temperature and some exhaust gas fraction in the combustion chamber and the soot
produced is remarkably low: in fact the highest value has been FSN=0.4, obtained with
5% exhaust gas fraction in the charge.
6. References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
International Standard, ISO 8178-4. Reciprocating Internal Combustion Engines –
Exhaust Emission Measurement, Genève 1996
Mikkonen, S. NExBTL, Second Generation Biodiesel, Handsout Neste Oil Oyj,
3007
Kuronen, M., Sepponen, M., Aakko, P., Murtonen, T. Hydrotreated Vegetable Oil
as Fuel for Heavy Duty Diesel Engines, SAE Technical Paper 2007-01-4031,
2007
Aatola, H, Larmi, M. Sarjovaara, T, Mikkonen, S. Hydrotreated Vegetable Oil
(HVO) as a Renewable Diesel Fuel: Trade-off between NOx, Particulate
Emission, and Fuel Consumption of a Heavy Duty Engine, SAE Technical Paper
2008-01-2500, 2008
Pflaum H., Hofmann P., Geringer, B., Weissel, W. Potential of Hydrogenated
Vegetable Oil (HVO) in a Modern Diesel Engine, SAE Technical Paper 2010-320081, 2010
Miller, R., Lieberherr, H.U. The Miller Supercharging System for Diesel and Gas
Engines Operating Characteristics, CIMAC Proceedings, Zurich 1957
Imperato, M., Sarjovaara, T., Antila, E., Kaario, O., Larmi, M., Kallio, I.,
Isaksson, S. NOx Reduction in a Medium-Speed Single Cylinder Diesel Engine
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The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
using Miller Cycle with Very Advanced Valve Timing, SAE Technical Paper
2009-01-0112, 2009
Heywood, J. B. Internal Combustion Engine Fundamentals, McGraw-Hill series
in mechanical engineering, p.930. ISBN 0-07-100499-8, Singapore, 1988
Edwards, S.P., Frankel, G.R., Wirbeleit, F., Raab, A. The Potential of a Combined
Miller Cycle and Internal EGR Engine for Future Heavy Duty Truck
Applications, SAE Paper Technical Paper 980180, 1998
Millo, F., Mallamo, F., Arnone, L. Bonanni, M., Franceschini, D. Analysis of
Different Internal EGR Solutions for Small Diesel Engines, SAE Paper Technical
Paper 2007-01-0128, 2007
Osada, H., Aoyagi, Y., Shimada, K., Goto, Y., Suzuku, H. Reduction of NOx and
PM for a Heavy Duty Diesel Using 50% EGR Rate in Single Cylinder Engine,
SAE Paper Technical Paper 2010-01-1120, 2010
Kallio, I., Rantanen, P., Imperato, M., Antila, E., Sarjovaara, T., Larmi, M.,
Huhtala, K., Liljenfeldt, G. The Design and Operation of the Fully Controllable
Medium-Speed Research Engine EVE, CIMAC Paper 163, 2007
Herranen, M., Huhtala, K., Vilenius, M., Liljenfeldt, G. The Electro-Hydraulic
Valve Actuation for Medium Speed Diesel Engines – Development Steps with
Simulation and Measurements, SAE Technical Paper 2007-01-1289, 2007
Tilli, A., Imperato, M., Aakko-Saksa, P., Larmi, M., Sarjovaara, T., Honkanen,
M. High Cetane Number Paraffinic Diesel Fuels and Emission Reduction in
Engine Combustion, CIMAC Paper 26, 2010
Nylund, N.O., Aakko-Saksa, P., Sipil, K. Status and Outlook for Biofuels, Other
Alternative Fuels and New Vehicles, VTT Research Notes 2426, Helsinki, 2008
7. ACKNOWLEDGEMENTS
The authors want to acknowledge to ReFuel project, funded by TEKES (Finnish
Technology Agency), Agency), Wärtsilä Finland Oy, Neste Renewable Fuels Oy, Aker
Arctic Technology Oy and Agco Sisu Power Oy. Special thanks to Neste Renewable
Fuels Oy for providing the fuel needed to accomplish the test runs.
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