Effects of H2 addition on combustion and exhaust emissions in a diesel engine.pdf

paper

1. Effects of H
2addition on combustion and exhaust emissions in a diesel
engine
Zhenzhong Yang
⇑
, Chaoyang Chu, Lijun Wang, Yan Huang
School of Mechanical Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450011, China
highlights
⇑Investigate the effect of H2addition on the performance of the 370 diesel.
⇑Establish the numerical model of the engine on the basis of the test.
⇑Extend the researching range of the effect of H2addition on performance by the model.
⇑Provide a method to study the effect of H2addition on diesel engine.
⇑Provide a method how to get a best ratio of H2addition.
article info
Article history:
Received 12 April 2014
Received in revised form 17 August 2014
Accepted 21 August 2014
Available online 6 September 2014
Keywords: Diesel engine The addition of H
2
Combustion characteristics Emission characteristics
Numerical simulation
abstract
The effect of the addition of H
2on combustion and emission characteristics of Cummins ISM370 diesel
engine was investigated. The cylinder pressure and heat release rate were tested under 70% load with
the addition of 6% H
2to the engine. On this basis, AVL FIRE software was used to construct numerical
model of the engine at 70% load with the addition of 6% H
2
. The simulation results and experimental
results were compared, and the results gave a good consistency. Whereby, the numerical model of Cum-
mins ISM370 engine was verified. Moreover, simulated the pressure, heat release rate, NO emissions and
particulate matter (PM) emissions at 70% load with the addition of H
2
from 6% to 20%, then analyzed the
effect of the addition of different H
2
on combustion and emission characteristics of the engine. The results
showed that the cylinder pressure and heat release rate first increased then decreased with the enhance-
ment of H
2addition, NO emissions increased, while PM emissions decreased. Both the pressure and heat
release rate reached the maximum value at the addition of 17% H
2
. Thus, this paper provided a method to
investigate the effect of the addition of H
2
on the performance of diesel engine and how to get a best ratio
of H
2
addition.
⇑2014 Elsevier Ltd. All rights reserved.
1. Introduction
Diesel engine has the advantages of high thermal efficiency,
strong power performance, good fuel economy and reliability,
which make it widely used in the field of engineering construction.
However, due to its diffusion combustion mode, NO
x
and PM emis-
sions are very high, and there is a trade-off between NO
x
emissions
and PM emissions, which leads to the difficulty to reduce the two
kinds of emissions at the same time. Hydrogen has long been rec-
ognized as a carbon-free fuel having excellent combustion charac-
teristics, such as its high calorific value, good physical and
chemical properties, environmental benefits and renewability.
These features make H
2
an excellent fuel for internal combustion
engine to potentially meet the ever increasingly stringent environ-
mental controls of exhaust emissions. The past research to H
2as
fuel has focused mainly on its application in S.I. engines utilizing
its fast flame propagation properties and also fuel cells. For exam-
ple, Ma et al.[1]investigated performance and emission character-
istics of a turbocharged spark-ignition hydrogen-enriched
compressed natural gas engine under wide open throttle operating
conditions. Ji et al.[2]examined how to enhance the performance
of a spark-ignition methanol engine with hydrogen addition. And Ji
and Wang[3]looked into combustion and emissions performance
of a hybrid hydrogen–gasoline engine at idle and lean conditions.
Nemati et al.[4]did numerical research of the effect of injection
timing under various equivalence ratios on energy and energy
terms in a direct injection SI hydrogen fueled engine.
There is increasing recent interest to explore the possibility of
using H
2
as supplement fuel in heavy-duty diesel engines[5–8].
http://dx.doi.org/10.1016/j.fuel.2014.08.057
0016-2361/⇑2014 Elsevier Ltd. All rights reserved.
⇑Corresponding author.
E-mail address:[email protected](Z. Yang).
Fuel 139 (2015) 190–197
Contents lists available atScienceDirect
Fuel
journal homepage: www.else vier.com/locate/fuel

2. In order to reduce the exhaust emissions of diesel engine, many
researchers[9–11]have proposed that adding H
2
in diesel engine
can enhance the mixture formation and improve the combustion
process in cylinder. In hydrogen–diesel dual fuel engine, lean
hydrogen–air mixture is ignited by multiple ignition points formed
during diesel combustion. And rapid propagation of H
2
flame can
improve diesel combustion process and achieve the goal of reduc-
ing emissions.
This paper researched the combustion and emissions charac-
teristics on Cummins ISM370 diesel engine with the addition of
H
2
. The research was carried out under 70% load with 6% H 2
concentration in the fresh intake mixture (H
2/(H
2+ Air), vol.%),
the pressure and heat release rate of the engine were tested.
Moreover, AVL FIRE software was used to construct numerical
model of this engine under 70% load with the addition of 6%
H
2
. And the three-dimensional numerical simulation was per-
formed. Further, the simulation results were compared with the
experimental results and the numerical model of Cummins
ISM370 engine was verified. On this basis, the simulations of,
such as the cylinder pressure, heat release rate, NO emissions
(the NO
x
emissions of diesel engine have many components,
among which NO accounts for a large proportion and the AVL-
FIRE software has a accurate calculation model for NO emissions,
so this paper calculates NO instead of NO
x
) and PM emissions,
have been carried out with H
2
concentration in the fresh intake
mixture (H
2
/(H2+ Air), vol.%) varied from 0% to 20%. And the
effect of the addition of different H
2
on the combustion process
and emissions characteristics of the diesel engine has been
analyzed. Considering the engine power, fuel economy and
emissions, the optimal proportion of H
2
addition to the Cummins
ISM370 diesel engine was analyzed at medium to high load,
which could provide reference for further experimental
research.
2. Experimental research of Cummins ISM370 diesel engine
with H
2addition
The experiment was completed by Hailin Li’s team[11]in the
United States of West Virginia University (the author did a visit-
ing research in the United States of West Virginia University in
2010). A Cummins ISM370 engine equipped with a hydrogen sup-
ply system was used for the combustion and emission testing.
The Cummins ISM370 was a turbocharged, inline 6 cylinders
heavy-duty diesel engine without an EGR system, whose rated
torque was 1830 Nm at 1200 rpm and rated power was
370 bhp at 2100 rpm. The structural parameters of the engine
were as follows: its compression ratio was 16.1, capacity was
10.8 L, bore and stroke were 125 mm and 147 mm. The test
engine was mounted to a 404.5 W (550 horsepower) General
Electric DC dynamometer, which was used to absorb engine load
and control engine speed. During the test, the engine load was
varied from 10% to 100% with the addition of 0–6% H
2
into the
intake mixture.
3. Numerical model
The three-dimensional numerical simulation of the hydrogen–
diesel dual fuel engine was mainly composed of three parts: using
single-phase fluid mechanics module to simulate the compression
process of hydrogen–air; using two phase fluid dynamics module
to simulate the spray and mixture process of diesel; using combus-
tion chemistry module to simulate the combustion process of dual
fuel. Therefore, it needs to establish the corresponding mathemat-
ical models to describe the process of compression, spray and
combustion.
3.1. Mathematical model
The control equations included the three basic law of conserva-
tion, the equation of continuity and turbulence model equations,
which were used to describe relationship of mass, momentum
and energy. Some specific mathematical models used in this paper
as shown inTable 1.
3.2. Physical model
Due to the symmetrical structure of Cummins ISM370 diesel
engine combustion chamber and nozzle holes, FIRE ESE-Diesel
module was chosen to divide the computational grid to ensure that
the calculation results were accurate. Spray nozzle had four holes,
so the final mesh was only a quarter of the whole combustion
chamber, and it was all hexahedral grids, which had a high quality.
3.3. Setting of major parameters
In order to highlight the focus of research, try to fit the actual
situation, and not make the simulation overly complicate, this
paper mainly investigated the effect of H
2
addition on the mixture
formation and combustion process in cylinder without considering
the effect of the intake and exhaust process. So setting calculation
range from 570CA (intake valve closed) to the end of 810CA
(exhaust valve open). Initial temperature and pressure in cylinder
were determined by combining test data with relevant experience.
In this paper, the boundary conditions were as follows: the piston
temperature was 545 K, cylinder liner temperature was 415 K, cyl-
inder head temperature was 515 K.
4. Calculation scheme
4.1. Definition of ratio of H
2addition
The ratio of H
2addition was defined as the volume of H
2divided
by the volume of H
2
-air in intake pipe, but taking into account the
fact that the mesh file does not contain the intake and exhaust pro-
cess, so in the parameters setting, the ratio of H
2
addition can be
defined as the volume fraction of H
2
in H
2–air when intake valve
was closed. And how to set the amount of H
2
addition in the
numerical simulation, this article assumed that the fresh air into
the cylinder were composed of O
2
and N2, their volume fraction
were 20.900% and 79.100% respectively, then regard H
2
as a com-
ponent added to air proportionally. The mass fraction of H
2
in
the air can be concluded by formula(1):
H2%¼
q
H
2
Va%
q
H
2
Va%þ0:209 q
O
2
ð1a%ÞVþ0:791 q
N
2
ð1a%ÞV
ð1Þ
Here:q
H
2
;q
O
2
;q
N
2
were density of H2,O2and N2, which can be
obtained from gas state equations;
Vwas for cylinder volume when intake valve closed, which can
be obtained from the movement rule of crank connecting rod
mechanism;
a% was for the ratio of H
2
addition;
The mass fraction of O
2and N
2can be concluded in the same
way.
4.2. Definition of diesel fuel injection quantity in a cycle with the
addition of H
2
In numerical simulation, in order to keep the engine running at
70% load and 1200 rpm operation, the diesel fuel injection quantity
in a cycle should be adjusted with H
2
addition at the same time,
Z. Yang et al. / Fuel 139 (2015) 190–197 191

3. that was to say, the effective gross calorific value of H2and diesel
should remain the same. Assumed that H
2
inside the cylinder can
burn completely, the diesel fuel injection quantity in a cycle can
be concluded by formulas(2) and (3):
Pe/Pmen/g
bg
et
n ð2Þ
Here:P
e,P
mewere for effective power and mean effective
pressure;
g
b
was for the diesel fuel injection quantity in a cycle;
getwas for the brake thermal efficiency;
nwas for speed;
Q¼Q
DþQ
H
2
ð3Þ
Here:Qwas for the effective gross calorific value which was
required when the engine running at 1200 rpm and 70% load
operation, andQcan be calculated byg
b
in formula(2);
Q
H2
was for the quantity of heat released by H
2, which can be
calculated by m
H2
in formula(1);
Q
D
was for the quantity of heat released by diesel, which can
calculate the diesel fuel injection quantity in a cycle under dif-
ferent ratio of H
2
addition;
4.3. Determination of calculation scheme
To examine the effect of the addition of different H2on combus-
tion and emission characteristics in the hydrogen–diesel dual fuel
engine running at a certain speed and load, and then obtain the
best ratio of H
2
addition under this condition, the operation condi-
tion at 1200 rpm and 70% load was selected. The maximum ratio of
H
2
addition was 6% during the test, so the combustion process in
two kinds of ratio of H
2
addition, 0% and 6%, were simulated to
calibrate and verify the numerical models under pure diesel fuel
and hydrogen–diesel dual fuel. When ratio of H
2
addition was
greater than 6%, in order to maintain the accuracy of simulation
calculation, some necessary parameters such as the effective ther-
mal efficiency, initial pressure, initial temperature, etc. were
assumed to be the same as they were in 6% H
2
addition. It can be
calculated from formulas(1)–(3)that the maximum ratio of H
2
addition was 20.374% when H
2
completely replaced diesel, so
when ratio of H
2
addition was greater than 6%, the calculation con-
dition was set as follows: the ratios of H
2
addition were 10%, 15%,
16%, 17%, 18%, 20% respectively at 1200 rpm speed and 70% load.
Mass fraction of H
2
,O
2and N
2in the air under each ratio of H
2
addition can be calculated by formula(1), and the results were
shown inTable 2. The cycle injection quantity of diesel fuel in
the computational domain under each ratio of H2addition can be
calculated by formulas(2) and (3), and the results were obtained
as shown inTable 3.
5. Calibration and verification of the numerical model
In order to guarantee the reliability of simulation results, the
international standard CFD Verification & Validation law which
defined by the United States AIAA (Air and Air Force Association)
was used to verify and validate the numerical model.
The comparison of simulation results and experimental results
with pure diesel and 6% H
2
addition were shown inFig. 1.
Fig. 1(a) and (b) showed the comparison of cylinder pressure
and heat release rate with pure diesel operation. As shown in
Fig. 1(a), the overall trend of the simulation curve and the experi-
mental curve matched well, the overall average errors were less
than 5%, and the start timing of combustion, the magnitude and
phase of the peak pressure were close to the experimental results.
Considering the fact that theoretical simulation did not calculate
heat transfer and thermal radiation between cylinder wall and
working medium in cylinder, the cylinder pressure curves were
in good agreement. As shown inFig. 1(b), the curves were in gen-
eral accord with each other, the simulation results were larger than
the experimental results before 735CA, then less than them after
735CA. This was mainly for the following two reasons: On one
hand, the errors caused by numerical simulation, calculation did
not take into account the heat transfer, thermal radiation, aperture
effect, and latent heat of vaporization, etc., so the simulated results
were larger in the early stage, and due to the calculation errors and
the influence of computational grid, the simulated results were
small in the later stage. On the other hand, the errors caused by
test results, there always was some errors during the experiment.
Fig. 1(c) and (d) showed the comparison of cylinder pressure
and heat release rate with 6% H
2
addition operation. The simulated
results agreed well with the experimental results as shown in
Fig. 1(c), the pressure peak phases were both at 732CA, and the
peak pressure error was 0.966%. InFig. 1(d), the curves were in
general accord with each other, but the simulation curve did not
have small peak in the early stage and its peak value was small
in the later stage. On one hand, this was due to the addition of
H
2
affected the pre-mixed combustion process. On the other hand,
because of the combustion chamber gap etc., H
2
inside cylinder did
not burn completely, and this reduced the heat release in some
ways, but H
2
was assumed to participate fully in the combustion
in simulation calculation which reduced the cycle injection quan-
tity of diesel fuel correspondingly, also reduced the heat quantity.
The results of the NO emissions obtained in simulation were
expressed in mass percentage, but the measured results were
expressed in brake specific emission. Although they can be mutu-
ally transformed, but considering the transfer and accumulation of
errors during the transformation, there would be big mistakes
between simulation results and measured results. So there was
no comparisons between the simulated results and the experimen-
tal results, similarly did not verify PM emissions.
It can be known from above results that the simulation curves
and experiment curves had almost the same change trend either
with pure diesel or the addition of 6% H
2
. Therefore, the established
numerical model was reasonable.
Table 1
Mathematical model.
Turbulence model Evaporation model Breakup model Ignition model Combustion model NO emission
k-zeta-f Multi-component WAVE ECFM-3Z Coherent flame model Extended zeldovich
Table 2
Mass fraction of H
2,O
2and N
2.
Ratio of hydrogen
addition (%)
Mass fraction of
H
2(%)
Mass fraction of
O 2(%)
Mass fraction of
N 2(%)
0 0.000 23.200 76.800
6 0.444 23.096 76.460
10 0.770 23.020 76.210
15 1.217 22.916 75.867
16 1.313 22.894 75.793
17 1.410 22.871 75.719
18 1.510 22.848 75.642
20 1.716 22.800 75.484
192 Z. Yang et al. / Fuel 139 (2015) 190–197

4. 6. Combustion and emission characteristics of the diesel engine
under different ratio of H
2addition
6.1. The effect of ratio of H
2addition on combustion characteristic of
the hydrogen–diesel dual fuel engine
In dual fuel engine, H2would be ignited by sparks which
produced by the spontaneous combustion of diesel fuel, and its
combustion process belonged to premixed combustion, the H
2
flame may affect the diffusion combustion of diesel. And what
was different from pure diesel’s premixed-diffusion combustion
was that dual fuel engine had a mix combustion process of H
2
and diesel dual fuel.
6.1.1. Cylinder pressure
Fig. 2showed the cylinder pressure curves under different ratio
of H
2
addition respectively. When ratio of H2addition was less
than 17%, the peak pressure gradually increased along with the
increase of the addition of H2at 70% load. Compared with the
addition of 6% H2, the increasement of the peak pressure under dif-
ferent ratio of H
2
addition were 7.456%, 7.437% and 12.342% in
turn, and the peak phases were slightly ahead of time. The peak
pressure, the increasement of which was 15.032% at the addition
of 17% H
2
operation, reached its maximum value. The peak pres-
sures were obviously decreased whether at the addition of 18%
or 20% H
2
. Compared to that of the addition of 17% H2, they
decreased 11.542% and 47.029% respectively. And the peak phases
lagged obviously at the addition of 18% or 20% H
2
operation.
The reasons for this phenomenon were that the more H
2would
be ignited by diesel spontaneous combustion in cylinder with the
increase of the addition of H
2
, and a large number of fast propaga-
tion of H
2
flame promoted the combustion process of hydrogen–
diesel–air, and improved the degree of perfection of combustion
process. But when the addition of H
2
continued to increase beyond
17%, the cycle diesel quantity should be reduced accordingly to
keep the engine running at a constant load. When the ratio of H
2
addition was 18%, the cycle diesel quantity reduced 83.481%
Table 3
The cycle injection quantity of diesel fuel under different ratio of H
2addition.
Parameter Ratio of hydrogen addition (%)
0 6 10 15 16 17 18 20
The cycle injection quantity of diesel fuel in the computational domain (mg) 35.634 24.021 17.337 8.981 7.310 5.639 3.968 0.626
Fig. 1.Comparison of cylinder pressure and heat release rate curves with pure diesel and 6% H
2addition.
Z. Yang et al. / Fuel 139 (2015) 190–197 193

5. compared with the addition of 6% H
2. This data became 97.394%
with the enhancement of the addition of H
2
to 20%. At the same
time, H
2
was introduced into cylinder from intake pipe, so the
more amount of H
2
meant the less amount of air, the both decrease
of diesel quantity and air volume formed the poor diesel–air mix-
ture which cannot develop enough healthy flame cores to ignite
the hydrogen–diesel–air mixture. That eventually led to the dete-
rioration of combustion process so that the peak pressures dropped
and phase lagged. And even at the addition of 20% H
2
, there maybe
no combustion in cylinder by observing its pressure curve.
6.1.2. Heat release rate and accumulated heat release
Figs. 3 and 4showed the heat release rate and accumulated heat
release under different ratio of H
2
addition. As shown inFig. 3, the
peak heat release rates increased along with the addition of H
2
until 17% operation, and the increasements were 21.258%,
95.061% and 135.755% in turn. And the peak phases lagged. The
peak heat release rate attained the maximum value at the addition
of 17% H
2
, the rate of increment was 136.212% compared to the
value at the addition of 6% H
2
. But when the addition of H2came
to 18% and 20%, the peak heat release rates fell by 34.911% and
73.567% compared to that of the 17% H
2
addition. And phases
lagged seriously. InFig. 4, when the addition of H
2
was less than
17%, the accumulated heat releases reduced along with the
enhancement of H
2
addition before 730CA, and the time to start
heat release delayed, and then the accumulated heat releases
increased after 730CA. And finally the curve reached the
maximum value at the addition of 17% H2. The accumulated heat
releases decreased and time to heat release delayed at the addition
of 18% and 20% H
2
.
This phenomenon can be explained as follows: Firstly, the addi-
tion of H
2
reduced the demand of diesel fuel, and thus formed the
poor diesel–air mixture which increased the ignition delay period,
hindered the premixed combustion of diesel and delayed the start
time of heat release, this can be verified by cylinder pressure curve
and heat release curve. Secondly, when diesel–air mixture caught
fire they produced multiple flame cores, and these cores ignited
the diesel–hydrogen–air mixture. The more the addition of H
2
,
the faster of its flame propagation speed, and the better of the com-
bustion quality in some extent. But when the addition of H
2
was
more than 17%, the volumetric efficiency decreased, and the cycle
diesel quality reduced too, those made the diesel–air mixture so
poor that sufficient flame cores cannot be provided, and then led
to the deterioration of combustion as well as the decrease of heat
release.
6.2. The effect of ratio of H
2addition on emission characteristics of the
hydrogen–diesel dual fuel engine
The addition of H2had effect on diesel premixed combustion
and diffusion combustion, which changed distribution,
development process and level of parameters such as pressure
and temperature, eventually led to the changes of NO and PM
emissions.
Fig. 2.Cylinder pressure curves under different ratio of H
2addition.
Fig. 3.Heat release rate curves under different ratio of H
2addition.
Fig. 4.Accumulated heat release curves under different ratio of H
2addition.
Fig. 5.NO emissions curves under different ratio of H
2addition.
194 Z. Yang et al. / Fuel 139 (2015) 190–197

6. Table 4
The development of NO mass fraction under different ratio of H
2addition.
6% 10% 15% 16% 17% 18% 20%
720°C
A
725°C
A
730°C
A
735°C
A
740°C
A
745°C
A
750°C
A
Table 5
The development of average temperature under different ratio of H
2addition.
6% 10% 15% 16% 17% 18% 20%
720°C
A
725°C
A
730°C
A
735°C
A
740°C
A
745°C
A
750°C
A
Z. Yang et al. / Fuel 139 (2015) 190–197 195

7. 6.2.1. NO emissions
Fig. 5showed the changes of NO emissions under different ratio
of H
2
addition. The NO emissions reached the maximum value at
the addition of 17% H
2
. When ratio of H2addition was equal to
or less than 17%, NO emissions increased with the enhancement
of H
2
addition, and theirs increase were 47.643%, 173.557%,
270.296%, 389.676% in turn. The NO emissions obviously decreased
at the addition of 18% and 20% H
2
, they decreased by 6.115% and
83.965% respectively compared to the value of 17% H
2
addition,
but NO emissions increased by 359.732% at the addition of 18%
H
2
compared to that of 6% H
2addition.
The changes of NO emissions with the addition of H2can be
analyzed from the NO formation conditions. The main factors
affecting the generation of NO emissions are the temperature,
the oxygen concentration and the duration of high temperature.
Tables 4–6respectively showed the development process of NO
mass fraction, average temperature in cylinder, and oxygen mass
fraction under different ratio of H
2
addition. As shown inFig. 5,
the formation time of NO delayed with the enhancement of H
2
addition, this can be verified byTable 4. AndTable 4also showed
that the mass fraction of NO was highest and its formation region
was largest at the addition of 6% H
2
before 730CA. However, the
mass fraction of NO and its formation region increased along with
the enhancement of H
2
addition after 730CA, and it reached its
maximum value at the addition of 17% H
2
. It was easy to see that
the temperature in-cylinder gradually decreased with the
enhancement of H
2
addition before 730CA, and although oxygen
mass fraction also reduced with the combustion proceeding, it was
still in relatively oxygen-rich state. These made temperature as a
major factor affecting NO emissions, and so NO emissions reduced.
A large amount of hydrogen–diesel–air mixture was ignited after
730CA, and the diffusion combustion of diesel fuel was promoted
on account of the fast speed of H2flame, which made the combus-
tion duration be relatively short. This would result in the reduction
of the duration of high temperature to some extent, but the tem-
perature in-cylinder was sharp rise, the oxygen mass fraction also
had a small increase, the NO emissions increased at dual role of the
temperature and oxygen concentration. When the addition of H
2
was more than 17%, the amount of diesel injected into the cylinder
should be significantly declined, and so less diesel fuel participated
in the initial premixed combustion that cannot form a better die-
sel–air mixture, and finally the combustion process worsened,
the temperature dropped. Although the oxygen concentration
increased, NO emissions still decreased.
6.2.2. PM emissionsFig. 6showed the PM emissions under different ratio of H2addi-
tion. The PM emissions drastically reduced with the enhancement
of H
2
addition, especially when the addition of H2was beyond 10%,
the decline was more sharply. The curve had three obviously peaks
at the addition of 6% H
2
, when the addition of H2reached to 10%,
the last two peaks had been significantly reduced, then when the
addition of H
2
was 15%, the curve had become bimodal and the sec-
ond peak decreased rapidly, finally when the addition of H
2
was
over 15%, the curve had only one peak.
High temperature and oxygen deficit were two primary factors
which resulted in PM formation, and here were the following three
main reasons that PM emissions reduced. Firstly, PM emissions
mainly came from the incomplete combustion of diesel fuel in
hydrogen–diesel dual fuel engine, and the amount of diesel fuel
would be reduced along with the enhancement of H
2
addition,
which radically reduced the PM emissions. It can be seen from
Table 6
The development of oxygen concentration under different ratio of H
2
addition.
6% 10% 15% 16% 17% 18% 20%
720°C
A
725°C
A
730°C
A
735°C
A
740°C
A
745°C
A
750°C
A
196 Z. Yang et al. / Fuel 139 (2015) 190–197

8. Table 3that the amount of diesel fuel was reduced by 27.826%,
62.612%, 69.568%, 76.525%, 83.481% and 97.394% in turn when
the addition of H
2
was more than 6%. Secondly, the addition of
H
2
improved the diffusion combustion process of diesel fuel. When
H
2
added into cylinder, it increased the diesel ignition delay period
and formed a more uniform diesel–air mixture to a certain extent.
So when burning started, the H
2
flame spread rapidly and
promoted the diffusion combustion process of diesel so that PM
emissions reduced. Thirdly, the rapid spread of H
2
flame acceler-
ated the oxidation of PM.
7. Conclusion
(1) The effect of the addition of H
2on combustion and emission
characteristics of the Cummins ISM370 diesel engine was
investigated. The cylinder pressure and heat release rate
were tested under 70% load with the addition of 6% H
2
to
the engine. By numerical simulation, the effects of the addi-
tion of 6%, 10%, 15%, 16%, 17%, 18% and 20% H
2
to the diesel
engine on combustion and exhaust characteristics, such as
cylinder pressure, heat release rate, accumulated heat
release, NO emissions and PM emissions were explored.
The addition of H
2
to the diesel engine had the effect on
the formation of the mixture and its combustion process.
The cylinder pressure, heat release rate and accumulated
heat release increased along with the enhancement of H
2
addition until 17% at 70% load. Combustion characteristics
of the diesel engine were best at 17% load, the cylinder peak
pressure, the peak heat release rate and accumulated heat
release all reached their maximum values. For emission
characteristics, the NO emissions and PM emissions showed
opposite changes. The NO emissions increased significantly
with the enhancement of H
2
addition, and reached their
maximum value at the addition of 17% H
2
, however, the
PM emissions dropped significantly.
(2) All things considered, the maximum ratio of the H
2
addition
of the diesel engine at high load (70% load) and rated torque
speed (1200 rpm) was 17%. Taking into account the fact that
EGR system can significantly reduce NO emissions without
causing a notable change of combustion characteristics, so,
the addition of 17% H
2
was the optimal rate if this diesel
engine rebalanced EGR system and run at medium speed
and high load.
Acknowledgements
This project was supported by National Nature Science Founda-
tion of China (51076046), and also was supported by Innovation
Scientists and Technicians Troop Construction Projects of Henan
Province (2011-39).
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Fig. 6.PM emissions curves under different ratio of H
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Z. Yang et al. / Fuel 139 (2015) 190–197 197