Article citation information:
Puškár,
M., Lavčák, M., Šoltésová, M., Kopas, M. Analysis
of advanced technology for combustion of homogeneous fuel mixture. Scientific Journal of Silesian University of
Technology. Series Transport. 2022, 117,
211-220. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2022.117.14.
Michal PUŠKÁR[1], Matúš
LAVČÁK[2], Marieta
ŠOLTÉSOVÁ[3], Melichar KOPAS[4]
ANALYSIS OF ADVANCED TECHNOLOGY FOR COMBUSTION OF HOMOGENEOUS FUEL
MIXTURE
Summary. The most
serious problems to overcome for a successful operation of the HCCI engine are
control of the combustion phase, limited operational range, cold start of
engine and high noise level during engine operation. This study aims at
describing the engine power output characteristics and emission characteristics
of HCCI engines under different testing conditions and the various challenges
associated with these engines. Furthermore, this study holds a potential guide
for overcoming these challenges and improvement of the engine power output as
well as the emission characteristics. Thus, it is possible to say, concerning
the performed investigation work, that HCCI combustion can be applied in
existing conventional engines after their modifications. The most significant
result of the HCCI process application is the reduction of NOx emissions and
soot emissions, keeping almost the same engine power output as the conventional
combustion process.
Keywords: analysis,
advanced technology, combustion, fuel mixture
1.
INTRODUCTION
Currently, there is an important modification of a
trend, which is presented in the form of pressure, to reduce engine emissions
significantly. However, this task requires
long-term, sustainable, and effective solutions, such as the advanced
combustion technology known as Homogeneous Charge Compression Ignition (HCCI). The HCCI technology is a combination of the gasoline
engine with spark ignition and the diesel engine with compression ignition,
using the best characteristic feature of both ignition systems. There is applied gasoline as a fuel, however, with a
higher efficiency, which is typical for the diesel engine. Combustion of the homogenous charge runs simultaneously in
the whole piston combustion area, that is, the charge is combusted almost
completely, without a rest. Exploration of gasoline
using HCCI technology is very efficient, as it offers a significant reduction
in fuel consumption and exhaust gas emissions, whereas NOx emissions are almost
negligible. However, the application of HCCI technology is connected
with some basic problems; for example, there is a high level of pressure values
during compression, thermal loading during combustion and complicated control
of the self-ignition process. It is necessary
and purposeful to inform the professional community about recent significant
advances reached in the area of HCCI combustion technology. This article is
focused on the power output characteristics and emission characteristics of
HCCI engines compared to conventional engines. Both categories of engines are
discussed in detail in different sections. Finally, the
results obtained from the individual investigations are summarized in tabular
form to present their comparison understandably. In addition,
appropriate solutions were proposed for specific challenges concerning HCCI
engines.
2. ENGINE POWER OUTPUT
The power output of an engine is a relevant parameter
that is important for the acceptability of the engine. The
engine power output is significantly influenced by the following factors: fuel
properties, fuel injection pressure, valve timing, fuel-air mixture, injected
amount of fuel, etc. This part of the article provides
a practical comparison of performance between HCCI engines and conventional
engines that use different kinds of fuel, taking into consideration the
cylinder pressures, Heat Release Rate (HRR), Specific Fuel Consumption (SFC),
thermal efficiency, exhaust gas temperatures, and ignition delay. The comparative results of each investigation, relating to
the performance characteristics of HCCI engines (using different fuels) and
conventional engines, are summarized in Table 1.
Tab. 1
The comparative results
|
Type of testing engine |
Conditions and fuel |
Increasing parameters |
Decreasing parameters |
|
4-stroke,1-cylinder,
air-cooled, compression ignition, direct-injection, compression ratio:
17.5:1, power: 4.4 kW, speed: 1500 rpm |
Constant
speed, different load, Diesel |
HC,
CO, NOx, Smoke |
Thermal
efficiency, Cylinder pressure |
|
4-stroke,1-cylinder,
naturally aspirated, compression ignition, direct-injection, compression
ratio: 18.5:1, speed: 1800 rpm |
Constant
speed, different load, Diesel |
HC,
CO, Soot, Cylinder
pressure, HRR |
Thermal
efficiency, NOx |
|
4-stroke,1-cylinder,
water-cooled, naturally aspirated, compression ignition, compression ratio:
14.8:1 |
Variable
speed, different percentages of EGR, Diesel |
HC,
CO |
Cylinder
pressure, SFC, NOx, Smoke |
|
4-stroke,1-cylinder,
compression ignition, compression ratio: 19:1, power: 11 kW, speed: 3000 rpm |
Variable
speed, different loads, Diesel |
SFC,
HC, CO |
NOx,
Smoke |
|
4-stroke,
2-cylinder, air-cooled, compression ignition, direct-injection, compression
ratio: 16.5:1, power: 5.85 kW, speed: 1500 rpm |
Constant
speed, different loads, different percentages of EGR, Biodiesel–diesel
blend |
Cylinder
pressure, HC, CO, Smoke |
Thermal
efficiency, SFC, HRR, NOx |
|
4-stroke,4-cylinder,
compression ignition, direct-injection, compression ratio: 16.5:1 |
Constant
speed, different loads, Biodiesel–diesel blend |
SFC,
Thermal efficiency, CO |
NOx,
HC |
|
4-stroke,1-cylinder,
compression ignition, direct-injection, compression ratio: 17.5:1, power: 4.4
kW, speed: 1500 rpm |
Constant
speed, different loads, different percentages of EGR, Biodiesel–diesel
blend |
Cylinder
pressure, HC, CO |
HRR,
NOx, Smoke |
|
4-stroke,1-cylinder,
naturally aspirated, compression ignition, direct-injection, compression
ratio: 16.1:1, power: 62 kW, speed: 1800 rpm |
Variable
speed, different loads, Gasoline |
SFC,
Thermal efficiency, Cylinder pressure, HRR, HC, CO |
NOx |
|
4-stroke,1-cylinder,
water-cooled, naturally aspirated, compression ignition, direct-injection,
compression ratio: 17:1 |
Variable
speed, different loads, Gasoline-Alcohol |
SFC,
HRR, HC, CO |
Cylinder
pressure, NOx |
2.1. Pressure in engine cylinder
Many researchers reported higher or lower cylinder
pressures in HCCI engines in comparison to conventional engines
[1-12]. For example, there were investigations on the
experimental characteristics of combustion, power output and emissions for HCCI
engines using different fuel mixtures and the obtained results were compared
with conventional engines. A two-cylinder,
four-stroke and air-cooled diesel engine was modified for the HCCI operation,
applying different percentages of the Exhaust Gas Recirculation (EGR), namely
the values 0, 15 and 30%. It was monitored that the
cylinder pressure was higher in the HCCI engines than in the conventional
engines. Several research works mentioned herein have
also used hydrogen as fuel to investigate the combustion characteristics of the
HCCI engine.
Higher pressure values were recorded in the engine cylinder, and using CO2
as an additive, kept the pressure in the cylinder at an adequate level. Several researchers, using different techniques, have found that
the values of cylinder pressures in HCCI engines are lower than in conventional
engines. There were also realized experiments to apply the
technology of internal and external exhaust gas recirculation for reduction of
the cylinder pressure. The recirculation of cooled
exhaust gases was very helpful in this case.
2.2. Heat release rate (HRR)
It is possible to state for the HCCI engine regime
that HRR during this kind of combustion is completely different from CI and SI
combustion [8, 9]. Most of the analyses
align with the works [2-12], which found that HRR is higher in HCCI engines
than in conventional engines.
Formation of a proper fuel-air mixture and suitable
self-ignition chemistry are critical factors for the reduction of HRR in HCCI
engines [4]. In addition, HRR in HCCI
engines is also influenced by the equivalence ratio of the fuel-air mixture and
the engine speed [13-15]. Careful control of the
HCCI process can ensure a suitable HRR in HCCI engines.
3. PROBLEMS OF HCCI COMBUSTION
3.1. Control of combustion phase
Although HCCI engines have many advantages compared to conventional
engines in terms of thermal efficiency and NOx emissions, they still have some
serious disadvantages. One of which is the problem with the timing of
ignition, which is a significant challenge, as it eventually affects engine
power output and efficiency. Unlike SI and CI
engines, HCCI engines do not have any direct mechanism for control of the
combustion beginning. Therefore, the HCCI process fully depends on the
self-ignition of the fuel-air mixture. According to [16,
17], this self-ignition is influenced by several factors. The most
important of them are: the chemical and thermodynamic properties of the fuel
self-ignition, burning time, temperature of the cylinder walls, concentration
of reactants, degree of fuel mixture homogeneity, suction temperature,
compression ratio, amount of EGR, engine speed, convective heat transfer to the
engine and others. Higher values of suction
temperatures and suction pressures cause the time advance of ignition moment
due to faster chemical kinetics [18-20]. A lower
specific heat due to higher equivalence ratios leads to ignition temperature,
assuming higher compression heating. However, in
real operation and for both types of applied fuels, higher equivalence ratios
increase the residual temperatures on the cylinder walls, influencing
combustion timing [20]. Utilization of EGR has
different effects on combustion timing in dependence on the EGR application
technique. Application of EGR with cooling causes a
delay in the ignition timing, after which the residual gases remain circulating
as non-reactive diluents in the fuel mixture inside the cylinder [5-7]. Internal EGR using, for example, the Negative Valve Overlap
(NVO), can achieve an earlier ignition timing given the fact that the
temperature of residual gases is much higher [2]. Most researchers opine that the HCCI process
is governed by chemical kinetics
[7]. Failure in the control of combustion timing
can also cause high levels of HC and CO emissions because these emissions
depend on ignition timing [6].
3.2. High level of engine noise
Another serious challenge for HCCI engines is the high
noise level of their operations, which is caused by the rapid increase of
pressure in the engine cylinder at high operational loads. During the compression stroke, instantaneous heat is
released by the self-ignition of the homogeneous fuel-air mixture, creating
high pressure values [7]. Failure in the control of this rapid heat
release, which is the main source of high pressure, can damage the engine [8]. A higher value of the heat release rate means not only a higher
value of pressure increase, which can lead to mechanical damage but also a higher
temperature of the cylinder wall and piston surface. These temperatures are high enough
to cause physical damage to the material. Also, high local
pressure peaks can damage the engine and the cylinder head gasket due to high
pressure. Pressure fluctuations can result in serious damage for two
reasons. The first reason is that the rate of
pressure increase in each oscillation cycle can physically damage the engine. And the
second reason is that pressure fluctuations may occur at the resonance
frequency of the engine cylinder. These excitations
due to oscillating fluctuations can also lead to physical damage of the engine.
The acceptable increase of the pressure limit for the noise is ≈ 8
bar / CA [9].
3.3. Engine operational range
Another obstacle to the successful commercialization
of HCCI engines and their establishment on the market is their limited
operational range. It is imperative
to extend this range for maximization of the benefits HCCI engines offer. However, it is necessary to control the ignition timing in the
extended operational area [9].
The self-ignition properties of the fuel and geometry
of the engine significantly influence the engine’s operational range
[2]. It is difficult to achieve self-ignition for
extremely low engine load, so there is a lower load limit for the HCCI
operation. In addition, the HCCI operation is also limited for higher
engine load [5] due to the knocking phenomenon. HCCI engines have
remarkably low levels of NOx and PM emissions; however, on the other hand, the
emissions values of CO and unburned hydrocarbon are increased because the
engine operational area is limited.
3.4. Cold start of engine
In connection with
the above-mentioned problems, the conditions required for a cold start of the
HCCI engine can cause serious complications. During the cold start of the
engine, the temperature of the compressed gas in the HCCl engine is low, as the
mixture is not pre-heated in the intake pipe, and the compressed mixture is
cooled rapidly due to the transfer of heat to the walls of the cold combustion
chamber. The low levels of temperatures of the
low-pressure mixture complicate the cold start of the HCCI engine if some
compensation mechanism is not applied.
A stable HCCI operation often primarily relies on
stable (and preferably high) suction temperatures. Therefore,
the cold start is quite challenging [7], and the initial ignition is
extraordinarily demanding without the help of thermal inertia. Possibility of a reliable cold start is vital for the high
and low operational load of the HCCI engines to utilize all their
benefits.
3.5. Preparation of homogeneous mixture
Efficient preparation of the fuel-air mixture is central to reducing HC
and PM emissions and achieving high fuel efficiency [2]. The fuel condensed on the surface of the combustion
chamber is a negative phenomenon regarding HC emissions, even for fuels with
moderate evaporability, such as petrol [2]. For fuels with low
evaporability, such as diesel fuel, it is very difficult to prepare a
homogeneous fuel-air mixture. The available time to
prepare a homogeneous fuel-air mixture in the combustion chamber is small due
to the short time interval of the thermodynamic cycle [1].
4. PROPOSED SOLUTIONS TO PROBLEMS
4.1. Control of engine noise
One of the many suitable solutions for controlling
ignition timing is changing the mixture temperature. The change in
the fuel-air mixture temperature can be achieved in several ways. The main possibilities are: Variable Valve Timing (VVT)
method, Variable Compression Ratio (VCR) method [6-9], method of variable EGR
[5], capturing of the residual exhaust gases [3-5], control of the injection
timing [10, 17], modulation of the suction temperature [5], water injection
[6], and changing of the coolant temperature [7]. Another possible
solution to ignition timing proposed by many researchers is the control of the
ignition timing using a change in the mixture reactivity. Also, this can
be achieved in many ways, for example, by the modulation of two or more fuels,
fuel stratification [4], the use of fuel additives and regulation [9].
4.2. Engine operational range
There were performed several measurements aimed at
enlarging the engine’s operational range at high and low engine loads. A possible solution for the extension of the high load
area can be increasing the air intake amount [11] using the residual gas
capture method with cooled EGR [1] or by employing the two-stroke operation in
the engine equipped with a possibility to switch between the two-stroke and
four-stroke operational regime [20]. The low load
area can be extended using the spark or the reduced coolant temperature.
4.3. Cold start of engine
Various mechanisms
were proposed to overcome the problem with the cold start of the engine. The most suggested solution is to start the engine in the
conventional mode and then switch the engine into the HCCI mode after a short
warm-up. Other proposed solutions are: the application of glow-plugs,
application of other kinds of fuel or fuel additive, increase of the
compression ratio based on the Variable Compression Ratio (VVR) method,
utilization of the VVT and also use of the Spark Assisted Compression Ignition
(SACI) [6]. Although the proposed solutions seem
suitable and feasible, it is necessary to perform another extensive research
and development focused on verifying these concepts and preparing them for the
production of engines [7].
4.4. Preparation of homogeneous mixture
Several successful
strategies were developed and determined as the solution to the problem of
creating a homogeneous fuel-air mixture. Low
Temperature Combustion (LTC) was already applied in various combustion
aggregates, for example, in internal combustion engines and gas turbines,
mainly to reduce NOx emissions. Classification of
LTC relating to strategies focused on the preparation of homogeneous mixture is
presented in Figure 1 [2]. Other suitable steps
intended for the solution of the given task are: fuel injection at
high-turbulent intake of air, application of highly evaporative fuels [8, 9]
and early fuel injection into the engine cylinder using sophisticated fuel
injectors [13].
Fig. 1. Low temperature combustion
5. CONCLUSION
·
Chemical kinetics plays a dominant role in
the HCCI combustion process.
·
For HCCI combustion, the amount of NOx and
PM emissions is negligible; this is similar to the trend of HC and CO emissions
compared to the conventional combustion process.
Acknowledgments
This work was supported
by the Slovak Research and Development Agency under Contract no. APVV-19-0328.
The article was written
within the framework of Grant Projects: VEGA 1/0318/21 “Research and
development of innovations for more efficient utilization of renewable energy
sources and reduction of the carbon footprint of vehicles” and KEGA
006TUKE-4/2020 “Implementation of Knowledge from Research Focused on
Reduction of Motor Vehicle Emissions into the Educational Process.”
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Received 20.06.2022; accepted in
revised form 02.09.2022
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[1] Faculty of Mechanical
Engineering, Technical university of Košice, Letná 9, 04001
Košice, Slovakia. Email: michal.puskar@tuke.sk. ORCID:
https://orcid.org/0000-0001-6042-8779
[2] Faculty of Mechanical
Engineering, Technical university of Košice, Letná 9, 04001
Košice, Slovakia. Email: matus.lavcak@tuke.sk. ORCID:
https://orcid.org/0000-0002-5632-8887
[3] Faculty of Mining,
Ecology, Process Control and Geotechnology, TU Košice, Park
Komenského 19, 040 01 Košice, Slovakia. Email: marieta.soltesova@tuke.sk.
ORCID: https://orcid.org/0000-0003-3402-3879
[4] Faculty of Mechanical Engineering, Technical university of Košice, Letná 9, 04001 Košice, Slovakia. Email: melichar.kopas@tuke.sk. ORCID: https://orcid.org/0000-0003-4013-0718