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Combustion and Emissions in Homogeneous Charge
Compression Ignition (HCCI) engines-A review
*1N.V.Mahesh Babu.Talupula, 2 Dr.P.Srinivasa Rao, 3 Dr.B.Sudheer Prem Kumar, 4Dr.A.P.Sathiyagnanam
*1Research Scholar, Department of Mechanical Engineering, JNTU Hyderabad,
Telangana-India. [email protected], #2Institute of Aeronautical Engineering, Dundigal, Hyderabad, Telangana, India.
[email protected], #3Department of Mechanical Engineering, JNTUH College of Engineering,
Hyderabad, Telangana, India. [email protected], [email protected], #4 Department of Mechanical Engineering, Annamalai University, Chidambaram,
Tamilnadu, India. [email protected],
Abstract
This paper describes the Homogeneous Charge Compression Engines (HCCI)
technology. The emissions from automobile engines have increased manifold since years
and need is inevitable to develop clean technology that reduces green house gases,
pollutants and to improve the quality of air. Major factors to consider for designing the
technology are high compression ratios, lean homogeneous air fuel mixture, complete and
instantaneous combustion, which lead to homogeneous charge compression ignition
(HCCI) engines. This technology leads to low Nitric Oxide (NOx) emissions, soot and
high volumetric efficiency. The technology combines both Spark Ignition (SI) and
Compression Ignition (CI) modes of combustion and characteristics of both are evident.
HCCI engines work on diesel fuel, gasoline, and most alternate fuels. This paper
discusses the recent trends on HCCI engine development and results of research on the
latest technology. The outlay of emissions in HCCI engines was discussed. The scope of
the technology are also emphasized also the developments are discussed in the paper.
Key words: HCCI engine, emissions, SI, CI, NOx , PM, CO, HC.
1. Introduction
The advent of internal combustion engines altered the face of the world to a great
extent for convenience in transport, logistics and power transmission. The extent of
emissions have been increased manifold, the depleting fossil fuels is a major problem in
global environment facing nowadays. Decreasing exhaust emissions and improving
economy of fuel of internal combustion engines are of major concerns. To pay off the
demand of fuel economy, minimize adverse environmental emissions, especially
carcinogenic NOx and greenhouse gases CO2, conservation of energy and greater thermal
efficiency, the new generation engines ought to have the following characteristics: less
fuel utilization, greater efficiency, reliability low price and low cost of usage. Inspite of
lack of direct ignition control, the HCCI technology is the best alternative to satisfy the
above requirements. The latest concept of engine which reduces NOx emissions, greater
efficiency, less fuel consumption has been proposed. Researchers have been studying the
type of engine, named Homogeneous Charge Compression Ignition (HCCI), which
utilises a wide variety of fuels such as fossil fuels, natural gas and other bio diesels from
vegetable and animal oils with minor modifications. The thermal efficiencies acquired by
HCCI engines are equivalent to those attained by high compression ratio throttle less
diesel engines likewise keeping up smoke free operation of spark ignited engines. In spite
of the fact that the calculated parameters, for example, efficiency, fuel consumption and
emissions concurred well with the experimental findings, HCCI generated more HC and
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CO than SI operation. The flexibility involved with usage of wide varieties of fuels that is
utilised in HCCI engines. The fuels range from bio-fuels, to hydrocarbon fuels and
reformed fuels. The applicability of HCCI engine technology is utilized over a wide
range of sizes of transportation engines from small motor cycle to large ship engines.
They can also be used in stationary applications such as power generation, oil and gas
production and pipe line pumping. Over the years, HCCI engines have been studied by
researchers to tackle challenges.
2. Fundamentals of HCCI engine
Automobiles play major role in global warming and air pollution. Majorly, the
oxides of Nitrogen (NOx) and smoke emitted by diesel engines is the main reason for the
air pollution and also fuel consumes more fuel. The problem of high emissions as well as
fuel consumption can be solved by Homogeneous Charge Compression Ignition Engines
[1-4]. HCCI engines produce more power in comparison with conventional diesel engines
under high load operation. The operation of HCCI engine combines both spark ignition
and compression ignition modes [5]. Homogeneous air-fuel charge is prepared by port
fuel injection system or direct in-cylinder injection system in HCCI engines [6]. HCCI
combustion involves at multiple points of the combustion chamber by the end of the
compression stroke with no flame front or diffusion flame [2]. Duration of combustion
and the beginning of Combustion in HCCI engines are controlled by exhaust gas
recirculation or by the inlet air temperature [7, 8]. Rate of combustion can be improved
using fuel additives or combustion improvers [9].
Using iso-octane, ethanol and nature gas as fuel, controlled by inlet air
temperature researchers have published papers on HCCI mode operation [2].The
temperature of the inlet air is varied at the start of combustion and the combustion
duration reduced. The compression ratio and inlet air temperature are varied in HCCI
engine by Christenson et al [10]. Using various test fuels such as pure iso- octane, n-
heptane, petrol and diesel fuels, tests were conducted. Combustion is started before the
piston reaches TDC position as the compression ratio and inlet air temperature are high.
As the combustion started in advance, heat release rate and engine power output are
increased.
The effect of charge temperature and exhaust gas re-circulation on combustion
and emission characteristics of acetylene fuelled HCCI engine were analyzed by Swami
Nathan et al [11]. The tests were conducted utilizing acetylene as fuel and the inlet air
temperature varied from 400C to 1100C from no load to full load. Also EGR was inducted
to HCCI engine, due to which, brake thermal efficiency improved, NOx and smoke
emissions were decreased. Auto ignition temperature and combustion process of
ethanol/n-heptane fuelled HCCI engine were studied and resulted in higher indicated
brake thermal efficiency than conventional diesel engine was about 50% at high load
engine operations by Lu et al [12]. Due to the high octane number of ethanol, the start of
ignition delayed. Experimental investigation resulted in high HC and CO emissions in the
exhaust, while increasing the engine load. Exhaust residual gases were used to control the
combustion of di-methyl ether and methanol fuelled HCCI engine by Mingfa et al. With
increase in EGR percentage combustion efficiency increased. It is inferred that EGR
could retard the start of combustion proximate to TDC position. But, exhaust emissions of
CO and HC were increased with EGR. Different percentages of EGR for controlling the
HCCI combustion were used by Ganesh [14] et al. Combustion of diesel fuel with
external mixture formation was studied by Ganesh. A fuel vaporizer was utilized to have
good HCCI combustion in a single cylinder air-cooled direct injection diesel engine. No
changes were made in the combustion system. A vaporized diesel fuel was mixed with air
to generate homogeneous mixture and induced into the cylinder during the intake stroke.
The cooled(300C) EGR method was adopted to control the early ignition of diesel vapour-
air mixture. Diesel vapour induction without EGR and with 10%, 20%, 30% experiments
were conducted and compared to those of conventional diesel fuel operation (DI at 23°
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BTDC and 200 bar injection pressure).Brake thermal efficiency was increased and oxides
of Nitrogen were decreased due to induction of exhaust gases into premixed fuel mixture.
Induction of EGR should be a limit , if increased the percentage of it, HCCI engines have
resulted a poor combustion efficiency and increased CO, HC and smoke emissions. This
is due to more EGR dilutes the fresh charge and increases CO2 molecules in the
combustion chamber [15]. Due to this issue on EGR induction, HCCI engine was
controlled by varying inlet charge temperature. The suction is heated by electric heater,
which is fixed in the suction pipe[16].The effect of air temperature and air-fuel ratio on
combustion and emission characteristics of HCCI engine were investigated by Maurya et
al. The test was performed on two cylinder HCCI engine using ethanol fuel. The inlet air
temperature was varied as 1200C, 1400C and 1600C. As a result, it was inferred that at
high air temperatures, the engine emitted high smoke and HC emissions due to deficiency
of air present in the combustion chamber. The effect of charge temperature and EGR on
HCCI engine fuelled with acetylene by Ramesh et al [18].
Epping et al. [19] and Christensen and Johansson[20] used iso-octane as a fuel in
HCCI engines, increased efficiency by as much as 37% applying high compression ratio
i.e. 18:1 and the emission levels are low. The efficiency and compression ratio are in the
order of CI engines. The technology can be utilized by altering either SI or CI engines
using any fuel and their combinations. Normally the air/fuel mixture quality in HCCI
engines is lean, it ignites at multiple locations and is then burnt volumetrically without
substantial flame propagation [21]. Combustion proceeds when the homogeneous fuel
mixture has attained the chemical activation energy and is completely controlled by
chemical kinetics [22] rather than spark or injection timing.
Low temperature combustion mode with diesel and biodiesel was studied by
Francisco J. et al [23] . In this paper, a methodology for HCCI combustion mode of
mixtures of biodiesel on a high swirl and EGR rate combined with late injection where
Heat Release Rate, NOx, CO,HC and soot emissions were analyzed. Due to tiis, fuel wall
impingement reduced when early injection is utilized. As EGR increased, Nox emissions
are reduced compared to conventional diesel combustion. When biodiesel percentage
increased, a small increase in NOx emissions were observed, although this is probably
related to ignition timing. Combustion, performance and emission characterization of
HCCI engine using biodiesel as a fuel using external mixture formation technique was
studied by Gajendra singh et al [24]. Effect of EGR was studied and was found to be a
very effective control in HCCI combustion.
Reduction in power output and increase in indicated specific fuel combustion
were observed as the content of biodiesel increased in the test fuel. With increase in
biodiesel content, a small increase in CO, HC and smoke emissions were observed due to
slow evaporation rate of biodiesel. A significant reduction in NOx emissions were
observed with for biodiesel blends.
Fig 1. Parameters that affect the performance of the HCCI engine
(From Sharma, T.K., Rao, G.A.P. & Murthy, K.M. Arch Computat Methods Eng
(2016) 23: 623)
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Fig 2. Heat deliverance curve for HCCI combustion of n-heptane fuel.[36]
3. HCCI combustion principle
The following are the features of HCCI /Controlled Auto Ignition (CAI) engines:
HCCI involves fuel and air mixed to form very lean homogeneous mixtures, auto
ignited and hence the combustion temperatures are low.
As the combustion temperatures are low, NOx formation is negligibly small.
Formation of NOx is lower by two orders than those from the current SI and CI
engines.
As the charge burnt is homogeneous, soot formation is very less.
Very lean mixtures are burned and hence high fuel efficiencies similar to DI
diesel engines are obtained.
Fig 3. Combustion in Gasoline, Diesel and HCCI engines (From Report of Basic Energy
Sciences workshop on Basic Research needs for Clean and Efficient combustion of 21st
Transportation fuels, 2006, DOI: 10.2172/935428)
HCCI engine concept which completely varies from other conventional concepts
like spark or compression ignition is proposed as an ultimate method of lean burn.
However, HCCI has features of two most familiar forms of combustion used in SI
engines- homogeneous charge spark ignition (gasoline engines) and CI engines: stratified
charge compression ignition (diesel engines). Fig. 3 shows the combustion modes in
Gasoline, Diesel and HCCI engines. Combustion in HCCI mode has potential to be highly
efficient and to produce low emissions. The limitation in both the combustion processes is
Heterogeneous temperature distribution resulting in local high temperatures compared to
mean bulk temperature of mixture. As in homogeneous charge spark ignition, the fuel and
oxidizer are mixed together. However, rather than using an electric discharge to ignite a
portion of the mixture, the density and temperature of the mixture are raised by
compression until the entire mixture reacts spontaneously.
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Table 1. Comparison between conventional SI engine and HCCI Engine [37]
Basis of comparison SI engine HCCI engine
Efficiency Less More
Throttle losses More No
Compression ratios Low High
Combustion duration More Less
NOx emissions Comparatively more Less
Table 2. Comparison between conventional diesel engine and HCCI engine [37]
Basis of comparison Diesel engine HCCI engine
Efficiency High Equally high
Combustion temperatures 1900–2100 K 800–1100 K
Cost Comparatively high Less
Combustion duration More Less
PM and NOx emissions More Less
4. Modes of HCCI Combustion Control
Controlling of combustion in HCCI mode engine is the challenging factor in the
development of HCCI. Successful operation of HCCI mode eliminates the challenges
faced. Major challenges are controlling auto ignition temperature mixture, control heat
release rate at high load operation, control of exhaust emissions and minimize the knock.
Combustion in HCCI can be controlled by preheating the inlet air, pressurizing inlet air,
varying compression ratio, increasing ignition pressure, varying equivalence ratio, using
ignition improver or fuel additives and exhaust gas recirculation.
Fig 4. Strategies to achieve HCCI combustion (From Sharma, T.K., Rao, G.A.P. &
Murthy, K.M. Arch Computat Methods Eng (2016) 23: 623)
4.1 Pre-Heat Inlet Air
Pre-heating the inlet air is one of the powerful techniques for control of combustion. The
inlet air is pre-heated by the heating coil, which is installed in the inlet manifold. The
temperature of inlet air is utilized to assist the fuel with being vaporized with less time
which is reduces the ignition delay and combustion starts before. The inlet air temperature
will influence the combustion process and formation of emissions. The incylinder peak
pressure can be varied with inlet air temperature, if the inlet temperature increases,
ignition delay decreases and combustion start earlier. The NOx emissions increase with
increase in air temperature. The heat release rate of HCCI mode can be varied with inlet
air temperature. The high inlet air temperature produces homogeneous charge within less
time and it reduces ignition delay of charge. Hence, combustion takes place before top
dead centre and combustion efficiency increases.
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4.2. Pressurized Intake Air
To extend the operating load range and reduced the exhaust emissions in HCCI operation
supercharging and turbo charging are used. The two operations provide high intake
pressure into the combustion chamber, increase in charge density and thereby engine
performance increases. The start of combustion (SOC) is advanced as the intake pressure
increases. This indicates the pressurized intake air improves the auto ignition of the fuel.
Increasing the intake pressure makes auto ignition to start combustion process before the
top dead centre (BTDC). Hence, supercharging increases engine efficiency.
4.3 Varying Compression Ratios
One of the parameter to control the combustion process is compression ratio. By
decreasing the compression ratio, the start of ignition timing extends, combustion occurs
after TDC. The late ignition reduces combustion efficiency and decreases heat release rate
from the charge. The lower compression ratio, HCCI engine has low peak in cylinder
pressure and temperature, and have lower NOx emission in the exhaust due to low
combustion temperature. Decreasing compression ratio from 18:1 to 16:1 was part of the
strategy used in the second generation of MK diesel engines to extend low temperature,
premixed combustion to higher load conditions. When compression ratio is reduced, the
accompanying reduction in temperature rise of the end gas prevents explosive self-
ignition from occurring.
4.4 Fuel Injection Pressure
Increase in fuel injection pressure promotes better mixing of in-cylinder charge especially
when used in combination with smaller nozzle orifice. At high fuel injection pressure,
injection speed increases leading to a high rate of air entrainment and mixing which
results in favourable spray structure and better combustion. If the fuel injection pressure
increases from 4 bar to 8 bar in the port fuel injected HCCI engine, the high injected
pressure can atomize the fuel and drizzle over the inlet air and create the homogenous
charge. The high homogeneity air-fuel mixture favour for complete combustion increases
the combustion efficiency. The well-mixed mixture reduces the ignition delay and
advance combustion happens at before TDC. The engine produces high heat release rate,
causes increase the peak in cylinder pressure and temperature.
4.5 Air-Fuel Ratio
The HCCI mode engine operates with lean air-fuel mixture under different operating
conditions. The HCCI engine combustion should be controlled by varying the ratio of air
and fuel in the mixture. The higher equivalence ratio (ε) of the charge reduces the ignition
delay and has the high flame velocity. If the value of ε = 0.421, the heat release rate
increases and combustion starts slightly advanced.
4.6 Internal and External EGR
For early injection HCCI combustion, EGR should be combined with some other
combustion control technology such as modification of fuel properties or adoption of
some other chemical approach. In the case of late injection system, EGR is typically
utilised as a NOx reduction measure with typical levels of approximately 40%. NOx is
reduced because of the lowering of flame temperature due to charge dilution and higher
heat capacity of the cylinder charge when EGR is introduced. For early injection HCCI
diesel combustion, EGR is used as a means of diluting the gas mixture in HCCI diesel
engine thereby retarding the ignition timing and reducing the combustion rate. The EGR
have been replaced the oxygen molecules with carbon-dioxide, it reduced the combustion
temperature.
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Fig 5. Limitations of HCCI engine (From Sharma, T.K., Rao, G.A.P. & Murthy, K.M.
Arch Computat Methods Eng (2016) 23: 623)
5 Emission characteristics of HCCI engines
5.1 Nitric Oxide (NOx) Emissions
Several formation mechanisms of NOx are Fenimore mechanism, Zeldovich
mechanism, fuel bound NOx, NO2 mechanism and N2O mechanisms. In Zeldovich
mechanism, due to resino nitrogen component in the fuel, NOx is not formed from the
fuel. NOx is formed in high temperature reaction, where nitrogen in the air dissociate s
into nitrogen radicals to form NO on reaction with oxygen. Part of NO is converted into
NO2 due to further reactions occulting in the combustion chamber. When combustion
temperature is below 1800K, the thermal NOx is not significant[25]. In Fenimore
mechanism, also known as prompt NOx, the NOx is promptly formed in laminar
premixed flames long before the NOx is formed by the thermal mechanism. Fenimore
mechanism explains the additional NOx produced over the Zeldovich mechanism in
hydrocarbon flames. Prompt NOx is important for hydrocarbon fuels in fuel-rich
conditions, where NOx is formed by rapid reactions of hydrocarbon radicals(CH, CH2,C2,
C2H and C) with molecular nitrogen.
In HCCI ignition, NOx emissions were exceptionally low due to low temperature
combustion and lean fuel/air mixture. Under all stable operation points, NOx emissions
were lesser than 10 ppm [26]. Concentration of nitrogen oxides is somewhat more for
auxiliary fuels injected at 250C BTDC of injection timing contrasted with other injection
timings as the carbons in the fuel burns completely. Less oxygen is inducted in the
cylinder with higher premixed ratio. Further, as the premixed ratio increments, less fuel is
directly injected and burnt under non-homogeneous conditions and thus avoiding the
formation of high temperature regions within the combustion chamber. As a result, the
nitrogen oxides decrease with increase in premixed ratio. Concentrations of nitrogen
oxides decrease with premixed ethanol than with premixed gasoline because of low gas
temperature and less oxygen left. Since ethanol has higher latent heat of vaporization and
lower heating value, the gas temperature is low with premixed ethanol. Further, the less
oxygen left with premixed ethanol ends up because of more ethanol oxidized within wider
flammability limits[27]. The maximum rate of production of NOx was zero at all
temperatures below 1300K at all equivalence ratios studied. Hence 1300K was predicted
as cutoff temperature of production of NOx for the particular study. NOx produced by
thermal reaction accounts for over 70% for all conditions studied. The second most
important source of NOx was the intermediate mechanism that accounts for 25% of the
total NOx. This attributes to use relatively lean mixtures in the examined conditions.
Because of the lean condition and absence of local rich zones from the homogeneous
charge, prompt NO mechanism ends up insignificant for those conditions and records for
around 5% of the total NOx[28]. Due to low combustion temperatures as a result of very
lean mixtures, the concentration of NO in the exhaust was always less than 25 ppm even
at maximum BMEP of 2.2 bar. The NO emissions increase with BMEP due to the
increase in equivalence ratio which increases in-cylinder peak temperature, for any given
charge temperature. However, NO emissions in all cases were far lower compared to
conventional engines [29]. As HCCI engines operate under leaner homogeneous mixtures,
NOx emissions reduced in HCCI combustion. Since NOx emissions were produced at
high combustion temperatures, NO formation mechanisms could not occur due to lower
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end of combustion temperature which was one of the most important advantages of HCCI
combustion. In the study, NO emissions obtained were 1 and 2 ppm with nheptane and
B20 at high inlet temperatures due to knocking. Due to knocking, the pressure rise is high
and combustion is faster. Knocking tendency is more at higher inlet temperatures[30].
Unlike diesel HCCI which depends on high intake dilution levels to decrease NOx
emissions, ultra-low emissions of NOx were accomplished with n-butanol HCCI
combustion without the utilization of EGR at low mid-engine loads. At higher loads, EGR
was commonly not required for NOx emission reduction during n-butanol HCCI
combustion, both boost and EGR were required to limit high pressure rise rates and to
modulate the combustion phasing for high thermal efficiency.
The load range was increased up to 10 bar IMPE with n-butanol HCCI while
maintaining ultra-low NOx emissions with improved performance characteristics
compared to diesel HCCI[31].With increase in air inlet temperatures, NO emissions first
decreased , while they tend to increase at high intake temperatures. The chemical reaction
is accelerated due to wormer air intakes and the in-cylinder gas temperature is increased
at the end of combustion[32]. In the experiments for E30/D70, E40/D60 and DEE test
fuels, almost zero emissions of NOx observed. Hence the emissions of NOx reduced
drastically for test fuels mentioned on HCCI combustion. The emissions of NOx were
only observed in E50/D50 test fuel[33]. HCCI engine has very low emissions compared
to conventional diesel engine due to lean air/fuel charge and low combustion temperature.
4 bar and 400C air temperature resulted in low NOx emissions compared to other
conditions operated HCCI engine. With increase in air temperature NOx emissions
increased [34].
5.2 Hydrocarbon (HC) emissions
When the mixture becomes leaner, HC emissions increase. The temperature of the
combustion chamber is lowered by the leaner mixture and thus emissions are more.
Maintaining constant air-fuel ratio with increase in intake air temperature, decreases the
unburnt hydrocarbon emissions[35]. When auxiliary fuels injected at 25° BTDC, the
hydrocarbon concentration was quite low than at other injection timings as the carbon in
the fuel burns completely. At high premixed ratios HC emissions increase due to more
fuel escapes from the flammable regions or trapped in the crevice volume in the
combustion chamber because of lower maximum temperature of bulk gas. The oxygen
self-contained in ethanol helps in burning fuel completely, and hence concentrations of
hydrocarbons became lower with premixed ethanol compared to premixed gasoline[27].
With increase in air inlet temperature, HC emissions decrease. This is due to the fact that
at high inlet temperatures, chemical reactions improve and rapid combustion occurs. With
increase in air inlet temperatures and combustion reactions production of radicals
accelerate. The cooling effects of homogeneous leaner charge mixture decreased by the
warmer inlet air temperature. Comparing n-heptane and B20 with other test fuels,
maximum HC emissions were measured. Using isopropanol as an additive fuel, HC
emissions were higher mostly at low inlet air temperatures. Apart from n-heptane,
minimum HC emissions were measured with B20. When each test fuel is blended with
alcohols, autoignition properties were deteriorated and HC emissions were generated.
Maximum HC emission obtained were 440 ppm and 438.88 ppm with P30 test fuel at 313
K and 333 K inlet air temperatures respectively [30]. As intake air temperature increased
HC emissions first increased until about 900C inlet air temperature. At λ = 0.6, higher HC
emissions were measured. Increase in HC emissions was due to the fact that low
volumetric efficiency owing to lower air flow to the engine at higher intake air
temperatures. One of the prominent reasons for the increase in HC emissions was richer
charge mixture. The whole fuel cannot be oxidised as the engine operates with richer
mixture. In order to have complete combustion in HCCI mode, the test engine needs more
air, due to which incomplete combustion occurs. In addition, the flame cannot enter the
piston and piston ring crevices. Hence, the flame goes out especially as the engine
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operates with leaner charge mixtures [32]. With increase in lambda, HC emissions were
decreased. The HC emissions were increased with increase in ethanol amount in the fuel
blends. Compared with other test fuels, the HC emissions were highest for E50/D50 for a
richer mixture. Higher the cetane number and oxygen content of DEE improved the
oxidation reactions which resulted in lower HC emissions. With increase in air inlet
temperature HC emissions were decreased[33]. Increasing inlet air temperature and
injection pressure, hydrocarbon emissions from HCCI engine were reduced. Owing to
lean premixed charge that leads to partial combustion at certain locations in the
combustion chamber, HC emissions form HCCI engine was higher. Injection pressure 5
bar and inlet air temperature 600C operated HCCI engine emitted low HC emissions in
comparison with other operating conditions of HCCI engine[34].
5.3 Carbon monoxide(CO) emissions
As the air –fuel ratio is increased, CO emission increased to a great extent. That is due to
the fact that combustion temperature lower and the later combustion phasing. The
temperature becomes too low for complete oxidation and large amount of CO was
generated at the end of combustion[26]. For auxiliary fuels injected 250 BTDC than the
other injection timings as enough oxygen is available for more completely mixed the air
with fuel and hence CO concentration is slightly lower. At high premixed ratios due to
lower maximum temperature of to react with oxygen incompletely, CO emissions
increase.The quantity of oxygen in ethanolhelps in combustion of fuel more fuel
completely and concentrations of CO are therefore lower with premixed ethanol than with
premixed gasoline[27]. Due to higher combustion temperature and faster combustion due
to knocking, minimum CO emissions were produced with n-heptane. With increase in
inlet air temperature CO emissions decrease, as CO could be oxidised due to higher inlet
temperatures. Hence, formation of CO2 improved and the amount of CO emissions
decreased. Maximum CO emissions were produced at 313 K inlet air temperature for all
the test fuels. With increase of amount of n-butanol in the test fuel, CO emissions
increase. Also CO emissions increase with increase of the amount of isopropanol except
for P40. Reduction in CO emissions occurred with P40 test fuel compared to P30 test fuel.
Due to octane number lower such as n-heptane, B20 and P20 can be easily ignited. Auto
ignition occurs rarely with other test fuels. Lower combustion temperature was obtained
B30, B40, P30 and P40 according to n-heptane , B20 and P20. Maximum CO emissions
were measured as 0.144% with B40, 0.138% with B30 at 313 K inlet air temperature [30].
With increase in air inlet temperature, CO emissions increased and then decreased at
higher intake air temperatures. When the intake air temperatures are high, in-cylinder gas
temperature increased and chemical reaction improved. Thus, CO oxidised to form CO2
[32].Increase in lambda causes reduction of emissions of CO for all test fuels. Increase
of ethanol in the fuel blends has led to an increase on CO emissions. The minimum
emissions of CO were measured that of other test fuels due to low ignition temperature
and oxygen within its chemical structure. Ethanol replaced with DEE resulted in
improvement of ignitability of the mixture[33].Compared to conventional diesel engine,
HCCI engine resulted in high CO emissions high CO emissions, which could be reduced
by increasing the inlet air temperature and injection pressure. For all operating conditions,
5 bar injection pressure and 600 C inlet air temperature operated HCCI engine has shown
low CO values[34].
5.4 Smoke emissions
Smoke concentration was different for injection timings variation in alternate
fuels; and at particular, at 250 BTDC, the smoke concentration occurred is minimum. This
is due to which auxiliary fuel injected into the intake port slightly later at 250 BTDC than
the intake valve opened at 21° BTDC to mix more homogeneously, and then to have local
high temperature regions less. The smoke concentration increases, when the intake valve
opening time is farther for auxiliary fuels injection. Smoke concentration reduces with
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increase in premixed auxiliary fuels because mixture consists of less carbon in fuel and
grater homogeneity during the compression than without premixed auxiliary fuel. Smoke
concentrations are less with premixed ethanol compared to premixed gasoline due to more
oxygen and less carbon in the fuel [27]. Less smoke density is also advantage in HCCI
engines, in that study, diesel fuelled HCCI engine resulted in smoke values between 8 to
24 HSU which are very low. 40°C inlet air temperature, 5 bar injection pressure and 50°C
inlet air temperature, 4 bar injection pressure operated HCCI engine resulted smoke
density values low[34].
6. Recent developments in HCCI engine
The development of Skyactiv-X Spark Controlled Compression Ignition (SPCCI)
engine enabled petrol to ignite in the similar way as diesel by compression instead of
spark. The concept also known by names as Auto Ignition, Combined Combustion
System and Gasoline Compression Ignition. For all these(including SPCCI) the
underlying concept is Homogeneous Charge Compression Ignition (HCCI). When diesel
is compressed without sparks than petrol, burning of fuel takes place uniformly
throughout the combustion chamber, which is a combination of petrol and diesel
technology. In HCCI engines, the fuel is burnt as a whole throughout the combustion
chamber homogeneously, also lean with extra oxygen supplied by a supercharger like a
pump.
On firing by HCCI mode, fuel-air charge burns throughout the combustion
chamber as a whole. In conventional SI engine, ignition of the fuel is started by the spark
plug and the flame front propagates forward and spreads in the combustion chamber. Due
to HCCI mode of combustion, fuel consumption is reduced, CO2 emissions lessen and
fewer oxides of nitrogen(NOx)
In 2001, Lotus Engineering developed a design of engine to utilise the hot
exhaust gases to re-ingest into the engine, to arrive at the same goal as Mazda. Research
engines are also produced by Ricardo and in 2007 a prototype Gasoline Compression
Ignition engine based on FSI petrol engine is developed by Volkswagen. Similar to Lotus
concept, high levels of exhaust gas recirculation (EGR) is utilised to ignite the fuel.
Fig 6 HCCI engine model developed by Mazda © Provided by Haymarket Media
Group
Because small amounts of fuel are used, HCCI engines using hot exhaust gases
work only when the engine bears fewer loads. For starting and full power, a conventional
spark is needed and a smooth switch between the two provided difficult to achieve.
SPCCI still involves a spark, which is used to ignite a small, fuel rich ‘detonator’ charge
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injected directly into the spark plug. The remaining part of the combustion chamber
contains lean charge brought to the vicinity of ignition by high compression.
On igniting the detonator charge by the spark, the expanding fireball from it
increases enough extra pressure on the main charge to make it combust spontaneously.
The pay-off is that SPCCI works when the engine is at full gallop and not just a gentle
canter-so during most normal driving. The difference between this and earlier concepts is
that combustion is guaranteed and stable.
Lean burn
The concept Lean burn means more amount of air and less fuel. In gasoline
engine, the chemically correct air fuel ratio is 14.6:1 i.e. for complete combustion, 14.6
parts of air is needed for one of fuel. The Skyactiv-X engine utilises Rootes- type
supercharger on the front of the engine, not to boost power but to provide enough air to
burn much leaner at 29.4:1.
7. Conclusion
Summarizing, HCCI is an hybrid engine technology, which combines the
advantages of Spark Ignition (SI) Engines and Compression Ignition (CI) Engines. It
provides efficiency as high as CI Engines and Ultra-low NOx and PM emissions. The
charge inducted in the cylinder is homogenous which burns volumetrically throughout the
combustion chamber simultaneously, thus reducing PM emissions however the
combustion temperature is also comparatively lower, therefore NOx formation is also
highly reduced.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the
research, authorship, and/or publication of this article.
Funding
The author(s) disclosed no receipt of the following financial support for the
research, authorship, and/or publication of this article.
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