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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

International Journal of Research

Volume VIII, Issue VI, JUNE/2019

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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 isooctane, 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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