a review of hcci engine using alternative fuels - zenodo

HBRP Publication -9 2020. All Rights Reserved Page 1

Research and Applications of Thermal Engineering

Volume 3 Issue 2

A Review of HCCI Engine Using Alternative Fuels

Manish Kumar1, Piyush Jasooja

2

1Assistant Professor,

2 Graduate

Department of Mechanical Engineering, Bhilai Institute of Technology, Durg, India

*Corresponding Author

E-mail Id:[email protected]

ABSTRACT

In today’s day and age, automotive as an industry is actively exploring alternative

combustion phenomenon’s to substitute Spark Ignition (SI) and Compression Ignition (CI)

systems for Internal Combustion (IC) engines. With a distinctive set of features,

Homogeneous Charge Compression Ignition (HCCI) engines hold a promising position as a

feasible alternative, as besides low emissions of Nitrogen Oxides (NOx) and soot, HCCI also

ensures plausible merits of high volumetric efficiency. In principle, the combustion auto-

ignites at multiple spots once the mixture has reached its chemical activation energy as there

is no spark plug or injector to assist the combustion. The important challenges encountered

in developing HCCI engines are: (i) Uncontrolled combustion and Self-ignition of the

working fluid with fuel,(ii) the maximum pressure at high load operations,(iii) To rectify the

problem of cold starting,(iv) reaching the emission standards and (v) to control the

phenomenon of knocking. At low engine speeds, a probable early auto-ignition can lead to

knocking, while at high engine speeds, delay auto-ignition would make HCCI susceptible to

misfire. The emission levels are greatly reduces by hydrogen but renders reduced power.

However, lower NOx, CO and particulate matter (PM) emissions levels can be achieved by

combining hydrogen with diesel in dual-fuel mode, and the efficiency of the engine can be

increased. HCCI engines performance (i.e. emissions levels, brake thermal efficiency and

combustion phasing) is usually predicted by numerical methods, which are cost effective

compared to entirely relying on empirical data.

Keywords:-HCCI, Energy, Biofuels, Emission Standards, Alternative fuels

INTRODUCTION

An incorporation of characteristics of the

conventional gasoline and diesel engines is

made possible in HCCI. The nomenclature

is derived from the composition of influx

charge and the combustion process,

namely, HCSI in case of gasoline engines

(a union of homogeneous charge (HC)

and spark ignition (SI)) and SCCI in case

of modern direct injection diesel engines

(a union of stratified charge (SC)

and compression ignition (CI)). During

last decades, the two main concerns have

become prominent in the energy

generation and transportation sectors,

namely the increasing energy demand

coupled with the lim3ited availability of

oil reserves and the detrimental

environmental effect produced by their

exploitation to deliver energy and motion.

One major detrimental effect of the use of

fossil fuels is related to the environmental

impact from their use for energy

production and transportation. The fuel

combustion in internal combustion engines

produces – among others – CO2, a major

greenhouse gas that is known to contribute

to the global warming effect. The

production of CO2 is not the primary

concern, as the combustion of most fuels

leads to its production, but the fact that in

the case of fossil fuels, the CO2 emitted

cannot be reconverted to the original fuel

by any short-term, natural – and therefore

https://en.wikipedia.org/wiki/Spark_ignition

https://en.wikipedia.org/wiki/Stratified_charge

https://en.wikipedia.org/wiki/Compression_ignition

HBRP Publication Page 1-9 2020. All Rights Reserved


Volume 3 Issue 2

energy neutral – process.

In an effort to gradually replace fossil

fuels, research has been guided towards

seeking alternative fuels to gradually

substitute conventional ones. Biofuels,

among them have gained increasing

attention due to their atypical qualities of

being renewable in nature and reducing the

net CO2 emissions. Furthermore, even

conventional diesel and gasoline engines

have utilized Biofuels as either neat fuels

or as supplements.

Moreover, besides the conventional

combustion modes, a relatively new

concept has been developing during the

last two decades, i.e. homogeneous charge

compression ignition combustion (HCCI).

In HCCI combustion, a mixture of fuel and

air is introduced into the combustion

chamber early during compression or

during the intake stroke. The mixture is

compressed and auto ignites at the point

where the local thermo- and fluid-dynamic

conditions are favourable [1]. The main

advantage of the HCCI concept relative to

conventional combustion modes is the

significant reduction of NOx emissions and

the absence of soot in the exhaust.

Moreover, the fast combustion usually

encountered and the unthrottled operations

are prone to yield high indicated thermal

efficiency. However, HC and CO

emissions are usually high due to bulk

quenching at low loads and wall

quenching and crevice flows at higher

loads [2].

Added to these unfavourable attributes of

HCCI is the inability to directly control the

combustion phasing in HCCI engines. This

is a major obstacle preventing the

implementation of the HCCI concept.

HCCI ignition relies heavily on chemical

kinetics and largely depends on the fuel

used. In an effort to control ignition and

the combustion rate, thereby expanding the

HCCI operation over a wide range of

load–engine speed, various means have

been used, such as variable compression

ratio, variable valve timing, variable inlet

temperature, internal or external EGR,

direct fuel injection, etc. [2].

The SI and CI engine have divergent

processes to achieve HCCI combustion in

an IC engine, while feeding the manifold

with premixed air-fuel mixture or by an

early infusion into the Direct Injectors (DI)

would result in a successful HCCI

combustion in the former, the CI

arrangement would require a rather higher

compression to attain an auto-ignition as a

consequence of notably higher degrees of

temperature. Anyhow, the limited

operating range of HCCI engines has been

a cause of constant concern as it has

detrimental effects on the engine, such as

knocking at high loads and speeds due to

enormous heat and pressure released and

misfire at low loads. [22].

A promising method for the combustion

control seems to be the use of alternative

fuels and fuel blends, depending on the

characteristics of the engine (compression

ratio, etc.) and the operating conditions [2–

4]. The main concept is that fuels with

different auto-ignition tendencies can be

blended at varying proportions to regulate

the ignition point at various load–speed

regions [26]. This is a fortunate evolution

since the fuels used in the blends may as

well be biofuels. Thus, hydrogen [4],

ethanol, ethers and biodiesel have been

used in HCCI engines, either as neat fuels

or in blends.

PERFORMANCE COMPARISON

Fuels Used in HCCI Engine An early or a chemically energetic auto-ignition causing an increase in the rate of combustion and in-cylinder pressures can potentially impair the Engine. For an auto-ignition, the reactants of the fuel air mixture should have sufficient concentrations and temperature required for ignition, as once triggered, auto-



Volume 3 Issue 2

ignition occurs at an alarming rate. Using of any fuel which is highly volatile and easily mix with air before ignition, can be operated in HCCI engine [5]. As HCCI engines are largely governed by chemical kinetics, the important is that to deduct the fuel’s self-combustion point to establish smooth engine operation, i.e.; free from knocking and misfiring. Every fuel have different auto ignition point. The intake temperatures required for various fuels to auto-ignite at different compression ratios when operating in HCCI mode [5] is depicted in Figure 1. It can be seen that methane needs a high initial pressure, temperature and compression ratio to self-ignition. The auto-ignition point decreases when the number of carbon atoms in the hydrocarbon increases, as shown in Figure 1, where methane has the fewest possible carbon atoms, while iso-octane has the most of those shown. The lack of well-defined combustion initiator in HCCI engines undermines the ability of a direct control over combustion resulting in an uncontrolled or rather difficult control relative to other combustion engines. In regards with ignition in conventional engines, the gasoline engine uses a spark in combination with a premixed charge of fuel and air whereas the diesel engine is reliant on the injection of fuel into the pre-

compressed air. Unlike SI or CI engines, where definitive control measures for combustion timing are in place, the HCCI engine experiences combustion as and when sufficient temperature and pressure is achieved by the compression of homogeneous fuel and air mixture. Three blends of unleaded gasoline, a Primary Reference Fuel (PRF) blend of 95% iso-octane and 5% n-heptane (95RON), methanol and ethanol were used in a study [22] to ascertain the difference between alcohols and hydrocarbons on HCCI combustion. The findings suggested that although the RONs of all the gasoline were poles apart, there was still some resemblance in their behaviour. While alcohols achieved highest thermal efficiencies, the emission levels were minimal for NOx followed by further lower in methanol. As derived from an experimental study, the fuels with octane numbers having greater differences can be used to obtain higher values of torque. In this study, a high octane homogeneous air/fuel mixture (propane or hydrogen) was infused with a low octane fuel (n-heptane) right before the intake valve. Nonetheless, due to its poor self-ignition tendency, a part of the high-octane fuel does not participate in oxidation reactions resulting in increased hydrocarbon emissions [23].

Fig.1:-Intake temperature required for fuels to operate under HCCI mode with varying

compression ratios, reproduced from [20]

https://www.intechopen.com/books/advances-in-internal-combustion-engines-and-fuel-technologies/homogenous-charge-compression-ignition-hcci-engines#B35



Volume 3 Issue 2

Table 1:-Natural gas emissions levels compared to oil and coal (Pounds per billion Btu of

energy) [20] Pollutant Natural gas Oil Coal

Carbon dioxide 117,000 164,000 208,000

Carbon monoxide 40 33 208

Nitrogen oxides 92 448 457

Sulphur dioxide 0.6 1122 2591

Particulates 7.0 84 2744

Mercury 0.000 0.007 0.016

From Table 1, the rate of production of

pollutant, Carbon dioxide is observed to be

1.4 and 1.75 times higher than natural gas

in case of crude oil and coal respectively.

Additionally, Natural gas is more readily

available than crude oil, with a cost that

has been competitive for a while now [3].

Though, suffers reduced power output in

HCCI engines, it is able to operate as an

independent fuel in an IC engine with

fairly lower HC and CO emissions[4]. Its

higher auto-ignition point (about 810 K)

ensures a significant precedence over

diesel–natural gas operation by

simultaneously maintaining the high CR of

a diesel engine and lowering emissions. As

claimed by Duc and Wattanavichien [6],

engines can operate at a high CR due to

the high octane number of methane (about

120). Deriving from the results of a four-

stroke HCCI engine simulation, a less than

400 K intake temperature with 15:1

compression ratio, methane can’t ignite

[7].This is shown in Fig. 1, where auto

ignition of methane will take place only

with an initial temperature is less than 400

K when compression ratio is greater than

18:1.

Knocking is often experienced due to the

unstable nature of the only gas capable of

operating as a single fuel in an HCCI

engine, Hydrogen. Besides a homogeneous

intake charge on being premixed with air,

hydrogen also catalyses the diffusivity of

any gas in air to about 3–8 times faster

than natural gas. With an approximate

three folds higher (119.93 MJ/kg

compared to 42.0035 MJ/kg) net heating

value than diesel, self-ignition temperature

to initiate combustion is as high as (858 K)

for hydrogen. Due to its imperative

instability, hydrogen has been ruled out a

single fuel in an HCCI engine by most

researchers, rather it is being used as an

additive: either to control the ignition

timing or to increase the engine

performance.

Iso-octane, a surrogate fuel for gasoline

and n-heptane, a surrogate fuel for diesel

are used in HCCI engine experiments.

Primarily due to the complexity in

production, and a subsequent high

manufacturing costs, Alcohol-derived

fuels, as shown in Fig. 1, are not widely

used. The fact that alcohol-derived fuels

like biofuels are still subject to worldwide

investigations, is in due consideration of

the consistent clogging issues in the engine

arising by their use. Biofuels, in spite of

the numerous challenges, have managed to

gather eminent consideration as a

potential renewable source from

researchers worldwide.

Peak Pressure and Temperature

Due to the high sensitivity to temperature,

HCCI's auto-ignition and combustion

timing are subjected to being largely

influenced by the intake charge

temperature. In light of these events, its

effects on HCCI combustion on-set have

been widely reported by many researchers.

The use of resistance heaters to vary the

inlet temperature is a rather simple but

quite slow approach of temperature control

method to obtain a cycle-to-cycle

frequency variation. The in-cylinder peak

pressure and temperature are directly



Volume 3 Issue 2

affected by the variations of heat release

rate during combustion. A directly

proportional relationship can be

established between load quantities (heat

release rate, temperature and pressure) and

the speed of the engine, equivalence ratio,

load, intake pressure, temperature and

energy content of the fuel configurations,

i.e., higher peak pressures are observed at

higher loads and richer mixtures. Table 4

gives an insight into the pattern of variance

among HCCI and conventional CI modes

in terms of the in-cylinder peak pressure

and temperature to evidently decide on the

mode that produces a higher peak.

Although the complete data for maximum

temperature is not reported, a general

conclusion drawn from the table can be the

proportional increase of pressure with rise

in temperature. The lower peak pressures

of HCCI configurations with respect to

conventional CI modes confirm significant

impacts on emission levels for every fuel

configuration. Addition of Hydrogen to

diesel causes a reduction in ignition delay

along with a directly proportional

influence on the peak pressure generation;

higher the addition, higher the peak

pressure [10]. A reasonable inference is

that the highest peak pressure along with

better power and efficiency values are

achieved with the hydrogen fuel

configuration.

Table 2:-In-cylinder peak pressure and temperature comparison for natural gas and

hydrogen in various configurations [8, 21] Mode Max pressure (MPa) Max temperature (K)

HCCI CI HCCI CI

H2 ~8 ~12 - -

NG ~7 ~7.5 ~1300 ~1850

Diesel ~6.1 ~6.6 - ~2300

NG+Diesel ~3 ~5.5 ~1450 -

H2+Diesel ~7 ~7.8 - -

Table 3:-Diesel Properties compared to hydrogen and natural gas. Properties Diesel Hydrogen Natural gas

Main component C12H23 H2 Methane (CH4)

Auto-ignition temperature (K) 553 858 923

Lower heating value (MJ/kg) 42.5 119.93 50

Density (kg/m3) 833-881 0.08 0.862

Molecular weight (g/mol) 170 2.016 16.043

Flammability limits in air (vol%) (LFL-UFL) 0.7-5 4-75 5-15

Flame velocity (m/s) 0.3 2.65-3.25 0.45

Specific gravity 0.83 0.091 0.55

Boiling point (K) 453-653 20.2 111.5

Cetane number 40-60 - -

Octane number 30 130 120

CO2 emissions 13.4 0 9.5

Diffusivity in air (cm2/s) - 0.61 0.16

Min ignition energy (mJ) - 0.02 0.28

Brake Thermal Efficiency

By definition, the brake thermal efficiency

(BTE) of an engine is the brake power

produced by an engine with respect to the

energy supplied by the fuel, but it is

represented as the ratio of brake output

power to input power. According to a

study, [8] the energy ratio of biogas and an

intake temperature of 80 1C, 100 1C and

135 1C, denotes the best BTE for biogas

fuels ranging from 40% to 57%. Anaerobic

fermentation of cellulose biomass

materials [6] was deployed to produce the

principal component of biogas, methane

(460%). 51% biogas and 135 1C intake

temperature were reported to be the best



Volume 3 Issue 2

energy ratio with diesel and the optimum

efficiency respectively, in HCCI mode,

conclusively, the heat release rate and the

efficiency [8] are inversely proportional to

the energy ratios. However, with the

exception of BTE which was higher for

diesel running in CI mode even when

operated at the optimum biogas energy

ratio. According to a report by Duc and

Wattanavichien [6], diesel single fuel in

either HCCI or non-HCCI mode recorded

higher efficiencies than the biogas-diesel

running in dual-fuel non-HCCI. Hydrogen,

on the contrary, having an increased BTE

by 13–16%, recorded a higher BTE than

pure diesel in non-HCCI mode. reported

An addition of 5% hydrogen lead to a

further increase in BTE from 30.3% to

32% as reported by Szwaja and

GrabRogalinski [9]. The uniformity of

mixing of hydrogen could be the prime

cause of the reported increase of BTE in

hydrogen– diesel mode.

Exhaust Gas Emissions

The lower peak temperature encountered

in HCCI relative to that in SI and diesel

engines is due to the fact that it operates on

lean mixtures, which results in incomplete

burning of fuel near the combustion

chamber walls along with reduced NOx

formation. An oxidising catalyst in the

oxygen-rich exhaust is deployed to

eradicate the high carbon monoxide and

hydrocarbons from the corollary

emissions.

As stringent vehicle emissions quality

standards have started being imposed by

numerous regulatory bodies, such as those

in Europe, the United States (US) and

Japan, emission levels have evolved to be

a major focus in new engine developments

recently. UHC, CO, NOx, and particulates

constitute the composition of emissions

from HCCI engines. Lower emission

levels of NOx and particulate matters and

higher levels of unburnt hydrocarbons

(HC) and carbon monoxide (CO) have

been claimed to have found in HCCI

engines emissions. As the emissions levels

of an engine depend upon operating

conditions of the engine, fuel quality and

the engine design, which are particular to

every engine, it is inappropriate and

ineffective to compare emission levels of

any two engines.

HC, CO and CO2

Among others, two major challenges that

need to be resolved are the higher UHC

and CO emissions in HCCI engines than

the conventional diesel engines as

reported. The low combustion

temperatures trigger incomplete

combustion that in turn leads to the

formation of UHC, consequently resulting

in deposition of fuel in boundary layers

and crevices. Total hydrocarbon

concentration (in parts per million carbon

atoms) is used to specify the level of UHC.

The cold area in any combustion wall

when the piston moves down the cylinder

wall leaving a thin layer of oil, known as

the crevice area, is reported to be the

source of UHC. The ability of diesel

engines to reduce UHC and CO emission

levels, is because of the presence of higher

concentrations of hydrogen and natural

gas, which in gaseous state, will reduce the

wall wetting effect on the cylinder liner.

NOx and particulate matter (PM)

The peak temperature which is a factor of

various parameters such as equivalence

ratio, fuel composition and the initial

temperature of the fuel–air mixture is

responsible for the determination of nearly

all of the NOx formation. Further, as

reported by Tanaka et al. An equivalence

ratio greater than 0.33, results in

significant increase in the NOx levels. The

maximum in-cylinder temperature exceeds

1800 K and produces more NOx when a

higher equivalence ratio is achieved. The

NOx and particulate matters (PM) are

reported to be very low by implementing

high CR engines in HCCI engines, which



Volume 3 Issue 2

can be further reduced by use of high EGR

rates and reduction in the local

temperature and the oxygen amount in the

cylinder However, a hike in unburned HC

and CO emissions could be a consequence

of insufficient oxygen. An effective way of

reducing the NOx, unburned HC and CO

by the use of internal EGR with VVT was

suggested by Bression et al. as he

experimented with high pressure-loop

EGR, to increase the combustion

temperature without employing coolers

and Variable Valve Timing (VVT).

The NOx level for a dual fuel engine, i.e.,

a blend of diesel (direct injected) and

natural gas (port injected) was found to be

lower than diesel CI engine when operated

for the combustion and exhaust emission

characteristics during an investigation by

Papagiannakis and Hountalas [11].

Addition of hydrogen would further reduce

the NOx. As hydrogen has a higher

temperature and flame speed compared to

natural gas and diesel, the concentration of

NOx between natural gas and hydrogen in

diesel HCCI mode is different due to

different combustion temperatures. The

higher level of NOx emissions from the

combustion of natural gas-hydrogen

mixtures was an after effect of the

combustion temperature and flame speed

of hydrogen which developed with

increase in NOx and hydrogen content in

the mixtures as indicated in a survey of

research papers by A.A. Hairuddin et al.

[12].

The NOx level in biogas–diesel HCCI

engines was inversely proportional to the

biogas energy. A higher homogeneity level

achieved between air and fuels could

possibly be the root cause for this. The

importance of a homogeneous and lean

mixture in eliminating the production of

NOx was repoorted by Van Blarigan [13]

in his study. According to Olsson et al.

[14], combining the NOx levels with

exhaust gas recirculation (EGR) would

further drop the already low NOx levels in

natural gas HCCI engines. The NOx level

even in natural gas–diesel non-HCCI mode

is lower than diesel conventional CI

engines. The absence of carbon in the fuel

sequences to the UHC, CO and CO2 free

emission with the use of hydrogen, yet it

produces NOx. Lower NOx levels were

recorded with the operation of hydrogen as

a single fuel in CI mode as compared to

diesel. Hydrogen addition to diesel in non-

HCCI mode yielded lower NOx emissions

amongst those obtained for all load ranges

compared to diesel in conventional mode

as inferred by Saravanan and Nagarajan

[19] in their study, they further reported

that the formation of NOx depends on

temperature more than the availability of

oxygen. Conclusively, extremely low NOx

emissions levels were observed with

hydrogen–diesel in HCCI mode with no

significant amount of PM.

PM is produced due to liquid fuel

nucleation from port fuel injection as

suggested by Kayes and Hochgreb [15]

according to their mechanism model for

PM. A gas-phase nucleation in fuel-rich

conditions also results in the production of

PM. Due to the substantial effects of air-

to-fuel ratio on PM and burning liquid

fuel, it is advisable to keep the fuel in an

atomized condition as this will

consequently increase the amount of PM

in the exhaust gas emissions. Addition of

hydrogen to a diesel fuelled engine and

heating the inlet [15] can significantly

reduce PM. No significant reduction in PM

was recorded [16] when the engine was

made to run on a dual fuel mode between

diesel and natural gas. However, addition

of hydrogen in reducing the PM emission

levels turned out to be an effective

method.

Knocking

A spontaneously ignition due to some un-

burnt gases ahead of the flame in an SI

engine leads to Engine knock or pinging.



Volume 3 Issue 2

The propagation of the flame compresses

the gas resulting in the rise in combustion

chamber pressure, this high pressure and

corresponding high temperature causes the

un-burnt reactants to spontaneously ignite.

A shock wave from the end gas region and

an expansion wave into the end gas region

are traversed as a result of this ignition. A

similar ignition process occurs in HCCI,

where the only difference being that the

ignition occurs due to piston compression

more or less simultaneously in the bulk of

the compressed charge instead of part of

the reactant mixture being ignited by

compression ahead of a flame front.

Knocking in SI engines is a phenomenon

where the un-burnt mixture in the

compressed gas ignites before it is reached

by the propagating flame front. An

excessive engine vibration and a pinging

sound heard outside as a result of the

combustion activity are physical

indications of Knocking. Besides, loss of

power, if not controlled, knocking could

lead to severe engine damage and shorten

its life. Knocking is a characteristic of

reciprocating engines. Governed by

chemical kinetics, HCCI engines are rather

prone to knock and with a lack of a fixed

mechanism to control knock in them. The

load range of an HCCI engine is also

confined within limits due to the Knocking

phenomena i.e., upper load limits have to

be applied as high load operations can

easily initiate knock. If the combustion

starts before the piston reaches TDC, it

results in knocking where as if it occurs

after TDC, misfire is observed. To prevent

deterioration of engine performance, it is

imperative to avoid Knocking and misfire

in engine operation as both of them can

significantly contribute to the damage

caused to the engine.

A high natural gas flow rate combined

with a low diesel flow rate in a natural

gas–diesel HCCI engines would lead to a

misfire while the opposite configuration

leads to knocking. An addition of high

amount of hydrogen, is expected to trigger

knocking in HCCI engines operating on

hydrogen–diesel fuels. As confirmed by

Guo et al. [17], it is essential for hydrogen

content to be more than 16% of the energy

ratio for Knocking to take place. He

further registered that hydrogen with mass

fraction less than 15% is necessary to

achieve stable combustion. An unstable

operation for high natural gas

concentrations is observed in an HCCI

engine running on dimethyl ether (DME),

resulting in the knock limit of an in-

cylinder pressure of 9 MPa and a very

limited load range.

Use of Computational Fluid Dynamics

(CFD) simulations with detailed chemical

kinetics lead to this discovery. An

equivalence ratio less than 0.45 and an

intake temperature of 380 K culminated to

knocking as revealed in an investigation of

auto-ignition and combustion of natural

gas HCC engines. Figure 3 illustrates the

area of knocking and misfiring of natural

gas HCCI engines with constant intake

pressure. Natural gas HCCI engines are

incapable of operating at high load

conditions, pertaining to this limitation.

A rapid increase in local pressure caused

by a rapid release of energy in the

remaining unburned mixture results in

Knocking. A combination of hydrogen and

diethyl ether (DEE) at 100% load in

hydrogen–diesel engines will cause a

highly unstable condition for working

resulting in severe knock. Besides a higher

probability of knock onset is anyways

expected with higher hydrogen content. By

observing a rapid instantaneous local

pressure rise in the in-cylinder pressure

variations, the Knocking phenomena can

be detected. The severity of the knock

corresponds to the frequency of the knock,

which is the principle to form the graph.

For a single combustion event in a

hydrogen HCCI engine with a



Volume 3 Issue 2

compression ratio of 17, the knocking

behaviour can be depicted in Figure 4 by

monitoring its in-cylinder pressure. It can

be seen that from the Figure, that the peak

pressure is higher than the normal in-

cylinder peak pressure without knocking,

which denotes the in-cylinder pressure

suddenly increases to the peak before

following the normal pressure trend. In an

incorrect operating condition, there is an

abnormality where the maximum increase

in pressure for knocking is unpredictable.

Due to the low temperature reactions in the

compression stroke, accumulation of H2O2

decomposition is found with HCCI

ignition, as described by Flowers et al.

[18]. Fuels in the cylinder get rapidly

consume by the enormous amount of OH

radicals formed by the rapid

decomposition of H2O2 into two radicals

once the in-cylinder temperature reaches

1050–1100 K. Incomplete combustion is a

result of reduction in OH concentration in

low temperature regions, which is also

responsible for delaying the high-

temperature oxidation. It is easier to

control knock in SI engines, for instance,

car manufacturers usually install knock

and oxygen sensors in the engine where

the sensors will automatically adjust the

spark timing if the mixture is prone to

generate knocking. Whereas, in HCCI

engines, since there is no such concrete

mechanism to define and control

knocking, a strong dependence is on the

right mixtures and conditions.

Fig.2:-Knocking and misfiring area for natural gas HCCI engines under constant intake

pressure, reproduced from [21]

Fig.3:-Knocking phenomena in a hydrogen HCCI engine for a single combustion event,

reproduced from [9]



Volume 3 Issue 2

CONCLUSION

CAI/HCCI engines have neither evolved to

phase of development nor to the

economical feasibility mandatory for a

market introduction as yet. The technical

challenges incurred with both gasoline and

diesel HCCI combustion are their limited

operational range and less optimized

combustion phasing, owing to the lack of

direct control over the start of ignition and

the rate of heat release.

To summarize, besides generating better

results for most major quantities, the

hydrogen-diesel combination in HCCI

engines also produce higher efficiency

compared to the single diesel mode and the

natural gas–diesel mode. A reduced fuel

consumption of the engine that could also

match to that of the hybrid engine is

credited to a higher value of BTE. HCCI

seems to be a promising choice of engines

in the near future as using it in a hybrid

configuration would result in a significant

reduction in fuel consumption.

The HCCI engine has low emissions levels

of NOx, soot and particulates. However,

knocking and high levels of unburned HC

and CO emissions are still some of the

unresolved issues of the HCCI engines.

Knocking in an HCCI engine will take

place on addition of hydrogen to diesel-air

mixture once the energy ratio is more than

16%. Since the combustion is no longer

controlled by the injection or the spark

timing, knocking is expected to be

influenced by chemical kinetics in HCCI

engines. Therefore, it is hypothesized that

the knocking in HCCI engines could be

due to the instantaneous initiation of local

combustion resulting in a local high heat

release rate in the region with the highest

OH concentrations in the high temperature

zone, i.e., the highest temperature region.

Further studies have to be performed to

have a resolution for these remaining

issues. The numerical method is far better

than the experimental method to simulate

the combustion behaviour in terms of cost

and time. To this end, a simulation model

to investigate the behaviour needs to be

developed and the numerical findings must

then be validated against empirical data.

The multi-zone numerical method

combined with advanced turbulent mixing

models demonstrates promising results

compared to the single-zone model. To

achieve even greater accuracy than multi-

zone model, CFD model can be used at the

expense of computational cost. But, one

could easily implement the LES or DES

model with CFD.

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Volume 3 Issue 2

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