This paper critically investigates both petroleum diesel and biodiesel emissions from the view point of LTC attaining strategies. Due to a number of differences of physical and chemical properties, petroleum diesel and biodiesel emission characteristics differ a bit under LTC strategies. LTC strategies decrease NOx and PM simultaneously but increase HC and CO emissions. Recent attempts to attain LTC by biodiesel have created a hope for reduced HC and CO emissions. Decreased performance issue during LTC is also being taken care of by latest ideas. However, this paper highlights the emissions separately and analyzes the effects of significant factors thoroughly under LTC regime.
Trang 1Impact of low temperature combustion attaining strategies on diesel
engine emissions for diesel and biodiesels: A review
Centre for Energy Sciences, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a significant technology to reduce emissions, LTC deserves a critical analysis of emission characteristicsfor both diesel and biodiesel
This paper critically investigates both petroleum diesel and biodiesel emissions from the view point ofLTC attaining strategies Due to a number of differences of physical and chemical properties, petroleumdiesel and biodiesel emission characteristics differ a bit under LTC strategies LTC strategies decrease NOxand PM simultaneously but increase HC and CO emissions Recent attempts to attain LTC by biodieselhave created a hope for reduced HC and CO emissions Decreased performance issue during LTC is alsobeing taken care of by latest ideas However, this paper highlights the emissions separately and analyzesthe effects of significant factors thoroughly under LTC regime
Ó 2014 Elsevier Ltd All rights reserved
1 Introduction
The diesel engine is the most efficient type of internal
combus-tion engine, offering good fuel economy and low carbon dioxide
(CO2) emission[1] Unfortunately, it is also a source of particulate
matter (PM) and nitrogen oxides (NOx), both of which are now
subjected to legislative limits because of their adverse effects on
the environment and human health[2] In the last few years, diesel
engines have been subjected to progressively stringent emission
control standards; especially as far as NOxand PM emissions are
concerned.Fig 1shows this trend for Europe (Euro 2, 1996–Euro
5, 2008), the United States (US04–US10) and Japan In order to meet
the requirements of future emission standards, emission of these
substances, as well as carbon monoxide (CO) and hydrocarbon
(HC) emissions must be reduced significantly Three general
meth-ods can be applied to the engines to meet lower regulated emission
limits, viz alternation of fuels [3,4], alternation of combustion
processes and after-treatment of the exhaust[5] Considerable gress has been made on both combustion and catalyst control path-ways to reduce emission Diesel particulate filters (DPF) for PMfiltration and selective catalytic reduction (SCR) of NOx are nowavailable for after-treatment of engine out emissions Nevertheless,
pro-to minimize the cost and complexity of exhaust after-treatment tems as well as for potential fuel economy penalties—considerableresearch efforts have also focused on the in-cylinder control of emis-sions through the application of low-temperature combustion (LTC)techniques
sys-LTC is now widely demonstrated covering light-duty[7–11]toheavy-duty[12–14]engines It is the concept at the heart of ad-vanced diesel combustion LTC is a general term for HomogeneousCharge Compression Ignition (HCCI) combustion, and PremixedCharge Compression Ignition (PCCI) combustion[5] To explain thetheory of LTC, Akihama et al.[15]simulated combustion by a com-pression ignition (CI) 3D-CFD KIVA2 model and plotted local equiv-alence ratio (U) vs flame temperature (T) for the stratifiedcombustion process This particular figure showed the NOx–PMtrade-off related to conventional diesel combustion, where at theedge of spray flame, fuel lean zones produce abundant NOxand fuelrich zones inside the spray flame produce abundant soot (an
http://dx.doi.org/10.1016/j.enconman.2014.01.020
⇑ Corresponding author Address: Department of Mechanical Engineering,
Uni-versity of Malaya, 50603 Kuala Lumpur, Malaysia Tel.: +60 146985294; fax: +60 3
79675317.
E-mail address: sayeed.imtenan@gmail.com (S Imtenan).
Contents lists available atScienceDirect
Energy Conversion and Management
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / e n c o n m a n
Trang 2element of PM) With their model andU–T map they explained that
LTC takes place at temperatures below the formation regime of NOx
and at local equivalence ratios below the formation regime of diesel
soot As mentioned earlier, these systems can be divided into two
categories [16] Those in which the combustion phasing is
decoupled from the injection timing and the kinetics of the chemical
reactions dominate the combustion, are in the first category which is
known as HCCI mode In the second category, combustion phasing is
closely coupled to the fuel injection event which is termed as PCCI
mode In the former category, air and fuel are thoroughly premixed
in such a way that at the start of the combustion, the mixture is
nearly homogeneous and characterized by an equivalence ratio,
which is lower than 1 everywhere For the second category,
pre-mix-ing occurs between the fuel injection and start of combustion event,
but significant regions exist where the equivalence ratio is greater
than unity at the start of the combustion.Fig 2shows the plot of
lo-cal equivalence ratio (U) vs flame temperature (T) with different
combustion mechanisms It can be seen that, NOxforms in the lean
mixture zone where flame temperature is above 2200 K, whereas
soot forms in the rich mixture zone above 1800 K Conventional
combustion overleaps the formation zones of NOx and soot, but
LTC techniques like HCCI and PCCI avoid these zones and reduce
NOx and soot simultaneously Recently, a new approach of LTC,
Reactivity Controlled Compression Ignition (RCCI) has been
pro-posed by several authors[17–19] This technology has the potential
to overcome some of the limitations of HCCI and PCCI
The objective of this article is to present the state of the art of
the effects of different LTC mode (HCCI, PCCI, RCCI) attaining
strategies on particular diesel emissions (NOx, PM, CO, UHC) usingboth petroleum diesel and biodiesel The attainment of these strat-egies primarily depends on some factors like, application of ex-haust gas recirculation (EGR), change in injection timing (IT) &injection pressure (IP), variation in compression ratio (CR) henceoperating load, changes in fuel blends, etc Therefore the analysishas been governed by these significant factors surely To provide
a complete overview of the whole scenario, more than 150 cal articles have been reviewed to collect significant informationrelated to this article’s objective At first, the article briefly intro-duces the LTC strategies and then analyzes how the attainment
techni-of these strategies may affect the emissions for petroleum dieseland biodiesel respectively Though LTC mode has a positive impact
on NOxand PM emissions but many of the researchers have ported reduced performance during LTC modes [20,21] due tohigher rates of EGR and incomplete combustion Impact of LTCmodes on engine performance is also briefly presented here in thisarticle
re-2 LTC strategies2.1 Homogeneous Charge Compression Ignition (HCCI)HCCI engine is a combination of SI (homogeneous charge sparkignition) and CI (stratified charge compression ignition) engineswith a sense that it uses premixed charge like SI engine but depends
on autoignition like CI engine[22] In HCCI, fuel is injected wellbefore the combustion event which allows the homogeneous mix-ture of air–fuel This homogeneous mixture initiates combustionsimultaneously at different sites of the combustion chamber unlike
SI (flame propagation) or CI (locally rich flame front) engines Withdiesel fuel, HCCI combustion shows two-stage heat release The firststage is low temperature kinetic reactions and the second stage ismain heat release regime[23] HCCI autoignition is controlled bylow temperature chemistry and the main heat release is dominated
by CO oxidation[24] The main advantage of the HCCI combustionover conventional combustion mode is the reduction of NOxand soot
in the exhaust Though the concept gives higher indicated thermalefficiency, inability to control the combustion phasing has led theresearchers to try different combustion control strategies e.g portfuel injection[25,26], early direct injection[27,28], multiple fuelinjection[29,30], compound combustion technology[31,32], nar-row angle injection[33–35], late direct injection[36,37], variable in-let temperature, variable valve timing, internal or external EGR, etc
[22] In addition, use of alternative fuels and fuel blends according tocompression ratios and operating conditions have much potential tocontrol the combustion phasing[22,38,39] Actually, fuels withdifferent autoignition points can be blended at varying ratios to con-trol the ignition point at various load–speed regions[40] This hasyield alternative fuels to be tested in HCCI engines[41–51] In die-sel–fueled HCCI engines, these combustion control technologiesare not often used alone The combination of several strategies helps
in achieving better effects on the combustion mechanism
2.2 Premixed Charge Compression Ignition (PCCI)
Premixed charge compression ignition or the partially premixedcharge compression ignition (PPCI) evolved from the HCCI combus-tion mode for the sake of better control over the start of combustion(SOC) In-cylinder homogeneity causes rapid combustion by simul-taneous ignition throughout the cylinder space and produces greatcombustion noise in the HCCI mode It is also very tough to controlthe combustion phases in HCCI mode PCCI process is introduced
to solve these problems It is not fully homogeneous like HCCI Itachieves desired ignition delay through enhanced charge motion,
Fig 1 Decreasing limit of NO x and PM [6]
Fig 2 Plot of local equivalence ratio vs flame temperature with different
Trang 3reduced compression ratio, higher injection pressure and extensive
use of EGR In the PCCI combustion process, fuel can be injected into
the combustion chamber in three ways, they are, advanced direct
injection, port fuel injection and late direct injection Advanced
direct injection and port fuel injection suffer from fuel spray
impingement on the cylinder walls and incomplete fuel evaporation
Consequently HC and CO emissions increased[52,53] However,
nar-row spray angle injectors and EGR reduce the wall impingement
[35,54,55] Late direct injection avoids the fuel-wall impingement
and gives a way to switch the combustion style to the conventional
at higher loads Researchers have tried to increase the high load
lim-its and reduce the emissions of PCCI by applying additives and
tun-ing fuel properties[56,57], variable valve timing, multiple injections
[58], and fuel–air mixing enhancement[10,59] A newer approach in
PCCI introduces air–fuel premixing by early injection followed by a
late injection of fuel pulse in the compression stroke, which governs
the onset of ignition Early injected fuel stratifies in the cylinder with
the air and as the compression stroke reaches near the TDC (top dead
center) it creates HCCI like condition When the late direct injection
occurs, the rich area of the late injection burns before the
fuel-lean homogeneous mixture This variable fuel–air mixture prevents
the entire charge from igniting instantaneously which gives a better
control over the combustion phase and rate Moreover adoption of
higher EGR permits longer ignition delay It permits better
premix-ing of air–fuel, results in less fuel-rich pockets followed by a low
temperature combustion, which simultaneously reduces NOxand
soot level[60]
2.3 Reactivity Controlled Compression Ignition (RCCI)
Reactivity controlled compression ignition is the newest
ap-proach where multiple fuels of different reactivity are injected at
scheduled intervals which governs the reactivity of the charge in
the cylinder for the desired combustion duration and magnitude
Mainly, in this approach, relatively low reactive fuel is injected
(port injection) very early in the engine cycle which mixes with
the air homogeneously Later on, a higher reactive fuel is injected
directly into the cylinder; it creates pockets of different air–fuel
ra-tios and reactivity, which govern the onset of combustion at
differ-ent times and rates
This process of combustion originated from the effort of the
researchers to reduce the EGR at higher loads while working on
the PCCI regime Inagaki et al.[61]investigated PCCI with two
dif-ferent reactive fuels and they succeeded to run the engine at higher
loads (up to 12 bar) with minimal EGR They reported,
stratifica-tion of fuel reactivity made it possible to reduce the heat release
rate and they achieved control over the combustion phasing
be-yond PCCI combustion In RCCI combustion process, the
combus-tion is staged[62]and proceeds from locally high reactivity fuel
areas to low reactivity fuel areas Such staging results in significant
expansion of the premixed combustion duration and consequently
produces high thermal efficiency, low pressure rise rate, low
emis-sion for higher loads up to 16 bar IMEP[63,64] Therefore, as the
combustion parameters are governed by the degree of reactivity
of the charge in RCCI process, it is likely that, different operating
conditions will need different fuel blends For this reason,
capabil-ity to operate with fuel blends covering the spectrum from neat
gasoline to neat diesel fuel (low reactive to high reactive) is
man-datory to get the best output from this kind of strategy
3 Emission analysis under LTC modes
This section investigates emission characteristics for diesel and
biodiesels under LTC modes Results are summarized inTables 1
of NOx, which is also called fuel NOx Formation of fuel NOxis quitecomplex because numerous intermediate species are there Severalhundred reversible reactions take place and still the true rate con-stant values are unknown Another process of NOx formation isprompt mechanism By this mechanism, the amount of NOxis quitelower than fuel and thermal NOx[66] Mainly, free radicals formed
in the flame front of the hydrocarbon flame generate this rapidproduction of NOx
Formation of NOxgenerally depends on oxygen concentration,in-cylinder temperature, air surplus coefficient and residence time
NOxforms both in the flame front as well as in the post flame gases
[67] In engines, flame reaction zone remains extremely thin, as thecombustion pressure is very high In addition, residence time isshort within this zone On the other hand, the burned gases, whichare produced early in the combustion process, are compressed to ahigher temperature than they reached just after the combustion.That is why NO formation on the post flame gases usually domi-nates over the flame-front-produced NO
3.1.2 NOxemission under LTC modes for diesel
In LTC modes, the combustion temperature is reduced by mixed or leaner mixture with moderated use of EGR, consequently
pre-NOxemission reduces[68] EGR hinders the O2flow rate into theengine and results in reduced local flame temperature, which helps
to reduce thermal NOx Again EGR extends the ignition delay whichindicates delayed start of combustion It results in lower pressureand temperature rise during the combustion The effect of lateinjection strategy on NOx emission is just like as ignition delay
[69] Many researchers have attained LTC modes like PCCI, HCCI
or RCCI, optimizing various parameters such as fuel reactivity tane number, CN of fuel), injection timing and pressure, dilution ofcharge by EGR, controlling the operating load Effects of theseparameters for attaining the LTC modes are discussed below con-cerning the literature review
(Ce-Valentino et al.[68]tested blends of fuels having lower cetanenumber, higher resistance to autoignition and higher volatilitythan diesel fuels to reach partially premixed LTC mode The fuelswere neat diesel, 20% and 40% blend of n-butanol with diesel.Along with EGR, late injection and higher injection pressure gavethem LTC mode for neat diesel They reported that higher injectionpressure allowed better mixing before the combustion and suffi-cient ignition delay provided by the use of EGR gave them a par-tially premixed LTC mode, which resulted in lower NOx Again,blends of n-butanol with diesel gave them premixed LTC mode
by elongating the ignition delay which can be attributed to thelower CN of n-butanol Longer ignition delay permitted earlierinjection as well as lower injection pressure with lower EGR rate
to achieve the LTC mode and obviously lower NOx Zhang et al
[21] also attempted lower cetane numbered gasoline and dieselfuel mixture (50:50) to reach premixed LTC mode They used singleadvanced injection (up to 28° BTDC, before top dead center) with
Trang 4Table 1
Emission for diesel at LTC.
Engine setup Operating
condition
Fuel Injection timing Percentage of
EGR/O 2 concentration
PCI D 8 to 25° BTDC Up to 50% E ; As EGR ", Increased Increased E ; As EGR ", [72]
(0.3 ppm) at 50%
EGR
Very low (0.03FSN)
at 50% EGR and 25° BTDC
torque:
54–80 N m 4S,1-cylinder, DI, super
E ; as IT advanced, 71% ; as IT swept from 9° to 15° BTDC
E ; as IT advanced, 92% ; as IT swept from 9° to 20° BTDC
[20]
50% load, charge air pressure:
2.4 bar (abs.)
Sweep of IT 9 to 20° BTDC
Up to 65% 0 g/kW h Very high for such EGR
0% and 40% Became 0 g/kW h as
40% EGR applied
E " with advancement
of IT and increment of EGR
E " with advancement
of IT and increment of EGR
At constant EGR remained almost same with the sweep
of IT
[74] Injection
71% n-heptane, 29% iso-octane, 1%
toluene
[124] 152° injection
injection angle
Lower soot than 152°
injection angle 160° injection
Lower soot than 152°
Trang 5Engine setup Operating
condition
Fuel Injection timing Percentage of
EGR/O 2 concentration
at 20° ATDC
360% increment as the 1st IT sweep from 50° to 60° BTDC while 2nd injection at 20° ATDC
40% increment as the 1st IT sweep from 50°
to 60° BTDC while 2nd injection at 20° ATDC
Injection angle: 60°
Sweep of 1st injection: 50 to 70°
BTDC Sweep of 2nd injection: TDC
20° ATDC 4S,1-cylinder, DI,
1500–
2500 rpm
Fischer–Tropsch fuel, FAME, N/A
About 50% On average 0.06 g/kW h E ; as speed and load ",
IMEP: 9.8 bar to 12.2 bar 4S,1-cylinder, DI, WC
70% heptane + 30%
E " as the% of heptane ", Commercial diesel gave highest emission
E " As the% of heptane ", Commercial diesel gave highest emission, On average
n-5 FSN
[166]
80% heptane + 20%
n-toluene, Commercial diesel fuel
Direct IP: 600 bar
Port IP: 4.14 bar
RCCI Port injected fuel:
iso-octane
Single direct injection: Sweep from 150° to 10°
BTDC
N/A Remained lower than 0.1 g/
kW h, advancement after 60°
BTDC caused rapid increment
Remained lower than 17.7 g/kW h, advancement after 70°
BTDC caused rapid increment
Remained lower than
5 g/kW h, advancement after 50° BTDC caused rapid increment.
Direct injected fuel:
n-heptane
Dual direct injection: 25°CA dwell in between, Sweep from 150°
to 10° BTDC
Remained lower than 0.1 g/
kW h, advancement after 50°
BTDC caused rapid increment.
Remained lower than
20 g/kW h, advancement after 60°
BTDC caused rapid increment.
Remained lower than
5 g/kW h, advancement after 40° BTDC caused rapid increment
Trang 6Engine setup Operating
condition
Fuel Injection timing Percentage of
EGR/O 2 concentration
4S,1-cylinder
DV: 2.44 L
CR: 16.1:1
IMEP: 9 bar
Direct IP: 400 bar
Port IP: 5.17ar
RCCI Port injected fuel:
No significant impact of% of DTBP,
0.75% DTBP gave the lowest E, Average E level was below 0.005 g/kW h.
[78]
On average 3 g/kW h.
Direct injected fuel:
gasoline + variable percentage of DTBP (di-tert-butyl peroxide)
No significant impact of DTBP percentage.
4S,1-cylinder, DI
DV: 1.9 L
CR: 16.7:1
RS: 1500 rpm
Rated IMEP: 4.5 bar
Fuel rail pressure: 860 bar
PPCI with advanced IT
Commercial diesel 26.6° BTDC Sweep of O 2
concentration from 15 to 9%
N/A E " 288% as the load ; to
48% "EGR and retarded IT gave;
NO x , 48% EGR gave lower than
1 g/kg-fuel all over the running condition.
"EGR and retarded IT gave " E, About 28% "for
IT sweep from 6.5° to 4.5° BTDC at 48%EGR
"EGR and retarded IT gave " E, About 80%
"for IT sweep from 6.5° to 4.5° BTDC at 48% EGR
Retarded IT gave ; E, About 53% ; as IT sweep from 6.5° to 4.5° BTDC at 48% EGR
[53]
Sweep from 8.5° to 2.5° BTDC
35–50% for 1.5 bar intake pressure
N/A Primarily dominated by
the mixture equivalence ratio Not affected significantly by ignition delay.
E " as the% of the gasoline "
Intake pressure 1.5 bar to 2 bar
40–60% for
2 bar intake pressure
N/A N/A 66% " as IT advanced
from 30° ATDC to
33° ATDC
185% " as IT advanced from 30°
Trang 7Engine setup Operating
condition
Fuel Injection timing Percentage of
EGR/O 2 concentration
Variable IP: 100–160 MPa
Injection cone angle: 148°
PPCI Low sulfur diesel, Retarded IT O 2
concentration 19.5%
Retarded IT, lower IP and EGR gave ; NO x for D Blends permitted slight advancement and reduction of IT and EGR respectively.
N/A " IP and " EGR
increased E Blends gave higher E than pure D
Higher% of n-butanol, retarded IT, higher IP and EGR gave ; PM
[68] 20% and 40% blend
of n-butanol with diesel
Injection system: high
pressure pump injection
PCCI Ultra-low sulfur
diesel
Sweep from 6° to 3° ATDC
Up to 45.4% E ; about 20% as CR ; N/A N/A Very high E for
higher load For lower load, E ; irrespective of CR
[76] Variable CR and
E " up to 100% as the 2nd IT retarded from TDC to 15° ATDC at a fixedU
E ; up to 64% as 2nd IT retarded from TDC to 15° ATDC at a fixedU
E " about 160% as the 2nd IT retarded from TDC to 15° ATDC at a fixedU
[71] Dual stage
injection Second injection:
sweep from TDC to 15° ATDC
(continued on next page)
Trang 8N/A As IT advanced eventually got
Diesel Sweep from
30.25° to 7.75°
ATDC
Up to 65% As IT retarded and EGR ", E ;
drastically EGR higher than 60% gave quite zero level E
As IT retarded and EGR
", E "
N/A As IT retarded and
EGR ", E ; drastically.
[16] EGR higher than 60%
gave quite zero level E
Direct IP: 800 bar
Port IP: 4.14 bar
RCCI operating loads: 9.6–
BTDC
Up to 47% Remained lower than 0.15 g/
kW h all through the running conditions.
At higher loads increased value.
Decreased as load increased
Constant low E throughout the operating conditions.
2nd direct injection: 36°
system: common rail
Intake pressure: 1.4 bar
Retarded injection assisted premixed homogeneous combustion.
ULSD Retarded single
injection close to TDC
25% " IP caused " E, As the IT
retarded emission ;
N/A N/A " IP caused ; E, At
retarded injection simultaneous ; of soot and NO x
Trang 9Emission for Biodiesels at LTC.
Engine setup Operating
condition
Fuel Injection
timing
Percentage of EGR/O 2 concentration
4S,1-cylinder, DI HCCI Diesel, 10° BTDC Up to 32% E " as BD content ", E ;
as EGR "
E " as BD content and EGR ",
E " as BD content and EGR "
E ; as BD content
", And E " as EGR "
[110] DV: 708 cm 3
100%, 65% and 30% blend of colza biodiesel.
Up to 32% increment for
BD than D
Average 16%
increment for BD than D
Up to 52% increment for
BD than D
Up to 61%
decrement for BD than D
17° BTDC Up to 60% E ; as EGR " Increased Increased E ; as EGR " [111] DV: 857 cm 3
Same trend like soy but relatively lower emission
Increased Increased Same trend like
soy but relatively higher emission 4S,4-cylinder, DI Late injection
and EGR enabled LTC
Various blends of soybean oil derived biodiesel.
Sweep from
20° ATDC to 5° ATDC
30% E " as IP and BD content
", E ; as IT retarded.
E ; as IP and BD content ", E " as IT retarded.
E ; as IP and BD content ",
Pure BD and higher IP gives ; emission,
[103] DV: 4.5 L
Up to 12% increment for higher IP for same blend Up to 22% decrement
for higher IP for same blend
Huge " at IT beyond
5° ATDC Up to 33%
decrement for higher IP for same blend.
Single and multiple injections with wide range sweep
of IT.
Up to 70%
according to the condition.
For low load retarded IT
up to 368°CA and higher EGR gave low emission.
At low load up to IT 368°CA remained very low and no effect of EGR
At low load Almost zero at
low load conditions
[105] DV: 1998 cm 3
Variable boost pressure.
up to IT 368°CA CR: 18.2:1
Single shot EGR assisted LTC for low loads.
remained very low and
E slight " as EGR "
RS: 1500 rpm
Multi-pulse EGR assisted HCCI for heavy loads.
At higher loads" EGR and; boost pressure gave" E,
At higher loads ;boost pressure gave" E,
At higher loads "
EGR gave " E for single injection, Two early injections gave good results
Lowest E for two early injections at 340°CA
Lowest E for two early injections
Lowest E for two early injections
4S,4-cylinder, DI EGR and late
injection assisted LTC mode
Biodiesel blend of Soy, Canola, Yellow grease and Tallow biodiesel.
Single shot injection (IMEP 8 bar)
Up to 70% For single shot injection
E got zero value at 50%
EGR For multi-pulse, retarded IT and reduced number of injection ; E.
Multiple injections (4 shots) gave reduced E
at lower EGR.
Otherwise increased.
Very " for single injection, Multiple (4shots) injections reduce the E
Very reduced value both for single and multiple injections.
[108] DV: 1998 cm 3
Variable intake pressure (1.2/
1.5 bar)
Multi-pulse injection (IMEP 6 bar) CR: 18.2:1
Wide range (347–367°CA) sweep of IT
Trang 104S,4-cylinder, DI Late injection
premixed LTC mode.
20, 50 and 100%
Soy-based methyl ester
E " for retarded IT and ; for " BD portion
Very low especially for 100% BD
[109] DV: 1.7 L
Up to 50% increment for
BD than ULSD at retarded IT
Up to 42% decrement for BD than ULSD at retarded IT
100% soy based methyl ester and 50% blend of soy based methyl ester with ULSD
5.9–7.1° BTDC (for LLTC)
45%(LLTC) N/A N/A E ; as BD content ",
ELTC, LLTC and CC gave 64%,25% and 66% ; respectively for B100 than ULSD
ELTC gave the highest E, 94% "
when used B100 than ULSD, CC gave lowest emission.
[143] DV: 1.7 L
17.3–24.1°
BTDC (for ELTC)
55%(ELTC) CR: 16:1
100% yellow grease based biodiesel.
17° BTDC (conventional single shot)
Up to 32% As load ;, EGR and BD
content " emission ; for single injection, Pilot- ignited HCCI ; emission.
Comparatively low E for pilot injection
Same trend as CO E " as load and
EGR " for single injection,
[112] DV: 857 cm 3
Variable BMEP:
3.3–8 bar
Pilot (4–8) injections starting at 17°
BTDC
Pilot-ignited HCCI gave very low E
Sweep of injection timing from
25° ATDC to 3° ATDC
N/A Retarded IT gave ; E
than early IT; except early IT, E " as BD content "
Very decreased emission for retarded injection
[107] DV: 300 cc
Up to 68% decrement than CC
Single injection (6°
BTDC)
Up to 38% As EGR " E ;, BD blend
showed slight " E than ULSD Single injection with EGR showed better results than double injections.
DV: 2.5 L
1600 rpm and 25% loading Double
injection (pilot: 25°
Trang 11Engine setup Operating
condition
Fuel Injection
timing
Percentage of EGR/O 2 concentration
4S, 1-cylinder, DI PPCI 20, 50 and 100%
palm oil methyl ester and sunflower methyl ester blended with diesel fuel.
Sweep from 32° BTDC to 4°
BTDC
O 2 concentration 9–10%
N/A At 19° BTDC E was
lowest for all the fuels.
PME gave less E than SME At " load Minimum E shifted to advanced CA o
At 21° BTDC E was lowest for all the fuels.100% SME gave lowest E At " load Minimum emission shifted to advanced CA°
DV: 0.477 L (1) IMEP: 3 bar,
1500 rpm, intake pressure: 1.5 bar CR: 14: 1
(2) IMEP: 6 bar,
2000 rpm, intake pressure:
ethanol (80–20%) blend.
Sweep from 8.3° to 7.5°
BTDC (for 100%
biodiesel)
40% for high load Up to 50%
for low loads.
E less than 1 g/kg-fuel at each loading for BD- ethanol
BD-ethanol blend less than 0.25FSN found almost all over engine load.
[106] DV: 425 cm 3
(1-cylinder)
Variable loading (IMEP: 0.25–
0.65 MPa)
13° to 10.5°
BTDC (for biodiesel–
N/A E ; as IP ", E ; as IP " up to 43% decrement
than D at higher IP
Tends to zero level of emission.
[144] DV: 522 cm 3
Variable injection pressure (400–
700 bar)
Up to 33% decrement than D.
N/A N/A E " with " intake air
50% burn point constant to 365°CA
N/A N/A E ; as the intake
temperature ;,
E ; as intake temperature ", E quite same for all the blends.
DV: 517 cc Variable intake
air temperature using 6 kW heater.
At 160–170 °C intake temperature E was 6 1 ppm
CR: 10.5:1
RP: 7.9 kW@3000 rpm
Injection system: air-assisted
partial-vaporization
port fuel injection.
(continued on next page)
Trang 124S,1-cylinder, DI, AC Fuel vaporizer
with port fuel injection assisted HCCI.
100% biodiesel 23° BTDC (for
direct injection)
N/A BD vapor induction gave
very low E,
As load ", E ;, up to 20% decrement for BD vapor induction
As load ", E ;, BD vapor induction emitted lowest
As load ", E ", BD vapor induction emitted almost 1/3 of the DI system
[113] DV: 662 cm 3
Variable loading
Up to 87% decrement at 2–4 bars BMEP than DI system
CR: 17.5:1
RP: 4.4 kW
RS: 1500 rpm
IP: 2 bar
Injection system: port
fuel injection with fuel
vaporizer, direct injection
4S,1-cylinder, DI Late injection
HCCI
Neat soybean biodiesel and 20–
50% blend of biodiesel with low sulfur diesel.
25°
ATDC,10°
ATDC and 3°
ATDC
N/A At IT 25° ATDC very "
E, At 3° ATDC the lowest
E for all fuel blends, E "
as BD content "
simultaneous reduction of soot and NO x
[142] DV: 300 cc
CR: 19.5:1
RS: 1500 rpm
IP: 600 bar
Injection system: common rail
Injection cone angle: 150°
4S,4-cylinder, DI Single late
injection Premixed LTC
Neat soy-based methyl ester.
Late injection 45% Very low E, Within the
range of 26–35 ppm
About 17% less emission than ULSD
About 30% less emission than ULSD
Engine out PM was over an order of magnitude higher than ULSD
[115] DV: 1.7 L
BMEP: 400 kPa CR: 16:1
MK type combustion
50% and 90%
blend of rapeseed methyl ester with European diesel
BMEP: 2 and
5 bar
At TDC for higher load.
Two stage EGR cooler configuration.
At " loads retarded IT gave low E like 30 ppm
900 bar for early injection,
1600 bar for late
Trang 13extensive EGR (50%) to gain PPCI mode and reported very low
amount of NOx(0.06 g/kW h) Masuda and Chen[70]also got same
type of results by the use of ethanol Mohammadi et al.[71]tried
two stage injection with 15–20% blend of ethanol with diesel fuel
to reach PCCI condition First injection was at 60° BTDC and they
reported that retarding the second injection along with 25% EGR
improved the NOxemission Low cetane number of ethanol
permit-ted very early injection giving a long ignition delay and moderapermit-ted
use of EGR suppressed the possible NOxemission from the second
injection
Not only the lower CN fuel blend but also late injection can
elon-gate the ignition delay to reach premixed LTC mode Jacobs and
Assanis[72]experimented with retarded injection and high EGR
rate which gave them increased ignition delay and combustion
duration Such experimental data confirmed the achievement of
PCI (premixed compression ignition) which showed reduced NOx
As the EGR rate increased and injection time retarded, emission of
NOxdecreased Bittle et al.[73]reported application of immense
EGR gave them about 94% decrement of NOxwhile they were trying
to get a universal determination of LTC mode attainment criteria
Retardation of injection timing with EGR gave them even better
re-sults of emission Han et al.[53]also reported same results by the
use of cooled EGR However, Kiplimo et al [74] reported lower
NOx even in early injection (20° BTDC) during PCCI combustion
strategy With EGR, they got about 75% decrement of NOxemission
and negligible difference of IMEP They worked out an optimum
spray-targeting zone where they got simultaneous reduction of
CO, HC and soot but could not manage to reduce the NOxwithout
EGR Without EGR, lower injection-pressure (80 MPa) with late
injection (2–15° BTDC) gave reduced amount of NOxwhile for
high-er injection pressure (140 MPa) advanced injection (20–40° BTDC)
resulted in reduced NOx This lower NOxfor higher injection
pres-sure at advanced injection timing can be attributed to the
achieve-ment of PCCI regime PCCI regime ensured lower in-cylinder
temperature and longer premixing time, which resulted in lower
NOx[75] On the contrary, Alriksson and Denbratt[20] reported
higher NOxwhen they tried advanced injection timing to keep the
BSFC (brake specific fuel consumption) low However, Kook et al
[16]investigated the effect of dilution and injection timing very
precisely on low temperature combustion emission In a fixed SOI
(start of injection) they observed that NOxemission decreased as
the dilution increased NOxemission was actually correlated with
the adiabatic flame temperature Again, in a fixed level of dilution,
retardation of injection timing gave lower NOx They commented
that earlier injection timing assisted by high level of dilution,
gener-ated greater adiabatic flame temperature than the late injection
even with less amount of dilution From NOx formation point of
view, we can infer that, as late injection LTC mode generates lower
temperature than early injection LTC mode, the former one is better
in this regard
Laguitton et al.[76]observed the effect of compression ratio on
NOxemission under a wide range of PCCI like combustion styles
Lowering the compression ratio gave them lowered NOx This effect
was more pronounced at higher loads They also observed that at
higher loads, combustion style proceeded to premixed-charge from
the combination of premixed and diffusion type combustion as the
injection timing was swept from very early to retarded Hence, NOx
emission was converged as the diffusion combustion suppressed
They also concluded that below a certain combustion temperature,
as in fully premixed charge combustion, NOxemission was
domi-nated by the air fuel ratio and the local oxygen concentration
rather than in-cylinder pressure and temperature
However, HCCI combustion mode has also been cited for reduced
NOxemission Pidol et al.[77]evaluated that
ethanol–diesel–biodie-sel blend could be used to keep NOx under the HCCI
acceptance criteria (<0.1 g/kW h) even at higher loads They used
two types of ethanol–diesel–biodiesel blends In one type of blendfossil diesel was used and to another synthetic Fischer–Tropsch die-sel was used In both cases 20% ethanol was used Use of ethanol ex-panded the ignition delay which helped to reach HCCI mode Theysucceeded to increase the IMEP to 12.2 bar and 11 bar for fossil die-sel and Fischer–Tropsch diesel blend respectively while for both ofthe cases NOxemission was below 0.10 g/kW h Reason for sustain-ing higher load with lower NOxof fossil diesel–biodiesel–ethanolblend can be attributed to the lower cetane number of the blendwhile Fischer–Tropsch diesel–biodiesel–ethanol blend sustained
at bit lower load for the reason of higher CN of Fischer–Tropsch sel Again higher volatility contributed by the ethanol permittedlower injection pressure to reach homogeneity which reduced thecombustion rate hence lowered NOx Kim and Lee[35]also cited verylow NOxat HCCI combustion style
die-While researchers were trying to increase the operating load der LTC mode keeping the NOxlower, RCCI combustion mechanismgave them very good results Splitter et al [19]got very reduced
un-NOx(below than 0.1 g/kW h) while they tried gasoline as the lowreactive fuel and diesel as the high reactive fuel at higher loads like14.5 bar Though use of ethanol–gasoline blend (85% ethanol) as thelow reactive fuel gave a bit higher NOxemission, but due to its lowercetane number it sustained even higher load (16.5 bar) than the gas-oline This dual-fuel mechanism needed comparatively lower EGR tokeep the NOxlower, which increased the thermal efficiency as well.Splitter et al.[78]also tried single fuel stock as the basis for both highand low reactive fuels They reported lower NOxbut the load level waslower than the former experiments They reported that at about 9 barload the emission characteristics were just similar to the dual fuel ap-proach of RCCI Again Splitter et al.[79]tried to reveal the impacts ofinjection timing on emission within RCCI combustion process Theyinjected iso-octane by port injection and n-heptane by direct injec-tion They reported, regardless of the single or double direct injection,injection timing had minimal effect on reduced NOxemission exceptbeyond 60° ATDC (after top dead center) NOxincreased beyond
60° ATDC for both injection style and that can be attributed to theless available mixing time Rapid ramp on the heat release rate at thatinjection timing clarified the scenario For double direct injection,they succeeded to retard the injection timing 10 more crank angledegrees as for such injection, fuel mass was more mixed
However, after such discussion we can come to some salientpoints which are following:
Regardless of PCCI, HCCI or RCCI combustion modes, lower NOx
depends on higher ignition delay and lower combustion ratewhich result in lower in-cylinder temperature and pressure riserate
Below a certain combustion temperature, NOxemission is trolled by air–fuel ratio and local oxygen concentration morethan in-cylinder temperature
con- Late injection premixed LTC is better than early injection mixed LTC mode regarding NOxemission
pre- Advanced injection assisted HCCI combustion mode needshigher EGR to control NOx emission as advanced injectioncauses higher in-cylinder temperature
RCCI combustion mode has succeeded to keep the NOx levellower at higher loads even with low EGR rate by the help of fuelreactivity gradient inside cylinder
3.1.3 NOxemission under LTC modes for biodieselsIncreased NOxis an established phenomenon while using bio-diesel in internal combustion engines[80–82] This increment isnot solely controlled by the change of a single fuel property, rathersome coupled mechanisms which may strengthen or cancel oneanother in various circumstances depending on combustion andfuel properties [83] Potentially contributing factors to make
Trang 14differences in NOx emission for biodiesel can be summarized as
injection timing, injection pressure-spray-mixing, ignition delay,
combustion stages and heat release, heat radiation from soot,
combustion temperature, fuel unsaturation and system response
issues[84]
Due to higher densities, bulk modulus of compressibility and
speed of sound, start of fuel injection is advanced for biodiesel
rel-ative to petroleum diesel in rotary/distributor-style fuel injection
systems[85,86] An advance in injection timing is considered as
a main reason for observed increases in NOxemissions with
biodie-sel as it helps to elevate diffusion reaction temperatures and
ulti-mately post flame gas temperature Of course, this incident is not
present in common rail fuel injection system[87] Szybist et al
[88] investigated NOx emission characteristics of different fuels
including biodiesel, altering injection timings at high and low load
conditions They observed at higher loads, relation between NOx
emission and injection timing was independent of fuel types but
at low loads emission characteristics were unique for each fuel
types Therefore, it can be said that, for higher loads increase in
NOxemission is due to the advancement of injection timing but
this is not true for low loads This confirms the existence of other
factors for increased NOxfor biodiesels along with the advanced
injection timing Such as, biodiesels have higher cetane number
which depicts shorter ignition delay[89–92] A short ignition delay
reduces the premixed burn, consequently increases the fraction of
diffusion burn[93] In the diffusion stage, the equivalence ratio at
the flame front is essentially always at a stoichiometric value[94]
Therefore, once the fuel is largely being consumed in a diffusion
flame, it is more relevant to consider the oxygen fraction within
it It is well-known that higher oxygen fractions yield higher diesel
combustion temperatures and NOx formation rates for diffusion
flame[95–97] Ullman et al.[98]has confirmed that, because of
in-creased oxygen content and dein-creased sulfur content, PM
forma-tion is comparatively low in the biodiesel combusforma-tion than
petroleum fuel Less PM depicts less radiation heat transfer which
increases post-flame gas temperature therefore increased NOx
emission[84] Again biodiesels have got higher degree of
unsatura-tion[99,100], and Graboski et al.[101]reported increase in NOx
emission, with the increase in unsaturation and decrease in mean
carbon chain length Finally, the changes in NOx emission for
biodiesel are largely dependent on pre-combustion chemistry
of hydrocarbon free radicals [102] It incorporates prompt
mechanism of NOxformation more in consideration, because it is
more sensitive to radical concentration within the reaction zone
whereas thermal mechanism remains quite unaffected by fuel
chemistry
Low temperature combustion is a promising technique for NOx
reduction not only for petroleum diesel but also for biodiesels,
though they produce much higher NOxthan petroleum diesel as
discussed earlier Veltman et al [103] experimented sweep of
SOI from 20° ATDC to TDC with a common rail injection with
moderated EGR to gain premixed LTC Electronically controlled
injection system ensured same injection timing for all the fuels
Still higher biodiesel content showed higher NOxemission which
contradicts the so called general clarification (advanced injection
for higher density) of the higher NOx They got reduced NOx(less
than 0.5 g/kW h) at 30% EGR at very retarded SOI due to lower
combustion temperature Though higher injection pressure caused
higher emission for increased combustion temperature, it was
insignificant at higher EGR as the emission was already low Weall
and Collings [104]also reported higher NOxemission for higher
injection pressure at premixed LTC Along with EGR and injection
pressure, intake pressure has also been cited for having command
on NO emission in premixed LTC NO Emission decreases as
intake pressure increases for biodiesels[105,106] Better premix
of charge was responsible for such results
Fang et al.[107]claimed that even in premixed LTC mode, gen content in biodiesel dominated the NOx emission more thanignition delay while they tried a sweep of SOI from 25° ATDC
oxy-to 3° ATDC They observed higher ignition delay of biodiesel thanEuropean low-sulfur diesel, which attributed to lower cetane num-ber and higher boiling point of biodiesel that slowed down thedroplet evaporation rate hence preparation of the ignitable air–fuelmixture In spite of higher ignition delay, increasing portion of bio-diesel showed increasing NOx at the conventional and late SOI.They attributed this phenomenon to the higher oxygen content
of the biodiesel They suggested a trade-off between ignition delayand oxygen concentration was responsible for this incident andconcluded commenting that late SOI was better to reduce the
NOxemission than early SOI Similarly, Zheng et al.[108]observedlower NOx for late injection but unlike Fang et al [107], theyobserved higher cetane number of biodiesels and commentedthat for this reason biodiesels sustained late SOI as well asEGR-incurred LTC better
Along with the oxygen concentration, injection timing and tion delay, combustion phasing has influence on NOxemission inthe case of premixed low temperature combustion From a com-mon baseline condition of combustion, created by keeping the50% mass fraction of the fuel burned point constant, Northrop
igni-et al.[109]got the NOxemission curves more or less same for allthe fuels they tested It proves the command of combustionphasing on NOx emission in premixed LTC modes They also ob-served the combustion location as a dominant factor of NOx
be higher
EGR assisted single injection LTC and pilot ignited HCCI bustion were investigated by Zheng et al [112] They reportedthat EGR was the instrumental factor to reduce the NOxat singleshot injection by reducing in-cylinder flame temperature anddiluting the oxygen concentration Pilot ignited HCCI with im-mense EGR reduced the NOx emission even more by helping toovercome mixing problem which led to homogeneity of the mix-ture Later, the same authors [111] experimented with variousbiodiesels with the same setup and got the same trend of resultsincluding lower emission for lower loads and higher emission forthe higher loads This can be attributed to the higher flame tem-perature for higher loads and vice versa However, they[105]got
com-an improvement at higher loads while they tried two early tions with higher boost pressure They mentioned better combus-tion process and improved combustion phasing due to enhancedfuel–air mixture responsible for such improvement Recently,Ganesan et al.[113] tried a unique technique to reach HCCI likecombustion and they succeeded to keep the NOx substantiallylow They used a fuel vaporizer with port fuel injection to achievethe mixture homogeneity as well as to attain HCCI like combus-tion process which gifted low level of NOx Pidol et al.[114]usedethanol–diesel blend, stabilized by biodiesel, and they got quite