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ORIGINAL CONTRIBUTION
Future Prospects of Low Compression Ignition Engines
M. A. Azim
Received: 29 June 2012 / Accepted: 24 February 2014 / Published online: 14 March 2014
� The Institution of Engineers (India) 2014
Abstract This study presents a review and analysis of the
effects of compression ratio and inlet air preheating on
engine performance in order to assess the future prospects
of low compression ignition engines. Regulation of the
inlet air preheating allows some control over the combus-
tion process in compression ignition engines. Literature
shows that low compression ratio and inlet air preheating
are more beneficial to internal combustion engines than
detrimental. Even the disadvantages due to low compres-
sion ratio are outweighed by the advantages due to inlet air
preheating and vice versa.
Keywords Low compression ignition �Inlet air preheating � Exhaust gas recirculation �Mixture homogeneity � Emission � Efficiency
Introduction
One of the most critical challenges for the future internal
combustion (IC) engine is to meet the near zero emission
with high cycle efficiency, which can be achieved through
homogeneous combustion. In this mode of combustion, fuel
mixture undergoes simultaneous heat release in the whole
combustion chamber characterized by homogeneous tem-
perature field [1]. Combustion in direct injection compres-
sion ignition (DICI) engines are unavoidably heterogeneous
because fuel is injected into combustion air only a few mil-
liseconds before the combustion is initiated, making homo-
geneity at the level relevant to chemical kinetics impossible.
So to obtain a desired level of homogeneity of fuel–air
mixture, decoupling of combustion from injection process is
a necessity for compression ignition engines that introduces
different approaches to fuel mixture formulation [2]. Com-
bustion emissions from IC engines are mostly NOx, CO, HC
(hydrocarbon), soot and smoke. High peak temperature in
combustion process governs the kinetics of NOx formation
while completeness of combustion governs the kinetics of
CO and HC formation. Soot forms in the fuel-rich zones of
the combustion chamber and its formation is influenced by
the mixing process rather than chemical kinetics, and smoke
mainly in the form of carbon are caused by the production of
soot. Complete microscale homogeneity of fuel, air and
residual gas mixture produces as much as 1000-fold reduc-
tion in NOx, which is close to the theoretical predictions by
chemical kinetics [3]. On the other hand, study on stratified
charge compression ignition engine [4] reveals that for a
fixed overall equivalence ratio / (fuel–air ratio to stoichi-
ometric fuel–air ratio), increasing homogeneity of fuel
mixture causes NOx emission to decrease to minimum and
then to increase and CO emission to increase to maximum
and then to decrease. However, emissions are known to
depend on the optimal combustion phasing and equivalence
ratio, while engine speed, load, exhaust gas recirculation
(EGR), compression ratio (CR), inlet air temperature and
fuel octane number influence the combustion phasing.
Homogeneous charge compression ignition (HCCI) and
premixed charge compression ignition (PCCI) are the two
potential approaches for mixture formation. HCCI relies on
early injection to achieve homogeneous air–fuel mixture.
Combustion is not directly controlled but begins to take place
whenever the appropriate conditions are reached. In HCCI
combustion, short duration of combustion and high rate of
heat release cause high rise in pressure and temperature,
which is detrimental to engine operation and life. While
M. A. Azim (&)
Department of Mechanical Engineering, Bangladesh University
of Engineering and Technology, Dhaka 1000, Bangladesh
e-mail: [email protected]
123
J. Inst. Eng. India Ser. C (January–March 2014) 95(1):25–30
DOI 10.1007/s40032-014-0103-7
PCCI relies on late injection to allow a certain degree of
stratification, it helps to restore control over the combustion
process partly. Air–fuel mixture is not completely homoge-
neous in PCCI as in HCCI, although combustion in both is
dominated by chemical kinetics instead of air–fuel mixing
[5]. The strategy in PCCI combines the efficiency of a diesel
engine and the low particulate emission of a gasoline engine.
In PCCI, combustion is slowed down because the mixture
does not ignite everywhere at once, especially in two-stage
injection. The lower rate of heat release in two-stage injec-
tion extends the operating range from low load to medium or
even full load. Its long duration of combustion and low rate of
heat release cause low rise in pressure and temperature which
is favourable to engine operation and life. For these reasons,
PCCI is the most promising combustion strategy for future
IC engines. It is noteworthy that recent research claims the
promising potential of HCCI approach for mixture formation
with stratified EGR. Test results for HCCI combustion with
EGR stratification show the decreased level of heat release
rate (HRR) as well as the decreased level of combustion
noise. This is because of delayed and slowed combustion
compared to homogeneous case due to the observed large-
scale stratification in the combustion chamber near the top
dead center (TDC) [6].
Combustion occurs in many different modes namely low
temperature combustion (LTC), high temperature com-
bustion (HTC) and diffusion combustion that influence the
engine emissions. Comparison of HCCI, PCCI (single
injection) and DICI combustion is shown in Fig. 1 against
crank angle (CA) for the same / [4, 7], hereafter zero
degree CA corresponds to piston position at TDC in com-
pression stroke. However, the nature of combustion is
greatly influenced by the EGR that controls the combustion
due to the reduction in oxygen concentration, presence of
combustion products and reduction in temperature rise
caused by the heat sink effect.
This study is intended to review and analyze the effects
of CR and inlet air preheating on engine performance
together with EGR in order to assess the future prospects of
low compression ignition engines.
Effects of Compression Ratio
It is generally expected that decrease in CR causes reduction in
gas pressure and temperature resulting in reduced rate of
combustion. Higher injection pressure in IC engine shows
more rapid combustion, and narrower and high peak heat
release patterns, while lower injection pressure that is asso-
ciated with low compression ignition results in broader and
low peak heat release patterns [7]. In other words, decrease in
CR causes reduction in charge density that reduces reactant
concentration and causes decrease in reaction kinetics. Thus
reduction in CR may be accompanied by LTC, which in
general causes increase in CO and HC emissions and decrease
in NOx emission. However, Christensen et al. [8] demon-
strated that efficiency of combustion in HCCI improves
almost linearly with reduced CR for any liquid fuel. Some
researchers e.g. Alkidas [9] and Damrongkijkosol [10]
showed combustion chamber crevices as source of HC
emission. This HC emission is found to reduce at lower CR as
less unburned fuel mixture is forced into the crevices during
compression and combustion, and emerges late in the
expansion and exhaust strokes. Some literature shows that CO
emission may be minimized by combustion with excess air,
rapid air–fuel mixing and long residence time at high tem-
perature. Kinetics of CO formation is not clearly understood
though it is mentioned earlier that completeness of combus-
tion governs its formation. Further, high CR accompanies
HTC where CO may originate by the dissociation of CO2 to
CO [11, 12]. It is possible that dissociation of CO2 to CO
occurs in the HTC zone to favor a lower Gibbs free energy
caused by the endothermic dissociation.
Goebel and Dutton [13] showed that transverse turbulence
intensity decreases with increasing compressibility, while
the streamwise turbulence intensity remains approximately
constant. In-cylinder velocity measurements show that at
high CR, turbulence intensity becomes less homogeneous
but its average intensity remains unaffected [5]. As turbu-
lence intensity affects the mechanism of mixing, it is
apparent that mixing of air–fuel through turbulence is not
affected by lower CR, rather may have some more mixing
due to increase in transverse turbulence intensity.
The test results of a DICI diesel engine by Kajitani [14]
show that reduction in CR causes decrease in minimum
brake specific fuel consumption, brake thermal efficiency
and NOx emission, and increase in HRR, exhaust gas
temperature and emission of CO and THC (total hydro-
carbon). The observed increase in HRR for the reduced CR
-30 -20 -10 0 10 20 30
0
60
120
180
240HTC
LTC
Diffusion combustion
DICI
PCCI
HCCI
HR
R (
J/ o C
A)
Crank angle ( oCA)
Fig. 1 Different modes of compression ignition combustion for /= 0.31.Continuous line HCCI, dashed line PCCI, dashed-dotted line
DICI. Data are from Lu et al. [4] and Sung et al. [7]
26 J. Inst. Eng. India Ser. C (January–March 2014) 95(1):25–30
123
may be due to moderate diffusion combustion after some
initial pre-mixture combustion. Effects of CR on the brake
thermal efficiency and NOx emission are shown in Fig. 2
where both are seen to decrease with the reduced CR.
Figure 3 shows that emissions of both CO and THC
increase with reduced CR, where the word ratio means a
value relative to the one at CR equals 17.7. So in DICI
diesel engine, reduction in CR is not beneficial to its per-
formance except reduction in NOx.
The test report of a HCCI dimethyl ether engine from
Hu et al. [15] demonstrates the decrease in peak pressure
and temperature in the first stage combustion, and increase
in brake thermal efficiency at low power level and increase
in CO and HC emissions with the reduction in CR, the HC
here referred to THC. Figures 4 and 5 show the increase of
brake thermal efficiency and CO emission with reduced
CR. It appears from Fig. 6 that change in CR has no
influence on NOx emission except at higher brake mean
effective pressure (BMEP). Figure 7 shows the effect of
CR on HC emission which is different from CO emission.
Low CR causes LTC that offers higher emission of HC. On
the other, combustion at low CR forces less unburned HC
into the crevices. HC emission from these two sources are
manifested such that THC emission is lower at CR = 8.0
than at CR = 10.7. This HCCI dimethyl ether engine
shows some benefits in terms of performance at low CR,
e.g. increase in brake thermal efficiency due to improved
combustion efficiency.
Beatrice et al. [16] carried out an exhaustive investiga-
tion on the effect of CR on the performance of a light duty
diesel engine. Their results show no significant gain in
PCCI application except drastic reduction in smoke emis-
sion. However, the test data of a PCCI diesel engine after
Laguitton et al. [17] show that reduction in CR causes
decrease in NOx and smoke emissions, and no significant
effect on fuel consumption. Figure 8 shows that fuel con-
sumption rate increases slowly with increasing CA at lower
CR. Effects of compression ratio on NOx and smoke
emissions are shown in Figs. 9 and 10 where the emissions
reduce with increasing CA at lower CR.
12 14 16 180.8
0.9
1.0
1.1
Thermal eff. NO
x
Rat
ios
of th
erm
al e
ff. ,
NO
x
CR
Fig. 2 Effect of compression ratio on brake thermal efficiency and
emission. Data are from Kajitani [14]
12 14 16 180
2
4
6
8
10 CO THC
Rat
ios
of C
O, T
HC
CR
Fig. 3 Effect of compression ratio on engine emissions. Data are
from Kajitani [14]
0.10 0.15 0.20 0.25 0.30 0.3510
15
20
25
30
35
CR=8.0 CR=10.7 CR=14.0
Bra
ke th
erm
al e
ff. (
%)
BMEP (MPa)
Fig. 4 Brake thermal efficiency under different CR. Data are from
Hu et al. [15]
0.1 0.2 0.30.0
0.5
1.0
1.5
CR=8.0 CR=10.7 CR=14.0
CO
(%
)
BMEP (MPa)
Fig. 5 Carbon monoxide emission under different CR. Data are from
Hu et al. [15]
J. Inst. Eng. India Ser. C (January–March 2014) 95(1):25–30 27
123
High injection pressure is found beneficial for better air-
fuel mixing and improved combustion in high compression
ignition. While low compression ignition requires low
injection pressure where it is desirable for cost effective
fuel injection system and for reduction in fuel pump
parasitics. Ignition in IC engines may be achieved at low
CR by increasing inlet air temperature using exhaust gas.
Regulation of inlet air temperature for achieving ignition
temperature by compression is described in the next sec-
tion. Increase in inlet air temperature causes reduction in
engine output, while decrease in operating pressure in low
compression engine causes reduction in its required weight,
e.g. some test results of diesel powered DICI engine show
that decrease in CR from 18 to 17.5 causes reduction in
weight/power ratio from 0.39 to 0.36 kg/kWh without
deterioration of performance. So low compression ignition
accompanied by inlet air preheating does not affect the use
of low compression ignition engine in land or space vehi-
cles considering weight/power ratio.
An overall assessment of the literature shows that lower
compression ratio is more beneficial to engines with regard
to its operation, life, combustion emissions and efficiency.
Now it appears that, to get some more control over the
combustion process, to reduce combustion emissions, and
to increase cycle efficiency in an IC engine, low com-
pression ignition is a promising avenue.
Effects of Inlet Air Preheating
Preheating of inlet air is known to induce faster vaporiza-
tion of fuel, better mixing of air–fuel and shorter ignition
delay in IC engines that result in less fuel adhering to in-
cylinder wall wetting. Less wall wetting reduces emission
of soot and THC, and better air–fuel mixing reduces
emission of CO. On the contrary, inlet air preheating is
detrimental to engine output. However, low operating
-2 0 2 4 61.8
1.9
2.0 CR=16.0 CR=18.4
Fue
l con
sum
ptio
n (k
g/h)
Crank angle ( oCA)
Fig. 8 Effect of compression ratio on fuel consumption. Data are
from Laguitton et al. [17]
0.1 0.2 0.30
200
400
600 CR=8.0 CR=10.7 CR=14.0
NO
x (%
)
BMEP (MPa)
Fig. 6 Nitrogen oxide emission under different CR. Data are from Hu
et al. [15]
0.1 0.2 0.3200
400
600
800
1000 CR=8.0 CR=10.7 CR=14.0
HC
(pp
m)
BMEP (MPa)
Fig. 7 Hydrocarbon emission under different CR. Data are from Hu
et al. [15]
-2 0 2 4 6
7
8
9
10
11 CR=16.0 CR=18.4
NO
x (g/
h)
Crank angle ( oCA)
Fig. 9 Effect of compression ratio on nitrogen oxide emission. Data
are from Laguitton et al. [17]
28 J. Inst. Eng. India Ser. C (January–March 2014) 95(1):25–30
123
pressure in low compression ignition engine compensates
the reduction in engine output as already mentioned.
Increase in inlet air temperature, in general, reduces
engine emissions and fuel consumption that are beneficial
to engines. Effects of inlet air temperature on the emissions
of NOx, CO and THC, and brake thermal efficiency without
EGR are shown in Figs. 11 and 12 from Kusaka et al. [18].
It appears in the figures that increase in inlet air tempera-
ture reduces CO and THC emissions, and increases brake
thermal efficiency and NOx emission.
Control of inlet air temperature is made by heating the
air with engine exhaust in a heat exchanger. Investigation
shows significantly different effects of hot and cold EGR
on engine performance. However, this effect of EGR
temperature diminishes as its percentage increases in the
oxidizer (mixture of inlet air and EGR) [19]. On the other
hand, literature shows that effect of inlet air preheating
together with EGR provides better result on engine per-
formance than their individual effect [18, 20]. Oxidizer
temperature To is the initial temperature of the compression
process in an IC engine which can be calculated from heat
balance for the mixture of preheated inlet air and EGR as
xCpe þ 1� xð ÞCpa
� �To ¼ xCpeTe þ 1� xð ÞCpaTpa ð1Þ
where Cpe and Cpa are specific heats for the exhaust gas and
preheated air, Te and Tpa are temperatures of exhaust gas
and preheated air, and x is the percentage of EGR in the
oxidizer. If the temperature at the end of compression is the
ignition temperature Tig then assuming isentropic
compression
To ¼ TigCR1�co : ð2Þ
Substituting Eq. (2) in Eq. (1), preheated air temperature
may be expressed as
Tpa ¼ ATigCR1�co � xCpeTe
� �= 1� xð ÞCpa ð3Þ
where A ¼ xCpe þ 1� xð ÞCpa and co equals specific heat
ratio of the oxidizer. A typical temperature of the preheated
air is calculated using Eq. (3) and found to be Tpa = 49 �C
for the given Tig = 700 �C, Te = 400 �C, CR = 10,
x = 0.2, Cpa = 1.005 kJ/kgK, Cpe = 1.15 kJ/kgK and
co ¼ xce þ 1� xð Þcpa ð4Þ
where the specific heat ratios of exhaust gas ce = 1.33 and
preheated air cpa = 1.4. The calculated preheat tempera-
ture 49 �C is quite small to produce knock compared to a
safe intake temperature of 180 �C with 50 % EGR at 40 %
load [18]. Further, Maurya and Agarwal [21] reported a
stable operating range in HCCI combustion in terms of /and inlet air temperature varying from 110–160 �C for
gasoline and 120–150 �C for methanol. Equation (3) can be
used to adjust preheat temperature and CR for achieving
auto-ignition for a variety of fuels. Christensen et al. [8]
have studied the relation between the inlet air temperature
and the CR needed to get auto-ignition for fuels with dif-
ferent octane numbers, e.g. auto-ignition of a fuel (80 %
0 40 80 1204
6
8
10
12
14 NO
x
NO
x (g/
kWh)
Inlet temperature ( oC)
10
15
20
25
30
COC
O (
g/kW
h)
Fig. 11 Effect of inlet temperature on exhaust emissions. Data are
from Kusaka et al. [18]
0 40 80 12015
30
45
60
75
TH C
TH
C (
g/kW
h)
Inlet temperature ( oC)
20
25
30
35
40
The
rmal
eff
(%
)
E ff
Fig. 12 Effect of inlet temperature on emission and thermal
efficiency. Data are from Kusaka et al. [18]
-2 0 2 4 610
15
20
25
30
C R= 16.0 C R= 18.4
Smok
e (m
g/m
3 )
Crank angle ( oCA)
Fig. 10 Effect of compression ratio on smoke emission. Data are
from Laguitton et al. [17]
J. Inst. Eng. India Ser. C (January–March 2014) 95(1):25–30 29
123
diesel and 20 % gasoline) at TDC requires CR = 10 for an
inlet air temperature of 90 �C while that requires CR = 14
for an inlet air temperature of 30 �C.
Conclusions
The prospects of low compression ignition engines are
reviewed and analyzed in this paper. Such engine requires
inlet air preheating for achieving the ignition temperature
and proves to be potential to meet stringent emission regu-
lations and high cycle efficiency of the future IC engine. The
effects of low CR and inlet air preheating together with EGR
on IC engine performance are summarized as follows:
(1) Change in CR does not affect average intensity of
turbulence, thus mixing of air–fuel and efficiency of
combustion are not affected by the reduction in CR.
While reduction in CR causes increase in brake
thermal efficiency in HCCI combustion.
(2) Low compression ignition offers low peak tempera-
ture and pressure that cause reduced emissions of
NOx and THC. Moreover, low operating pressure is
useful for lighter engines and reduction in fuel pump
parasitics.
(3) Inlet air preheating reduces emission of CO and THC
and consumption of fuel, and increases thermal
efficiency with reduced engine output. This reduction
in output may be compensated in low compression
ignition engine by reduction in its required weight
due to low operating pressure.
(4) Inlet air preheating allows some more control over the
combustion process in compression ignition engine
and thus control over start of ignition.
(5) Varieties of fuels with wide range of ignition
temperatures may be found suitable for low com-
pression ignition by adjusting preheat temperature of
the inlet air.
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