Part-load performance and emissions of a spark ignition engine

Energy Conversion and Management 88 (2014) 928–935
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Part-load performance and emissions of a spark ignition engine fueled
with RON95 and RON97 gasoline: Technical viewpoint on Malaysia’s fuel
price debate
Taib Iskandar Mohamad a,⇑, How Heoy Geok b
a
Department of Mechanical and Materials Engineering & Centre for Automotive Research, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia,
43600 Bangi, Selangor, Malaysia
b
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history:
Received 10 March 2014
Accepted 3 September 2014
Keywords:
Automotive engine
Gasoline
RON
Part load operation
Emission
Performance
Heat release
Fuel price
Public perception
a b s t r a c t
Due to world crude oil price hike in the recent years, many countries have experienced increase in gasoline price. In Malaysia, where gasoline are sold in two grades; RON95 and RON97, and fuel price are regulated by the government, gasoline price have been gradually increased since 2009. Price rise for RON97
is more significant. By 2014, its per liter price is 38% more than that of RON95. This has resulted in escalated dissatisfaction among the mass. People argued they were denied from using a better fuel (RON97).
In order to evaluate the claim, there is a need to investigate engine response to these two gasoline grades.
The effect of gasoline RON95 and RON97 on performance and exhaust emissions in spark ignition engine
was investigated on a representative engine: 1.6L, 4-cylinder Mitsubishi 4G92 engine with CR 11:1. The
engine was run at constant speed between 1500 and 3500 rpm with 500 rpm increment at various partload conditions. The original engine ECU, a hydraulic dynamometer and control, a combustion analyzer
and an exhaust gas analyzer were used to determine engine performance, cylinder pressure and emissions. Results showed that RON95 produced higher engine performance for all part-load conditions
within the speed range. RON95 produced on average 4.4% higher brake torque, brake power, brake mean
effective pressure as compared to RON97. The difference in engine performance was more significant at
higher engine speed and loads. Cylinder pressure and ROHR were evaluated and correlated with engine
output. With RON95, the engine produces 2.3% higher fuel conversion efficiency on average but RON97
was advantageous with 2.3% lower brake specific fuel consumption throughout all load condition. In
terms of exhaust emissions, RON95 produced 7.7% lower NOx emission but higher CO2, CO and HC emissions by 7.9%, 36.9% and 20.3% respectively. Higher octane rating of gasoline may not necessarily beneficial on engine power, fuel economy and emissions of polluting gases. Even though there is some
advantage using RON97 in terms of emission reduction of CO2, CO and HC, the 38% higher price and
higher NOx emission is more expensive in the long run. Therefore using RON95 is economically better
and environmentally friendlier. The findings provide some techno-economic evaluation on the fuel price
debate that surround the Malaysia’s population in the recent years. The increased of fuel price may have
limited their ability to use higher octane gasoline but it did not negatively affecting the users as they
perceive.
Ó 2014 Elsevier Ltd. All rights reserved.
Abbreviations: ATDC, after top dead centre; BMEP, brake mean effective
pressure; BSFC, brake specific fuel consumption; BTDC, before top dead centre;
CFR, Cooperative Fuel Research; CO, carbon monoxide; CO2, carbon dioxidel; COV,
coefficient of variation; CR, compression ratio; DOHC, double overhead camshaft;
FCE, fuel conversion efficiency; HC, hydrocarbon; MON, motor octane number; NOx,
oxides of nitrogen; OHV, overhead valve; PRF, primary reference fuels; RON,
Research octane number; rpm, revolution per minute; SI, spark ignition; SIDI, spark
ignition direct injection.
⇑ Corresponding author.
E-mail address: [email protected] (T.I. Mohamad).
http://dx.doi.org/10.1016/j.enconman.2014.09.008
0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Producing optimal balance between power density, fuel economy and exhaust emission have always been one of the major challenges in automotive technology. This challenge must address the
complex relation between fuel characteristics and engine architecture. Combustion behavior, energy conversion and emission formation of a certain formulated fuel can easily change with
different engine design and operating parameters. The reverse is
T.I. Mohamad, H.G. How / Energy Conversion and Management 88 (2014) 928–935
also true where specific engine reacts differently with different
fuels. Studies on automotive fuels are extensive but far from
exhausted because fuel formulations and engine designs are evolving with technological advancement. Therefore investigating these
complex relations will always relevant as new breeds of engine
architecture and advance fuel formulation are continuously
explored.
Gasoline is the most widely used fuel for on-road vehicles
worldwide and therefore the main power source for this energy
user. Demand for gasoline is continually expanding with the
increasing number of on-road vehicles supported by the development of fuel efficient direct gasoline injection and hybrid engines
[1]. Gasoline engine formulation is mainly indexed with respect
to octane numbers; RON or MON. Octane ratings are considered
among the most important parameters in determining fuel quality
and influencing engine performance and emissions of the vehicle
[2]. RON and MON are determined by Cooperative Fuel Research
engine according to the ASTM procedures D-2699 and D-2700
respectively. An alternative method to determined RON and MON
was realized by associating distillation curves (ASTM D86) with
multivariate calibration (PLS – Partial Least Squares) [3]. Both the
RON and MON scales are based on two paraffinic hydrocarbons
which define the two ends of the scale; iso-octane is assigned
the value of 100 and n-heptane the value of zero in both the scales.
Primary reference fuels (PRF) are the blends of these two components which define the intermediate points in the RON or MON
scale [4]. Thus theoretically, RON95 and RON97 are blend of 95%
and 97% iso-octane with 5% and 3% of n-heptane by volume respectively. In both RON and MON scales they are PRF 95 and PRF 97.
Many studies have been conducted to investigate the effects of
RON number to engine performance and emissions. Even though
many results have been published, the studies are far from
exhausted since fuel-engine relation is complex and sensitive to
fuel formulations and engine architecture. Each new engine model
requires different range of octane number for optimal performance
due to tolerances in production and variations in design and the
expected driving conditions. At the market end, users are attracted
to use higher RON gasoline due to the belief that the higher-octane
rating produces better engine performance.
An experimental investigation on a low compression ratio (CR)
engine (8.0:1) with fuel carburetion with gasoline RON91 and
RON95 showed that the RON91 produces 4.2–4.8% more power
and 5.6% less brake specific fuel consumption (BSFC) while emitting 5.7% and 3.4% lower CO and HC respectively. The outcomes
were attributed to the concentration of tetra alkyl in the fuel.
The study concluded that using higher-octane rating gasoline than
the engine requirement did not increase engine performance but
increased exhaust emissions, which led to increased maintenance
expenses [5].
Positive effects of higher RON were realized in engines with
higher CR and with turbocharged intake. A study on a high CR
(13.0:1) spark ignition direct injection (SIDI) showed that a high
RON fuel with high aromatics content produced significant torque
improvement under high load. Engine torque and efficiency
increased by 13% and 21% respectively compared to the original
engine CR of 9.8:1 [6]. The impact of RON and MON on a turbocharged multi-point injection spark ignition (MPISI) engine was
studied and favourable power, consumption and emissions results
were found with higher RON [7]. The effect of turbocharging
resembles increasing CR. It benefit from better knock resistance
of higher RON. Using RON higher than engine requirement with
the original spark timing setting not only reduces brake thermal
efficiency but also increased brake specific fuel consumption, CO
and HC emission but can be reduced with a more advanced spark
timing [8]. Further improvement of turbocharged engine with high
octane gasoline was realized with direct fuel injection and adaptive
929
intake valve lift [9]. Inclusion of exhaust gas recirculation (EGR) in
the DISI engine with RON97 gasoline was studied which resulted in
increased thermal efficiency [10]. A study on engine response to
lower RON than engine requirement (RON91 vs. RON95) showed
that emission behavior is slightly affected [11]. Only a very slight
increase in total unburned hydrocarbon and decrease in CO emissions with increasing RON was realized. However, NOx had
revealed no direct correlation with RON.
Gasoline octane improvement were achieved with various fuel
blends. In a work done to improve RON using olefins, ETBE and
MTBE, power increase with higher RON gasoline was achieved by
increasing CR, modifying spark timing, varying intake system and
valve timing [12]. Increasing RON with the addition of MTBE alone
showed that the optimal engine performance and emission reduction occurred at 10% MTBE content [13]. Improving octane number
by blending gasoline with 2,5-dimethylfuran (DMF) and ethanol
was studied and results showed that DMF blend produces higher
cylinder peak pressure and higher NOx, while both blends reduces
HC and CO emissions [14]. Gasoline RON was increased with small
fraction of oxygenated additives; ethyl acetate and methyl acetate
[15]. Wide distillation fuel is an option to improve compression
ignition engine by blending diesel with gasoline at different octane
number. The blend resulted in low soot emission, improved thermal efficiency and little sensitivity to gasoline octane number [16].
The effects of high RON on low temperature combustion (LTC)
Homogeneous Charge Compression Ignition (HCCI) engine was
studied numerically using zero-dimensional single-zone combustion model. Availability balance model was used and results
showed that rising iso-octane volume fraction in primary reference
fuel (PRF) has negative effects of increased irreversibility and
decreased heat transfer availability [17]. Optical techniques were
used to study combustion behavior of gasoline. For example Merola et al. investigated knock behavior of a SI engine fuel with various
gasoline octane rating with flame and radical species detection
[18]. Detailed chemical chemistry of gasoline with respect to
octane rating was studied. Laminar burning velocities of various
blend of iso-octane and n-heptane have been measured using the
heat flux method [19]. Turbulent burning velocity was investigated
with Schlieren imaging technique by measuring the expanding
flame kernel [20].
According to the mentioned works, using the right gasoline
requirement by the engine is more beneficial than using a higher
or lower-octane gasoline for different type of engines and vehicles.
1.1. Vehicle and gasoline perspective in Malaysia
By the end 2013, there were nearly 23 million on-road vehicles
in Malaysia. According to the statistic produced by Malaysian Institute of Road Safety Research (MIROS), vehicle ownership has doubled since 1997 from 2.8 to 1.3 person per vehicle in 2012. Fig. 1
shows the growth of passenger car in Malaysia between 1980
and March 2014. During this period, nearly 5.5 million of new passenger cars were registered. The total new car increased significantly between 1980 and 2005 with high annual growth. After
that period the annual counts fluctuated slightly. This is reflected
in the annual growth was reduction from peak 111% in 1995 to
3–4% in the current time, making the total period average growth
of 19%. The trend of new car purchase is closely related to the economy of the country, like most developing countries. Apart from
that fuel price also affects the buying trend. Vast majority of passenger cars are fueled with gasoline while the remaining minority,
mainly taxis, are powered with diesel and natural gas.
Since 2009 the gasoline was sold in Malaysia only in two grades;
RON95 and RON97. Gasoline price is controlled by the government.
In the recent years, the government increased the gasoline price as
a result of global oil price hike. Fig. 2 shows the trend of gasoline
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T.I. Mohamad, H.G. How / Energy Conversion and Management 88 (2014) 928–935
Fig. 3. Gap between RON95 and RON97 price.
Fig. 1. Passenger car growth trend 1980–2014. Source: Malaysia Automotive
Association.
From historical data, the 4GXX engine represents the biggest fraction of engines used in passenger cars of the studied vehicle owner
group to this date. The ownership of the cars using these engines
falls mainly to the lower and medium income people, whose main
concerns are running and maintenance cost. The 4G92 model is the
middle of the range and selected for the experimental investigation. The outcomes of this experimental investigation can be used
to verify people view about RON number related to fuel economy,
performance and emission.
2. Experimental set up and procedures
Fig. 2. Fuel price trend in Malaysia from 2009.
price since mid-2009 when the significant price increase started to
take place. RON97 price are being increased from MYR1.80/liter to
MYR2.90/liter, while price for RON95 only increased from
MYR1.75/liter to MYR2.10/liter. Fig. 3 shows the price difference
between the two gasoline grades where at time this difference went
up to 60%. There have been major complain responding to this. People argued that they were denied from using higher grade (thus
more power and better fuel economy) gasoline. This has ignited
an interest to look into this matter from technical point of view.
In order to select the representative engine model for the study,
a careful selection must be done taking into account the most
widely used engine models and the income group of people owning the corresponding car models. The Mitsubishi 4G-series, which
can be found in majority of the national car, Proton was selected
for investigation. Proton car represents more than 30% of passenger
car fleet in the country. This national car company of Malaysia was
established in 1985 in cooperation with Mitsubishi Motor of Japan.
Between 1985 and the early 2000s, Protons cars were mainly
badge engineered of Mitsubishi models. Mitsubishi series engine
4Gxx series have been used until 2004. The 4Gxx engines were
then replaced gradually by the Campro engine which was first
introduced in 2004 on the Gen-2 model, the first all-Malaysia built
car. The 4G engines comes in 6 variants; 4G13 (1.3L), 4G15 (1.5L),
4G92 (1.6L), 4G93 (1.8L), 4G63 (2.0L) and 4G68 (2.0L turbo diesel).
The engine used in the experiments was a 1.6-liter, 4-cylinder
Mitsubishi 4G92 DOHC engine with compression ratio 11:1. Engine
specifications are listed in Table 1. The experimental schematic
diagram shown in Fig. 4. A hydro-kinetic dynamometer was used
to absorb the power produced by the engine and has the capability
of applying brake load to engine shaft. Engine torque, rotational
speed and air intake were measured by the dynamometer control
unit. A throttle actuator and original engine ECU provide control
to engine operation. Gasoline consumption was measured using a
volume-scaled pipette and time recording. A Kistler 6125B pressure sensor measures the engine in-cylinder pressure which was
then processed by a Dewetron DEWE5000 combustion analyzer.
The time and crank angle resolved cylinder pressure data with
1000 Hz sampling rate were processed by DEWE5000 software.
In order to assure minimum spark advance for best torque (MBT)
operation during the whole course of engine test, power, torque
and IMEP were monitored from the combustion analyzer. A 4-gas
Kane-May exhaust gas analyzer was used to measure the exhaust
emissions of CO2, CO, HC and NOx. The probe of gas analyzer was
Table 1
Specifications of the Mitsubishi 4G92 engine.
Descriptions
Parameter
Type
Fuel system
Combustion chamber
Number of cylinder
Number of valve
Compression ratio
Firing order
Displacement volume (cm3)
Bore (mm)
Stroke (mm)
Maximum rated power (kW/rpm)
Maximum rated torque (N m/rpm)
In-line OHV, DOHC
Sequential port injection
Pent-roof type
4
16
11.0:1
1–3–4–2
1597
81
77.5
108/7000
149/4500
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T.I. Mohamad, H.G. How / Energy Conversion and Management 88 (2014) 928–935
1. Engine
2. Dynamometer
3. Throttle valve
4. Pressure sensor
5. Combustion analyzer
6. Main controller
7. Oxygen sensor
8. Exhaust gas analyzer
Fig. 4. Experimental set up.
3. Results and discussion
Engine performance in response to RON95 and RON97 gasoline
were investigated for under various engine speeds and loads (LL,
ML, HL). Parameters determined were brake torque, brake power,
brake mean effective pressure (BMEP), fuel conversion efficiency
(FCE), brake specific fuel consumption (BSFC) and exhaust emissions. All results were derived either directly from measured
experimental data or calculated based on Heywood [21]. Each
point of the measured data were taken from 10 sets of experiments
from which 1000 thermodynamic cycles were averaged, showing
reliable outcomes with low error (±2%) and COV (<1).
Engine performance are represented by brake torque (Fig. 5),
brake power (Fig. 6) and BMEP (Fig. 7) as they are directly correlated. RON95 produced 0.4–9% more brake torque, brake power
Table 2
Fuel properties.
Descriptions
RON95
RON97
Research octane number
Initial boiling point (°C)
Final boiling point (°C)
Density @ 15 °C (kg/L)
Reid vapour pressure (kPa)
95
35.7
197.2
0.764
66
97
35.7
200.5
0.7639
65.5
100
Brake torque, Nm
90
R95 LL
R97 LL
80
R95 ML
70
R97 ML
60
R95 HL
50
R97 HL
40
30
20
10
0
1500
2000
2500
3000
3500
Engine speed, rev/min
Fig. 5. Brake torque vs. engine speed at various engine loads.
40
Brake power, kW
placed 1 m downstream the exhaust manifold from exhaust valves.
An oxygen sensor was placed in the exhaust manifold to monitor
the equivalence ratio of cylinder charge.
The engine was run at constant speed ranging from 1500 to
3500 rpm with 500 rpm increment at three different engine load
conditions: 1.25 N m, 2.5 N m and 5.0 N m engine loads, denoted
by low load (LL), medium load (ML) and high load (HL) respectively. The engine load is supplied by the frictional braking force
from the dynamometer. During the test, all data are taken after
the engine oil has reach a stable temperature range between 80
and 90 °C. At each speed and load condition, minimum spark
advance was set for best torque from the ECU where spark timing
can be controlled and monitored from the combustion analyzer.
For each data point, a set of 10 experiment data was average, in
which 1000 thermodynamic cycles were averaged and the error
was found within ±2%.
The fuels used in the experiment were PETRONAS (the Malaysia’s National Petroleum Company) commercial RON95 and
RON97 gasoline. Detailed gasoline additives (aromatics, etc.) were
not obtained from the producer as it is considered confidential.
Some thermal and physical properties of gasoline used are shown
in Table 2. These properties were derived from lab measurement.
R95 LL
35
R97 LL
30
R95 ML
25
20
R97 ML
R95 HL
R97 HL
15
10
5
0
1500
2000
2500
3000
3500
Engine speed, rev/min
Fig. 6. Brake power vs. engine speed at various engine loads.
and BMEP than that of RON97 throughout the speed range. At
low load and low speed, the brake torque and brake power shows
little difference but when engine speed is greater than 2500 rpm
and load increases, the differences in performance increases. In
all load conditions beyond 2500 rpm engine output were better
with RON95. Similar trend was found in BMEP with the exception
to low load conditions where superiority of RON95 was only visible
at 3500 rpm. On average, RON95 produces 4.4% higher brake torque, brake power and BMEP compared to RON97. The results are
in agreement with the findings of Sayin et al. [5]. The higher performance found in RON95 operations is attributed to the original
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700
R97 LL
600
R95 ML
500
400
800
R95 LL
R95 LL
R97 LL
700
R97 ML
BSFC, g/kWh
BMEP, kPa
800
R95 HL
R97 HL
300
R95 ML
600
R97 ML
500
R97 HL
R95 HL
400
200
300
100
0
1500
2000
2500
3000
3500
Engine speed, rev/min
200
1500
2000
2500
3000
3500
Engine speed, rev/min
Fig. 7. Brake mean effective pressure vs. engine speed at various engine loads.
Fig. 9. Brake specific fuel consumption vs. engine speed at various engine loads.
engine construction optimized for lower fuel octane rating, with
relatively low compression ratio, in which the advantage of higher
RON cannot be realized to its maximum.
Fig. 8 presents the fuel conversion efficiency of the engine in
response to the fuels and different speed and loads. The general
trend showed that FCE increased as engine speed and load
increased. The FCE of RON95 is 0.7–7.7% higher than that of
RON97 fuel throughout the speed range. The results show that
on average RON95 achieves 2.3% better FCE compared to RON97.
Reflecting the average output power superiority of 4.4% with
RON95 but 2.3% average benefit in FCE, in the absence of fuel heating value data, one may suggest that RON95 calorific value is more
than that of RON97.
BSFC behavior between the two fuels did show predictable
trend. Generally, fuel consumption decreased with increasing load
and speed. RON97 showed better fuel consumption with lower
BSFC at higher engine loads condition as shown in Fig. 9. On average, RON97 resulted in 2.3% lower BSFC than RON95. The opposing
behavior of these two fuels with respect to FCE and BSFC can be
explained by the variation of its properties. RON97 have 97% of
its content as iso-octane, but remaining percentage includes many
anti-knocking substances, not limiting to n-heptane. With 97% isooctane, calorific value is generally higher thus the engine requires
less fuel mass per unit power produced, resulting in lower BSFC. On
the other hand, because RON95 possesses lower calorific value but
produces higher power on average, the resulting FCE is higher. This
analysis is however opposing to the discussion of calorific value in
the previous paragraph. Both explanation has the merit but could
only be confirmed with the presence of fuel calorific value data
and detailed knowledge of additives.
In order to better understand the better performance of RON95,
cylinder pressure and rate of heat release at selected test conditions are shown in Figs. 10–14. Table 3 summarizes the important
parameters from Figs. 10–12. As shown in Fig. 10, at 2500 rpm and
medium load, the peak pressure of RON95 is higher than RON97
(29.8 bar vs. 28.6 bar) and happens at later crank angle (12.5° ATDC
vs. 11.8° ATDC) which is favorable in producing more power. At
high load, even though the peak pressure of RON97 is higher and
occurs at later CA (37 bar @ 14.7° ATDC vs. 34.2 bar @ 12.5° ATDC),
higher pressure can be seen before TDC thus more work is wasted
during compression stroke (CA < 360°). This resulted in less torque
and power compared to RON95. Similar trends can be seen at
3000 rpm and 3500 rpm, shown in Figs. 11 and 12 respectively.
At higher speeds, peak cylinder pressure increased as a result of
decreased combustion durations and intensified burning rate. At
the highest tested speed, the peak pressure of RON95 become closer to RON97 peak. Lower overall pressure during compression
stroke, mainly due to longer ignition delay, is advantageous to
RON95. At high load, the peak cylinder pressure of RON97 are more
than those of RON95 but the behavior is reversed at medium load.
It is also noted that the peak pressure of RON97 happened earlier at
medium load but later than RON95 at high load except at
3500 rpm.
The heat release rate pattern at 3500 rpm and 2500 rpm were
chosen for comparison and further explain the phenomena. Plot
of ROHR between 40° BTDC and 40° ATDC are shown in Figs. 13
35
R95 LL
R97 LL
30
R95 ML
FCE, %
R97 ML
25
R95 HL
R97 HL
20
15
10
1500
2000
2500
3000
3500
Engine speed, rev/min
Fig. 8. Fuel conversion efficiency vs. engine speed at various engine loads.
Fig. 10. Cylinder pressure at 2500 rpm.
T.I. Mohamad, H.G. How / Energy Conversion and Management 88 (2014) 928–935
933
Table 3
Cylinder peak pressure.
Engine
1speed
Fuel-load
Peak cylinder
pressure, bar
Peak cylinder pressure
location, °CA ATDC
2500 rpm
R95
R97
R95
R97
ML
ML
HL
HL
29.8
28.6
34.3
37
12.5
11.8
12.5
14.7
3000 rpm
R95
R97
R95
R97
ML
ML
HL
HL
37
34.8
42.5
49.4
14.8
13.2
14.4
15.4
3500 rpm
R95
R97
R95
R97
ML
ML
HL
HL
45.4
42.9
52.1
52.8
10.7
9.3
13.3
11.3
Fig. 13. Rate of heat release at 3500 rpm at high and medium loads.
Fig. 11. Cylinder pressure at 3000 rpm.
Fig. 14. Rate of heat release at 2500 rpm at high and medium loads.
Fig. 12. Cylinder pressure at 3500 rpm.
and 14. Table 4 summarizes the peak ROHR values and locations
with respect to each discussed engine operation and fuels. ROHR
is a reflection of cylinder pressure behavior. The ROHR increased
with increasing engine speed and load. The peak ROHR location
with respect to TDC showed mixed behavior. Except for RON97
at high load where peak ROHR is advanced, in all other cases peak
load location is delayed with increasing engine speed and load. At
3500 rpm and high load, even though RON97 produces higher peak
value of 30.8 J/°CA, less power is produced as bigger fraction of
heat is release happened during compression stroke. However at
2500 rpm, peak ROHR for RON97 at high load shifted towards
the later part of power stroke (>10° ATDC) while the fraction during compression stroke is reduced. This particular RON97 high load
heat release at 2500 rpm behavior did not produce better engine
outputs as compared to RON95 because of unstable heat release
pattern during and after the peak value, which may indicate the
occurrence of knock.
The exhaust emissions of CO2, CO, HC, and NOx from RON95 and
RON97 operations at all test conditions are shown in Figs. 15–18.
The observations indicate that emissions of CO2 and CO of
RON97 gasoline are less than RON95 throughout the speed and
load range as shown in Figs. 15 and 16. The CO2 emissions do
not vary much with speed and load for RON95 with only 12% variation between maximum and minimum. RON95 operation
released more CO2 at medium load between 2000 and 3000 rpm.
CO2 emission from RON97 showed bigger variation especially
between 2000 and 3000 rpm with highest emission produced at
high load and lowest by medium load, where significant decrease
was recorded at 2000 and 2500 rpm.
The emission patterns of CO are more uniform for both fuels
and directly proportioned to engine speed and load especially at
speed greater than 2000 rpm as shown in Fig. 16. In all cases
RON97 produced lower CO than RON95. This is particularly significant at medium and low loads. On average, CO2 and CO emissions
of RON97 are lower than that of RON95 by 7.9% and 36.9% respectively. This signalled incomplete combustion tends to be more with
RON95.
Unburned hydrocarbons (HC) emission is shown in Fig. 17. The
presence of HC in the exhaust gas is a result of unburned or
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300
Table 4
Peak ROHR.
R95 LL
R95 ML
R95 HL
280
Engine
speed
Fuel-load
Peak ROHR,
J/°CA
Peak ROHR location,
°CA ATDC
2500 rpm
R95
R97
R95
R97
ML
ML
HL
HL
108
10.2
12.8
15.3
5.7
3.2
7.4
9.3
R95
R97
R95
R97
ML
ML
HL
HL
16.8
15
20.3
30.8
2.9
3.9
4.5
3.5
240
HC (ppm)
3500 rpm
260
R97 LL
R97 ML
R97 HL
220
200
180
160
140
120
100
1500
16
15
R95 LL
R95 ML
R95 HL
2000
2500
3000
3500
Engine speed, rev/min
R97 LL
R97 ML
R97 HL
Fig. 17. HC emission vs. engine speed at various engine loads.
CO2 (%)
14
4500
13
4000
3500
12
2500
3000
3500
Fig. 15. CO2 emission vs. engine speed at various engine loads.
0.8
0.7
R95 LL
R95 ML
R95 HL
NOx (ppm)
2000
Engine speed, rev/min
0.9
R97 LL
R97 ML
R97 HL
3000
11
10
1500
R95 LL
R95 ML
R95 HL
2500
2000
1500
1000
500
0
1500
R97 LL
R97 ML
R97 HL
2000
2500
3000
3500
Engine speed, rev/min
Fig. 18. NOx emission vs. engine speed at various engine loads.
CO (%)
0.6
0.5
0.4
0.3
0.2
0.1
0
1500
2000
2500
3000
3500
Engine speed, rev/min
Fig. 16. CO emission vs. engine speed at various engine loads.
partially burned fuels, incomplete combustion, and presence of
lubricating engine oil in the fuel or combustion chamber [10].
Result shows that HC emission decreased with increasing speed.
However the engine load has mixed response to HC emission. High
load tend to produce more HC at low speed but as speed increases,
the values reduced significantly for both RON95 and RON97. However RON97 records a much bigger reduction at higher speed. The
least change in HC emissions happens at medium load. On average,
RON97 produces 20% lower HC emission throughout the speed
range as compared to RON95. Even though results shows that
increasing octane number reduces HC emission, the effects of
octane number on HC emission cannot be simply explained since
it has many other contributing factors, i.e. further oxidation during
exhaust process, temperature distribution within cylinder, etc.
The formation rate of NOx is directly dependant to cylinder temperature and the amount of NOx generated also sensitive to the
location within the cylinder; i.e. highest concentration usually
found around the spark plug vicinity, at which temperature and
pressure are highest [11]. The emission of NOx from RON95 and
RON97 gasoline is presented in Fig. 18. The result showed that
RON97 produces higher NOx emission compared to RON95 fuel
by an average of 7.7%. The higher NOx emission from RON97 is
more apparent at higher engine speed. The emissions are increased
with increasing engine speed because cylinder temperature
increases with speed as a result of intensified combustion. Higher
cylinder temperature in RON97 operation is predicted due to high
in-cylinder pressure shown in Figs. 10–12 particularly at high load.
This is the reason for higher NOx emission as compared to RON95.
The BSFC and emissions (except for NOx) from RON97 operations are lower than that of RON95. It is in contrast to the outcomes
in some of previous works by Sayin et al. [4,5] as their study indicated that lower RON fuel (RON91) produced lower BSFC and cleaner emissions compared to RON95. However in that work the
engine use has a lower compression ratio (8.0:1), which is advantageous to lower octane fuel.
Variation of emission data with respect to octane rating cannot
be easily concluded due to the absence of a more comprehensive
fuel properties data. However, a simple explanation for lower
CO2, CO and hydrocarbon emission from RON97 operations are
due to higher combustion efficiency, as inferred from lower HC
emission RON97, but this is penalized with higher NOx due to
T.I. Mohamad, H.G. How / Energy Conversion and Management 88 (2014) 928–935
the expected higher combustion temperature from better
combustion.
Looking from the technical and economic point of views simultaneously, it is clear that the use of RON95 in this widely used and
representative engine model in Malaysia is more favorable, economically sound and environmentally cleaner compared to
RON97. Even though RON97 produced lower CO2, CO and HC emissions by the average of 7.9%, 36.9% and 20% respectively, it cannot
compensate the fact that unit price is 38% higher. In addition,
RON95 produced higher power and more fuel efficient. From the
point of view of end-users these are the clear advantages. Lower
NOx emission is also environmentally favorable in reducing the
potential of acid raid. Furthermore, the after-treatment of this
gas requires expensive catalytic converter in which excessive clogging to it results in higher maintenance cost.
4. Conclusion
The effects of engine performance, fuel economy and exhaust
emissions of RON95 and RON97 gasoline grades were investigated
experimentally on a representative engine model with respect to
Malaysia’s situation. Analysis of experimental results were performed and cross-related to fuel price. The major findings derived
from thus study and experimental investigation are as follows:
1. Fuel price increase due to global crude oil price hike has
resulted in increase of gasoline unit price including in Malaysia.
With 16% increase in RON95 price and 42% increase in RON97
price since 2009, negative effects on new vehicle sales growth
can be seen in recent years.
2. Under the same engine specification and operations, RON95
resulted in an average of 4.4% higher brake torque, brake power
and BMEP compared to that of RON97 and the differences were
significantly revealed at higher engine speed and loads.
3. The BSFC with RON97 is 2.3% lower than RON95 but RON95 is
2.3% more fuel efficient especially at higher engine speeds and
loads.
4. Emissions of CO2, CO and HC were significantly lower with of
RON97 with the average of 7.9%, 36.9% and 20.3% respectively.
However, RON97 fuel produced 7.7% higher NOx emission compared to RON95 which is more environmentally harmful.
5. The cylinder pressure patterns and ROHR behavior of these two
gasoline grade explained the difference in engine performance
and energy efficiency. Original ECU which controls engine operation and engine architecture proved to be favouring RON95
burning characteristics.
6. RON97 is 38% more expensive than RON95 in Malaysia’s market. With the average of 4.4% better performance, 2.3% more
fuel efficient and 7.7% less NOx emitted, RON95 is a better
choice economically and environmentally. The advantage of
RON97 in terms of lower CO2, CO and HC emission better BSFC
may be attractive but less of concern to many users. However,
reducing NOx from vehicle tailgate using current technology
such as catalytic converter is more costly to manufacturer and
vehicle owners in the long run.
This work provides some valuable techno-economic viewpoint
of the surrounding issue of ‘increased fuel price causing less quality
935
fuel affordable by most Malaysian vehicles’ owners’. For an engine
model that is used by a very large group of vehicle in Malaysia, it is
shown that the use of RON95 gasoline not only economically sound
but is technically more favourable over RON97 in terms of engine
power and fuel economy and overall emission.
Acknowledgements
This work was supported by Universiti Kebangsaan Malaysia
through the Center for Automotive Research, the Engine Laboratory at Department of Mechanical and Materials Engineering and
partially funded by Research University Fund UKM-GUP-BTT-0725-157.
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