EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND

EXHAUST EMISSIONS ON A SMALL SI ENGINE 1

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
1 INTRODUCTION
The engine performance can be enhanced by
introducing leaner air fuel mixture which caused by higher air intake pressure to improve
the combustion process. Reducing the restriction of air filter element is the simplest
method to improve the flow of air intake. Air filter reduces the pollutant particles in the
air intake into a safe level for engine to operate. However, the air filter element restricts
the flow of air intake and leads to the pressure difference between atmospheric pressure
and pressure at the outlet of the intake system. In this project, the air intake pressure was
observed at the downstream of the air filter. This helps in identifying the best
characteristics of the air intake for improving engine performance with a better fuel
economy and lower exhaust emissions. The testing was conducted in a control chamber
which has clean air to avoid the polluted air damaging the internal engine components. In
a complete combustion, the hydrocarbon fuel reacts completely with the dry atmospheric
air as explained on Equation 1. In incomplete combustion generally, it will has such
unburned components of C, H2, CO, NOx, CH4 and OH [1].
C
N
H
2N+2
+ (3N+1)(O
2
+3.76N
2
)NCO
2
+ (N+1)H
2
O + 3.76(3N+1)N
2
(1)
An ideal tube length of the air intake system, L (m) is inversely proportional to the engine
speed, n (rpm) with respective to the constant acoustic speed, a (m/s) as explained in
Equation 2 [2]. The theory of ideal tube length is applied on the Variable Intake Manifold
(VIM) system. The system uses a long intake runner at low engine speed as it makes low
velocity of air intake which optimizes the air-to-fuel mixing coefficient. The short intake
runner is switched at higher engine speed to increase the flow of air intake at higher
engine speed so that the combustion efficiency is increased within a short period of the
intake stroke. Ceviz and Akin 2010 state that the longer intake plenum length is
averagely suitable for 1000 rpm to 3000 rpm of engine speed and the shorter intake
plenum length is averagely suitable for engine speed higher than 4000 rpm.
L(10a)/n (2)
A clogged air filter restricts the air flow, increases the fuel consumption and reduces air-
to-fuel mixing coefficient. Despite a clogged air filter in injection engine, a lean air-to-
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 2

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
fuel mixture is maintained thanks to closedloop algorithm in controlling the flow rate of
injected fuel [4]. Hence, the condition of air filter does not affect the air-to-fuel mixture
of the fuel injected engine. On the other hand, a traditional carbureted engine does not
have the closed-loop algorithm fuel control. Insufficient air-to-fuel mixture is supplied
into the combustion chamber of the carbureted engine when the air filter is clogged. In
order to study the fuel consumption characteristics due to the clogged filter effect, a
carbureted engine that was taken out from Subaru EX17 was used. In this project, an air
filter was installed into the engine to demonstrate the clog effect on the engine
performance. Thereafter, the air filter was taken out to demonstrate on clogging effect
and the data was measured so that results can be compared. Based on Rizalman Mamat,
et al., higher pressure drop in air intake system which occurs during part load and low
load reduces the engine efficiency and increase the engine brake specific fuel
consumption, BSFC. The NOx emission is majorly influenced by the air fuel ratio, AFR
compared to the ignition delay especially during part load. The engine combustion and
emissions behaviours are affected by the magnitude of inlet manifold pressure drop on
both usages of ultra low sulphure diesel, ULSD and rapeseed methyl ester, RME [5].
Reduction on both carbon monoxide, CO and hydrocarbon, HC with constant on both
oxygen, O2 and carbon dioxide, CO2 are caused by the reduction of engine altitude,
increment in air intake pipe diameter, higher atmospheric pressure and higher engine
speed [6]. The reduction in HC emission is due to improved combustion process because
of larger air intake pipe diameter at higher engine speed. Meanwhile, the CO emission is
affected by the volume of injected fuel which burnt based on the amount of oxygen
during the combustion process.





EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 3

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
2 EXPERMENTAL SETUP AND PROCEDURE
The experiment is carried out by using 169 cm3 single cylinder
gasoline engine mounted with 5 kW eddy current type electric dynamometer. Each
experiment variable is running at increasing engine speed and load to study the effects
throughout the engine operation condition by obtaining various output data. The overall
experiment instruments layout is explained in Figure 1 below.

Figure 1: Overall Experiment Instruments Layout
The mentioned engine is the Subaru Robin EX17 single cylinder gasoline
engine potentials in producing 4.2 kW of maximum power and mounted with the PS-
DEC-1 5 kW small engine electric dynamometer eddy current type which capable up to
4000 rpm maximum speed on the engine test bench. PS-SOE-1B fuel tank with graduated
tube is used for measuring the fuel consumption. PS-SOE-2 air consumption device with
manometer are used to measure the air intake pressure and temperature. The manometer
is from Dwyer which senses the air intake pressure. The manometer fluid used has a
specific gravity of 0.826 which makes its density to be 826 kg/m3. The instrument panel
of PS-SOE-7 electronic digital temperature meter with six thermocouples and selector
switch equipped with torque meter and tachometer gives the temperature reading at 6
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 4

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
points and magnitude of load applied and engine speed. The 6 points temperature
readings are at air cooler inlet, exhaust port, air cooler outlet, engine oil, air intake and
ambient air. The energy supplied to the dynamometer is given by Lodestar power supply
model 8112 capable of producing 60 volts and 5 ampere at maximum. In measuring the
atmospheric air temperature and humidity, the hygro-thermometer is used with its sensor
is placed near to the air filter to obtain the exact data in avoiding effects from surrounding
elements. MRU 95/3 CD Flue Gas Analyzer is used to analyze the exhaust emissions
compositions. It capable to produce the emissions compositions volume percentage in
unit % vol and ppm for O2, CO2, CH4, CO, SO2, NO, and NOX compositions and
temperature of exhaust gas and environment. The CH4 and CO sensors in this gas
analyzer are malfunctioning.

On each run, multiple data such as temperatures, atmospheric air
temperature and humidity, engine speed and load, voltage and current supplied to
dynamometer, fuel consumption and exhaust emissions are taken. The fuel consumption
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 5

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
in unit of liter per hour (l/hour) is measured by dividing a volume of fuel consumed with
a period of time as explained in Equation 3.
(3)
By using the MRU 95/3 CD Flue Gas Analyzer, at least 40 seconds
of time period from the time the probe sense the exhaust gases is needed for the gas
analyzer to produce the consistence and reliable data in obtaining the exhaust emissions
compositions data. According to its manual, O2 sensor needs approximately 20 seconds,
30 seconds for CO sensor, 30 seconds for SO2 sensor and 30 seconds for the NO sensor
(MRU 2005). The gas analyzer probe is located at 120 cm from the exhaust port because
the exhaust port is connected with a long pipe to channel the exhaust gas out from the lab
in avoiding human suffocation.
In obtaining the air intake absolute pressure, the atmospheric pressure
is ignored as it is assumed to be constant through entire experiment process. Only air
intake gauge pressure is measured by using the water column height, h from manometer
as explained in Equation 5. The point of measuring the air intake gauge pressure is 23 cm
after the air filter. After 5 runs, the spark plug with gas analyzers probe and filter are
cleaned in maintaining the engine efficiency and consistency of gas analyzer.
P
gauge =
.g.h





EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 6

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
3 RESULTS AND DISCUSSION
Analysis of Output Data on Constant Engine Speed
This analysis of data is carried out based on constant engine speed.
The approach of this data analysis is carried out in order to study the pattern of output
data behavior towards the manipulation of load at constant engine speed.

Table 3: The Constants Load and Engine Speed for Output Data Analysis


3.1 Fuel Consumption
Based on Figure 2, the fuel consumption is clearly decreasing on
both variables of without air filter and with air filter based on constant engine speed
analysis. The fuel consumption is decreasing by about 13.1 % which is from 1.233 l/hour
to 1.071 l/hour for the variable with air filter. While for the variable without air filter, the
reduction is only about 10.8 % which is from 1.191 l/hour to 1.062 l/hour. The reduction
of the fuel consumption is due to the increasing load acted to the constant engine speed
which lowers the engine speed that consumed lower amount of fuel.
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 7

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING

Figure 2: Graph of Fuel Consumption at Constant Engine Speed
The fuel consumption is affected by the air fuel ratio which leaner combustion
will less consuming the fuel while richer combustion will more consuming the fuel
(Heywood 1988). Without air filter, the discharge air capacity is larger than the variable
with air filter. Moreover, in this case of constant engine speed, the increasing magnitude
of load will reduce the fuel consumed by the engine because larger load will reduce the
engine speed to the lower engine speed which less consuming the fuel. In reality, we
feel that more load increase the fuel being consumed because we are usually increase
the throttle opening in order to increase the engine speed to overcome the higher load
act on the vehicle.

3.2 Air Intake Gauge Pressure
The differences between both variables of air intake gauge
pressures are on conditions 2 and 3 which are 0.54 Pa and 1.621 Pa respectively. In
addition, for condition 1, the air intake gauge pressure is similar on both variables
without air filter and with air filter which is 17.827 Pa. The reason of this similarity is
that when the engine is running at high speed for example 3000 rpm and being loaded
with 1 Nm of load, the engine is still capable to maintain high speed but surely less than
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 8

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
3000 rpm and that final speed require a large air intake flow rate which able to overcome
the restriction of air filter media. Due to this reason, the air intake gauge pressure
between both variables of without air filter and with air filter on condition 3 shows a very
huge difference. On condition 3, the engine speed is 3000 rpm and engine is loaded with
4 Nm which reduces its final engine speed greater than the condition 1. The lower final
engine speed on condition 3 require small air intake flow rate which capable to be more
restricted by the air filter media which drops down more the air intake gauge pressure on
the variable with air filter.

Figure 3: Graph of Air Intake Gauge Pressure at Constant Engine Speed
The engine speed is the main factor that affects the air intake pressure in
naturally-aspirated engine. Higher air intake pressure will occur on higher engine speed.
But, with higher engine speed, the time period for the air fuel mixture to enter the
cylinder is very concise. Thus, it leads to insufficient supply of air fuel mixture into the
cylinder during higher engine speed which can be overcome by using forced induction
system and also variable valve timing and lifting. In forced induction engine, the air
intake pressure is boosted with the help of compressor which being rotated by the exhaust
gas in turbocharged engine or crankshaft in supercharged engine. The gas exchange in
combustion chamber is also can be optimized with the overlap intake and exhaust valves
opening during limited period of time on high engine speed. During both intake and
exhaust valves are both opening, the low pressure caused by the opening exhaust valve
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 9

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
helps the air fuel mixture to flow easily into the combustion chamber which is very useful
during high engine speed.
In term of volumetric efficiency, supplying the higher amount of air into
the cylinder is able to increase the engine power output. The problem is to obtain the
bigger volume of gaseous air that reacts with the fuel is much harder than to obtain the
smaller volume of liquid fuel in the engine cylinder. Theoretically, the mass of induced
air into the cylinder for each cycle is equal to the atmospheric air density times with the
engine cylinder displacement volume. The reality is with the limited period of cycle
available and restricted air flow caused by air filter, intake manifold and intake valve; the
exact amount of air is usually less than the theoretical volume (Pulkrabek 2004).

3.3 CO
2
Emissions
For the analysis on constant engine speed, there is similarity in volume
percentage of carbon dioxide composition on condition 2 for both variables which is 3.8
vol %. Only 2.4 % extra on volume percentage of carbon dioxide emission for variable
without air filter respective to the variable with air filter on condition 1. For condition 3,
the variable without air filter is having extra 12.5 % more in volume percentage of carbon
dioxide emission than the variable with air filter.

Figure 4: Graph of CO2 Composition at Constant Engine Speed
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 10

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
The carbon dioxide volume percentage is higher on the condition without air
filter and lower on the condition with air filter. More complete combustion produces
more carbon dioxide as the product of oxygen inside air combines with the carbon inside
the fuel. Hence, the volume percentage of carbon dioxide is higher on the condition
without the air filter as it induces more air into carburetor forming leaner air fuel mixture
for better combustion. While, the condition with the air filter will have slightly lower
amount of carbon dioxide because the combustion quality is lower due to richer air fuel
mixture caused by less amount of induced air into the carburetor.
The higher amount of CO2 is an indicator for better combustion of fuel in the combustion
chamber which also relates to the exhaust gas temperature (Mohanraj and Kumar 2013).
CO
2
emission will increase because of better reaction between fuel and oxygen during
combustion process (Sayin, Gumus et al. 2010). But, Sayin, Gumus et al. enhance the
reaction between fuel and oxygen by means of advanced injection timing in order to have
better combustion.

3.4 NOx Emissions
In comparing on both variables of without air filter and with air filter,
the variable without air filter is more producing NOx than variable with air filter. The
NOx composition of variable without air filter is fluctuating through condition 1 until 3.
But, on the variable with air filter, the NOx formation is decreasing from condition 1
until 3. The most obvious difference between both variables on this NOx formation is on
condition 3, where the NOx composition of variable without air filter is almost 30%
more than variable with air filter. The NOx formation is similar for both variables
oncondition 1.While for condition 2; it is only 3.4% additional of NOx composition for
variable without air filter based on variable with air filter.
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 11

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING

Figure 5: Graph of NOX Composition at Constant Engine Speed
The Zeldovich mechanism is supported with this pattern of NOx result
which NOx formation is influenced by the maximum temperature and pressure of the
combustion process (Aithal 2012). The improved combustion process causes to the
higher in-cylinder temperature and pressure on the variable without air filter which then
leads to the higher NOx composition (Abdullah, Mamat et al. 2009). Thus by comparing
the theory of Zeldovich mechanism with the experiment result, it is agreed that the
increase in combustion pressure and temperature on the condition without air filter
increases the formation of NOx. Increasing in the pressure drop through the air intake
system which makes less efficient combustion reduces the number of NOx formations.

3.5 Exhaust Gas Temperature
The data of exhaust gas temperature on the variable without air filter
is fluctuating through condition 1 until 3. For the variable with air filter, the exhaust gas
temperature is decreasing through condition 1 until 3 as the load is increased with the
constant engine speed. The biggest difference is on condition 2, where there is 50%
increment of exhaust gas temperature for variable without air filter based on exhaust gas
temperature of variable with air filter. Only small increment of exhaust gas temperature
EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 12

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
for variable without air filter compared to variable with air filter on conditions 1 and 3
which are17.37% and 14.16% respectively.

Figure 6: Graph of Exhaust Gas Temperature at Constant Engine Speed
The energy is released in many forms and one of them is heat produced
during the combustion process. More efficient combustion with better chemical reactions
will produce more energy such as heat which raises the exhaust gas temperature to be
higher. Higher exhaust gas temperature leads to higher exhaust gas pressure as
explained in the ideal-gas equation of state,

. The temperature of exhaust gas

reflects the in-cylinder temperature which is directly proportional to the in-cylinder
pressure. According to Abdullah, Mamat et al., the exhaust gas temperature is
increasing along with the increased injection pressure due to more heat is generated
inside the combustion chamber caused by more complete combustion (Abdullah, Mamat
et al.2009). Both higher injection pressure and higher air intake pressure lead to better
combustion process and higher exhaust gas temperature. The difference is that thehigher
air intake pressure improves the combustion process by having leaner air fuel mixture to
be combusted more efficiently while the higher injection pressure improves the
combustion process by having smaller fuel droplet particles that able to penetrate more
deeply through the pressurized air inside the CI engine combustion chamber.

EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 13

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
CONCLUSION
The difference in air intake pressure caused by variables with air filter
and without air filter variables influence the engine behavior in aspects of engine
performance, fuel consumption and exhaust emissions. Combustion process burns entire
combustible elements in the fuel by combining the carbon and hydrogen with oxygen. To
extract more energy from the combustion process, it needs to form leaner air fuel
mixture. The large amount of air increases the potentiality of fuel chemical elements to
be burned with oxygen. As a result, the engine performance and fuel economy are
increased while the unburned exhaust emissions components are reduced.
1. Higher air intake pressure increases the engine performance in against the applied
load better than lower air intake pressure.
2. Higher air intake pressure improves the fuel economy by introducing leaner air
fuel mixture. The fuel consumption is directly influenced by the air fuel ratio.
3. Higher air intake pressure supplies more O2 to be combined with fuel elements I
improving the combustion process
4. Higher air intake pressure increases the CO2 emissions by optimizing the
chemical reaction between carbon from fuel with the oxygen from air.
5. Higher air intake pressure increases the NOX emission. Zeldovich mechanism
explains that the NOx formation is directly proportional to maximum pressure and
temperature of the combustion process.
6. Higher air intake pressure enhances the conversion of chemical energy into heat
energy which raises the exhaust gas temperature.



EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 14

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
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EFFECTS OF AIR INTAKE PRESSURE TO THE FUEL ECONOMY AND
EXHAUST EMISSIONS ON A SMALL SI ENGINE 15

SNGIST,MANJALLY, NORTH PARAVUR DEPT OF MECHANICAL ENGINEERING
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