chapter 10 the internal combustion engine...
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CHAPTER 10
THE INTERNAL COMBUSTION ENGINE EXPERIMENTS
10.1 INTRODUCTION
Ceramic coating of IC engine components, especially the piston
head, skirt and the cylinder head of diesel engines has been practiced for some
years using YSZ (yttria stabilized zirconia) as the coating material. Results
have been encouraging and studies have shown improvement in the
performance parameters and emission levels of the engines. This study
pertains to the use of mullite as a coating material to coat on petrol engine
components, to study the performance and compare with those of the
uncoated engine.
10.2 TBC’S IN PETROL ENGINES
Metal substrates in petrol engine components such as cylinder head,
piston and valves are generally made of aluminum –silicon (eutectic and
hyper eutectic) alloys. Current research is focused on successful coating of
these components, especially to optimize the coating adhesion characteristics
during long duration performance on road. Coating of mullite on aluminum
poses a challenge, due to the mismatch in thermal expansion coefficients
between the substrate and the coating. Thus aluminum silicon alloys present
the most challenging task of adapting to the environment of engine with
insulated combustion chamber. Petrol engine TBC’s operate at much lower
temperatures compared to aircraft engines and hence it is not reasonable to
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expect any significant improvement in performance efficiency. However,
their effect on reducing the particulate emission is expected to be worth a
mention.
Thermal barrier coatings are highly advanced material systems
applied to metallic surfaces, such as gas turbine or aero-engine parts,
operating at elevated temperatures. These coatings serve to insulate metallic
components from large and prolonged heat loads by utilizing thermally
insulating materials which can sustain an appreciable temperature difference
between the load bearing alloys and the coating surface. In doing so, these
coatings can allow for higher operating temperatures while limiting the
thermal exposure of structural components, extending part life by reducing
oxidation and thermal fatigue.
Figure 10.1 Coated piston crown
The thermal barrier crown coating is applied to the top of the piston
as shown in figure 10.1 and is designed to reflect heat into the combustion
chamber, thereby increasing exhaust gas velocity and greatly improving
scavenging potential. The 300 m thick coating can also assist in extending
piston life by decreasing the rate of thermal transfer (Pierz 1993).
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Ceramic-based thermal barrier coatings are primarily used to control the
movement of heat. When applied to the top of a piston, heat is evenly
distributed across the dome. This reduces hot spots that cause detonation. The
coating protects and insulates the piston against heat soak and this keeps the
heat on top of the piston building more power, and at the same time reduces
the amount of stress on the piston rings allowing a better seal. The coating can
also be applied to the combustion chamber, valve faces, and exhaust ports. By
holding more heat in the combustion chamber power is increased. Detonation
can also be controlled by the following ways:
1. Using a higher octane fuel like grade 89 instead of the regular
87 grade.
2. Keeping the compression ratio within limits. A static
compression ratio of 9:1 is usually the recommended limit for
most naturally aspirated street engines.
3. Checking for over-advanced ignition timing. Too much spark
advance can cause cylinder pressures to rise too rapidly.
4. Using the correct heat range spark plug.
5. Checking for engine overheating. In coated engine, the engine
temperatures are lower and hence this may not be a problem.
6. Using a richer air fuel mixture.
10.3 EXPERIMENTAL WORK
In the present study, a two stroke petrol engine with components
plasma spray coated with mullite was evaluated for performance efficiency
and endurance on road. Mullite ceramics is a good alternative to stabilized
zirconia and in some characteristics even better than stabilized zirconia. The
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properties of mullite most critical for TBCs are a very low thermal
conductivity and a thermal expansion close to that of super alloys.The 100 µm
thick mullite coating on 100 µm nickel chrome ( NiCr) bond coat was
deposited on the top surface of the piston and the cylinder head of a two
stroke engine of a two wheeler. The engine with the coated piston and the
cylinder head was subjected to a preliminary performance evaluation on an
engine dynamometer. The fuel efficiency, brake specific fuel consumption
(BSFC) and hydrocarbon emission (HC) were measured and the performance
parameters were compared with those of standard combustion chamber
containing uncoated components. Six different sets of coated piston and
cylinder heads were used for the test. The engine was tested at different
speeds and load conditions. The compression and air fuel ratios were
maintained constant for all combinations. Engine performance tests were
performed at full throttle openings on a mechanical dynamometer, specially
designed and fabricated to handle lighter loads, applicable for a two stroke
petrol engine of a two wheeler. The carburetor was set to the richest condition
to prevent detonation from taking place.
The performance of the engine using uncoated engine components
and coated components were compared, the results plotted and the reasons for
improved performance of the coated engines highlighted.
10.3.1 Materials
10.3.1.1 Piston
Pistons are manufactured from cast aluminum alloy A356.0
containing silicon which makes the piston harder improving its strength and
wear properties. All pistons are heat treated by T6 (solution quenching and
aging) treatment to improve their strength before machining and helps retain
their shape. Generally, higher the silicon content, higher the strength and wear
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properties. In the present study, pistons are made of cast aluminum A 356.0,
T6 treated, silicon and magnesium alloy with an Fe (iron) content of 0.1 %,
which will impart good strength. The ceramic coating on the piston improves
the thermal shock resistance, wear resistance, oxidation and corrosion
resistance of the piston, thus enhancing the operating life of the piston.
10.3.1.2 Cylinder head
Similarly the cylinder heads are also manufactured from cast
aluminum alloy A 356.0 and T6 treated. The ceramic coating on the side
facing the combustion chamber helps retain the heat of combustion and
minimizes the heat transfer towards the inside of the head. The coating
imparts similar properties mentioned in the case of the piston.
10.3.1.3 Bond Coat
Bond coat of nickel chrome is applied on the aluminum substrate to
a thickness of 100 m + 50 m before applying the top coat of mullite to
improve the adhesion of the coating to the substrate.
10.3.1.4 Top coat
The top coat faces the hot combustion gases and hence should be a
thermal barrier.Mullite ceramic of 100 m + 50 m thickness is plasma
sprayed on the bond coat.
10.3.2 Coating process
The METCO ROBOT plasma spraying equipment was used to
fabricate the specimens in this study using optimized plasma spraying
parameters as detailed earlier. Six sets of pistons and cylinder heads supplied
by M/s TVS company, Hosur, were coated for the study.
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10.3.3 Experimental Setup
The experimental setup is shown in the Figure 10.2. The engine
setup is rigidly fixed on the chassis of the fixture. The carburetor setting was
set to the richest condition throughout the testing to avoid any chances of
detonation. The power is transmitted from the engine to the pulley of the
mechanical dynamometer through a V-belt. The loading arrangement is
mounted on a frame. The load is applied to the engine using spring balance
connected to a rope wound around the brake drum of the setup. The fixture
consists of a supporting block, which is a cylindrical hub that carries ball
bearings through which a shaft is inserted as it supports the transmission of
power from the pulley. The supporting block used here is a cylindrical hub
that is bolted to the chassis of the setup. It is made of cast iron to provide a
stronger support to avoid vibration of the test rig at higher speed runs of the
engine. A stop watch is used for measuring the time and a calibrated
thermocouple is used for the temperature measurements. The air inlet of the
carburetor is connected to a surge tank which in turn is connected to the
manometer to measure the air intake head. The surge tank has an orifice of
diameter 14 mm. A 500ml burette is filled with petrol through which the fuel
is supplied to the carburetor of the engine. The test rig constructed with IC
engine is operated to measure various test parameters namely, brake horse
power (BHP), total fuel consumption (TFC), specific fuel consumption (SFC),
heat transfer, indicated power (IP), brake thermal efficiency , indicated
thermal efficiency , mechanical efficiency and volumetric efficiency. This test
is conducted for both coated and uncoated component of the engine.
A comparative study is made between the two results and the performance
improvement is presented. The various graphs are charted like brake power vs
total fuel consumption, brake power vs efficiencies, specific fuel consumption
vs brake power, buildup of temperature vs duration of running the engine and
torque vs speed.
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Figure 10.2 Experimental setup
10.3.4 Engine specification
STROKE = 2
BORE = 46 mm
STROKE LENGTH = 42 mm
DISPLACEMENT = 69.9 cc
MAX. POWER@ 5000 rpm = 2.61 KW
TORQUE@ 3750rpm = 5.0 Nm
COMPRESSION RATIO = 8.3:1
10.3.5 Testing Arrangements
The experimental tests are carried out by constructing a test rig as
shown in Figure 10.3 with the following instruments. Manometer, stopwatch,
surge tank, thermocouple, spring balance, Brake drum, and burette.
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Figure 10.3 Fixture setup
10.3.6 Test Procedure
The various testing procedures are done between the coated and the
uncoated engine of same specification under similar conditions. These test
results helps us to compare the engine performance between the coated and
uncoated engines. Initially the air inlet of the carburetor is connected to a
surge tank setup. The manometer is connected to the surge tank which shows
the pressure difference. A 500 ml burette is filled with petrol through which
the fuel is supplied to the carburetor of the engine. The load is applied to the
engine using spring balance on the brake drum of the test rig setup.
10.3.7 Performance Testing
Initially the engine is run at no load. The time taken for fuel
consumption of 10 ml is observed and tabulated. The pressure
difference in manometer is observed and tabulated as h1 , h2.
The load of 1kg is applied on the brake drum and again the
time taken for fuel consumption for 10ml, and the pressuredifference is tabulated.
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Similarly for gradual increase in load the readings are noted
and tabulated.
Using the above readings the engine performance parameters
like BHP, Indicated power, SFC, TFC, mechanical efficiency,
thermal efficiency and volumetric efficiency are calculated.
The obtained results are charted like BP VS TFC, BP VS
efficiencies.
Fuel consumption at various speeds -The engine is made to
run at no load condition for testing the fuel consumption rate
at various speed. The speed is increased gradually and their
respective fuel consumption rate is calculated for every 10
ml.SFC vs speed graph is charted for comparison with coated
and uncoated engine performance.
Torque vs speed - The engine is initially let to run with no
load condition. The load is gradually increased and speed at
various load are observed and tabulated. The torque value for
the particular speed is calculated and thus a torque vs speed
graph is charted for comparison with coated and uncoated
engine performance.
10.3.8 Temperature Testing
10.3.8.1 Calibration of thermocouple
Initially a thermocouple is calibrated by keeping one of the
junctions in cold water and the other in hot water. The cold water is made
constant were as the temperature of the hot water is increased gradually. The
multi meter is made use for measuring the voltage at its terminals. The
observed value is tabulated to calculate the average ‘k’ value.
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10.3.8.2 Temperature testing procedure
The temperature of the engine is measured by keeping the cold
junction of thermocouple as water and the hot junction at the surface point
were the temperature to be measured. This is done by positioning the hot
junction probe on the surface of engine like head and cylinder portion, their
respective voltage are tabulated. The observed values are calculated for its
temperatures. Thus the results are tabulated for comparison and charted.
10.3.9 Emission Tests
Emission test was conducted for CO (carbon monoxide) and HC
(Hydrocarbon ) emissions for various speeds and graphs plotted for the coated
and uncoated engines and compared. The photos of the coated piston and
cylinder head are shown in Figure 10.4.
(a) (b) (c)
Figure 10.4 (a) Coated piston top view (b) Coated cylinder head(c) Coated piston elevation
10.3.10 Endurance Test of Coated IC Engine
An endurance test of the engine with coated piston and cylinder
head was conducted to make a feasibility study of the coating, viz its adhesion
strength after prolonged usage. The coated engine was mounted in a two
wheeler and was run on road for 1000 kms, at various speeds and loads.
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10.4 RESULTS AND DISCUSSION
In the present study, mullite TBC successfully withstood the severe
thermo-mechanical stresses during simulated and accelerated burner rig
conditions. The thermal shock resistance of the coating composition in an
actual working engine was required to be tested. This was done to understand
the endurance capabilities of the coating by not only carrying out the
performance evaluation (fuel efficiency, engine power) of the engine with
coated components but also by running a two wheeler on road with the coated
components. Petrol engine components are subjected to milder thermal
stresses and shocks in their combustion chamber environment compared to
those in diesel engines.
10.4.1 Visual Examination
The test fixture was run in the laboratory for 100 hrs at varying
speeds and loads. The coated surface of the piston was examined for surface
defects and signs of spallation. The visual examination of the piston surface
revealed no significant defect of the coating surface or the interface. The
surface had a deposit of carbon as expected.
10.4.2 Performance Tests
The performance curves of the petrol engine at different speeds and
load conditions are shown graphically and the tables of measurements taken
are shown. On examination of the various parameters evaluated the general
conclusions are drawn and summarized after each test.
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10.4.2.1 BP vs TFC characteristics
Brake horse power (BP) is the actual power delivered at the crank
shaft. It is obtained by deducting various power losses in the engine from the
indicated horse power. In other words, BP is the usable power produced by
the engine. (TFC)Total fuel consumed is the amount of fuel used by the
engine for one hour duration of running. The measured values are shown
graphically below in Figure 10.5.
(a)
(b)
Figure 10.5 BP vs TFC characteristics of a) uncoated and b) coated engine
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The results of the BP Vs TFC characteristics are shown below.
1. The total fuel consumption is more flat in the coated engine,
when compared to the uncoated engine. Also the fuel
consumption is lower (Chan and Khor 2000).
2. Generally, the TFC increases with increase in BP.
3. By extending the points to the second quadrant in the X-axis,
the friction power calculated is 1.5 kw for the uncoated engine
and 1.9 kw for the coated engine.
10.4.2.2 BP vs efficiency (mechanical, volumetric, indicated thermal and
brake thermal) characteristics
Mechanical efficiency is the ratio of the brake power (delivered
power) to the indicated power. Volumetric efficiency is an indication of the
breathing ability of the engine and is given by the ratio of volume of air
actually inducted at ambient conditions to the swept volume of the engine.
Indicated thermal efficiency is the ratio of the indicated power (IP) to the
input fuel energy in appropriate units. The power developed within the engine
cylinder is known as indicated power. In other words, it is the power the
engine can theoretically produce. Similarly brake thermal efficiency is the
ratio of the brake power (BP) to the input fuel energy in appropriate units.
The measured and calculated values are shown tabulated below in
Tables 10.1and 10.2 and the performance curves are shown in Figure 10.6.
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Table 10.1 Observed values of performance studies on uncoated engine
S.No. BP (kW) bt % It % mech % vol %1 0 0 48.5 0 842 0.1 3.2 50.5 6.3 853 0.2 5.7 48.2 12 904 0.3 8.5 50.8 16.7 875 0.4 9.9 46.9 21 866 0.5 12.4 49.6 25 867 0.6 14.2 49.8 28.6 888 0.7 16.9 53 31.8 889 0.8 18.5 53.2 34.8 86
10 0.9 20.5 54.5 37.5 8811 1 21.6 53.9 40 8812 1.1 22.4 53.1 42.3 8313 1.2 22.2 50 44.4 8914 1.3 23.7 51 46.4 8415 1.4 23.7 49.2 48.3 8916 1.5 24.6 49.2 50 85
Table 10.2 Observed values of performance studies on coated engine
S.No. BP (kW) bt % It % mech % vol %1 0 0 62.3 0 752 0.1 3.03 60.8 5 813 0.2 6.07 63.8 9.5 804 0.3 9.5 69.4 13.6 725 0.4 11.7 67.3 17.4 896 0.5 14.6 70.2 20.8 747 0.6 16.9 70.6 24 888 0.7 20 73.4 27 859 0.8 22.6 76.3 30 9110 0.9 24.6 76.5 32 8911 1 27.3 79 34 8412 1.1 29.1 79.4 37 8313 1.2 31.7 82 39 8014 1.3 33.3 82 40.6 7915 1.4 34.7 82 43.4 8016 1.5 36.1 82 44 83
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Figure 10.6 Comparison of brake horse power Vs efficiences for boththe coated and uncoated engines
The results of the BP vs efficiency characteristics are given below.
1. The maximum brake thermal efficiency bt is 24.6 % for theuncoated engine and 36.1 % for the coated engine.
2. The maximum Indicated thermal efficiency It is 54.5 % forthe uncoated engine and 82 % for the coated engine.
3. The maximum mechanical efficiency mech is 50 % for theuncoated engine and 44 % for the coated engine.
4. The maximum volumetric efficiency vol is 88 % for theuncoated engine and 91 % for the coated engine. Coating onthe piston and cylinder head has increased the combustiontemperature and hence the improvement in volumetricefficiency. James A. liedel (1997) and Tamil Porai (2003)have confirmed this in their study.
5. Reddy et al (1990), Taymaz et al (2005) , James A. liedel(1997) and Stephen Bernard (2009) in their study on coated diselengines confirm that the thermal efficiencies have improved.
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10.4.2.3 Speed vs SFC characteristics
The fuel consumption characteristics of an engine are generally
expressed in terms of SFC (Specific fuel consumption) in kilograms of fuel
per kW-hr. It is an important parameter that reflects the performance of the
engine.The Speed of the engine is varied and the SFC noted for each speed
and tabulated in table 10.3 and the performace curve is shown in figure 10.7.
Table 10.3 Speed vs SFC data a-uncoted engine, b- coated engine
Sl. No. Speed(rpm)
SFC(kg/kW.hr) Sl.No. Speed
(rpm)SFC
(kg/kW.hr.)1 950 0.22 1 950 0.222 1200 0.18 2 1200 0.183 1600 0.16 3 1600 0.164 1800 0.14 4 1800 0.155 2000 0.16 5 2000 0.186 2300 0.16 6 2300 0.18
Figure 10.7 Comparison of speed Vs specific fuel consumptioncharacteristics of coated and uncoated engines
There is no major change in the SFC vs Speed characteristics.
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10.4.2.4 Torque vs speed characteristics
The engine is loaded graduallyand the torque is calculated for each
load and the speed measured for various throttle openings. The measured and
calculated values are shown below in tables 10.4 and 10.5 and the
characteristic curves are shown in figure 10.8.
Table 10.4 Torque Vs speed observed values of the uncoated engine at25%,50 % and 100 % throttle openings
Speed (rpm)
Torque(Nm)
25 %throttleopening
50 %throttleopening
100 %throttleopening
0 1800 2000 2200
1 1730 1935 1922
2 1620 1620 1764
3 1421 1540 1630
Table 10.5 Torque Vs speed observed values of the coated engine at25 %, 50 % and 100 % throttle openings
Speed (rpm)
Torque(Nm )
25 %throttleopening
50 %throttleopening
100 %throttleopening
0 1800 2000 22001 1789 1960 21852 1750 1940 20313 1735 1917 1995
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(a)
(b)
(c)
Figure 10.8 Comparison of torque vs speed characteristics of coated anduncoated engines (a) 25 % throttle opening, (b) 50 %throttle opening, (c) 100 % throttle opening
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The results of the torque vs speed chracteristics are given below.
The coated engine performs better in providing a higher torque at
various throttle openings and also there is less variation in speed as load is
increased.Hence the power produced in the coated engines are higher.
10.4.3 Temperature Measurements
Measurements were done on the cylinder head and the cylinder
close to the conbustion chamber using thermocouples after calibration. The
measured values are tabulated and shown in tables 10.6 and 10.7 and also
shown graphically in Figures 10.9 and 10.10.
Table 10.6 Temperature measurements taken for the Uncoated engine
Duration (minutes)SpeedTemperature (°C) 10 20 30 40 50
Head 82 86.2 94.6 104.2 104.2
Cylinder 83.2 89.8 94.6 98.8 98.8
Head 120.8 130 134.2 137.8 137.8
Cylinder 124.6 124.6 127.6 129.4 129.4
Table 10.7 Temperature measurements taken for the coated engine
Duration (minutes)SpeedTemperature (°C) 10 20 30 40 50 60
Head 54.6 79.2 84.6 94.2 103.2 103.2
Cylinder 57.6 81.6 97.2 93 94.8 94.8
Head 103.2 110.4 113.4 118.8 122.4 122.4
Cylinder 97.2 102 106.8 110.4 118.8 118.8
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Figure 10.9 Comparison of temperature measurements on cylinder head
of coated and uncoated engines
Results : The temperature reduction of the coated cylinder head is apparent
from the graphs, indicating a higher combustion temperature.
Figure 10.10 Comparison of temperature measurements on cylinder ofcoated and uncoated engines
Results : The temperature reduction of the cylinder in the vicinity of the
combustion chamber is apparent from the graphs, indicating a
higher combustion temperature.
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10.4.4 Emission Tests
The CO (carbon monoxide) and HC (hydrocarbon) emission levels
are measured using in an engine emission test rig, for various speeds and
tabulated below, in table 10.8 and the characteristic curves are shown
graphically in Figure 10.11.
Table 10.8 Emission levels of CO and HC (a) CO % (b) HC (ppm)
Speed(rpm )
CO ( % vol ) HC ( ppm )Uncoated
EngineCoatedEngine
UncoatedEngine
CoatedEngine
0 2.6 2 1250 900500 2.65 2.05 1500 900
1000 2.7 2.1 1750 9001500 2.75 2.25 2100 9502000 2.8 3 2500 10002300 3.5 3.2 2900 900
(a)
Figure 10.11 Comparison of CO and HC emission characteristics of
coated and uncoated engines a) CO % b) HC ( ppm)
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(b)
Figure 10.11 (Continued)
The results of the emission tests are given below.
The CO and HC emissions are lower for the various speeds in thecoated engines. A trend of significant improvement in the carbon monoxideand hydrocarbon emissions is observed in the case of engine with coatedcomponents compared to that of the standard engine. The reduced hydrocarbon isseen to be more pronounced at higher engine speeds (> 1000 rpm) for theengine with coated components.
10.4. 5 Endurance Test of Coated IC Engine
The endurance test on road was completed for duration of 1000 kmsfor various speed and loads and the coated components were visuallyexamined for any peel off of the coating and any other coating defects. Onvisual examination, it was found that the coating was able to withstand thetemperature variations in actual use and found to be satisfactory.
10.5 CONCLUSION
1. The visual examination of the coated piston crown and thecylinder head showed no spallation of the coating after testingin the test rig.
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2. Petrol engines are not expected to exhibit superior performance
when incorporated with ceramic coated components, but the
results are encouraging and as such significant improvement is
observed in performance efficiency by a two stroke petrol engine
with coated components. The load carrying capacity of the coated
engine increased by 10 to 15 %, the brake thermal efficiency ( bth)
increased by 12 %, indicated thermal efficiency ( ith) increased by
28%, volumetric efficiency ( vol) increased by 3%.The
mechanical efficiency ( m) of coated engine reduced by 6%.
3. The maximum temperature of cylinder head and the cylinder
of coated engines were lower than that of uncoated engine by
20°C on an average, for the conditions described.
4. Also, it is very clear that the coating has not deteriorated the
performance of the petrol engine. Coating has in fact
improved the emission characteristics of the engine which is
due to the better burning of the fuel thereby reducing the
amount of unburnt fuel, ie. hydrocarbon in the exhaust. The
CO and HC emissions were lower by 30% for the various
speeds in the coated engines.
5. No visible defects were noticed after the endurance tests were
conducted on the petrol engine. Mullite duplex coatings on
engine components have withstood the long distance
endurance tests carried out. Mullite coatings have not failed as
anticipated due to the high creep strength of mullite. Similar
studies conducted on diesel engines by Pierz (1993), gave
good results. Hence, mullite coatings can be successfully
applied on spark ignition engines.
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