25-08-2014, 04:40 PM
THE INFLUENCE OF THERMAL BARRIER COATING SURFACE ROUGHNESS ON SPARK-IGNITION ENGINE PERFORMANCE AND EMISSIONS PROJECT REPORT
THE INFLUENCE OF THERMAL.pdf (Size: 6.55 MB / Downloads: 53)
Abstract
The effects on heat transfer of piston crown surface finish and use of a metal based
thermal barrier coating (TBC) on the piston crown were studied in an SI engine. Measured
engine parameters such as power, fuel consumption, emissions and cylinder pressure were
used to identify the effects of the coating and its surface finish. Two piston coatings were
tested: a baseline copper coating and a metal TBC. Reducing surface roughness of both
coatings increased in-cylinder temperature and pressure as a result of reduced heat transfer
through the piston crown. These increases resulted in small improvements in both power and
fuel consumption, while also having measurable effect on emissions. Oxides of nitrogen
emissions were increased while total hydrocarbon emissions were decreased. Improvements
attributed to the TBC were found to be small, but statistically significant. At an equivalent
surface finish, the TBC performed better than the baseline copper finish
Importance of Heat Transfer
Heat loss is one of the primary loss mechanisms in an internal combustion engine and
plays a critical role in all aspects of engine operation: performance, efficiency and emissions.
The primary means of heat loss from an engine is through the engine cooling system which
absorbs combustion- and friction-generated heat energy into a cooling medium (liquid- or
air-cooled) and dissipates it to the surroundings to ensure engine temperatures remain below
the material and tribological limits of the engine. The heat energy of the exhaust gases
accounts for the remainder of the heat losses from the combustion process.
Heat losses from an engine result in decreased engine performance and efficiency.
This is due to a portion of the thermal energy from the combustion gases being rejected to the
surroundings rather than being converted to increased in-cylinder gas temperatures and
pressure, and thus useful engine output. Therefore, thermodynamically, a reduction in engine
heat transfer results in increases in overall engine performance
TBCs in Engines
he purpose of applying thermal barrier coatings to engine components is to impede
heat flow by increasing the thermal resistance at the interface between high temperature
combustion gases and metallic surfaces. Thermal barrier coatings provide a means of doing
this due to their very low thermal conductivities in comparison to the metallic substrates
upon which they are applied. This reduction in heat loss to the metallic components translates
to higher gas temperatures, which can lead to greater work output and therefore
improvements in efficiency. Likewise, the metallic substrates will be exposed to lower peak
temperatures as a result of the coatings, and thus reduce the thermal stresses in the engine
components
Motivations/Objectives
Studies have shown that TBCs have the potential for improving engine performance
and emissions in both diesel and SI engines (refer to Chapter 2: Literature Review). Though
the ability of thermal barrier coatings to reduce heat transfer in an engine and improve
performance seem encouraging, a clear understanding of the true effects these coatings have
on engine behaviour has yet to be achieved in the engine research community. After a large
influx of work in the 1980’s focussing on the development of the “adiabatic” compression
ignition engine using thick (>1mm) ceramic coatings, interest in TBCs in IC engines
diminished due to coating durability issues and inconclusive heat transfer/emissions data.
The fears of engine knock and irregular combustion resulting from thermal insulation has
also deterred any serious attention from using thin TBCs in SI engines, even though studies
have shown promising results. In general, a clear understanding of the true reasons for the
inconsistency seen in these studies has yet to be agreed upon and has led to an apparent loss
of interest in the field
Roughness Studies
In the following, a brief overview of various studies that have been conducted in the
field of heat transfer on rough surface is provided. As a result of the extensive use of TBCs in
gas turbines and the rough surface characteristics of TBCs, the majority of the studies
reviewed focus on the effects of roughness on heat transfer in gas turbines using both flat
plate and airfoil experiments. The review will then focus on IC engine roughness studies
Roughness Effects on Convective Heat Transfer in CFR Engine
As demonstrated by the turbine blade and flat plate studies in Section 2.3.1, surface
roughness has an important effect on convective heat transfer. The relative effect of surface
roughness is defined by the roughness Reynolds number, which is a function of roughness
height and fluid velocity (see Equation 2.6 on p.17). To apply the conclusions from the
previous roughness studies and quantify the significance of the roughness introduced into the
combustion chamber of an IC engine, the non-dimensional Rek values for the engine and
airfoil/flat plate studies should be comparable
Experimental Setup and Procedures
The following chapter provides a description of the general apparatus used
throughout the testing regime. Any specific changes made to the apparatus for individual
tests will be described in the appropriate chapters. A brief overview of the testing procedure
used to study the effects of TBC surface roughness on engine performance is also presented.
Temperature and Pressure Measurement
Four basic temperature measurements were used to define an engine operating
condition: intake air, oil, coolant and exhaust temperatures. Each of these temperatures was
measured using standard Type K thermocouples from Omega Engineering (P/N KQXL-
116G-12). The intake air, oil and coolant temperatures were used as inputs to PID controller
heaters set to 30 oC, 80 oC and 100oC, respectively throughout testing unless otherwise
specified. Each measurement was recorded by the data acquisition system (refer to Section
3.3 - Data Acquisition System) and displayed at the engine control panel.
A piezoelectric pressure gage (Kistler 6125B) was used to measure engine in-cylinder
pressures. The voltage output of the transducer was passed through a dual mode charge
amplifier (Kistler 5010) equipped with a 33 kHz low pass filter (Kistler 5311A) to minimize
unwanted signal noise. Intake air pressure was measured using an OEM manifold air pressure
(MAP) sensor (P/N GM 866 7212) salvaged from an old automobile.
Data Acquisition System
Experimental data was gathered using a National Instruments cDAQ-9178 8-slot USB
chassis populated with five modules used to acquire the varying signals. A 16-bit NI-9205
module gathered all low speed voltage signals including the emissions analyzer, flow meter
and speed/torque readings. An NI-9211 thermocouple module was used to record oil,
coolant, intake and exhaust temperature. These low-speed measurements were taken at a
sampling rate of 200 Hz, averaged over a 0.5 second period and recorded. Four cDAQ-9215
modules acquired the high-speed pressure and temperature measurements which were
sampled as a function of engine speed using the crankshaft encoder. The high-speed
measurements were triggered using the one pulse per revolution “Z-pulse” which was
manually adjusted to begin signal acquisition at TDC. The encoder “A-pulse” then provided
a sampling frequency of 1800 samples per revolution, or a sample every 0.2 CAD, for the
high speed data. This sampling rate, which was dependant on engine speed, thus translated to
a maximum rate 50,000 kS/second per data channel at an engine speed of 16
Thermocouple Development
Temperature measurements are the primary pieces of information needed to conduct a
heat transfer analysis, as by definition, heat transfer describes energy transport across a
temperature gradient. The basics of the heat transfer analysis that will be conducted in this
study and the development of the temperature measurement devices using finite element
modelling will be discussed in the following chapter
Surface Thermocouple Design and Construction
Figure 4.6 shows the basic design of the surface thermocouple used in this study. The
thermocouple consisted of a single constantan wire (Omega Engineering TFCI series) fed
through an aluminum (AL 2618) substrate. Constantan was selected due to its high Seebeck
coefficient (S = -38.3mV/K, see Table 4.1 on p.51) relative to other typical thermocouple
materials [110]. The thermocouple’s hot junction was formed using a thin plasma-sprayed
copper coating which damped the hotspot created at the junction due to differences in
material properties, and thus increasing the one dimensionality and accuracy of the device.
The optimization of the copper layer is presented in Section 4.4.
FEA Sensitivity Analysis
There was some uncertainty pertaining to the properties of the sprayed copper coating
which may impact the accuracy of the surface thermocouple. Properties such as thermal
contact resistance between substrate and coating, porosity and oxidation of the coating each
created a deviation from the ideal physical properties of the copper film implemented in the
FEA model.
An attempt was made to address these issues by using the finite element model to
determine a lower bound for thermocouple accuracy assuming less-than-ideal properties for
the thin copper film. Through thickness and weight measurements, the porosity of a thin
copper coating was estimated to be approximately 30%. This large concentration of air in the
copper coating would result in a proportional reduction in density of the coating and was
accounted for in the specific heat capacity of the coating. The thermal conductivity of the
coating to be implemented in this worst-case model was estimated to be 25% of the bulk k
value of copper, or 100 W/mK. To account for the uncertainty in the k value for the Aremco
potting compound, the value k = 10 W/mK was reduced to 5 W/mK as a worst case scenario.
In-Depth Thermocouple Development
In addition to the surface temperature boundary condition, a second temperature
measurement a fixed distance from the surface was required to conduct a heat transfer
analysis of the piston. The second boundary condition should have minimal temperature
fluctuations, and thus thermocouple response time is not crucial in this application. Using a
similar approach to that taken by Marr [113], a second constantan-aluminum thermocouple
hot junction was created by affixing a single constantan wire to the underside of the piston
crown. Figure 4.17 depicts a schematic of the in-depth thermocouple layout
Experimental Thermocouple Testing
Experimental tests using instrumented plugs installed in the cylinder wall of the CFR
engine were used to validate the surface thermocouple design. These tests helped determine
the effects different thermocouple configurations had on temperature measurements while
also helping develop the experimental apparatus for the final piston tests. In the following,
the manufacturing, calibration and testing of the instrumented test plugs will be presented. A
method for testing thermocouple response time is also verified discussed
Piston Testing Setup and Methodology
The following chapter outlines the development of the experimental apparatus used to
conduct the in-cylinder heat transfer analysis of the thermal barrier coatings. A detailed
description of the piston instrumentation process will also be provided. Finally, a summary of
the test procedure to be implemented during the engine testing will be discussed
Piston Thermocouple Calibration
Since piston surface temperatures in the CFR engine were expected to be significantly
greater than cylinder wall temperatures based on the results seen by Marr [113], the piston
was calibrated for a larger temperature range than that used during the plug tests. However,
due to the temperature limitations of the Teflon insulation used on the constantan wires, a
preliminary calibration of the aluminum-constantan thermocouple pairing was conducted in
one of the test plugs ensure the instrumented piston was not damaged. This test thus provided
a baseline calibration curve for the thermocouples used throughout the piston testing.
For both tests described below, the same calibration apparatus used in the
thermocouple plug tests discussed in Section 5.2 was used. An ice bath was once again used
as the cold junction for the aluminum-constantan thermocouples being calibrated. The
reference temperatures and measured thermocouple voltages were allowed to stabilize prio
Piston Surface Roughness Test Results
The following chapter outlines the results of the piston surface roughness tests
conducted for two coatings (copper and metal based TBC). Each coating surface finish was
run at a total of 12 different load/speed conditions in an attempt to isolate changes in heat
transfer, and thus engine performance, resulting from the interaction between in-cylinder
flows and the varying surface finishes. The general effectiveness of the thermal barrier
coating due to its insulating properties, and the effect of surface roughness on its
performance, will also be discussed.
Throughout the discussion of the results, an emphasis will be placed on trying to
explain engine measurements in terms of changes in heat transfer as a result of 1) surface
finish and 2) coating material. The measurements will also be compared to estimates
Thermocouple Calibration - Circuit Setup
The analog filtering circuit was designed to allow for versatility in the thermocouple
calibration process. The combination of the trim potentiometer and the variable output gains
are used to maximize the resolution of the data acquisition system when small peak-to-peak
temperature fluctuations are measured during TBC-coated tests. However, as the output gain
is increased, the temperature range available for calibration is reduced. Also, due to the
inherent DC offset of the ICs used, the trim pot must be used to shift the circuit output to the
±10V range read by the DAQ. As a result, a new thermocouple calibration must be
completed at each gain setting
MATLAB Code Descriptions
The following section provides further detail on each of the MATLAB functions used
in the processing of the collected data. Three types of data files are processed using these
MATLAB programs: instantaneous thermocouple voltages, instantaneous in-cylinder
pressure measurements and low speed control panel data (i.e. speed, torque, flow rates,
emissions etc). All high speed temperature and pressure files contain 100 cycles of data
measured at 3600 samples per engine cycle (1800 measurements per revolution). Note that
the programs were written for the batch processing of multiple data files collected through a
test run, but codes can also be modified to process individual data files.