0
Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Impact of a Yttria-Stabilized Zirconia Thermal Barrier Coating on HCCI Engine Combustion, Emissions, and Efficiency OPEN ACCESS

[+] Author and Article Information
Tommy Powell

Department of Automotive Engineering,
Clemson University,
4 Research Drive,
Greenville, SC 29607
e-mail: trpowel@g.clemson.edu

Ryan O'Donnell

Department of Automotive Engineering,
Clemson University,
4 Research Drive,
Greenville, SC 29607
e-mail: rodonne@g.clemson.edu

Mark Hoffman

Department of Automotive Engineering,
Clemson University,
4 Research Drive,
Greenville, SC 29607
e-mail: mhoffm4@clemson.edu

Zoran Filipi

Department of Automotive Engineering,
Clemson University,
4 Research Drive,
Greenville, SC 29607
e-mail: zfilipi@clemson.edu

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received February 22, 2017; final manuscript received March 13, 2017; published online August 1, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(11), 111504 (Aug 01, 2017) (9 pages) Paper No: GTP-17-1077; doi: 10.1115/1.4036577 History: Received February 22, 2017; Revised March 13, 2017

In-cylinder surface temperature has significant impacts on the thermo-kinetics governing the homogeneous charge compression ignition (HCCI) process. Thermal barrier coatings (TBCs) enable selective manipulation of combustion chamber surface temperature profiles throughout a fired cycle. In this way, TBCs enable a dynamic surface temperature swing, which prevents charge heating during intake while minimizing heat rejection during combustion. This preserves volumetric efficiency while fostering more complete combustion and reducing emissions. This study investigates the effect of a yttria-stabilized zirconia (YSZ) coating on low temperature combustion (LTC), efficiency, and emissions. This is an initial step in a systematic effort to engineer coatings best suited for LTC concepts. A YSZ coating was applied to the top of the aluminum piston using a powder air plasma spray (APS) process; final thickness of the YSZ was approximately 150 μm. The coated piston was subsequently evaluated in the single-cylinder HCCI engine with exhaust re-induction. Engine tests indicated significant advancement of the autoignition point and reduced combustion durations with the YSZ coating. Hydrocarbon and carbon monoxide emissions were reduced, thereby increasing combustion efficiency. The combination of higher combustion efficiency and decreased heat loss during combustion produced tangible improvements in thermal efficiency. When the effects of combustion advance were removed, the overall improvements in emissions and efficiency were lower, but still significant. Overall, the results encourage continued efforts to devise novel coatings for LTC.

Homogeneous charge compression ignition engines (HCCI) have distinct advantages in both emissions and efficiency over spark-ignited engines. Lean, unthrottled operation enables HCCI engines to simultaneously have higher cycle efficiency and lower NOx emissions [1,2]. However, low temperature combustion (LTC) is driven by chemical kinetics, making HCCI engines highly sensitive to in-cylinder charge temperature history [3]. As an added difficulty, in-cylinder temperatures are subject to strong spatial variation. Gas temperatures measured via planar laser-induced florescence (PLIF) showed the development of late-compression thermal stratification in a chamber that was homogenous in temperature at bottom dead center [47]. The significant cycle-to-cycle variability in thermal stratification can be seen by comparing an ensemble-averaged PLIF image (Fig. 1(a)) with one of a single cycle (Fig. 1(b)) [4]. Volumes of gas influenced by wall temperatures were not limited to near wall regions by the end of compression, and these wall-affected regions exhibited temperatures 50 K below the bulk average temperature. Other parcels of gas mixture were as much as 50 K warmer than the bulk average temperature for a single cycle [4].

Chang et al. [8] showed a strong dependence of HCCI autoignition and combustion on in-cylinder wall temperatures. By increasing coolant temperature from 80 °C to 95 °C, combustion stability improved, autoignition advanced, and peak rates of heat release increased up to 35%. Thus, large changes in combustion duration and ignition timing were possible with small changes in combustion chamber surface temperatures, providing important evidence of the influence that wall-affected-gas temperatures have on HCCI combustion and autoignition.

Additional evidence of wall temperatures influencing HCCI combustion was found with the buildup of combustion chamber deposits (CCDs) [911]. Increasing deposit accumulation in an HCCI engine advanced autoignition and reduced combustion duration. It was found that deposits act as thermal insulation, having a low thermal diffusivity and increasing combustion chamber surface temperatures. The increased wall temperature swing decreases the temperature differential between the charge and the chamber walls, reducing heat transfer, increasing the bulk gas temperature during compression, and advancing the autoignition process. The cumulative effect was a stabilization of combustion and increased rates of heat release, ultimately enabling the engine to operate at leaner equivalence ratios and shifting the low load limit [911]. The buildup of the deposit layer reached equilibrium after 40 h of testing at a constant HCCI operational point [911]. This equilibrium thickness is reached when the CCD surface temperature increases to the point where condensation of new deposits equals their rate of burn-off. Thus, a shortcoming of relying on CCDs as thermal insulation is that their thickness depends on engine operational history.

Thermal barrier coatings (TBCs) can offer thermal insulation similar to combustion deposits with reduced drive cycle dependence. Hoffman et al. [11] utilized a magnesium zirconate coated piston in an HCCI engine. Combustion phasing advanced relative to a standard engine, resulting in increased combustion efficiency. The phasing advance makes it difficult to isolate the fundamental effects coatings have on LTC. Advanced combustion increases gas temperatures, promoting lower emissions and higher efficiency. Overall findings encourage further research efforts related to TBCs for HCCI.

A newly completed single-cylinder HCCI engine setup enables experimental measurements of in-cylinder heat flux and coating surface temperatures when used in conjunction with TBCs. In this study, yttria-stabilized zirconia (YSZ) was applied to the piston top. This coating has been proven both effective and durable in gas turbine applications. However, YSZ was not previously evaluated in an HCCI engine. Two experimental campaigns were conducted to isolate YSZ impacts on HCCI combustion, emissions and efficiency. The first group of experiments compared the metal-engine baseline to the engine operated with the YSZ coated piston over the same conditions. As expected, the use of the YSZ coated piston advanced autoignition and combustion phasing. The second set of experiments intentionally prevented the advancement of combustion, and isolated the purely thermal impacts of the coating on heat loss, as well as thermal and combustion efficiencies. Cooled exhaust gas recirculation (EGR) was added to maintain combustion phasing as the baseline when operating with the YSZ TBC. In both cases, coating surface temperatures were computed using a recently developed inverse heat conduction solver. By combining measurements of thermal boundary conditions, engine performance metrics, and combustion phasing control, a deeper understanding of the impact TBCs have on LTC was developed.

This paper is organized as follows: first, a brief background on the use of TBCs is presented, followed by a discussion of the experimental setup and TBC process. Next, the experimental results with the YSZ coated piston are presented and discussed, followed by concluding statements about the effects of TBCs in HCCI engines.

Thick “monolithic” TBCs were previously pursued as a potential solution for achieving a low heat rejection engine [12]. The coatings were thick to increase the thermal resistance and reduce the heat transferred to the coolant [1214]. However, the high thermal mass of the coatings stored combustion heat and released it during gas exchange, causing substantial charge heating and losses in volumetric efficiency [12]. The surface temperature of the thick TBCs followed a similar behavior to the “traditional insulation” in Fig. 2.

In contrast, thin TBCs are suggested to avoid problematic charge heating while still reducing combustion heat transfer [1618]. These coatings differ from previous TBCs in that the thermal resistance is provided by a low thermal conductivity material instead of by the coating's thickness. A thin coating with low conductivity reduces thermal energy storage and enables the coating to respond quickly to gas temperature changes, displaying behavior comparable to the “temperature swing insulation” in Fig. 2 [1518]. As a result, coating surface temperature is closer to the burned gas temperatures during combustion and similar to the fresh air temperature during gas exchange. A finite element model of a thin TBC subjected to experimentally measured heat fluxes and temperatures confirmed the ability of thin, low conductivity coatings to both reduce combustion heat transfer and minimize charge heating in a LTC engine [15].

HCCI Engine.

The engine used for testing is a modified single-cylinder Ricardo Hydra, seen in Fig. 3, with four valves per cylinder, direct fuel injection, and a rebreathe style exhaust cam. Additional information regarding the engine geometry and timing specifications are found in Table 1.

The re-induction of hot residual is required to facilitate HCCI combustion due to the moderate 12.5:1 compression ratio employed. The exhaust valve re-opens during the intake stroke to reintroduce hot exhaust gasses into the cylinder with the aid of backpressure. The hot residual comprises approximately 45% of the total charge mass at intake valve closing (IVC). Exhaust backpressure is controlled through a downstream gate valve, which is used to adjust the hot residual fraction. The in-cylinder residual fraction is estimated through the use of the state estimation method given by Eq. (1) below [19]. The estimated residual mass, mres, is a function of the temperature in the exhaust runner, Texh, along with the cylinder pressure, Pcyl,EVC, and cylinder volume, VEVC, at the time of exhaust valve closing (EVC). Display Formula

(1)mres=Pcyl,EVCVEVCRTexh

Exhaust gas is sampled by a Horiba MEXA 7100D-EGR to provide unburned hydrocarbon (HC), CO, CO2, O2, and NOx emissions measurements. Emissions are normalized by the injected fuel quantity and presented as emissions index values [20]. Additionally these emission concentrations are used to calculate both combustion efficiency [20] and the exhaust air-to-fuel ratio. Lambda measurements are also taken in the exhaust plenum via an ETAS ES630.1 and a Bosch LSU4.9 wideband lambda sensor. The accuracy of the lambda sensor measurement is ± 3% of the reading between the equivalence ratios of 0.6 and 1. Additionally, crank angle resolved exhaust runner pressure is measured by a Kistler 4045 A pressure sensor.

Intake air is supplied from a compressed air line through a critical orifice system, and the supply pressure is adjusted to control the mass flowrate. The flow measurements are provided downstream by a Fox FT2 flowmeter, and the intake pressure is measured in the runner with a Kistler 4007 A pressure sensor. Intake air is heated by means of a 2.7 kW heater to facilitate autoignition and control combustion phasing. As an additional method to control combustion phasing, exhaust gas can be recirculated upstream of the intake heater using an external EGR loop. The fresh air and EGR both pass through the charge heater, ensuring a uniform temperature intake charge. The mass of EGR is calculated using Eq. (2) and the intake EGR concentration   reported by the emissions bench. Display Formula

(2)m˙EGR=m˙air(1[EGR]conc)m˙air

The EGR concentration used in Eq. (2) is computed using intake and exhaust CO2 measurements (Eq. (3)) from the emissions bench. The background concentration is the concentration of atmospheric carbon dioxide. Display Formula

(3)[EGR]=[CO2]int[CO2]bkgnd[CO2]exh[CO2]bkgnd

Additional combustion phasing control is available through coolant and oil temperature control. However, in this study, coolant and oil temperatures are held at a constant 95 °C.

Fuel is delivered by means of direct injection at 10.4 MPa and is timed to inject at 333 deg before combustion top dead center, allowing sufficient time for near homogenous mixing. A piston accumulator and high pressure nitrogen are used to supply fuel pressure. The test fuel used in this experiment is Gage Products 91 RON test fuel (40665-55f). Fuel flow is measured to an accuracy of ±0.2% by a Max Machinery 213 piston type flow meter.

Fuel-Matched Operation.

For this experimental testing regimen, combustion phasing was allowed to vary, showing the real-world coating effects. Exhaust backpressure, the difference between the average exhaust and intake pressures, was adjusted with the gate valve in the exhaust to obtain a pressure differential of 3.7 kPa at wide open throttle with the baseline metal engine. This provides 45% hot residual gas fraction (RGF) via the rebreathe cams at the 2000 RPM, 11 mg fuel/cycle operating point. Subsequent tests with the YSZ coated piston were completed with the same gate valve position, though the residual fraction varied slightly due to changes in the exhaust temperature. The intake temperature was set to 90 °C, while coolant and oil temperatures were held constant at 95 °C. Fuel injected per cycle and air to fuel ratio were kept constant for both pistons at each operating point. Additional operating conditions are found in Table 2.

Fuel-Matched, Phasing-Matched Operation.

Further experiments were conducted to remove the effect of combustion phasing. For this testing, CA50 was set to 7 deg after top dead center (ATDC) for the YSZ coated and uncoated cases. Fueling, AFR, airflow, exhaust gate valve position, coolant, and oil temperatures were identical to the fuel-matched (FM) points. Combustion phasing for the metal engine was achieved by increasing the intake temperature until CA50 was 7 deg ATDC. This intake temperature was also used with the YSZ coated piston at the same load points. However, CA50 for the YSZ coated piston was retarded to 7 deg by adding external EGR. Normally, combustion phasing is achieved through modification of the intake temperature. However, imposing different intake temperatures for the metal and YSZ coated cases would confound the effects of the coating on combustion through alteration of the cylinder temperature at IVC and subsequent heat transfer phenomena.

An overview of the FMPM operating conditions is presented in Table 2. Additionally, EGR is given as a percent of the incoming fresh air mass, and the RGF is presented as a fraction of the total charge mass in the cylinder during the closed portion of the cycle. Likewise, the fresh air equivalence ratio phi (ϕ) and equivalence ratio with dilution, phi prime (ϕ′), are also included in Table 2.

Postprocessing Procedure.

The data were collected with a 0.5 crank angle degree resolution. The first step in postprocessing is ensemble averaging 200 consecutive cycles. The cylinder pressure is pegged to intake pressure in a ±10 deg window at bottom dead center during the intake stroke. The heat release rate is calculated by performing a first law analysis on the ensemble-averaged cylinder pressure [2]. The heat transfer coefficient is computed using a correlation specifically designed for use with HCCI engines by Chang et al. [21].

Yttria Stabilized Zirconia Coating.

For this experiment, a thin, 150 μm YSZ coating was applied to a piston. The piston was chosen because it occupies a large portion of the combustion chamber area and requires little preparation to receive the coating. In order to maintain compression after coating application, a total of 150 μm was first machined off the piston crown. Next, an air plasma spray (APS) technique was used to apply a base layer of NiCrAl, and then, a YSZ low conductivity topcoat. Typical properties for APS YSZ coatings are included in Table 3.

Coating thickness was measured with a Fischer MP-20 Dualscope. The measured thicknesses are provided in Table 4, and the corresponding locations for the APS-YSZ piston are shown in Fig. 4. The thickness measurements were accurate to 0.5% of the thickness measurement. The measurements were taken “N” times at each spatial location, and subsequently the mean and standard deviation were recorded. Some spatial variation of coating thickness was noted, owing to the complex piston geometry. In addition to the piston, one of two cylinder head heat flux probes was also coated with YSZ. The average coating thickness on the probe was 157 μm with a standard deviation of 10.4 μm (n = 21).

Yttria-stabilized zirconia is often chosen for TBCs due to its high fracture toughness and high coefficient of thermal expansion [23,24]. The ASP process uses a high temperature plasma arc to melt powdered coating components [25]. The melted coating droplets are then accelerated with a gas jet toward the workpiece. The droplets impact the work surface forming lamellae, and layered accumulation of these “splats” builds the coating thickness [25]. The final coating composition is comprised of 7% Y2O3 and 93% ZrO2, often called “yttria-stabilized zirconia.”

Heat Flux Probes.

The engine is equipped with two Medtherm fast-response heat flux probes capable of providing subcrank-angle resolved temperatures. The cylinder head is provisioned for mounting two of these heat flux probes, denoted H1 and H2 in Fig. 5. The probes have a co-axial construction, with a central wire of constantan, an electrically insulating dielectric separator, and an outer casing made of iron. The ends of the tube then covered with a 10 μm layer of vacuum-deposited metal that provides an electrical connection between the thermocouple [8,9]. The microsecond-order temporal response of these probes is sufficient to measure with resolution finer than the 0.5 CAD at engine speeds relevant to this study. Additionally, these probes contain a second coaxial J-type junction 4 mm into the probe body, enabling surface heat flux calculation through a Fourier analysis. Refer to Ref. [21] for more details on the heat flux computation.

The surface thermocouple junction is normally exposed to the combustion chamber in the baseline metal engine, allowing surface temperature and heat flux measurements. However, with the addition of the YSZ coating atop the probe, the surface thermocouple junction is then located at the probe/TBC interface and measures the subcoating temperature. The YSZ surface temperature and heat flux are estimated from the subsurface temperature and heat flux measurements using an inverse heat conduction solver. More details on this solver are provided in Ref. [26].

Impact of a Yttria-Stabilized Zirconia Coated Piston.

The engine was first operated in the baseline configuration (also denoted as “uncoated” and “metal”) at the operating points listed in Table 2. Next, the engine was run with the YSZ coated piston, while all major engine inputs such as fueling, AFR, and temperatures were kept constant between the two cases. The purpose of this testing methodology was to see the real-world effects of using TBCs in HCCI engines.

The YSZ coating advanced combustion, increasing peak cylinder pressures and peak heat release rates. The coating's effect on cylinder pressures is seen in Fig. 6(a) and rates of heat release in Fig. 6(b). The combustion advance exhibited by the YSZ results from decreased compression heat transfer, leading to higher gas temperatures near top dead center.

The combustion advance illustrated in Figs. 6(a) and 6(b) occurred over the entire range of tested speeds and load. Figure 7 quantifies the YSZ-induced combustion advance as between 2.2 and 4.9 CA deg. Additionally, Fig. 7 shows that the combustion duration is shortened by 2.6–5.0 deg when utilizing the YSZ-coated piston. This trend in combustion duration is directly influenced by the YSZ-induced ignition advance. It is noteworthy that the TBC-induced combustion advance decreases as engine speed increases, as the real time for heat transfer decreases with engine speed.

The YSZ coating advanced combustion phasing and caused increased peak heat release rates and cylinder temperatures. These effects, combined with the decreased heat transfer from the YSZ coating and increased temperature in the wall-affected regions, led to enhanced autoignition, and increased combustion efficiency [6].

The main pathway for decreased HC emissions is more complete combustion. The YSZ coating produces a 25–37% decrease in unburned hydrocarbons over metal engine operation, as shown in Fig. 8. Additionally, the magnitude of hydrocarbon emission improvement decreased with increasing engine speed. This trend aligned with the effect of engine speed on time available for heat transfer and diminishing combustion phasing advance from the YSZ coating.

Carbon monoxide emissions decreased at all engine speeds for the YSZ–TBC equipped engine, see Fig. 9. The decrease in CO emissions was between 29% and 37% across the operating range; however, the change was not monotonic. The trend in carbon monoxide emissions was not a strong function of engine speed. Instead, CO emissions follow the trend in combustion advance. The reduction in carbon monoxide was highest for 1600 RPM and then for 1200 RPM, corresponding to the largest and second largest combustion advances. Advanced combustion phasing led to increased gas temperatures in the wall-affected regions, which improved carbon monoxide oxidation.

The large decrease in hydrocarbon and carbon monoxide emissions with the YSZ coating increased combustion efficiency. Combustion efficiency improves at all engine speeds during YSZ operation, as shown in Fig. 10. The increase in efficiency is between 1.0% and 1.9% over the baseline metal HCCI engine. This can be attributed to reduced heat transfer, and consequently increased charge temperatures in the near-wall region. This, in turn, causes more of the fuel-air charge to react.

However, the improvement in combustion efficiency diminishes with increasing engine speed. This effect is a direct result from the trend in the combustion advance and decreasing time for heat transfer impacts. The advance in combustion phasing, increase in cylinder pressure, and increased combustion efficiency combine to improve the cyclic work output during YSZ operation. This increases the gross-indicated thermal efficiency between 1.9% and 4.1%, as shown in Fig. 11. However, because the combustion advance is intermingled with the increased YSZ combustion efficiency, the YSZ impact is difficult to decipher from this testing. In particular, the combustion phasing of the metal engine at 1200 RPM was retarded beyond optimal. Hence, the advanced phasing achieved with the YSZ coated piston produced an added benefit, i.e., part of the 4.1% improvement should be attributed to combustion phasing rather than direct impact of oxidation reactions and reduced heat loss. For a more rigorous examination, the YSZ impact must be isolated from combustion phasing.

Fueling-Matched, Phase-Matched Operation.

The effect of the TBC on combustion provided a number of benefits, such as reduced emissions and increased combustion and thermal efficiencies. However, to isolate the thermal impact of the YSZ–TBC, combustion phasing must be held constant. A fixed combustion phasing, CA50, of 7 CA deg ATDC was chosen for this investigation. The intake charge temperature was adjusted with the metal engine until CA50 of 7 deg ATDC was achieved. During YSZ operation, the same intake temperature was utilized, and external EGR was added until CA50 retarded to the design point. The addition of EGR occurs prior to the intake heater, so the mixture of fresh air and recirculated exhaust gas are the same intake temperature. It should be noted that the 1200 RPM operating point is absent from this section due to the inability to add sufficient EGR from a combination of low backpressure and physical size limitations of the EGR circuit. By maintaining a constant intake temperature, the charge temperature at IVC is constant between tests, isolating the impact of the YSZ coating on combustion and heat transfer.

Figure 12(a) illustrates the phase-matched pressure curves at 2000 RPM, and the respective rates of heat release are presented in Fig. 12(b). The cylinder pressure trace for the YSZ coated case is similar to the metal one. The peak rate of heat release is slightly higher for the YSZ case than for the baseline engine due to a slight decrease in combustion duration with the coating.

The relative phasing and combustion durations for the phase-matched points are presented in Fig. 13. With combustion phasing tightly controlled, it is apparent that combustion duration decreases by 0.1–0.6 deg CA due to the YSZ coating. Notably, the addition of the coating advances the CA90 point for the same CA50, shortening the CA50-90 duration between 0.4 deg and 0.5 deg. The coating increases the temperatures of the wall-affected regions, increasing the combustion rate, and shortening its duration. However, additional testing is planned to verify this effect.

After the effects of combustion phasing are removed, the improvement in hydrocarbon emissions from the coating alone is still present, but lower in magnitude when compared to the fueling-matched case. Namely, the YSZ coating caused a decrease in HC emissions between 2.3% and 11%, as shown in Fig. 14.

The change in carbon monoxide emissions with the YSZ coating was consistent with the hydrocarbon emission trend. Reductions in CO emissions of 4.3% and 8.8% were observed, as seen in Fig. 15. This reduction is from increased gas temperatures in the wall-affected regions, improving CO oxidation. Similarly to the HC emissions trends, the magnitudes of YSZ induced CO emission reduction is less than the fuel matched results shown in Fig. 9.

The decrease in hydrocarbon and carbon monoxide emissions led to increased combustion efficiency for the YSZ–TBC. For the phase matched points, combustion efficiency increased between 0.1% and 0.4%, as shown in Fig. 16.

The same trend seen in the FM operating points is present where shorter timescales decrease the impact of the coating at higher speeds.

Finally, through the use of the YSZ coating, the HCCI engine experienced between a 1.9% and 3.0% increase in gross indicated cycle efficiency as depicted in Fig. 17. A decrease in heat transfer during combustion increased the expansion work and compounded with the previously described impact of the TBC on combustion efficiency. The increase in indicated efficiency was slightly higher for the FMPM points than for the FM points. At 1600 RPM, this was caused by retarding combustion phasing from 3.9 deg to 7.1 deg CA with the YSZ coating, resulting in a reduction in heat transfer [2]. Meanwhile, at 2000 RPM, FMPM combustion with the YSZ coating was only advanced 0.3 deg CA toward more optimal combustion phasing when compared to FM operation.

The mechanism responsible for the observed improvements can be confirmed with directly measured heat flux. The crank angle-resolved YSZ surface heat flux, Fig. 18(a), and YSZ surface temperature, Fig. 18(b), are calculated using an inverse conduction solver and the two junction temperatures of the probe. For more information on the calculation methodology, please consult O'Donnell et al. [26]. The YSZ coated probe, Fig. 18(b), demonstrated a surface temperature similar to an idealized thin TBC (see “temperature swing insulation” in Fig. 2).

The YSZ surface temperature profile is similar in magnitude to the metal surface temperature during gas exchange, thereby avoiding any adverse effect on volumetric efficiency. The YSZ surface then undergoes a 36 °C temperature swing during compression and combustion, much higher than in the case of the metal piston.

The heat transfer shown in Fig. 19(a) was calculated using the instantaneous surface heat flux from the cylinder head probe location and the exposed cylinder surface area. A cumulative sum of the heat transferred during combustion and expansion is included in Fig. 19(b).

The large coating temperature swing helps to reduce the difference between gas temperature and the wall temperature, reducing the driving force for combustion heat transfer and contributing to the increased thermal efficiency for the YSZ case. Eventually the instantaneous heat loss with the YSZ piston exceeds that of the baseline metal configuration, illustrating a well-known heat storage effect, see Fig. 19(a). Consequently the difference in cumulative heat loss increases early in the expansion process and eventually diminishes close to 450 CA deg ATDC of gas exchange. In short, the key to achieving the desired effect on the cycle efficiency is a dynamic change of the surface temperature, rather than excessive insulation.

Application of a YSZ–TBC to the piston of an LTC engine advanced combustion phasing between 2.2 deg CA and 4.9 deg CA. The combustion advance boosted peak cylinder pressures and temperatures, causing increased peak rates of heat release. The higher cylinder temperatures, especially in the wall-affected zones, caused more of the fuel-air charge to autoignite. The result was 25–37% less unburnt hydrocarbons in the exhaust and 1.0–1.9% higher combustion efficiency. Carbon monoxide emissions also decreased between 29% and 37%. The increase in the cylinder pressure combined with the additional energy released from higher combustion efficiency to increase gross-indicated thermal efficiency by 1.9–4.1% over the metal engine.

Subsequent experimentation with cooled EGR was designed to hold combustion phasing constant and isolate the thermal effect of the coating on cycle parameters. With constant combustion phasing, the magnitude of YSZ benefits in combustion efficiency and emissions decreased relative to FM operation, but were still present. With the effects of the coating isolated, the YSZ coating lowered hydrocarbon emissions by 2–11% and improved combustion efficiency by 0.1–0.4%. Carbon monoxide emissions decreased between 4.3% and 8.8%. Gross indicated efficiency improved by 1.9–3.0% due to the thermal effects of the coating on instantaneous heat loss during combustion.

The advance in combustion phasing with the YSZ coating has a large, positive impact on HC and CO emissions, combustion efficiency, and thermal efficiency. After removing the combustion advance, many of the key benefits of the TBC are preserved, but have a reduced magnitude. Further improvements in these parameters are likely with additional reductions in heat transfer. Use of engineered coatings with reduced thermal conductivities would likely offer enhanced benefits when used in LTC engine applications [28,29], this will be investigated in future efforts.

The authors want to thank Dr. Eric Jordan at the University of Connecticut for providing the YSZ coating and General Motors R&D for providing the single-cylinder engine used in this investigation.

Funding for this research was provided by the National Science Foundation (NSF)/Department of Energy (DOE) Partnership on Advanced Combustion Engines Grant No. 1258714, “Thermal Barrier Coatings for the LTC Engine—Heat Loss, Combustion, Thermal versus Catalytic Effect, Emissions, and Exhaust Heat”.

  • National Science Foundation (NSF)/Department of Energy (DOE) Partnership on Advanced Combustion Engines (Grant No. 1258714).

  • ATDC =

    after top dead center

  • BTDC =

    before top dead center

  • CA50 =

    crank angle at 50% mass fraction burned relative to top dead center, firing

  • CCD =

    combustion chamber deposits

  • CO =

    carbon monoxide

  • CO2 =

    carbon dioxide

  • EGR =

    exhaust gas recirculation

  • FM =

    fuel matched

  • FMPM =

    fuel matched, phase matched

  • HC =

    hydrocarbon

  • HCCI =

    homogenous charge compression ignition

  • IVC =

    intake valve closing

  • LTC =

    low temperature combustion

  • NOx =

    nitrogen oxides

  • O2 =

    oxygen

  • PLIF =

    planar laser-induced florescence

  • RGF =

    residual gas fraction

  • TBC =

    thermal barrier coating

Thring, R. , 1989, “ Homogeneous-Charge Compression-Ignition (HCCI) Engines,” SAE Paper No. 892068.
Heywood, J. , 1988, Internal Combustion Engine Fundamentals, McGraw-Hill, New York.
Dec, J. , and Sjöberg, M. , 2003, “ A Parametric Study of HCCI Combustion—The Sources of Emissions at Low Loads and the Effects of GDI Fuel Injection,” SAE Paper No. 2003-01-0752.
Dronniou, N. , and Dec, J. , 2012, “ Investigating the Development of Thermal Stratification From the Near-Wall Regions to the Bulk-Gas in an HCCI Engine With Planar Imaging Thermometry,” SAE Paper No. 2012-01-1111.
Sjöberg, M. , Dec, J. , Babajimopoulos, A. , and Assanis, D. , 2004, “ Comparing Enhanced Natural Thermal Stratification Against Retarded Combustion Phasing for Smoothing of HCCI Heat-Release Rates,” SAE Paper No. 2004-01-2994.
Dec, J. , Hwang, W. , and Sjöberg, M. , 2006, “ An Investigation of Thermal Stratification in HCCI Engines Using Chemiluminescence Imaging,” SAE Paper No. 2006-01-1518.
Snyder, J. , Dronniou, N. , Dec, J. , and Hanson, R. , 2011, “ PLIF Measurements of Thermal Stratification in an HCCI Engine Under Fired Operation,” SAE Int. J. Eng., 4(1), pp. 1669–1688. [CrossRef]
Chang, J. , Filipi, Z. , Assanis, D. , Kuo, T. , Najt, P. , and Rask, R. , 2005, “ Characterizing the Thermal Sensitivity of a Gasoline Homogeneous Charge Compression Ignition Engine With Measurements of Instantaneous Wall Temperature and Heat Flux,” Int. J. Engine Res., 6(4), pp. 289–309. [CrossRef]
Güralp, O. , Hoffman, M. , Assanis, D. N. , Filipi, Z. , Kuo, T. W. , Najt, P. , and Rask, R. , 2006, “ Characterizing the Effect of Combustion Chamber Deposits on a Gasoline HCCI Engine,” SAE Paper No. 2006-01-3277.
Güralp, O. , Hoffman, M. , Assanis, D. N. , Filipi, Z. , Kuo, T. W. , Najt, P. , and Rask, R. , 2009, “ Thermal Characterization of Combustion Chamber Deposits on the HCCI Engine Piston and Cylinder Head Using Instantaneous Temperature Measurements,” SAE Paper No. 2009-01-0668.
Hoffman, M. , Lawler, B. , Güralp, O. , Najt, P. , and Filipi, Z. , 2015, “ The Impact of a Magnesium Zirconate Thermal Barrier Coating on Homogeneous Charge Compression Ignition Operational Variability and the Formation of Combustion Chamber Deposits,” Int. J. Engine Res., 16(8), pp. 968–981. [CrossRef]
Kamo, R. , and Bryzik, W. , 1978, “ Adiabatic Turbocompound Engine Performance Prediction,” SAE Paper No. 780068.
Zucchetto, J. , Myers, P. , Johnson, H. , and Miller, D. , 1988, “ An Assessment of the Performance and Requirements for ‘Adiabatic’ Engines,” Science, 240(4856), pp. 1157–1162. [CrossRef] [PubMed]
Assanis, D. , and Heywood, J. , 1986, “ Development and Use of a Computer Simulation of the Turbocompounded Diesel System for Engine Performance and Component Heat Transfer Studies,” SAE Paper No. 860329.
Powell, T. , Hoffman, M. , Killingsworth, N. , O'Donnell, R. , Prucka, R. , and Filipi, Z. , 2015, “ Predicting the Gas-Wall Boundary Conditions in a Thermal Barrier Coated Low Temperature Combustion Engine Using Sub-Coating Temperature Measurements,” Int. J. Powertrains, epub.
Wong, V. , Bauer, W. , Kamo, R. , Bryzik, W. , and Reid, M. , 1995, “ Assessment of Thin Thermal Barrier Coatings for I.C. Engines,” SAE Paper No. 950980.
Kamo, R. , Assanis, D. , and Bryzik, W. , 1989, “ Thin Thermal Barrier Coatings for Engines,” SAE Paper No. 890143.
Kosaka, H. , Wakisaka, Y. , Nomura, Y. , Hotta, Y. , Koike, M. , Nakakita, K. , and Kawaguchi, A. , 2013, “ Concept of ‘Temperature Swing Heat Insulation’ in Combustion Chamber Walls, and Appropriate Thermo-Physical Properties for Heat Insulation Coat,” SAE Int. J. Engines, 6(1), pp. 142–149. [CrossRef]
Ortiz-Soto, E. , Vavra, J. , and Babajimopoulos, A. , 2012, “ Assessment of Residual Mass Estimation Methods for Cylinder Pressure Heat Release Analysis of HCCI Engines With Negative Valve Overlap,” ASME J. Eng. Gas Turbines Power, 134(8), p. 082802. [CrossRef]
Stivender, D. , 1971, “ Development of a Fuel-Based Mass Emission Measurement Procedure,” SAE Paper No. 710604.
Chang, J. , Güralp, O. , Filipi, Z. , Assanis, D. , Kuo, T. , Najt, P. , and Rask, R. , 2004, “ New Heat Transfer Correlation for an HCCI Engine Derived From Measurements of Instantaneous Surface Heat Flux,” SAE Paper No. 2004-01-2996.
Brandt, R. , Pawlowski, L. , Neuer, G. , and Fauchais, P. , 1986, “ Specific Heat and Thermal Conductivity of Plasma Sprayed Yttria-Stabilized Zirconia and NiAI, NiCr, NiCrAI, NiCrAIY, NiCoCrAlY Coatings,” High Temp. High Pressures, 18(1), pp. 65–77.
Jordan, E. , Jiang, C. , Roth, J. , and Gell, M. , 2014, “ Low Thermal Conductivity Yttria-Stabilized Zirconia Thermal Barrier Coatings Using the Solution Precursor Plasma Spray Process,” J. Therm. Spray Technol., 23(5), pp. 849–859. [CrossRef]
Gell, M. , Jordan, E. , Teicholz, M. , Cetegen, B. , Padture, N. , Xie, L. , and Roth, J. , 2008, “ Thermal Barrier Coatings Made by the Solution Precursor Plasma Spray Process,” J. Therm. Spray Technol., 17(1), pp. 124–135. [CrossRef]
Davis, J. , 2004, Handbook of Thermal Spray Technology, ASM International, Materials Park, OH, pp. 3–13.
O'Donnell, R. , Powell, T. , Filipi, Z. , and Hoffman, M. , 2017, “ Estimation of Thermal Barrier Coating Surface Temperature and Heat Flux Profiles in a Low Temperature Combustion Engine Using a Modified Sequential Function Specification Approach,” ASME. J. Heat Transfer, 139(4), p. 041201. [CrossRef]
O'Donnell, R. , Powell, T. , Hoffman, M. , Jordan, E. , and Filipi, Z. , 2017, “ Inverse Analysis of In-Cylinder Gas-Wall Boundary Conditions: Investigation of a Yttria-Stabilized Zirconia Thermal Barrier Coating for Homogeneous Charge Compression Ignition,” ASME. J. Eng. Gas Turbines Power, 139(10), p. 102808.
Jadhav, A. , Padture, N. , Jordan, E. , Gell, M. , Miranzo, P. , and Fuller, E. , 2006, “ Low-Thermal-Conductivity Plasma-Sprayed Thermal Barrier Coatings With Engineered Microstructures,” Acta Mater., 54(12), pp. 3343–3349. [CrossRef]
Jordan, E. , Xie, L. , Gell, M. , Padture, N. , Cetegen, B. , Ozturk, A. , and Bryant, P. , 2004, “ Superior Thermal Barrier Coatings Using Solution Precursor Plasma Spray,” J. Therm. Spray, 13(1), pp. 57–65. [CrossRef]
Copyright © 2017 by ASME
View article in PDF format.

References

Thring, R. , 1989, “ Homogeneous-Charge Compression-Ignition (HCCI) Engines,” SAE Paper No. 892068.
Heywood, J. , 1988, Internal Combustion Engine Fundamentals, McGraw-Hill, New York.
Dec, J. , and Sjöberg, M. , 2003, “ A Parametric Study of HCCI Combustion—The Sources of Emissions at Low Loads and the Effects of GDI Fuel Injection,” SAE Paper No. 2003-01-0752.
Dronniou, N. , and Dec, J. , 2012, “ Investigating the Development of Thermal Stratification From the Near-Wall Regions to the Bulk-Gas in an HCCI Engine With Planar Imaging Thermometry,” SAE Paper No. 2012-01-1111.
Sjöberg, M. , Dec, J. , Babajimopoulos, A. , and Assanis, D. , 2004, “ Comparing Enhanced Natural Thermal Stratification Against Retarded Combustion Phasing for Smoothing of HCCI Heat-Release Rates,” SAE Paper No. 2004-01-2994.
Dec, J. , Hwang, W. , and Sjöberg, M. , 2006, “ An Investigation of Thermal Stratification in HCCI Engines Using Chemiluminescence Imaging,” SAE Paper No. 2006-01-1518.
Snyder, J. , Dronniou, N. , Dec, J. , and Hanson, R. , 2011, “ PLIF Measurements of Thermal Stratification in an HCCI Engine Under Fired Operation,” SAE Int. J. Eng., 4(1), pp. 1669–1688. [CrossRef]
Chang, J. , Filipi, Z. , Assanis, D. , Kuo, T. , Najt, P. , and Rask, R. , 2005, “ Characterizing the Thermal Sensitivity of a Gasoline Homogeneous Charge Compression Ignition Engine With Measurements of Instantaneous Wall Temperature and Heat Flux,” Int. J. Engine Res., 6(4), pp. 289–309. [CrossRef]
Güralp, O. , Hoffman, M. , Assanis, D. N. , Filipi, Z. , Kuo, T. W. , Najt, P. , and Rask, R. , 2006, “ Characterizing the Effect of Combustion Chamber Deposits on a Gasoline HCCI Engine,” SAE Paper No. 2006-01-3277.
Güralp, O. , Hoffman, M. , Assanis, D. N. , Filipi, Z. , Kuo, T. W. , Najt, P. , and Rask, R. , 2009, “ Thermal Characterization of Combustion Chamber Deposits on the HCCI Engine Piston and Cylinder Head Using Instantaneous Temperature Measurements,” SAE Paper No. 2009-01-0668.
Hoffman, M. , Lawler, B. , Güralp, O. , Najt, P. , and Filipi, Z. , 2015, “ The Impact of a Magnesium Zirconate Thermal Barrier Coating on Homogeneous Charge Compression Ignition Operational Variability and the Formation of Combustion Chamber Deposits,” Int. J. Engine Res., 16(8), pp. 968–981. [CrossRef]
Kamo, R. , and Bryzik, W. , 1978, “ Adiabatic Turbocompound Engine Performance Prediction,” SAE Paper No. 780068.
Zucchetto, J. , Myers, P. , Johnson, H. , and Miller, D. , 1988, “ An Assessment of the Performance and Requirements for ‘Adiabatic’ Engines,” Science, 240(4856), pp. 1157–1162. [CrossRef] [PubMed]
Assanis, D. , and Heywood, J. , 1986, “ Development and Use of a Computer Simulation of the Turbocompounded Diesel System for Engine Performance and Component Heat Transfer Studies,” SAE Paper No. 860329.
Powell, T. , Hoffman, M. , Killingsworth, N. , O'Donnell, R. , Prucka, R. , and Filipi, Z. , 2015, “ Predicting the Gas-Wall Boundary Conditions in a Thermal Barrier Coated Low Temperature Combustion Engine Using Sub-Coating Temperature Measurements,” Int. J. Powertrains, epub.
Wong, V. , Bauer, W. , Kamo, R. , Bryzik, W. , and Reid, M. , 1995, “ Assessment of Thin Thermal Barrier Coatings for I.C. Engines,” SAE Paper No. 950980.
Kamo, R. , Assanis, D. , and Bryzik, W. , 1989, “ Thin Thermal Barrier Coatings for Engines,” SAE Paper No. 890143.
Kosaka, H. , Wakisaka, Y. , Nomura, Y. , Hotta, Y. , Koike, M. , Nakakita, K. , and Kawaguchi, A. , 2013, “ Concept of ‘Temperature Swing Heat Insulation’ in Combustion Chamber Walls, and Appropriate Thermo-Physical Properties for Heat Insulation Coat,” SAE Int. J. Engines, 6(1), pp. 142–149. [CrossRef]
Ortiz-Soto, E. , Vavra, J. , and Babajimopoulos, A. , 2012, “ Assessment of Residual Mass Estimation Methods for Cylinder Pressure Heat Release Analysis of HCCI Engines With Negative Valve Overlap,” ASME J. Eng. Gas Turbines Power, 134(8), p. 082802. [CrossRef]
Stivender, D. , 1971, “ Development of a Fuel-Based Mass Emission Measurement Procedure,” SAE Paper No. 710604.
Chang, J. , Güralp, O. , Filipi, Z. , Assanis, D. , Kuo, T. , Najt, P. , and Rask, R. , 2004, “ New Heat Transfer Correlation for an HCCI Engine Derived From Measurements of Instantaneous Surface Heat Flux,” SAE Paper No. 2004-01-2996.
Brandt, R. , Pawlowski, L. , Neuer, G. , and Fauchais, P. , 1986, “ Specific Heat and Thermal Conductivity of Plasma Sprayed Yttria-Stabilized Zirconia and NiAI, NiCr, NiCrAI, NiCrAIY, NiCoCrAlY Coatings,” High Temp. High Pressures, 18(1), pp. 65–77.
Jordan, E. , Jiang, C. , Roth, J. , and Gell, M. , 2014, “ Low Thermal Conductivity Yttria-Stabilized Zirconia Thermal Barrier Coatings Using the Solution Precursor Plasma Spray Process,” J. Therm. Spray Technol., 23(5), pp. 849–859. [CrossRef]
Gell, M. , Jordan, E. , Teicholz, M. , Cetegen, B. , Padture, N. , Xie, L. , and Roth, J. , 2008, “ Thermal Barrier Coatings Made by the Solution Precursor Plasma Spray Process,” J. Therm. Spray Technol., 17(1), pp. 124–135. [CrossRef]
Davis, J. , 2004, Handbook of Thermal Spray Technology, ASM International, Materials Park, OH, pp. 3–13.
O'Donnell, R. , Powell, T. , Filipi, Z. , and Hoffman, M. , 2017, “ Estimation of Thermal Barrier Coating Surface Temperature and Heat Flux Profiles in a Low Temperature Combustion Engine Using a Modified Sequential Function Specification Approach,” ASME. J. Heat Transfer, 139(4), p. 041201. [CrossRef]
O'Donnell, R. , Powell, T. , Hoffman, M. , Jordan, E. , and Filipi, Z. , 2017, “ Inverse Analysis of In-Cylinder Gas-Wall Boundary Conditions: Investigation of a Yttria-Stabilized Zirconia Thermal Barrier Coating for Homogeneous Charge Compression Ignition,” ASME. J. Eng. Gas Turbines Power, 139(10), p. 102808.
Jadhav, A. , Padture, N. , Jordan, E. , Gell, M. , Miranzo, P. , and Fuller, E. , 2006, “ Low-Thermal-Conductivity Plasma-Sprayed Thermal Barrier Coatings With Engineered Microstructures,” Acta Mater., 54(12), pp. 3343–3349. [CrossRef]
Jordan, E. , Xie, L. , Gell, M. , Padture, N. , Cetegen, B. , Ozturk, A. , and Bryant, P. , 2004, “ Superior Thermal Barrier Coatings Using Solution Precursor Plasma Spray,” J. Therm. Spray, 13(1), pp. 57–65. [CrossRef]

Figures

Grahic Jump Location
Fig. 1

In-cylinder PLIF images illustrating the influence of wall temperature throughout the combustion chamber near TDC for an (a) ensemble averaged and (b) single cycle. Adapted from Ref. [4].

Grahic Jump Location
Fig. 3

HCCI engine test setup

Grahic Jump Location
Fig. 2

Wall surface temperatures for an uninsulated metal wall, a monolithic TBC, and a thin TBC, showcasing the reduction in temperature difference between the coating surface and the bulk gas during both combustion and gas exchange through the application of temperature swing insulation. Adapted from Ref. [15].

Grahic Jump Location
Fig. 4

YSZ coating measurement locations

Grahic Jump Location
Fig. 13

Combustion phasing and burn durations for phase-matched operating points relative to top dead center firing, see operating conditions in Table 2 for cases with CA50 at 7 deg CA

Grahic Jump Location
Fig. 5

Heat flux probe mounting locations, H1 and H2 [11]

Grahic Jump Location
Fig. 6

The YSZ–TBC had higher (a) peak cylinder pressure at 2000 RPM and a correspondingly higher (b) rate of heat release due to advanced combustion phasing. Pintake = 104 kPa, 10.3 mg fuel/cycle, 90 °C Tintake, 95 °C Tcoolant, Toil, 20.9–21.1 AFR, 47.7–48.9% RGF, 0% external EGR.

Grahic Jump Location
Fig. 7

Combustion phasing and burn durations measured relative to top dead center firing, showing an advancement of combustion when the YSZ coating is implemented. CA50 is located at the intersection of the two burn durations. See operating conditions in Table 2 for FM cases.

Grahic Jump Location
Fig. 8

Decrease in unburnt HC emissions index when using the YSZ TBC. See operating conditions in Table 2 for FM cases.

Grahic Jump Location
Fig. 12

Comparison of the (a) cylinder pressures at 2000 RPM and (b) rates of heat release for the phase-matched points with CA50 at 7 deg CA. Pintake = 104 kPa, 10.3 mg fuel/cycle, 109 °C Tintake, 95 °C Tcoolant, Toil, 21 AFR, 47.5% RGF (metal), 45.6% RGF, and 7.0% external EGR (APS-YSZ).

Grahic Jump Location
Fig. 14

Hydrocarbon emissions for phase-matched points illustrating a general decrease in HC emission due to the YSZ coating. See operating conditions in Table 2 for cases with CA50 at 7 deg CA.

Grahic Jump Location
Fig. 15

Change in carbon monoxide emissions for the phase-matched points. See operating conditions in Table 2 for cases with CA50 at 7 deg CA.

Grahic Jump Location
Fig. 16

Combustion efficiency for phase-matched, FM points, showing an increase in combustion efficiency during YSZ operation. See operating conditions in Table 2 for cases with CA50 at 7 deg CA.

Grahic Jump Location
Fig. 17

Change in the indicated thermal efficiency for phase-matched, FM operation with the addition of the YSZ–TBC. See operating conditions in Table 2 for cases with CA50 at 7 deg CA.

Grahic Jump Location
Fig. 9

Decrease in carbon monoxide emission index due to YSZ implementation. See operating conditions in Table 2 for FM cases.

Grahic Jump Location
Fig. 10

Increase in combustion efficiency when using the YSZ TBC. See operating conditions in Table 2 for FM cases.

Grahic Jump Location
Fig. 11

Effect of the YSZ TBC on gross-indicated thermal efficiency. See operating conditions in Table 2 for FM cases.

Grahic Jump Location
Fig. 18

(Measured) (a) heat flux and (b) (calculated) surface temperatures for the metal surface and the APS-YSZ surface at the 1600 RPM, phase match point. Adapted from Ref. [27].

Grahic Jump Location
Fig. 19

(a) Instantaneous heat loss and (b) cumulative heat loss for the metal surface and APS-YSZ at the 1200 RPM phase match point [27]

Tables

Table Grahic Jump Location
Table 2 Engine operating points for fueling and phase match cases for the YSZ and baseline engine configurations
Table Grahic Jump Location
Table 1 Engine specifications, valve timings measured relative to TDC compression
Table Grahic Jump Location
Table 3 Coating thicknesses and design properties
Table Grahic Jump Location
Table 4 Measured YSZ coating thicknesses (x¯), standard deviations (σ), and sample sizes (N). All measurements are in micrometers. Average coating thickness is 160 μm.

Errata

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In