Paul M. Najt

New Heat Transfer Correlation for an HCCI Engine Derived from Measurements of Instantaneous Surface Heat Flux

An experimental study has been carried out to provide qualitative and quantitative insight into gas to wall heat transfer in a gasoline fueled Homogeneous Charge Compression Ignition (HCCI) engine. Fast response thermocouples are embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux. Heat flux measurements obtained at multiple locations show small spatial variations, thus confirming relative uniformity of incylinder conditions in a HCCI engine operating with premixed charge. Consequently, the spatially-averaged heat flux represents well the global heat transfer from the gas to the combustion chamber walls in the premixed HCCI engine, as confirmed through the gross heat release analysis. Heat flux measurements were used for assessing several existing heat transfer correlations. One of the most popular models, the Woschni expression, was shown to be inadequate for the HCCI engine. The problem is traced back to the flame propagation term which is not appropriate for the HCCI combustion. Subsequently, a modified model is proposed which significantly improves the prediction of heat transfer in a gasoline HCCI engine and shows very good agreement over a range of conditions.

Characterizing the thermal sensitivity of a gasoline homogeneous charge compression ignition engine with measurements of instantaneous wall temperature and heat flux

Abstract An experimental study was performed to provide qualitative and quantitative insight into the thermal effects on a gasoline-fuelled homogeneous charge compression ignition (HCCI) engine combustion. The single-cylinder engine utilized exhaust gas rebreathing to obtain large amounts of hot residual gas needed to promote ignition. In-cylinder pressure, heat release analysis, and exhaust emission measurement were employed for combustion diagnostics. Fast response thermocouples were embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux, thus providing critical information about the thermal boundary conditions and a thorough understanding of the heat transfer process. Two parameters determining thermal conditions in the cylinder, i.e. intake charge temperature and wall temperature, were considered and their effect on ignition and burning rate in an HCCI engine was investigated through systematic experimentation. The approach allowed quantitative analysis, and separating qualitatively different effects on the core gas temperature from the effects of near-wall temperature stratification. The results show great sensitivity to changes in wall temperature and such like, but a somewhat weaker effect of intake charge temperature on HCCI combustion. Variations of combustion phasing and peak burn rates due to wall temperature changes can be compensated if the intake charge temperature is varied in the opposite direction and with a factor of 1.11. The combustion stability limit of the HCCI engine depends more on wall temperature than on intake charge temperature. Analysis of a large number of individual cycles indicates that decreasing intake temperature retards timing, and the burn rates change primarily as a function of ignition timing. In contrast, lowering the wall temperature led to greater reduction in the bulk burn rate and greater increase in cyclic variability than expected simply as a result of retarded ignition, thus indicating significance of the thermal stratification in the near-wall boundary layer.

Characterizing the Effect of Combustion Chamber Deposits on a Gasoline HCCI Engine

Homogenous Charge Compression Ignition (HCCI) engines offer a good potential for achieving high fuel efficiency while virtually eliminating NOx and soot emissions from the exhaust. However, realizing the full fuel economy potential at the vehicle level depends on the size of the HCCI operating range. The usable HCCI range is determined by the knock limit on the upper end and the misfire limit at the lower end. Previously proven high sensitivity of the HCCI process to thermal conditions leads to a hypothesis that combustion chamber deposits (CCD) could directly affect HCCI combustion, and that insight about this effect can be helpful in expanding the low-load limit. A combustion chamber conditioning process was carried out in a single-cylinder gasoline-fueled engine with exhaust rebreathing to study CCD formation rates and their effect on combustion. Burn rates accelerated significantly over the forty hours of running under typical HCCI operating conditions. Variations of burn rates diminished after approximately 36 hours, thus indicating equilibrium conditions. Observed trends suggest that deposits change dynamic thermal boundary conditions at the wall and this in turn strongly affects chemical kinetics and bulk burning. In addition, this work presents a methodology for investigating the thermal diffusivity of deposits without their removal. The experimental technique relies on a combination of instantaneous surface temperature and CCD thickness measurements. Results demonstrate a strong correlation between deposit thickness and the diffusivity of the CCD layer.

A numerical study of high-pressure non-equilibrium streamers for combustion ignition application

We present a computational simulation study of non-equilibrium streamer discharges in a coaxial electrode and a corona geometry for automotive combustion ignition applications. The streamers propagate in combustible fuel-air mixtures at high pressures representative of internal combustion engine conditions. The study was performed using a self-consistent, two-temperature plasma model with finite-rate plasma chemical kinetics. Positive high voltage pulses of order tens of kV and duration of tens of nanoseconds were applied to the powered inner cylindrical electrode which resulted in the formation and propagation of a cathode-directed streamer. The resulting spatial and temporal production of active radical species such as O, H, and singlet delta oxygen is quantified and compared for lean and stoichiometric fuel-air mixtures. For the coaxial electrode geometry, the discharge is characterized by a primary streamer that bridges the inter-electrode gap and a secondary streamer that develops in the wake of the ...

Fuel unmixedness effects in a gasoline homogeneous charge compression ignition engine

Abstract Fuel stratification, independent of thermal and residual gas stratification, was studied in a gasoline homogenous charge compression ignition (HCCI) engine. The unmixed charge was created by injecting fuel (iso-octane) into the intake port after being prevaporized and heated to the same temperature as the intake stream. Planar laser-induced fluorescence measurements showed that local equivalence ratios in the charge differed from the mean equivalence ratio by up to 50 per cent for the latest possible injection timing. Experimental results showed little to no change in combustion phasing and performance between prevaporized port (unmixed) or premixed (homogeneous) fuelling. Increases in NO x and CO emissions were observed with the prevaporized port fuelling and are believed to result from the regions richer or leaner than the mean equivalence ratio. These results indicate that fuel stratification in the absence of thermal and residual stratification does not appear to be a viable method for HCCI combustion control for gasoline-type fuels.

The effects of intake charge preheating in a gasoline-fueled hcci engine

Experiments were performed on a homogeneously fueled compression ignition gasoline-type engine with a high degree of intake charge preheating. It was observed that fuels that contained lower end and/or non-branched hydrocarbons (gasoline and an 87 octane primary reference fuel (PRF) blend) exhibited sensitivity to thermal conditions in the surge tanks upstream of the intake valves. The window of intake charge temperatures, measured near the intake valve, that provided acceptable combustion was shifted to lower values when the upstream surge tank gas temperatures were elevated. The same behavior, however, was not observed while using isooctane as a fuel. Gas chromatograph mass spectrometer analysis of the intake charge revealed that oxygenated species were present with PRF 87, and the abundance of the oxygenated species appeared to increase with increasing surge tank gas temperatures. No significant oxygenated species were detected when running with isooctane. The presence of the oxygenated species for PRF 87 fueling indicated that reactions were occurring in the intake surge tanks which resulted in needing lower intake charge temperatures to achieve autoignition.

Development of a Postprocessing Methodology for Studying Thermal Stratification in an HCCI Engine

Naturally occurring thermal stratification significantly impacts the characteristics of homogeneous charge compression ignition (HCCI) combustion. The in-cylinder gas temperature distributions prior to combustion dictate the ignition phasing, burn rates, combustion efficiency, and unburned hydrocarbon and CO emissions associated with HCCI operation. Characterizing the gas temperature fields in an HCCI engine and correlating them to HCCI burn rates is a prerequisite for developing strategies to expand the HCCI operating range. To study the development of thermal stratification in more detail, a new analysis methodology for postprocessing experimental HCCI engine data is proposed. This analysis tool uses the autoignition integral in the context of the mass fraction burned curve to infer information about the distribution of temperature that exists in the cylinder prior to combustion. An assumption is made about the shape of the charge temperature profiles of the unburned gas during compression and after combustion starts elsewhere in the cylinder. Second, it is assumed that chemical reaction rates proceed very rapidly in comparison to the staggering of ignition phasing from thermal stratification. The autoignition integral is then coupled to the mass fraction burned curve to produce temperature-mass distributions that are representative of a particular combustion event. Due to the computational efficiency associated with this zero-dimensional calculation, a large number of zones can be simulated at very little computational expense. The temperature-mass distributions are then studied over a coolant temperature sweep. The results show that very small changes to compression heat transfer can shift the distribution of mass and temperature in the cylinder enough to significantly affect HCCI burn rates and emissions.

The impact of a magnesium zirconate thermal barrier coating on homogeneous charge compression ignition operational variability and the formation of combustion chamber deposits

The accumulation and burn-off of combustion chamber deposits create uncontrolled shifting of the homogeneous charge compression ignition operability range. This combustion chamber deposit–created operational variability places increased control burden on a multi-mode engine. However, the operational variability can be mitigated by manipulating combustion chamber deposit accumulation. A magnesium zirconate thermal barrier coating was applied to the piston of a homogeneous charge compression ignition engine in an effort to reduce combustion chamber deposit accumulation through elevated piston surface temperatures. While reduced combustion chamber deposit thicknesses were observed on the magnesium zirconate piston periphery, combustion chamber deposit accumulation in the bowl region increased relative to aluminum piston operation. Additionally, combustion chamber deposit thicknesses on the aluminum cylinder head were reduced during operation with the magnesium zirconate coated piston. Chamber-wide alterations to combustion chamber deposit accumulation taken together with the increased burn duration and hydrocarbon emissions measured during operation with the magnesium zirconate piston indicate significant interaction between the directly injected fuel spray and thermal barrier coating porosity. The porosity and surface roughness of the magnesium zirconate thermal barrier coating are speculated to create fuel pooling/absorption within the piston bowl, increasing combustion chamber deposit accumulation in the bowl and leaning the remaining fuel–air charge. The charge leaning lengthens the magnesium zirconate burn duration and reduces cylinder head combustion chamber deposit accumulation. Furthermore, hydrocarbon emissions were increased during magnesium zirconate operation due to late desorption and subsequent incomplete burning of fuel from piston bowl and the presence of incombustibly lean areas in the remaining cylinder charge.

Method for Determining Instantaneous Temperature at the Surface of Combustion Chamber Deposits in an HCCI Engine

Homogeneous charge compression ignition (HCCI) combustion is widely regarded an attractive option for future high efficiency gasoline engines. HCCI combustion permits operation with a highly dilute, well mixed charge, resulting in high thermal efficiency and extremely low NOx and soot emissions, two qualities essential for future propulsion system solutions.Because HCCI is a thermo-kinetically dominated process, full understanding of how combustion chamber boundary thermal conditions affect the combustion process are crucial. This includes the dynamics of the effective chamber wall surface temperature, as dictated by the formation of combustion chamber deposits (CCD). It has been demonstrated that, due to the combination of CCD thermal properties and the sensitivity of HCCI to wall temperature, the phasing of auto-ignition can vary significantly as CCD coverage in the chamber increases.In order to better characterize and quantify the influence of CCDs, a numerical methodology has been developed which permits calculation of the crank-angle resolved local temperature profile at the surface of a layer of combustion chamber deposits. This unique predictor-corrector methodology relies on experimental measurement of instantaneous temperature underneath the layer, i.e. at the metal-CCD interface, and known deposit layer thickness. A numerical method for validation of these calculations has also been devised. The resultant crank-angle resolved CCD surface temperature and heat flux profiles both on top and under the CCD layer provide valuable insight into the near wall phenomena, and shed light on the interplay between the dynamics of the heat transfer process and HCCI burn rates.Copyright

Particulate Matter Sampling and Volatile Organic Compound Removal for Characterization of Spark Ignited Direct Injection Engine Emissions

More stringent emissions regulations are continually being proposed to mitigate adverse human health and environmental impacts of internal combustion engines. With that in mind, it has been proposed that vehicular particulate matter (PM) emissions should be regulated based on particle number in addition to particle mass. One aspect of this project is to study different sample handling methods for number based aerosol measurements, specifically, two different methods for removing volatile organic compounds (VOCs). One method is a thermodenuder (TD) and the other is an evaporative chamber/diluter (EvCh). These sample handling methods have been implemented in an engine test cell with a spark ignited direct injection (SIDI) engine. The engine was designed for stoichiometric, homogeneous combustion. SIDI is of particular interest for improved fuel efficiency compared to other SI engines, however, the efficiency benefit comes with greater PM emissions and may therefore be subject to the proposed number based PM regulation. Another aspect of this project is to characterize PM from this engine in terms of particle number and composition.