Search Results

The post-flame oxidation of unburned hydrocarbons released from the ring-pack crevice was investigated for a small, air-cooled, spark-ignition utility engine. Spark timing sweeps were performed at 50, 75 and 100% load and speeds of 1800, 2400 and 3060 RPM while operating at a 12:1 air-fuel ratio, which is typical for these engines. A global HC consumption rate (GCR) was introduced based on the temporal profile of the mass released from the ring pack; the mass release after CA90 and up to the point where the remainder of the ring pack HC mass is equal to the exhaust HC level was taken as the mass oxidized, and a rate was defined based on this mass and the corresponding crank angle period over which this took place. For all conditions tested, the GCR varied with the spark timing; advanced spark timing gave higher GCR.

An experimental study has been conducted to provide insight into heat transfer to the piston of a light-duty single-cylinder research engine under Conventional Diesel (CDC), Homogeneous Charge Compression Ignition (HCCI), and Reactivity Controlled Compression Ignition (RCCI) combustion regimes. Two fast-response surface thermocouples embedded in the piston top measured transient temperature. A commercial wireless telemetry system was used to transmit thermocouple signals from the moving piston. A detailed comparison was made between the different combustion regimes at a range of engine speed and load conditions. The closed-cycle integrated and peak heat transfer rates were found to be lower for HCCI and RCCI when compared to CDC. Under HCCI operation, the peak heat transfer rate showed sensitivity to the 50% burn location.

An experimental investigation was conducted to explore the impact in-cylinder pressure oscillations have on piston heat transfer. Two fast-response surface thermocouples embedded in the piston top measured transient temperature and a commercial wireless telemetry system was used to transmit thermocouple signals from the moving piston. Measurements were made in a light-duty single-cylinder research engine operated under low temperature combustion regimes including Homogeneous Charge Compression Ignition (HCCI) and Reactivity Controlled Compression Ignition (RCCI) and Conventional Diesel (CDC). The HCCI data showed a correlated trend of higher heat transfer with increased pressure oscillation strength, while the RCCI and CDC data did not. An extensive HCCI data set was acquired. The heat transfer rate - when corrected for differences in cylinder pressure and gas temperature - was found to positively correlate with increased pressure oscillations.

High resolution planar laser-induced fluorescence (PLIF) measurements were performed in an optically accessible internal combustion (IC) engine to investigate the behavior of scalar dissipation and the fine-scale structures of the turbulent scalar field. The fluorescent tracer fluorobenzene was doped into one of the two intake streams and nitrogen was used as the carrier gas to permit high signal-to-noise ratio fluorescence measurements without oxygen quenching effects. The resulting two-dimensional images allowed for an analysis of the structural detail of the scalar and scalar dissipation fields defined by the mixing of the two adjacent intake streams. High levels of scalar dissipation were found to be located within convoluted, sheet-like structures in accordance with previous studies. The fluorescence data, which were acquired during the intake stroke, were also used to examine the scalar energy and dissipation spectra.

The effect of the ring pack storage mechanism on the hydrocarbon (HC) emissions from an air-cooled utility engine has been studied using a simplified ring pack model. Tests were performed for a range of engine load, two engine speeds, varied air-fuel ratio and with a fixed ignition timing using a homogeneous, pre-vaporized fuel mixture system. The integrated mass of HC leaving the crevices from the end of combustion (the crank angle that the cumulative burn fraction reached 90%) to exhaust valve closing was taken to represent the potential contribution of the ring pack to the overall HC emissions; post-oxidation in the cylinder will consume some of this mass. Time-resolved exhaust HC concentration measurements were also performed, and the instantaneous exhaust HC mass flow rate was determined using the measured exhaust and cylinder pressure.

In-cylinder velocity measurements were acquired in a two-valve, single-cylinder research engine to study the bulk fluid motion and small-scale turbulence. Different port geometries (two), different port orientations (two) and both shrouded and non-shrouded intake valves were tested to vary the intake-generated flow. Tests were performed at engine speeds of 300, 600, 900 and 1200 RPM with an atmospheric intake pressure. Prior to testing on the engine, the different head configurations were tested on a steady flow bench. Particle image velocimetry data were taken on a single plane, parallel to the piston surface, in the engine using both a low magnification to characterize the large-scale flow phenomena, and a high magnification to characterize the turbulence field. The low-magnification results showed that the swirl center location was relatively insensitive to engine speed, but did change position throughout the cycle.

Reactivity controlled compression ignition combustion was investigated for three fuel combinations: isooctane-diesel, PRF90-diesel, and E85-diesel. Experiments were conducted at 1200 rpm, 160 kPa absolute intake pressure, and fixed total fuel energy using ‘optimal’ operating condition for each fuel combination that were chosen based on combustion performance from SOI timing and premixed energy fraction sweeps. The heat release duration was found to scale with the difference in reactivity between the premixed and direction injected fuel; a small difference gives rise to short heat release duration, similar to that of HCCI combustion. Conversely, as the difference increases, the heat release period lengthens. The high-speed optical data confirmed that the combustion occurred in a staged manner from the high-reactivity zones, which were located at the periphery of the chamber, to low-reactivity zones in the field of view.

The effect of the presence of liquid fuel in the intake manifold on unburned hydrocarbon (HC) emissions of a spark-ignited, carbureted, air-cooled V-twin engine was studied. To isolate liquid fuel effects due to the poor atomization and vaporization of the fuel when using a carburetor, a specially conditioned homogeneous, pre-vaporized mixture system was developed. The homogeneous mixture system (HMS) consisted of an air assisted fuel injection system located approximately 1 meter upstream of the intake valves. The results from carburetor and HMS are compared. To verify the existence of liquid fuel in the manifold, and to obtain an estimate of its mass, a carburetor-mounted liquid fuel injection (CMLFI) system was also implemented. The conditions tested were 10% and 25% load at 1750 RPM, and 25%, 50%, and 100% at 3060 RPM. The results of the comparison show that the liquid fuel in the intake manifold does not have a statistically significant influence on the averaged HC emissions.

The effects of residual gas mixing were studied in a single-cylinder, air-cooled utility engine using both external exhaust gas recirculation (EGR) and internal residual retention. EGR was introduced far upstream of the throttle to ensure proper mixing. Internal residual was changed by varying the length of the valve overlap period. EGR was measured in the intake system; the total in-cylinder diluent was directly measured using a skip-fire, cylinder dumping technique. A sweep of diluent fraction was performed at different engine speeds, engine loads, fuel mixture preparation systems, and ignition timings. An optimum level of diluent, where the combined hydrocarbon and NOx emissions were minimal, was found to exist for each operating condition. Higher levels of diluent, either through internal retention or external recirculation, caused the combined emissions to increase.

The contribution of fuel adsorption in engine oil and its subsequent desorption following combustion to the engine-out hydrocarbon (HC) emissions of a spark-ignited, air-cooled, V-twin utility engine was studied by comparing steady state and cycle-resolved HC emission measurements from operation with a standard full-blend gasoline, and with propane, which has a low solubility in oil. Experiments were performed at two speeds and three loads, and for different mean crankcase pressures. The crankcase pressure was found to impact the HC emissions, presumably through the ringpack mechanism, which was largely unaltered by the different fuels. The average and cycle-resolved HC emissions were found to be in good agreement, both qualitatively and quantitatively, for the two fuels. Further, the two fuels showed the same response to changes in the crankcase pressure. The solubility of propane in the oil is approximately an order of magnitude lower than for gasoline.

The effects of fuel composition on homogeneous charge compression ignition (HCCI) combustion were studied experimentally in an engine employing negative valve overlap (NVO). Three test fuels, varying in ignition quality and volatility, were investigated for their effect on engine performance and combustion phasing; comparisons were made to a full-run 87-octane base fuel. The three test fuels, which varied in research octane number from 69 to 98, were all found to advance the combustion timing slightly relative to the base fuel, suggesting some differences in the ignition chemistry. The combustion performance at a fixed combustion phasing, however, was found to be comparable, within the limits of the system, for all of the fuels. A major testing issue that limited the system repeatability was the formation of combustion chamber deposits under some operating conditions. A methodology to mitigate these effects was employed with some success.

The KIVA 3V code has been applied to predict combustion chamber heat flux in an air-cooled utility engine. The KIVA heat flux predictions were compared with experimentally measured data in the same engine over a wide range of operating conditions. The measured data were found to be approximately two times larger than the predicted results, which is attributed to the omission of chemical heat release in the near-wall region for the heat transfer model applied. Modifying the model with a simple scaling factor provided a good comparison with the measured data for the full range of engine load, heat flux sensor location, air-fuel ratio and spark timings tested. The detailed spatially resolved results of the KIVA predictions were then used to develop a simplified model of the combustion chamber temporally integrated heat flux using an artificial neural network (ANN).

Planar laser-induced fluorescence of 3-pentanone doped into the fuel (iso-octane) and OH, which is present in the combustion products, was performed in an optically accessible direct-injection spark-ignition (DISI) engine under stratified and homogeneous operating conditions. A wall-guided, swirl-based combustion chamber was utilized, and experiments were performed for light load, where the fuel-air equivalence ratio was 0.3, and high load conditions, with an equivalence ratio of 0.7, at speeds of 600 and 1200 rpm. The 3-pentanone images were calibrated through the use of a premixed charge condition of known equivalence ratio, with corrections applied for number density changes due to combustion. At the light load condition combustion of the highly stratified fuel cloud was directly measured for the first time. The equivalence ratio of the mixture at the flame front was found to be in the range from 0.5 - 0.8 for optimized combustion conditions in this engine.

Highly time resolved measurements of cylinder pressure acquired simultaneously from three transducers were used to investigate the nature of knocking combustion and to identify biases that the pressure measurements induce. It was shown by investigating the magnitude squared coherence (MSC) between the transducer signals that frequency content above approximately 40 kHz does not originate from a common source, i.e., it originates from noise sources. The major source of noise at higher frequency is the natural frequency of the transducer that is excited by the impulsive knock event; even if the natural frequency is above the sampling frequency it can affect the measurements by aliasing. The MSC analysis suggests that 40 kHz is the appropriate cutoff frequency for low-pass filtering the pressure signal. Knowing this, one can isolate the knock event from noise more accurately.

The current work investigates the in-cylinder mixing of a fluorescent tracer species inducted into the engine through a small-diameter tube mounted along the inner port wall and the remaining inlet stream in a small-bore utility engine. Planar laser-induced fluorescence (PLIF) measurements were acquired on a single plane, parallel to and approximately 4 mm below the cylinder head deck, throughout the intake and compression strokes. The data were analyzed to qualitatively and quantitatively describe the evolution of the mixture stratification. The highest degree of stratification in the mean field was observed at a timing of 90 crank angle (CA) degrees after top dead center (aTDC) of the intake stroke, which corresponds closely to the point of maximum intake valve lift (105 CA degrees aTDC).

In this paper, three models for the prediction of knock onset timing are compared: an ignition-integral model using a simple ignition delay correlation, an ignition-integral model using a pre-computed lookup table of ignition delays, and the direct integration of a detailed chemical kinetic mechanism. All three models were found to compare well with experimentally measured results; the correlation-based knock-integral model was found to be as accurate as the other methods and was computationally far more efficient. The direct integration approach correlated very well with the experimental data but was delayed by 1-2 crank angles. The simplified models have been used in conjunction with a Monte-Carlo approach to assess the cycle-by-cycle variations in knock onset timing. A statistical comparison between the Monte-Carlo predictions and experimental results showed a good prediction of the distribution widths, and some modest phasing issues over a wide range of ignition timing.

Fuel film temperature and thickness were measured on the piston crown of a DISI engine under both motored and fired conditions using the fiber-based laser-induced fluorescence method wherein a single fiber delivers the excitation light and collects the fluorescence. The fibers were installed in the piston crown of a Bowditch-type optical engine and exited via the mirror passage. The fuel used for the fuel film temperature measurement was a 2×10-6 M solution of BTBP in isooctane. The ratio of the fluorescence intensity at 515 to that at 532 nm was found to be directly, but not linearly, related to temperature when excited at 488 nm. Effects related to the solvent, solution aging and bleaching were investigated. The measured fuel film temperature was found to closely follow the piston crown metal temperature, which was measured with a thermocouple.

A spectroscopic diagnostic system was designed to study the effects of different engine parameters on the chemiluminescence characteristic of HCCI combustion. The engine parameters studied in this work were intake temperature, fuel delivery method, fueling rate (load), air-fuel ratio, and the effect of partial fuel reforming due to intake charge preheating. At each data point, a set of time-resolved spectra were obtained along with the cylinder pressure and exhaust emissions data. It was determined that different engine parameters affect the ignition timing of HCCI combustion without altering the reaction pathways of the fuel after the combustion has started. The chemiluminescence spectra of HCCI combustion appear as several distinct peaks corresponding to emission from CHO, HCHO, CH, and OH superimposed on top of a CO-O continuum. A strong correlation was found between the chemiluminescence light intensity and the rate of heat release.

An axisymmetric one-dimensional finite difference model has been developed to investigate the optimization of external fins on a cylindrical tube with a non-uniform heat flux applied to the inner surface. The heat flux boundary condition applied to the inner surface was determined from detailed 3-dimensional calculations using the KIVA code. The external convective boundary condition was determined from published correlations. This model encompasses the basic geometry of an air-cooled engine cylinder. The model was computationally efficient and allowed for the optimization of the fin length of each fin and its location. A genetic algorithm optimization procedure was utilized. The results show that optimum usage of material is obtained from fins of comparable length distributed over the entire outer cylinder, in spite of the concentrated heat flux at the upper end of the cylinder. The results indicate the important role of axial conduction in the thermal energy balance of this system.

High frame rate particle image velocimetry (PIV) measurements were performed in a motored engine at speeds of 600 and 1200 rpm under both throttled and unthrottled conditions. Data were acquired at 1 kHz throughout the entire engine cycle, allowing the temporal and spatial evolution of the flow to be observed. The data were both temporally and spatially filtered to study the turbulent flowfield. The mean (over the spatial domain) kinetic energy of the high-pass filtered data, and its evolution with cutoff frequency or length, was used to quantitatively compare differences between operating conditions and different cycles at the same condition. The difference in fluctuation kinetic energy, when normalized, between different operating conditions was found to be comparable to the difference between cycles. A comparison between spatially and temporally filtered data at the same level of fluctuation kinetic energy was performed.