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The objective of this study was to observe and attempt to understand the effects of equivalence ratio and simulated exhaust gas recirculation (EGR) on the exhaust emissions and performance of a L-head single cylinder utility engine. In order to isolate these effects and limit the confounding influences caused by poor fuel mixture preparation and/or vaporization produced by the carburetor/intake port combination, the engine was operated on a premixed propane/air mixture. To simulate the effects of EGR, a homogeneous mixture of propane, air, and nitrogen was used. Engine measurements were obtained at the operating conditions specified by the California Air Resources Board (CARB) Raw Gas Method Test Procedure. Measurements included exhaust emissions levels of HC, CO, and NOx, and engine pressure data.

To study the near-wall velocity characteristics, gas velocity measurements have been made near the cylinder head of a motored four-stroke engine using Laser Doppler Velocimetry (LDV), and near-wall flow characteristics have been observed in three different two-stroke geometries using Particle Image Velocimetry (PIV) and particle photographs. The results of these studies show that the behavior of the fluid near the wall depends on the engine intake geometry, combustion chamber geometry, and operating condition. The near-wall velocity characteristics tend to be one of two forms. In one form, the behavior is one of an extended region of low momentum fluid, where an imbalance in radial pressure gradient forces and centripetal forces exists because of the combined effects of fluid rotation and shear. Such a flow can be seen in engines with gas exchange systems that do not promote scrubbing of the wall, and in cylinder geometry that does not cause flow normal to the wall.

Particle image velocimetry (PIV) was used to study the velocity field characteristics in motored two-stroke ported engines. Measurements of the two-dimensional velocity field were made at the midplane of the clearance volume for bowl-in-head and disk combustion chamber geometries. Measurements were also obtained for two scavenging port geometries, i.e. a loop-scavenged engine and a loop-scavenged engine with a boost port. Results from this study show that in-cylinder geometry had a dominant effect on the flow structure observed at TDC. For example, with the boost-port scavenging crankcase, the disk-shaped chamber showed a turbulent flow-field at TDC with little large scale motion. In contrast, addition of a squish flow from the bowl-in-head geometry produced an organized cross-chamber flow. The addition of a boost port also changed the flow structure markedly. A large-scale swirl flow was observed in the engine that did not contain a boost port.

Instantaneous heat flux and flame position were measured on a silicon nitride diesel engine head. Ionization probes and thin-film platinum temperature detectors were applied directly to the head surface. The ionization probes showed that the flame exited the bowl and propagated asymmetrically from the centerline of the combustion bowl. The temperature measurements revealed that average surface temperatures varied with position by more than 200°C. Spatial variations in the temperature swings were also present with large swings resulting from direct combustion effects on heat transfer at locations near the lip of the piston bowl. Peak instantaneous heat flux values varied from 0.3 to 2.0 MW/m2. Five of the seven probe locations exhibited heat transfer rates that were limited due to the combustion rate. At three different positions, the peak heat flux magnitude and phasing were independent of load.

Two sets of comparisons were made in an attempt to provide a mechanism for understanding the behavior of transient sprays. First, detailed measurements of drop size and velocity in a transient spray were compared to established stability criteria for different droplet breakup mechanisms, specifically criteria for bag breakup and boundary layer stripping. Then, probability-density-functions were determined from the experimental data and compared, where appropriate, to different computed distributions (such as the Chi-square or log-hyperbolic distributions). Comparison with the stability criteria indicates that the a majority of droplets in the spray are susceptible to both breakup mechanisms near the injector tip. However, downstream, the spray appears to stabilize and any redistribution of droplet size must apparently be a result of collisions. The experimentally-determined PDF's for size and velocity are functions of both position and time in the spray.

Consideration of the heat transfer effects in low-heat-rejection engines has prompted further study into engine heat transfer phenomena. In a previous study, an approximate solution of the one-dimensional energy equation was acquired for transient, compressible, low-Mach number, turbulent boundary layers typical of those found in engines. The current study shows that an approximate solution of the one-dimensional energy equation with arbitrarily-distributed heat release can also be obtained. Using this model, the effects of high temperature walls, combustion, and autoignition on heat transfer can be studied. In the case of high temperature walls, the model predicts the expected behavior unless the quench distance gets very small. For combustion, the reaction must occur close to the wall for a direct effect on the heat transfer to be observed. With autoignition, instantaneous values of heat flux reach levels as high as 6 MW/m2, and oscillate in phase with the pressure wave.

Simultaneous droplet sizes and velocities were obtained for a transient diesel fuel spray in a quiescent chamber at atmospheric temperature and pressure. Instantaneous injection pressure, needle lift, and rate of injection were also measured, allowing calculation of the instantaneous nozzle discharge coefficient. Short-exposure still photographs were obtained at various chamber pressure and densities to further investigate this spray. Correlations between droplet size and velocity were determined at each crank angle to observe the detailed nature of the transient events occurring in this transient diesel fuel spray. As expected, peak mean and rms velocities are observed in the center of the spray. Measured average velocities are consistent with a calculated value, using the discharge coefficient for the nozzle and the known rate of fuel injection.

Pilot injection and two other forms of auxiliary fuel introduction have been studied for their effects on diesel engine combustion and emissions. A two-stroke diesel has been equipped with an electronic solenoid-controlled unit injector such that the injector can operate with pilot injection. In addition, the engine has been fitted with experimental air-blast atomizing injectors in the inlet port and intake manifold. In-cylinder pressure, Bosch smoke, exhaust hydrocarbons, NO and NOx emissions measurements have been made for a range of engine conditions. In addition, two fuels have been tested to observe the effects of fuel blend on the auxiliary fuel behavior. In general, the effect of auxiliary fuel introduction is to reduce ignition delay and rate-of-pressure rise. This tends to result in a decrease in NO emissions. Unburned hydrocarbons and smoke tend to increase, although not in every case.

An in-cylinder optical sensor has been developed and tested for use in spark-ignition engine combustion analysis and control, This sensor measures the luminous emission in the near infrared region. Results of these tests show good correlation between the measured luminosity and traditional combustion parameters, such as location and magnitude of maximum cylinder pressure, and location and magnitude of maximum heat release. Engine performance indicators, such as the indicated mean effective pressure (IMEP), also can be determined accurately with the measured luminosity combined with other engine operating parameters, e.g. intake manifold pressure. In-cylinder air-fuel ratio can be determined with accuracy over an ensemble of 100 cycles.

In the first part of this study, a one-dimensional code was used to compare predictions from six different two-equation turbulence models. It is shown that the application of the traditional k-ε models to the viscous-dominated region of the boundary layer can produce errors in both the calculated heat flux and surface friction. A low-Reynolds-number model does not appear to predict similar non-physical effects. A new one-dimensional model, which includes the effect of compression, has been formulated by multiparameter fit to the numerical solution of the energy equation. This model can be used in place of the law-of-the-wall to calculate the surface heat flux. The experiments were performed in a specially-instrumented engine, allowing optical access to the clearance volume. Measurements of heat flux, swirl velocities, and momentum boundary layer thickness were made for different engine speeds.

Cyclic variation is examined by: (1) conditional grouping and heat-release analysis to reveal different modes of combustion, (2) considering the order in which the burn modes occur to establish prior-cycle effects and (3) comparing the measured variation in IMEP with data generated by simple models. Results show that several burn modes may exist, particularly under fuel-lean conditions. Prior-cycle effects also become more obvious as the air-fuel ratio is increased. Finally, comparisons with data generated by simple models show that the nature of cyclic variation may range from completely stochastic to a superposition of a non-chaotic deterministic process on a stochastic process.

Conditional sampling of cylinder-pressure data is used to investigate cyclic variability in a premixed-charge spark-ignited engine operating under fuel-lean conditions. Unlike straight ensemble averaging of pressure data, conditional sampling applies a set of constraints to the pressure data such that like combustion events can be identified and grouped together. Ensemble averaging of pressure data from an engine that exhibits significant cycle-to-cycle variation is shown to produce a mean pressure history that is not representative of the combustion process. Conditional sampling provides a means of identifying and analyzing the different groups of pressure histories and therefore the different types of combustion processes that occur in an engine that exhibits cyclic variability.

Measurements are presented for the turbulence intensities and mean velocities obtained in a research engine in which a grid was used to create a flow field characterized by negligible mean motions and homogeneous and isotropic turbulence at the time of ignition. Pressure measurements for homogeneous stoichiometric combustion indicate a very low level of cyclic variation. The combustion-induced mean flow field is shown to be characteristic of a one-dimensional compression of the unburned gases, which produces a small increase in the bulk turbulent kinetic energy ahead of the flame. Most of the effect of combustion appears to occur locally, as the turbulence in the preflame gases close to the flame front is strongly amplified in the direction of flame propagation. Parallel to the flame surface there is little effect until the flame has propagated nearly all the way across the chamber.

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured during combustion in a research engine. Simultaneously with each LDV measurement, the cylinder pressure and gas state (unburned or burned) were measured so that conditional sampling techniques could be used in the data-averaging procedure. Measurements of the mean velocity component in the direction of flame propagation agree well with a computer simulation of the induced velocities generated by the volume expansion of the burned gases. Mean velocities measured parallel to the flame surface are shown to be complex because a small amount of swirl was present. Conditional sampling on the time of flame arrival at the LDV probe volume revealed a thirty percent cyclic-variation bias error in the turbulence component normal to the flame.

Laser Doppler velocimeter results are presented for the mean velocity and turbulence intensity measured in a motored research engine. The compression of complex bulk motions created during induction produces turbulence as the piston approaches top dead center. The turbulence field is shown to be isotropic but nonhomogeneous. A zero-dimensional computer simulation based on an averaged k-ϵ model is shown to adequately predict the decay of turbulence at a point in the flow after the production phase is completed. Cylinder pressure measurements were recorded for homogeneous stoichiometric combustion for a range of engine speeds and ignition locations. A two-zone (burned and unburned gases) thermodynamic model accurately predicts the measured pressure histories when the turbulence results determined from the motored tests are used to establish initial conditions for the combustion model.