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This study investigates the relationship between intake port geometry, flame position and stability, and combustion rate and emissions in an air-cooled four-stroke engine where three intake ports of differing geometry have been tested. In particular, the production intake port geometry, and directed and helical intake port geometries were tested. It was a specific intent of this study to investigate the interaction between inlet port geometry, equivalence ratio and ignition timing, without interference from mixture preparation effects. Thus all tests were performed using a homogeneous mixture of propane and air. Significant differences in combustion stability, flame position and stability, burn rate and emissions were observed. For example, the flow induced by the helical port, which should be characterized by a dominant swirl motion, resulted in stabile, asymmetric flames at many of the operating conditions studied.

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.

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.

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.

A two-stroke diesel engine was outfitted for operation with an electronic solenoid-controlled unit injector and an additional solenoid-controlled air-assisted injector at the inlet ports. Access through an existing pressure transducer port allowed installation of a sapphire window to the combustion chamber with very little disturbance to the combustion system. A coherent fiber optic bundle permitted remote visualization of the combustion event. Use of a gateable intensified solid-state camera permitted imaging at high effective shutter speeds at arbitrary times in the engine cycle. Imaging and two-color temperature and soot concentrations measurements were performed. Imaging results indicated a low-intensity diffuse ignition, away from the injector tip, for both the pilot spray in pilot-main tests and the main spray in the main-only runs. Remnants of the burning pilot spray congregated near the injector tip where a region of flame remained until main injection arrived.

This paper describes the development of diagnostics and testing methods for the characterization of carburetor exit flow conditions in small utility engines. These diagnostics include: 1) Three different methods of acquiring intake flow photographs. 2) A technique to measure the thickness of the fuel film running along the bottom of the intake manifold using the electrical properties of the fuel. 3) A system for measuring the A/F ratio across the carburetor exit using a heated catalyst to oxidize the sampled mixture and a wide-range oxygen sensor to determine the A/F of the reacted sample.

A laboratory-based fuel mixture preparation system has been developed that is capable of generating a wide range of fuel/air mixtures, including production of a premixed, prevaporized homogeneous charge, beginning with liquid gasoline fuel. This system has been developed to allow the study of the effects of fuel/air mixture preparation characteristics on engine combustion, in-cylinder pressure, and exhaust emissions. For the study to be described here, engine combustion behavior and emissions measurements were obtained for a wide range of A/F's with the fuel mixture preparation being produced in one case, by the stock carburetor operating with fixed throttle position, and the other case, with the custom-built system producing a homogeneous mixture (HM.) A four-stroke, spark-ignited, single-cylinder, overhead valve-type utility engine was used for all tests.

A laboratory-based fuel mixture system capable of delivering a range of fuel/air mixtures has been used to observe the effects of differing mixture characteristics on engine combustion through measurement and analysis of incylinder pressure and exhaust emissions. Fuel air mixtures studied can be classified into four different types: 1) Completely homogeneous fuel/air mixtures, where the fuel has been vaporized and mixed with the air prior to entrance into the normal engine induction system, 2) liquid fuel that is atomized and introduced with the air to the normal engine induction system, 3) liquid fuel that is atomized, and partially prevaporized but the air/fuel charge remains stratified up to introduction to the induction system, and 4) the standard fuel metering system. All tests reported here were conducted under wide open throttle conditions. A four-stroke, spark-ignited, single-cylinder, overhead valve-type engine was used for all tests.

To help understand the fundamental processes involved in direct injection, a research project was conducted using a single-cylinder, two-stroke research engine at a mid-speed, boat load operating condition. A 24 statistical factorial experimental design was applied. Of the factors tested at this operating condition, spray type was the most important factor affecting hydrocarbon emissions, followed by in-cylinder flow-related factors. Injection spray was also most important for nitrogen oxide emissions, carbon monoxide emissions, and efficiency. The dominant mechanism influencing the results was misfire, with other mechanisms present for specific responses.

Previous experiments using an air-assisted spray in a two-stroke direct-injected engine demonstrated a significant improvement in combustion stability at part-load conditions when a wide injection spray was used. It was hypothesized that the decrease in variability was due to the spray following the combustion chamber wall, making it less affected by variations in the in-cylinder gas flows. For this study, experiments were conducted to investigate engine spray combustion for cases where engine performance was not dominated by cyclic variation. Combustion and emission performance data was collected for a wide range of injection timings at several speed/load conditions. Experimental data for combustion shows that combustion stability is relatively unaffected by injection timing changes over a 40 to 100 degree window, and tolerant to spark gap projections over a range of 0.7 to 5.2 mm, depending on operating conditions.

Gas-phase in-cylinder mixing was examined by two different methods. The first method for observing mixing involved planar Mie scattering measurements of the instantaneous number density of silicon oil droplets which were introduced to the in-cylinder flow. The local value of the number density was assumed to be representative of the local gas concentration. Because the objective was to observe the rate in which gas concentration gradients change, to provide gradients in number density, droplets were admitted into the engine through only one of the two intake ports. Air only flowed through the other port. Three different techniques were used in analyzing the droplet images to determine the spatially dependent particle number density. Direct counting, a filtering technique, and autocorrelation were used and compared. Further, numerical experiments were performed with the autocorrelation method to check its effectiveness for determination of particle number density.