Richard C. Peterson

A visual study of divided-chamber diesel combustion using a rapid compression machine

Abstract The processes of air introduction, fuel injection, ignition, and combustion are investigated optically in simulated diesel engine prechambers mounted on a rapid compression machine. During the compression stroke air velocities at the edge and center of one of the prechambers are on the order of 20 and 15 m/s, respectively. The airflow, therefore, is more complex than solid-body rotation. During fuel injection the fuel spray is strongly deflected by the airflow and hits the glow plug and the wall opposite the injector. Fuel evaporation and air-fuel mixing begin 0.3–0.5 ms after the start of injection but are not complete within the ignition delay period (1–1.5 ms). Hence ignition and combustion occur at times when both liquid and gaseous fuel exist in the prechamber. This liquid fuel is located both within the volume and on the surfaces of the prechamber. Combustion is strongly luminous and implies significant soot formation in the prechamber.

The effect of operating conditions on flame temperature in a diesel engine

A two-color optical procedure was used to measure temperatures during combustion in the prechamber of an automotive-type divided-chamber diesel engine. The prechamber design was cylindrical, instead of spherical, with quartz windows at the ends to provide optical access. The temperature was measured for engine conditions in which the combustion timing, overall air-fuel ratio, engine speed, intake-manifold pressure and intake-air temperature were varied. At each condition, the temperature was measured along a single line of sight conincident with the axis of the prechamber and was measured as a function of engine crank angle in at least 92 engine cycles.

Correlation of Nitric Oxide Emission from a Diesel Engine with Measured Temperature and Burning Rate

The exhaust nitric oxide (NO) emission from a divided-chamber diesel engine was correlated with a function which included the effects of temperature, mixing time, and phasing between the temperature and fuel-burning histories. The correlation was evaluated for variations in combustion timing, overall air-fuel ratio, engine speed, intake-manifold pressure and intake-air temperature.

Piston geometry effects in a light-duty, swirl-supported diesel engine: Flow structure characterization:

This work studied how in-cylinder flow structure is affected in a light-duty, swirl-supported diesel engine when equipped with three different piston geometries: the first two featuring a conventional re-entrant bowl, either with or without valve cut-outs on the piston surface and the third featuring a stepped-lip bowl. Particle image velocimetry experiments were conducted inside an optical engine to measure swirl vortex intensity and structure during the intake and compression strokes. A full computational model of the optical diesel engine was built using the FRESCO code, a recently developed object-oriented parallel computational fluid dynamics platform for engine simulations. The model was first validated against the measured swirl-plane velocity fields, and the simulation convergence for multiple cycles was assessed. Flow topology was studied by addressing bulk flow and turbulence quantities, including swirl structure, squish flux, plus geometric and operating parameters, such as the presence of valve cut-outs on the piston surface, compression ratio and engine speed. The results demonstrated that conventional re-entrant bowls have stronger flow separation at intake, hampering bowl swirl, but higher global swirl than for stepped-lip bowls thanks to a stronger and more axisymmetric squish mechanism and less tilted swirl. Stepped-lip bowls have larger inhomogeneities (tilt and axisymmetry) and higher turbulence levels, but also faster turbulence dissipation toward top dead center. They have weaker squish flux but larger squish inversion momentum as a result of the smaller inertia.

On the Reduction of Combustion Noise by a Close-Coupled Pilot Injection in a Small-Bore DI Diesel Engine

For a pilot-main injection strategy in a single cylinder light duty diesel engine, the dwell between the pilot- and main-injection events can significantly impact combustion noise. As the solenoid energizing dwell decreases below 200 μs, combustion noise decreases by approximately 3 dB and then increases again at shorter dwells. A zero-dimensional thermodynamic model has been developed to capture the combustion-noise reduction mechanism; heat-release profiles are the primary simulation input and approximating them as top-hat shapes preserves the noise-reduction effect. A decomposition of the terms of the underlying thermodynamic equation reveals that the direct influence of heat-release on the temporal variation of cylinder-pressure is primarily responsible for the trend in combustion noise. Fourier analyses reveal the mechanism responsible for the reduction in combustion noise as a destructive interference in the frequency range between approximately 1 kHz and 3 kHz. This interference is dependent on the timing of increases in cylinder-pressure during pilot heat-release relative to those during main heat-release. The mechanism by which combustion noise is attenuated is fundamentally different from the traditional noise reduction that occurs with the use of long-dwell pilot injections, for which noise is reduced primarily by shortening the ignition delay of the main injection. Band-pass filtering of measured cylinder-pressure traces provides evidence of this noise-reduction mechanism in the real engine.When this close-coupled pilot noise-reduction mechanism is active, metrics derived from cylinder-pressure such as the location of 50% heat-release, peak heat-release rates, and peak rates of pressure rise cannot be used reliably to predict trends in combustion noise. The quantity and peak value of the pilot heat-release affect the combustion noise reduction mechanism, and maximum noise reduction is achieved when the height and steepness of the pilot heat-release profile are similar to the initial rise of the main heat-release event. A variation of the initial rise-rate of the main heat-release event reveals trends in combustion noise that are the opposite of what would happen in the absence of a close-coupled pilot. The noise-reduction mechanism shown in this work may be a powerful tool to improve the tradeoffs among fuel efficiency, pollutant emissions, and combustion noise.Copyright

Effects of flame temperature and burning rate on nitric oxide emission from a divided-chamber diesel engine

A correlation was developed which related the NO emission from a diesel engine to a parameter which considered the variation of both temperature and fuel-burning rate during the engine cycle. The correlation was evaluated for several engine conditions representing variations in combustion timing, overall air-fuel ratio, engine speed, intake-manifold pressure and intake-air temperature. The data were obtained from an automotive-type, divided-chamber diesel engine which had a cylindrically shaped prechamber with quartz windows. The correlation had two forms. The variable-temperature correlation (VTC) used the measured temperature as a function of crank angle as one of the input data. The constant-temperature correlation (CTC) was a simplified form of the first correlation and used the maximum measured temperature as one of the input data. The temperature was measured by a two-color method along a single line of sight coincident with the axis of the prechamber. The other input data for the correlation were the engine speed and the fuel-burning rate derived from the measured cylinder pressure. These techniques could correlate (1) the exhaust emission index of NO and (2) the normalized mass of NO formed as a function of engine crank angle during the combustion process. Both schemes gave satisfactory correlation of the exhaust NO emission. However, comparison of the normalized mass of NO as a function of crank angle during combustion with experimental data suggested that the VTC was superior. The activation temperature used for successful correlation, 38 000 K, is substantially lower than the overall activation temperature for the extended Zeldovich mechanism if O is in equilibrium with O2 existing in the post-flame gases. This observation and the sensitivity of NO emission to variations in the flame temperature suggest that most of the NO is formed in the vicinity of the flame zone via the extended Zeldovich mechanism with super-equilibrium radical concentrations.