Effects of Fuel Overpenetration and Overmixing During Ignition-Delay Period on Hydrocarbon Emissions From a Small Open-Chamber Diesel Engine

Abstract

A quasi-steady gas-jet model was applied to examine the spray trajectory in swirling flow during the ignition-delay period in an open-chamber diesel engine timed to start combustion at top dead center. Spray penetration, deflection, and the fractions of too-lean-mixed, burnable, and overpenetrated fuel at the start of combustion were calculated by employing the measured ignition delay and mean fuel-injection velocity. The calculated parameters were applied to correlate the measured exhaust hydrocarbon (HC) emissions. The engine parameters examined were bowl geometry, compression ratio, overall air-fuel ratio, and speed. Both the ignition delay and the relative spray-penetration parameter, defined as the ratio of the spray-penetration distances at the moments of start of combustion and wall impingement, gave good correlations for some of the engine parameters examined but could not explain all the measured trends. However, good correlation of the measured exhaust HC emissions was obtained by using the calculated too-lean-mixed and overpenetrated fuel fractions at the start of combustion. Correlation of the overpenetrated fuel with the measured HC indicated that approximately 2 percent of the fuel mass that overpenetrated before start of combustion emitted from the engine as unburned HC. This could account for 0 to 65 percent of the total HC emission from this engine. Additionally, it was found that the too-lean-mixed fuel could contribute 10 to 30 percent of the total HC emission, as found in a previous study on a somewhat similar engine. The remaining HC emission is caused by other sources such as bulk quenching.

Development and Validation of Predictive Emissions Schemes for Quasi-Dimensional Combustion Models

“…JET OVERPENETRATION -As found by Kuo et al [29], another important source of HC can be the spray impingement against the cylinder walls and piston bowl. As a matter of fact, though this contribution can be neglected in the case of large bore engines, it plays an important role when dealing with smaller high speed DI engines, where the jet impingement can cause up to 65% of the total unburned hydrocarbon emissions.…”

“…OVERLEANED FUEL -As far as fuel overmixing is concerned, it applies to the fraction of fuel spray injected into the cylinder that, during the ignition delay, mixes below the lean auto-ignition limit. As illustrated in Kuo [29] for alkanes, and extended in [30] to the case of diesel fuel, the lean auto-ignition limit is not appreciably affected by any change in pressure, and the temperature effect on it is even less significant. For this reason, a constant lean flammability limit has been assumed for diesel fuel, and, in accordance with [29] chosen to be Φ= 0.2.…”

“…As illustrated in Kuo [29] for alkanes, and extended in [30] to the case of diesel fuel, the lean auto-ignition limit is not appreciably affected by any change in pressure, and the temperature effect on it is even less significant. For this reason, a constant lean flammability limit has been assumed for diesel fuel, and, in accordance with [29] chosen to be Φ= 0.2. For computing the total amount of fuel overmixed during the ignition delay period, it is necessary to estimate the distribution of fuel concentrations across the zones of each parcel.…”

“…The unburned hydrocarbons (HC) being emitted from diesel engines are usually represented in terms of a variety of phenomenological models which capture the different sources causing some amount of fuel not to be burnt during the engine cycle [14,28,29]. Among the others, three main sources of HC have been identified: the first one is the overlean mixture, which is due to the non-uniform spray jet composition during the ignition delay period; the second one is due to fuel released by the injector sac volume, during the expansion stroke, in the form of large drops with very low momentum; the last one is the effect of fuel overpenetration and impingement against the piston bowl and cylinder walls.…”

“…, (29) where the values for the molar standard free energy of formation, Δg o , are computed from the knowledge of the coefficients for polynomial interpolation [17,20] According to (28,29), the equilibrium constants, K p,j , for each of the equilibria, are known as a function of temperature and pressure, and the only unknown quantities are the molar concentrations of the species. In this way, a non-linear system of dimension 11 x 11 is derived.…”