Piston Crevice Research Articles

The Effects of Piston Crevices and Injection Strategy on Low-Speed Pre-Ignition in Boosted SI Engines

The Effects of Piston Crevices and Injection Strategy on Low-Speed Pre-Ignition in Boosted SI Engines

Modified one-step reaction equation for modeling the oxidation of unburned hydrocarbons in engine conditions

The oxidation of unburned hydrocarbons from piston crevices was modeled using a modified one-step reaction equation. This new one-step oxidation model was developed by modifying the Arrhenius reaction rate coefficients of the conventional one-step reaction equation. The predictions of the new one-step oxidation model agree well with the results of the detailed chemical reaction mechanism in terms of the 90% oxidation time of the fuel. The effects of pressure and intermediate species in the burnt gas on the oxidation rate were also investigated and included as additional multiplying factors in the modification of the equation. To simulate the oxidation process of unburned hydrocarbons from a piston crevice, a two-dimensional computational mesh, based on the conventional engine geometry, was constructed with a fine mesh density at the regions of the piston crevice and cylinder wall. The number of cell layers in the cylinder was controlled according to the piston motion to model the out-flow of unburned hydrocarbons from the piston crevice during the expansion stroke. The effects of engine operational conditions on the oxidation rate were examined at several engine speeds and load conditions, and the sensitivity of the oxidation rate to the piston crevice volume was also evaluated. Finally, the new one-step oxidation model was applied to a three-dimensional computational mesh that modeled the three-dimensional engine geometry and piston-valve motions to simulate the oxidation of unburned hydrocarbons in a real engine condition.

Modeling of emissions from HCCI engines using a consistent 3-zone model with applications to validation of reduced chemistry

The piston crevice and the quench zone, the area of fluid close to the cylinder walls, are responsible largely for emissions in Homogeneous Charge Compression Ignition (HCCI) engines. For proper modeling of emissions from HCCI engines, the piston crevice and quench zone need to be included in the numerical model. For HCCI applications, we have developed an improved numerical model with three zones: the piston crevice zone, the quench zone close to the cylinder walls, and the cylinder zone comprising up of the rest of the cylinder. With the assumption of equal pressure for all zones, an analytical model is developed for the mass transfer between the core fluids and those in the piston crevice. The numerical model then solves for the coupled mass, energy, and species equations simultaneously leading to consistent solutions at every time step. Comparisons of the calculated results are made between this stand-alone multi-zone code, a 3-D CFD model coupled with multiple zones, and experimental data which show encouraging agreement. The present stand-alone model also provides a useful platform for development/validation of reduced chemistry for emissions.

CFD modeling of two-stage ignition in a rapid compression machine: Assessment of zero-dimensional approach

In modeling rapid compression machine (RCM) experiments, zero-dimensional approach is commonly used along with an associated heat loss model. The adequacy of such approach has not been validated for hydrocarbon fuels. The existence of multi-dimensional effects inside an RCM due to the boundary layer, roll-up vortex, non-uniform heat release, and piston crevice could result in deviation from the zero-dimensional assumption, particularly for hydrocarbons exhibiting two-stage ignition and strong thermokinetic interactions. The objective of this investigation is to assess the adequacy of zero-dimensional approach in modeling RCM experiments under conditions of two-stage ignition and negative temperature coefficient (NTC) response. Computational fluid dynamics simulations are conducted for n-heptane ignition in an RCM and the validity of zero-dimensional approach is assessed through comparisons over the entire NTC region. Results show that the zero-dimensional model based on the approach of ‘adiabatic volume expansion’ performs very well in adequately predicting the first-stage ignition delays, although quantitative discrepancy for the prediction of the total ignition delays and pressure rise in the first-stage ignition is noted even when the roll-up vortex is suppressed and a well-defined homogeneous core is retained within an RCM. Furthermore, the discrepancy is pressure dependent and decreases as compressed pressure is increased. Also, as ignition response becomes single-stage at higher compressed temperatures, discrepancy from the zero-dimensional simulations reduces. Despite of some quantitative discrepancy, the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the ignition delay simulation.

Predictions of the effects of piston-liner crevices on flow motion and emissions in three-dimensional diesel engine simulations

Computational fluid dynamics predictions of the flow field and pollutant emissions in a high-speed direct-injection diesel engine operating at high load and rated speed are presented. Piston-liner crevice flows during the expansion stroke can give rise to vortex motions whose characteristics depend on the piston crevice configuration. Both a realistic and a simplified crevice volume that consists of a neck area and a reservoir to keep the surface—volume ratio realistic are considered. In the simplified crevice approach, with moderate intensity of the crevice flow, the vortex enhances mixing and engine-out carbon monoxide (CO) emissions are reduced. However, when the crevice flow intensity is above a certain level, a strong tumble motion is created in the bulk mixture in the squish area, which isolates a region of high CO concentration and prevents oxidation. The results also show that proper timing of the crevice flow into the combustion chamber lowers the mixture temperature and reduces nitrogen oxide emissions. Calculations with a realistic crevice flow geometry and a crevice flow submodel also predict squish region vortex motion.

Piston crevice hydrocarbon oxidation during expansion process in an SI engine

Combustion chamber crevices in SI engines are identified as the largest contributors to the engine-out hydrocarbon emissions. The largest crevice is the piston ring-pack crevice. A numerical simulation method was developed, which would allow to predict and understand the oxidation process of piston crevice hydrocarbons. A computational mesh with a moving grid to represent the piston motion was built and a 4-step oxidation model involving seven species was used. The sixteen coefficients in the rate expressions of 4-step oxidation model are optimized based on the results from a study on the detailed chemical kinetic mechanism of oxidation in the engine combustion chamber. Propane was used as the fuel in order to eliminate oil layer absorption and the liquid fuel effect. Initial conditions of the burned gas temperature and in-cylinder pressure were obtained from the 2-zone cycle simulation model. And the simulation was carried out from the end of combustion to the exhaust valve opening for various engine speeds, loads, equivalence ratios and crevice volumes. The total hydrocarbon (THC) oxidation in the crevice during the expansion stroke was 54.9% at 1500 rpm and 0.4 bar (warmed-up condition). The oxidation rate increased at high loads, high swirl ratios, and near stoichiometric conditions. As the crevice volume increased, the amount of unburned HC left at EVO (Exhaust Valve Opening) increased slightly.

Schlieren Observation of Spark-Ignited Premixed Charge Combustion Phenomena Using a Transparent Collimating Cylinder Engine

The schlieren photographs of in-cylinder processes in a spark-ignited premixed charge gasoline engine were observed via a transparent collimating cylinder and were presented in comparison with a pressure analysis. The schlieren photographs of the spark, the initial flame and the unburned gas ejection from the piston crevice, which is unable to be observed by direct photography, were clearly taken. It shows that the small difference in the initial combustion process among cycles is intensified by the movement of the piston during the expansion stroke. Finally, this difference appears as the cycle by cycle variation in the pressure and the rate of heat release. The observed flame size increased faster and was larger than the burned gas estimated from the pressure. The difference between them is large enough and can not be explained without considering the mixing of burned and unburned gases inside the flame front.

Effects of the oil and liquid fuel film on hydrocarbon emissions in spark ignition engines

A model was developed to assess the absorption and desorption of fuel in oil film and in the binary diffusion of oil and fuel films. This was investigated with a parametric study according to engine speed, load and oil film temperature. The results show that Henry's constant, which is related to solubility, is the most dominant parameter in the absorption/desorption mechanism of fuel into the oil film. Under warm-up conditions, engine speed had little influence on the amount of fuel absorbed/desorbed, but when the oil film temperature was low, the quantity of fuel absorbed/desorbed decreased with increasing engine speed. Liquid fuel on the oil film and piston head caused higher hydrocarbon (HC) emissions, and under base conditions (a simulated cold engine), the amount of fuel vaporized from fuel film and desorbed from wetted oil film was 24.5 per cent of the stoichiometric fuel mass. The effect of oil film with liquid fuel was 5.3 times larger than that of oil film without liquid fuel. The amount of fuel that escaped from the piston crevice was 1.3 times larger than that of fuel in the oil film. However, the fuel trapped in the oil film desorbed into the combustion chamber more slowly than the fuel that escaped from the piston crevices under cold engine conditions.

Modeling of the Dynamic Process of Fuel Absorption/Desorption in the Oil Film in SI Engines.

A modeling for absorption / desorption of the fuel in the oil film has been developed. The effect of oil film on fuel absorption / desorption was investigated according to engine speed, load, and mean oil film temperature. Multi-component fuel was employed to quantify the oil film effect. The results showed that Henry's constant, which is related to solubility, is the most dominant parameter in absorption / desorption mechanism of the fuel into oil film. The oil segments close to the top of the cylinder liner have significant contribution to the fuel absorption and desorption process than other oil segments. At warm-up conditon, engine speed has a little influencse on the amount of fuel absorbed / desorbed. However, when the oil film temperature is low, the quantity of fuel absorbed / desorbed decreases with the increasing engine speed. The amount of fuel trapped in piston crevice is 2 to 2.3 times as large as that of fuel in the oil film. However, the fuel trapped in the oil film desorbs slower into the combustion chamber than that escaped from the piston crevices at the cold engine conditon.

Rapid Compression Machines: Heat Transfer and Suppression of Corner Vortex

A method to suppress the piston corner vortex generated in compressing gas mixtures in rapid compression machines has been developed. A piston crevice was designed to swallow the thermal boundary layer along the wall, allowing better definition of core conditions in the reacting mixtures by confining the cold gases to the wall. Axisymmetric calculations of the flow and temperature fields show that the proposed design indeed suppresses the vortex formation, keeping the core reacting cases intact. A simple thermodynamic model based on the piston displacement history was formulated, incorporating the predicted heat transfer to the walls and mass transfer to the crevices. The model predictions agree very well with experimental pressure history under a range of initial pressures and types of different gases. The new experimental device and model allows the incorporation of complex chemical kinetics with early heat release without the need for experimental approximations for the heat transfer terms.

Oil Layer as Source of Hydrocarbon Emissions in SI Engines

The role of lubrication oil film on the cylinder liner as a source of hydrocarbon emissions in spark-ignition engines is assessed. First, the “source strength” is examined via an analytical model of the gasoline vapor absorption/desorption process. The solution shows that depending on engine operating conditions, there are three regimes. The process could be (1) limited by the gas side diffusion process, (2) limited by the liquid phase diffusion process, with the absorbed fuel fully penetrating the oil layer thickness (thin oil film regime), and (3) again limited by the liquid phase diffusion process, but with the absorbed fuel penetration depth small compared to the oil layer thickness (thick oil film regime). In regime (1), the source strength (the integrated absorption or desorption flux over one cycle) is proportional to the inverse of the square root of the rpm, but independent of oil layer parameters. In regimes (2), the strength is proportional to the oil film thickness divided by the Henry’s constant. In regime (3), the strength is independent of the oil film thickness, but is proportional to the fuel penetration depth divided by the Henry’s constant. Then, the oxidation of the desorbed fuel (using iso-octane as fuel) is examined with a one-dimensional reaction/diffusion model. The novel feature of the model is that the desorbed fuel is being exposed to the piston crevice hydrocarbon, which is laid along the liner as the piston descends. At stoichiometric conditions, the oxidation of the crevice HC is reduced by the presence of the desorbed HC from the oil layer.

Characteristics of Quenching Crevice Widths in Combustion Chamber of a Spark Ignition Engine.

Exhaust HC emissions from spark ignition engines are still creating a major environmental problem. It is difficult to reduce HC emissions during engine warm-up because the catalysts cannot work well at low temperatures. The source of unburned HCs from engines is thought to be crevices in the combustion chamber, oil layer and deposits on the cylinder wall surfaces. Quenching crevice widths in the combustion chamber were measured under various engine operating conditions with an ion probe that could detect flame arrival. Then, an experimental equation of quenching crevice width was estimated as a function of cylinder pressure and cylinder wall temperature. Schlieren photographs of a combustion chamber with a lubricant-free piston were also taken to monitor the flow of gas of unburned HCs emitted from the piston crevice during the expansion and exhaust processes.

Oxidation of the Piston Crevice Hydrocarbon During the Expansion Process in a Spark Ignition Engine

Abstract Engine-out HC emissions were measured in SI engine experiments in which the piston topland crevice size was changed systematically. For a warmed-up engine, the HC emissions were found to be modestly sensitive to the piston crevice size-a 10% change in size results in approximately a 2% change in HC emissions. This low sensitivity is explained in terms of a crevice HC diffusion/oxidation model in the expansion process. When the piston crevice is sufficienlly small, however, the model shows that the HC emissions have a one-to-one correspondence with the crevice size.

Laminar flame propagation/quench for a parallel-wall duct

Steady laminar isobaric two-dimensional propagation of a fuel-lean large-activation-temperature premixed flame between plane parallel (isothermal noncatalytic) walls is examined theoretically. A Shvab-Zeldovich engineering-type formulation, with a treatment of convective transport of species and heat in the manner of Oseen, is integrated numerically by quasilinearization/alternating-direction-implicit techniques. Of particular interest in this transcendentally nonlinear elliptical (convective-diffusive-reactive) boundary-value problem is how the flame speed (an eigenvalue) decreases as the wall separation decreases, for different wall temperatures. At a finite wall separation the quenching distance is achieved, such that the flame speed goes abruptly to zero; this critical quenching distance is found to be smaller for hotter walls. This investigation seeks to lend credibility of principle to one possible strategy for reducing the level of unburned-hydrocarbon species exhausted from Otto-cycle-type cylinders. The primary source for hydrocarbon emissions is now generally acknowledged to be the failure of flame to propagate into the narrow piston crevice owing to heat loss to cold walls. Higher wall temperature at the piston crown and rings would seem to abet flame propagation and assist burn-up during the combustion event; also, persistent fuel vapor would be discharged at higher temperature into the hot, oxygen-rich bulk cylinder contents during the powerstroke expansion.