Piston Crevice Research Articles

Evaluating potential of increasing flow intensity and reducing crevice volume to improve thermal efficiency and hydrocarbon emission in spark-ignition natural gas engines

Low thermal efficiency and high hydrocarbon emissions caused by slow combustion rates and high methane emissions hinder the development of natural gas (NG) engines. Engine structure modifications have always been an important means of improving the combustion effects of engines. This study investigates the effects of combustion chamber improvements on the thermal efficiency and hydrocarbon emissions of stoichiometric spark-ignition NG engines through numerical simulations. The improvements aimed to enhance squish flow, organize high tumble flow, and reduce crevice volume. The results indicate that strengthening the squish intensity by increasing the squish area of the piston can accelerate flame propagation and reduce exhaust loss. However, this improvement is hindered by a significant rise in heat transfer loss, which limits further gains in thermal efficiency. Compared to the traditional high swirl combustion chamber, a high tumble combustion chamber (dual straight intake port combined with a hemispherical piston) can promote flame propagation in the early stages of combustion and increase thermal efficiency by 1.43%. However, the accelerated flame propagation rate resulting from increased flow intensity has little potential to reduce hydrocarbon emissions due to the small contribution of flame quenching to unburned hydrocarbon production. The primary source of hydrocarbon emissions from NG engines is unburned methane in the piston crevice. Therefore, when addressing hydrocarbon emissions from NG engines, the first consideration should be to minimize the volume of the piston crevice, which will significantly reduce methane emissions and incomplete combustion loss.

Effects of fuel trapping in piston crevice on unburned hydrocarbon emissions in early-injection compression ignition engines

Early injection strategy is employed in many advanced combustion concepts of modern compression-ignition engines to promote the fuel-air premixing, but it could result in fuel spray impingement on the cylinder wall and potential fuel trapping in the crevice regions. The impact of the fuel trapping on the formation and distribution of unburned hydrocarbon (UHC) in the advanced compression-ignition concepts is not well understood. In this study, the planar laser-induced fluorescence (PLIF) technique was applied in a single-cylinder optical engine to visualize the fuel distribution before combustion and the formaldehyde formation during combustion. A three-dimensional computational model was established to explore the detailed mechanism of UHC formation in the piston crevice. Three cases with injection timings of 20°, 40° (SOI-40), and 100° (SOI-100) were compared. The PLIF and simulation results indicate that, under early injection conditions, the squish region and piston crevice trap a considerable amount of fuel, resulting in increased UHC emission and reduced engine work, and the main combustion zone resides in the squish region. The simulation shows that the amount of UHC formation from the trapped fuel is highly dependent on the local equivalence ratio distribution formed by different injection timings. The local equivalence ratio within the piston crevice region for the SOI-40 case exceeds 2 before combustion; the charge sequentially undergoes both low- and high-temperature heat release (LTHR and HTHR) processes and produces less UHC in the crevice region. However, the SOI-100 case produces an overall leaner mixture with a local equivalence ratio lower than 1 before combustion; the charge undergoes LTHR without noticeable HTHR and becomes UHC near the cylinder wall. The injector dribbling results in more UHC formation in the central part of the cylinder under very early injection timing, and thus an accurate injector dribbling model is required to better reproduce the UHC emissions.

Numerical investigation of natural gas-diesel dual-fuel engine with different piston geometries and radial clearances

Piston crevices are the major source of hydrocarbon emissions for spark ignition engine. It is similar for natural gas-diesel (NG-diesel) dual fuel engine, maybe even worse in high compression ratio and lean burn NG-diesel engine at low load. The aim of this paper is to numerically investigate combustion and methane emissions of YC-6K NG-diesel engine with different radial clearances. Four piston clearances (0.5 mm, 0.76 mm, 1.0 mm and 1.54 mm) and four different piston geometries were employed. Three diesel-spray-orientated (DSO) piston bowls were designed based on diesel injectors of YC-6K with the purpose of supplying more natural gas and air mixture to diesel spray jets, in addition, a protrusion-ring was designed at the bowl rim of DSO pistons to enhance methane flame propagation. The results showed that both in-cylinder pressure and heat release rate (HHR) of all three DSO pistons increased due to higher turbulence kinetic energy comparing to the original piston design (OPD). Methane emissions of pistons with radial clearance of 0.76 mm were lower than that of other radial clearances for DSO pistons.

Impact of spray-wall interaction on the in-cylinder spatial unburned hydrocarbon distribution of a gasoline partially premixed combustion engine

Partially premixed combustion (PPC) often adopts the early fuel-injection strategy that could result in spray-wall interaction involved with piston top-land crevice. This interaction may produce a significant impact on engine combustion and unburned hydrocarbons (UHC) emission, which is still not well understood. In this study, we investigated the detailed spray-wall interaction and its effects on the two-stage ignition, i.e. low- and high-temperature heat release (LTHR and HTHR), and the in-cylinder spatial UHC distribution of PPC in a full-view optical engine at low engine load. The PRF 70 fuel was used as the gasoline surrogate. The high-speed imaging of the natural flame luminosity was acquired to quantify the flame probability distribution. The qualitative fuel-tracer, formaldehyde, and UHC planar laser-induced fluorescence (PLIF) imaging techniques were employed to reveal the fuel, LTHR and UHC distribution characteristics, respectively. The LTHR, HTHR and UHC distribution formed by the fuel trapped in the piston top-land crevice were visualized by PLIF imaging techniques for the first time. The PLIF results indicate that the main UHC formed in the PPC engine comes from the central part of the cylinder close to the injector nozzle, where the overall equivalence ratio is low and the injector dribbling is an important source of UHC. The UHC formed in the piston crevice of the PPC engine depends on the local equivalence ratio of the fuel trapped in the crevice. When the overall equivalence ratio of the charge in the crevice is relatively high, the trapped fuel undergoes both LTHR and HTHR and produces negligible UHC. However, the UHC from the piston crevice becomes considerable when the fuel injection timing is too early so that an overly lean mixture is generated. Based on the above findings, three implications of the PPC operation at low engine load for low UHC emission and high engine efficiency are proposed.

An assessment of gas power leakage and frictional losses from the top compression ring of internal combustion engines

A multi-physics integrated analysis of piston top compression ring of a high-performance internal combustion engines is presented. The effects of transient ring elastodynamics, thermal gas flow through piston crevices upon chamber leakage pressure and parasitic frictional losses are investigated. The multi-physics analysis comprises integrated flexible ring dynamics, ring-liner thermo-mixed hydrodynamics and gas blow-by, an approach not hitherto reported in literature. The predictions show close conformance to frictional measurements under engine motored dynamometric conditions. It is shown that power losses due to gas leakage can be as much as 6 times larger than frictional losses, which are usually considered as the main sources of inefficiency.

Effect of Piston Crevices on the Numerical Simulation of a Heavy-Duty Diesel Engine Retrofitted to Natural-Gas Spark-Ignition Operation

Three-dimensional computational fluid dynamics internal combustion engine simulations that use a simplified combustion model based on the flamelet concept provide acceptable results with minimum computational costs and reasonable running times. Moreover, the simulation can neglect small combustion chamber details such as valve crevices, valve recesses, and piston crevices volume. The missing volumes are usually compensated by changes in the squish volume (i.e., by increasing the clearance height of the model compared to the real engine). This paper documents some of the effects that such an approach would have on the simulated results of the combustion phenomena inside a conventional heavy-duty direct injection compression-ignition engine, which was converted to port fuel injection spark ignition operation. Numerical engine simulations with or without crevice volumes were run using the G-equation combustion model. A proper parameter choice ensured that the numerical results agreed well with the experimental pressure trace and the heat release rate. The results show that including the crevice volume affected the mass of a unburned mixture inside the squish region, which in turn influenced the flame behavior and heat release during late-combustion stages.

Combustion stability study of partially premixed combustion by high-pressure multiple injections with low-octane fuel

This work is the second part of a study on low-load combustion stability for gasoline partially premixed combustion. In part 1, we investigated the sensitivity of the intake air temperature to combustion stability. In part 2, we evaluate the potential of the multiple-injection strategy along with the intake air temperature sensitivity to promote low-load combustion stability using low-octane gasoline fuel. The experiments were carried out in a fully transparent, single-cylinder, compression-ignition engine. The spray/wall interaction, particularly the fuel trapping in the piston crevice zone, was visualized by fuel-tracer planar laser-induced fluorescence for the first time in experiments. The in-cylinder combustion process of natural flame luminosity was captured by a high-speed color camera. By employing a multiple-injection strategy, the minimum intake air temperature can be further reduced from 70 °C (single injection) to 50 °C for target stable combustion. The combustion stability and engine performance were further improved by increasing the fuel injection pressure. For instance, with the triple-injection strategy at a higher fuel-injection pressure of 800 bar, the indicated mean effective pressure was increased by 24% when compared to that of the single-injection strategy. A stronger interaction among fuel spray jets, the piston, and the cylinder wall was observed for multiple injections with higher injection pressure, leading to higher unburned hydrocarbon (UHC) and carbon monoxide (CO) along with a more pronounced pool fire in the squish zone. The double-injection strategy resulted in lower UHC and CO emissions when compared to the triple-injection strategy. Applying a narrow spray angle injector with re-entrant combustion chamber is suggested for optimizing the spray/wall interaction.

Experimental Study on the Effects of Spray–Wall Interaction on Partially Premixed Combustion and Engine Emissions

We investigated the detailed spray–wall interaction in partially premixed combustion (PPC) in an optical engine with a wide-angle injector at low engine load. The fuel-trapping effect of the piston top-land crevice was visualized by fuel-tracer planar laser-induced fluorescence (PLIF) for the first time. In agreement with the fuel distribution shown by PLIF, the combustion region first moves into the squish region and then tends to move back to the piston bowl when advancing the injection timings. Results indicate that a considerable portion of the fuel is trapped in the piston top-land crevice for the cases with injection timings at −30° (SOI-30 case) and −40° (SOI-40 case) crank angle after top dead center. The SOI-40 case with earlier injection timing presents a backflow process of the trapped fuel from the piston top-land crevice to the squish region. However, there is not enough time for the fuel vapor to flow back before the start of combustion for the SOI-30 case, resulting in a relatively lower fuel–air equivalence ratio in the squish region and thus higher CO emission than that of the SOI-40 case. This study provides insights into the fuel distribution characteristics in the piston crevice and the potential effects on the engine emissions under PPC condition.

Promoting “adiabatic core” approximation in a rapid compression machine by an optimized creviced piston design

Rapid compression machine (RCM) is a widely-used experimental instrument for measuring ignition delay time (IDT) in the low-to-intermediate temperature range. In this work, to enhance the validity of the “adiabatic core” approximation in RCM, a piston crevice for suppressing the piston-driven vortex and therefore promoting the temperature homogeneity in the combustion chamber was firstly optimized by numerical simulation with a dynamic mesh strategy. Results show that crevice volume should be large enough to contain most of the boundary layer cold gas during the compression so as to assure the temperature homogeneity in the reaction chamber. While the shape of the crevice (with sufficiently large volume) only weakly affects the temperature homogeneity. An optimized piston was tested across wider ranges of pressure, the chamber length, and inert gas under experimental conditions. Based on the optimized piston crevice, a RCM was established and tested. Test results have confirmed that the crevice shape does not affect the IDT measurements, provided the crevice volume fixed. In addition, the present IDTs show consistency with those using other RCMs, as well as with the numerical predictions obtained by using the well-recognized and validated kinetic mechanisms, which support the validity of the present RCM facility for ignition delay measurement.

CFD study of the Effect of Piston Crevice Volume on the Temperature Distribution in a Rapid Compression Machine

CFD study of the Effect of Piston Crevice Volume on the Temperature Distribution in a Rapid Compression Machine

How to ensure the interpretability of experimental data in Rapid Compression Machines? A method to validate piston crevice designs

In most Rapid Compression Machine (RCM) operations, it is sought to have a homogeneous temperature field inside the core region of the reaction chamber so that the adiabatic core assumption may be appropriately applied. Pistons with crevice on their side have shown to be effective in producing a uniform temperature field at the end of the compression. Although the efficiency of the crevice to produce a homogeneous environment is highly dependent on the operational conditions (pressure, temperature, gases composition, compression time, etc.), crevice designs are seldom adapted to the intended experiments. This is problematic as there are potentially many experiments which do not respect the conditions to obtain a uniform temperature field required to correctly interpret the data. Actually, no method other than CFD simulations has been proposed to this day to validate a specific crevice design, which is in practice most often an obstacle to the systematic validation of crevices. The model proposed in this study aims to fill this gap in allowing to check if a crevice design yields a homogeneous temperature field in the considered operational conditions. It is observed that as soon as a critical mass is transported from the chamber to the crevice, the crevice fulfills its role and guarantees the temperature homogeneity of the core region. The two key findings of the present work are therefore to be able to predict this critical mass for any possible configuration, and to propose an easy method to assess whether a specific crevice volume ensures this critical mass to be effectively sucked from the chamber. The whole process proposed in this study could then be applied as a certification of crevice design prior to any RCM experimental campaign.

A multizone model of the combustion chamber dynamics in a controlled trajectory rapid compression and expansion machine (CT-RCEM)

A novel, controlled trajectory rapid compression and expansion machine (CT-RCEM) with the unique capability of precise motion control of the piston has been developed. The compression and expansion profile can be varied, in the CT-RCEM, by digitally changing the piston reference trajectory assigned to the active motion controller. Besides the ease of operation, and wider selection of operating parameters with higher resolution, this capability lowers the turnaround time between experiments by eliminating the need for any hardware intervention. More importantly, this capability allows us to tailor the thermodynamic path of compression (and expansion) – essentially the pressure and temperature profile – by appropriate selection of the piston trajectory. However, to effectively leverage these features of the CT-RCEM, a numerical model is essential – for selection of appropriate thermodynamic path for chemical-kinetic investigations, as well as for the interpretation of the corresponding experimental results.In this work, we present a computationally efficient, physics based multi-zone model of the combustion dynamics of the CT-RCEM, which accounts for the heat loss and piston crevice flows. We show that the concurrent use of the proposed model with the CT-RCEM allows, for the first time, a systematic investigation of the effect of changing piston trajectory – and consequently – the thermodynamic path of compression on the auto-ignition characteristics of fuels. The effect of changing piston trajectory on autoignition characteristics of dimethyl-ether (DME) observed in CT-RCEM experiments has been explained using the proposed model. The study clearly shows that changing the piston trajectory can significantly affect the measured ignition delay due to the resulting change in the thermodynamic path. Also, a shorter compression time for a given compression ratio does not necessarily guarantee smaller reaction progress during compression. The paper concludes by summarizing the unique insight obtained from this study —the use of CT-RCEM for auto-ignition investigation involving thermodynamic paths with similar pressure and temperature conditions at the end of compression but different intermediate species buildup can potentially provide additional information vital for further understanding of the chemical kinetics.

Optimal piston crevice study in a rapid compression machine

Multi-dimensional effects such as vortex generation and heat losses from the gas to the wall of the reactor chamber have been an issue to obtaining a reliable RCM data. This vortex initiates a flow in the relatively cold boundary layer, which may penetrate the core gas. This resulting non-uniformity of the core region could cause serious discrepancies and give unreliable experimental data. To achieve a homogenous temperature field, an optimised piston crevice was designed using CFD modelling (Ansys fluent). A 2-Dimensional computational moving mesh is assuming an axisymmetric symmetry. The model adopted for this calculation is the laminar flow model and the fluid used was nitrogen. To get the appropriate crevice volume suitable for the present design, an optimisation of the five different crevice volume was modelled which resulted to about 2-10% of the entire chamber volume. The use of creviced piston has shown to reduce the final compressed gas temperature and pressure in the reactor chamber. All the crevice volumes between 2-10% of the chamber volume adequately contained the roll up vortexes, but the crevice volume of 282 mm3 was chosen to be the best in addition to minimising the end gas pressure and temperature drop. The final pressure trace from experiment shows a reasonable agreement with the CFD model at compression and post compression stage.

Improvement of diesel combustion with multiple injections at cold condition in a constant volume combustion chamber

A series of diesel spray combustion tests was carried out in a constant volume combustion chamber (CVCC) to investigate the effect of multiple injection strategies on cold startability. The experiments were performed under a simulated low temperature cold start condition. In-chamber pressure analysis and high speed flame imaging were conducted to compare the effectiveness of each injection strategy on cold startability. Spray targeting visualization was also performed to examine the wall impingement of injected fuel. The diesel fuel was injected into the CVCC with an injection pressure of 35MPa. Multiple injection strategies with different amounts of pilot injection quantities were applied to improve the diesel combustion under simulated cold start ambient condition. The flame imaging and in-chamber pressure results showed that the multiple injection strategy provided better cold startability than the single injection condition. According to the pilot injection quantity, the peak of the flame luminosity and in-chamber pressure were gradually increased with larger pilot injection quantity cases. The peak of the variation rate of the in-chamber pressure with a multiple injection strategy was approximately 2 times higher than that of the single injection case. In terms of spray targeting imaging, the results indicated that the fuel impingement and flow to the piston crevice volume were increased with a larger amount of pilot2 injection. Therefore, the increment of the pilot1 injection quantity rather than the pilot2 injection quantity was suggested not only to improve cold startability, but also to reduce unburned hydrocarbon emissions under a cold start condition in a real engine.

Optical study of spray-wall impingement impact on early-injection gasoline partially premixed combustion at low engine load

Spray-wall impingement caused by early fuel injection for gasoline partially premixed combustion (PPC) can lead to low combustion efficiency and a significant rise of UHC emissions. But the influence of spray-wall impingement on the in-cylinder combustion process is not well understood. In this study, multiple optical diagnostics were applied to investigate the ignition, flame development and UHC formation of gasoline PPC with early single fuel-injection in a light-duty optical engine under low engine load. Natural combustion luminosity images and emission spectra were obtained. Planar laser-induced fluorescence (PLIF) of the fuel-tracer and formaldehyde were used to explore the fuel/air mixing and UHC formation in PPC, respectively. The results indicated that there was a fuel-injection time window (about −30° to −60° ATDC in the present study), during which the spray-impingement led to a decrease in combustion efficiency. The fuel-trapping effect in the squish region and piston crevice was shown to be the main reason because it prevented the fuel/air mixture from entering the combustion chamber. Two typical fuel injection timings of −35° (PPC-35) and −60° (PPC-60) were chosen for further study. For both cases, ignition sites first emerged in the fuel-rich regions and then the flames developed to the fuel-lean regions. The formaldehyde PLIF images revealed distinct flame front in the flame development process. For the PPC-35 case, residual formaldehyde persisted in the fuel-lean regions late during the power stroke and might become a source of UHC emissions. When misfire happened, the combustion chamber was filled with formaldehyde. For the PPC-60 case, the flame development was composed of initial flame front propagation and following sequential auto-ignition, and the flame expansion speed of the initial flame front propagation was much higher than that in SI (spark ignition) or SACI (spark assisted compression ignition) combustion. When the injection timing was further advanced (earlier than −60°), the impact of spray-wall impingement on PPC was reduced because of more time being available for fuel premixing.

Interpretation of auto-ignition delays from RCM in the presence of temperature heterogeneities: Impact on combustion regimes and negative temperature coefficient behavior

Rapid compression machines (RCMs) have often been used to study auto-ignition phenomena and to validate chemical kinetic models at conditions relevant to engines. Many RCM designs were historically used for the latter purpose. First, the flat piston configuration was extensively used and contributed to valuable ignition delay data. Temperature heterogeneity in these devices was later addressed, and new designs were proposed. In particular, creviced piston configurations have been demonstrated to enhance the temperature homogeneity in the bulk volume of RCM chambers. Nevertheless, data from flat piston RCMs are still used in the community. Thus, additional understanding of experimental results from such devices is still useful, and it also helps to enhance the understanding of temperature stratification effects on auto-ignition phenomena.This paper addresses the interpretation of measured ignition delays from a flat piston RCM (without a circumferential crevice in the piston). First, we characterize local and temporal temperature evolution with a toluene planar laser induced fluorescence (PLIF) technique applied in oxygen-free gas under different test conditions. At a compression pressure (Pc) of 11bar and an adiabatic core temperature (Tc) of 750K, we find that the maximum measured temperature is very close to Tc up to 30ms after the compression stroke. In the second step, we qualitatively compare temperature fields with chemiluminescence images during the auto-ignition of n-hexane and an n-heptane/methyl-cyclohexane (MCH) blend for a fuel-air equivalence ratio of 0.4, Pc values of 11bar and 16bar and Tc values of 700–900K. The chemiluminescence results of n-hexane show a fast sequential auto-ignition which strongly suggest that the combustion propagation regime is mainly a reaction fronts regime. In the n-heptane/MCH case, the tracking of ignition kernels demonstrates that the auto-ignition initiates in the cold vortex at Tc values of 787K and 834K and in the adiabatically compressed region at Tc values of 742K and 900K.In this study, flat piston RCM experiments demonstrate temperature heterogeneities with ranges up to 120K and gradients of approximately 160K/mm. This significantly influences the ignition delay measurement in the negative temperature coefficient (NTC) range. Through the example of n-heptane/MCH auto-ignition, it seems that despite temperature heterogeneities, the lower limit of the NTC range is well detected, whereas the higher limit is over-estimated. The determination of temperature thresholds in which ignition delay data may be confounded highly depends on the stratification level and on complex thermo-kinetic interactions. Outside these thresholds, when combustion is dominated by auto-ignition in regions at the adiabatic core temperature, the experimental data are easier to compare with simulation frameworks assuming a homogeneous hypothesis. In chemical kinetics-oriented studies, temperature heterogeneity should be considered when flat piston RCMs are used and when the piston crevice is not adequate.

Application of a Phenomenological Model for the Engine-Out Emissions of Unburned Hydrocarbons in Driving Cycles

Abstract In this work, the application of a phenomenological model to determine engine-out hydrocarbon (HC) emissions in driving cycles is presented. The calculation is coupled to a physical-based simulation environment consisting of interacting submodels of engine, vehicle, and engine control. As a novelty, this virtual calibration methodology can be applied to optimize the energy conversion inside a spark-ignited (SI) internal combustion engine at transient operation. Using detailed information about the combustion process, the main origins and formation mechanisms of unburned HCs like piston crevice, oil layer, and wall quenching are considered in the prediction, as well as the in-cylinder postoxidation. Several parameterization approaches, especially, of the oil layer mechanism are discussed. After calibrating the emission model to a steady-state engine map, the transient results are validated successfully against measurements of various driving cycles based on different calibration strategies of engine operation.

Experimental and numerical study of the pollution formation in a diesel/CNG dual fuel engine

A dual fuel combustion model comprising a modified turbulent flame speed closure (TFSC) model and a partially premix reactor model is developed and incorporated into the KIVA-3V code to simulate the combustion processes within the dual fuel engine. Numerical results are validated by the cylinder pressures, heat release rates and emissions of the corresponding experimental data. The effects of engine speeds on the combustion process of dual fuel engines are investigated and the spatial and temporal distributions of the in-cylinder temperature and pollutions are compared. The results reveal that most of the pilot fuel vapor remains within the bowl, and the combustion of the pilot fuel forms a high temperature region. Moreover, some of pilot fuel vapor enters the squish volume, and forms a high temperature region above the piston top. NO formation region follows the development of the high temperature field, which is generated by the combustion of the pilot diesel. At low engine speed, the piston crevices are largely responsible for the creation of the unburned CH4 emissions. However, at high engine speed, bulk gas partial oxidation in the cylinder center region is the major source of the CH4 emissions. The flame quench zone in the squish volume close to the cylinder walls is the main source of the CO emission at low engine speed. However, the bulk gas partial oxidation zone in the cylinder center region is the major source of the CO emission at high engine speed.

A computationally efficient, physics-based model for simulating heat loss during compression and the delay period in RCM experiments

A computationally efficient, physics-based model is developed to account for the heat loss that occurs during the compression process and delay period in rapid compression machine (RCM) experiments. The model is formulated using multiple zones (MZs) and accounts for conductive losses in the main reaction chamber as well as convective losses in the piston crevice and ringpack regions. Blowby past the piston ringpack is also considered. The loss terms can be incorporated into the widely-utilized ‘adiabatic core’ model in order to perform ‘volumetric expansion’ calculations during chemical kinetic simulations that correct for pressure and temperature reductions resulting from these energy losses. An advantage of this approach over existing models and ‘ad-hoc’, or empirical methods is that the MZM constants are only a function of the machine geometry and therefore do not vary from experiment to experiment. As such, fewer experiments need to be conducted (e.g., non-reacting tests+reacting tests at each condition), while existing literature data can be more reliably utilized for kinetic model validation and improvement. This formulation also has the potential to account for effects of multi-stage ignition, as well as phase change that can occur during experiments with involatile, transportation-relevant fuels. The performance of the new MZM is validated here via comparisons with multi-dimensional RCM simulations as well as experiments employing non-reacting and reacting mixtures. Excellent agreement is demonstrated for the overall pressure decay experienced in the reaction chamber during the delay period, as well as the computed gas temperature and velocity behavior within the crevice. Predicted thermal stratification within the reaction chamber also agrees well when high pressure, low viscosity conditions are utilized, i.e., when no vorticular motion is present. Computed ignition delay times for H2/O2/diluent mixtures using the new model agree favorably with the reactive CFD simulations and experimental measurements.

Methodology to account for multi-stage ignition phenomena during simulations of RCM experiments

RCM experiments are used to investigate the ignition behavior of fuels at engine relevant conditions. Modern designs utilize pistons with crevice volumes machined around the circumference of the crown in order to suppress boundary layer effects during the volumetric compression process. While piston crevices have been successful in controlling heat loss from the reaction chamber gases and improving the overall homogeneity of the reacting mixture, multi-stage ignition events can be sufficiently perturbed by spatial non-uniformities and there can be substantial gas flow into the crevice volume due to the preliminary, or low-temperature heat releases. Ignition delay times can be lengthened by up to 25% as a result of these effects. These features are difficult to incorporate into 0D chemical kinetic simulations where volumetric expansion curves from non-reacting experiments are often used to prescribe the heat loss characteristics of reacting chamber mixtures. A new methodology is presented here to account for multi-stage ignition phenomena during simulations of RCM experiments. The approach and a range of demonstrative examples are presented in this study.