Haiqiao Wei

Interaction of Flame Propagation and Pressure Waves During Knocking Combustion in Spark-Ignition Engines

Knocking combustion in spark-ignition (SI) engines is a typical abnormal combustion phenomenon that severely limits engine performance and thermal efficiency. However, its mechanism has not so far been completely revealed, such as the origin of pressure oscillation with tremendous amplitude and broken mechanisms of the engine body when knock occurs. This article systematically reviews the series of physical and chemical phenomena involved in knocking combustion, including auto-ignition, gas-dynamic waves, cavity resonance, and auto-ignited flame propagation. Significant research has shown that the rapid heat release of end-gas auto-ignition and subsequent strong pressure waves play a crucial role during knocking combustion. A methodology of the interaction of flame propagation and pressure waves has been proposed to reveal knock formation in terms of positive feedback between strengthened pressure waves and auto-ignited flame propagation. Also, new suggestions on the weakness in prior research of knocking combustion have also been discussed in the present article.

A predictive Livengood–Wu correlation for two-stage ignition

The Livengood–Wu correlation has been widely used to predict the state of auto-ignition in internal combustion engines, although its application to two-stage ignition processes remains unresolved. In this study, the original Livengood–Wu integral is extended to such two-stage ignition process and applied to simulations of typical operations within homogeneous charge compression ignition engines. Specifically, based on recent understanding of the global and detailed kinetics of low-temperature chemistry leading to ignition, simplified Arrhenius-based global reaction expressions were developed for both stages of constant-state auto-ignition. It is shown that the original Livengood–Wu integral works well for the first-stage ignition delay, as demonstrated in previous studies. Furthermore, by also accurately describing the cool flame temperature and pressure increment at the end of the first-stage ignition, the second-stage ignition delay can in addition be coupled with the first-stage ignition and predicted satisfactorily with a second integral. This formulation is then applied to extensive homogeneous charge compression ignition engine operation conditions, showing satisfactory predictive capability.

Toward Efficient Chemistry Calculations in Engine Simulations Through Static Adaptive Acceleration

ABSTRACT The incorporation of detailed chemistry in internal combustion engine (ICE) simulations is challenging due to the large number of chemical species and the wide range of chemical timescales involved, which is further complicated by the highly transient nature of the combustion process. The performance of chemistry acceleration methods such as in situ adaptive tabulation (ISAT) deteriorates dramatically in ICE simulations because of the large variation in the accessed composition space. In this study, a dynamic pruning (DP) strategy for ISAT and an automated initial searching species method together with adaptive reduction thresholds for dynamic adaptive chemistry (DAC) are first proposed to enhance the capabilities of ISAT and DAC for transient ICE simulations, respectively. For ISAT, the dynamic pruning for infrequently accessed tabulated points is achieved by pruning the least recently used tabulated entries. It is observed that DP strategy can improve the computational efficiency in chemistry for the compression and post-combustion stages by an order of magnitude. For DAC, the automated specification of searching-species is based on species reactivity, which is characterized by the species rates of change. The two-stage n-heptane/air autoignition illustrates that it can dynamically select the species of importance according to the progress of the combustion process. The dynamic specification of reduction thresholds based on local thermodynamic conditions can achieve an additional 20% reduction in species and reactions for high-temperature combustion. A static adaptive chemistry acceleration approach is then proposed to further improve the chemistry calculation, in which the enhanced ISAT or DAC is employed at different combustion stages based on the encountered composition inhomogeneity and reaction activity. Specifically, ISAT is employed during the compression and post-combustion stages and DAC is employed during the combustion stage if composition inhomogeneity is significant. It is found that compared to the fixed coupled ISAT-DAC approach, the static adaptive approach can improve the efficiency of chemistry calculations by an order of magnitude for the compression and post-combustion stages and a factor of 2 for the combustion stage. For the simulated HCCI combustion of primary reference fuel with significant composition inhomogeneity, the proposed static adaptive approach achieves a speed-up factor of more than 5 for chemistry calculation with a 171-species mechanism while maintaining accurate predictions of pressure, temperature, and species. It is three time more efficient than the fixed coupled ISAT-DAC approach, demonstrating the great potential of dynamic chemistry acceleration approaches for ICE simulations.

Experimental Investigation of Turbulent Flame Propagation and Pressure Oscillation in a Constant Volume Chamber Equipped With an Orifice Plate

ABSTRACT In this work, the main contribution is an understanding of different combustion phenomena involving flame acceleration, flame propagation, and the pressure oscillation resulting from flame-shock interactions. These physical phenomena were experimentally studied using a newly developed confined combustion chamber equipped with one or two orifice plates. The results showed that there are five stages of flame propagation when a laminar flame passes through the orifice plate in a confined space. These include the deceleration of the laminar flame, jet flame formation and rapid acceleration, deceleration of the flame, turbulent flame formation and acceleration, and turbulent flame propagation in the end-gas region. Flame acceleration and pressure oscillation were found to be strongly related to the aperture size of the orifice plate. The high amplitudes of pressure oscillations were found to be the results of two combustion mechanisms: the end gas auto-ignition and the interactions between the accelerated turbulent flame and shock wave. To further accelerate the flame and promote stronger disturbance in the end gas, another identical orifice plate was employed. Subsequently, strong flame-shock interaction caused end-gas auto-ignition with an extremely high-amplitude pressure oscillation. Eventually, the maximum amplitude of pressure oscillation exceeded 8 MPa as end-gas auto-ignition occurred in the end region of the combustion chamber.

Experimental Study on the Burning Rate of Methane and PRF95 Dual Fuels

Natural gas as an alternative fuel offers the potential of clean combustion and emits relatively low CO2 emissions. The main constitute of natural gas is methane. Historically, the slow burning speed of methane has been a major concern for automotive applications. Literature on experimental methane–gasoline Dual Fuel (DF) studies on research engines showed that the DF strategy is improving methane combustion, leading to an enhanced initial establishment of burning speed even compared to that of gasoline. The mechanism of such an effect remains unclear. In the present study, pure methane (representing natural gas) and PRF95 (representing gasoline) were supplied to a constant volume combustion vessel to produce a DF air mixture. Methane was added to PRF95 in three different energy ratios 25%, 50% and 75%. Experiments have been conducted at equivalence ratios of 0.8, 1, 1.2, initial pressures of 2.5, 5 and 10 bar and a temperature of 373K. At stoichiometric conditions, experiments in an SI engine have been also performed. It has been found that methane and all DFs have their fastest burning rate at stoichiometric conditions whereas PRF95 at rich conditions (Φ=1.2). At lean conditions (Φ=0.8), all DFs resulted in faster combustion than PRF95, whereas methane is the slowest of all. At rich conditions, DF75 and DF50 are slower than methane. The transition mechanism between the constant volume combustion experiments and those in the engine environment resulted in a larger increase in the burning speed of methane and all DFs in comparison to that of the liquid fuel.

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.

Numerical investigation of diesel spray flame structures under diesel-engine relevant conditions using large eddy simulation

ABSTRACT Large eddy simulation coupled with the third-order Monotone Upstream-centered Schemes for Conservation Laws (MUSCL) differencing scheme was employed for investigating the ignition processes and flame structures of the reacting n-heptane spray over a wider range of diesel engine-relevant conditions. First, the effects of numerical schemes on the mixing and combustion processes are analyzed in detail. Comparisons of the mixture fraction profiles with experimental data from the Engine Combustion Network website show that the MUSCL gives a better prediction compared with Quasi-second-order upwind scheme. The mixing between fuel and air is much better using MUSCL scheme. As a result, ignition is initiated at fuel-leaner regions, but still richer than stoichiometric equivalence ratio. Second, the predicted ignition delay times and flame lift-off lengths are compared with experimental data under different initial conditions. The predicted results show good agreements with experimental results at different temperatures, oxygen concentrations, and densities. Finally, the effects of initial conditions on the spray flame structures are comprehensively analyzed using the temperature-equivalence ratio maps. The effects of initial parameters on the reacting spray structures and important radicals, such as OH and CO are investigated carefully. Results show that initial oxygen concentration has the greatest influence on the flame structures than ambient gas temperature and density.

Thermodynamic Analysis of a Novel Combined Power and Cooling Cycle Driven by the Exhaust Heat Form a Diesel Engine

A novel combined power and cooling cycle based on the Organic Rankine Cycle (ORC) and the Compression Refrigeration Cycle (CRC) is proposed. The cycle can be driven by the exhaust heat from a diesel engine. In this combined cycle, ORC will translate the exhaust heat into power, and drive the compressor of CRC. The prime advantage of the combined cycle is that both the ORC and CRC are trans-critical cycles, and using CO₂ as working fluid. Natural, cheap, environmentally friendly, nontoxic and good heat transfer properties are some advantages of CO₂ as working fluid. In this paper, besides the basic combined cycle (ORC-CRC), another three novel cycles: ORC-CRC with an expander (ORC-CRCE), ORC with an internal heat exchanger as heat accumulator combined with CRC (ORCI-CRC), ORCI-CRCE, are analyzed and compared. The cycle parameters, including the coefficient of performance (cop), the cooling capacity (Qro) and expansion power of CRC (We) have been analyzed and optimized as the variation of the high pressure of ORC, the high pressure and the outlet temperature of gas cooler of CRC, and temperature drop of heat source in heat accumulator of ORC. The results indicate that there is an optimal high pressure of CRC (about 8.6 MPa to 8.8 MPa) for the combined cycles, at which the combined cycles achieve the optimal performance. The results also show that both the expander and heat accumulator could improve the system performance. The higher ΔTi could improve the system performance, but also resulting the more insufficiency of waste heat recovery.

Simulation research on the effect of cooled EGR, supercharging and compression ratio on downsized SI engine knock

Knock in spark-ignition(SI) engines severely limits engine performance and thermal efficiency. The researches on knock of downsized SI engine have mainly focused on structural design, performance optimization and advanced combustion modes, however there is little for simulation study on the effect of cooled exhaust gas recirculation(EGR) combined with downsizing technologies on SI engine performance. On the basis of mean pressure and oscillating pressure during combustion process, the effect of different levels of cooled EGR ratio, supercharging and compression ratio on engine dynamic and knock characteristic is researched with three-dimensional KIVA-3V program coupled with pressure wave equation. The cylinder pressure, combustion temperature, ignition delay timing, combustion duration, maximum mean pressure, and maximum oscillating pressure at different initial conditions are discussed and analyzed to investigate potential approaches to inhibiting engine knock while improving power output. The calculation results of the effect of just cooled EGR on knock characteristic show that appropriate levels of cooled EGR ratio can effectively suppress cylinder high-frequency pressure oscillations without obvious decrease in mean pressure. Analysis of the synergistic effect of cooled EGR, supercharging and compression ratio on knock characteristic indicates that under the condition of high supercharging and compression ratio, several times more cooled EGR ratio than that under the original condition is necessarily utilized to suppress knock occurrence effectively. The proposed method of synergistic effect of cooled EGR and downsizing technologies on knock characteristic, analyzed from the aspects of mean pressure and oscillating pressure, is an effective way to study downsized SI engine knock and provides knock inhibition approaches in practical engineering.

Influence of injection strategies on knock resistance and combustion characteristics in a DISI engine

The combustion of a direct injection spark ignition engine is significantly affected by the fuel injection strategy due to the impact this strategy has on the gas-mixture formation and the turbulence flow. However, comprehensive assessments on both knock and engine performances for different injection strategies are generally lacking. Therefore, the main objective of the present study is to provide an experimental evidence of how a single injection strategy and a split injection strategy compare in terms of both knock tendency and engine performances like thermal efficiency, torque and combustion stability. Starting from the optimization of a single injection strategy, a split injection strategy is then evaluated. Under the present operating conditions, an optimum secondary injection timing of 100 CAD BTDC is found to have significant improvements on both the knock resistance and the overall engine performances. It should be noted that the present results indicate that the relationship between double injection and anti-knock performance is not monotonous. In addition, the double injection shows superior potential in improving fuel economy and power performance in contrast with the single injection thanks to a more stable combustion when a late injection timing is applied.