Spark-assisted Ignition Research Articles

Combustion characteristics and performance analysis of a heavy-fuel rotary engine by designing fuel injection position

The heavy fuel rotary engine commonly operates under high-pressure direct injection, lean-burn and spark-assisted ignition modes, this characteristic feature further highlights the significance of the distance between fuel injection position (FIP) and assisted spark plug in engine combustion organization and performance improvement. Thus, a computational fluid dynamics (CFD) means coupled with chemical reaction kinetics is used to investigate the FIP effects on the fuel stratification rule and combustion performance of a heavy-fuel rotary engine. Meanwhile, the flow and combustion transient information in the narrow combustion chamber is analyzed by considering five specially designed FIPs, whose distances to the assisted spark plug along the X-axis (major axis) are 25, 30, 35, 40 and 45 (mm). The results indicate that the distance increases to 40 mm (FIP-40) and 45 mm (FIP-45), the fuel stratified distribution rule is more rational, the mixture burn rate accelerates, combustion time reduces, and combustion performance improves significantly. The FIP-45 scheme is the preferred engineering application scheme after comprehensively assessing engine performance parameters, and when compared to the FIP-35 (original scheme) its peak cylinder pressure (Pmax) increases by 5.3 %, CA10-90 accelerates by 20.4 %, and main emissions of Soot, NO and CO reduce by 48.1 %, 50.0 % and 36.2 % respectively, while CO2 increase.

Development of a new configuration and optimization of the 5-kW class energy conversion system combining a solid oxide fuel cell and internal combustion engine using a spark-assisted ignition

For the optimal operation of an energy conversion system combining a solid oxide fuel cell (SOFC) and an internal combustion engine using spark-assisted ignition (SAI), a novel configuration based on natural gas was analyzed. This is the first study to determine the optimal configuration of an SOFC–SAI engine hybrid system through a case study that includes all possible configurations, considering the heat transfer quality for balance of plant (BOP) operation. A system-level analysis was conducted by integrating the single-cylinder engine experimental results with a simulation model for the SOFC and BOPs. The optimal operating conditions affecting power generation, such as load distribution and air supply related to the electrochemical and combustion reactions in the SOFC and SAI engine, were investigated using the response surface method (RSM) based on a system-level analysis. Through a comparative analysis of the configurations, the most efficient configuration was developed, which included three external reformers using the thermal energy of the anode/cathode off-gas and a heat exchanger to recover the residual heat after steam generation, equivalent to 3–4 % of the lower heating value of the fuel in the system. Based on the response surface, which is a multiple linear regression model using the least squares method (LSM), the optimal design point was investigated by considering the system operability, such as combustion stability and thermal self-sustainability. The combustion stability was confirmed through an engine experiment, and the SAI engine achieved over 90 % combustion efficiency under all operable ranges, such as fuel utilization and equivalence ratio. Consequently, the 5-kW class SOFC–SAI engine hybrid system can achieve an efficiency of ∼63.2 % at optimal operation with the newly developed design, that is, fuel utilization of ∼75 %, air utilization of ∼25 %, and equivalence ratio of ∼0.55. The SAI engine had responsibility for approximately 9 % of the hybrid system power.

Analysis on the operating performance of 5-kW class solid oxide fuel cell-internal combustion engine hybrid system using spark-assisted ignition

The objective of the proposed hybrid system is to increase the system efficiency by using the residual fuel of the anode off-gas from a solid oxide fuel cell in an internal combustion engine (ICE). In this study, a novel hybrid system using spark-assisted ignition (SAI) in an ICE operation is proposed. Since this is the first attempt of using a new combustion concept in a hybrid system, feasibility of SAI and its effect on the system operation are investigated in this study. To analyze the effect of the combustion on the system, the engine experiments on the SAI were conducted by changing the engine operating parameters, such as the intake temperature, equivalence ratio, and spark timing. For the system-level analysis, a zero-dimensional model for the fuel cell and the balance of plants was developed and validated. The results of the engine experiments were integrated directly with the system model. The performance of the hybrid system using SAI was analyzed from the energy and exergy perspectives. Under the operating conditions of this study, the anode off-gas combustion can be controlled stably (COV: 5–7%) through the SAI, even though the intake temperature is decreased to ~280 °C at the low compression ratio of 8.2. It leads to an increase in exergy efficiency of the engine to ~37%. Consequently, thermal self-sustainability is improved and the indicated efficiency of ~61.6% is achieved in the hybrid system. The SAI engine is responsible for ~14% of the system power and produces considerably low NOX emissions (<~3 ppm at 15% O2 on a dry basis).

End-gas autoignition behaviors under pressure wave disturbance

The present study addresses effects of pressure wave disturbance on end-gas autoignition behavior and subsequent pressure wave development, which are of particular interest for understanding a fundamental process of knocking combustion in spark-assisted ignition engines. Such pressure wave disturbance is initiated by a compression wave generated from spark-like ignition and continuously exists in a combustion chamber. The investigation is conducted through a direct numerical simulation with a 1-D constant volume reactor, where the compressible Navier–Stokes equations are solved with detailed reaction mechanisms of n−C7H16 and n−C4H10/air mixtures. The width of an ignition kernel parametrically changes in order to control the strength of an initial compression wave and thus pressure wave disturbance. The result showed that stronger compression waves with larger kernel widths enhance the progress of the autoignition process at the wall through the transient increase in pressure and temperature due to the wave reflection. Consequently, stronger pressure waves such as a developing detonation are generated with the enhanced inhomogeneous end-gas autoignition starting from the wall. In contrast, weak pressure wave disturbance produces no strong pressure waves due to the lower inhomogeneity of autoignition timing in an end-gas region. An analysis with temperature variation in an end-gas region indicates that cool flame generation helps maintain the superiority of the wall for earlier autoignition through the heat release at an earlier stage. Therefore, a stronger pressure wave associated with inhomogeneous end-gas autoignition is likely generated around 650 K in the case of an n−C7H16/air mixture, which corresponds to a distinct peak generation of knocking intensity in the negative temperature coefficient regime. For lower temperature conditions such as 500 K, the end-gas autoignition first takes place in a region away from the wall. This is because the initially induced temperature inhomogeneity in the end-gas region is considerably weakened due to wave interactions and dissipation effects during longer ignition delay times. Thus, multiple end-gas points including the wall can become a preferred point for earlier end-gas autoignition, resulting in a variation of end-gas autoignition locations. The result for an n−C4H10/air mixture is finally shown and the difference from that for the n−C7H16/air mixture is highlighted.

Evaluation of Homogeneous Charge Compression Ignition (HCCI) autoignition development through chemiluminescence imaging and Proper Orthogonal Decomposition

Homogeneous Charge Compression Ignition (HCCI) engines deliver high thermal efficiency and, therefore, low CO2 emissions, combined with low NOX and particulate emissions. However, HCCI operation is not possible at all conditions due to the inability to control the autoignition process and new understanding is required. A high-swirl low-compression-ratio, optically accessed engine that can produce overall fuel lean, axially stratified charge (richer fuel mixture close to the cylinder head was achieved using port injection against open valve and homogeneous mixture during injection against closed valve timing) was operated in HCCI mode without and with spark-assist mixture ignition. The present study investigates the differences in the HCCI autoignition process and the propagation of the autoignition front with homogeneous mixture or fuel charge stratification, internal Exhaust Gas Recirculation (iEGR) (introduced by utilizing different camshafts) and spark-assisted iEGR lean combustion. In order to visualize the HCCI process, chemiluminescence flame images, phase-locked to a specific crank angle, were acquired. In addition, time-resolved images of the developing autoignition flame front were captured. Proper Orthogonal Decomposition (POD) was applied to the acquired images to investigate the temporal and spatial repeatability of the autoignition front and compare these characteristics to the considered scenarios. The eigenvalues of the POD modes provided quantitative measure of the probability of the corresponding flame structures. The first POD mode showed higher probability of single autoignition sites originating from a particular location (depending on the scenario). However, the contribution from other modes cannot be neglected, which signified multiple locations of the single autoignition and also, multiple sites of self-ignition of the fuel-air mixture. It was found that increasing iEGR resulted in random combustion (multiple autoignition sites and fronts), which, however, became significantly non-random due to addition of spark-assisted ignition. It was identified in the POD analysis of the time-resolved flame images that the presence of inhomogeneity either in the temperature or the mixture fraction distribution increases the probability of random combustion during the very early stages of flame development. Thus, the fluctuations of heat release is higher during this period.

Combustion and emission characteristics of alcohol fuels in a CAI engine

The combustion and emission characteristics of alcohol fuels in a controlled auto-ignition (CAI) combustion engine have been investigated. Combustion characteristics were studied by heat releases analysis through in-cylinder pressure measurement and auto-ignition reactions through high-speed chemiluminescence imaging. By means of the negative valve overlap, varied amount of residual gas was trapped in the cylinder to achieve the controlled auto-ignition with/without spark-assisted ignition. Total chemiluminescence images were captured with a high speed camera equipped with a high repetition image intensifier. Methanol, ethanol and their blends with gasoline were tested and their combustion and emission characteristics were compared with the results of gasoline.Alcohol fuels, in particular methanol, resulted in advanced auto-ignition and faster combustion than that of gasoline. In addition, their use could lead to substantially lower HC, NOX and CO exhaust emissions. Spark-assisted ignition was most effective on gasoline combustion by advancing ignition timing and initiating flame kernel at the centre of combustion chamber but it had marginal effect on alcohol fuels. Auto-ignition always took place at the perimeter of the chamber and occurred earlier with alcohol fuels because of their greater heat release and fuel reforming reactions during the negative valve overlap period. Chemiluminescence of the gasoline fuel was most visible and it decreased with increasing percentage of oxygenated fuels.