Pressure Common Rail Injection System Research Articles

GPIO‐based rail pressure control for diesel high pressure common rail injection system

The rail pressure control in diesel high-pressure common rail (HPCR) system is considered as one of the key issues for precise control of fuel injection. This study investigates the application of generalised proportional integral observer (GPIO)-based control in diesel HPCR system. Firstly, a non-linear mathematical model of HPCR system which uses the metering unit as the actuator is established based on hydromechanics under some proper simplifications. Secondly, a GPIO is developed to estimate the mismatched disturbance caused by the injection process. Then a composite controller combining GPIO and backstepping control method is designed. By introducing the disturbance estimations into the control design process, the disturbance rejection capability of the system is promisingly enhanced. The effectiveness of the proposed control method is validated by the simulation results.

Effect of cold start on engine performance and emissions from diesel engines using IMO-Compliant distillate fuels

Emissions from ships at berth are small compared to the total ship emissions; however, they are one of the main contributors to pollutants in the air of densely-populated areas, consequently heavily affecting public health. This is due to auxiliary marine engines being used to generate electric power and steam for heating and providing services. The present study has been conducted on an engine representative of a marine auxiliary, which was a heavy duty, six-cylinder, turbocharged and after-cooled engine with a high pressure common rail injection system. Engine performance and emission characterisations during cold start are the focus of this paper, since cold start is significantly influential. Three tested fuels were used, including the reference diesel and two IMO (International Maritime Organization) compliant spiked fuels. The research engine was operated at a constant speed and 25% load condition after 12 h cooled soak. Results show that during cold start, significant heat generated from combustion is used to heat the engine block, coolant and lubricant. During the first minute, compared to the second minute, emissions of particle number (PN), carbon monoxide (CO), particulate matter (PM), and nitrogen oxides (NOx) were approximately 10, 4, 2 and 1.5 times higher, respectively. The engine control unit (ECU) plays a vital role in reducing engine emissions by changing the engine injection strategy based on the engine coolant temperature. IMO-compliant fuels, which were higher viscosity fuels associated with high sulphur content, resulted in an engine emission increase during cold start. It should be taken into account that auxiliary marine diesel engines, working at partial load conditions during cold start, contribute considerably to emissions in coastal areas. It demonstrates a need to implement practical measures, such as engine pre-heating, to obtain both environmental and public health advantages in coastal areas.

High efficiency ethanol-diesel dual-fuel combustion: A comparison against conventional diesel combustion from low to full engine load

Comparisons between dual-fuel combustion and conventional diesel combustion (CDC) are often performed using different engine hardware setups, exhaust gas recirculation rates, as well as intake and exhaust manifold pressures. These modifications are usually made in order to curb in-cylinder pressure rise rates and meet exhaust emissions targets during the dual-fuel operation. To ensure a fair comparison, an experimental investigation into dual-fuel combustion has been carried out from low to full engine load with the same engine hardware and identical operating conditions to those of the CDC baseline. The experiments were executed on a single cylinder heavy-duty diesel engine at a constant speed of 1200 rpm and various steady-state loads between 0.3 and 2.4 MPa net indicated mean effective pressure (IMEP). Ethanol was port fuel injected while diesel was direct injected using a high pressure common rail injection system. The start of diesel injection was optimised for the maximum net indicated efficiency in both combustion modes. Varied ethanol energy fractions and different diesel injection strategies were required to control the in-cylinder pressure rise rate and achieve highly efficient and clean dual-fuel operation. In terms of performance, dual-fuel combustion attained higher net indicated efficiency than the CDC mode from 0.6 to 2.4 MPa IMEP, with a maximum value of 47.2% at 1.2 MPa IMEP. The comparison also shows the use of ethanol resulted in 26% to 90% lower nitrogen oxides (NOx) emissions than the CDC operation. At the lowest engine load of 0.3 MPa IMEP, the dual-fuel operation led to simultaneous low NOx and soot emissions at the expense of a relatively low net indicated efficiency of 38.9%. In particular, the reduction in NOx emissions introduced by the utilisation of ethanol has the potential to decrease the engine running costs via lower consumption of aqueous urea solution in the selective catalyst reduction system. Moreover, the dual-fuel combustion with a low carbon fuel such as ethanol is an effective means of decreasing the use of fossil fuel and associated greenhouse gas emissions.

Experimental investigation on effect of ethanol and di-ethyl ether addition on the spray characteristics of diesel/biodiesel blends under high injection pressure

In this work, a comprehensive experimental investigation on spray characteristics of four blended fuels, including diesel (D100), diesel-biodiesel (DB), diesel-biodiesel- ethanol (DBE), and diesel-biodiesel-diethyl ether (DBDE) has been conducted by using high pressure common rail injection system (up to 200 MPa). The transient spray behavior under various conditions was recorded by high speed photography with scattering light illumination. It is shown that higher injection pressure significantly accelerates the spray tip penetration (STP) evolution due to increased inertia of spray while increase in ambient pressure reduces the STP evolution due to higher gas resistance. With the addition of diethyl ether (DEE) into biodiesel, the STP of blended fuel tends to go down and corresponding projected area increases a lot when compared to DB. By means of particle droplet image analysis (PDIA) optical diagnostic method, spray microscopic parameters such as Sauter Mean Diameter (SMD), droplet diameter distribution probability curve, cumulative volume curve and characteristic diameter have been investigated. Results show that both the injection pressure and ambient pressure have significant influence on the spray microscopic characteristics. In addition, for fixed injection pressure and ambient pressure, when DEE is added into DB blends, the number fraction of smaller droplets increases, though the statistic diameter with peak probability is fixed at a certain value. Furthermore, SMD of the four tested fuels decreases sequentially in the order of DB, D100, DBE, and DBDE, indicating that DEE addition favors the atomization process.

Improvement of dual-fuel biodiesel-producer gas engine performance acting on biodiesel injection parameters and strategy

Dual-fuel biodiesel-producer gas combustion has shown potential in reducing nitric oxides and particulate emission levels compared to only diesel operation; however, engine overall efficiency is slightly penalized, while the main drawbacks are represented by the higher levels of total hydrocarbons and carbon monoxide emissions.In this work, the improvements in the combustion development deriving from the splitting of the liquid fuel injection at low loads have been assessed using a 0.51L single-cylinder research diesel engine equipped with a high pressure common rail injection system and operated in dual-fuel mode. In this case, a synthetic producer gas was used as inducted gaseous fuel, while biodiesel was used as pilot fuel.Initially, the spray morphology was characterized in a constant-volume vessel for different values of injection duration and pressure, as well as vessel backpressure.Then, the experimental campaign, run on the engine at 1500rpm, was divided in two sessions. During the former, only one pilot injection of constant fuel amount (11mm3/cycle) was performed, the rail pressure was set equal to 500 or 1000bar, the injection timing was varied in the range −50÷5 degrees crank angle after top dead center while the amount of gaseous fuel inducted in the cylinder was varied on three levels. During the latter, the pilot fuel amount, kept equal to the one pilot injection tests, was split in two smaller injections and the effect of the dwell between them – varied in the range 5÷50 degrees crank angle – was investigated as well.The results of the first set of experiments revealed that pilot injection timing and pressure both affect the combustion development. This resulted in sensible variations on thermal and combustion efficiencies, and therefore on fuel conversion efficiency, the last one exhibiting higher values with pilot injection timing slightly advanced respect to top dead center and lower injection pressure. In these conditions, total hydrocarbons and carbon monoxide are lowered, while nitric oxides are increased. The amount of gas demonstrated to have a secondary effect on combustion development and emissions levels at the exhaust.Splitting pilot injection, demonstrated to be an effective way to increase fuel conversion efficiency and to reduce the levels of all the pollutant species compared to the single pilot injection strategy. Based on the extensive experimental activity described in this paper, a dwell ranging between 10 and 30 degrees of crank angle, combined with a first injection timing ranging between 35 and 20 degrees of crank angle before top dead center guarantee the highest fuel conversion efficiency and the lowest pollutants emission levels. Injection pressure confirmed to be a significant factor in affecting the combustion development, while a secondary effect was determined by the gaseous mass inducted in the cylinder.Ultimately, pilot injection splitting demonstrated to be an effective way for improving gaseous fuel combustion in dual-fuel mode at low load (lean mixture) conditions.

Effects of injection pressure on ignition and combustion characteristics of diesel in a premixed methanol/air mixture atmosphere in a constant volume combustion chamber

The experiments on the ignition and combustion characteristics of diesel in air atmosphere (AA) and premixed methanol/air mixture atmosphere (MAA) were conducted in a constant volume combustion chamber (CVCC) equipped with a high pressure common-rail injection system. The ignition and combustion processes were recorded by a high-speed camera and a combustion pressure acquisition system. Results show that with the increase of injection pressure from 40 to 160MPa, the flame lift-off length (FLoL) becomes longer; and the ignition delay is shortened from 2.80ms to 2.03ms (AA) and 2.90ms to 2.27ms (MAA) respectively; and correspondingly the combustion duration is shortened from 5.17ms to 2.47ms (AA) and 4.83ms to 2.13ms (MAA). Compared those in AA, the ignition delay is prolonged by 0.14ms on average, while the combustion duration is shortened by 0.27ms in MAA. At high injection pressure, the moments that the maximum combustion pressure occurs and the apparent heat release rates (AHRR) starts to rise are advanced; while in MAA both of them are delayed; and the peak value of AHRR increases. With the increase of injection pressure, both spatially integrated natural luminosity (SINL) and time integrated natural luminosity (TINL) reduce significantly, but the reduction trends of TINL weaken. Due to the addition of methanol, the FLoL of diesel becomes longer, and the SINL and TINL in MAA are both lower than those in AA.

Experimental study on the effect of injector nozzle K factor on the spray characteristics in a constant volume chamber: Near nozzle spray initiation, the macroscopic and the droplet statistics

In this work, the effects of the injector K factor on the spray characteristics have been investigated by using a high pressure common rail injection system and a constant volume chamber. The near nozzle region spray tip behavior, the spray macroscopic characteristics, and the droplet statistics in a fixed spray location is sequentially examined. Specifically, in the near nozzle region, laser pulse illumination and microscopic imaging technique was used to capture the very initial spray evolution morphology and it is found to take on the form of a mushroom like structure which quickly breaks up. Increasing K factor advances the spray tip evolution and favors the formation of the shockwave which might be caused the accelerated nozzle inner flow. For different K factor nozzles, the spray tip penetration (STP) evolution before the primary breakup is all found to include two sub-stages: in the very initial stage when the needle valve open is relatively small, the breakup of the mushroom like tip results in a slightly decelerated STP evolution. Subsequently, when the needle valve is fully opened, the STP evolution is gradually accelerated by the increased mass flux. The classic Hiroyasu and Arai [1] empirical model, which shows constant STP evolution speed before the primary breakup, is then refined to account this unsteady STP evolution behavior. Secondly, the spray macroscopic behavior data such as the spray tip penetration and the spray cone angle from varying K factor nozzles were collected by high speed visualization. It is found that as the K factor is increased, the STP evolution is accelerated and spray cone angle decreases. The classic model for the STP by Hiroyasu and Arai [1] which is applied K=0 nozzle was extended to accommodate the effect of K factor on the STP evolution after the primary breakup and the revised empirical model show perfect performance in predicting the STP behavior after the primary breakup for all different K factor nozzles. Finally, by using the particle/droplet image analysis (PDIA) system, the droplet statistics and the Sauter Mean Diameter (SMD) that characterize the atomization behavior at a certain spray location were obtained. It is seen that high K factor nozzle yields a slightly smaller SMD and it’s believed to enhance the atomization process. However, our experimental results show that this K nozzle effect is relatively weak and it vanishes for nozzles with larger outlet diameter.

Effects of Injection Rate Profile on Combustion Process and Emissions in a Diesel Engine

When multi-injection is implemented in diesel engine via high pressure common rail injection system, changed interval between injection pulses can induce variation of injection rate profile for sequential injection pulse, though other control parameters are the same. Variations of injection rate shape which influence the air-fuel mixing and combustion process will be important for designing injection strategy. In this research, CFD numerical simulations using KIVA-3V were conducted for examining the effects of injection rate shape on diesel combustion and emissions. After the model was validated by experimental results, five different shapes (including rectangle, slope, triangle, trapezoid, and wedge) of injection rate profiles were investigated. Modeling results demonstrate that injection rate shape can have obvious influence on heat release process and heat release traces which cause different combustion process and emissions. It is observed that the baseline, rectangle (flat), shape of injection rate can have better balance between NOx and soot emissions than the other investigated shapes. As wedge shape brings about the lowest NOx emissions due to retarded heat release, it produces the highest soot emissions among the five shapes. Trapezoid shape has the lowest soot emissions, while its NOx is not the highest one. The highest NOx emissions were produced by triangle shape due to higher peak injection rate.

Effect of di-n-butyl ether blending with soybean-biodiesel on the near-nozzle spray characteristics

In this work, the near-nozzle spray characteristics of soybean biodiesel, di-n-butyl ether (DBE)/biodiesel blends and 0# diesel were investigated by using a high pressure common-rail injection system and the effect of experiment conditions and fuel physical properties on the near-nozzle spray structures were explored. Microscopic spray images at the near-nozzle field were captured by the high-resolution microscopy. The micro spray cone angle and micro projected spray area were obtained from the processed spray images for quantitative analysis. The results show that the initial spray of diesel is easier to breakup into ligaments and droplets because of smaller surface tension and viscosity compared to that of biodiesel under atmospheric condition. The spray with more ligaments was easily restrained by the ambient gas and promoted to evolve along the radial direction, leading to a wider micro spray cone angle of diesel. The breakup of liquid jet also tends to increase the micro projected spray area of diesel. When 30% of DBE was added into biodiesel, both the micro spray cone angle and projected spray area were increased due to the decreased surface tension and viscosity of the blended fuel. The initial spray images reveal that the high injection pressure favors the spray primary breakup while the high ambient pressure results in worse spray primary breakup. The quantitative analysis shows that the injection pressure has little influence on the micro spray cone angle but results in the increase in micro projected spray area, while both parameters increase significantly under high ambient pressures.

ESO-based iPI Common Rail Pressure Control of High Pressure Common Rail Injection System

ESO-based iPI Common Rail Pressure Control of High Pressure Common Rail Injection System

Experimental investigation on spray and atomization characteristics of diesel/gasoline/ethanol blends in high pressure common rail injection system

The effects of gasoline and ethanol addition on the spray and atomization characteristics of diesel spray from a common rail injection system, was investigated in a constant volume chamber at different ambient and injection pressures. The fuel spray evolution process was recorded with high spatial and time resolution and the corresponding spray tip penetration (STP), the cone angle were extracted. The results show that increased gasoline proportion in the test blends causes decreased STP and increased spray cone angle. Additionally, the microscopic spray characteristics such as the local average droplet diameter and statistical size distributions were measured by particle/droplet image analysis (PDIA) technique. As gasoline blending ratio increases, significantly smaller droplet size was observed which indicates that the spray breakup and atomization processes were promoted. Further adding ethanol slightly increased the droplet size, but it is still much smaller than the droplet size of neat diesel spray. Moreover, the local droplet size increases along the radial direction but the local droplet volume fraction and normalized droplet number density decreases, indicating a reduced fuel concentration along the radial direction. Along the axial direction, the droplet size, the local droplet volume fraction and the normalized droplet number density were almost constant.

High pressure common rail injection system modeling and control

In this paper modeling and common-rail pressure control of high pressure common rail injection system (HPCRIS) is presented. The proposed mathematical model of high pressure common rail injection system which contains three sub-systems: high pressure pump sub-model, common rail sub-model and injector sub-model is a relative complicated nonlinear system. The mathematical model is validated by the software Matlab and a virtual detailed simulation environment. For the considered HPCRIS, an effective model free controller which is called Extended State Observer – based intelligent Proportional Integral (ESO-based iPI) controller is designed. And this proposed method is composed mainly of the referred ESO observer, and a time delay estimation based iPI controller. Finally, to demonstrate the performances of the proposed controller, the proposed ESO-based iPI controller is compared with a conventional PID controller and ADRC.

Experimental study on injection characteristics of diesel-bioethanol fuel blends

This article presents the test results of injection processes of diesel-bioethanol fuel blends on a high pressure common rail injection system. The injection characteristics were analyzed using the injection rate measuring instrumentation. The injection rate, cycle injection quantity, injection delay and injection duration were analyzed across a range of injection pressure and injection energizing time. As the results show, the peak injection rate and delay of diesel-ethanol blends are lower compared to diesel fuel. The injection duration and discharge coefficients of diesel-ethanol blends were lower than those of diesel fuel. It was observed that fuel density and fuel viscosity have significant influence on the injection characteristics.

Effect of Premixed Fuel Preparation for Partially Premixed Combustion With a Low Octane Gasoline on a Light-Duty Multicylinder Compression Ignition Engine

Gasoline compression ignition (GCI) concepts with the majority of the fuel being introduced early in the cycle are known as partially premixed combustion (PPC). Previous research on single- and multicylinder engines has shown that PPC has the potential for high thermal efficiency with low NOx and soot emissions. A variety of fuel injection strategies have been proposed in the literature. These injection strategies aim to create a partially stratified charge to simultaneously reduce NOx and soot emissions while maintaining some level of control over the combustion process through the fuel delivery system. The impact of the direct injection (DI) strategy to create a premixed charge of fuel and air has not previously been explored, and its impact on engine efficiency and emissions is not well understood. This paper explores the effect of sweeping the direct injected pilot timing from −91 deg to −324 deg ATDC, which is just after the exhaust valve closes (EVCs) for the engine used in this study. During the sweep, the pilot injection consistently contained 65% of the total fuel (based on command duration ratio), and the main injection timing was adjusted slightly to maintain combustion phasing near top dead center. A modern four cylinder, 1.9 l diesel engine with a variable geometry turbocharger (VGT), high pressure common rail injection system, wide included angle injectors, and variable swirl actuations was used in this study. The pistons were modified to an open bowl configuration suitable for highly premixed combustion modes. The stock diesel injection system was unmodified, and the gasoline fuel was doped with a lubricity additive to protect the high pressure fuel pump and the injectors. The study was conducted at a fixed speed/load condition of 2000 rpm and 4.0 bar brake mean effective pressure (BMEP). The pilot injection timing sweep was conducted at different intake manifold pressures, swirl levels, and fuel injection pressures. The gasoline used in this study has relatively high fuel reactivity with a research octane number of 68. The results of this experimental campaign indicate that the highest brake thermal efficiency (BTE) and lowest emissions are achieved simultaneously with the earliest pilot injection timings (i.e., during the intake stroke).

Effect of di-n-butyl ether blending with soybean-biodiesel on spray and atomization characteristics in a common-rail fuel injection system

In this work, the spray and atomization characteristics of soybean biodiesel, di-n-butyl ether (DBE)/biodiesel blends and 0# diesel were investigated by using a high pressure common-rail injection system. The macroscopic spray characteristics such as the spray tip penetration (STP), the cone angle and the spray projected area were obtained from the spray images captured through high speed schlieren photography. The results show that as DBE is blended into biodiesel, the STP is decreased and the spray cone angle, projected area are increased and when the DBE volume fraction in the DBE/biodiesel blends reaches 30%, the STP, the spray cone angle and the projected area are comparable to that of diesel. For all the tested fuels, the microscopic spray characteristics such as the Sauter Mean Diameter (SMD) and statistical size distributions were measured by particle/droplet image analysis (PDIA) technique. The droplets number density distribution shows that for all the tested fuels, the region near the central of spray has the largest droplets number density and it decreases sharply as the position shifts from the center to the edge of the spray. As DBE is blended into biodiesel, smaller SMD is observed, which indicates that DBE addition can promote the atomization of biodiesel.

가시화 엔진을 이용한 직접 분사식 압축착화 디젤엔진에서 폐식용유 바이오디젤과 디젤의 분무 및 화염 특성 비교

Spray and combustion process with waste cooking oil (WCO) biodiesel and commercial diesel were analyzed in an optically-accessible single-cylinder compression ignition diesel engine equipped with a high pressure common-rail injection system. Direct imaging method was applied to investigate spray and combustion characteristics. From the mie-scattering results, it was verified that WCO biodiesel had a longer injection delay compared to diesel. Spray tip penetration length of WCO biodiesel was longer and spray angle was narrower than those of diesel due to poor atomization characteristics. In terms of combustion, WCO biodiesel showed later start of combustion, while flame was vanished more rapidly. Analysis of flame luminosity showed that WCO biodiesel combustion had lower intensity and lasted for shorter duration.

Experimental study on injection characteristics of fatty acid esters on a diesel engine common rail system

The injection processes of three fatty acid esters (methyl laurate, methyl oleate and ethyl oleate) are investigated on a high pressure common rail injection system and compared to that of diesel fuel. The cycle injection rate, cycle injection quantity and pressure fluctuation at the injector inlet during and after the injection event are studied across a range of injection pressure and injector energizing time. Test fatty acid esters show smoother rising slopes at the start of injection and lower injection rates at the stable injection period in the volumetric injection rate curves, but the mass injection rates among all test fuels are quite close. Fatty acid esters have longer injection delay than diesel fuel; while increased injection pressure causes reduced injection delay but prolonged injection duration. Injector energizing time significantly influences the shape of injection rate curve and the pressure fluctuation at the injector inlet. After the needle valve closure, pressure oscillation damps more rapidly for methyl oleate and ethyl oleate, due to their high density, viscosity and bulk modulus of compressibility.

飞秒激光高精细加工柴油机喷油嘴倒锥孔法

Femtosecond laser processing has litle-thermal effects,high precision,and is not particular about materials.Aiming at the machining requirement for nozzle′s spray holes,as a key part of the high pressure common rail injection system for diesel engine,a femtosecond laser processing apparatus with three-wedge scanning system was designed for technical research of taper holes′machining.Then tests of the taper holes′physical dimensions and the flow rate of the fuel jet nozzles were carried out with three coordinates measuring machine(measurement accuracy 1.2μm)and high-pressure liquid flow test bench for the injector(measurement accuracy ±0.1%).The experimental results show that the designed apparatus can machine nozzles,and the performance is able to meet the application requirement of Euro V emission regulations.The technique has important significance for improving the level of automobile industry and domestic emission standard of diesel engined car.

Combustion process of JP-8 and fossil Diesel fuel in a heavy duty diesel engine using two-color thermometry

An experimental study was performed to analyze the combustion processes of JP-8 and fossil Diesel fuel in an optically-accessible single-cylinder heavy-duty diesel engine equipped with a high pressure common-rail injection system. In terms of emission, JP-8 emitted less smoke with more HC and NOx. Direct imaging and two-color thermometry were applied to verify the emission trend for both fuels. The combustion process was characterized by means of image analysis focusing on the luminosity intensity and its spatial distribution (flame spatial fluctuation (FSF) and flame non-homogeneity (FNH)). The results from the two-color thermometry were analyzed by the flame temperature and KL factor distribution. From the combustion process analysis of the direct imaging, it was verified that JP-8 had a longer ignition delay compared to fossil Diesel fuel regardless of injection pressure. However, flame luminosity of JP-8 was vanished more rapidly. The flame luminosity intensity analysis showed that fossil Diesel fuel had stronger flame luminosity overall and duration of visible flame luminosity was longer than JP-8. This implies that fossil Diesel fuel had more diffusion dominant combustion. From the flame luminosity variation rate analysis, decreasing rate of flame luminosity for JP-8 was higher compared with fossil Diesel fuel, showing that oxidation rate of JP-8 was much higher than fossil Diesel fuel. From FSF and FNH analysis, JP-8 showed lower value for both FSF and FNH in the later stage of combustion, because the later stage of combustion with JP-8 has less jet structure in comparison with fossil Diesel fuel. The flame temperature field from two-color thermometry showed that locally high temperature region existed with JP-8. KL factor distribution of JP-8 was distributed more uniformly with a relatively lower level of KL intensity in comparison with fossil Diesel fuel in the late stage of combustion.

Application of JP-8 in a heavy duty diesel engine

An experimental study was carried out to investigate the combustion and emission of JP-8 and diesel fuel in an optically-accessible single-cylinder heavy duty diesel engine equipped with a high pressure common-rail injection system in order to evaluate the feasibility of JP-8 application in diesel engine. The basic spray characteristics, including spray tip penetration and spray angle, were investigated with macroscopic spray images obtained by the Mie-scattering method. The combustion and emission characteristics were analyzed on the basis of the results obtained from the spray experiments. Visualization by direct imaging was used to characterize the combustion process. It was found that JP-8 had a shorter spray tip penetration and wider spray angle than diesel fuel mainly due to the faster vaporization characteristic of JP-8. The peak heat release rate was higher and the premixed burn portion was larger with JP-8 due to its superior mixing rate through faster vaporization characteristics. Furthermore, ignition delay with JP-8 was longer than that with diesel fuel due to the lower cetane number of JP-8. In terms of emission, JP-8 showed a benefit in smoke reduction while it produced larger amounts of HC and NO x . Longer ignition delay and accelerated oxidation in the late stage of JP-8 combustion were verified by direct imaging.