Direct Injection Combustion Research Articles

Numerical investigation of mixture formation and combustion in a hydrogen direct injection plus natural gas port injection (HDI + NGPI) rotary engine

Hydrogen direct injection (HDI) in cylinder is considered as an effective method to improve natural gas engine performance. The present study aims to bridge the gap on the HDI in rotary engine, and to investigate the effect of hydrogen injection timing (IT) and hydrogen injection duration (ID) on mixture formation and combustion process of a hydrogen direct injection plus natural gas port injection (HDI + NGPI) rotary engine. Numerical approach was used in this study for obtaining some critical information, which was difficult to obtain through experiment, such as flow field, fuel distribution and some intermediate concentration fields in cylinder. The research results showed that for mixture formation, the distribution law of the hydrogen and the natural gas at the late stages of the compression stroke (100°CA (BTDC)), was as follows: at a fixed ID of 24°CA, with retarded hydrogen IT, the stratification phenomenon of hydrogen became obvious increasingly, and the hydrogen distribution area moved towards the back of the combustion chamber continuously. At a fixed IT of 210°CA (BTDC), with the extension in ID, the accumulation area of hydrogen reduced significantly, and the hydrogen continued to gather in the middle of the combustion chamber. For combustion process, the overall combustion rate for the hydrogen injection strategy which had an IT of 210°CA (BTDC) and ID of 40°CA (case ID5), was the fastest. This was due to the fact that compared with the leading spark plug (LSP), the combustion condition around the trailing spark plug (TSP) has a great influence on the combustion process. For case ID5 at ignition timing, the hydrogen concentration near the TSP is high enough for the rapid formation of flame kernel. Compared with case IT1 which had an IT of 390°CA (BTDC) and an ID of 24°CA, the improved combustion rate of case ID5 had a 11.7% increase in peak pressure, and a 7% decrease in NO emissions.

Alcohol fuels for spark-ignition engines: Performance, efficiency and emission effects at mid to high blend rates for binary mixtures and pure components

The paper evaluates the results of tests performed using mid- and high-level blends of the low-carbon alcohols, methanol and ethanol, in admixture with gasoline, conducted in a variety of test engines to investigate octane response, efficiency and exhaust emissions, including those for particulate matter. In addition, pure alcohols are tested in two of the engines, to show the maximum response that can be expected in terms of knock limit and efficiency as a result of the beneficial properties of the two alcohols investigated. All of the test work has been conducted with blending of the alcohols and gasoline taking place outside the combustion system, that is, the two components are mixed homogeneously before introduction to the fuel system, and so the results represent what would happen if the alcohols were introduced into the fuel pool through a conventional single-fuel-pump (dispenser) approach. While much has been written on the effect of blending the light alcohols with gasoline in this way, the results present significant new findings with regard to the effect of the enthalpy of vaporization of the alcohols in terms of particulate exhaust emissions. Also, in one of the tests, two mid-level blends are tested in a highly downsized prototype engine – these blends being matched for stoichiometry, enthalpy of vaporization and volumetric energy content. The consequences of this are discussed and the results show that this approach to blend formulation creates fuels which behave in the same manner in a given combustion system; the reasons for this are discussed. One set of tests using pure methanol alternately with cooled exhaust gas recirculation and with excess air shows a significant increase in thermal efficiency that can be expected as the blend level is increased. The effect on nitrous oxide emissions is shown to be similarly beneficial, this being primarily a result of the enthalpy of vaporization of the alcohol cooling the charge coupled with the lower adiabatic flame temperature of the alcohol. Whereas it is normally a beneficial characteristic in spark-ignition combustion systems, one disadvantage of a high value of enthalpy of vaporization is shown in another series of tests in that, in admixture with gasoline, it is a driver on flash boiling of the hydrocarbon component in the blend in direct injection combustion systems. In turn, this causes the particulate emissions of the engine to increase quite markedly over those of ethanol–gasoline blends at the same stoichiometry. The paper shows for the first time the dichotomy of the potential efficiency improvement with the challenge of particulate control. This effect poses a challenge for the future introduction and use of such high blend rate fuels in engines without particulate filters. Although it must be stated that the overall particulate emissions of the methanol–gasoline blend are lower than for gasoline, the effect is only present at extremely low load, and that there is a likelihood that particulate filters will be adopted in production vehicle anyway, nullifying this issue.

Preliminary aerodynamic design methodology for aero engine lean direct injection combustors

ABSTRACTThe Lean Direct Injection (LDI) combustor is one of the low-emissions combustors with great potential in aero-engine applications, especially those with high overall pressure ratio. A preliminary design tool providing basic combustor sizing information and qualitative assessment of performance and emission characteristics of the LDI combustor within a short period of time will be of great value to designers. In this research, the methodology of preliminary aerodynamic design for a second-generation LDI (LDI-2) combustor was explored. A computer code was developed based on this method covering the design of air distribution, combustor sizing, diffuser, dilution holes and swirlers. The NASA correlations for NOxemissions are also embedded in the program in order to estimate the NOx production of the designed LDI combustor. A case study was carried out through the design of an LDI-2 combustor named as CULDI2015 and the comparison with an existing rich-burn, quick-quench, lean-burn combustor operating at identical conditions. It is discovered that the LDI combustor could potentially achieve a reduction in liner length and NOxemissions by 18% and 67%, respectively. A sensitivity study on parameters such as equivalence ratio, dome and passage velocity and fuel staging is performed to investigate the effect of design uncertainties on both preliminary design results and NOxproduction. A summary on the variation of design parameters and their impact is presented. The developed tool is proved to be valuable to preliminarily evaluate the LDI combustor performance and NOxemission at the early design stage.

Numerical and experimental analysis of the spray momentum flux measuring on a GDI injector

In direct injection combustion systems, the spray momentum flux measurement can provide significant insight in the fuel jet development and the in-cylinder fuel-air mixing potential. The spray momentum flux can be determined by means of the impact force method, which was proved to be completely consistent when the basic measuring hypotheses are fulfilled, i.e. during the steady phase of the injection process. Conversely, in the transients the measurement technique details can significantly affect the results. An appropriate analysis of the possible effects exerted by the experimental setup is thus mandatory. In order to deepen the knowledge about this measurement technique applied to GDI (Gasoline Direct Injection) systems and to support the design of the experiment, in this paper a CFD 3-D model of a single-hole, research injector was developed and assessed with experimental data in terms of spray penetration curve, overall shape and droplet sizing and velocity.The validated numerical tool was then used to simulate the spray momentum measuring procedure in order to investigate the possible effects of the experimental set-up details on the momentum flux results. To this end, numerical results were compared to experimental data for different values of the main measurement parameters: target size, nozzle/target distance, discharge ambient pressure. This analysis confirmed both the experimental procedure sensitivity to some setup details and the model ability to capture the spray-target interaction phenomenon, supporting the need of a combined use of numerical and experimental approaches to obtain an adequate insight in the spray evolution.

Simulation Investigation on Combustion Characteristics in a Four-Point Lean Direct Injection Combustor with Hydrogen/Air

To investigate the combustion characteristics in multi-point lean direct injection (LDI) combustors with hydrogen/air, two swirl–venturi 2 × 2 array four-point LDI combustors were designed. The four-point LDI combustor consists of injector assembly, swirl–venturi array and combustion chamber. The injector, swirler and venturi together govern the rapid mixing of hydrogen and air to form the mixture for combustion. Using clockwise swirlers and anticlockwise swirlers, the co-swirling and count-swirling swirler arrays LDI combustors were achieved. Using Reynolds-Averaged Navier–Stokes (RANS) code for steady-state reacting flow computations, the four-point LDI combustors with hydrogen/air were simulated with an 11 species and 23 lumped reaction steps H2/Air reaction mechanism. The axial velocity, turbulence kinetic energy, total pressure drop coefficient, outlet temperature, mass fraction of OH and emission of pollutant NO of four-point LDI combustors, with different equivalence ratios, are here presented and discussed. As the equivalence ratios increased, the total pressure drop coefficient became higher because of increasing heat loss. Increasing equivalence ratios also corresponded with the rise in outlet temperature of the four-point LDI combustors, as well as an increase in the emission index of NO EINO in the four-point LDI combustors. Along the axial distance, the EINO always increased and was at maximum at the exit of the dump. Along the chamber, the EINO gradually increased, maximizing at the exit of chamber. The total temperature of four-point LDI combustors with different equivalence ratios was identical to the theoretical equilibrium temperature. The EINO was an exponential function of the equivalence ratio.

Simulation of combustion characteristics in a hydrogen fuelled lean single-element direct injection combustor

Combustion characteristics in a hydrogen fuelled single-element lean direct injection (LDI) combustor for low emission gas turbine engine are computationally investigated. A single-element LDI combustor was produced and simulated with steady-state Reynolds-averaged Navier–Stokes reacting computations. Reacting computations of the single-element LDI combustor were performed with an augmented reduced 23-step reaction mechanism for hydrogen/air. A realizable k − ε model was used to obtain turbulence closure. For the single-element LDI combustor with different equivalence ratio, the effective area, axial velocity, total pressure drop coefficient, total temperature, mass fraction of OH, emission index of pollutant NO (EINO) were achieved and discussed.

Development of a High Turbulence, Low Particle Number, High Injection Pressure Gasoline Direct Injection Combustion System

Development of a High Turbulence, Low Particle Number, High Injection Pressure Gasoline Direct Injection Combustion System

Evaluating the Potential of a Direct-Injection Constant-Volume Combustion Chamber as a Tool to Validate Chemical-Kinetic Models for Liquid Fuels

ABSTRACTChemical-kinetic mechanisms for low-volatility hydrocarbons and alternative fuels are mostly scarce or not validated, primarily due to the lack of a convenient experimental platform to investigate fundamental ignition and combustion behavior for such fuels. This work reports experimental measurements of ignition delays and related combustion behavior of n-heptane in a direct-injection constant-volume combustion chamber, for pressures ranging from 0.2 MPa to 1.0 MPa and temperatures ranging from approximately 390°C to 500°C while holding the global equivalence ratio constant at 0.62. Results showed clear two-stage ignition and negative temperature coefficient behavior, and data at higher pressures agreed closely with predictions from a perfectly-stirred-reactor model. At lower pressures, impingement of liquid fuel on the chamber wall likely caused experimental data to deviate significantly from model predictions. Overall, this work strongly suggests that this experimental platform can be used to validate chemical kinetics of liquid fuels.

Development of a three-step hybrid simulation approach (THSA) for engine combustion investigation coupled with a multistep phenomenon soot model and energy balance analysis

A comprehensive simulation approach is important in order to better replicate the complex combustion process over a wide range of engine operating conditions. This will allow a more accurate understanding of crucial factors that affect engine combustion. In this study, the entire engine combustion process was systematically considered and a three-step hybrid simulation approach (THSA) was proposed in order to achieve a more accurate engine combustion simulation. For this approach, a so-called “full cavitation” model was selected for 3-dimensional (3D) internal nozzle flow study and a Kelvin-Helmholtz/Rayleigh-Taylor (KH-RT) model was used for 3D spray prediction wherein the flow variables at the nozzle outlet obtained through internal nozzle flow simulations were used as the input information in the KIVA4 code. Besides, a compact and accurate primary reference fuel (PRF) mechanism with 46 species and 144 reactions, which is coupled with a multi-step phenomenon soot model and energy balance analysis, was used for engine combustion simulation. Based on it, a numerical study was conducted for a comparison between conventional direct injection combustion (CDIC) and partially premix combustion (PPC) fueled with diesel, gasoline and diesel/gasoline blend fuel (GD). The final result indicates that with PPC and gasoline fuel, an optimized and high thermal efficiency of 52.5% can be realized along with extremely low NOx and soot emissions.

Investigation of Performance and Emission Characteristics of VCR Engine by Using Ceramic Coating on the Piston Crown with Using Methanol Fuel

The world faces the energy demand, exaggerated crude rates and reduce of fuel resources. Methanol has been considered as an alternate fuel. . The current study takes out an experiment of using methanol in VCR engine. They are blended with diesel at various mixing ratios. The results of methyl alcohol works in engine were checked in an exceedingly single cylinder, direct injection, water cooled and high speed VCR internal combustion engine at varied engine load with constant engine speed of 1500rpm conjointly we tend to mistreatment the limit coated piston with MgO-ZrO2 and Al2O3-13%TiO2 to investigation on performance and emission characteristics. The limit coating materials has low thermal conduction, low thermal expansion and high young’s modules. The influences of blends on CO, CO2, HC, NOX were investigated by emission tests. The brake thermal efficiency of mix M20 was found on top of diesel. The emissions of smoke, hydro carbon and chemical element oxides of methyl alcohol blends were on top of that of diesel. However the CO2 emission is higher once we compare with diesel.

Lower particulate matter emissions with a stoichiometric LPG direct injection engine

The direct injection (DI) engine is considered as a promising technology that can improve fuel economy compared to the conventional port fuel injection engine. However, studies have shown that relatively high levels of particulate matter (PM) are emitted by gasoline fuel engines. This is a concern in view of more stringent emission regulations expected in the future. In this respect, the use of Liquefied Petroleum Gas (LPG) in direct injection engines has the following advantages: (1) lower carbon dioxide (CO2) emissions due to its low carbon content; and (2) reduced PM emission because it is a premixed mixture. This study used an LPG direct injection engine that was converted from a 2000-cm3, 4-cylinder gasoline port fuel injection (PFI) engine. The engine head was redesigned for the spray-guided direct injection combustion system. The conventional LPG for transportation in South Korea was used as the fuel, and the engine was operated under stoichiometric condition. In this study, the engine operation map that covers the entire operating envelope was provided to investigate its particle number concentrations and exhaust emissions, including total hydrocarbon (THC), carbon monoxide (CO), carbon dioxide (CO2) and nitrogen oxides (NOx) as a function of engine operating points. Also, the particle number size distribution was shown as a function of engine operating conditions. The results of this study indicated that a lower occurrence of THC and NOx emissions compared to previous gasoline studies. The main finding in this study was that particulate matter emissions of a LPG direct injection engine were substantially lower than those from a gasoline direct injection engine.

Experimental investigation of spray characteristics of a modified bio-diesel in a direct injection combustion chamber

Macroscopic and microscopic characteristics of spray of a Modified Bio-diesel Fuel (MBF), applying direct injection system have been explored and compared with those of conventional diesel fuel. MBF is a new combination of bio-diesel, molasses bio-ethanol, and water, which has been kept as a single-phase bio-fuel, employing an emulsifier. Lower emissions and production costs, higher oxygen content and cetane number are the key advantages of the MBF to be replaced by conventional fossil fuels in internal combustion engines. Applying atomization model, the spray atomization properties such as Ohnesorge number and Sauter Mean Diameter (SMD) have been investigated. Air entrainment analysis has been studied employing quasi-steady jet theory. Furthermore, considering physics of atomization of droplets and employing dimensional analysis, a new non-dimensional number namely Atomization Index (AI) has been developed. AI number, which is the ratio of square of inertia forces to product of viscous and surface tension forces, can predict the atomization level of the spray. Results show that increasing AI number decreases SMD and improves atomization behavior of the spray as well. Since outcomes of Ohnesorge and AI numbers verify each other, and AI number can clarify the relative atomization level of different cases without drawing a diagram, AI as a proper number can be applied in further studies.

Control of the low-load region in partially premixed combustion

Partially premixed combustion (PPC) is a low temperature, direct-injection combustion concept that has shown to give promising emission levels and efficiencies over a wide operating range. In this concept, high EGR ratios, high octane-number fuels and early injection timings are used to slow down the auto-ignition reactions and to enhance the fuel and are mixing before the start of combustion. A drawback with this concept is the combustion stability in the low-load region where a high octane-number fuel might cause misfire and low combustion efficiency. This paper investigates the problem of low-load PPC controller design for increased engine efficiency. First, low-load PPC data, obtained from a multi-cylinder heavy- duty engine is presented. The data shows that combustion efficiency could be increased by using a pilot injection and that there is a non-linearity in the relation between injection and combustion timing. Furthermore, intake conditions should be set in order to avoid operating points with unfavourable global equivalence ratio and in-cylinder temperature combinations. Model predictive control simulations were used together with a calibrated engine model to find a gas-system controller that fulfilled this task. The findings are then summarized in a suggested engine controller design. Finally, an experimental performance evaluation of the suggested controller is presented.

Investigation of reactivity-controlled compression ignition combustion in a two-stroke engine

In this study, a two-stroke outboard engine was modified to accommodate two direct fuel injectors for reactivity-controlled compression ignition combustion in one cylinder, while the production direct-injection spark ignition combustion system was maintained in the other cylinder. This setup enabled direct comparisons of the performances at equivalent operating conditions. The engine’s octane requirement for homogeneous charge compression ignition combustion was studied with primary reference fuels for ranges of engine speed, load, and delivery ratio. The resulting primary reference fuel requirement was used to determine the baseline ratio of low-reactivity fuel to high-reactivity fuel. Experiments using gasoline and diesel fuel were unsuccessful as they resulted in unstable combustion and rapid accumulation of particulate matter in the emissions-sampling equipment. Reactivity-controlled compression ignition experiments with gasoline and n-heptane (higher volatility) proved to be successful. The low-reactivity fuel fraction and the high-reactivity start-of-injection timing were found to be independent combustion-control levers. At 1500 r/min, an indicated mean effective pressure of 2.5 bar and nitrogen oxides emissions of 1.25 g/kW h, reactivity-controlled compression ignition resulted in a lower coefficient of variation in the indicated mean effective pressure, lower carbon monoxide emissions and a significantly higher gross indicated efficiency than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (36.3% versus 27.0% and 25.7% respectively); at 1200 r/min, an indicated mean effective pressure of 2.0 bar and hydrocarbon + nitrogen oxide emissions of about 16.5 g/kW h, the reactivity-controlled compression ignition efficiency was still significantly better than those for the direct-injection spark ignition homogeneous mode and the direct-injection spark ignition stratified mode (32.9% versus 25.2% and 26.6% respectively). Overall, the viability of reactivity-controlled compression ignition combustion in a two-stroke engine was demonstrated; with further design optimization, it will probably be possible to use a standard diesel fuel instead of n-heptane.

Experimental Study on In-Cylinder Pressure Oscillations of Homogenous Charge Compression Ignition–Direct Injection Combustion Engine Fueled With Dimethyl Ether

The in-cylinder pressure oscillations of a homogeneous charge compression ignition (HCCI)-DI engine fueled with dimethyl ether (DME) have been investigated using discrete wavelet transform (DWT) based on four different wavelet functions. The in-cylinder pressure is decomposed into three levels that contain three details D1, D2, and D3, and an approximation A1. In normal combustion, there are no obvious pressure impacts in three details due to smooth combustion process. The abnormal pressure oscillations occur in three details in knocking combustion, and the oscillation is most intense in D1. Its frequency band 5–10 kHz is the knock frequency band, and most high-frequency pressure oscillations and wavelet energy are in this frequency band. The pressure oscillations are located in the premixed combustion stage and diffusion combustion stage. Characteristics of in-cylinder pressure oscillations can be extracted using four wavelet functions “db4,” “db8,” “sym4,” and “sym8.” Extract abilities of four wavelet functions are different and wavelet db4 is suitable for pressure oscillation detection.

Effect of injection timing on mixture formation and combustion in an ethanol direct injection plus gasoline port injection (EDI+GPI) engine

Ethanol direct injection plus gasoline port injection (EDI+GPI) is a new technology to utilise ethanol fuel more effectively and efficiently in spark-ignition engines by taking the advantages of ethanol fuel and direct injection, such as the cooling effect and anti-knock ability. A full cycle numerical modelling including both port and direct injection sprays was performed to understand the mechanisms behind the experimental results of the EDI+GPI engine. The turbulence-chemistry interaction of the two-fraction-mixture partially premixed combustion was solved by a five-dimensional presumed Probability Density Function table. Effects of direct injection timing on fuel evaporation, mixing, wall-wetting, combustion and emission processes were investigated. The results showed that when the direct injection timing was retarded, the mixture around the spark plug became leaner and the distribution of equivalence ratio became more uneven. Moreover, late direct injection resulted in severe fuel impingement and caused local over-cooling effect and over-rich mixture. Consequently, the combustion speed and temperature were decreased by retarded direct injection timing, leading to reduced NO emission and increased HC and CO emissions. Finally, numerical modelling was performed to investigate the strategy of injecting small amount of ethanol fuel on reducing the fuel impingement and incomplete combustion caused by late direct injection.

Sensors Based Measurement Techniques of Fuel Injection and Ignition Characteristics of Diesel Sprays in DI Combustion System

Innovative sensor based measurement techniques like needle lift sensor, photo (optical) sensor and piezoresistive pressure transmitter are introduced and used to measure the injection and combustion characteristics in direct injection combustion system. Present experimental study is carried out in the constant volume combustion chamber to study the ignition, combustion and injection characteristics of the solid cone diesel fuel sprays impinging on the hot surface. Hot surface ignition approach has been used to create variety of advanced combustion systems. In the present study, the hot surface temperatures were varied from 623K to 723K. The cylinder air pressures were 20, 30 and 40bar and fuel injection pressures were 100, 200 and 300bar. It is found that ignition delay of fuel sprays get reduced with the rise in injection pressure. The ignition characteristics of sprays much less affected at high fuel injection pressures and high surface temperatures. The fuel injection duration reduces with the increase in fuel injection pressures. The rate of heat release becomes high at high injection pressures and it decreases with the increase in injection duration. It is found that duration of burn/combustion decrease with the increase in injection pressure. The use of various sensors is quite effective, reliable and accurate in measuring the various fuel injection and combustion characteristics. The study simulates the effect of fuel injection system parameters on combustion performance in large heavy duty engines.

Multi-objective Numerical Investigation of a Generic Airblast Injector Design

In combustor design for aero-engines, engineers face multiple opposing objectives with strict constraints. The trend toward lean direct injection (LDI) combustors suggests a growing emphasis on injector design to balance these objectives. Decades of empirical and analytical work have produced low-order methods, including semi-empirical and semi-analytical correlations and models of combustors and their components, but detailed modeling of injector and combustor behavior requires computational fluid dynamics (CFD). In this study, an application of low-order methods and published guidelines yielded generic injector and combustor geometries, as well as CFD boundary conditions of parameterized injector designs. Moreover, semi-empirical correlations combined with a numerical spray combustion solver provided injector design evaluations in terms of pattern factor, thermoacoustic performance, and certain emissions. Automation and parallel coordinate visualization enabled exploration of the dual-swirler airblast injector design space, which is often neglected in published combustor design studies.

Load expansion of a dieseline compression ignition engine with multi-mode combustion

This paper presents a study on the upper load expansion and combustion characteristics of a multi-mode combustion engine with dieseline. The partially premixed compression ignition (PPCI) mode with early direct injection and late-injection low temperature combustion (L-LTC) mode with near-top dead center (TDC) direct injection are studied and compared at various loads with sweeps of injection parameters and exhaust gas recirculation (EGR) rates. A single injection strategy and a double injection strategy with small pilot ratios are tested. For both combustion modes, the criteria are set to identify the load limit of clean combustion. The NOx limit is 200ppm, and the particulate matter number limit is 100×106/cm3. The maximum pressure rise rate is lower than 0.8MPa/°, and the maximum cylinder pressure is below 16MPa. In addition, the coefficient of variation for the indicated mean effective pressure (IMEP) should be less than 3% combustion stability. The results indicate that L-LTC has a higher load limit than PPCI for all the test fuels; therefore, the upper load limit of PPCI can be increased using L-LTC. However, the thermal efficiency of L-LTC is lower than that of PPCI. Therefore, a multi-mode combustion strategy is proposed to be the best option for the whole load range. For the small to medium load range, PPCI can achieve a low emissions level and with a higher thermal efficiency than L-LTC; therefore, PPCI is an ideal choice for this range. For the medium to high load range, the L-LTC mode is the only suitable mode due to its low emissions. In the high load range, a simultaneous reduction in NOx and PM is not possible for both PPCI and L-LTC; therefore, the conventional CI mode has to be used to ensure high fuel economy performance.

초희박 LPG 직접분사식 엔진에서 질소산화물 저감을 위한 배기재순환 적용성 평가

Abstract - In order to keep the competitiveness of LPG fuel for transportation fuel, the difference in fuel consumption with gasoline and cost for an aftertreatment system should be reduced with continuous development of technology for LPG engine. In the present study, spray-guided type direct injection combustion system, whose configuration is composed of direct injector in the vicinity of spark plug, was employed to realize stable lean combustion. A certain level of nitrogen oxides(NO x ) emits due to a locally rich mixture regions in the stratified mixture. With the application of EGR system for the reduction of NO x , 15% of NO x reduction was achieved whereas fuel consumption and hydrocarbon emission increased. By the application of EGR, the combustion speed reduced especially appeared at initial flame development period and peak heat release rates and increasing rates for heat release rate decreased as EGR rate increased due to the dilution effect of intake air.Key words : LPG engine, Lean combustion, Spray-guided type direct injection system, Exhaust gas recirculationKIGAS Vol. 19, No. 4, pp 22~28, 2015 http://dx.doi.org/10.7842/kigas. 2015.19.4.22(Journal of the Korean Institute of Gas)