Compressed Natural Gas Direct Injection Research Articles

Numerical analysis of performance, combustion, and emission characteristics of PFI gasoline, PFI CNG, and DI CNG engine

Compressed natural gas (CNG) has gained widespread acceptance in the automobile industry due to its clean-burning properties, lower emissions, and superior anti-knock properties. Typically, CNG is used in bi-fuel mode, where it partially replaces gasoline in a conventional gasoline engine. However, because of its gaseous nature, CNG displaces intake air, which reduces the engine's overall power output. One possible solution is to directly inject CNG into the cylinder during the late intake stroke or at the beginning of the compression stroke, which increases the engine's volumetric efficiency and power output. In a previous study, we examined the impact of injection timing on the performance of direct injection (DI) CNG engines under various engine load and speed conditions. However, the intrinsic combustion process, such as flame speed and flame front propagation, could not be investigated. In this study, we attempted to understand DI CNG engines' in-cylinder flame speed and propagation behavior. We developed a combustion model for the DI CNG engine using computational fluid dynamics. We used methane as the surrogate fuel and six cylindrical nozzles with inlet velocity boundary conditions for CNG direct injection. We studied the CNG combustion process and flame speed propagation using a validated chemical mechanism and a look-up table. We made comparisons to evaluate the numerical model against experimental reference data. We also conducted a comparative simulation of DI CNG, port fuel injection (PFI) CNG, and gasoline engines to assess combustion behavior. The combustion duration was longer for the PFI CNG engine than the gasoline engine, but it was reduced for the DI CNG engine. This was due to the DI CNG engine's 40% higher turbulent flame speed than the PFI CNG engine. The combustion of the air-fuel charge was completed within 40° CA after spark ignition. As a result, DI CNG requires a retard in ignition timing. The DI CNG engine showed a 7% higher power output with a 10% increase in volumetric efficiency compared to the PFI CNG engine. However, the power output was still lower than that of the PFI gasoline engine. The DI CNG engine emitted lower NOx and CO emissions than the PFI CNG engine. This study demonstrates the higher turbulent flame speed and rapid flame front propagation characteristics of DI CNG engines over PFI CNG engines, increasing engine efficiency.

Numerical Comparative Study on the In-Cylinder Mixing Performance of Port Fuel Injection and Direct Injection Gas-Fueled Engine

In recent decades, research on alternative fuel engines is becoming more and more popular. Compressed natural gas (CNG) has the advantages of abundant reserves and a lower cost. It can reduce vehicle emissions relatively quickly and has little impact on the entire transportation infrastructure. As the fourth generation of a gas fuel supply method, gas fuel direct injection (DI) technology can effectively avoid volumetric efficiency reduction and power reduction problems of the port fuel injection (PFI) method. However, the former’s mixing path and duration are shortened greatly, which often leads to poor mixing uniformity. In order to improve the in-cylinder mixing uniformity, the in-cylinder mixing process of the CNG-fueled engine is taken as the research object in this study. The computational fluid dynamics (CFDs) models of the mixing process for the PFI and DI modes are established, and their mixing uniformities are compared. Besides, based on the authors’ previous research, the influence mechanism of the piston crown shape and fuel injection angle on the mixing process of the CNG DI engine is explored. The results show that the probability distribution frequency (PDF) of the best mixture concentration region (BMCR) is as high as 72% for the PFI mode, which is much higher than for the DI mode. The shorter jet impingement distance of the flat top piston leads to higher turbulent kinetic energy (TKE) intensity, and the in-cylinder mixing uniformity will be improved. When gas fuel is injected into an area with a higher in-cylinder TKE, the average in-cylinder TKE will be higher, and the in-cylinder mixture will be more homogeneous.

Study on Combustion and Emissions of a Combined Injection Spark Ignition Engine with Natural Gas Direct Injection Plus Ethanol Port Injection under Lean-Burn Conditions.

Conventional ethanol spark ignition (SI) engines have poor fuel atomization and mixture formation. The objective of this paper is to improve the combustion and emission performance of ethanol SI engines under lean-burn conditions through the dual-injection mode with ethanol port injection and compressed natural gas (CNG) direct injection (CDI+EPI). This paper studies the engine performance at 1500 rpm under five CNG direct injection ratios (CDIr) and five excess air ratios (λ). The results show that as the CDIr increases under lean-burn conditions, the following occurs: the minimum advance for best torque (MBT), the coefficient of variation (CoVIMEP), and CO and HC emissions decrease; the crankshaft rotation or time with cumulative heat release rate ranging from 10% to 90% (CA 10–90) and NOx emissions first decrease and then increase; and torque, peak in-cylinder pressure (Pmax), and the λ limit first increase and then decrease. The larger the CDIr is, the less influence λ has on the MBT. When CDIr = 15%, the CoVIMEP can be effectively reduced, the engine can still work stably in all lean-burn conditions, and the λ limit will reach the maximum value of 1.73, 19.31% higher than that of the original engine (CDIr = 0). When λ = 1.1, CO emissions decrease the most and HC emissions decrease the least. At this time, CO and HC emissions decrease by 1.56 vol % and 30 ppm, respectively, on average for every 0.1 decrease in λ. For CA 10–90, torque, and Pmax, λ = 1.1, 15% CDI, and 85% EPI is the optimal combination under lean-burn conditions. When CDIr ≥ 15%, NOx emissions are at an ideal level. Under lean-burn conditions, direct-injection CNG can form a good stratified natural gas/ethanol mixture in the cylinder, effectively improving the engine’s power and stability and reducing emissions. The λ = 1.1, 15% CDI, 85% EPI combination provides a cutting-edge and outstanding solution for a natural gas/ethanol combined injection SI engine.

Combined effect of boost pressure and injection timing on the performance and combustion of CNG in a DI spark ignition engine

There is an increasing interest in supercharging spark ignition engines operating on CNG (compressed natural gas) mainly due to its superior knock resisting properties. However, there is a penalty in volumetric efficiency when directly injecting the gaseous fuel at early and partial injection timings. The present work reports the combined effects of a small boost pressure and injection timing on performance and combustion of CNG fueled DI (direct injection) engine. The experimental tests were carried out on a 4-stroke DI spark ignition engine with a compression ratio of 14. Early injection timing, when inlet valves are still open (at 300°BTDC), and partial injection timing, in which part of the injection occurs after the inlet valves are closed (at 180°BTDC), were varied at each operating speed with variation of the boost pressure from 2.5 to 10 kPa. A narrow angle injector (NAI) was used to increase the mixing rate at engine speeds between 2000 and 5000 rpm. Similar experiments were conducted on a naturally aspirated engine and the results were then compared with that of the boosting system to examine the combined effects of boost pressure and injection timing. It was observed that boost pressure above 7.5 kPa resulted in an improvement of performance and combustion of CNG DI engine at all operating speeds. This was manifested in the faster heat release rates and mass fraction burned that in turn improved combustion efficiency of the boosting system. An increased in cylinder pressure and temperature was also observed with boost pressure compared to naturally aspirated engine. Moreover, the combustion duration was reduced due to concentration of the heat release near to the top dead center as the result of the boost pressure. Supercharging was also found to reduce the penalty of volumetric efficiency at both the simulated port and partial injection timings.

Modeling of the fuel injection and combustion process in a CNG direct injection engine

This paper describes a methodology to model the gaseous fuel injection process in a compressed natural gas (CNG) direct injection (DI) engine. Simulations were conducted using KIVA-3V Release 2 code by modifying the liquid fuel injection model to function as a gaseous fuel injection model. This model simulated gaseous fuel injection using a coarse mesh. In this model, gaseous spheres are injected similar to liquid fuel injection model and the gaseous spheres evaporate together without the latent heat of evaporation. Therefore, it does not require a very fine mesh and saves calculation time.The experiments were performed for gas-jet visualization using the planar laser induced fluorescence (PLIF) method. The compressed nitrogen was used instead of CNG fuel for safety reasons. The gaseous fuel injection model was validated by comparing the simulation and experimental results. Furthermore, engine experiments were performed using a single cylinder CNG DI engine. The experimental results of the in-cylinder pressure were compared to calculated results for validation of the combustion model. The fuel injection and combustion process were simulated using a three-dimensional engine mesh with four valves from intake valve open (IVO) to exhaust valve open (EVO).

Development of a single cylinder CNG direct injection engine and its performance, emissions and combustion characteristics

This paper presents experimental results of a newly developed compressed natural gas (CNG) fuelled direct injection (DI) engine, which was developed by modifying a single cylinder CI engine. Major modifications included: a) modifications in cylinder head and piston; b) development and installation of electronic CNG DI injection system; c) installation of a capacitive discharge ignition system. Experiments were conducted at a constant fuel injection pressure and engine speed and performance, emission and combustion characteristics of this CNG DI engine with varying fuel injection timings [i.e., start of injection (SOI)] and varying engine loads [i.e., brake mean effective pressure (BMEP)] were investigated. Moderate engine loads lead to faster and more complete combustion of the fuel, thus improving engine performance and reducing emissions for specific injection timings. Advanced fuel injection improved engine performance (lower BSFC, higher BTE); reduced emissions and produced faster ROHR whereas retarded injections delivered opposite trends for each engine load. [Received: July 25, 2013; Accepted: February 16, 2014]

Effect of Two-Stage Injection Timing on a Gas-Jet Ignition CNG Engine

Compressed natural gas (CNG) engines normally operate in lean condition to take the advantage of higher efficiency and better fuel economy. Several studies have shown that gas-jet ignition with two-stage injection technique is effective to extend the lean combustible range of CNG engines. This paper investigates the effectiveness of such technique using a prototype lean burn direct injection CNG engine. The experiment was conducted at speed of 900 rpm, fuel injection pressure of 3 MPa, equivalence ratio φ=0.8, and ignition timing at top dead center. The effect of first injection timing on the test engine performance and exhaust emission was analyzed. The result shows that the first injection timing is crucial in determining the performance of the engine. First injection timings when the piston is near to bottom dead center produced relatively stable combustion. First injection timings when the piston is at midpoint produced misfire. First injection timings near the gas-jet ignition produced unstable combustion except at a certain timings which produced acceptable combustion with low hydrocarbon and carbon monoxide emissions.

Cycle-by-cycle Variations in a Direct Injection Hydrogen Enriched Compressed Natural Gas Engine Employing EGR at Relative Air-Fuel Ratios.

Since the pressure development in a combustion chamber is uniquely related to the combustion process, substantial variations in the combustion process on a cycle-by-cycle basis are occurring. To this end, an experimental study of cycle-by-cycle variation in a direct injection spark ignition engine fueled with natural gas- hydrogen blends combined with exhaust gas recirculation at relative air-fuel ratios was conducted. The impacts of relative air-fuel ratios (i.e. λ = 1.0, 1.2, 1.3 and 1.4 which represent stoichiometric, moderately lean, lean and very lean mixtures respectively), hydrogen fractions and EGR rates were studied. The results showed that increasing the relative air-fuel ratio increases the COVIMEP. The behavior is more pronounced at the larger relative air-fuel ratios. More so, for a specified EGR rate; increasing the hydrogen fractions decreases the maximum COVIMEP value just as increasing in EGR rates increases the maximum COVIMEP value. (i.e. When percentage EGR rates is increased from 0% to 17% and 20% respectively. The maximum COVIMEP value increases from 6.25% to 6.56% and 8.30% respectively). Since the introduction of hydrogen gas reduces the cycle-by-cycle combustion variation in engine cylinder; thus it can be concluded that addition of hydrogen into direct injection compressed natural gas engine employing EGR at various relative air-fuel ratios is a viable approach to obtain an improved combustion quality which correspond to lower coefficient of variation in imep, (COVIMEP ) in a direct injection compressed natural gas engine employing EGR at relative air-fuel ratios. Keywords—Coefficient of variation in indicated mean effective pressure (COVIMEP); Direct injection compressed natural gas (DI-CNG); Exhaust gas recirculation (EGR); Hydrogen fractions; Relative air-fuel ratios.

A Study on the Spray Characteristics of SIDI CNG Engine Based on a Visualization System

Compressed natural gas (CNG) is regarded as one of the most promising alternative fuels, is widely used for the automobile engines. In order to improve the thermal efficiency of CNG engine, the direct-injection (DI) technology has been adopted. The stratified CNG mixture can be ignited by the spark plug which is installed near the injector nozzle, so the lean combustion can be subsequently achieved. For the direct-injection spark-ignition (DISI) engine, the reliable ignition is a foundational problem for the reliable operation, so it is very important to study on the spray characteristics of DISI engine. In our study, a combustion chamber is designed and a visualization system is built. The DI CNG spray's injection process was digital recorded with the schlieren optical system under different experiment conditions. The spray characteristics of the DI CNG engine were analyzed with the experimental results.

Combustion Characteristics of Compressed Natural Gas in a Direct Microchannel-Injection Engine under Various Operating Conditions

The combustion characteristics of compressed natural gas (CNG) in a direct microchannel-injection engine under various operating conditions were investigated. In this study, a novel idea for direct CNG microchannel injection was realized with spark plug fuel injector (SPFI). It is a device developed to convert engine to CNG direct injection (DI) operation with minimal cost and technical simplicity. It was installed and tested on a Ricardo E6 single cylinder engine with compression ratio of 10.5:1 without modification on the original engine structure. The engine test was carried out under various operation conditions at 1100 rpm. Burning rates of CNG were measured using normalized combustion pressure method by which the normalized pressure rise due to combustion is equivalent to the mass fraction burned (MFB) at the specific crank angle. The results showed that the MFB of CNG direct injection is substantially faster but initially slower than the ones of port injection. The optimal fuel injection and ignition timings are 190 °CA ATDC and 25 °CA BTDC respectively. The optimal injection pressure was 6 MPa. Combustion durations were not changed with different injection pressures but ignition delay was affected. There was no direct correlation between injection pressure and ignition delay which is most probably due to the effect of charge flow difference. Changing mixture stoichiometry affects the magnitude of ignition delay. Combustion duration, on the other hand increases with leaner mixture.

Power boosting of a modified natural gas engine

This paper presents the experimental results on brake power enhancement of a modified natural gas engine. Conventional natural gas engine produces 15 to 20% less brake power than that of gasoline fired engine. This lacking in compressed natural gas (CNG) engine needs to be recovered through modifications and optimization of fuel injection system to provide complete gas combustion in the engine cylinder. A multi-cylinder gasoline engine was modified to bi-fuel engine for operating in several test conditions, such as constant full and half throttle condition. Variation in power production with corresponding fuel flow rate and emission gases (such as carbon monoxides, CO; unburnt hydrocarbon, HC and nitrogen oxides, NOx) were studied by using two fuels (gasoline and CNG). Engine tune up information like variation in air fuel ratio (AFR) for lean burn operation can be known from those tests. Performance of engine was studied with both the fuels at fixed load and the corresponding fuel flow rate and emissions were measured for evaluation and optimization. The engine produced 10% higher brake power with CNG fuel as compared to that produced with gasoline fuel at full load, but at partial load gasoline fuel produced more brake power than CNG. Emission results revealed that CNG fuelled engine emits less CO and HC, showing more complete combustion than gasoline fuel. On the contrary, higher combustion temperature of CNG fuel produced more NOx than gasoline. The results of these investigations can be used to develop a new compressed natural gas (CNG), direct injection (DI) and higher efficiency engine in the near future to build an environment friendly fuel economic clean burning automotive vehicle with less emission and similar power rating like gasoline engine. Key words: Brake power, compressed natural gas-direct injection (CNG-DI), bi-fuel engine, fuel consumption, emission.

A study of spray development and combustion propagation processes of spark-ignited direct injection (SIDI) compressed natural gas (CNG)

Compressed natural gas (CNG) is regarded as one of the most promising alternative fuels, and may be the cleanest fuel for the spark-ignited (SI) engine. Its high octane number permits a high compression ratio, thereby leading to higher thermal efficiency, and lower emissions could be realized by using CNG. In the SI engine, direct injection (DI) technology significantly increases the engine volumetric efficiency and decreases the need of throttle valve for control purposes. During low load and speed conditions, DI allows engine operation under ultra-lean conditions by realizing the stratified charge. This enables the use of extremely lean air–fuel mixtures, thereby reducing fuel consumption.For the design and optimization of an SI engine adopting DI technology with CNG fuel, it is necessary to investigate the spray development process, the ignition probability, and the combustion propagation process of DI CNG. In this study, a combustion chamber with a visualization system is designed and built. CNG is injected into the combustion chamber by a gasoline direct injection (GDI) injector and ignited by a spark plug placed near the injector. The close arrangement of the injector and spark plug provides a stratified charge of CNG around the spark discharge position. Images of the CNG spray development and combustion propagation processes were digitally recorded and analyzed. The results of this study can contribute important data for the design and optimization of spark-ignited direct injection (SIDI) CNG engines.

An experimental investigation of high performance natural gas engine with direct injection

This paper presents experimental results of a new compressed natural gas direct injection (CNG-DI) engine that has been developed from modification of a multi cylinder gasoline port injection (PI) engine. The original gasoline-PI engine was also modified to a CNG bi-fuel system. The test results obtained from CNG fuel using two different systems (i.e. bi-fuel and DI) have been investigated and compared with the original gasoline engine. The objective of this investigation is to compare the test results between CNG-DI, with CNG-BI and gasoline-PI engines with the same displacement volume. It was found that the CNG-DI engine produces similar brake power at 6000 rpm and wide open throttle (WOT) but produces higher brake power at part load condition as compared to the original gasoline. The CNG-BI engine produces 23% lower brake power than the CNG-DI engine. The average brake specific fuel consumption (BSFC) of the CNG-DI engine was 0.28% and 8% lower than gasoline-PI and CNG-BI engines respectively. The CNG-DI engine reduces 42% NOx emission as compared to the base engine. However, the CNG-DI engine produces higher HC and CO emissions as compared to the base engine. This paper discusses a review on the direct injection (DI) natural gas engine with new information along with other investigations.

Emission analysis of a compressed natural gas directinjection engine with a homogenous mixture

In an era in which environmental pollution and depletion of world oil reserves are of major concern, emissions produced by automotive vehicles need to be controlled and reduced. An ideal solution is to switch to a cleaner fuel such as natural gas, which generates cleaner emissions. In addition, control over the in-cylinder air-fuel mixture can be best achieved through a direct-injection mechanism, which can further improve combustion efficiency. This need for cleaner automobiles provides the motivation for this paper’s examination of the use of computational fluid dynamic (CFD) simulations to analyze the concentrations of the exhaust gases produced by a compressed natural gas engine with a direct-fuel-injection system. In this work, a compressed natural gas direct-injection engine has been designed and developed through a numerical simulation using computational fluid dynamics (CFD) to provide an insight into complex in-cylinder behavior. The emissions analyzed in this study were carbon monoxide (CO), nitric oxide (NO) and carbon dioxide (CO2), i.e. the main pollutants produced by natural gas combustion. Based on a stoichiometric mixture, the concentrations of CO and NO were computed using the dissociation of carbon dioxide and the extended Zeldovich mechanism. CO2 was calculated using a mass balance of the species involved in the combustion process. The simulation results were then compared with the experimental data generated by a single-cylinder research engine test rig. A good agreement was obtained with the experimental data for the engine speeds considered for all emissions concentrations.

English

An experimental study was carried out on a dedicated compressed natural gas direct injection (CNG-DI) engine with a compression ratio (CR) of 14 and a central injection system. Several injection timing parameters from early injection timing (300 BTDC) to partial direct injection (180 BTDC) to full direct injection (120 BTDC) were investigated. The 300 BTDC injection timing experiment was carried out to simulate the performance of a port injection engine and the result is used as a benchmark for engine performance. The full DI resulted in a 20% higher performance than the early injection timing for low engine speeds up to 2750 rpm. 180 BTDC injection timing shows the highest performance over an extensive range of engine speed because it has a similar volumetric efficiency to full DI. However, the earlier injection timing allowed for a better air–fuel mixing and gives superior performance for engine speeds above 4500 rpm. The engine performance could be explained by analysis of the heat release rate that shows that at low and intermediate engine speeds of 2000 and 3000, the full DI and partial DI resulted in the fastest heat release rate whereas at a high engine speed of 5000 rpm, the simulated port injection operation resulted in the fastest heat release rate.

Experimental Test of a New Compressed Natural Gas Direct Injection Engine

This paper presents experimental test results of a new compressed natural gas direct injection (CNGDI) engine that has been developed by modifying a multicylinder gasoline port injection engine. The major modifications done are (1) a modification of the injection system to gas direct injection using new high-pressure gas injectors, (2) a change of the compression ratio from 10 to 14 through modification of piston and cylinder head, (3) and new long edge spark plugs. The CNG pressure at the common rail is kept constant at 20 bar. The engine has been operated at wide open throttle (WOT) condition to compare all the results with the original base engine. Hence, two engines are tested in this investigation. The test was conducted on a computer-controlled dynamometer to measure brake power; specific fuel consumption; volumetric efficiency; and exhaust emissions such as carbon monoxide (CO), oxides of nitrogen (NOx), and unburned hydrocarbon (HC). The objective of this investigation is to compare the test results between a CNGDI engine and a gasoline engine. It was found that the CNGDI engine produces a 4% higher brake power at 6000 rpm as compared to the original gasoline-fueled engine. The average brake specific fuel consumption (BSFC) of the CNGDI engine was slightly lower (about 0.28%) than the base engine. The CNGDI engine reduces 50% NOx emission as compared to base engine. However, the CNGDI engine produces higher HC and CO emissions as compared to the base engine by 34 and 48%, respectively. The results of this experiment will be used to further improve the CNGDI engine as well as to develop a new CNGDI car. The detailed results including fuel mapping are presented with discussions.

Gear Ratios Strategy of PROTON Waja CNG-DI Vehicle for Improved Performance

A 1597cc gasoline CamPro engine was modified to adapt a Direct Injection (DI) technology that uses Compressed Natural Gas (CNG) as a fuel to form Compressed Natural Gas Direct Injection (CNGDI) engine. The modification includes increasing the compression ratio, redesigning the piston crown and cylinder head and a new engine control systems. These changes resulted in engine performance characteristics which is very different from its gasoline origin. This CNGDI engine is to be used with PROTON Waja vehicle body. Due to this change in the characteristics, the transmission systems utilizing an existing gear ratio combination, appears to be unsuitable, particularly for use in automatic transmission. Therefore, new gearbox with appropriate transmission matching needs to be developed. A computer based algorithm was developed for the purpose of predicting the PROTON Waja’s vehicle dynamic performance when CNGDI engine is used. The parameters being considered are maximum speed, acceleration, and elapsed time and these were optimized depending on the engine characteristics such as power, torque, gear ratios, and vehicle design parameters. The results recommended that the gear ratios of 3.58, 1.95, 1.34, 0.98, 0.8 and 4.33 for first, second, third, fourth, fifth and Final Drive (FD) respectively were the most suitable.

Numerical analysis of the combustion process in a four-stroke compressed natural gas engine with direct injection system

In this paper, a numerical study to simulate and analyze the combustion process occurred in a compressed natural gas direct injection (CNG-DI) engine by using a multi-dimensional computational fluid dynamics (CFD) code was presented. The investigation was performed on a single cylinder of the 1.6-liter engine running at wide open throttle at a fixed speed of 2000 rpm. The mesh generation was established via an embedded algorithm for moving meshes and boundaries for providing a more accurate transient condition of the operating engine. The combustion process was characterized with the eddy-break-up model of Magnussen for unpremixed or diffusion reaction. The modeling of gaseous fuel injection was described to define the start and end of injection timing. The utilized ignition strategy into the computational mesh was also explained to obtain the real spark ignition timing. The natural gas employed is considered to be 100% methane (CH4) with three global step reaction scheme. The CFD simulation was started from the intake valves opening until the time before exhaust valves opening. The results of CFD simulation were then compared with the data obtained from the single-cylinder engine experiment and showed a close agreement. For verification purpose, comparison between numerical and experimental work are in the form of average in-cylinder pressure, engine power as well as emission level of CO and NO.

Study on cycle-by-cycle variations of combustion in a natural-gas direct-injection engine

In this paper, the cycle-by-cycle variations of a compressed-natural-gas (CNG) direct-injection (DI) engine were investigated. The results show that the CNG DI engine has a better lean burn capability, and misfire cycles and partial burn cycles exist when the engine operates at small equivalence ratio (>0.4). Meanwhile, the indicated mean effective pressure (IMEP) has a low value, and the high value of the coefficient of variation in the IMEP is presented in comparison with those operating at a high equivalence ratio. Cycles with a high maximum cylinder pressure correspond to the cycles of fast burning, and parameter interdependence is observed between the maximum cylinder gas pressure and its corresponding crank angle, between the maximum rate of pressure rise and its corresponding crank angle, and between the maximum cylinder pressure and the indicated mean effective pressure. Better parameter interdependence exists between the maximum cylinder pressure and the flame- developing period, between the maximum cylinder pressure and the rapid-burning period, and between the accumulated heat release amount per cycle and the indicated mean effective pressure. A small variation in the flame-developing duration will lead to a large variation in the rapid-burning duration under lean mixture combustion; the slow flame propagation speed of the lean mixture combustion is considered to lead to this phenomenon.

A study of the combustion and emission characteristics of compressed-natural-gas direct-injection stratified combustion using a rapid-compression-machine

The objective of the present study is to determine the characteristics of combustion and emissions of compressed-natural-gas (CNG) direct-injection combustion using a rapid-compression-machine which has a compression ratio of 10 and a disc-shaped combustion chamber. Combustion and emission characteristics are compared for three types of fuel injection (single side, parallel side and opposed side injection) and a homogeneous mixture. The results show that with fuel injection, the fuel could be burned up to an equivalence ratio φ of 0.2 with sufficiently high combustion efficiency except for the case of φ = 1.0, while with a homogeneous mixture, the lean burn limit was only φ = 0.6 with poor combustion producing higher unburned CH4 By adjusting the location of the spark plug and fuel injectors, the combustion limit was extended to φ = 0.02. The Combustion efficiency of the injection modes is over 0.95 except for φ = 1.0 and φ < 0.06 which gave a lower combustion efficiency. Incomplete combustion in the stratified rich zone reduced the combustion efficiency at large values of φ, and possible occurrence of bulk quenching resulted in the lower combustion efficiency for very lean mixtures. Combustion efficiency for the homogeneous mixture decreases greatly with leaner mixtures, which is probably due to the thicker quenching layer near the wall. Combustion duration with fuel injection was insensitive to φ and was much shorter than for the homogeneous mixture. It was also shown that the number and location of the injectors and the injection rate had little influence on the combustion and the exhaust emissions including NOx. The pressure rise due to combustion in the case of fuel injection is higher compared to that of homogeneous mixture combustion due to the lower heat loss to the combustion chamber walls resulting from a short combustion duration. Thus it is shown that stratified-combustion with extremely lean burn capability can be realized with CNG direct injection.