Natural Gas Direct Injection Research Articles

Towards optimized excess air ratio and substitution rate for a dual fuel HPDI engine

The diesel-piloted high-pressure direct-injection (HPDI) natural gas engine is a significant application of natural gas in heavy-duty engines. However, there is limited research focusing on quantitative optimization of the excess air coefficient and natural gas substitution rate of HPDI engines. In order to support the design and development of HPDI engines, this article investigates the effects of excess air coefficient and substitution rate on a dual fuel HPDI engine combustion performance and emission characteristics. Results show that increasing the excess air coefficient results in higher cylinder pressure, but it has minimal effect on the heat release rate. A higher excess air coefficient can also lead to higher nitrogen oxides (NOx) emissions, lower soot emissions, methane (CH4) and total hydrocarbon (THC) emissions significantly decrease at 50% loads (Case2 and Case4), but these increase at 70% loads (Case1 and Case3). Furthermore, increasing the substitution rate markedly reduces greenhouse gas emissions, though it comes with the drawback of heightened combustion instability, and there is a trade-off between engine combustion cycle variations and greenhouse gas emissions as the substitution rate changes. The above research findings will provide optimization guidance for the engineering development of HPDI engines.

Diesel-natural gas dual fuel injection strategy effects on engine ignition delay and cylinder pressure evolution

Diesel pilot high-pressure direct injection (HPDI) natural gas technology has been noticed and applied in medium and heavy-duty engines due to its outstanding economy and power performance. However, the combustion of two kinds of fuels will involve more injection parameters, which increases the difficulty of designing injection strategy and calibrating injection parameters. This paper investigates comprehensively the effects of three key engine injection parameters (injection timing, injection duration of the pilot diesel, interval of injection between diesel and natural gas) on the engine ignition delay and cylinder pressure evolution using the experimentation method, which are utilized to guide the design of injection strategy and optimize injection parameters. Results show that the maximum in-cylinder pressure and its position are determined by the injection timing of natural gas, but little affected by the injection parameters of diesel. As the injection timing advances and the duration of diesel injection shortens, the ignition delay increases. To avoid rough combustion in the cylinder, it is necessary to optimize the injection timing, injection interval, and ignition diesel injection duration to ensure that the pressure rise rate does not exceed the threshold.

Engine Performance Technology and Applications

The aim of this paper is to summarize and consolidate the optimization and improvements in engine types, new technologies, and existing technologies over the past few years. Additionally, it effectively analyzes the impact of improved engines on automotive performance and driving experience. Regarding direct-injection technology, this paper primarily highlights the new technology in the field of natural gas direct-injection engines. Among these, high-pressure direct-injection natural gas engines excel in improving fuel economy and emissions, effectively suppressing the limitations of knocking on such engines' output power. On the other hand, low-pressure direct-injection natural gas engines exhibit lower injection pressures when the engine is in the intake stroke. Concerning variable valve timing technology, the study investigates parameters such as the angle adjustment of the variable valve timing mechanism, unlocking methods, and unlocking pressures. Furthermore, the dual VVT-I technology enhances intake efficiency, ensuring ample torque output within the low and mid-range RPMs, guaranteeing sufficient power performance in various operating conditions. In today's rapidly advancing industrial technology landscape, many technologies have made significant progress. However, the current societal trend is moving towards energy conservation and carbon reduction, gradually phasing out traditional engines from the historical stage. The future development trend will be new energy engines, such as new energy materials like pure electric, hydrogen, and natural gas. Additionally, artificial intelligence will rapidly emerge in various industries.

Investigation on jet controlled diffusion combustion (JCDC) mode applied on a marine large-bore two-stroke engine

Ships with large-bore marine engines are essential for international trade and transportation. However, a large number of fuel energy consumption and waste emissions are produced. Future lower fuel consumption and cleaner marine natural gas engines needs technological innovation. One approach is the use of a prechamber, providing high ignition energy. In this study, diesel injected into the prechamber and the prechamber jet controlled diffusion combustion (JCDC) mode was applied to a marine two-stroke natural gas/diesel engine. A three-dimensional simulation method was employed to optimize the combustion control parameters and compare the performance of JCDC mode with the traditional pilot controlled diffusion combustion mode (PCDC). The diesel compression ignition in the prechamber generated turbulent jet in the main chamber, which ignited the direct-injection natural gas. The key parameters influencing the JCDC mode, including the direct-injection interval and diesel energy percentage, were optimized in this study. A small direct-injection interval of 0.15°CA between diesel and natural gas was achieved, resulting in good ignition performance. By utilizing a 1.8% diesel energy percentage in the prechamber, a high indicated thermal efficiency (ITE) of 53.6% was obtained. Compared to the PCDC mode, the JCDC mode demonstrated a maximum reduction of approximately 1.4g/kWh in equivalent indicated specific fuel consumption (EISFC), indicating very large improved fuel economy for the marine two-stroke engine. Furthermore, the JCDC mode exhibited notable reductions of greenhouse gas in CO2 and unburned CH4 emissions, with a maximum decrease of 1.6% and 89%, respectively. The JCDC mode indicates a cleaner marine natural gas engine.

Numerical study on the effect of the combustion system parameters on combustion and emission characteristics for a HPDI natural gas/diesel dual fuel engine

Replacing diesel by natural gas in diesel engine is of great interest for transportation and power station industry due to larger availability and lower environmental impact of natural gas compared to diesel fuel. In addition, the high-pressure direct injection (HPDI) natural gas engine is widely recognized as a positive development prospect and an ideal alternative to conventional diesel engine. However, the trade-off relationship between indicated thermal efficiency (ITE) and NOx emissions limits the development of dual-fuel engines. In order to resolve this problem, we established a three-dimensional model combined with a multicomponent reduced chemical kinetic model and a phenomenological soot model. Using the 3D simulation model, we jointly optimized the combustion chamber (CC) structure, diesel/natural gas (NG) injection pressure (IP), and exhaust gas recirculation (EGR) strategy for HPDI NG engine in the two combustion modes. The results show that the adoption of the straight crater type (SCT) CC improves the mixture space equivalent ratio gradient distribution, slightly increases NOx emissions, rises ITE and reduces CO emissions in the two combustion modes. In NG mixture-limited combustion (NMLC) mode, increasing the diesel injection pressure (DIP) while appropriately increasing the NG injection pressure (NIP) promotes multi-point ignition and faster flame propagation and further optimizes CO emissions. In NG slightly premixed combustion (NSPC) mode, high ITE and low CO emissions can be maintained by increasing DIP while slightly increasing NIP, but the NOx increases substantially. In the moderate NOx emissions region (1.0 g/kW·h ∼ 2.0 g/kW·h), as the EGR rate increases, an overall improvement in trade-off relationships of NOx-CO, NOx-soot, and NOx-ITE in NMLC mode can be achieved. In NSPC mode, as the EGR rate increases, although CO and soot emissions can maintain lower values, it is difficult to improve the trade-off relationship of NOx-ITE.

Numerical study on the combustion and emission characteristics of a direct injection natural gas engine ignited by diesel/n-butanol blends

Adjusting the composition of pilot fuel has been regarded as an effective method for the improvement of performance and emissions of pilot ignited direct injection natural gas engines. In this paper, the effects of employing diesel/n-butanol blends as the pilot fuel in a direct injection natural gas engine were assessed by numerical simulations. The results indicated that adding n-butanol into the pilot fuel will result in notable extensions in ignition delay and obvious reductions in combustion duration at low and medium engine loads; at high engine loads, the effects of n-butanol blending on combustion parameters are relatively small. In terms of engine performance, thermal efficiency could be improved by up to 2.3% and 6.5% by adding n-butanol into the pilot fuel at low and medium engine loads, whereas lower than 2% reduction in thermal efficiency would be induced by n-butanol addition at high engine load. With regard to emissions, more than 30% reductions in CO and soot emissions could be achieved by adjusting n-butanol blending ratio at low and medium engine loads; at low and high engine loads, blending n-butanol into the pilot fuel is favorable for the control of NOx emissions while at medium engine load, n-butanol blending could result in up to 34.7% increases in NOx emissions.

Investigation on combustion characteristics and gas emissions of a high-pressure direct-injection natural gas engine at different combustion modes

In the present study, the potential of natural gas pre-injection, mixing-limited combustion, and slightly premixed combustion modes were evaluated to achieve high thermal efficiency and low nitrogen oxides and carbon monoxide emissions. To this end, a dual direct injection model was established to perform numerical simulations. The obtained results revealed that in the natural gas pre-injection mode, the optimized natural gas pre-injection proportion and injection interval between the end of natural gas pre-injection and the start of diesel injection can significantly reduce carbon monoxide emissions and improve indicated thermal efficiency with a slight increase in nitrogen oxides. In the mixing-limited combustion mode, combustion phasing and indicated thermal efficiency are not sensitive to the variations of the injection interval between diesel and natural gas. Moreover, a larger injection interval between diesel and natural gas is beneficial to reduce carbon monoxide. In the slightly premixed combustion mode, as the injection interval between diesel and natural gas decreases, combustion phasing delays, carbon monoxide emissions reduce, and nitrogen oxides emissions increase first and then decrease. Advancing the start of natural gas injection increases maximum pressure rise rate and nitrogen oxides in both natural gas mixing-limited combustion and slightly premixed combustion modes but decreases carbon monoxide dramatically. The natural gas mixing-limited combustion mode has more potential to meet carbon monoxide limits imposed by the Euro VII standard with no need for extra after-treatment devices while improving thermal efficiency.

Effects of operating parameters on combustion and soot emissions in a pilot ignited HPDI natural gas engine for different combustion modes

In this paper, a comprehensive numerical study of the combustion and emission characteristics of a direct injection natural gas engine operating at the high load condition was presented. First, two combustion modes, natural gas mixing-limited combustion (NMLC) and natural gas slightly premixed combustion (NSPC), were defined separately. Subsequently, the potential of combustion chamber type, spray angle of natural gas (SAN), diesel injection pressure (DIP), and natural gas injection pressure (NIP) to achieve high thermal efficiency and low soot emissions are evaluated in both NMLC and NSPC modes. The results showed that in both NMLC and NSPC modes, the straight-sided combustion chamber (SSCC) has more potential to improve the indicated thermal efficiency (ITE) and reduce soot emissions. In NMLC mode, an appropriate increase in SAN can promote the development of vortex at the bowl bottom and improve the stratification state of concentration gradient, thus enhancing ITE, inhibiting the production of pyrene (A4) and acetylene (C2H2), and reducing soot emissions. However, an excessive increase in SAN will lead to fuel accumulation, thus deteriorating soot emissions. In NSPC mode, when SAN increases to 15°, ITE reaches the maximum. Meanwhile, the production of soot precursors (A4 and C2H2) reduces, thus soot emissions reach an extremely low level. However, when SAN exceeds 15°, soot emissions increase rapidly along with the rising SAN. In both combustion modes, when NIP is low, increasing DIP can significantly reduce soot emissions. In NMLC mode, the increase of NIP is more effective in enhancing ITE and inhibiting soot formation. In NSPC mode, when NIP reaches 30 MPa, the maximum pressure rise rate (MPRR) exceeds the limit (1.5 MPa/°CA). When NIP exceeds 30 MPa, ITE begins to decrease. Therefore, NIP should be controlled within 30 MPa for NSPC mode.

Large-eddy simulation of a natural gas direct injection spark ignition engine with different injection timings

This paper presents a large-eddy simulation (LES) study of a compressed natural gas (CNG) direct injection spark ignition (DISI) engine with different injection timings. Three cases with late, moderate and advanced injection timings are investigated to examine fuel/air mixing and combustion characteristics of a CNG DISI engine. A transition flame speed approach is used in which the flame propagates like a laminar flame until it reaches a specified kernel size and then develops as a turbulent flame. Changing the transition kernel size significantly affects both the in-cylinder pressure trace and the mass fraction burned (MFB) profile, highlighting the importance of correctly capturing the early stage of flame development. Different injection timings result in varying levels of mixing and homogeneity within the cylinder. Although more time is available for mixing in the most advanced injection case, the jet impingement on the piston crown increases the level of mixture in-homogeneity. It is shown that the variation of the laminar flame speed as a result of mixture in-homogeneity is the primary factor affecting flame propagation for different injection timings.

A Study on Performance and Emission Characteristics of Direct Injection Natural Gas Engine

Natural gas is well known as a promising alternative fuel for internal combustion engine. In this study, a numerical model was built based on the experimental CNG converted engine. Direct injection and port fuel injection method was operated in the model on the same conditions such as design parameter, engine speed, compression ratio. The effect of injection method on engine performance and emission characteristics were compared. The mixing and burning properties were also investigated to have further insight about the fuel injection effects. The results have illustrated that the injection strategy had strongly effects on the mixing and combustion process. Due to the better mixture, the direct injection engine has showed the obviously improvement on the emission as well as engine performance. All these results have provided the data and theory to improve the engine performance for the development of the direct injection system for the engine using CNG.

Effect of natural gas direct injection (NGDI) on the performance and knock behavior of an SI engine

The unique properties of natural gas (NG), including high availability and lower cost compared with other fossil fuels, make it attractive in internal combustion engine (ICE) application. NG is composed mainly of methane and has greater knock resistance than gasoline, enabling higher compression ratios (CR). In contrast with the distinctive advantages, the NG fueled engines suffer from lower power and torque outputs. To address the subject, this study proposes an approach employing NG direct injection (NGDI) strategy (with higher volumetric efficiency unlike port injection), enabling a higher CR irrespective of knock limit. This work applies reactive computational fluid dynamics (CFD) to investigate spark ignited co-combustion of direct-injected NG with port-admitted gasoline. The results are validated against experimental data. In all simulated cases, the equivalence ratio (i.e., ∅ = 1) and the total input energy are kept constant. Engine performance is evaluated for three CRs (10.5, 11.5, and 12.5:1), five proportion of CNG (RCNG) and at part- and full-load conditions at an engine speed of 1500 rpm. Results indicated that while running RCNG = 100 % with a CR of 10.5:1, carbon monoxide (CO) and carbon dioxide (CO2) emissions were decreased by 29.3 % and 23.5 % respectively, compared to RCNG = 0 %. The corresponding emission reduction at CR = 11.5:1 was 27.1 % and 24 %; at CR = 12.5:1 they were 29.6 % and 23.5 % respectively. At each CR, the knock intensity at full load fell significantly as the percentage of NG increased. At a CR of 12.5:1, ringing intensity (RI) at full load decreased by 88.6 % when using RCNG = 100 %, instead of RCNG = 0 %. Under the same conditions, RCNG = 25 % cut RI by 56 %.

Numerical analysis of performance and soot emissions of a natural gas engine operating in HPDI and SPC combustion modes

High-pressure direct injection (HPDI) natural gas (NG) engines can help significantly improve thermal efficiency and reduce harmful emissions. In this study, we established a three-dimensional (3D) model combined with a multicomponent reduced chemical kinetic model and a phenomenological soot model. Using the 3D simulation model, we investigated the influence of pilot diesel mass on the combustion and emission characteristics of an HPDI NG engine at different start of injection of NG (NSOI). Subsequently, we evaluated the influence of diesel and NG injection interval (DNT) on the combustion characteristics and soot emissions in the HPDI and slightly premixed combustion (SPC) combustion modes at different NSOI. The simulation results indicated that with an increase in the pilot diesel mass at different NSOI, the peak values of cylinder pressure (Pmax) and nitrogen oxides (NOx) emissions increase, but carbon monoxide (CO) and soot emissions decrease simultaneously. Advancing NSOI leads to higher NOx emissions and increased Pmax and maximum pressure rise rate (MPRR) at different pilot diesel masses; by contrast, delaying NSOI leads to decreased indicated thermal efficiency (ITE). In the HPDI mode, combustion parameters such as Pmax and CA50% are not sensitive to variations in the DNT at different NSOI. Furthermore, the variations in the DNT do not notably influence the reduction of soot emissions. In the SPC mode, the ignition delay significantly increases with the decrease in the DNT, whereas CA50% moves away from the top dead center (TDC) and Pmax decreases. Meanwhile, the peak values of pyrene (A4) and acetylene (C2H2) simultaneously decrease, and the OH radicals are more extensively distributed in the cylinder, thereby resulting in a considerable reduction in final soot emissions. Considering the combined influence of Pmax, MPRR, ITE, and soot emissions, the operating point with the NSOI of −12°CA ATDC and DNT of −4°CA is considered the optimized point.

Spark-Induced Breakdown Spectroscopy In A Direct-Injected Natural Gas Spark-Ignition Engine

The purpose of this study is to investigate the fuel concentration around the spark plug obtained using spark-induced breakdown spectroscopy (SIBS) method with the spark plug with an optical fiber in the direct-injected natural gas SI engine. Effect of exposure duration on measurement accuracy of fuel concentration was discussed. Equivalence ratio around the spark plug in the direct-injected natural gas SI engine were measured under different natural gas injection timing. When the exposure duration is shortened, the emission of potassium elements near the emission wavelength of oxygen elements can be suppressed, and the measurement accuracy in the calculation of the emission intensity ratio can be improved. The relationship between the luminescence intensity ratio of hydrogen and oxygen (Hα/O) and the equivalence ratio of homogeneous mixture of natural gas and air was investigated and it was found that there was a positive correlation between the luminescence intensity ratio of hydrogen and oxygen (Hα/O) and the equivalence ratio. As a result of measuring the equivalence ratio around the spark plug of direct-injected natural gas SI engine using the SIBS method, an equivalence ratio of 0.67 was measured for a set value of 0.8 using the luminescence intensity ratio of hydrogen and oxygen.

Optical characterization of stratified-premixed natural gas direct-injection combustion regimes.

Gaseous fuels for heavy-duty internal combustion engines provide inherent advantages for reducing CO2, particulate matter (PM), and NOX emissions. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a late-cycle main direct injection of NG, resulting in significant reduction of unburned CH4 emissions relative to port-injected NG. Previous works have identified NG premixing as a critical parameter establishing indicated efficiency and emissions performance. To this end, a recent experimental investigation using a metal engine identified six general regimes of PIDING heat release and emissions behavior arising from variation of NG stratification through control of relative injection timing (RIT) of the NG with respect to the pilot diesel. The objective of the current work is to provide comprehensive description of in-cylinder fuel mixing of direct injected gaseous fuel and its impacts on combustion and pollutant formation processes for stratified PIDING combustion. In-cylinder imaging of OH*-chemiluminescence (OH*-CL) and PM (700 nm), and measurement of local concentration of fuel is considered for 11 different , representing 5 regimes of stratified PIDING combustion (performed with MPa and ). The magnitude and cyclic variability of premixed fuel concentration near the bowl wall provides direct experimental validation of thermodynamic metrics ( , , ) that describe the fuel-air mixture state of all 5 regimes of PIDING combustion. The local fuel concentration develops non-monotonically and is a function of RIT. High indicated efficiency and low CH4 emissions previously observed for stratified-premixed PIDING combustion in previous (non-optical) investigations are due to: (i) very rapid reaction zone growth ( m/s) and (ii) more distributed early reaction zones when overlapping pilot and NG injections cause partial pilot quenching. These results connect and extend the findings of previous investigations and guide the future strategic implementation of NG stratification for improved combustion and emissions performance.

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.

Implementation of various bowl designs in an HPDI natural gas engine focused on performance and pollutant emissions

The air-fuel mixture preparation in pilot spray-ignited natural gas engines is primarily dominated by piston bowl profiles and fuel injection strategy. Piston bowl geometry is regarded as the crucial point in controlling engine pollutant emissions. In the present work, the SAGE combustion model was applied coupled with a general reaction kinetic mechanism. The engine model was validated with experimental data achieved from a Cummins ISX 400 engine, and good agreement between predicted and measured in-cylinder pressure and heat release rate was obtained. The influence of various piston bowl designs, including Mexican-hat geometry, double-lip geometry, bow geometry, and toroidal geometry, on the combustion process, engine performance, and pollutant emissions of a high-pressure direct-injection natural gas engine have been studied and analyzed numerically. The present study confirms the benefit of the piston bowl design as a beneficial tool to enhance the performance and pollutant emissions of the pilot diesel-ignited natural gas engine. Results showed that different chamber shapes slightly influence the combustion initiation, and the difference in in-cylinder pressure presents noticeable as the combustion continues. A higher turbulent kinetic energy improves the flow movement and facilitates the mixture formation in the cylinder. However, the combustion behavior is unwished caused by the improper injection angle of natural gas. Increasing the recess depth of combustion chambers reduces NOx formations at the price of sacrificing fuel economy. For the bow combustion chamber design, the NOx emission declined by 31.1%, while the indicated specific fuel consumption increased by 5.5% compared with the original engine. Although the indicated mean effective pressure and specific fuel consumption of the optimal double-lip geometry almost remain the same, NOx emissions can be reduced by 16.7% compared with the base design.

Effects of scavenging port angle and combustion chamber geometry on combustion and emmission of a high-pressure direct-injection natural gas marine engine

ABSTRACT HPDI natural gas (HPDI-NA) engines can improve nitrogen oxides and PM emissions while getting better in-cylinder combustion in the comparison of the conventional engines fueled with diesel. In this paper, a three-dimensional (3D) of a HPDI natural gas marine engine is set up. The influence of geometry parameters on mixture flow, combustion, and emissions were presented and discussed. The geometry parameters include scavenging port angle (SPA) and three combustion chamber geometries (shallow-basin-shaped (SBC), shallow-ω-shaped (SWC), and deep-pit-shaped combustion chamber (DPC)). The results show that increasing the SPA can significantly swell the swirl ratio, but too large the swirl ratio is not conducive to improving the scavenging effect. As SPA is varied from 30° to 60°, a richer-than-flammable region gets greater, and the penetration distance is shortened by the larger vortex. Then, the mixing and combustion process get worse. With the increase of the SPA, the emissions of soot, carbonic oxide (CO), and unburned hydrocarbons (HC) can be optimized. Also, a larger scavenge port angle is not conducive to improving nitrogen oxides emission and the fuel consumption efficiency. The combustion chamber geometries have fewer effects on the mixture flow and the pressure profiles and temperature than that of SPA. The CO/HC and soot emissions of the three tested combustion chamber geometries are similar. The effect of the shallow basin-shaped combustion chamber (SBC) is much better than that of the other combustion chamber geometries with respect to NOx emission.

Approaches for emission reduction in natural gas fueled direct-injection engines

A glow plug (GP) can serve as an effective ignition source in a heavy-duty direct-injection natural gas (DING) engine. Ignition performance can be improved by shielding the GP. The shield protects the GP from cooling by the injected natural gas (NG) jets and also traps NG next to the GP surface, acting as kind of pre-combustion chamber. Achieving rapid filing of the shield by a fuel jet and facilitating flame propagation out are key goals of shield design. However, previous studies found that emissions such as unburned CH4 and soot particles could be trapped inside the shield especially near its top end, contributing a large fraction to the engine exhaust. In this paper, new designs either controlling the NG amount entering the shield or improving the emission propagation out of the shield are proposed to solve the emission issues, and the impacts of these design changes on fuel injection and combustion emissions were computationally investigated. The base case includes an injector with nine 0.2 mm diameter nozzles and a diamond-pattern multi-opening GP shield. A new injector combining one smaller nozzle directed towards the GP shield and eight regular size nozzles was compared, while a new five-opening shield design adding a small opening on the top end of the diamond-pattern shield was studied. Compared to the base case, the new two-nozzle-size injector greatly reduced the amount of fuel injected into the shield, which solved the emission issues inside the shield. Over 35% and 95% reductions in the total mass of unburned CH4 and soot particles respectively were achieved. Furthermore, the additional opening for the new shield provides an alternative route for fuel flowing out of the shield during fuel injection. However, fuel injection is the main force for fuel motion inside the shield and highly affects the new flow route provided by the additional opening. The unburned CH4 and formed soot particles could not flow out of the shield through the additional opening once fuel injection is finished. Generally, an increase was found for the unburned CH4 and soot mass by using the new five-opening shield, compared to the base case.

Computational studies of hydrogen post-injection in direct-injection natural gas engines

The impact of using a hydrogen (H2) post-injection on combustion emission characteristics in a glow plug (GP) assisted direct-injection natural gas (DING) engine was investigated in this paper. This study was conducted by using the KIVA-3V code integrated with a detailed kinetic chemical model and a modified phenomenological soot model. Previous studies have indicated two sources of unburned methane (CH4) and soot particles in such an engine, namely the injector sac volume and the glow plug shield. The emissions were reduced in previous work by improving the shield design. However, the contributions from the injector sac volume were not solved. In these simulations, a small amount of hydrogen was injected following the major injection of natural gas (NG), and the unburned CH4 was pushed out of the injector into the combustion chamber. The injected hydrogen then affects the chemical reactions to form more hydroxyl (OH) radical, and the unburned methane from the sac volume and that originally left in the combustion chamber were oxidized to reduce its final value, compared to the baseline NG only case. As well, the soot particles contributed from the glow plug shield and from the injector sac volume and thereby the total particle emissions were reduced, compared to the baseline case. The reduction is due to the additionally formed OH which reduces the soot precursor acetylene (C2H2) and enhances the soot oxidation. The hydrogen post-injection also highly affected the CO–CO2 reaction, leading to more carbon monoxide (CO) and less carbon dioxide (CO2) emissions in the DING engine. In addition, more simulations were conducted by injecting different amounts of hydrogen fuels, and the emission values were generally changed as expected. The simulations also indicated that there is a minimum requirement for the hydrogen amount, below which the formed OH radical is not enough to oxidize the soot caused by the unburned CH4 from the injector sac volume, compared to the NG only case.

DI-CNG injector nozzle design influence on SI engine standard emissions and particulates at different injection timings

Compressed natural gas direct injection (DI-CNG) systems in spark ignition (SI) internal combustion engines have shown that it can give several benefits compared to CNG port fuel injection systems. However, the DI-CNG injector nozzle head design and gas jet formation may greatly influence engine exhaust gas emissions and performance. Present experimental study investigated the influence of 7 different nozzle head designs of spray-guided DI-CNG injectors on the combustion process, engine performance, standard emissions, and particulate number (PN) when methane fuel was injected at different injection timings (SOI) and injection pressures (18 bar and 50 bar). The nozzle heads had two main design patterns – heads with small multi holes/orifices and heads with larger crevices (swirl or umbrella spray pattern). Naturally aspirated SI engine tests were conducted at part load (6 bar IMEP) and wide-open throttle (WOT) at 2000 rpm engine speed. The results revealed that the difference between the nozzle heads was small when the fuel was injected at an early stage of the intake stroke (310–350 CAD bTDC) either at part load or high load. However, for late injection timing (130–190 CAD bTDC), the design of the DI-CNG injector nozzle head had a large impact on the combustion stability, standard emissions formation and particulates. Multi-hole nozzle heads showed improved CO2, CO, THC, total PN, and slightly higher NOx emissions compared to nozzle heads with larger crevices. For some of the nozzles, the SOI could be retarded more than for other injector head designs at higher injection pressure whilst still ensuring an acceptable engine performance in terms of combustion stability, power output and emissions formation. Overall, 50-bar injection pressure and a late injection timing under WOT conditions achieved higher engine load levels with all injector nozzle types. Images acquired using an optical endoscope technique with a high-speed video camera showed that a yellow flame was present for all nozzle types at a low injection pressure and late SOI. Increasing the injection pressure reduced the injection duration, improved air/fuel mixing which resulted in the reduced yellow flame formation and lower PN for most of the nozzle heads.