Direct Injection Natural Gas Engine Research Articles

Evaluation of the use of ammonia/hydrogen blends in turbocharged gas engines

The potential for ammonia/hydrogen (NH3/H2) blends as fuel in modern, turbocharged, gas engines is evaluated by comparing the combustion properties of these blends with those of methane. To ensure proper combustion phasing in existing spark-ignited (SI) engines, the laminar burning velocity (LBV) is used, while the ignition delay times (IDT) of the mixtures are compared to assess the risk of engine knock. The chemical mechanisms used for the computation of IDT and/or LBV of NH3/H2 mixtures are selected from 8 chemical mechanisms based on their ability to predict experimental data. The application of a turbocharger is shown to “boost” the temperature in the cylinder after compression to values that admit the use of NH3/H2 mixtures in natural-gas SI engines without increasing the compression ratio (CR). The analysis also identifies regions of temperature and H2 fraction that increase the risk of knock. Using the conditions in a practical turbocharged SI engine as an example, 40–50% H2 in NH3 yields combustion phasing equivalent to that obtained with methane, while avoiding the risk of knock. Analysis of the IDT for the conditions in a high-pressure direct-injection natural-gas engine also shows that the use of a turbocharger in compression-ignition (diesel-type) engines strongly reduces the demands on CR for achieving the conditions necessary for reliable ignition of NH3/H2 mixtures. Increasing the temperature by only a few tens of degrees above the conditions studied will yield an IDT for ∼20% H2 in NH3 that satisfies the ignitibility requirements for use in a turbocharged diesel engine.

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.

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.

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.

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.

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.

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.

Investigation on online perception method of gas injection characteristics for the high-pressure natural gas direct injection engine

To realize the online perception of the gas injection process for high-pressure natural gas direct injection engines. This paper presents an innovative perception method for gas injection characteristics. The injection characteristics are divided into time characteristics and quantify characteristics. The gas inlet pressure is used to be the sensing signal. Firstly, time characteristics are highlighted on the frequency domain signal by short-time Fourier transform, and all kinds of time characteristics are defined in the time domain. To improve the real-time performance of the algorithm, the mean instantaneous frequency (MIF) was selected as the representative of the frequency at each moment to reduce the dimension of the time–frequency signal. Based on the zero and local maximum points of the MIF signal, a recognition method of the injector time characteristics is proposed. The neural network is used to build the prediction model of the quantified characteristics. The results show that: In all the ranges of the injector working conditions, the error of the time characteristics is less than 5%. The root mean square error of the injection mass is 2.68 mg, and the regression coefficient between the predicted value and the measured value is 0.99878. The method proposed in this paper has relatively high accuracy.

Experimental study on the two-phase fuel transient injection characteristics of the high-pressure natural gas and diesel co-direct injection engine

In this paper, an innovative fuel injection rate measurement methodology for the natural gas and diesel co-direct injection injector was designed to study the dual-fuel injection characteristics for a high-pressure natural gas direct injection engine. The results indicate that the injection mass shows two different upward trends as the energizing time (ET) increases, and it appears more sensitive to the length of ET when it is short. The gas injection duration is affected by both the injection pressure and ambient pressure under the same ET, thus any increase in either will significantly increase the injection duration, resulting in a pronounced increase in gas injection mass. The process of diesel pilot injection has a remarkable effect on the gas injection rate. According to the actual engine working condition, provided that the pilot diesel ET is 0.8 ms, the gas injection mass reaches the minimum value when the split time is about 0.8 ms, meanwhile, it reaches the maximum value when the split time is about 0.2 ms or 2 ms. The results of this study can provide an accurate database for the performance of the injector and accurate fuel injection boundary conditions for the combustion process in the engine cylinder.

Numerical Study on the Combustion and Emission Characteristics of a Direct Injection Natural Gas Engine Ignited by Diesel/N-Butanol Blends

Numerical Study on the Combustion and Emission Characteristics of a Direct Injection Natural Gas Engine Ignited by Diesel/N-Butanol Blends

Experimental Study on Entrainment Characteristics of High-Pressure Methane Free Jet.

Entrainment occurs during the high-pressure gas jet process, which is crucial for a natural gas direct injection engine. This study presents an experimental investigation on the high-pressure methane jet from one single-hole injector and proposes a method to obtain the entrainment mass flow rate based on kinetic energy conservation. The entrainment is related to three variables, i.e., spring plate moving distance Δx, gas jet mass at the nozzle outlet mn, and gas jet velocity u1. A spring-set test rig is built to measure the spring plate moving distance Δx, and the schlieren method is adopted to test the gas jet velocity u1 based on a constant-volume bomb (CVB) optical test rig; finally, the weight method is used to obtain the methane gas jet mass at the nozzle outlet mn. This combined measuring method is verified to be valid in the near field to the nozzle. The results show that the methane jet mass flow rate gradually increases along the jet direction and has a two-zone entrainment process. Zone I: near field (Lr < 10), the methane jet mass flow rate linearly increases up to the maximum; in the nozzle exit field (Lr < 1), it is conserved, and no entrainment occurs. Zone II: far field (Lr ≥ 10), the jet mass flow rate maintains the maximum, and the entrainment becomes saturated with a saturation value larger than the initial value at the nozzle outlet. The entrainment rate experiences three stages, linearly increasing in stage I and early stage II but not in late stage II and stage III. The methane injection pressure causes great effects on the mass flow rate and entrainment. As the injection pressure increases, the methane jet mass flow rate increases linearly, but the entrainment rate decreases.

Performance and emissions of a pilot ignited direct injection natural gas engine operating at slightly premixed combustion mode

Due to the increased attention on energy saving and carbon emission reduction, the research on techniques for the improvement of engine efficiency has become more attractive. Slightly premixed combustion mode has been considered as an effective method for raising thermal efficiency. In this paper, the performance and emissions of a pilot ignited direct injection natural gas engine operating at slightly premixed combustion mode were evaluated by experiments and compared to those of mixing-limited combustion mode. The results indicated that brake thermal efficiency could be improved by up to 1.1% to 3.6% when slightly premixed combustion mode is utilized at different engine loads. At BMEP higher than 6.5 bar, adoption of slightly premixed combustion mode could cause NOx emissions to rise by more than 7.3% while at BMEP lower than 13.0 bar, significant increases in THC emissions may be accompanied. It is also revealed that using slightly premixed combustion mode could reduce CO emissions by more than 35.2% at BMEP higher than 6.5 bar and reduce soot emissions by more than 55.8% at all engine loads. Moreover, the effects of the injection parameters are highly dependent on engine load, which means that the trends for thermal efficiency and emissions are different at different engine loads.

Knocking characteristics of a high pressure direct injection natural gas engine operating in stratified combustion mode

Abstract Knocking becomes an increasingly important issue in direct injection natural gas engines with the application of new combustion modes. In this article, the knocking characteristics of natural gas engine operating in stratified combustion mode were studied with the aid of cylinder pressure oscillations and combustion parameters. The results indicated that knocking tendency will be stronger when operating in stratified combustion mode. The first to the fourth circumferential modes and the first radial mode are the featured modes for knocking behavior, while knocking is more serious when the duration of 10–50% of total energy released is shorter.

Effects of injection strategy on the knocking behavior of a pilot ignited direct injection natural gas engine

Pilot ignited direct injection natural gas engines have become a research focus due to the high thermal efficiency and low emissions. However, when stratified combustion mode is adopted, more attention should be paid to the knocking behavior. In this paper, the effects of injection strategy on the knocking behavior of a pilot ignited direct injection natural gas engine have been studied. The results indicated that the knocking intensity will be increased at advanced natural gas injection timing, delayed diesel injection timing and raised injection pressure. The three major resonance modes that dominate the pressure oscillations are the first circumferential mode, the third circumferential mode and the first radial mode. Advancing natural gas injection has negative effects on cyclic variations of indicated mean effective pressure while have negligible effects on the cyclic variations of combustion duration. After correlation analysis of parameters, it was found that knocking behavior is highly correlated with maximum heat release rate, combustion duration and maximum combustion pressure, whereas has weak correlations with combustion phasing and cyclic variations.

Experimental investigation on the combustion and emissions in a pilot ignited direct injection natural gas engine using HCDI strategy

Pilot ignited direct injection natural gas engines have been increasingly attractive owing to their advantages in thermal efficiency and emissions. For conventional pilot ignited direct injection natural gas engines, soot emissions have been considered as an urgent issue, which could be possibly resolved by the adoption of homogenous charge direct injection strategy. To prove this viewpoint, this paper experimentally studied the effects of homogenous charge direct injection strategy on the combustion, emissions, thermal efficiency and operation stability of a pilot ignited direct natural gas engine. The results indicated that at low load, the application of homogenous charge direct injection strategies has negative effects on NOx, CO and HC emissions; though soot emissions could be improved, the negative effects seem to be more predominant. At high load, though NOx and HC emissions are impaired with the application of homogenous charge direct injection strategies, CO and soot emissions could be substantially improved with optimized injection parameters. Thermal efficiency could be slightly improved at low load condition and could be greatly raised at high load condition. Additionally, the adoption of homogenous charge direct injection strategies would increase the instabilities of engine operation, however, this behavior is more significant at low load condition.

Effect of the HPDI and PPCI Combustion Modes of Direct-Injection Natural Gas Engine on Combustion and Emissions

Natural gas (NG) engines have very broad application prospects. The pilot-ignited NG diesel engine can be organized into two combustion modes according to the sequence of oil and gas injection: (1) High-pressure direct injection, where NG is mainly diffused combustion; and (2) partially premixed compression ignition, where NG is mainly premixed combustion. In this study, we used CONVERGE to explore the influence of the NG injection timing on the distribution of the mixture equivalence ratio, ignition characteristics, thermal efficiency, emission, and combustion reaction rate under the two combustion modes. We also used a multi-step soot model to analyze the particle mass and quantity. We showed herein that the NG injection timing significantly affects the mixture distribution in the cylinder, thereby consequently affecting the combustion process. Both very early and very late injection times were not conducive to NG combustion. In addition, the mass, quantity, and diameter of the soot produced by diffusion combustion were larger than those produced with premixed combustion.