Ignition Natural Gas Engine Research Articles

Experimental and computational study on the effects of exhaust gas recirculation on thermodynamics, combustion and emission characteristics of a diesel pilot ignition natural gas engine

This study investigates the impact of diesel pilot ignition (DPI) natural gas (NG) engines on combustion and emission characteristics across various exhaust gas recirculation (EGR) volumes.

Combustion Process of the Compound Supply CNG Engine

Objective: In order to study the lean combustion process of a natural gas engine by separating the combustor, a spark ignition natural gas engine with separated combustors was retrofitted from a S195 single-cylinder diesel engine. Methods: The electronic control system controlled the gas supply and the spark plug ignition. A low pressure injection valve was set in the inlet pipe to form a lean mixture while a high pressure injection valve was placed in the subsidiary chamber to create a rich mixture, which was then ignited and injected into the main combustor, where the lean mixture was subsequently ignited again to achieve stratified combustion. Results: The test results showed that steady ignition is feasible in the system and verified the impact of the shape of the main combustor on HC, the impact of channel diameter on NOX production, and the impact of the ratios of high-pressure gas and low-pressure gas on HC and NOX. The combustion conditions of high-pressure gas and low-pressure gas in the engine combustor vary greatly. Our results signify that the shape of the main combustor has a great impact on the performance of the engine, that is, a shorter propagation distance can reduce the generation of HC. Conclusion: The best ignition advance angle under different conditions was determined using a spark ignition natural gas engine. The ratios of high-pressure gas and low-pressure gas greatly impact the performance and emission of the engine. The reduced diameter of the channels between the main and subsidiary combustors can enhance the stratification and facilitate the secondary ignition.

Effects of Wobbe Index on the Combustion and Emission Characteristics of a Natural Gas/Diesel RCCI Engine

Abstract Due to energy shortage and environmental problems, the application of natural gas in internal combustion engine has attracted extensive attention. Therefore, diesel pilot ignition natural gas engine is a promising technology. However, the different sources of natural gas lead to the change in composition, which has a great impact on engine combustion and emission. In this study, the relationship between Wobbe index (WI) and swirl ratio (SR) of six different natural gas mixtures was studied by numerical simulation method. Besides, reactivity controlled compression ignition (RCCI) combustion strategy was evaluated. The results showed that increasing the WI increased the in-cylinder pressure and temperature, increased the ignition delay, and shortened the combustion duration, the gross indicated efficiency (GIE) of the six gases exceeded 50%. In addition, the increase of WI increased the nitrogen oxide (NOx) emissions and reduced the hydrocarbon (HC) and carbon monoxide (CO) emissions. Moreover, the peak pressure rise rate (PPRR) increased with the rise of WI, which may lead to engine knock. The results also showed that the increase of SR increased the in-cylinder pressure and temperature and improved the PPRR. When the SR was 0.7 and the WI was 51.7, the combustion and emission performance of the RCCI engine was relatively better.

Multi-scale dynamics for a lean-burn spark ignition natural gas engine under low load conditions

This work investigates the multi-scale dynamics of the combustion system in a lean-burn spark ignition natural gas engine using different gas injection timings (GIT). The in-cylinder pressure time series are measured, and the indicated mean effective pressure (IMEP) time series are calculated. The influence of GIT on the combustion system is investigated through wavelet analysis, multi-resolution analysis, and the return maps with the GIT covering from 0 to 90°CA after top dead center at the intake stage. Results show that the combustion system has multi-scale chaotic characteristics, and it is sensitive to the change of GIT. The unreasonable GIT will result in serious combustion variations. Meanwhile, the combustion instability of the engine evolves on multiple time scales, showing evident multi-scale oscillation characteristics. All the wavelet power spectrums (WPS) present the characteristics of intermittent short-time periodic oscillations and persistent large-scale periodic oscillations concealed inside the IMEP time series. When the GIT approaches the medium value, the persistence of large-scale periodic oscillations is weakened, while the characteristics of high-frequency intermittent oscillations are enhanced. Under all working conditions, the contribution rate of high-frequency signal D1 decomposed by the IMEP time series to the overall time series fluctuation is about 40% or even higher. The contribution rate of the signal D1 also increases with the aggravation of combustion instability. The return map structures of high-frequency signals D1 and D2 show bifurcation structures, and the bifurcation characteristics of the signal D1 are more evident under medium GIT conditions, indicating a certain correlation between the 2–8 engine cycles, and the correlation between the adjacent engine cycles is stronger. The deterministic relationship between the multiple engine cycles found can help in developing a more reasonable and efficient combustion control strategy, which provides a theoretical basis for improving the stability of a natural gas engine.

Corrigendum to “Effects of gas injection timing on combustion instability for a spark ignition natural gas engine under low load conditions” [Appl. Therm. Eng. 206 (2022) 118144

Corrigendum to “Effects of gas injection timing on combustion instability for a spark ignition natural gas engine under low load conditions” [Appl. Therm. Eng. 206 (2022) 118144

Effects of gas injection timing on combustion instability for a spark ignition natural gas engine under low load conditions

• The entire cycle of in-cylinder pressure time series is applied to study the CCV. • The return maps and phase space reconstruction are used as the analysis methods. • The gas injection timing is used as the impact factor. • The chaotic behavior is stronger when the GIT is near 60°CA ATDC. Experiments are carried out with different GITs under 10% and 25% load conditions to reveal the influence of gas injection timing (GIT) on combustion instability for a spark ignition natural gas engine under low load conditions. The phase space reconstruction and coefficient of cyclic variation (CCV) of the entire cycle in-cylinder pressure time series, the return maps of indicated mean effective pressure (IMEP) and CA50, the distributions of CA5 and CA90 are applied to analyze the combustion instability of the natural gas engine qualitatively and quantitatively. Results show that the combustion process of the natural gas engine presents chaotic characteristics. With increasing GIT from 0°CA to 90°CA ATDC, the CCV and the separation degree of phase space attractor initially increase and then decrease. The distributions of return map points for IMEP and CA50 initially disperse and then concentrate, and the combustion instability and chaotic characteristics are initially enhanced and then weaken. When the GIT is near 60°CA ATDC, the chaotic characteristics of combustion instability for the natural gas engine is the strongest. The reasons for the influence of GIT on combustion instability of the natural gas engine are identified based on 3D computational fluid dynamic (CFD) simulation, showing that the pressure difference between in-cylinder and the intake manifold under different GITs leads to the difference in the mixing degree of the natural gas and air as well as the CH 4 concentration distribution of in-cylinder. This finding provides a theoretical basis for improving the lean-burn stability of natural gas engines.

A Study on the Correlation of Factors Influencing the Cyclic Variation of Marine Natural Gas Engines

As the most reliable alternative energy source for traditional fuel engines, natural gas has been widely used in inland river marine engines. The natural gas engine is faced with the problem of increased cyclic variations under the condition of lean combustion. In this paper, a multi-point injection spark ignition natural gas engine is tested under different lean burn degrees to investigate the correlation between different parameters and cyclic variation, including accumulated heat release, combustion phase and burning rate. The purpose is to clarify the optimized technical route for marine natural gas engine. A new method to quantify the correlation between parameters and cyclic variation is proposed. The maximum explosion pressure of combustion and its phase are used as the parameters to characterize the cyclic variation. Different parameters are then nonlinearly fitted to it, and the R-S value of the fitting is used to quantify the correlation between parameters and cyclic variation. The results show that the correlation between accumulated heat release and cyclic variation is less than 10%. The main factors causing the cyclic variation are the fluctuation of ignition delay and initial flame propagation, whose correlation with cyclic variation is over 80% and 70%, respectively.

An experimental investigation on performance of converted CNG engine by varying piston bowl geometry: A case study

ABSTRACT Using natural gas as an alternative fuel will be reduced exhaust emissions and dependence on petroleum. The practical solution for Vietnam’s conditions is to convert diesel engines into natural gas engines. Understanding the effect of piston geometry on the performance of converted CNG engines helps domestic manufacturers come up to spark ignition natural gas engines with high performance and low emission. In this work, three different types of combustion chamber were made from milling piston head and varying thickness of cylinder head gasket. The experiment results showed that torque and power of ε = 11.5 were more significant than ε = 12.5. During experiment processes, the stable operation state of the converted engine with ε = 12.5 was challenging to control, and the noise levels were intense. The results of mass fraction burned were presented the effect of squish gap on performance was more substantial than that of the squish area. It can be concluded that for modifying the original diesel engines to run on CNG fuel, the maximum compression ratio should be set to 12.5 and must be redesigned accordingly to use the natural results. Implications: This manuscript shows the new technology to convert traditional engines into engines fueled by renewable energy. It contributes to reducing the fossil fuel crisis and environmental emissions.

Experimental investigation the effects of spark discharge characteristics on the heavy-duty spark ignition natural gas engine at low load condition

The fuel economy of natural gas (NG) spark ignition (SI) engine could improve significantly when using high exhaust gas recirculation (EGR) dilution. However, the combustion processes become instability and even occur misfire at higher EGR rate. In this paper, the high energy ignition system, which was the technology of variable spark, was utilized to extend the EGR tolerance of a SI engine. Extensive experimental tests were conducted to investigate the effects of spark discharge current and duration on combustion and emissions characteristics at low load condition. The results showed that the fuel economy was improved with the spark discharge energy at the high EGR rate of 30%. The increase amplitude of fuel economy was 1.32% when the spark discharge energy increased from 210 to 320 mJ. The effective spark discharge energy was existed in consideration of the fuel economy. The ignition delay (CA0-5) and the flame propagation period (CA10-90) shortened with an increase in spark discharge energy. Besides, NOx emissions were more sensitive to the spark discharge energy than that of HC and CO emissions. As the spark discharge energy increased, the increase extent of NOx emissions was 26.9%, while HC and CO emissions decrease slightly.

Investigation of cycle skipping methods in an engine converted to positive ignition natural gas engine

In this study, a single cylinder of 1.16 L, naturally aspirated engine was converted to a spark ignition engine, which was a diesel engine operating with natural gas as fuel. By placing electronic throttle, electronic ignition module, gas fuel injectors and proximity sensors on the test engine, the engine has been turned into a positive ignition engine that can work with natural gas as fuel, thanks to the electronic control unit developed by our project team. Then, in the study performed, different cycle skipping strategies were experimentally investigated at a constant engine speed of 1565 rpm, in accordance with the generator operating conditions. Engine performance, emissions (CO, HC, and NOx), and combustion characteristics (cylinder pressure, rate of heat release, etc.) of cycle skipping strategies were experimentally investigated with natural gas as fuel in Normal, 3N1S, 2N1S, and 1N1S engine operating modes. According to the results obtained, specific fuel consumption, CO and HC values improved in all cycle skipping operating conditions, except for NOx, but the best results were obtained in 2N1S operating conditions; it was concluded that the specific fuel consumption, CO and HC values improved by 11.19%, 61.89%, and 65.60%, respectively.

Parametric Knocking Performance Investigation of Spark Ignition Natural Gas Engines and Dual Fuel Engines

Both spark ignition (SI) natural gas engines and compression ignition (CI) dual fuel (DF) engines suffer from knocking when the unburnt mixture ignites spontaneously prior to the flame front arrival. In this study, a parametric investigation is performed on the knocking performance of these two engine types by using the GT-Power software. An SI natural gas engine and a DF engine are modelled by employing a two-zone zero-dimensional combustion model, which uses Wiebe function to determine the combustion rate and provides adequate prediction of the unburnt zone temperature, which is crucial for the knocking prediction. The developed models are validated against experimentally measured parameters and are subsequently used for performing parametric investigations. The derived results are analysed to quantify the effect of the compression ratio, air-fuel equivalence ratio and ignition timing on both engines as well as the effect of pilot fuel energy proportion on the DF engine. The results demonstrate that the compression ratio of the investigated SI and DF engines must be limited to 11 and 16.5, respectively, for avoiding knocking occurrence. The ignition timing for the SI and the DF engines must be controlled after −38°CA and 3°CA, respectively. A higher pilot fuel energy proportion between 5% and 15% results in increasing the knocking tendency and intensity for the DF Engine at high loads. This study results in better insights on the impacts of the investigated engine design and operating settings for natural gas (NG)-fuelled engines, thus it can provide useful support for obtaining the optimal settings targeting a desired combustion behaviour and engine performance while attenuating the knocking tendency.

Effects of n-heptane enrichment on in-cylinder thermochemical fuel reforming (TFR) characteristics and performances of spark ignition natural gas engine: A comparison with natural gas and methanol enrichment

In this study, the effects of n-heptane enrichment on in-cylinder thermochemical fuel reforming (TFR) characteristics and performances of a spark ignition (SI) natural gas (NG) engine were examined. Methanol and NG enrichment cases were also involved for comparative analyses. It was found that n-heptane enrichment substantially improved NG in-cylinder thermochemical fuel reforming as methanol enrichment did. Under n-heptane enrichment case, H2 and CO in the reformate could reach up to 4%(v) and 6.5%(v), respectively, when equivalence ratio of TFR cylinder reaches 1.40. At the same time, very slight fraction of unburnt hydrocarbon exists in the reformate, indicating a robust fuel reforming in TFR cylinder. With n-heptane enrichment, more stable TFR cylinder combustion was achieved compared with NG enrichment with a concentrated CA50-IMEP distribution for TFR cylinder and lowered standard deviation of IMEP for cylinder #2 (typical of normal cylinders). Besides, compared with NG enrichment, a significantly weaker discrepancy between TFR cylinder and cylinder #2 was achieved with n-heptane enrichment. Similar to methanol enrichment case, n-heptane enrichment did not obviously affect engine CO and THC emissions while remarkably reduced engine NOx emission. With n-heptane enrichment, more than 10 g/kWh BSFC discount could be achieved, though slight knock happens, relative to either NG or methanol enrichment case, indicating better fuel economy.

Effects of propanol isomers enrichment on in-cylinder thermochemical fuel reforming (TFR) in spark ignition natural gas engine

This paper investigates the effects of propanol isomers enrichment on natural gas TFR engine via experiment and simulation. The results indicate that the reforming effect of TFR cylinder with n-propanol enrichment is the best, followed by that with iso-propanol, and with that of the natural gas being the worst. The impact of negative effect due to lack of fire was the most significant on the reforming effects with n-propanol enrichment, less on those with iso-propanol, and the least on those with natural gas. According to the kinetics analysis, the rate of H2 formation depends on the rates of H, O, OH, and HO2 formation. The n-propanol had the fastest production rate for H, O, OH, and HO2 radicals in the early stage of combustion, compared with iso-propanol and methane. The performance analysis of the normal cylinder shows that n-propanol enriched normal cylinders produced the lowest fuel consumption.

Influences of natural gas energy fraction on combustion and emission characteristics of a diesel pilot ignition natural gas engine based on a reduced chemical kinetic model

In this research, a reduced chemical kinetic mechanism was coupled in computational fluid dynamics (CFD) model to study the detailed influence mechanism of natural gas (NG) energy fraction on the diesel pilot ignition (DPI) NG engines. Based on the CFD model calibrated by experimental data, the impacts of NG energy fraction on the combustion and emission generation processes in DPI NG engine were investigated. The results show that, the start of combustion (SOC) in DPI NG engine is advanced as the NG energy fraction declines and the maximum advance comes up to 4.1°CA at 1000 rpm and 100% load. The deep analysis reveals that the positive influence of R342 on the temperature is the main reason for the shorter ignition delay. The 10–90% combustion duration increases first as the NG energy fraction changes from 50% to 60%, and then decreases if the NG energy fraction further increases. Furthermore, the combustion mode is changed from premixed combustion to non-premixed combustion with the NG energy fraction decreasing. For the emissions, the NOx emission drops with the decline of NG energy fraction, and the maximum decrease of NOx emission comes up to 0.96 mg/kJ at 1000 rpm and 100% load. The CO emission increases first and then decreases as the NG energy fraction rises from 50% to 90%.

Numerical investigation on the effects of valve timing on in-cylinder flow, combustion and emission performance of a diesel ignition natural gas engine through computational fluid dynamics

In the present study, a diesel engine was modified to a diesel pilot ignited natural gas engine and the influences of intake valve closing timing on in-cylinder flow, combustion and emission performance of engine were investigated by three-dimensional computational fluid dynamics simulation. Based on the geometric model and basic parameters of this engine, the simulation model was built under three operating conditions and then validated by experimental data. On this basis, the validated model was applied to investigate the effects of the intake valve closing timing strategy. The simulation results indicated that, the volumetric efficiency decreases with the retarding of intake valve closing timing in three cases while it increases as the intake valve closing timing is advanced by 10°CA at 1200 rpm. The in-cylinder turbulence kinetic energy decreases with the retarding of intake valve closing timing. The peak in-cylinder pressure decreases when the intake valve closing timing is either advanced or retarded at 50% load. Nevertheless, the maximum peak in-cylinder pressure occurs at advancing 10°CA intake valve closing timing at 1200 rpm and 100% load, which rises by 4.5 bar compared with that at the original intake valve closing timing. Additionally, the maximum heat release rate appears at advancing 10°CA intake valve closing timing at 100% load, which is 26 J/deg higher than that at the original intake valve closing timing. Simultaneously, the shortest combustion duration occurs at advancing 10°CA intake valve closing timing at 1200 rpm and 100% load. For the emissions, the NOx emissions decrease with the retarding of intake valve closing timing but the variation becomes unobvious with the advancing of the intake valve closing timing. Besides, the intake valve closing timing strategy has little effect on HC and CO emissions.

Influences of excess air coefficient on combustion and emission performance of diesel pilot ignition natural gas engine by coupling computational fluid dynamics with reduced chemical kinetic model

In the presented study, the influence of lean-burn on combustion and emission performance of diesel pilot ignition natural gas engine was investigated by using the method of computational fluid dynamics coupling with the reduced chemical kinetic model. Based on bench tested results, the computational fluid dynamics model was validated in four typical conditions, and then it was used for the simulation at different excess air coefficient. Due to the visibility of computational fluid dynamics results, the combustion medium process and emissions medium products were obtained, which then were used to explain the influence mechanism of excess air coefficient. The simulated results show that, under 50% load, the maximum cylinder pressure becomes larger and the start of combustion is advanced when the excess air coefficient increases from 1.0 to 1.5, and the maximum advance of the start of combustion reaches 9.5 °CA. Nevertheless, under 100% load, the start of combustion is advanced first and then retarded. Meanwhile, the higher the excess air coefficient is, the earlier the heat release rate shoots up. When the excess air coefficient increases from 1.2, the 10–50%, 50–90% and 10–90% combustion duration become longer. The nitrogen oxide emission increases as the excess air coefficient rises from 1.0 to 1.1 but decreases if it continues to increase. The unburned methane emission decreases first and then increases with the increase of the excess air coefficient. Nevertheless, at 1500 rpm and full load, the unburned methane emission shoots up as the excess air coefficient changes from 1.3 to 1.5 and the maximum difference of unburned methane emission reaches 3233 ppm.

An Experimental Investigation on the Combustion Process of a Simulated Turbocharged Spark Ignition Natural Gas Engine Operated on Stoichiometric Mixture

This research investigated the combustion process of an AVL Model LEF/Volvo 5312 single cylinder engine configured to simulate the operation of a heavy-duty spark ignition (SI) natural gas (NG) engine operated on stoichiometric mixture. The factors affecting the combustion process that were examined include intake pressure, spark timing (ST), and the addition of diluents including nitrogen (N2) and carbon dioxide (CO2) to the NG to simulate low British thermal unit (BTU) gases. The mixing of diluents with NG is able to slow down the flame propagation speed, suppress the onset of knock, and allow the engine to operate on higher boost pressure for higher power output. The addition of CO2 was more effective than N2 in suppressing the onset of knock and slowing down the flame propagation speed due to its high heat capacity. Boosting intake pressure significantly increased the heat release rate (HRR) evaluated on J/°CA basis which represents the rate of mass of fuel burning. However, its impact on the normalized HRR evaluated on %/°CA basis, representing the flame propagation rate, was relatively mild. Boosting the intake pressure from 1.0 to 1.8 bar without adding diluents increased the peak HRR to 1.96 times of that observed at 1.0 bar. The increase was due to the burning of more fuel (about 1.8 times), and the 12.9% increase in the normalized HRR. The latter was due to the shortened combustion duration from 23.6 to 18.2 °CA, a 22.9% reduction. The presence of 40% CO2 or N2 in their mixture with NG increased the peak cylinder pressure (PCP) limited brake mean effective pressure (BMEP) from 17.2 to about 20.2 bar. The combustion process of a turbocharged SI NG engine can be approximated by referring to the HRR measured under a naturally aspirated condition. This makes it convenient for researchers to numerically simulate the combustion process and the onset of knock of turbocharged SI NG engines using combustion process data measured under naturally aspirated conditions as a reference.

Improving cold start, combustion and emission characteristics of a lean burn spark ignition natural gas engine with multi-point hydrogen injection

The cold start, combustion and emission characteristics of a four-cylinder spark ignition (SI) natural gas (CNG) engine were optimized using multi-point hydrogen gas (H2) injection, exhaust gas recirculation (EGR), and an oxidation catalyst (OC). The transient HC emission was also collected with Fast FID measurement within the period of cold start. The results indicate that multi-point H2 injection could improve the cold start performance of CNG SI engine. The peak cylinder pressure and HC emissions maintain stable quickly with hydrogen addition in the stage of cold start. The maximum in-cylinder pressure and the heat release rate increased linearly with the H2 injection duration. Multi-point H2 injection shortens the flame development duration and the combustion duration. The coefficient of variation (COV) of the indicated mean effective pressure (IMEP) increased with the EGR rate and decreased with H2 multi-point injection. Multi-point H2 injection can improve EGR tolerance for CNG engine. The OC could effectively reduce CO emissions for all of the different conditions, but the conversion efficiency for the HC emissions was insufficient. The combination of lean burn combustion, EGR, multi-point H2 injection, and a more effective OC is a potential pathway for CNG engine optimization.

Effects of ethanol enrichment on in-cylinder thermochemical fuel reforming (TFR) spark ignition natural gas engine

In this study, the concept of in-cylinder thermochemical fuel reforming (TFR), which involves running one cylinder rich of stoichiometric and routing its entire exhaust back into the intake manifold, were examined on a four-cylinder spark ignition natural gas (NG) engine. Extra natural gas fuel (NG enrichment) was port injected into cylinder #4 (TFR cylinder) to realize its rich running. Results indicated that the combustion of cylinder #2 (typical of cylinder #1, 2 and 3) was significantly improved with substantial H2 and CO fraction in the reformed gas when equivalence ratio of TFR cylinder (ΦTFR) was about 1.25. However, not only the stability of TFR cylinder was worsened, but also the discrepancy of IMEP between cylinder #2 and TFR cylinder remarkably increased with the increase of ΦTFR. To improve engine performances, a trial of enriching TFR cylinder using ethanol instead of NG was performed. Results manifested that ethanol enrichment improved H2 and CO concentration in reformed gas and led to a more stable engine operation. Besides, ethanol enrichment also lowered the discrepancy of IMEP between cylinder #2 and TFR cylinder, indicating a better engine uniformity. However, ethanol enrichment did not severely affect engine HC and CO emissions while remarkably lowered NOx emission compared with NG enrichment.

Extending the lean operating range of a premixed charged compression ignition natural gas engine using a pre-chamber

One major drawback about premixed charge compression ignition (PCCI) engines is incomplete combustion and producing high levels of carbon monoxide (CO) emission in lean operating conditions. This issue becomes even more serious when fuels with high octane number, e.g. natural gas, are used. In the present work, the combustion process of premixed charge natural gas engines is studied by means of numerical tools. The studied engine concepts are: conventional homogenous charge compression ignition (HCCI), HCCI with a pre-chamber and PCCI with a pre-chamber. The numerical model is validated against the experimental data and then, combustion characteristics and engine emissions in various operating regions, are discussed and compared. The results reveal that the PCCI case has the earliest start of combustion. It is also shown that in the ultra lean operating conditions (fuel equivalence ratio of 0.1) decreasing the intake temperature from 450 K results in an efficiency increment for the PCCI engine, while for the HCCI case this causes misfiring and more CO emission. Nevertheless, the PCCI engine has higher levels of NO emission compared to the HCCI cases and only in the ultra lean condition this level reaches amounts less than the acceptable limits set by Euro 6 emission regulations.