Fundamental characterization of backfire in a hydrogen fuelled spark ignition engine using CFD and experiments

Backfire, an abnormal combustion phenomenon, in a hydrogen fuelled spark ignition (SI) engine was analyzed using computational fluid dynamics (CFD) and experimental tests. One of the main causes of backfire origin is the presence of any high temperature heat source including hot spot in the combustion chamber of the engine during intake process. A CFD based parametric study was carried out by varying the temperature of hot spot and its location in the combustion chamber of the engine in order to analyze their effects on backfire origin and its propagation in the intake manifold of the engine. The temperature of hot spot was varied from 800 K to till the temperature of backfire occurrence. The minimum temperature of hot spot at which backfire occurred was observed as 950 K and beyond. The probability of backfire occurrence increases with increase in hot spot temperature. The CFD simulations were also carried out by varying the location of hot spot (spark plug tip and exhaust valve) and the results indicate that the location of hot spot does not influence the characteristics of backfire but it affects the timing of its origin. The average backfire velocity is 230 m/s based on the average turbulent flame velocity during backfire propagation in the intake manifold and the value agreed reasonably well with the experimental observations of backfire propagation on the engine with the transparent intake manifold. Backfire propagation is under the category of deflagration based on its velocity (subsonic), and the maximum pressure gradient (<0.3 bar). The backfire phenomenon is characterized into three stages namely ignition delay for backfire, backfire propagation and its termination. The study results provide a better in-depth understanding of backfire origin and its propagation and would be helpful for developing a robust control strategy. Based on this study, it is recommended that the spark plug and exhaust valves of hydrogen fuelled SI engine should be customized in such a way that the temperature of spark plug tip and exhaust valves should not exceed 900 K during suction process in order to eliminate backfire occurrence.     

Development of online control system for elimination of backfire in a hydrogen fuelled spark ignition engine

Backfire is one of the major technical issues in a port injection type hydrogen fuelled spark ignition engine. It is an abnormal combustion phenomenon (pre-ignition) that takes place in combustion chamber and intake manifold during suction stroke. The flame propagates toward the upstream of the intake manifold from combustion chamber during backfire and thus can damage the intake and fuel supply systems of the engine, and stall the engine operation. The main cause of backfire could be the presence of any hot spot, lubricating oil particle's traces (HC and CO due to evaporation of the oil) and hot residual exhaust gas present in the combustion chamber during suction stroke which could act as an ignition source for fresh incoming charge. Monitoring the temperatures of the lubricating oil and exhaust gas during engine operation can reduce the probability of backfire. This was achieved by developing an electronic device which delays the injection timing of hydrogen fuel with the inputs of engine oil temperature (T_{lube oil}) and exhaust gas temperature (T_exh). It was observed from the experimental results that the threshold values of T_{lube oil} and T_exh were 85 °C and 540 °C respectively beyond which backfire occurred at equivalence ratio (φ) of 0.82. The developed device works based on the algorithm that retards the hydrogen injection to 40 ⁰aTDC whenever the temperatures (T_{lube oil} and T_exh) reached to the above mentioned values and thus the backfire was controlled. Delaying injection of hydrogen increased the time period at which only air is inducted during the early part of the suction stroke, this allows cooling of the available hot spots in the combustion chamber, hence the probability of backfire would be reduced.     

Experimental investigation on effects of knocking on backfire and its control in a hydrogen fueled spark ignition engine

The experimental study was carried out on a multi-cylinder spark ignition engine fueled with hydrogen for analyzing the effect of knocking on backfire and its control by varying operating parameters. The experimental tests were conducted with constant speed at varied equivalence ratio. The equivalence ratio of 0.82 was identified as backfire occurring equivalence ratio (BOER). The backfire was identified by high pitched sound and rise in in-cylinder pressure during suction stroke. In order to analyze backfire at equivalence ratio of 0.82, the combustion analysis was carried out on cyclic basis. Based on the severity of in-cylinder pressure during suction stroke, the backfire can be divided into two categories namely low intensity backfire (LIB) and high intensity backfire (HIB). From this study, it is observed that there is frequent LIB in hydrogen fueled spark ignition engine during suction stroke, which promotes instable combustion and thus knocking at the end of compression stroke. This knocking creates high temperature sources in the combustion chamber and thus causes HIB to occur in the subsequent cycle. A notable salient point emerged from this study is that combustion with knocking can be linked with backfire as probability of backfire occurrence decreases with reduction in chances of knocking. Retarding spark timing and delaying injection timing of hydrogen were found to reduce the chances of backfire occurrence. The backfire limiting spark timing (BLST) and backfire limiting injection timing (BLIT) were found as 12 ⁰bTDC and 40 ⁰aTDC respectively.     

Monitoring of hydrogen-fueled engine backfires using dual manifold absolute pressure sensors

The application of hydrogen in internal combustion engines (ICE) has attracted widespread attention. However, due to the low ignition energy, high flame propagation speed, and wide combustible range of hydrogen, it is easy to cause abnormal combustion phenomena such as backfire in the port fuel injected (PFI) hydrogen-fueled engine. When a backfire occurs, the combustible mixture burns in the intake, resulting in a decrease in the volumetric efficiency of the engine, which may cause it to misfire or shut down in severe cases. Fast and accurate detection of backfire events is essential to take targeted control measures. In this research, a backfire detection system based on dual intake manifold absolute pressure (MAP) sensors were designed, with two MAP sensors installed on the intake manifolds of the first and fourth cylinders, and four gas injectors were installed on the intake manifold to convert the gasoline ICE into a hydrogen-fueled ICE. During the experiment, the engine speed was stabilized at 1000 rpm, the throttle valve was fully opened, and the intake pressure was maintained at 100 kPa.The test results show that the system can accurately determine the location and intensity of backfire occurrence. This method provides a basis for precise control of backfiring.     

The effect of engine speed and cylinder-to-cylinder variations on backfire in a hydrogen-fueled internal combustion engine

The port-injection-type hydrogen engine is advantaged in that hydrogen gas is injected into the intake pipe through a low-pressure fuel injector, and the mixing period with air is sufficient to produce uniform mixing, improving the thermal efficiency. A drawback is that the flame backfires in the intake manifold, reducing the engine output because the amount of intake air is reduced, owing to the large volume of hydrogen. Here, the backfire mechanism as a part of the development of full-load output capability is investigated, and a 2.4-liter reciprocating gasoline engine is modified to a hydrogen engine with a hydrogen supply system. To secure the stability and output performance of the hydrogen engine, the excess air ratio was controlled with a universal engine control unit.The torque, excess air ratio, hydrogen fuel, and intake air flow rate changes in time were compared under low- and high-engine speed conditions with a wide-open throttle. The excess air ratio depends on the change in the fuel amount when the throttle is completely opened, and excess air ratio increase leads to fuel/air-mixture dilution by the surplus air in the cylinder. As the engine speed increases, the maximum torque decreases because the excess air ratio continues to increase due to the occurrence of the backfire. The exhaust gas temperature also increases, except at an engine speed of 6000 rpm. Furthermore, the increase in exhaust gas temperature affects the backfire occurrence. At 2000 rpm, under low-speed and wide-open throttle conditions, backfire first occurs in the No. 4 cylinder because the mixture is heated by the relatively high port temperature. In contrast, at 6000 rpm, under high-speed and wide-open throttle conditions, the backfire starts at the No. 2 cylinder first because of a higher exhaust gas temperature, resulting in a lower excess air ratio in cylinders 2 and 3, located at the center of the engine.     

Conversion of a commercial spark ignition engine to run on hydrogen: Performance comparison using hydrogen and gasoline

The modifications performed to convert the spark ignition gasoline-fueled internal combustion engine of a Volkswagen Polo 1.4 to run with hydrogen are described. The car is representative of small vehicles widely used for both city and interurban traffic. Main changes included the inlet manifold, gas injectors, oil radiator and the electronic management unit. Injection and ignition advance timing maps were developed for lean mixtures with values of the air to hydrogen equivalence ratio (λ) between 1.6 and 3. The established engine control parameters allowed the safe operation of the hydrogen-fueled engine (H₂ICE) free of knock, backfire and pre-ignition as well with reasonably low NO_x emissions. The H₂ICE reached best brake torque of 63 Nm at 3800 rpm and maximum brake power of 32 kW at 5000 rpm. In general, the brake thermal efficiency of the H₂ICE is greater than that of gasoline-fueled engine except for the H₂ICE working at very lean conditions (λ = 2.5) and high speeds (above 4000 rpm). A significant effect of the spark advance on the NO_x emissions has been found, specially for relatively rich mixtures (λ < 2). Small changes of spark advance with respect to the optimum value for maximum brake torque give rise to an increase of pollutant emissions. It has been estimated that the hydrogen-fueled Volkswagen Polo could reach a maximum speed of 140 km/h with the adapted engine. Moreover, there is enough reserve of power for the vehicle moving on typical urban routes and routes with slopes up to 10%.     

Effects of ignition advance on a dual sequential ignition engine at lean mixture for hydrogen enriched butane usage

In this study, the effects of ignition advance on dual sequential ignition engine characteristics and exhaust gas emissions for hydrogen enriched butane usage and lean mixture were investigated numerically and experimentally. The main purpose of this study is to reveal the effects of h-butane application in a commercial spark ignition gasoline engine. One cylinder of the commercially dual sequential spark ignition engine was modeled in the Star-CD software, taking into account all the components of the combustion chamber (intake-exhaust manifold connections, intake-exhaust valves, cylinder, cylinder head, piston, spark plugs). Angelberger wall approximation, k-ε RNG turbulence model and G-equation combustion model were used for analysis. In the dual sequential spark ignition, the difference between the spark plugs was defined as 5° CAD. At the numerical analysis; 10.8:1 compression ratio, 1.3 air-fuel ratio, 2800 rpm engine speed, 0.0010 m the flame radius and 0.0001 m the flame thickness were kept constant. The hydrogen-butane mixture was defined as 4%–96% by mass. In the analysis, the optimal ignition advance was determined by the working conditions. In addition, the effects of changes in ignition advance were examined in detail at lean mixture. For engine operating conditions under investigation, it has been determined that the 50° CAD ignition advance from the top dead center is the optimal ignition advance in terms of engine performance and emission balance. It has also been found that the NO_x formation rises up as the ignition advance increases. The BTE values were approximately 12.01% higher than butane experimental results. The experimental BTE values for h-butane were overall 3.01% lower than h-butane numerical results.     

Experimental investigation of the effect of spark plug gap on a hydrogen fueled SI engine

In this work, an experimental study on the performance and exhaust emissions of a commercial hydrogen fueled spark ignition engine (HFSIE) was performed at partially and full wide open throttle (50% and 100% WOT) positions. The engine is a four-stroke cycle six-cylinder, engine volume of 4.9 L, port fuel injection, hydrogen fueled SI engine with a bore of 102.1 mm, a stroke of 101.1 mm and a compression ratio of 13.5:1. The experiments were performed using 3 different spark plug gaps (SPG) (0.4, 0.6 and 0.8 mm), varied engine speeds of 1000–3000 rpm and two ignition timing values (10 and 15° CA BTDC) at 50% and 100% wide open throttle (WOT). SPG is a factor affecting the performance of the engine depending on the engine structure. Maximum power values were obtained at 0.6 mm SPG for both 50% and 100% WOT at ignition timing values of 10 and 15° CA BTDC. The maximum efficiency values were obtained with a 0.8 mm SPG at 50% WOT. At 100% WOT position, the maximum efficiency values were obtained with a 0.6 mm spark plug gap (SPG) at ignition timing values of 10 and 15° CA BTDC. A significant decrease in NO emission was observed using hydrogen for all WOT and SPGs.     

Effect of pre-chamber geometrical parameters and operating conditions on the combustion characteristics of the hydrogen-air mixtures in a pre-chamber spark ignition system

The pre-chamber spark ignition system is a promising advanced ignition system adopted for lean burn spark ignition engines as it enables stable combustion and enhances engine efficiency. The performance of the PCSI system is governed by the turbulent flame jet ejected from the pre-chamber, which is influenced by the pre-chamber geometrical parameters and the operating conditions. Hence, the current study aims to understand the effects of pre-chamber volume, nozzle hole diameter, equivalence ratio, and initial chamber pressure on the combustion and flame jet characteristics of hydrogen-air mixture in a passive PCSI system. Pre-chamber with different nozzle hole diameters (1 mm, 2 mm, 3 mm, and 4 mm) and volumes (2%, 4%, and 6% of the engine clearance volume) were selected and manufactured in-house. The experimental investigation of these pre-chamber configurations was carried out in a constant-volume combustion chamber with optical access. The flame development process was captured using a high-speed camera at a rate of 20000 fps, and the images were processed in MATLAB to obtain quantitative data. The combustion characteristics of hydrogen-air mixtures with the PCSI system improved when compared to the conventional SI system; however, the improvement was more significant for ultra-lean mixtures. Early start of combustion and shorter combustion duration were observed for PCSI – D2 and PCSI – D3 configurations, respectively and improved combustion and flame jet characteristics were also noted for these configurations. With the increase in pre-chamber volume, ignition energy associated with the flame jet increases, which reduces the combustion duration and the ignition lag.     

On the ignition and flame development in a spray-guided direct-injection spark-ignition engine

High-speed fuel, flow, and flame imaging are combined with spark discharge measurements to investigate the causes of rare misfires and partial burns in a spray-guided spark-ignited direct-injection (SG-SIDI) engine over a range of nitrogen dilution levels (0–26% by volume). Planar laser induced fluorescence (PLIF) of biacetyl is combined with planar particle image velocimetry (PIV) to provide quantitative measurements of equivalence ratio and flow velocity within the tumble plane of an optical engine. Mie scattering images used for PIV are also used to identify the enflamed region to resolve the flame development. Engine parameters were selected to mimic low-load idle operating conditions with stratified fuel injection, which provided stable engine performance with the occurrence of rare misfire and partial burn cycles. Nitrogen dilution was introduced into the intake air, thereby displacing the oxygen, which destabilized combustion and increased the occurrence of poor burning cycles. Spark measurements revealed that all cycles exhibited sufficient spark energy and duration for successful ignition. High-speed PLIF, PIV, and Mie scattering images were utilized to analyze the spatial and temporal evolution of the fuel distribution and flow velocity on flame kernel development to better understand the nature of poor burning cycles at each dilution level. The images revealed that all cycles exhibited a flammable mixture near the spark plug at spark timing and a flame kernel was present for all cycles, but the flame failed to develop for misfire and partial burn cycles. Improper flame development was caused by slow flame propagation which prevented the flame from consuming the bulk of the fuel mixture within the piston bowl, which was a crucial step to achieve further combustion. The mechanisms identified in this work that caused slower flame development are: (1) lean mixtures, (2) external dilution, and (3) convection velocities that impede transport of the flame into the fuel mixture.     

Influence of laser ignition on characteristics of an engine for hydrogen enriched CNG and iso-octane usage

The study has focused on determining the laser plug effects on engine characteristics and the laser plug usage results have compared with spark plug usage. The laser ignition technique is a type of new ignition technique and an important solution that can make combustion systems more efficient. The testing of an engine with a laser plug is the novelty of the study and the tests were carried out with reference to equivalence ratio and plug power ranges. The behaviors of the engine at full load were examined so experimentally for both ignition techniques at hydrogen enriched CNG and iso-octane mixture usage. The tests were carried out for variations of 0.4–2.0 equivalence ratio and 20–120 W plug power. A mixture that 90% iso-octane and 10% HCNG in mass was used at two ignition modes in tests for 3300 rpm maximum engine torque speed. Also, the flame formation and propagation for both ignition techniques were detected via a high-speed camera. The tests have shown the laser ignition leads to more energy consumption in the rich mixture conditions and also, less energy is required in the lean conditions. The laser ignition discharge has extended the engine's lean combustion limits via a small energy input at the tests. The high-speed camera images have shown that the laser ignition reduces the Kernel flame formation and propagation time. The laser ignition technique was produced less NO_x than the conventional spark ignition method.     

Backfire prediction in a manifold injection hydrogen internal combustion engine

Hydrogen internal combustion engine (H2ICE) easily occur inlet manifold backfire and other abnormal combustion phenomena because of the low ignition energy, wide flammability range and rapid combustion speed of hydrogen. In this paper, the effect of injection timing on mixture formation in a manifold injection H2ICE was studied in various engine speed and equivalence ratio by CFD simulation. It was concluded that H2ICE of manifold injection have an limited injection end timing in order to prevent backfire in the inlet manifold. Finally, the limit of injection end timing of the H2ICE was proposed and validated by engine experiment.     

Experience and special aspects on mixture formation of an otto engine converted for hydrogen operation

As mentioned in previous papers, the two liter four cylinder BMW 520 engine, as used in the DFVLR experimental vehicle, was investigated with respect to different types of external mixture formation. The most convenient compromise was continuous port injection of gaseous hydrogen. Relative to gasoline operation, the hydrogen operated engine studied here shows more sensitivity against deviation from optimum equivalence ratio and proper ignition timing. Especially during transitions, the equivalence ratio should be kept as small as possible and the spark advance as large as possible. The maximum permissible response time must not exceed about 100 ms, and no intake manifold vacuum was available because of quality control. It was therefore very difficult to develop a pure mechanically operated system for ignition timing and mixture control. A modified Bosch L-Jetronic for water injection and mixture formation proved useful and application of a digital electronic system such as Bosch-Motronic enabled mixture formation as well as proper ignition timing. Based on preliminary results, cryogenic external mixture formation may act as a replacement for intake port water injection for suppression of uncontrolled preignition. An essential condition for proper engine operation is to eliminate the influence of the cold hydrogen temperature on the equivalence ratio at all operational conditions. From these experiments, it appears that future hydrogen engines may employ cryogenic mixture formation techniques controlled by digital motor electronics systems.     

Effect of turbocharger on performance and thermal efficiency of hydrogen-fueled spark ignition engine

This study systematically investigated the application of a turbocharger system to a hydrogen spark ignition engine to extend operating limitations under high loads. The exhaust system of a commercial 2.4-L natural aspiration spark ignition engine was modified by adopting a turbocharger system. Engine test speeds were 2000–6000 rpm at intervals of 1000 rpm. The intake pressure was fixed for each experimental case, however, the quantity of hydrogen and spark advance timings were varied before the back-fire occurred. High load conditions under natural aspiration and turbocharging conditions were compared. The results indicated that distinctly higher boosts with the turbocharging system helped extend high load conditions, however, the high exhaust pressure obstructed the increasingly high load conditions under high speeds.     

Research on optimum method to eliminate backfire of hydrogen internal combustion engines based on combining postponing ignition timing with water injection of intake manifold

Preignition or backfire occurs easily in hydrogen internal combustion engines (HICE) of manifold injection type, especially, the bigger equivalence ratio is, the more serious backfire happens. And decreasing equivalence ratio will reduce engine's power output. So to analyze and resolve the contradiction between abnormal combustion and power output in HICE is the key of promoting the progress of research on HICE. Postponing ignition timing is helpful to reducing the occurrence degree or inclination of pre-ignition, and water injection of intake manifold can be used to eliminate backfire. But postponement of ignition has a lesser effect on power output and brake thermal efficiency than water injection of intake manifold, That is to say water injection would bring power output to drop obviously, and water injection will also has many disadvantages, such as, the worse corrosion degree of cylinder and deteriorated lubrication performance. It is necessary to combine postponing ignition timing with water injection of intake manifold to give full play to their advantages, and avoid their disadvantages to the greatest extent. In the paper, the concept of pre-ignition strength and backfire strength were presented, and the inhibition degree of pre-ignition and the elimination degree of backfire was introduced. The functional relationship between inhibition degree of pre-ignition and ignition timing was established, and the functional relationship between the elimination degree of backfire and water injection rate was also established for quantitative analysis and research into inhibiting pre-ignition and eliminating backfire. A optimal control method was put forward about resolving contradiction between eliminating backfire and improving performance of HICE, which not only eliminates backfire, but also take into account the power output and economy.     

The effects of ethanol–unleaded gasoline blends and ignition timing on engine performance and exhaust emissions

In this study, the effects of using unleaded gasoline (E0) and unleaded gasoline–ethanol blends (E10, E20 E40 and E60) on engine performance and exhaust emissions have been experimentally investigated. The investigation was conducted on a Hydra single-cylinder, four-stroke, spark ignition engine. The experiments were performed by varying the compression ratio (8:1, 9:1 and 10:1) and ignition timing at a constant speed of 2000 rpm at wide open throttle (WOT). The experimental results showed that blending unleaded gasoline with ethanol slightly increased the brake torque and decreased carbon monoxide (CO) and hydrocarbon (HC) emissions. It was also found that blending with ethanol allows increasing the compression ratio without knock occurrence.     

Assessment of elliptic flame front propagation characteristics of hydrogen in an optically accessible spark ignition engine

The absence of carbon content of hydrogen fuel makes it an attractive candidates for future energy carriers. Hydrogen or dual fuelled engines are a practical alternative to pure hydrocarbon fuelling modes. However, fine tuning of current engines is necessary. In this study premixed hydrogen flame propagation is investigated in a single-cylinder, spark-ignited, four-stroke optically accessible spark ignition test engine using high-speed imaging. Ellipses were fitted on the flame contours during the analysis to obtain flame speeds and flame centre motion. The test conditions covered a range of engine speeds from 1000 rpm to 2000 rpm with 100 rpm increments using a lean mixture, ? = 0.67. The fine temporal resolution allowed the time, at which spark governed kernel formation becomes a function of engine parameters to be determined. The few data that have been published in the literature regarding hydrogen flame speeds were compared with the finding of this study.     

Control of backfire and NOx emission reduction in a hydrogen fueled multi-cylinder spark ignition engine using cooled EGR and water injection strategies

The experimental study was carried out on a constant speed multi-cylinder spark ignition engine fueled with hydrogen. Exhaust gas recirculation (EGR) and water injection techniques were adopted to control combustion anomalies (backfire and knocking) and reduce NOx emission at source level. The experimental tests were conducted on the engine with varied EGR rate (0%–28% by volume) and water to hydrogen ratio (WHR) (0–9.25) at 15 kW load. It was observed from the experiments that both the strategies can control backfire effectively, but water injection can effectively control backfire compared to EGR. The water injection and EGR reduce the probability of backfire occurrence and its propagation due to the increase in the requirement of minimum ignition energy (MIE) of the charge, caused mainly due to charge dilution effect, and reduction in flame speed respectively. The NOx emission was continuously reduced with increase in EGR rate and WHR, but at higher rates (of EGR and WHR), there was an issue of stability of engine operation. It was found from the experimental results that at 25% EGR, there was 57% reduction in NOx emission without drop in brake thermal efficiency whereas, with WHR of 7.5, the NOx emission was reduced by 97% without affecting the efficiency. The salient point emerging from the study is that water injection technique can control backfire with ultra-low (near zero) NOx emission without compromising the performance of the hydrogen fueled spark ignition engine.     

Effect of compression ratio and spark timing on the power performance and combustion characteristics of an HCNG engine

This paper investigates the effect of compression ratio and spark timing on the power performance and combustion characteristics of a hydrogen enriched compressed natural gas engine. The experimental data was conducted under variable compression ratios (i.e. 10:1, 11:1 and 12:1) by varying the spark ignition timing. The engine was kept running at a constant speed of 1200 rpm with a constant excess air ratio of 1.6 and a constant manifold absolute pressure of 50 kPa. It has been found from the results that the higher compression ratio, the higher the indicated thermal efficiency. Increase in compression ratio leads to higher brake torque and lower break specific fuel consumption. But the improvements are weakened between higher compression ratios. The peak pressure is higher with higher compression ratio. Faster heat release rates and lesser coefficient of variations of indicated mean effective pressure can also been observed at higher compression ratios.     

Effect of different excess air ratio values and spark advance timing on combustion and emission characteristics of hydrogen-fueled spark ignition engine

This study investigated the effect of varying the spark advance timing and excess air ratio (air excessive ratio; λ) on the combustion and emission of nitrogen oxide (NOx) in a hydrogen-fueled spark ignition engine under part load conditions. The engine test speed was fixed at 2,000 rpm and the torque condition was 60 Nm. Excess air ratio was varied from the stoichiometric (λ = 1) to the lean mixture condition (λ = 2.2) by throttling. The spark advance timing was controlled to determine the maximum brake torque timing (MBT) for each excess air ratio value. Subsequent to the determination of the spark advance timing for MBT, the spark timing was varied from MBT timing to top dead center. Based on the results, it is concluded that the leanest mixture condition (λ = 2.2) with MBT spark timing exhibited the highest brake thermal efficiency of 34.17% and the NOx emissions were as low as 14 ppm.