Pilot Diesel Research Articles

Direct injection of hydrogen main fuel and diesel pilot fuel in a retrofitted single-cylinder compression ignition engine

Up to 90% hydrogen energy fraction was achieved in a hydrogen diesel dual-fuel direct injection (H2DDI) light-duty single-cylinder compression ignition engine. An automotive-size inline single-cylinder diesel engine was modified to install an additional hydrogen direct injector. The engine was operated at a constant speed of 2000 revolutions per minute and fixed combustion phasing of −10 crank angle degrees before top dead centre (°CA bTDC) while evaluating the power output, efficiency, combustion and engine-out emissions. A parametric study was conducted at an intermediate load with 20–90% hydrogen energy fraction and 180-0 °CA bTDC injection timing. High indicated mean effective pressure (IMEP) of up to 943 kPa and 57.2% indicated efficiency was achieved at 90% hydrogen energy fraction, at the expense of NOx emissions. The hydrogen injection timing directly controls the mixture condition and combustion mode. Early hydrogen injection timings exhibited premixed combustion behaviour while late injection timings produced mixing-controlled combustion, with an intermediate point reached at 40 °CA bTDC hydrogen injection timing. At 90% hydrogen energy fraction, the earlier injection timing leads to higher IMEP/efficiency but the NOx increase is inevitable due to enhanced premixed combustion. To keep the NOx increase minimal and achieve the same combustion phasing of a diesel baseline, the 40 °CA bTDC hydrogen injection timing shows the best performance at which 85.9% CO2 reduction and 13.3% IMEP/efficiency increase are achieved.

Coupling combustion simulation and primary evaluation of an asymmetric motion diesel pilot hydrogen engine

The thermal efficiency and combustion of conventional hydrogen engines cannot be optimized and improved by its symmetric reciprocating. This article introduces an asymmetric motion hydrogen engine (AHE) and investigates its combustion characteristics using diesel pilot ignition. A dynamic model is firstly proposed to describe the asymmetric motion of the AHE, and then it is coupled into a multidimensional model for combustion simulation. The effect of asymmetric motion on the AHE combustion is also analyzed by comparing with a corresponding conventional symmetric hydrogen engine (SHE). The results show that the AHE moves slower in compression and faster in expansion than the SHE, which brings about higher hydrogen-air mixing level for combustion. The asymmetric motion delays diesel injection to ignite the AHE and its combustion appears later than the SHE, which leads to lower pressure and temperature for reducing NO formation. However, the AHE faster expansion has a more severe post-combustion effect to reduce isovolumetric heat release level and decrease the energy efficiency.

Modeling-optimization of performance and emission characteristics of dual-fuel engine powered with pilot diesel and agricultural-food waste-derived biogas

Gaseous-liquid fuel combinations of biogas as primary fuel and diesel as pilot fuel were employed to power a compression ignition (CI) engine in this study. The effects of variable input factors such as engine load, compression ratio, and pilot fuel injection advance on the dual-fuel engine's combustion, thermal, and emission performance were investigated. The data acquired during this lab-based testing was used to develop a prognostic model using a Bayesian optimization strategy for hyperparameter optimization and a contemporary ensemble Gaussian process regression (GPR) technique. During model testing, the error analysis revealed that the developed model could predict the experimental data rather correctly, with R2 values ranging from 0.9995 to 0.9999 and MSE values ranging from 0.00018 to 0.1334. The mean absolute error was discovered to be between 0.0091 and 0.3065. Consequently, the developed GPR model can effectively simulate the on-board performance, emission, and combustion characteristics of a diesel-biogas dual-fuel setting. As a consequence, the current work established the GPR model in the existing CI engine meta-modeling framework as a consistent, trustworthy, and robust system analytical approach for diesel-biogas dual fuel mode of operation.

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.

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

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

The effect of diesel pilot injection strategy on combustion and emission characteristic of diesel/methanol dual fuel engine

The effects of diesel fuel pilot injection timing, pilot mass percent and methanol substitution rate on the combustion and emissions of a diesel-methanol dual fuel engine were experimentally investigated. The experiments were conducted on a single-cylinder, naturally aspirated common rail diesel engine modified with a methanol injection system. The results indicated that the addition of pilot injection advanced the ignition time and increased the heat release rate. With the advancement of pilot injection timing, the ignition delay increased, and the indicated thermal efficiency reduced. The NOx emission first decreased and then increased while the smoke emission showed the opposite trend. With the increase in the pilot mass percent, the peak in-cylinder pressure and the heat release rate increased. Meanwhile, the ignition time and the CA50 advanced. At different pilot injection timing, the combustion duration and the indicated thermal efficiency showed different trends. Meanwhile, with the increase in the pilot mass percent, the NOx emission increased and the smoke emission decreased. When the MSR increases from 20% to 40%, the indicated thermal efficiency increases by 1.9 percentage points, and the NOx and smoke emissions decrease by 88% and 61% respectively by advancing the PIT and increasing the pilot mass percent with the EGR combined. It can be concluded that, at high MSR, advancing the pilot injection timing and increasing the pilot mass percent with EGR combined is a better injection strategy to improve the performance of combustion and emissions.

Ignition and combustion characteristics of diesel piloted ammonia injections

Ammonia is considered a potential carbon-free alternative to fossil fuels. However, its unfavorable combustion characteristics and propensity to form fuel NOx pose a challenge for its use as fuel for internal combustion engines. The high-pressure dual fuel (HPDF) direct-injection of ammonia could offer the potential to reduce ammonia slip and decrease NOx formation. The feasibility of this combustion process has not yet been shown experimentally in literature. This work examines the ignition and combustion characteristics of diesel piloted liquid ammonia sprays under engine-relevant conditions in a rapid-compression-expansion-machine (RCEM). By examining heat release rates (HRRs) under a variety of spatial and temporal spray interaction configurations, charge conditions as well as different diesel pilot amounts and injection durations, the fundamental prerequisites for successful combustion of liquid ammonia sprays are revealed. Strong interaction of the two fuels is found necessary to properly ignite ammonia. Misfiring due to deterioration of the pilot mixture formation can be avoided by injecting diesel first. A strong correlation between main ignition delay and burnout rate suggests a significant influence of wall quenching effects. An investigation of less reactive charge conditions suggests poor suitability of the combustion process for low-load engine operation. While reliable ammonia ignition was achieved for diesel pilot amounts as small as 3.2% of the total injected LHV, ignition is increasingly delayed for smaller pilot amounts. For an operating point, which showed favorable ignition behavior and high conversion rates, pilot fuel amount and injection duration are found to have a major influence on the combustion process.

Effects of Injection Overlap and EGR on Performance and Emissions of Natural Gas HPDI Marine Engine

ABSTRACT Due to the increasingly stringent emission regulations and the growing concern about energy hazard, natural gas (NG) has emerged as an alternative fuel with abundant reserves, low cost, high combustion performance and low pollutant emissions. In this study, a NG high pressure direct injection (HPDI) model is embedded in a Cummins ISX marine medium-speed four-stroke natural gas-diesel dual fuel (NGDDF) engine model. The strategies with different injection overlap of NG and pilot diesel oil (PDO) and different exhaust gas recirculation (EGR) were investigated, the following conclusions are drawn. At the same EGR rate, the injection start point of NG (SOING) has impact on the compression end pressure to some extent, while the injection end point of the pilot diesel oil (EOIPDO) makes little difference to the compression end pressure. With the increase of SOING, the maximum pressure (PMax) rises gradually. When the SOING remains unchanged, the longer the pilot diesel oil (PDO) injection lasts, the higher the PMax. The heat release rate (HRR) of HPDI engine has two peaks. The first one decreases as the EGR rate rises, while the other is inversely proportional only to the EGR rate. As the EGR rate increasing, the position of the second peak of HRR is increasingly delayed, irrespective of the injection strategy used. At the same EGR rate, the overlapping injection strategy has only a limited impact on the maximum in-cylinder temperature (about 42 K) and the soot emission (about 0.003 g/kwh). The first stage of nitrogen oxides (NOX) generation is closely related to the EGR rate. The higher the EGR rate, the longer the interval between the first and second stage of NOX generation. Though both reasonable injection overlap and delay of EOIPDO are conducive to reduce pollutant emissions, it remains necessary to delay the position of the overlap angle as much as possible, and the emission reduction effect of injection overlap will diminish as the EGR rate increasing. Either the injection overlap formed by delaying EOIPDO or advancing SOING has a positive impact on indicated mean effective pressure (IMEP). The introduction of EGR will increase indicated specific fuel consumption (ISFC) of the engine, while the injection overlap strategy can be used to reduce it by 5.35 g/kwh. In order to meet the Tier III standard, the combination of injection strategy B and 18% EGR can be considered as the most effective solution for soot, ISFC and IMEP factors.

Numerical Study on the Effects of Pilot Diesel Quantity Coupling EGR in a High Pressure Direct Injected Natural Gas Engine

ABSTRACT The natural gas high pressure direct injection (HPDI) engines have attracted attention due to their high combustion stability and thermal efficiency comparable to diesel. However, the nitrogen oxides (NOx) emissions from HPDI engines remain high. In this study, the coupling effects of the exhaust gas recirculation (EGR) and pilot diesel quantity were investigated on a four stroke HPDI engine by simulation. Three pilot diesel quantities and four EGR rates were involved, and the combustion and emission characteristics of the engine were compared and discussed. The results show that with increasing EGR rate, the maximum combustion pressure (Pmax) decreases with different pilot diesel quantity, and the crank angle corresponding to the maximum combustion pressure (CPmax), the crank angle corresponding to the maximum heat release rate (CHRRmax) and the crank angle corresponding to the 50% cumulative heat release (CA50) are all pushed back. NOx emissions decrease gradually with the increase of EGR rate, but soot, unburned hydrocarbon (HC) and carbon monoxide (CO) emissions and methane escape all increase and the indicated thermal efficiency (ITE) decreases. The traditional “trade-off” relationship between NOx and soot emissions still exists on HPDI engine and becomes more obvious as the quantity of pilot diesel increases. NOx and soot emissions can be well balanced at moderate EGR rate. Increasing the quantity of pilot diesel can partially offset the hysteresis effect of EGR on the combustion process. And when the EGR rate is controlled within 30%, increasing the quantity of pilot diesel is beneficial to reduce HC and CO emissions and methane escape. By increasing the quantity of pilot diesel coupled with a moderate level of EGR rate, it is possible to significantly reduce NOx emissions while maintaining the ITE, methane escape and unburned diesel, HC and CO emissions at the same level as without EGR. But soot emissions will increase substantially.

Evaluating the effect of variable methanol injection timings in a novel co-axial fuel injection system equipped locomotive engine

Methanol has been identified as a potential substitute for conventional fuels (diesel, petrol) from scalability, sustainability, and an economic viewpoint. Economically, methanol could be cheaper than conventional fuels based on the price per MJ energy. This study evaluated the possibility of displacing the diesel (90% on an energy basis) by methanol in the locomotive engine using a novel co-axial injection concept while evaluating the different methanol injection timings. Pilot diesel injection (10% on an energy basis) ensured the combustion of injected methanol. A 1-D model of a 16-cylinder locomotive engine was developed for the parametric study to identify a suitable methanol injection strategy corresponding to co-axial injector configuration. Three different parameters were selected to investigate the effect of the new co-axial injection system on the engine performance, combustion, and emissions. These were (i) methanol injection timing, (ii) nozzle design parameters (hole diameters and the number of holes) for methanol injection, and (iii) nozzle design for diesel injection. Three different methanol injection strategies (ITM1, ITM2, ITM3) were evaluated at eight different notches of the locomotive engine for identical diesel injection timings of the validated base model. ITM1 emerged as the best strategy to inject methanol into the engine cylinder because of relatively higher in-cylinder pressure and comparable heat release rate (HRR). The maximum in-cylinder pressure (Pmax), the crank angle at maximum pressure (°CA Pmax), the start of combustion (SoC) duration, combustion duration (CD), and torque were comparable to the baseline diesel-only fuelling. For this strategy, co-axial injector design parameters [diesel's nozzle diameters (HDD) = 0.2 mm, number of nozzles for diesel (NHD) = 4, methanol nozzle diameters (HDM) = 0.35, number of nozzles for methanol (NHM) = 9)] exhibited promising results. Overall, the first injection strategy emerged as the best for direct methanol injection in the locomotive engine at all engine loads. The main finding of this study was that since methanol had a higher latent heat of vaporization, it should not be injected too late after the diesel pilot injection. This means that methanol must be injected in a sufficiently hot environment to easily vaporize it to ensure its participation in combustion without struggling for evaporation due to the colder in-cylinder environment. The methanol showed relatively lower environmental impact in all three strategies by significantly reducing the NOx emissions. NOx control is a major challenge for the surface transport industry. It could be resolved using methanol, which may expedite its use in internal combustion engines (ICEs), cleaning up the combustion engine based transportation. This simulation study demonstrated that petroleum usage and its negative environmental impact could be reduced by using cleaner/greener methanol.

Exploration of engine characteristics in a CRDI diesel engine enriched with hydrogen in dual fuel mode using toroidal combustion chamber

The impact of dual fuel (diesel/hydrogen) on different performance aspects of CRDI diesel engines is investigated in this study. Amongst the fuel alternatives for IC (internal combustion) engines, the research described in this study recommended hydrogen as the least polluting and renewable in the long term. A CNG-LPG injector feeds hydrogen into the intake manifold, while diesel injectors pump pilot diesel to a DI engine adapted to hydrogen and diesel (dual-fuel mode). By maintaining 5.2 KW of consistent IP (Indicated Power) and engine speed at 1500 ± 10 rotations per minute (RPM), the hydrogen energy was varied in the dual fuel at 0% (100% diesel), 6%, 12%, 18% and 24%. With the increase in H2 energy proportion, a decrease (5.2% decrease at 24% HES) in the BSEC (brake specific energy consumption) and the engine's BTE (brake thermal efficiency) is improved (7.85% increase at 24% HES). When emissions are considered, indicated NOx increased (3.42%) while indicated CO2 (3.61%), CO (2.84%), and smoke (4.85%) decreased with an increase in the proportion of hydrogen. Along with this, it was noted that the peak HRR (heat release rate) of 69.8 J/deg and in-cylinder pressure of 80.8 bar which increased significantly with the increase in hydrogen rate.

A measurement method for the dual fuel coupling injection characteristics of the high-pressure natural gas and diesel co-direct injection engine

This paper presents an innovative measurement method for a two-phase fuel transient flow rate in the same space–time dimension. Based on the jet momentum measurement, a spray filter is designed to realize the space extraction of the target jet. The optimized jet baffle structure is proposed, which greatly improves the quality of the test signal. Compared with the pressure–time method, this method has higher precision in the gas injection rate shape measurement and the recognition precision of gas injection beginning and ending moment. The coupling effect of two-phase fuel injection and the influence of different ambient pressures on injection performance can be studied by this method. The results show that the pilot diesel injection will cause more than 20% fluctuation of gas injection mass, and the ambient pressure can promote the gas needle opening, which will significantly affect the fuel injection mass of the engine.

Large-eddy simulation of diesel pilot spray ignition in lean methane-air and methanol-air mixtures at different ambient temperatures

In dual-fuel compression-ignition engines, replacing common fuels such as methane with renewable and widely available fuels such as methanol is desirable. However, a fine-grained understanding of diesel/methanol ignition compared to diesel/methane is lacking. Here, large-eddy simulation (LES) coupled with finite rate chemistry is utilized to study diesel spray-assisted ignition of methane and methanol. A diesel surrogate fuel ( n-dodecane) spray is injected into ambient methane-air or methanol-air mixtures at a fixed lean equivalence ratio [Formula: see text] = 0.5 at various ambient temperatures ([Formula: see text] = 900, 950, 1000 K). The main objectives are to (1) compare the ignition characteristics of diesel/methanol with diesel/methane at different [Formula: see text], (2) explore the relative importance of low-temperature chemistry (LTC) to high-temperature chemistry (HTC), and (3) identify the key differences between oxidation reactions of n-dodecane with methane or methanol. Results from homogeneous reactor calculations as well as 3 + 3 LES are reported. For both DF configurations, increasing [Formula: see text] leads to earlier first- and second-stage ignition. Methanol/ n-dodecane mixture is observed to have a longer ignition delay time (IDT) compared to methane/ n-dodecane, for example ≈ three times longer IDT at [Formula: see text] = 950 K. While the ignition response of methane to [Formula: see text] is systematic and robust, the [Formula: see text] window for n-dodecane/methanol ignition is very narrow and for the investigated conditions, only at 950 K robust ignition is observed. For methanol at [Formula: see text] = 1000 K, the lean ambient mixture autoignites before spray ignition while at [Formula: see text] = 900 K full ignition is not observed after 3 ms, although the first-stage ignition is reported. For methanol, LTC is considerably weaker than for methane and in fully igniting cases, heat release map analysis demonstrates the dominant contribution of HTC to total heat release rate for methanol. Reaction sensitivity analysis shows that stronger consumption of OH radicals by methanol compared to methane leads to the further delay in the spray ignition of n-dodecane/methanol. Finally, a simple and novel approach is developed to estimate IDT in reacting LES using zero-dimensional IDT calculations weighted by residence time from non-reacting LES data.

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.

Coupling Combustion Simulation and Primary Evaluation of an Asymmetric Motion Diesel Pilot Hydrogen Engine

Coupling Combustion Simulation and Primary Evaluation of an Asymmetric Motion Diesel Pilot Hydrogen Engine

A numerical study of the influence of pilot fuel injection timing on combustion and emission formation under two-stroke dual-fuel marine engine-like conditions

Stricter regulations imposed on emissions are motivating the scientific community to consider studying alternative fuels to achieve low emission, high efficient dual-fuel (DF) marine engines. In this context, three dimensional computational fluid dynamic (CFD) simulations are performed to study the combustion and emission formation under two-stroke, dual-fuel marine engine-like conditions. The DF engine configuration consists of a pilot diesel fuel and a high-pressure, direct injection (HPDI) of natural gas (NG). The simulation results are validated under both high load (high charge density) and low load (low charge density) operating conditions. Detailed analysis of the flame development and emission formation are performed. The interaction between the pilot diesel jets and the methane flame jets is studied. Based on the results, the further methane jets penetration in the low load case leads to better air–fuel mixing and a higher combustion intensity than that in the high load. Effects of the pilot fuel injection timing on combustion and emission formation and the governing mechanisms are also investigated in detail. Results indicate that the intense combustion of the accumulated methane expands the methane flame towards the piston when the pilot injection timing is retarded. The NO formation is lower in the high load case with higher charge density due to the lower combustion intensity. Also, retarding the pilot injection timing decreases the NO formation.

An optical investigation of dual fuel and RCCI pilot ignition in a medium speed engine

Driven by the Paris agreement and tightening IMO regulations, the marine sector is focusing on lowering engine emissions and moving to low-carbon fossil or renewable fuel such as methane. The dual-fuel concept allows the usage of methane as main energy-source in diesel engines. A small pilot diesel injection acts as an ignition source for the premixed methane. It was investigated how NOx formation, mainly taking place during combustion of the pilot fuel, could be minimized.To better understand the process of ignition of a pilot injection in dual-fuel engines, optical research has been performed on a medium speed dual fuel marine engine. A unique Bowditch 200 mm single cylinder setup enabled high speed recordings of natural luminescence. Both Reactivity Controlled Compression Ignition (RCCI) and Conventional Dual Fuel (CDF) combustion was investigated. The RCCI combustion was created by an early pilot injection, allowing a long mixing time. The CDF cases had a late injection timing.In RCCI operation the higher degree of premixing was recognized by combustion luminescence starting further away from the injector, at a varying location. The diluted pilot combustion generated a limited brightness. The heat release profile was Gaussian/bell-shaped, without the typical diesel premixed peak. In CDF operation the recorded images show that combustion follows the shape of the diesel injector jet. The heat release profile was showing a strong initial peak, resembling the premixed peak known from conventional diesel combustion. This heat release peak in CDF combustion, correlated to NOx emissions, is absent in RCCI mode.

Study on the ignitability of a high-pressure direct-injected methane jet using a diesel pilot, a glow-plug, and a pre-chamber

Natural gas is a promising alternative fuel for internal combustion engines, it allows for a reduction of engine-out emissions without impairing high engine efficiencies. Although this approach is already utilized from small to large engine classes, it is almost exclusively based on the combustion of a premixed, homogeneous charge. For ignition, small engines use standard spark-plugs or pre-chambers, while large and lean-operated engines use pre-chambers and pilot injections. Direct high-pressure gas injection is a more recent, alternative way to operate gas engines which offers benefits compared to premixed operation such as high compression ratio, high combustion pressures, lean operation, quantity regulation, among others. However, in contrast to diesel direct injection, the compression temperatures are too low for the auto-ignition of the gas jets. Therefore, an additional ignition system is required, usually a pilot injection system is used. In this study, the usability and performance of three ignition strategies for direct injected high-pressure gas jets have been investigated in an optically accessible test-rig that is able to operate at engine-like conditions. The first type of ignition system is a pilot injection with a liquid fuel, the second is a glow-plug located near the main gas jet, and the third system is a pre-chamber with a nozzle hole aimed at the main gas jet. Results show that all three strategies are feasible options under the studied conditions. Ignition by a pilot fuel injection is a safe and reliable way to ignite the main fuel. The glow-plug is less reliable and leads to high cycle-to-cycle variation. The best option in the present study is the pre-chamber, it is very reliable, delivers the highest peak cylinder pressure and exhibits the lowest cyclic variability. The good performance is attributed to the intense mixing of the main gas jet with the hot jet exiting the pre-chamber.

Effect of pilot fuel properties on engine performance and combustion stability in a tri-fuel engine powered by premixed methane-hydrogen and diesel pilot

The present work investigates the effect of pilot fuel properties on TF combustion using premixed methane-hydrogen-air (CH4-H2-air) mixtures ignited by a small amount of diesel pilot. Especially, we are investigating the effect of the cetane number (CN) and aromatic content (AC) on TF combustion in a single-cylinder compression ignition (CI) engine at varying charge air temperatures (Tair = 25 °C, 40 °C, 55 °C) and H2 volume fractions (MH2 = 10%, 20%, 40% and 60%) at lean premixed charge mixture conditions (equivalence ratio φ = 0.5). The novelty and main findings of the work consist of the following features: 1) besides the effect of H2 concentration and charge-air temperature, pilot fuel properties also play a crucial role in TF combustion, even a small amount of diesel pilot could dramatically affect the engine performance and combustion stability, 2) the CN and AC are the key factors affect the ignition delay time (IDT) and indicated thermal efficiency (ITE), 3) the in-cylinder pressure oscillation analysis based on a novel Superlets (SL) approach indicates that pilot fuel properties are important to the combustion states and combustion stability.

Heat release rate and emissions regimes of stratified pilot-ignited direct-injection natural gas combustion.

Natural gas (NG) is an attractive fuel for heavy-duty internal combustion engines because of its potential for reduced CO2, particulate, and NOX emissions and lower cost of ownership. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a main direct injection of NG. Recent studies have demonstrated that increased NG premixing is a viable strategy to increase PIDING indicated efficiency and further reduce particulate and CO emissions while maintaining low CH4 emissions. However, it is unclear how the combustion strategies relate to one another, or where they fit within the continuum of NG stratification. The objective of this work is to present a systematic evaluation of pilot combustion, NG combustion, and emissions behavior of stratified-premixed PIDING combustion modes that span from fully-premixed to non-premixed conditions. A sweep of the relative injection timing, , of NG and pilot diesel was performed in a heavy-duty PIDING engine with = 140-220 bar, = 0.47-0.71, and a constant NG energy fraction of 94%. Apparent heat release rate and emissions analyses identified interactions between the pilot fuel and NG, and qualitatively characterized the impact of NG stratification on combustion and emissions. Changes in the resulted in six distinct PIDING combustion regimes, for all considered injection pressures and equivalence ratios: (i) RIT-insensitive premixed, (ii) stratified-premixed (early-cycle injection), (iii) NG jet impingement transition, (iv) stratified-premixed (late-cycle injection), (v) variable premixed fraction, and (vi) minimally-premixed. Parametric definitions for the bounds of each regime of combustion were valid for the wide range of and investigated, and are expected to be relevant for other PIDING engines, as previously identified regimes agree with those identified here. This conceptual framework encompasses and validates the findings of previous stratified PIDING investigations, including optimal ranges of operation that provide significantly increased efficiency and lower emissions of incomplete combustion products.