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This study examines the use of hydrogen as a fuel for internal combustion engines to decrease greenhouse gas emissions. The focus is on hydrogen combustion at leaner mixture conditions, which has the potential to increase efficiency and reduce NOx emissions. While metal engine experiments have established these benefits, there are only a few optical studies on pure hydrogen combustion under lean operating conditions. This study reports optical measurements performed in a heavy-duty optical diesel engine converted to spark-ignition operation with port-fuel injections and varying spark timing, at air-excess ratios (lambda) of 2.5 and 3. The engine was equipped with a flat-shaped optical piston that allowed for bottom-view imaging of the combustion process. High-speed natural combustion luminosity images were recorded, along with in-cylinder pressure measurements.

Lean combustion technologies show promise for improving engine efficiency and reducing emissions. Among these technologies, prechamber-assisted combustion (PCC) is established as a reliable option for achieving lean or ultra-lean combustion. In this study, the effect of engine speed on PCC was investigated in a naturally aspirated heavy-duty optical engine: a comparison has been made between analytical performances and optical flame behavior. Bottom view natural flame luminosity (NFL) imaging was used to observe the combustion process. The prechamber was fueled with methane, while the main chamber was fueled with methanol. The engine speed was varied at 1000, 1100, and 1200 revolutions per minute (rpm). The combustion in the prechamber is not affected by changes in engine speed. However, the heat release rate (HRR) in the main chamber changed from two distinct stages with a faster first stage to more gradual and merged stages as the engine speed increased.

To understand key practical aspects of ammonia as a fuel for internal combustion engines, three-dimensional computational fluid dynamics (CFD) simulations were performed using CONVERGETM. A light-duty single-cylinder research engine with a geometrical compression ratio of 11.5 and a conventional pentroof combustion chamber was experimentally operated at stoichiometry. The fumigated ammonia was introduced at the intake plenum. Upon model validation, additional sensitivity analysis was performed. The combustion was modeled using a detailed chemistry solver (SAGE), and the ammonia oxidation was computed from a 38-specie and 262-reaction chemical reaction mechanism. Three different piston shapes were assessed, and it was found that the near-spark flow field associated with the piston design in combination with the tumble motion promotes faster combustion and yields enhanced engine performance.

The pre-chamber combustion (PCC) concept is a proven lean or diluted combustion technique for internal combustion engines with benefits in engine efficiency and reduced NOx emissions. The engine lean operation limit can be extended by supplying auxiliary fuel into the pre-chamber and thereby, achieving mixture stratification inside the pre-chamber over the main chamber. Introducing liquid fuels into the pre-chambers is challenging owing to the small form factor of the pre-chamber. With a conventional injector, the fuel penetrates in liquid form and impinges on the pre-chamber walls, which leads to increased unburned hydrocarbon emissions from the pre-chamber. In this study, a prototype liquid fuel injector is introduced which preheats the fuel within a heated chamber fitted with an electrical heating element before injecting an effervescently atomized spray into the pre-chamber.

The pre-chamber combustion can extend the lean limit of internal combustion engines (ICE) and hence increase their overall efficiency. Compared to active pre-chambers equipped with an auxiliary fuel supply system, passive pre-chambers have lower manufacturing costs and require minimal or no design modifications to the conventional spark plug engines. The major challenge of the passive pre-chamber is to extend the lean limit as much as the active pre-chamber. Computational fluid dynamics (CFD) simulations were conducted on a light-duty single-cylinder engine geometry fitted with a passive pre-chamber and using iso-octane as fuel to investigate and optimize the passive pre-chamber fuel enrichment through the pre-chamber nozzles. The non-reacting flow simulations were performed from the intake valve open (IVO) to spark timing.

In recent years lean-burn technologies have acquired center stage in engine research due to stringent emission norms. Among such technologies, pre-chamber assisted combustion (PCC) has gained much attention for its ability to allow ultra-lean engine operation (λ > 2). The spark-ignited pre-chambers engines allow such lean operation by inducing a strong charge stratification, enhancing turbulence generation, and multipoint ignition. Adding a pre-chamber igniter to the engine alters the in-cylinder flow fields as mass is exchanged between the pre-chamber and the main chamber. This study reports the main chamber flow fields of methane fuelled heavy-duty optical engine fitted with a narrow throat active prechamber. Particle image velocimetry (PIV) at 10 Hz is performed from the side view using TiO2 particle seeding.

In modern compression ignition engines, the dense liquid fuel is directly injected into high pressure and temperature atmosphere, so the spray transitions from subcritical to supercritical conditions. To gain better control of the spray-combustion heat release process, it is important to have a physically accurate description of the spray development process. This work explored the effect of real-fluid thermodynamics in the computational prediction of multiphase flow for two non-ideal situations: the cryogenic nitrogen and non-cryogenic n-dodecane and ammonia sprays. Three real-fluid equations of state (EoS) such as the Soave-Redlich-Kwong (SRK), Peng-Robinson (PR), and Redlich-Kwong-Peng-Robinson (RKPR) coupled with the real-fluid Chung transport model were implemented in OpenFoam to predict the real-fluid thermodynamic properties. Validations against the CoolProp database were conducted.

To elucidate the complex characteristics of pre-chamber combustion engines, the interaction of the hot gas jets initiated by an active narrow throated pre-chamber with lean premixed CH4/air in a heavy-duty engine was studied computationally. A twelve-hole KAUST proprietary pre-chamber geometry was investigated using CONVERGE software. The KAUST pre-chamber has an upper conical part with the spark plug, and fuel injector, followed by a straight narrow region called the throat and nozzles connecting the chambers. The simulations were run for an entire cycle, starting at the previous cycle's exhaust valve opening (EVO). The SAGE combustion model was used with the chemistry modeled using a reduced methane oxidation mechanism based on GRI Mech 3.0, which was validated against in-house OH chemiluminescence data from the optical engine experiments.

Pre-chamber combustion (PCC) has re-emerged in recent last years as a potential solution to help to decarbonize the transport sector with its improved engine efficiency as well as providing lower emissions. Research into the combustion process inside the pre-chamber is still a challenge due to the high pressure and temperatures, the geometrical restrictions, and the short combustion durations. Some fundamental studies in constant volume combustion chambers (CVCC) at low and medium working pressures have shown the complexity of the process and the influence of high pressures on the turbulence levels. In this study, the pre-chamber combustion process was investigated by combustion visualization in an optically-accessible pre-chamber under engine relevant conditions and linked with the jet emergence inside the main chamber. The pre-chamber geometry has a narrow-throat. The total nozzle area is distributed in two six-hole rows of nozzle holes.

Pre-chamber Combustion (PCC) extends the lean operation limit operation of spark ignition (SI) engines, thus it has been of interest for researchers as a pathway for increased efficiency and reduced emissions. Optical diagnostic techniques are essential to understand the combustion process, but the engine components such as the piston geometry, are often different from real engines to maximize the optical access. In this study, ignition and subsequent main chamber combustion are compared in an optically accessible PCC engine equipped with a “flat” and a real engine-like “bowl” piston geometry. An active fueled narrow throat pre-chamber was used as the ignition source of the charge in the main-chamber, and both chambers were fueled with methane. Three pre-chamber fuel effective mean pressure (FuelMEP) ratios (PCFR) namely 6%, 9% and 11% of the total amount of fuel were tested at two global excess air ratios (λ) at values of 1.8 and 2.0.

Heavy-duty vehicles face increasing demands of emission regulations. Reduced carbon-dioxide (CO2) emission targets motivate decreased fuel consumption for fossil fuel engines. Increased engine efficiency contributes to lower fuel consumption and can be achieved by lower heat transfer, friction and exhaust losses. The double compression expansion engine (DCEE) concept achieves higher efficiency, as it utilizes a split-cycle approach to increase the in-cylinder pressure and recover the normally wasted exhaust energy. However, the DCEE concept suffers heat losses from the high-pressure approach. This study utilizes up to three injectors to reduce the wall-gas temperature gradient rendering lower convective heat losses. The injector configuration consists of a standard central injector and two side-injectors placed at the rim of the bowl. An increased distance from side-injector to the wall delivered lower heat losses by centralizing hot gases in the combustion chamber.

In this study, the authors investigated the effect of fuel properties on the combustion characteristics by employing methane, methanol, ethanol, and primary reference fuels (PRFs) as the main chamber fuel while using methane for the pre-chamber. Global excess air ratios (λ) from 1.6 to lean limit were tested, while 13% of total fuel energy supplied to the engine was delivered via the pre-chamber. The gaseous methane was injected into the pre-chamber at the gas exchange top-dead-center (TDC). Port fuel injection was tested with both open and closed inlet valves. The pre-chamber assembly was designed to fit into the diesel injector pocket of the base engine, which resulted in a narrow throat diameter of 3.3 mm. The combustion stability limit was set at 5% of the coefficient of variation of gross IMEP, and the knock intensity limit was set at 10 bar. GT-Power software was used to estimate the composition of pre-chamber species and was used in heat release analysis of the two chambers.

Engine knock is an abnormal phenomenon, which places barriers for modern Spark-Ignition (SI) engines to achieve higher thermal efficiency and better performance. In order to trigger more controllable knock events for study while keeping the knock intensity at restricted range, various spark strategies (e.g. spark timing, spark number, spark location) are applied to investigate on their influences on knock combustion characteristics and pressure oscillations. The experiment is implemented on a modified single cylinder Compression-Ignition (CI) engine operated at SI mode with port fuel injection (PFI). A specialized liner with 4 side spark plugs and 4 pressure sensors is used to generate various flame propagation processes, which leads to different auto-ignition onsets and knock development. Based on multiple channels of pressure signals, a band-pass filter is applied to obtain the pressure oscillations with respect to different spark strategies.

Pre-chamber combustion (PCC) allows an extension on the lean limit of an internal combustion engine (ICE). This combustion mode provides lower NOx emissions and shorter combustion durations that lead to a higher indicated efficiency. In the present work, a narrow throat pre-chamber was tested, which has a unique nozzle area distribution in two rows of six nozzle holes each. Tests were carried out in a modified heavy-duty engine for optical visualization. Methane was used as fuel for both the pre-chamber and the main chamber. Seven operating points were tested, including passive pre-chamber mode as a limit condition, to study the effect of pre- and main-chamber fuel addition on the pre-chamber jets and the main chamber combustion via chemiluminescence imaging. A typical cycle of one of the tested conditions is explained through the captured images. Observations of the typical cycle reveal a predominant presence of only six jets (from the lower row), with well-defined jet structures.

Stringent emission regulations and the anticipated climate change call for a paradigm shift in the design of the conventional internal combustion engines. One way to combat this problem is oxy-fuel combustion in which the combustion products are mainly water vapor and carbon dioxide. Water vapor can be easily separated by condensation and carbon dioxide is then easily captured and stored. However, many technical challenges are associated with this mode of combustion. There are many challenges facing oxy-fuel combustion before it find its way to commercial production especially for internal combustion engines. One such challenge is the relatively high temperature of the oxy-fuel combustion. A solution to this problem is the recirculation of the generated CO2 to moderate the in-cylinder temperature. Therefore, careful study of the effect of recirculating the CO2 back to combustion chamber is needed before the implementation of such a concept.

A new concept utilizing multiple fuel injectors was proven effective at reducing heat transfer losses by directing spray plumes further away from the combustion chamber walls. In this concept, two injectors are mounted close to the rim of the piston bowl and point in opposite directions to generate swirling in-cylinder bulk motion. Moreover, a new flat-bowl piston design was also proposed in combination with the multiple fuel injectors for even larger improvements in thermal efficiency. However, all tests were performed at low-to-medium load conditions with no significant EGR. Modern engine concepts, such as the double compression-expansion engine (DCEE), have demonstrated higher thermal efficiency when operated at high-load conditions with a large amount of EGR for NOx control. Thus, this study aims to assess the effectiveness of the multiple-fuel-injector system under such conditions. In this study, a number of 3-D CFD simulations are performed using the RANS technique in CONVERGE.

Isobaric combustion has demonstrated a great potential to reach high thermodynamic efficiency in the advanced Double Compression Expansion Engine (DCEE) concept. It appears as one of few viable choices for applications with high-pressure combustion. At these conditions, releasing heat at a constant pressure minimizes the peak in-cylinder pressure and, hence, mitigates excessive mechanical stress on the engine. This study focuses on the effect of fuels on the multiple-injection isobaric combustion. A single-cylinder heavy-duty engine was utilized to test and compare the isobaric combustion with pure isooctane and n-heptane fuels. The engine was equipped with an optical piston to allow a bottom-view of the combustion chamber. The interactions of multiple injections and the combustion behavior were studied using high-speed acquisition of chemiluminescence. The examined isobaric cases have a peak pressure of 70 bar.

To provide insights into the fundamental characteristics of pre-chamber combustion engines, the ignition of lean premixed CH4/air due to hot gas jets initiated by a passive narrow throated pre-chamber in a heavy-duty engine was studied computationally. A twelve-hole pre-chamber geometry was investigated using CONVERGETM software. The numerical model was validated against the experimental results. To elucidate the main-chamber ignition mechanism, the spark plug location and spark timing were varied, resulting in different pressure gradient during turbulent jet formation. Different ignition mechanisms were observed for turbulent jet ignition of lean premixed CH4/air, based on the geometry effect. Ignition behavior was classified into the flame and jet ignition depending on the significant presence of hot active radicals. The jet ignition, mainly due to hot product gases was found to be advanced by the addition of a small concentration of radicals.

Pre-chamber spark ignition (PCSI) combustion is an emerging lean-burn combustion mode capable of extending the lean operation limit of an engine. The favorable characteristic of short combustion duration at the lean condition of PCSI results in high efficiencies compared to conventional spark ignition combustion. Since the engine operation is typically lean, PCSI can significantly reduce engine-out NOx emissions while maintaining short combustion durations. In this study, experiments were conducted on a heavy-duty engine at lean conditions at mid to low load. Two major studies were performed. In the first study, the total fuel energy input to the engine was fixed while the intake pressure was varied, resulting in varying the global excess air ratio. In the second study, the intake pressure was fixed while the amount of fuel was changed to alter the global excess air ratio.

High-pressure isobaric combustion used in the double compression expansion engine (DCEE) concept was proposed to obtain higher engine brake thermal efficiency than the conventional diesel engine. Experiments on the metal engines showed that four consecutive injections delivered by a single injector can achieve isobaric combustion. Improved understanding of the detailed fuel-air mixing with multiple consecutive injections is needed to optimize the isobaric combustion and reduce engine emissions. In this study, we explored the fuel spray characteristics of the four-consecutive-injections strategy using high-speed imaging with background illumination and fuel-tracer planar laser-induced fluorescence (PLIF) imaging in a heavy-duty optical engine under non-reactive conditions. Toluene of 2% by volume was added to the n-heptane and served as the tracer. The fourth harmonic of a 10 Hz Nd:YAG laser was applied for the excitation of toluene.