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A hydrogen-diesel dual direct injection (H2DDI) combustion strategy in a compression-ignition engine is investigated numerically, reproducing the configuration of previous experimental investigations. These experiments demonstrated the potential of up to 50% diesel substitution by hydrogen while maintaining high engine efficiency; nevertheless, the emission of NOx increased compared with diesel operation and was strongly dependent on the hydrogen injection timing. This implies the efficiency and NOx emission are closely associated with hydrogen charge stratification; however, the underlying mechanisms are not fully understood. Aiming to highlight the hydrogen injection-timing influence on hydrogen/air mixture stratification and engine performance, the present study numerically investigates the mixture formation and combustion process in the H2DDI engine concept using Converge, a three-dimensional fluid dynamics simulation code.

The combustion process of a homogeneous hydrogen charge in a small-bore compression ignition engine with diesel-pilot ignition was simulated using the CONVERGE computational fluid dynamics code. Analysis of the simulation results aimed to understand the processes leading to NOx production and heat loss in this combustion strategy, and their dependence on the hydrogen fuel energy fraction. Previous experimental results demonstrated promising performance, but this comes with a penalty in increased NOx emissions and potentially higher heat losses. The present study aims to enhance understanding of the mechanisms governing these phenomena. The simulated engine was initialised with a lean homogeneous hydrogen-air mixture at BDC and n-dodecane was injected as a diesel surrogate fuel near TDC. The simulations were validated based on experimental results for up to 50% hydrogen energy fraction, followed by an exploratory study with variation of the energy fraction from 0% to 90%.

Implementing triple injection strategies in partially premixed charge-based gasoline compression ignition (GCI) engines has shown to achieve improved engine efficiency and reduced NOx and smoke emissions in many previous studies. While the impact of the triple injections on engine performance and engine-out emissions are well known, their role in controlling the mixture homogeneity and charge premixedness is currently poorly understood. The present study shows correspondence between the triple injection strategies and mixture homogeneity/premixedness through the experimental tests of second/third injection proportion and their timing variations with an aim to explain the observed GCI engine performance and emission trends. The experiments were conducted in a single cylinder, small-bore common-rail diesel engine fuelled with a commercial gasoline fuel of 95 research octane number (RON) and running at 2000 rpm and 830 kPa indicated mean effective pressure conditions.

Stringent particulate emission regulations are applied to spark-ignition direct-injection (SIDI) engines, calling for a significant in-cylinder reduction of soot particles. To enhance fundamental knowledge of the soot formation and oxidation process inside the cylinder of the engine, a new in-flame particle sampling system has been developed and implemented in a working optical SIDI engine with a side-mounted, wall-guided injection system. Using the sampling probes installed on the piston top, the soot particles are directly sampled from the petrol flame for detailed analysis of particle size distribution, structure, and shape. At the probe tip, a transmission electron microscope (TEM) grid is stored for the soot collection via thermophoresis, which is imaged and post-processed for statistical analysis. Simultaneously, the flame development was recorded using two high-speed cameras to evidence the direct exposure of the sampling grids to the soot-laden diffusion flames and pool fires.

Recent trend in gasoline-powered automobiles focuses heavily on reducing the CO2 emissions and improving fuel efficiency. Part of the solutions involve changes in combustion chamber geometry to allow for higher turbulence, higher compression ratio which can greatly improve efficiencies. However, the changes are limited by the ignition-source and its location constraint, especially in the case of direct injection SI engines where mixture stratification is important. A new compact microwave plasma igniter based on the principle of microwave resonance was developed and tested for propane combustion inside a constant volume chamber. The igniter was constructed from a thin ceramic panel with metal inlay tuned to the corresponding resonance frequency. Microwaves generated by semiconductor based oscillator were utilized for initiation of discharge. The small and flat form factor of the flat panel igniter allows it to be installed at any locations on the surface of the combustion chamber.

Exhaust gas recirculation (EGR) has proven to be beneficial for not only fuel economy improvement but also knock and emissions reduction. Combined with lean burning, it can assist gasoline engines to become cleaner, more efficient and to meet the stringent emissions limit. However, there is a practical limit for EGR percentage in current engines due to many constraints, one of which being the ignition source. The Microwave Discharge Igniter (MDI), which generates, enhances and sustains plasma discharge using microwave (MW) resonance was tested to assess its ability in extending the dilution limit. A combination of high-speed Schlieren imaging and pressure measurements were performed for propane-air mixture combustion inside a constant volume chamber to compare the dilution limits between MDI and conventional spark plug. Carbon dioxide addition was carried out during mixture preparation to simulate the dilution condition of EGR and limit the oxygen fraction.

The present study aims to evaluate the effects of engine speed on gasoline compression ignition (GCI) combustion implementing double injection strategies. The double injection comprises of near-BDC first injection for the formation of a premixed charge and near-TDC second injection for the combustion phasing control. The engine performance and emissions testing of GCI combustion has been conducted in a single-cylinder light-duty diesel engine equipped with a common-rail injection system and fuelled with a conventional gasoline with 91 RON. The double injection strategy was investigated for various engine speeds ranging 1200~2000 rpm and the second injection timings between 12°CA bTDC and 3°CA aTDC.

Ethanol has been selected as a fuel for gasoline compression ignition (GCI) engines realising partially premixed charge combustion, considering its higher resistance to auto-ignition, higher evaporative cooling and oxygen contents than widely used gasoline, all of which could further improve already high efficiency and low smoke/NOx emissions of GCI engines. The in-cylinder phenomena and engine-out emissions were measured in a single-cylinder automotive-size common-rail diesel engine with a special emphasis on double injection strategies implementing early first injection near BDC and late second injection near TDC.

Some soot particles emitted from common-rail diesel engines are so small that can penetrate deep into the human pulmonary system, causing serious health issues. The analysis of nano-scale internal structure of these soot particles sampled from the engine tailpipe has provided useful information about their reactivity and toxicity. However, the variations of carbon fringe structures during complex soot formation/oxidation processes occurring inside the engine cylinder are not fully understood. To fill this gap, this paper presents experimental methods for direct sampling and nanostructure analysis of in-flame soot particles in a working diesel engine. The soot particles are collected onto a lacey carbon-coated grid and then imaged in a high-resolution transmission electron microscope (HR-TEM). The HR-TEM images are post-processed using a Matlab-based code to obtain key nanostructure parameters such as carbon fringe length, fringe-to-fringe separation distance, and fringe tortuosity.

This paper presents engine performance and emissions of coconut oil-derived 10% biodiesel blends in petroleum diesel demonstrating simultaneous reduction of smoke and NOx emissions and increased brake power. The experiments were performed in a single-cylinder version of a light-duty diesel engine for three different fuels including a conventional diesel fuel and two B10 fuels of chemical-catalyst-based methyl-ester biodiesel (B10mc) and biological-catalyst-based ethyl-ester biodiesel (B10eb). The engine tests were conducted at fixed speed of 2000 rpm and injection pressure of 130 MPa. In addition to the fuel variation, the injection timing and rate of exhaust gas recirculation (EGR) were also varied because they impact the combustion and thus the efficiency and emissions significantly.

Extending the lean limit or/and exhaust-gas-recirculation (EGR) limit/s are necessary for improving fuel economy in spark ignition engines. One of the major problems preventing the engine to operate at lean conditions is stable and successful initial ignition kernel formation. A repeatable, stabilized ignition and early flame development are quite important for the subsequent part of the combustion cycle to run smooth without partial burn or cycle misfire. This study aims to develop an innovative plasma ignition system for reciprocating combustion engines with an aim to extend lean limit and for high pressure applications. This ignition system utilizes microwaves to generate plasma as an ignition source. This microwave plasma igniter is much simplified device compared to conventional spark plug. The microwave plasma ignition system consists of microwave oscillator, co-axial cable and microwave discharge igniter (MDI).

The spray development of ethanol, gasoline and iso-octane has been studied in an optically accessible, spark-ignition direct-injection (SIDI) engine. The focus is on how fuel properties impact temporal and spatial evolution of sprays at realistic ambient conditions. Two optical facilities were used: (1) a constant-flow spray chamber simulating cold-start conditions and (2) a single-cylinder SIDI engine running at normal, warmed-up operating conditions. In these optical facilities, high-speed Mie-scattering imaging is performed to measure penetrations of spray plumes at various injection pressures of 4, 7, 11 and 15 MPa. The results show that the effect of fuel type on the tip penetration length of the sprays depends on the injection conditions and the level of fuel jet atomisation and droplet breakup.

This study aims to clarify the spray development of ethanol, gasoline and iso-octane fuel, delivered by a multi-hole injector and spark-ignition direct-injection (SIDI) fuelling system. The focus is on how fuel properties impact temporal and spatial evolution of sprays at realistic ambient conditions. Two optical facilities were used: (1) a constant-flow spray chamber simulating cold-start conditions and (2) a single-cylinder SIDI engine running at normal, warmed-up operating conditions. In these optical facilities, high-speed Mie-scattering imaging is performed to measure penetrations of spray plumes at various injection pressures of 4, 7, 11 and 15 MPa. The results show that the effect of fuel type on the tip penetration length of the sprays depends on the injection conditions and the level of fuel jet atomisation and droplet breakup.

The present study focuses on the observation of knock phenomena in a small-bore optical diesel engine. Current understanding is that a drastic increase of pressure during the premixed burn phase of the diesel combustion causes gas cavity resonances, which in turn induce a high frequency pressure ringing. The frequency and severity of this ringing can be easily measured by using a pressure transducer. However, visual information of flames under knocking conditions is limited especially for a small-bore diesel engine. To fill this gap, high-speed imaging of soot luminosity is performed in conjunction with in-cylinder pressure measurement during knocking cycles in an automotive-size optical diesel engine. From the experiments, flames were observed to oscillate against the direction of the swirl flow when the pressure ringing occurred.

Future fuels will come from a variety of feed stocks and refinement processes. Understanding the fundamentals of combustion and pollutants formation of these fuels will help clear hurdles in developing flex-fuel combustors. To this end, we investigated the combustion, soot formation, and soot oxidation processes for various classes of fuels, each with distinct physical properties and molecular structures. The fuels considered include: conventional No. 2 diesel (D2), low-aromatics jet fuel (JC), world-average jet fuel (JW), Fischer-Tropsch synthetic fuel (JS), coal-derived fuel (JP), and a two-component surrogate fuel (SR). Fuel sprays were injected into high-temperature, high-pressure ambient conditions that were representative of a practical diesel engine. Simultaneous laser extinction measurement and planar laser-induced incandescence imaging were performed to derive the in-situ soot volume fraction.

For a better understanding of soot formation and oxidation processes in conventional diesel and biodiesel spray flames, the morphology, microstructure and sizes of soot particles directly sampled in spray flames fuelled with US#2 diesel and soy-methyl ester were investigated using transmission electron microscopy (TEM). The soot samples were taken at 50mm from the injector nozzle, which corresponds to the peak soot location in the spray flames. The spray flames were generated in a constant-volume combustion chamber under a diesel-like high pressure and high temperature condition (6.7MPa, 1000K). Direct sampling permits a more direct assessment of soot as it is formed and oxidized in the flame, as opposed to exhaust PM measurements. Density of sampled soot particles, diameter of primary particles, size (gyration radius) and compactness (fractal dimension) of soot aggregates were analyzed and compared. No analysis of the soot micro-structure was made.

A plasma combustion system was developed to improve fuel economy and efficiency without modifying the engine configuration. Non-thermal plasma generation technology with microwave was applied. Plasma was generated by spark discharge and expanded using microwaves that accelerated the plasma electrons, generating non-thermal plasma. Even at high pressures, spark discharge occurred, allowing plasma generation under high pressures. The durability and practicality of previous plasma combustion systems was improved. The system consisted of a spark plug without a resistor, a mixer circuit, and a control system. The mixer unit used a standard spark plug for plasma combustion and functioned as a high-voltage and high-frequency isolator. A commercially available magnetron produced microwaves of 2.45 GHz. The spark and microwave control system used a trigger signal set to the given crank angle, from the engine control unit.

Schlieren and shadowgraph imaging have been used for many years to identify refractive index gradients in various applications. For evaporating fuel sprays, these techniques can differentiate the boundary between spray regions and background ambient gases. Valuable information such as the penetration rate, spreading angle, spray structure, and spray pattern can be obtained using schlieren diagnostics. In this study, we present details of a z-type schlieren system setup and its application to port-fuel-injection gasoline sprays. The schlieren high-speed movies were used to obtain time histories of the spray penetration and spreading angle. Later, these global parameters were compared to specifications provided by the injector manufacturer. Also, diagnostic parameters such as the proportion of light cut-off at the focal point and the orientation of knife-edge (schlieren-stop) used to achieve the cut-off were examined.

The liquid and vapor-phase spray penetrations of #2 diesel and neat (100%) soybean-derived biodiesel have been studied at late expansion-cycle conditions in a constant-volume optical chamber. In modern diesel engines, late-cycle staged injections may be used to assist in the operation of exhaust stream aftertreatment devices. These late-cycle injections occur well after top-dead-center (TDC), when post-combustion temperatures are relatively high and densities are low. The behavior of diesel sprays under these conditions has not been well-established in the literature. In the current work, high-speed Mie-scatter and schlieren imaging are employed in an optically accessible chamber to characterize the transient and quasi-steady liquid penetration behavior of diesel sprays under conditions relevant for late-cycle post injections, with very low densities (1.2 - 3 kg/m 3 ) and moderately high temperatures (800 - 1400 K).

This paper presents experimental results for two fuel-related topics in a diesel engine: (1) how fuel volatility affects the premixed burn and heat release rate, and (2) how ignition quality influences the soot formation. Fast evaporation of fuel may lead to more intense heat release if a higher percentage of the fuel is mixed with air to form a combustible mixture. However, if the evaporation of fuel is driven by mixing with high-temperature gases from the ambient, a high-volatility fuel will require less oxygen entrainment and mixing for complete vaporization and, consequently, may not have potential for significant heat release simply because it has vaporized. Fuel cetane number changes also cause uncertainty regarding soot formation because variable ignition delay will change levels of fuel-air mixing prior to combustion.