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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.

Demand for transport energy is growing but this growth is skewed heavily toward commercial transport, such as, heavy road, aviation, marine and rail which uses heavier fuels like diesel and kerosene. This is likely to lead to an abundance and easy availability of lighter fractions like naphtha, which is the product of the initial distillation of crude oil. Naphtha will also require lower energy to produce and hence will have a lower CO₂ impact compared to diesel or gasoline. It would be desirable to develop engine combustion systems that could run on naphtha. Many recent studies have shown that running compression ignition engines on very low Cetane fuels, which are very similar to naphtha in their auto-ignition behavior, offers the prospect of developing very efficient, clean, simple and cheap engine combustion systems. Significant development work would be required before such systems could power practical vehicles.

In the engine community, gasoline compression ignition (GCI) engines are at the forefront of research and efforts are being taken to commercialize an optimized GCI engine in the near future. GCI engines are operated typically at Partially Premixed Combustion (PPC) mode as it offers better control of combustion with improved combustion stability. While the transition in combustion homogeneity from convectional Compression Ignition (CI) to Homogenized Charge Compression Ignition (HCCI) combustion via PPC has been comprehensively investigated, the physical and chemical effects of fuel on GCI are rarely reported at different combustion modes. Therefore, in this study, the effect of physical and chemical properties of fuels on GCI is investigated. In-order to investigate the reported problem, low octane gasoline fuels with same RON = 70 but different physical properties and sensitivity (S) are chosen.

Pre-chamber combustion (PCC) enables leaner air-fuel ratio operation by improving its ignitability and extending flammability limit, and consequently, offers better thermal efficiency than conventional spark ignition operation. The geometry and fuel concentration of the pre-chamber (PC) is one of the major parameters that affect overall performance. To understand the dynamics of the PCC in practical engine conditions, this study focused on (i) correlation of the events in the main chamber (MC) with the measured in-cylinder pressure traces and, (ii) the effect of fuel concentration on the MC combustion characteristics using laser diagnostics. We performed simultaneous acetone planar laser-induced fluorescence (PLIF) from the side, and OH* chemiluminescence imaging from the bottom in a heavy-duty optical engine. Two different PC Fueling Ratios (PCFR, the ratio of PC fuel to the total fuel), 7%, and 13%, were investigated.

Direct injection compression ignition engines running on gasoline-like fuels have been considered an attractive alternative to traditional spark ignition and diesel engines. The compression and lean combustion mode eliminates throttle losses yielding higher thermodynamic efficiencies and the better mixing of fuel/air due to the longer ignition delay times of the gasoline-like fuels allows better emission performance such as nitric oxides (NOx) and particulate matter (PM). These gasoline-like fuels which usually have lower octane compared to market gasoline have been identified as a viable option for the gasoline compression ignition (GCI) engine applications due to its lower reactivity and lighter evaporation compared to diesel. The properties, specifications and sources of these GCI fuels are not fully understood yet because this technology is relatively new.

Pre-chamber combustion (PCC) is a promising engine combustion concept capable of extending the lean limit at part load. The engine experiments in the literature showed that the PCC could achieve higher engine thermal efficiency and much lower NOx emission than the spark-ignition engine. Improved understanding of the detailed flow and combustion physics of PCC is important for optimizing the PCC combustion. In this study, we investigated the gas exchange and flame jet from a narrow throat pre-chamber (PC) by only fueling the PC with methane in an optical engine. Simultaneous negative acetone planar laser-induced fluorescence (PLIF) imaging and OH* chemiluminescence imaging were applied to visualize the PC jet and flame jet from the PC, respectively. Results indicate a delay of the PC gas exchange relative to the built-up of the pressure difference (△ P) between PC and the main chamber (MC). This should be due to the gas inertia inside the PC and the resistance of the PC nozzle.

Compared to conventional diesel combustion (CDC), isobaric combustion can achieve a similar or higher indicated efficiency, lower heat transfer losses, reduced nitrogen oxides (NOx) emissions; however, with a penalty of soot emissions. While the engine performance and exhaust emissions of isobaric combustion are well known, the overall flame development, in particular, the flow-field details within the flames are unclear. In this study, the performance analysis of CDC and two isobaric combustion cases was conducted, followed by high-speed imaging of Mie-scattering and soot luminosity in an optically accessible, single-cylinder heavy-duty diesel engine. From the soot luminosity imaging, qualitative flow-fields were obtained using flame image velocimetry (FIV). The peak motoring pressure (PMP) and peak cylinder pressure (PCP) of CDC are kept fixed at 50 and 70 bar, respectively.

Gasoline compression ignition (GCI) pertains to high efficiency lean burn compression ignition with gasoline fuels, where ignition is controlled by mixture’s auto-ignition chemistry as well as local mixture strength. The presented GCI combustion strategy is based on a multi-mode combustion strategy at various operating conditions. This study presents a part of work on the development of an optimum combustion strategy at medium loading condition for commercial gasoline fuel with research octane number (RON) = 91. The single cylinder engine with a compression ratio (CR) = 16 features a centrally mounted multi-hole injector with a spark plug at a distance from the injector under shallow pent-roof combustion chamber design. The design of combustion chamber and piston was previously optimized based on CFD numerical analysis.

The effects of intake pressure (Pin), start of injection (SOI), injection pressure (Pinj), injection split ratio (Qsplit), internal and external exhaust gas recirculation rates were varied to optimize several key parameters at a partially pre-mixed combustion low load/low speed condition using CFD. These include indicated specific fuel consumption (ISFC), combustion phasing (CA50), maximum rate of pressure rise (MRPR), maximum cylinder pressure (Pmax), indicated specific NOx (sNOx), indicated specific hydrocarbons (sHC) and Filter Smoke Number (FSN) emissions. Low-load point (6 bar indicated mean effective pressure (IMEP), 1500 revolutions per minute (RPM)) was selected where Pin varied between 1.25 and 1.5 bar, SOI between -100 and -10 crank angle degree (CAD) after top dead center (aTDC), Pinj between 100 and 200 bar, split ratio between 0 and 0.5, EGR between 0 and 45% and internal EGR measured by rebreathing valve height was varied between 0 and 4.5 mm.

The combustion instability at low loads is one of the key technology risks that needs to be addressed with the development of gasoline compression ignition (GCI) engine. The misfires and partial burns due to combustion instability leads to excessive hydrocarbon (HC) and carbon monoxide (CO) emissions. This study aims to improve the combustion robustness and reduce the emissions at low loads. The GCI engine used in this study has unique hardware features of a spark plug placed adjacent to the centrally mounted gasoline direct injector and a shallow pent roof combustion chamber coupled with a bowl in piston geometry. The engine experiments were performed in a single cylinder GCI engine at 3 bar indicated mean effective pressure (IMEP) and 1500 rpm for certified gasoline with research octane number (RON) = 91.

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.

Compared to conventional diesel combustion (CDC), isobaric combustion can achieve higher thermal efficiency while lowering heat transfer losses and nitrogen oxides (NOx). However, isobaric combustion suffers from higher soot emissions. While the aforementioned trends are well established, there is limited literature about the high-temperature reaction zones, the liquid-phase penetration distance, and the flame tip propagation velocity of isobaric combustion. In the present study, the line-of-sight integrated imaging of Mie-scattering, combustion luminosity, and CH* chemiluminescence were conducted in an optically accessible single-cylinder heavy-duty diesel engine. The engine was equipped with a flat-bowl-shaped optical piston to allow bottom-view imaging of the combustion chamber. The experiments were conducted using n-heptane fuel for CDC and isobaric combustion modes.

A multi-objective optimization scheme based on stochastic global search is developed and used to examine the performance of an HCCI model containing a reduced chemical kinetic mechanism, and to study interrelations among different model responses. A stochastic reactor model of an HCCI engine is used in this study, and dedicated HCCI engine experiments are performed to provide reference for the optimization. The results revealed conflicting trends among objectives normally used in mechanism optimization, such as ignition delay and engine cylinder pressure history, indicating that a single best combination of optimization variables for these objectives did not exist. This implies that optimizing chemical mechanisms to maintain universal predictivity across such conflicting responses will only yield a predictivity tradeoff. It also implies that careful selection of optimization objectives increases the likelihood of better predictivity for these objectives.

Octane-on-Demand (OoD) is a promising technology for reducing greenhouse emissions from automobiles. The concept utilizes a low-octane fuel for low and mid load operating conditions, and a high-octane additive is added at high load operating conditions. Researchers have focused on the minimum ethanol content required for operating at high load conditions when the low-octane fuel becomes knock limited. However, it is also widely known that ethanol has a high tendency to pre-ignite, which has been linked with its high laminar flame speed and surface ignition tendency. Moreover, ethanol has a lower stoichiometric air-fuel ratio, requiring a larger injected fuel mass per cycle. A larger fuel mass increases the potential for oil dilution by the liquid fuel, creating precursors for pre-ignition. Hence, the limits on ethanol addition owing to pre-ignition also need consideration before the technology can be implemented.

Lean combustion is one of the most applied methods to increase engine efficiency and maintain a good trade-off with engine emissions. The pre-chamber combustion (PCC) is one of the most promising combustion concepts to extend the lean operating limits of the engine. The Narrow throat pre-chamber has shown better lean limit extension compared to other ignition sources. The pre-chamber jets and the main-chamber combustion were studied in a Heavy-Duty optical engine using methane fuel. The tested conditions covered global excess air ratios (λ), between 1.9 to 2.3. The combustion process was recorded using three collection systems: (a) Natural Flame Luminosity (NFL) with a temporal resolution of 0.1 CAD; (b) OH* Chemiluminescence, and (c) CH* Chemiluminescence with a temporal resolution of 0.2 CAD for both. The propagating velocity of the reacting jets was studied using Combustion Image Velocimetry (CIV) based on bottom view images of the main chamber.

Isobaric combustion has been proven a promising strategy for high efficiency as well as low nitrogen oxides emissions, particularly in heavy-duty Diesel engines. Previous single-cylinder research engine experiments have, however, shown high soot levels when operating isobaric combustion. The combustion itself and the emissions formation with this combustion mode are not well understood due to the complexity of multiple injections strategy. Therefore, experiments with an equivalent heavy-duty Diesel optical engine were performed in this study. Three different cases were compared, an isochoric heat release case and two isobaric heat release cases. One of the isobaric cases was boosted to reach the maximum in-cylinder pressure of the isochoric one. The second isobaric case kept the same boost levels as the isochoric case. Results showed that in the isobaric cases, liquid fuel was injected into burning gases. This resulted in shorter ignition delays and thus a poor mixing level.

This paper is the first of a two part study which investigates the use of advanced combustion modes as a means of improving the efficiency and environmental impact of conventional light-duty vehicles. This first study focuses on the application of so-called Octane-on-Demand combustion, whereby the fuel anti-knock quality is customized to match the real-time requirements of an otherwise conventional spark-ignition engine. Methanol is utilized as the high octane fuel, while three alternative petroleum-derived fuels with Research octane numbers (RONs) ranging from 61 to 90 are examined as candidates for the lower octane fuel. Experimental engine calibration maps are first developed to quantify the minimum amount of methanol that must be added to each lower octane fuel in order to reproduce the baseline engine performance attained on a market gasoline (RON 95). The properties of the lower octane fuel are shown to affect the engine performance significantly.

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.

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.

Conventional fossil fuels will constitute the majority of automotive fuels for the foreseeable future but will have to adapt to changes in engine technology. Unconventional transport fuels such as biofuels, gas-to-liquid fuels, compressed natural gas, and liquid petroleum gas will also play a role. Hydrogen might be a viable transport fuel if it overcomes barriers in production, transport, storage, and safety and/or if fuel cells become viable. This book opens by considering these issues and then introduces practical transport fuels. A chapter on engine deposits follows, which is an important practical topic about how fuels affect engines that is not usually considered in other books. The next three chapters discuss auto-ignition phenomena in engines. The auto-ignition resistance of fuels is the most important fuel property since it limits the efficiency of spark ignition engines and determines the performance of compression ignition engines.