In future high-efficiency engines that implement high compression ratios, the occurrence of knock/super knock phenomena, primarily triggered by the end gas auto-ignition (EGAI), will pose a significant challenge. To address this issue, ion sensing technology has emerged as a highly promising approach for real-time detection of knock events. However, the reliability of ion sensing based EGAI detection is still poor due to the electron ambipolar diffusion process. When EGAI occurs at the far end of the engine combustion chamber, the electrons produced in the EGAI zone are bound to positive ions due to electrostatic force. In such case, the electrons can hardly diffuse out of the zone and be collected by the anode of spark plugs (which functioned as the ion probes in production engines), consequently leading to the fault of ion sensing method. To overcome this challenge, the behaviors of electron transport under various external electric field configurations are analyzed by simulating the end gas ignition process in a constant volume combustion chamber (CVCC). The findings demonstrate that an external electric field can surmount the electrostatic force acting on electrons and positive ions. Specifically, when the voltage applied from the DC power source in the ion sensing circuit exceeds 10 kV, numerical analysis suggests a transition from electron ambipolar diffusion to unipolar diffusion. Consequently, electrons can successfully diffuse out of the EGAI zone in the CVCC. This enables the collection of ion current signals by an ion probe positioned outside of that zone. Thus, the potential of early-stage engine knocking detection based on ion sensing is highlighted, particularly since high voltages can be configured by incorporating engine ignition modules into the ion sensing systems.

Fuel injection strategies often have a considerable impact on pre-ignition in high specific output gasoline engines. Splitting the injection event into two or more pulses has been widely explored as one means of reducing pre-ignition. As effective as these strategies can be with respect to pre-ignition suppression, they often introduce other compromises into the combustion process, for example, reduced indicated mean effective pressure or greater cycle-to-cycle variation. This study examines a split injection strategy with up to three injection pulses for suppressing pre-ignition, while optimizing the start of injection and duration of injection to minimize the associated compromises on the combustion process. The results demonstrate that splitting the injection event generally lowers the in-cylinder temperature and reduces the fuel mass that reaches the cylinder liner. This leads to a lower probability of creating oil-fuel droplets, which may act as a precursor for pre-ignition. The split injection strategy with a late injection when the piston is close to top dead center is shown to perform even better in terms of pre-ignition suppression, while providing comparable indicated mean effective pressure and cycle-to-cycle variation to the baseline case with a single injection pulse. Finally, the injection pressure is varied to establish an optimal combination of operating parameters for avoiding pre-ignition in high specific output gasoline engines.

Pre-ignition remains a significant bottleneck to further downsizing and downspeeding technologies employed for reducing CO2 emissions in modern turbocharged spark-ignited engines.Pre-ignition, which occurs rarely, may lead to high peak pressures that auto-ignite the entire charge before TDC.The resulting high-pressure oscillations are known as super-knock, leading to sudden and permanent hardware damage to the engine.Over the years, numerous researchers have investigated the stochastic phenomenon's source and concluded that there is a role of lubricant additives, deposits, gasoline properties, and hot surfaces in triggering pre-ignition.No single source has been identified; the research continues.Here, we take a different approach; rather than continue the search for the source(s) of super-knock, we explore mitigating super-knock by detecting pre-ignition early enough to take immediate evasive action.Such evasive action is expected to suppress knock intensity, thereby saving the engine from any permanent damage.In this regard, the current work offers ways to detect pre-ignition (using ion sensors) and then mitigate engine damage by using immediate fuel enrichment.We present three related explorations.In exploration #1, we explore if the occurrence of ions products from the exhaust can warn that the next cycle has a high probability of preignition.For this next cycle, the intake fuel injection can be suspended or increased to operate engine fuel-rich.We find strong ion activity on every cycle.However, there is a weak correlation between the ion signal and pre-ignition occurrence.In exploration #2, an in-cylinder ion-current sensor is used to discover pre-ignition event unfolding during the compression stroke.When such a rare event is detected, more fuel is immediately injected, making the end gas far less reactive and avoiding autoignition and knock.These explorations #1 and #2 were conducted with a DC-based ion sensor.These explorations showed exciting and promising findings.However, our DC-based ion sensors are prone to low signal-to-noise ratio SNR, leading to false positives (unacceptably high number of false positives.)In Exploration #3, the signal-to-noise ratio improvement is explored by replacing the DC-based system with a novel AC-based system.We find the bandpass filtering of the ion signal is key to improved SNR.

Petroleum coke (petcoke) is a by-product of heavy petroleum refining, with heating values comparable to that of coal. It is readily available in oil-producing countries such as the United States of America (USA) and the Kingdom of Saudi Arabia (KSA) at minimum costs and can be used as an inexpensive fossil fuel for power generation. Oxy-petcoke combustion is an attractive CO2 capture option as it avoids the use of additional absorption units and chemicals, and results in a CO2 + H2O flue gas stream that is compressed and dehydrated in a CO2 capture and purification unit (CO2CPU). The additional cost of the CO2CPU can be reduced through high pressure combustion. Hence, this paper reports a techno-economic analysis of an oxy-petcoke plant with CO2 capture simulated at pressures between 1 and 15 bars in Aspen PlusTM based on USA and KSA scenarios. Operating at high pressures leads to reduced equipment sizes and numbers of units, specifically compressors in CO2CPU, resulting in increased efficiencies and decreased costs. An optimum pressure of ~10 bars was found to maximize the plant efficiency (~29.7%) and minimize the levelized cost of electricity (LCOE), cost of CO2 avoided and cost of CO2 captured for both the USA and KSA scenarios. The LCOE was found to be moderately sensitive to changes in the capital cost (~0.7% per %) and increases in cost of petcoke (~0.5% per USD/tonne) and insensitive to the costs of labour, utilities and waste treatment.

The CO2 capture performance of sorbents derived from three distinct limestones, including a metamorphosed limestone, is studied under conditions relevant for calcium looping CO2 capture from power plant flue gas. The combined and individual influence of flue gas H2O and SO2 content, the influence of textural changes caused by sequential calcination/carbonation cycles, and the impact of CaSO4 accumulation on the sorbents’ capture performance were examined using bubbling fluidized bed reactor systems. The metamorphosed limestone-derived sorbents exhibit atypical capture behavior: flue gas H2O negatively influences CO2 capture performance, while limited sulfation can positively influence CO2 capture, with space time significantly impacting CO2 and SO2 co-capture performance. The morphological characteristics influencing sorbents’ capture behavior were examined using imaging and material characterization tools, and a detailed discussion is presented. This insight into the morphology responsible for metamorphosed limestone-derived sorbent’s anomalous capture behavior can guide future sorbent selection and design efforts.

Increasingly stringent emission norms have always brought forth innovative measures to improve engine efficiency. Spark-ignited engines have been limited in efficiency, traditionally by knock, and more recently by preignition too. Water injection has recently regained interest as a knock suppressant. The current work explored water injection via port and direct injection at a fixed engine speed of 2000 rpm and varying engine loads. The data presented in this work emphasize that the gains from using water injection are best realized at a specific injection timing (neither too early nor too late), and the effectiveness of water in suppressing knock decreases rapidly with increasing water mass injected. In general, direct water injection offers a more significant knock reduction because of better utilization of the charge cooling effect than port water injection. Engine-out emission confirms a reduction in NOx and CO, while the HC emissions increased when using water injection. No preignition events were observed at the engine load up to 27 bar. Chemical kinetics simulations confirm the role of water in suppressing reactivity under the operating conditions considered in the current study.

Formic acid is a promising fuel candidate that can be generated by reacting renewable hydrogen with carbon dioxide. However, the burning characteristics of formic acid/air mixtures have not been extensively studied. Furthermore, due to its low reactivity, the addition of hydrogen to formic acid/air mixtures may help with improving burning characteristics. This paper presents the first extensive study of formic acid/air premixed laminar burning velocities, as well as mixtures with hydrogen and carbon dioxide. Unstretched laminar burning velocities and Markstein lengths of formic acid in air for two different unburnt gas temperatures and equivalence ratios are presented. Measurements of formic acid mixed with various proportions of hydrogen and carbon dioxide in air are also studied as a potential renewable fuel for the future. Experimental results demonstrate the low burning velocities of formic acid and the ability to significantly enhance flame speeds by hydrogen addition. A modified detailed kinetic model for combustion of formic acid and its mixtures with hydrogen is proposed by merging well-validated literature models. The proposed model reproduces the experimental observations and provides the basis for understanding the combustion kinetics of formic acid laminar premixed flames, as well as mixtures with hydrogen. It is shown that the HOCO radical is the principal intermediate in formic acid combustion, and hydrogen addition accelerates the decomposition of HOCO radical thereby accelerating burning velocities.

Carbon capture has been deemed crucial by the Intergovernmental Panel on Climate Change if the world is to achieve the ambitious goals stated in the Paris agreement. A deeper integration of renewable energy sources is also needed if we are to mitigate the large amount of greenhouse gas emitted as a result of increasing world fossil fuel energy consumption. These new power technologies bring an increased need for distributed fast dispatch power and energy storage that counteract their intermittent nature. A novel technological approach to provide fast dispatch emission free power is the use of the Argon Power Cycle, a technology that makes carbon capture an integral part of its functioning principle. The core concept behind this technology is a closed loop internal combustion engine cycle working with a monoatomic gas in concert with a membrane gas separation unit. By replacing the working fluid of internal combustion engines with a synthetic mixture of monoatomic gases and oxygen, the theoretical thermal efficiency can be increased up to 80%, more than 20% over conventional air cycles. Furthermore, the absence of nitrogen in the system prevents formation of nitrogen oxides, eliminating the need for expensive exhaust gas after-treatment and allowing for efficient use of renewable generated hydrogen fuel. In the case of hydrocarbon fuels, the closed loop nature of the cycle affords to boost the pressure and concentration of gases in the exhaust stream at no penalty to the cycle, providing the driving force to cost effective gas membrane separation of carbon dioxide. In this work we investigated the potential benefits of the Argon Power Cycle to improve upon current stationary power generation systems regarding efficiency, air pollutants and greenhouse gas emissions. A cooperative fuel research engine was used to carry out experiments and evaluate engine performance in relation to its air breathing counterpart. A 30% efficiency improvement was achieved and results showed a reduction on engine heat losses and an overall increase on the indicated mean effective pressure, despite the lesser oxygen content present in the working fluid. Greenhouse gas emissions were reduced as expected due to a substantial increase in efficiency and nitric oxides were eliminated as it was expected. Numerical simulation were carried out to predict the performance and energy penalty of a membrane separation unit. Energy penalties as low as 2% were obtained capturing 100% of the carbon dioxide generated.

Natural gas is a high-octane fuel that produces lower CO2 emissions per kilowatt hour than liquid transport fuels, with essentially zero sulfur emissions. Historically, natural gas has mostly been used in power generation and industrial applications. However, there has been a recent shift towards employing natural gas in the transport sector. In many regions, vehicles are retrofitted with compressed natural gas (CNG) systems, enabling operation on both gasoline and natural gas (and theoretical mixtures thereof). This work examines the effect of leveraging the secondary natural gas fuel system on the performance, efficiency and broader environmental impact of a high specific output gasoline engine. Firstly, mixture sweeps are presented for varying gasoline/natural gas ratios (100% gasoline to 100% natural gas) at wide open throttle (WOT) with both fixed and variable spark timing. This baseline information is then used to optimize the engine calibration for varying gasoline/natural gas ratios over a wider range of operating conditions. Finally, the CO2-equivalent emissions are computed to investigate the broader environmental impact of an optimized gasoline/natural gas vehicle.

Autoignition in HCCI engines is known to be controlled by the combustion kinetics of the in-cylinder fuel/air mixture which is highly influenced by the amount of low-temperature and intermediate-temperature heat release (LTHR and ITHR) that occurs. At lower intake pressures (typically <1.4 bar absolute), it has been observed that gasoline behaves as a single-stage heat release fuel, while at higher intake pressures (typically >1.8 bar absolute) gasoline behaves as a two-stage heat release fuel. Furthermore, ethanol blending into gasoline strongly affects heat release characteristics, and this warrants further investigation. This paper experimentally investigates the conditions under which gasoline transitions from a single-stage heat release fuel to a two-stage heat release fuel as intake pressure is increased. Experiments were performed in single-cylinder HCCI engine fueled with two research-grade gasolines, FACE A and FACE C. These gasolines were tested neat, and with 10% and 20% (by volume) ethanol addition. In addition, these results were compared to results previously obtained for PRF 85, and new results for PRF 84 with 10% and 20% ethanol addition. Moreover, the engine experiments were supported by rapid compression machine (RCM) ignition delay data for the same fuels. The engine experiments revealed that there were minimal differences between the heat release profiles of the two gasolines, FACE A and FACE C, a result which was supported by the RCM experiments that showed similar ignition delay times for the two FACE fuels and PRF 84. On the other hand, with ethanol addition to these gasolines and PRF 84, the occurrence of LTHR shifted to higher intake pressures compared to ethanol-free cases, from 1.4 bar intake pressure for neat fuel to 2.2 bar with 20% ethanol. Consequently, the intake temperatures required to achieve constant combustion phasing for all mixtures were drastically altered. Simulations using a detailed chemical kinetic model were utilized to understand the effects of ethanol blending on the ignition characteristics of PRF 84. The addition of ethanol was found to act as a radical sink where it inhibits the radical pool formation during the low (<850 K) and intermediate (850-1050 K) temperature chemistry regimes resulting in lower reactivity. These results help explain ethanol's significant antiknock qualities under boosted conditions in spark-ignition engines.

Ion current sensing technology has the potential to be a low cost and real time combustion phasing solution for HCCI or HCCI-like engine control. Based on a primary reference fuel oxidation mechanism and a C1–C4 hydrocarbon flame ionization mechanism, a skeletal mechanism for gasoline flame ionization process prediction on HCCI engines was developed in this paper. Since the ion concentrations significantly affect the aroused ion current signals, the mechanism is targeted on accurately predicting both the ion production concentrations and other key combustion characteristics. Through the comparison with the results from the detailed gasoline flame ionization mechanism and experimental results, the predicted maximum hydronium (H3O+) ion concentration and the concentration variation tendency are validated. Additionally, the auto-ignition delay time (tign) accurately predicted under HCCI engine conditions. Through coupling with a 3D-CFD engine model, the skeletal mechanism was applied to predict the important information of in-cylinder ion species, which are validated by the experimental ion current amplitudes and phases. The results show that the ion current phase (Ion50) matches well with the positions where the predicted ion concentration reaches its maximum, and the ion current amplitudes are well predicted under the conditions of different equivalence ratios (Φ) and fuel injection ratios (Injratio).

This work was supported by King Abdullah University of Science and Technology (KAUST) with Saudi Aramco under the Fuelcom II program and made use of KAUST supercomputing resources. The authors thank convergent science for providing their code for the simulations.

The use of a spark plug ion sensor to detect combustion timing in a homogeneous charge compression ignition (HCCI) engine is a technique that could alleviate the need for pressure transducers, a more expensive alternative. One disadvantage of this approach is the difficulty in obtaining a strong signal at lower equivalence ratios. This article addresses and compares three ion sensing circuitries, namely a voltage follower, a notch filter circuit that removes the 60-Hz wall noise, and a notch filter whose output is coupled to a custom-built "integrator" circuit. The circuit optimizations are aimed at improving signal strength and reliability. The ion signal present in the combustion chamber is experimentally investigated in a 1.9-L Volkswagen engine, modified for HCCI operation and fueled with gasoline. Experiments are conducted across different intake temperatures, pressures, and equivalence ratios. It was found that the custom- built circuit provided the best ion signal strength and reliability.