Mixing controlled combustion of alcohol fuels has been identified as a promising technology based on their low propensity for particulate and NOx production, but the higher heats of vaporization and auto-ignition temperatures of these fuels make their direct use in diesel engine architectures a challenge. To realize the potential of alcohol-fueled combustion, a computational fluid dynamics (CFD) modeling framework is developed, validated and exercised to identify designs that maximize engine thermal efficiency. To evaluate the use of thermal barrier coatings, a simplified 1-D conjugate heat transfer (CHT) modeling framework is employed. The addition of the 1-D CHT model only increases the computational expense by 15% relative to traditional approaches, yet offers more accurate heat transfer predictions over constant temperature boundary conditions. The validated model is then used to explore a range of injector orientations and piston bowl geometries. Using a design of experiments approach, several designs were identified that improved fuel-air mixing, shortened the combustion duration, and increased thermal efficiency. The most promising design was fabricated and tested in a Caterpillar 1Y3700 Single Cylinder Oil Test Engine. Engine testing confirmed the findings from the CFD simulations, and found that the co-optimized injector and piston bowl design yielded over 2-percentage point increase in thermal efficiency at the same equivalence ratio (0.96) and over 6-percentage point increase at the same engine load (10.1 bar indicated mean effective pressure), while satisfying design constraints for peak pressure and maximum pressure rise rate.

New ignition delay time measurements of natural gas mixtures enriched with small amounts of n-hexane and n-heptane were performed in a rapid compression machine to interpret the sensitization effect of heavier hydrocarbons on auto-ignition at gas-turbine relevant conditions. The experimental data of natural gas mixtures containing alkanes from methane to n-heptane were carried out over a wide range of temperatures (840–1050 K), pressures (20–30 bar), and equivalence ratios (φ = 0.5 and 1.5). The experiments were complimented with numerical simulations using a detailed kinetic model developed to investigate the effect of n-hexane and n-heptane additions. Model predictions show that the addition of even small amounts (1–2%) of n-hexane and n-heptane can lead to an increase in reactivity by ∼40–60 ms at compressed temperature (T C ) = 700 K. The ignition delay time (IDT) of these mixtures decreases rapidly with an increase in concentration of up to 7.5% but becomes almost independent of the C 6 /C 7 concentration beyond 10%. This sensitization effect of C 6 and C 7 is also found to be more pronounced in the temperature range 700–900 K compared to that at higher temperatures (>900 K). The reason is attributed to the dependence of IDT primarily on H 2 O 2 (+M) ↔ 2ȮH(+M) at higher temperatures while the fuel-dependent reactions such as H-atom abstraction, RȮ 2 dissociation, or Q ˙ OOH + O 2 reactions are less important compared to 700–900 K, where they are very important.

In this paper, a zero-dimensional (0D) single-zone combustion model was applied for predicting combustion and indicated engine parameters in a spark ignition (SI) engine. Three different shale gas mixtures, methane, and liquefied petroleum gas (LPG) (30% C 3 H 8 –70% C 4 H 10 ), were studied as SI engine fuel. The shale gas compositions included shale gas-1 (86% CH 4 –14% C 2 H 6 ), shale gas-2 (81% CH 4 –10% C 2 H 6 –9% N 2 ), and shale gas-3 (58% CH 4 –20% C 2 H 6 –2% C 3 H 8 –10% CO 2 ). Experimental results of the SI engine operated with LPG were used in the model verification phase and provided to the validation of the theoretical model. In addition, the operational parameters of the LPG engine were used as the model input values. The results show that shale gas-1 has the potential to be a good alternative fuel to LPG for SI engines. Shale gas-1 has an indicated mean effective pressure (IMEP) value of 5.7–7.3%, which is lower than LPG in the range of ϕ = 0.83–1.2. Furthermore, LPG has a 27.7% higher indicated specific fuel consumption (ISFC) compared to shale gas-1. On the other hand, LPG has 1.2–2.4 units lower indicated thermal efficiency (ITE) values than shale gas-1 in the range of ϕ = 0.83–1.2. However, Methane, Shale gas-2, and Shale gas-3 have 7.55–9.62%, 20.35–20%, 22.19–21.47 lower IMEPs than LPG in the range of ϕ = 0.83–1.2, respectively.

This work describes the development of a transported Livengood-Wu (L-W) integral model for computational fluid dynamics (CFD) simulation to predict auto-ignition and engine knock tendency. The currently employed L-W integral model considers both single-stage and two-stage ignition processes, thus can be generally applied to paraffin, olefin, aromatics and alcohol. The model implementation is first validated in simulations of homogeneous charge compression ignition combustion for three different fuels, showing good accuracy in prediction of auto-ignition timing for fuels with either single-stage or two-stage ignition characteristics. Then, the L-W integral model is coupled with G-equation model to indicate end-gas auto-ignition and knock tendency in CFD simulations of a direct-injection spark-ignition engine. This modeling approach is about 10 times more efficient than the ones that based on detailed chemistry calculation and pressure oscillation analysis. Two fuels with same Research Octane Number but different octane sensitivity are studied, namely Co-Optima Alkylate and Co-Optima E30. The CFD model is validated against experimental data in terms of pressure traces and heat release rate for both fuels under a wide range of operating conditions. The knock tendency-indicated by fuel energy contained in the auto-ignited region-of the two fuels at different load conditions correlates well with the experimental results and the fuel octane sensitivity, implying the current knock modeling approach can capture the octane sensitivity effect and can be applied to further investigation on composition of octane sensitivity.

Lean combustion in an internal combustion engine is a promising strategy to increase thermal efficiency by leveraging a more favorable specific heat ratio of the fresh mixture and simultaneously suppressing the heat losses to the cylinder wall. However, unstable ignition events and slow flame propagation at fuel-lean condition lead to high cycle-to-cycle variability and hence limit the high-efficiency engine operating range. Pre-chamber ignition is considered an effective concept to extend the lean operating limit, by providing spatially distributed ignition with multiple turbulent flame-jets and enabling faster combustion rate compared to the conventional spark ignition approach. From a numerical modeling perspective, to date, still the science base and available simulation tools are inadequate for understanding and predicting the combustion processes in pre-chamber ignited engines. In this paper, conceptually different RANS combustion models widely adopted in the engine modeling community were used to simulate the ignition and combustion processes in a medium-duty natural gas engine with a pre-chamber spark-ignition system. A flamelet-based turbulent combustion model, i.e., G-equation, and a multi-zone well-stirred reactor model were employed for the multi-dimensional study. Simulation results were compared with experimental data in terms of in-cylinder pressure and heat release rate. Finally, the analysis of the performance of the two models is carried out to highlight the strengths and limitations of the two formulations respectively.

In choosing the lubricating oil for a gas turbine system, properties such as viscosity, viscosity index, corrosion prevention, and thermal stability are chosen to optimize turbine longevity and efficiency. Another property that needs to be considered is the lubricant's reactivity, as the lubricant's ability to resist combustion during turbine operation is highly desirable. In evaluating a method to define reactivity, the extremely low vapor pressure of these lubricants makes conventional vaporization by heating impractical. To this end, a new experiment was designed and tested to evaluate the reactivity of lubricating oils using an existing shock-tube facility at Texas A&M University equipped with an automotive fuel injector. This experiment disperses a premeasured amount of lubricant into a region of high-temperature air to study auto-ignition. To ensure proper dispersal, a laser extinction diagnostic was used to detect the lubricant particles behind the reflected shock as they are dispersed and vaporized. An OH* chemiluminescence diagnostic was used to determine ignition delay time. Using this method, various 32-, 36-, and 46-weight lubricants identified as widely used in the gas turbine industry were tested. Experiments were conducted in postreflected shock conditions around 1370 K (2006 °F) and 1.2 atm, where ignition delay time, peak OH* emission, and time-to-peak values were recorded and compared. Ignition was observed for all but one of the lubricants at these conditions, and mild to strong ignition was observed for the other lubricants with varying ignition delay times.

An experimental investigation was performed in a premixed annular combustor equipped with multiple swirl, bluff body burners to assess the ignition probability and to provide insights into the mechanisms of failure and of successful flame propagation. The experiments are done at conditions that are close to the lean blow-off (LBO) limit, and hence, the ignition is difficult and close to the limiting condition when ignition is not possible. Two configurations were employed, with 12 and 18 burners, the mixture velocity was varied between 10 and 30 m/s, and the equivalence ratio ( ϕ ) between 0.58 and 0.68. Ignition was initiated by a sequence of sparks (2 mm gap, 10 sparks of 10 ms each) and “ignition” is defined as successful ignition of the whole annular combustor. The mechanism of success and failure of the ignition process and the flame propagation patterns were investigated via high-speed imaging (10 kHz) of OH * chemiluminescence. The lean ignition limits were evaluated and compared to the LBO limits, finding the 12-burner configuration is more stable than the 18-burner. It was found that failure is linked to the trapping of the initial flame kernel inside the inner recirculation zone (IRZ) of a single burner adjacent to the spark, followed by localized quenching on the bluff body probably due to heat losses. In contrast, for a successful ignition, it was necessary for the flame kernel to propagate to the adjacent burner or for a flame pocket to be convected downstream in the chamber to grow and start propagating upward. Finally, the ignition probability (P ign ) was obtained for different spark locations. It was found that sparking inside the recirculation zone resulted in P ign ∼ 0 for most conditions, while P ign increased moving the spark away from the bluff body or placing it between two burners and peaked to P ign ∼ 1 when the spark was located downstream in the combustion chamber, where the velocities are lower and the turbulence less intense. The results provide information on the most favorable conditions for achieving ignition in a complex multiburner geometry and could help the design and optimization of realistic gas turbine combustors.

In this paper, we explore the operational map of a lean axial-staged combustor of premixed and partially premixed reacting jet-in-crossflow flames at high -pressure (5 atm). This study attempts to expand the data to relatively high pressure and could significantly aid scaling to real gas turbine engine conditions at 20–30 atm. High-speed camera, particle image velocimetry (PIV), CH* chemiluminescence, temperature, and pressure measurements were taken and processed to allow accurate reconstruction of six operating points relative to computational fluid dynamics (CFD) simulations under minimal adjustments. Variation of lean main stage ( φ = 0.575 and 0.73) and rich jet ( φ = 1.1, 4, and 8) equivalence ratio has been investigated for a four mm axial jet. The fully premixed flames were found to be controlled by the crossflow temperature before ignition and the crossflow oxygen content during combustion. Analysis of flame shape and position for the partially premixed operating points describes a lee stabilized as well as a more unsteady windward flame branch. Adjustment of added jet fuel and crossflow temperature along with its corresponding oxygen level is required to attain a compact flame body. The risk of delaying combustion progress is significantly increased at a richer jet φ = 8 and an overshooting, spatially divided flame was attained with a main stage φ = 0.73. Control toward a compact flame body is critical to allow combustion at reasonable reaction rate.

During the ignition of a swirled single-injector combustor, two phases have been identified experimentally. In the first, the flame penetrates the injection unit, while in the second, the flame lifts off after a substantial delay before stabilizing at a distance from the injector. This transient phenomenon is investigated using Large Eddy Simulations based on an Euler–Lagrange description of the liquid spray, an energy deposition model to mimic ignition, and the thickened flame combustion model. It is shown that the initial penetration of the flame in the injector unit is linked with the positive pressure excursion induced by the rapid volumetric expansion of burnt gases. This sudden expansion is itself due to the fast increase in heat release rate that occurs during the initiation of the process. The corresponding positive and negative pressure disturbances induce a rapid reduction of the mass flow rate through the injector, followed by an acceleration of the flow and a return to the nominal value. It is also shown that the flame root disappears after another delay, which results in the flame edge lifting and stabilization at a distance from the injector exhaust corresponding to steady operation of the device. The relatively long delay time before this liftoff takes place is found to correspond to the residence time of the cooled burnt gases in the vicinity of the chamber walls, which are ultimately entrained by the internal recirculation zone and quench the lower flame foot.

Low calorific value (LCV) gaseous fuels are generated as by-products in many commercial sectors. Their efficient exploitation can be a considerable source of primary energy. Typically, product gases from biomass are characterized by low lower heating values (LHV) due to their high concentration of inert gases and steam. At the same time, their composition varies strongly based on the initial feedstock and may contain unwanted components in the form of tars and ammonia. These properties make the design of appropriate combustion systems very challenging and issues such as ignition, flame stability, emission control, and combustion efficiency must be accounted for. By employing a proprietary gas turbine burner at the TU Berlin, the combustion of an artificial LCV gas mixture at stoichiometric conditions has been successfully demonstrated for a broad range of steam content in the fuel. The current work presents the stability maps and emissions measured with the swirl-stabilized burner at premixed conditions. It was shown that the flame location and shape primarily depend on the steam content of the LCV gas. The steam content in the fuel was increased until flame blow-out occurred at LHVs well below the target condition of 2.87MJ/kg (2.7MJ/Nm3). The exhaust gas is analyzed in terms of the pollutants NOx and CO for different fuel compositions, moisture contents, and thermal powers. Finally, OH* measurements have been carried out in the flame. A simple reactor network simulation was used to confirm the feasibility of the experimental results.

Biogas, which is a renewable alternative fuel, has high antiknocking properties with the potential to substitute fossil fuels in internal combustion engines. In this study, performance characteristics of a spark-ignition (SI) engine operated under methane (baseline case) and biogas are compared at the compression ratio (CR) of 8.5:1. Subsequently, the effect of CR on operational limits, performance, combustion, and emission characteristics of the engine fueled with biogas is evaluated. A variable compression ratio, spark-ignition engine was operated at various CRs of 8.5:1, 10:1, 11:1, 13:1, and 15:1 over a wide range of operating loads at 1500 rpm. Results showed that the operating range of the engine at 8.5:1 CR reduced when biogas was utilized in the engine instead of methane. However, the operating range of the engine for biogas extended with an increase in CR—an increase from 9.6 N-m-16.5 N-m to 2.8 N-m-15.1 N-m was observed when CR was increased from 8.5:1 to 15:1. The brake thermal efficiency improved from 13.7% to 16.3%, and the coefficient of variation (COV) of indicated mean effective pressure (IMEP) reduced from 12.7% to 1.52% when CR was increased from 8.5:1 to 15:1 at 8 N-m load. The emission level of carbon dioxide was decreased with an increase in CR due to an improvement in the thermal efficiency and the combustion process.

This study demonstrates the effects of technologies applied for the development of gasoline direct injection (GDI) engine for improving the brake thermal efficiency (BTE). The test engine has a relatively high stroke to bore ratio of 1.4 with a displacement of 2156 cm 3 . All experiments have been conducted for stoichiometric operation at 2000 RPM. First, since compression ratio (CR) is directly related to the thermal efficiency, four CR were explored for operation without exhaust gas recirculation (EGR). Then, for the same four CR, EGR was used to suppress the knock occurrence at high loads, and its effect on initial and main combustion duration was compared. Second, the shape of intake port was revised to increase tumble flow for reducing combustion duration, and extending EGR-stability limit further. Then, as an effective method to ensure stable combustion for EGR-diluted stoichiometric operation, the use of twin spark ignition (SI) system is examined by modifying both valve diameters of intake and exhaust, and its effect is compared against that of single spark ignition. In addition, the layout of twin spark ignition was also examined for the location of front-rear and intake-exhaust. To get the maximum BTE at high load, 12 V electronic super charger (eSC) was applied. Under the condition of using 12 V eSC, the effect of intake cam duration was identified by increasing from 260 deg to 280 deg. Finally, 48 V eSC was applied with the longer intake camshaft duration of 280 deg. As a result, the maximum BTE of 44% can be achieved for stoichiometric operation with EGR.

Using a split injection of wet ethanol, where a portion of the fuel is injected during the compression stroke, has been shown to be an effective way to enable thermally stratified compression ignition (TSCI), an advanced combustion mode that aims to control the heat release process by enhancing thermal stratification, thereby extending the load range of low temperature combustion (LTC). Wet ethanol is the ideal fuel candidate to enable TSCI because it has a high latent heat of vaporization and low equivalence ratio sensitivity. Previous work has shown “early” compression stroke injections (−150 to −100 deg aTDC) have the potential to control the start of combustion (SOC) while “mid” compression stroke injections (−90 to −30 deg aTDC) have the potential to control in-cylinder thermal stratification, thereby controlling the heat release rate. In this work, a mixture of 80% ethanol and 20% water by mass is used to further study the injection strategy of TSCI combustion. Additionally, the impact of external, cooled exhaust gas recirculation (EGR), and intake boost on the effectiveness of a TSCI with wet ethanol to control the heat release process are investigated. It was found that neither external, cooled EGR, nor intake boost level has any impact on the effectiveness of the compression stroke injection(s) at controlling the burn rate of TSCI. External, cooled EGR has the potential to increase the overall tailpipe combustion efficiency, while intake boost has the potential to decrease NO x emissions at the expense of combustion efficiency by lowering the global equivalence ratio.

Future emission regulations for internal combustion engines require increasingly stringent reductions of engine-out emissions, especially NO x and particulate matter (PM), together with the continuous improvement of engine efficiency. In the current scenario, even though compression-ignited engines are still considered the most efficient and reliable technology for automotive applications, the use of diesel-like fuels has become a critical issue, since it is usually not compatible with the required emissions reduction. A large amount of research and experimentation is being carried out to investigate the combined use of compression-ignited engines and gasoline-like fuels, which proved to be very promising, especially in case the fuel is directly injected in the combustion chamber at high pressure. This work investigates the combustion process occurring in a light-duty compression-ignited engine while directly injecting only gasoline. A specific experimental setup has been designed to guarantee combustion stability over the whole operating range that is achieved controlling boost pressure and temperature together with all the injection parameters of the multijet pattern. The analysis of the experimental data clearly highlights how the variation of the control parameters affect the ignition process of small amounts of directly injected gasoline and the maximum achievable efficiency. In particular, the analysis of the sensitivity to the injection parameters allows identifying an ignition delay model and the key control parameters that might be varied to guarantee a robust control of combustion phasing within the cycle.

Lean staged combustion can reduce the NO x emissions by prevaporizing and premixing fuel with air, which is considered the state-of-the-art solution strategy in achieving low emission in aeronautical combustors. However, lean premixed combustion is subjected to combustion stability problems, which restrict the ground and altitude operation limits of the commercial engine. In this work, the effect of the swirl intensity of pilot inner swirler on combustion stability of a lean staged injector is experimentally and numerically studied. The lean staged injector is piloted by a dual swirler prefilm atomizer. The swirl intensity of the pilot inner swirler is varied by parameterizing the vane angle as +20 deg, −20 deg, and −35 deg, with −20 deg selected as the baseline with a counterswirling design. A single sector model combustor is designed, and the nonreacting flow field and fuel concentration distributions are measured by particle image velocimetry (PIV) and kerosene planar laser induced fluorescence (kerosene-PLIF) techniques. The alteration of swirl direction from counterswirling to coswirling induces a negligible effect on flow structures, but the spray distribution changes from a solid pattern to a hollow pattern. The increase in the pilot inner swirl intensity causes a shrunk cyclone recirculation zone (CRZ) and a reduction of kerosene concentration in the central region. The influences of the pilot inner swirler angle on combustion stability are evaluated. The ignition and lean blow-out (LBO) results show that the baseline injector exhibits excellent combustion stability, while the coswirling design holds the highest ignition and LBO fuel–air ratio (FAR). In order to find out the physical mechanisms dominating the ignition and LBO processes, nonreacting numerical simulations are conducted to provide information regarding the flow structures and kerosene concentrations at ignition limits. Moreover, the ignition sequences are redefined as the radial flame propagation phase, the axial flame propagation phase, and the flame stabilization phase. The comparison of kerosene concentration along the radial and axial propagation routes concludes that the fuel enrichment in the two processes improves the ignition performance. On the other hand, the Karlovitz number of flame anchoring points in the flame rooting region is calculated to evaluate the flame stabilization characteristics. The results indicate that promoting the number of flame anchoring points and their radial range benefits the LBO performance.

Due to recent regulation changes to restricted fuel usage in various motor-sport events, motor-sport engine manufacturers have started to focus on improving the thermal efficiency and often claim thermal efficiency figures well above equivalent road car engines. With limited fuel allowance, motor-sport engines are operated with a lean air–fuel mixture to benefit from higher cycle efficiency, requiring an ignition system that is suitable for the lean mixture. Prechamber ignition is identified as a promising method to improve lean limit and has the potential to reduce end gas auto-ignition. This paper analyses the full-load performance of a motor-sport lean-burn gasoline direct injection (GDI) engine and a passive prechamber is developed with the aid of a computational fluid dynamics (CFD) tool. The finalized prechamber design benefited in a significant reduction in burn duration, reduced cyclic variation, knock limit extension, and higher performance.

In this work, a simplified kinetic model of the SO 3 generation reaction is proposed with a test diesel engine, and the semikinetic model was further developed and applied to predict the acid dew point (ADP) temperature. In the model, the one-dimensional combustion model and the reaction kinetic model of O-radicals and SO 3 generation were considered. The recommended kinetic constant equation was also provided. In addition, the evolution of O-radicals and SO 3 and the effects of both on the semikinetic ADP model were also discussed. To the best of my knowledge, the introduction of dynamics into the ADP model is a new and noteworthy contribution. The research results indicated that the ADP model based on the semikinetic method improved the prediction accuracy of the thermodynamic ADP model. The key factors in the O 2 dissociation reaction were the high-temperature environment and the presence of flame and ignition. The O-radical concentration played a leading role in the SO 3 formation reaction, and the SO 3 /SO 2 conversion ratio was less than 10% in the cylinder.

Spark ignition aeropiston engines have good prospects due to light weight and high power to weight ratio. Both gasoline and kerosene can be utilized on these engines by using either traditional port fuel injection (PFI) or the novel air-assisted fuel injection (A2FI). In this article, the effects of different fuels and injection methods on the performance of a four-cylinder opposed aeropiston engine were studied. The spray performance test rig and the engine performance test rig were established. First, the influence of different injection methods on engine performance were compared, which indicated that A2FI is superior to PFI in engine power and starting performance. Furthermore, the fuel performance comparison by using A2FI was conducted, which demonstrates that kerosene is inferior to gasoline in terms of spray characteristics and power performance. Finally, detailed working characteristics of A2FI system using kerosene were studied, which indicated that the stable and reliable operation of the spark-ignition operation can be realized and the kerosene's spark-ignition combustion process can be optimized similar to that of gasoline. Results shows that the use of kerosene combined with A2FI is the best technical way to achieve ideal working process of the spark ignition aeropiston engine.

Although engines fueled with diesel/gasoline blends show excellent combustion and emission performance, its low-temperature flame development characteristics under cold-start conditions remain to be further verified. To clarify the details, experiments were conducted in an optical constant volume combustion chamber using Mie-scattering and direct photography methods at different ambient temperatures. Results show that the ignition delay of pure diesel during spray combustion shows a zero temperature coefficient (ZTC) region, and the addition of gasoline weakens the ZTC behavior until it disappears. The cool flame initiates the ignition, and the hot flame tends to far from the base of the cool flame as the gasoline content increases. In addition, the addition of gasoline to diesel increases the ratio of cool flames because the high evaporation reduces the temperature in the mixing zone, so only cool flame occurs in the G45 blends. Consequently, the total flame intensity presents an order of magnitude decrease. At lower ambient temperatures, the addition of gasoline significantly increases ignition instability. It is difficult to convert a cool flame into a hot flame due to the inhomogeneity of temperature and species field, which results in various unstable ignition phenomena, such as a short flash cool flame and intermittent cool and hot flame. Therefore, it is essential to directly target the cool flame and pay attention to the intrinsic mechanism of the evolution from the cool flame to the hot flame during the spray combustion process.

Nitromethane has a stoichiometric air–fuel ratio of 1.7, which is 8.5 times lower than gasoline. For the same amount of air being drawn by the engine, more amount of nitromethane blends and hence more energy can be added. Methanol was used as a medium to mix nitromethane and gasoline, which are normally immiscible. Engine performance tests were carried out to study the effect of nitromethane addition to the methanol-gasoline blend. A large rise in engine torque and brake thermal efficiency (BTE) was obtained during the investigation. However, the brake specific fuel consumption (BSFC) also increased for the nitromethane blends. The engine parameters like spark timing, equivalence ratio, and compression ratio were optimized to further increase the engine power and also bring down the BSFC. A net torque improvement of 42%, BTE improvement of 35%, and BSFC rise of 9% were obtained by adding nitromethane and methanol in small fractions to gasoline. Combustion analysis was carried out using the cylinder pressure trace. High heat release rate and shorter combustion duration with nitromethane addition were observed. Emission measurements showed decrease in HC and CO emissions with nitromethane addition. However, a drastic rise in NO emissions was observed. Hence, it can be concluded that the specific power of small two-stroke spark ignition (SI) engines can be enhanced using nitromethane as a fuel additive to increase the payload of the unmanned aerial vehicles.