Gasoline Compression Ignition Engine Research Articles

Experimental study on combustion and emissions of a compression ignition engine fueled with gasoline

Gasoline compression ignition (GCI) is an effective way to achieve both high thermal efficiency and low emission. The combustion and emission performances of GCI and DCI (diesel compression ignition) were compared on a 2.0 L diesel engine equipped with Three-way catalyst-Lean NOx trap/Passive selective catalytic reduction (TWC-LNT/PSCR) aftertreatment system. In order to further clarify the advantages and disadvantages of GCI, this paper first studies the combustion and emission at 1500 rpm and braking average effective pressure (BMEP) of 4–9 bar. Secondly, six small map points of worldwide harmonized light vehicles test cycle (WLTC) are studied. The results show that the braking thermal efficiency (BTE) of GCI is lower than that of DCI at low load. When BMEP is greater than 5 bar, the BTE of GCI is significantly improved. GCI achieves a maximum BTE of 43%, which is 3% higher than DCI. Compared with DCI, the NOx emission of GCI is slightly lower, the smoke emission of filter smoke number (FSN) is significantly improved, and the CO and HC emissions are significantly increased. GCI engine equipped with TWC-LNT/PSCR system with high aftertreatment efficiency has the potential to meet China’s VI B emission regulations.

Aerobic oxidation of hydrocarbons using N-hydroxyphthalimide (NHPI) catalysts for cetane enhancement

Gasoline compression ignition (GCI) utilizes gasoline-like fuels in low temperature compression ignition (CI) engines. As a result, thermal efficiency is increased compared to that of a conventional spark ignition (SI) engine, while NOx and soot tailpipe emissions are reduced compared to those from a diesel engine. This combustion strategy requires gasoline fuel with a moderate ignition delay for cold start and high load demand conditions. This work proposes an approach to control the ignition characteristics of gasoline to widen GCI engine operation conditions. Aerobic oxidation of gasoline using N-hydroxyphthalimide (NHPI) catalyst has been suggested as an effective method to produce high cetane components, such as hydroperoxide. NHPI catalyzed oxidation reactions of toluene and market gasoline were studied at different reaction conditions using an autoclave batch reactor. The produced hydroperoxide (HP) was quantified by titration using triphenylphosphine (Ph3P) followed by Gas Chromatography (GC) analysis. In addition, an ignition quality tester (IQT) was used to estimate the derived cetane number (DCN) of the oxygenated product. The gasoline cetane number was enhanced by nine digits using gaseous oxygen at moderate conditions (10 bar, 110 °C for 3 h). Aerobic oxidation of gasoline using NHPI catalyst was found to be a promising approach to control the ignition characteristics of fuel and hence an enabler of the GCI engine concept.

Machine Learning Model for Spark-Assisted Gasoline Compression Ignition Engine

<div class="section abstract"><div class="htmlview paragraph">The study showcases the strength of machine learning (ML) models in imitating the operation of an advanced engine concept - the gasoline compression ignition (GCI) - at low loads. The GCI engine is prone to exceeding the limits of criteria emissions at such loads, especially at the cold start when the catalyst is not activated. One proposition to accelerate catalyst light-off is using spark-ignition. This, however, adds an extra level of complexity in identifying an optimum operation point. The ML models can be a useful tool in guiding the engine calibration process. In this study, the ML models are trained on GCI engine experiments, covering different intake conditions, injection strategies, and spark settings. The models can predict seven engine performance parameters: fuel consumption, four engine-out emissions, exhaust temperature, and coefficient of variation (COV) in indicated mean effective pressure (IMEP). The study considered four architectures to train the dataset, namely linear regression, support vector machine (SVM), random forest and CatBoost regressor. CatBoost, which is a gradient boosting tree-based regressor, outperformed all of the considered models. The models were evaluated using the leave-one-out-cross-validation method to obtain the most representative results of the model’s accuracy. This paper shows that the seven models have successfully captured the complex relationship between the input calibration parameters and the seven desired outputs. The developed models have the potential to be utilized in optimizing GCI engine performance - especially at low loads where the engine has issues lighting off the catalyst. Coupling ML models with suitable optimization algorithms can pave the way to pinpoint the global optimum operation point in less time and with less cost than traditional calibration approaches.</div></div>

Numerical analysis of soot emissions from gasoline-ethanol and gasoline-butanol blends under gasoline compression ignition conditions

In the present work, computational fluid dynamics (CFD) simulations of a single-cylinder gasoline compression ignition (GCI) engine were performed to investigate the impact of blending two biofuels, ethanol and n-butanol, with gasoline on the trade-off between combustion phasing and soot emissions under low load conditions. In order to represent market gasoline (RD5-87), a four-component toluene primary reference fuel (TPRF) + ethanol (ETPRF) surrogate (with 20% ethanol by mole; E20) was formulated using a neural network based octane predictor such that the surrogate had the same ethanol content, Research Octane Number (RON) and Octane Sensitivity (S). In addition, a novel skeletal kinetic mechanism for ETPRF and TPRF + n-butanol (BTPRF) blends, incorporating polycyclic aromatic hydrocarbon (PAH) chemistry, was developed. A three-dimensional (3D) engine CFD formulation employing the skeletal mechanism, adaptive mesh refinement (AMR), finite-rate chemistry approach, and hybrid method of moments (HMOM) was adopted to capture the in-cylinder combustion phenomena and soot emissions. The engine CFD model was validated against RD5-87 experimental data for a broad range of start-of-injection (SOI) timings (-21/-27/-36/-45 crank angle degrees (CAD) after top-dead-center (aTDC)), with respect to in-cylinder pressure, heat release rate, combustion phasing, and soot emissions. The closed-cycle simulation results were analyzed to elucidate the non-monotonic trend of soot emissions versus SOI timing: SOI-36 > SOI-45 > SOI-21 > SOI-27. Thereafter, the validated CFD model was employed to simulate the combustion of a gasoline-ethanol blend with 45% (by mole) ethanol (E45) and a gasoline-butanol blend with 45% (by mole) n-butanol (B45) under the same operating conditions to study the effects of fuel composition and SOI timing on combustion phasing and soot emissions. The sooting propensity followed the trend: B45 > E20 > E45 at all SOI timings. Overall, it was observed that the autoignition propensity was primarily related to fuel chemistry. On the other hand, sooting propensity showed strong coupling with both fuel chemistry and physical properties, with greater impact of fuel physical properties at advanced SOI timings.

Variable Valve Strategy Evaluation for Low-Load Operation in a Heavy-Duty Gasoline Compression Ignition Engine

By harnessing gasoline’s low reactivity for partially premixed combustion promotion, gasoline compression ignition (GCI) combustion shows the potential to produce markedly improved NOx-soot trade-off with high fuel efficiency compared to conventional diesel combustion. However, at low-load conditions, gasoline’s low reactivity poses challenges to attaining robust combustion with low unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions. Increasing the in-cylinder charge temperature by using variable valve actuation (VVA) can be an effective means to address these challenges. In this numerical investigation, VVA strategies, including (1) early exhaust valve opening (EEVO), (2) positive valve overlap (PVO), and (3) exhaust rebreathe (ExReb), were investigated at 1375 RPM and 2 bar brake mean effective pressure in a heavy-duty GCI engine using a market-based gasoline with a research octane number (RON) of 93. The total residual gas level was kept over 50% to achieve an engine-out NOx target of below 1.5 g/kWh. For a complete engine system analysis, one-dimensional (1-D) system-level modeling and three-dimensional (3-D) computational fluid dynamics (CFD) analysis were close-coupled in this study. Performance of the VVA strategies was compared in terms of in-cylinder charge and exhaust gas temperatures increase versus brake-specific fuel consumption (BSFC). The EEVO strategy demonstrated in-cylinder charge and exhaust temperature increase up to 130 and 180 K, respectively. For similar in-cylinder charge temperature gains, the ExReb strategy demonstrated 11% to 18% lower BSFC compared to the EEVO strategy. This benefit primarily originated from a more efficient gas-exchange process. The PVO strategy, due to the valve–piston contact constraint, required excessive exhaust back-pressure valve (BPV) throttling for hot residuals trapping, thereby incurring higher BSFC compared to ExReb. In addition, the ExReb strategy demonstrated the highest potential for exhaust temperature increase (up to 673 K) among the three strategies. This was achieved by ExReb’s maximum air-fuel ratio reduction from high internal residuals mass and BPV throttling. Finally, the ExReb profile was optimized in terms of the peak lift, the duration, and the location for maximizing the fuel-efficiency potential of the strategy.

Effects of different injection strategies on mixing, combustion and emission behavior of gasoline compression ignition (GCI) engines

Gasoline compression ignition (GCI) engines have the advantages of high thermal efficiency and low emissions, but show poor combustion stability under low-load conditions. Adjusting the injection strategy can improve the fuel–air mixing process, which is an effective way to solve the fluctuation of GCI combustion. In this paper, the effects of single injection and pre-main injection strategy on the combustion performance and emission characteristics of GCI engines were investigated using a combination of experiment and simulation. The results showed that advancing the injection timing of the single injection could significantly increase the thermal efficiency and reduce the emissions of carbon monoxide (CO), total hydrocarbons (THC), and soot, but also increase the nitrogen oxide (NOX) emissions due to the rise of peak temperature. Compared with the single-injection strategy, the pre-injection strategy could reduce the overall inhomogeneity in the cylinder and increase the OH radicals before the main injection, thus increasing the in-cylinder pressure and heat release rate, and advancing the combustion phase. Moreover, advancing the main-injection timing (MIT) and increasing the pre-injection ratio (PIR) could increase the flame propagation rate and help reduce combustion instability. Under all pre-injection timing (PIT) conditions, when the PIT = 100CAD BTDC, the inhomogeneity of the mixture was the lowest, thereby reducing the generation of soot precursor A4 and lowering the soot nucleation rate. Meanwhile, higher combustion temperature increased the soot oxidation rate, thus reducing the final soot production. By selecting appropriate PIT, advancing the MIT, and increasing the PIR, the GCI engine can obtain the comprehensive benefits of high thermal efficiency, high combustion stability, and low emissions.

Effects of injection strategies coupled with gasoline-hydrogenated catalytic biodiesel blends on combustion and emission characteristics in GCI engine under low loads

The Gasoline Compression Ignition (GCI) engine is one among the most advanced combustion mode engines, having high thermal efficiency with lower nitrogen oxides and particle emissions simultaneously targeted for upcoming emission regulations. The engine's practical applications are limited by its igniting difficulty and combustion stability at low loads. In order to address these issues, a novel type of high reactive fuel known as hydrogenated catalytic biodiesel is mixed with gasoline as fuel in various ratios (20–40%) and its combustion and emission parameters are studied at low loads. Experiments were carried out in single and double injection mode to find out optimum engine conditions by varying start of injection timings (−7 to −13 deg. CA aTDC). Further, in double injection mode with different pilot fuel injection ratios from 20 to 40% and pilot fuel injection timings from −60 to −40 deg. CA aTDC. The experimental results show that stable combustion is achieved at low loads by using this test fuel without having any ignition difficulty or combustion assistance. The findings of the single-injection mode show that delaying the injection timing resulted in lower maximum pressure rise rate, nitrogen oxide emissions and nucleation mode number concentration emissions under low load conditions. The double injection mode controls the maximum pressure rise rate and cyclic variations within the controlled range under all conditions. From different test conditions, a lower pilot injection ratio (20%) during the advanced start of injection (−60 deg. CA aTDC) condition is recommended because it shows better combustion and emission results. Overall, the gasoline-hydrogenated catalytic biodiesel blend could be the fuel for GCI engines and it is effective at tackling low load difficulties.

An Experimental and Computational Investigation of Tailor-Developed Combustion and Air-Handling System Concepts in a Heavy-Duty Gasoline Compression Ignition Engine

This study investigates using tailor-developed combustion and air-handling system concepts to achieve high-efficiency, clean gasoline compression ignition (GCI) combustion, aimed at addressing a future heavy-duty ultralow NOx standard of 0.027 g/kWh at the vehicle tailpipe and the tightening CO2 limits around the world by combining GCI with a cost-effective engine aftertreatment system. The development activities were conducted based on a 15 L heavy-duty diesel engine. By taking an analysis-led design approach, a first-generation (Gen1) GCI engine concept was developed and tested, encompassing tailor-designed piston bowl geometry, fuel spray pattern, and swirl motion paired with a customized, fixed-geometry, two-stage turbocharging system and a high-pressure EGR loop with two-stage cooling. Across four key steady-state operating points, the Gen1 GCI concept demonstrated 85–95% lower smoke and 2–3% better diesel-equivalent gross indicated fuel consumption compared to the diesel baseline at 1 g/kWh engine-out NOx. By upgrading to a Gen2 air-handling concept that was composed of a prototype, single-stage, variable-geometry turbocharger and a less restrictive EGR loop, 1D system-level analysis predicted that the pumping mean effective pressure was reduced by 43–54% and the diesel-equivalent brake-specific fuel consumption was improved by 2–4%, thereby demonstrating the performance enhancement potential of refining the air-handling system.

CFD-guided optimization of the injector and injection parameters for light-duty GCI engine

In this study, computational fluid dynamics (CFD) and machine learning (ML) were used to investigate the effects of and optimize the injector design parameters for light-duty gasoline compression ignition (GCI) engine. This study was performed at part- (6 bar) and high-load (22 bar) indicated mean effective pressure (IMEP) conditions. The effects of number of nozzles (nNoz), spray angle (SA) and plume angle (PA) while maintaining total nozzle area (TNA) and start of injection (SOI) were first investigated. The increased nNoz and PA enhanced fuel/air mixing, especially in the bowl region, by even spray distribution at part-load condition, but the effects are negligible at high-load mixing-driven combustion due to high in-cylinder temperature and pressure at the time of main injection. On the other hand, SA had significant effect on the air utilization and hence engine performance and emissions at both part- and high-load conditions. The best design from this manual parametric study produced a balanced air utilization between piston bowl and squish region resulting from the fuel spray targeting the upper lip of the bowl. The second phase of this study focused on the optimization of the injector and injection parameters including nNoz, SA, nozzle hole diameters (dNoz) and SOI using design of experiment (DoE) approach. 32 DoE cases were generated and best design was selected at each load point. It was found that the SA and SOI are the most influential injection parameters. Specifically, two optimum SOI regimes at part-load conditions have been identified. These are a) early injection resulting in retarded combustion and low pressure rise rate and b) late injection yielding high combustion efficiency and low hydrocarbon emission. At high-load condition, SOI right before top dead center (TDC) is most preferable and early injection should be avoided to minimize pressure rise rate. The narrow SA (90 degrees (deg) in this study) as well as wide SA were found to produce optimum performance. Subsequently, ML algorithm was used to further optimize the injector and injection parameters. As a result of this optimization study, 10.7% reduction of fuel consumption at part-load and 7.5% reduction at high-load were achieved.

Experimental and modeling study of the autoignition characteristics of gasoline/hydrogenated catalytic biodiesel blends over low-to-intermediate temperature

Gasoline/Hydrogenated catalytic biodiesel blending fuel is a prospective alternative fuel for gasoline compression ignition (GCI) engine which has the potential to solve the ignition difficulties of GCI especially at low load conditions. However, the autoignition behavior of gasoline/hydrogenated catalytic biodiesel blends is unknown. In this study, the ignition delay times of gasoline/hydrogenated catalytic biodiesel (HCB) blends were measured in a heated rapid compression machine over a wide range of conditions (the temperature range of 655–870 K, pressures of 10 and 15 bar, equivalence ratios of 0.5–1.5). Experimental results show that the measured ignition delay time of the gasoline/HCB blends exhibits negative-temperature coefficient behaviors under the investigated conditions. The total ignition delay times diminish with rising pressure, equivalence ratio and HCB ratio in the blends. The simulation results of ignition delay times for gasoline/HCB blends using a detailed mechanism are compared with experimental data. The detailed model can predict the trends of ignition delay time well during low-to-intermediate temperature region. Finally, kinetic analyses were performed under different compressed pressure, temperature, equivalence ratios and blending ratios. The dominating reactions and decisive factors of ignition are explored to gain a deeper understanding of the ignition characteristic of gasoline/HCB blends. It is found that the autoignition of fuel is completely caused by HCB related reactions at low temperature, which explains why the blending of HCB can expand the working condition downward from the perspective of chemical kinetic.

Analysis of ignition temperature range and surrogate fuel requirements for GCI engine

Gasoline compression ignition (GCI) technology aims to improve internal combustion engine (ICE) efficiency. However, much is still unknown about the detailed combustion behavior of fuels under this novel ICE technology, such as relevant ignition temperature regimes and surrogate requirements under various engine load and speed conditions as well as for different fuels. Engine and spray simulations require the use of carefully designed surrogate fuels that mimic the properties of real fuels. Generally, these surrogates range from simple primary reference fuels (PRFs) to complex multi-component mixtures. This study aims to identify the relevant temperature conditions and the surrogate requirements for GCI engines covering the entire engine speed-load map; engine loads between 1 and 22 bar indicated mean effective pressure (IMEP) and engine speeds between 800 and 2500 revolutions per minute (RPM). Simulations were performed using the constant volume zero-dimensional (0-D) and homogeneous charge compression ignition (HCCI) modules in Chemkin-Pro for two gasoline fuels with widely different sensitivities (11 and 0). Furthermore, homogeneous ignition delay times and Livengood-Wu integrals were used to analyze the fundamental characteristics of GCI combustion. It was found that, except at excessively high-load, the relevant ignition temperature in a GCI engine is in the 750–1000 K range. The low-temperature (less than700 K) reactivity exhibited negligible contribution to the ignition of the high-sensitivity fuel. Two stage ignition and cool flame formation were found to dominate mid-load kinetics of the low-sensitivity fuel. Simple PRF surrogate was found to be sufficient to mimic the ignition requirement of the low-sensitivity gasoline fuel at almost the entire GCI engine speed-load map. However, for the high-sensitivity gasoline fuel, TPRF and TPRFE surrogates were needed for a better representation of the auto-ignition behavior. The obtained knowledge of the surrogate requirement and the ignition temperature regimes inside GCI engine will aid in guiding GCI engine optimization and fuel design.

Model predictive combustion control of a Gasoline Compression Ignition engine

Gasoline Compression Ignition is a novel combustion concept that derives is superiority from the high compression ratio of a compression ignition engine as well as the properties of gasoline fuel, such as longer ignition delay and higher volatility compared to diesel. This combustion concept was experimentally tested on a 12.4L Class 8 truck engine which is equipped with unique features that include variable geometry turbine and variable valve actuation. Based on these experimental data, prior efforts by the authors focused on the development of an engine model for a heavy-duty engine operating on a low-reactivity fuel. This engine model was leveraged within this study to investigate a combustion control strategy at different engine conditions and injection methods and was augmented to incorporate cycle-to-cycle combustion variations. State estimation is performed by means of a Kalman filter which feeds into a model predictive controller. The model predictive controller chooses control actions based on a predefined cost function under consideration of bounds reflecting physical constraints. The engine model was utilized to establish a state–space model that serves the Kalman filter and model predictive controller for estimation and prediction. A comparative study investigating control actions and engine behavior was performed with and without limiting in-cylinder peak pressure as well as combustion noise, which is of particular interest for early pilot injection strategies. In addition, the proposed control architecture was investigated at two different levels of cycle-to-cycle variations and compared to the performance of a control structure with input disturbance rejection For increased cycle-to-cycle variations, disturbance estimation reduces state fluctuations and control effort. In general, this investigation highlights control aspects specific to a compression-ignited combustion regime with low-reactivity fuel, which heavily relies on the impact of the variable geometry turbine and variable valve actuation on states and constraints. The control algorithm is able to maintain the desired references for brake mean effective pressure and combustion phasing while controlling peak in-cylinder pressure and combustion noise.

Collaborative optimization of fuel composition and operating parameters of gasoline compression ignition (GCI) engine in a wide load range

Through the integration of three-dimensional computational fluid dynamics (CFD) simulation and genetic algorithm, the operating parameters and fuel composition of a gasoline compression ignition (GCI) engine were simultaneously explored in the full load range. The blends of n-heptane, iso-octane, and toluene were utilized as the substitute for real gasoline fuels, in which toluene can efficiently diminish the heat release rate so that excessively high ringing intensity (RI) can be avoided at high load by extending combustion duration. The synergistic effects of fuel properties and operating parameters on combustion and emissions were analyzed. In addition, the fuel composition requirements of GCI combustion were further discussed. The optimization results indicate that the collaborative optimization can achieve low nitrogen oxides (NOx) emissions, near-zero soot emissions, and satisfactory fuel consumption under the GCI strategy. The combustion procedure was dominated by initial temperature and pressure at low load, and the effect of fuel characteristics is strengthened with higher load. The optimal combustion phasing of the GCI engine is postponed with the increase of load, which can enhance the temperature and fuel stratifications in the cylinder to prevent unacceptable RI. To minimize NOx emissions, a high proportion of n-heptane is preferred at low load and the contents of iso-octane and toluene approach to zero. Fuel research octane number (RON) has an inconspicuous impact on GCI at low load, and the fuel with higher RON can perform well at low load, except for the slightly higher NOx and soot emissions for high RON fuels. With the increase of load, the proportion of n-heptane decreases, which is compensated by the higher proportion of iso-octane. At high load, a certain amount of toluene is required, and high soot emissions are the main factor to limit the adoption of low RON fuels. Overall, GCI engine can utilize the fuel with RON between 70 and 80 in a wide load range.

A Method and System for Combining the Advantages of Gasoline Compression Ignition (GCI) Engine Technologies into Hybrid Electric Vehicles (HEVs)

By combining a clean fuel such as gasoline with a high efficiency thermodynamic cycle (compression ignition), it is possible to demonstrate a powertrain that is clean and efficient, thus breaking the historical trade-off between decreasing CO2 and reducing criteria pollutants. The gasoline compression ignition (GCI) engine is a promising technology that can be used to improve thermal efficiency while reducing emissions. Its low temperature combustion does however lead to several problems that need to be overcome. The present study relates to a method and system for combining the advantages of GCI engine technology into a hybrid electric vehicle (HEV) to maximize the benefits. A plausible path is to operate the GCI engine at conditions where the benefits of a GCI engine could be maximized and where an electric motor can supplement the conditions where the GCI is less beneficial. In this study, GCI engines with different cetane number (CN) fuels were selected, and a hybrid simulation tool was used to address the potential of the GCI engines into hybrid electric vehicles. Co-developments can demonstrate efficiency and emission solutions through the achievements of the study, which will address examples of the competitive powertrain and will introduce more than 30% of CO2 reduction vehicle by 2030.

Numerical study of spray combustion and soot emission of gasoline–biodiesel fuel under gasoline compression ignition-relevant conditions

A blend of biodiesel and gasoline fuel has been proposed for powering gasoline compression ignition (GCI) engines to improve overall reactivity, which is essential for the auto-ignition quality of GCI engines. Previous studies emphasized the differences in the physicochemical properties of hydrogenated catalytic biodiesel (HCB) and gasoline, which significantly altered the combustion characteristics. This study is a further step in this direction to gain a deeper understanding on the spray combustion and emission behavior of this promising fuel based on computational fluid dynamic simulation in a constant volume chamber. N-Hexadecane and toluene reference fuel were selected as surrogates for HCB and gasoline fuel, respectively. A reduced kinetic mechanism for HCB-gasoline surrogate was developed and combined with a reduced PAH mechanism to simulate HCB-gasoline spray combustion and soot formation. Liquid penetration length, ignition delay, lift-off length, and soot formation for HCB and HCB/gasoline surrogates were validated against available experimental data in the literature. The proposed mechanism along with a multi-step phenomenological soot model could capture the spray characteristics and soot formation of the tested fuels with acceptable accuracy under GCI engine combustion conditions. The numerical results showed larger spray/flame development for HCB fuel as compared with that of HCB/gasoline blend prior to ignition and at ignition stage, which was mainly due to the less fuel spray mixing with the surrounding fresh air. Furthermore, the outer diffusion flames for HCB and HCB/gasoline blend were identical and existed on the stoichiometric region (at ϕ ∼ 1), while the rich pre-mixed core of HCB was located at higher equivalence ratio, which was more favorable for precursors formation. HCB/gasoline fuel had a lower soot mass yield compared with HCB because of its lower C2H2 concentration, which slowed down the surface growth step. In addition, the effects of ambient oxygen level and ambient temperature on flame structure and soot formation were also investigated. The current study provides a useful information about spray combustion and soot formation of HCB–gasoline blend, which has practical applications in GCI engines.

Effect of swirl ratio on charge convection, temperature stratification, and combustion in gasoline compression ignition engine

Various low-temperature combustion strategies (namely, homogeneous charge compression ignition, reactivity controlled compression ignition, and partially premixed charge compression ignition) have shown the potential to comply with upcoming and prevailing stringent emissions legislations. Low octane gasoline has emerged as an ideal fuel candidate for premixed charge combustion under diesel-like conditions in gasoline compression ignition (GCI) engines. GCI is an excellent technology to rectify future global energy demand imbalance, because it aims to replace diesel (which is in short supply) with low octane fractions/naphtha (which is in surplus supply) in compression ignition engines. However, this novel combustion concept requires modifications in the conventional design of diesel engines. The combustion chamber shape and in-cylinder flows play a crucial role in charge distribution and temperature stratification. Therefore, understanding the combined effect of combustion chamber geometry and in-cylinder flows is essential for future engine designs. GCI combustion engine simulations for varying swirl ratios (SRs) were performed in CONVERGE CFD software to understand the effect of in-cylinder air motion on the mixture stratification and combustion. A 1/7th sector geometry for a conventional re-entry piston bowl was modeled and then simulated. Two different mechanisms were used for model validation. The results indicated that the large-scale flow structures govern the fuel distribution in the combustion chamber. The charge convection because of increased swirl has a substantial effect on the combustion characteristics of the engine. A distinguished ignition kernel was observed for all test cases. An interfacial region with counter-rotating vortices formed a lean mixture zone, hindering flame propagation and combustion. A lower SR, shallow depth piston, and modifications to avoid flame quenching in the squish zone need to be further investigated to optimize the engine performance.

Experimental Study on Combustion and Emission Characteristics of Gasoline Compression Ignition Engines Under Cooperative Control of Operating Parameters

Abstract This study investigated the effects of cooperative-control of the start of injection (SOI), excess air ratio (λ), internal exhaust gas recirculation (I-EGR), and intake air temperature (IAT) on the combustion and emission characteristics of gasoline compression ignition (GCI) engines, especially regards to the combustion stability and knock characteristics. And optimizing the GCI engine combustion and emissions through the cooperative control of multiple parameters is the innovation of this research. The results showed that advancing the SOI and increasing the I-EGR ratio can significantly expand the low-load limit, but the heating effect of 20% I-EGR only worked when the SOI was earlier. An appropriate increase of λ could increase the maximum brake thermal efficiency (BTE) to 40.06%, but resulted in high-knock probability and high NOx emissions. Rising the IAT was more effective than advancing the SOI in improving combustion fluctuations, but the knock probability and knock intensity were more sensitive to the early SOI. When the SOI varied from 26 °CA BTDC to 30 °CA BTDC, λ was 1–1.5, the I-EGR ratio was 5%–20%, and the IAT was 40–50 °C; the GCI engine can obtain the balance among high thermal efficiency, high combustion stability, low knock probability, and low emissions.

Effects of intake conditions and octane sensitivity on GCI combustion at early injection timings

Gasoline compression ignition (GCI) engines favor low-octane fuels (RON = 60–80). However, low-octane fuels possessing low-temperature reactions strongly interact with in-cylinder thermodynamic conditions, which in turn significantly affect GCI combustion characteristics. In this work, using a toluene primary reference fuel (TPRF) with RON = 75, the effects of intake pressure and temperature on combustion evolutions, low-temperature heat release (LTHR), high-temperature heat release (HTHR), and indicated thermal efficiency were numerically investigated in a single-cylinder GCI engine, allowing for the role of octane sensitivity. An early injection timing was considered to obtain relatively homogeneous mixtures and large variations in intake pressure were employed to simulate low to medium loads. The results show that for a given equivalence ratio, the peak of the LTHR and HTHR and the main combustion phasing positively correlates with intake pressure. However, an opposite tendency in the peak of the HTHR is observed for a fixed amount of fuel injection. When octane sensitivity is elevated, the peak of in-cylinder pressure and heat release rate is reduced and ignition timing is retarded, especially for the HTHR. Elevating intake temperature facilitates high-temperature combustion while suppresses low-temperature reactions, which results in nonlinear variations in combustion phasing at a certain range of intake temperatures. The correlations of operating conditions and combustion evolutions are evaluated by pressure-temperature trajectory and Livengood-Wu integration, which indicates the significance of intake pressure in GCI combustion. Finally, combustion evolutions show that low-temperature reactions first take place at fuel-lean regions and hot flame pockets develop in fuel-rich regions, which suggests that low-temperature ignition is more sensitive to ambient temperatures while high-temperature ignition is more dependent on equivalence ratio.

A Computational Investigation of PPCI-Diffusion Combustion Strategy at Full Load in a Light-Duty GCI Engine

<div class="section abstract"><div class="htmlview paragraph">A two-stage PPCI-diffusion combustion process recently showed good potential to enable clean and fuel-efficient gasoline compression ignition (GCI) combustion at medium-to-high loads. By conducting closed-cycle 3-D CFD combustion analysis, a further step was undertaken in this work to evaluate and optimize the PPCI-diffusion combustion strategy at a full load operating point (2000rpm-23.5 bar IMEPcc) while keeping engine-out NOx below 1 g/kWh.</div><div class="htmlview paragraph">The light-duty GCI engine used in this investigation featured a custom-designed piston bowl geometry at a 17.0 compression ratio (CR), a high pressure diesel fuel injection system, and advanced single-stage turbocharging. A split fuel injection strategy was used to enable the two-stage PPCI-diffusion combustion process.</div><div class="htmlview paragraph">First, the injector spray pattern and swirl ratio effects were evaluated. In-cylinder air utilization and the PPCI-diffusion combustion process were notably influenced by the closed-cycle combustion system design. Among the different spray patterns at a swirl ratio of 0, the one with 120° spray inclusion angle, 8-hole, and 1.5 times total nozzle area (TNA) was favored due to enhanced late-stage fuel-air mixing and more rapid diffusion combustion. In the second step, a fuel injection strategy optimization campaign was performed through a space-filling Design of Experiments (DoE) approach. Overall, the optimized injector spray pattern and the optimized fuel injection strategy together were predicted to produce 5.1% lower ISFC and 50% soot reduction over the baseline. A competitive analysis showed the optimized PPCI-diffusion combustion strategy had the potential to generate substantially lower NOx and soot than a modern light-duty diesel engine at full load.</div></div>

Tailored Air-Handling System Development for Gasoline Compression Ignition in a Heavy-Duty Diesel Engine

The simultaneous application of new low-NOx emissions standards and greenhouse gas (GHG) rules has placed great pressure on the commercial vehicle industry and has driven demand for innovative solutions. One potential solution, gasoline compression ignition (GCI), utilizes gasoline’s lower reactivity to promote partially premixed combustion and extract efficiency while reducing the PM-NOx trade-off curve. Gasoline’s volatility allows for the use of higher levels of exhaust gas recirculation (EGR), a key enabler of GCI combustion. In order to deliver higher levels of EGR while maintaining sufficient boost pressure, a tailored and efficient air-handling system is critical. This work presents the analysis-led development of a low-NOx GCI air-handling system including both turbocharger matching and EGR configuration for a prototype heavy-duty GCI engine based on a model year 2013 Cummins ISX diesel engine using low octane gasoline (RON80). In the analysis-driven development process, a 1D engine system-level analysis was closely coupled with closed-cycle 3D CFD GCI combustion development. Three different boost systems were investigated using a validated 1D engine model: 1) the production turbocharger; 2) an off-the-shelf single-stage waste-gate turbocharger; 3) a prototype single-stage variable geometry turbocharger. For each boost system, three EGR configurations were evaluated: 1) a high-pressure EGR route; 2) a low-pressure EGR route; 3) a dual-loop EGR route. The air-handling system performance was first investigated over five steady-state engine operating conditions extracted from the ramped modal cycle supplemental emissions test. Then, through cosimulation using a Simulink-based engine controls model, the best performing candidates under transient operation through the Heavy-Duty Federal Test Procedure certification cycle were identified. The production turbocharger, designed for 4–6 g/kWh engine-out NOx, suffered from low combined turbocharger efficiency under the low-NOx GCI thermal boundary conditions. The prototype 1-Stage variable geometry turbocharger, when used with a high-pressure EGR configuration, demonstrated higher combined efficiencies, while the waste-gate turbocharger showed the best results when used with a dual-loop EGR system. All low-pressure only EGR configurations were found to incur additional pumping penalties due to the need for a back pressure valve to drive sufficient EGR levels. In the transient test cycle analysis, the single-stage high-pressure EGR system was capable of delivering the target boost and EGR, while the off-the-shelf waste-gate turbocharger, with its higher mass inertia, showed slower turbine response and a resulting lag in boost response. Unsurprisingly, the dual-loop EGR system also suffered from delays in EGR delivery during engine acceleration. In summary, the prototype single-stage variable geometry turbocharger with a high-pressure EGR system produced the best performance over both the steady-state and transient engine cycles and was identified as the best candidate for the prototype low-NOx heavy-duty GCI engine.