Comparison of Stratified and Homogeneous Combustion in a Direct-Injected Two-Stroke Engine for Snowmobile Applications

Gasoline direct injection (GDI) two-stroke engine technology has been developed for use in snowmobile applications. Applying GDI to a two-stroke engine significantly reduces emissions of unburned hydrocarbons and improves fuel economy by reducing or eliminating the short circuiting of fuel that occurs in conventional carbureted two-stroke engines. The GDI design allows for two different modes of combustion, stratified and homogeneous. Stratified combustion is typically used during idle and light to moderate loads at low engine speeds while homogeneous combustion is used at moderate to high loads and medium to high engine speeds. This work presents the process and results of determining which mode of combustion provides better fuel economy during cruise point operation, and where the transition from stratified to homogeneous combustion should occur in snowmobile operation. Results show that homogeneous calibration is the superior mode of combustion for the cruise points of a snowmobile.

3D CFD Simulations of Hydrogen Fuelled Spark Ignition Engine

By supporting hydrogen as an alternative fuel to the conventional fuel i.e. gasoline, new era of renewable and carbon neutral energy resources can be introduced. Hence, development of hydrogen fuelled internal combustion engine for improved power density and less emission of NOx has become today’s need and researchers are continuously extending their efforts in the improvement of hydrogen fuelled internal combustion engine. In this work, three dimensional CFD simulations were performed using CFD code (AVL FIRE) for premixed combustion of hydrogen. The simplified 3D geometry of engine with single valve i.e. inlet valve was considered for the simulation. Various combustion models for spark ignition for hydrogen i.e. Eddy Breakup model, Turbulent Flame Speed Closure Combustion Model, Coherent Flame model, Probability Density Function model were tested and validated with available simulation results. Results obtained in simulation indicate that the properties of hydrogen i.e. high flame speed, wide flammability limit, and high ignition temperature are among the main influencing factors for hydrogen combustion being different than that of gasoline. Different parameters i.e. spark advance angle (TDC to 40° before TDC in the step of 5°), rotational speed (1200 to 3000 rpm in the step of 300 rpm), equivalence ratio (0.5 to 1.2 in the step of 0.1), and compression ratio (8, 9 and 10) were used to simulate the combustion of hydrogen in spark ignition engine and to investigate their effects on the engine performance, which is in terms of pressure distribution, temperature distribution, species mass fraction, reaction progress variable and rate of heat release for complete cycle. The results of power output for hydrogen were also compared with that of gasoline. It has been observed that power output for hydrogen is almost 12–15% less than that of gasoline.

Numerical Simulation of Convective Heat Transfer in a Spark Ignition Engine

A computational fluid dynamics code is applied to simulate fluid flow and combustion in a four-stroke single cylinder engine with flat combustion chamber geometry. Heat flux and heat transfer coefficient on the cylinder head, cylinder wall, piston, intake and exhaust valves are determined. Result for a certain condition is compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurement in the literature and close agreement is observed. It is observed that the value of heat flux and heat transfer coefficient varies considerably in different positions of the combustion chamber, but the trend with crank angle is almost the same.

Modeling Soot Formation Using Reduced PAH Chemistry in n-Heptane Lifted Flames With Application to Low Temperature Combustion

A numerical study of in-cylinder soot formation and oxidation processes in n-heptane lifted flames using various soot inception species has been conducted. In a recent study by the authors, it was found that the soot formation and growth regions in lifted flames were not adequately represented by using acetylene alone as the soot inception species. Comparisons with a conceptual model and available experimental data suggested that the location of soot formation regions could be better represented if polycyclic aromatic hydrocarbon (PAH) species were considered as alternatives to acetylene for soot formation processes. Since the local temperatures are much lower under low temperature combustion (LTC) conditions, it is believed that significant soot mass contribution can be attributed to PAH rather than to acetylene. To quantify and validate the above observations, a reduced n-heptane chemistry mechanism has been extended to include PAH species up to four fused aromatic rings (pyrene). The resulting chemistry mechanism was integrated into the multidimensional CFD code KIVA-CHEMKIN for modeling soot formation in lifted flames in a constant volume chamber. The investigation revealed that a simpler model that only considers up to phenanthrene (three fused rings) as the soot inception species has good possibilities for better soot location predictions. The present work highlights and illustrates the various research challenges toward accurate qualitative and quantitative predictions of soot for new low emission combustion strategies for I.C. engines.

Application of a Phenomenological Soot Model for Diesel Engine Combustion

The application of the improved CFD code for the simulation of combustion and emission formation in a high-speed diesel engine has been presented and discussed. The soot concentration transport equation is found and solved together with all other flow equations. A slip correction factor is introduced into this equation. In turbulent combustion, the soot particles are contained within the turbulent eddies, and burnt up swiftly with the dissipation of these eddies in the soot oxidation zone. However, the chemical reactions always process except the dissipation of turbulent eddies and the intermixing of soot particles and turbulent eddies. The soot oxidation rate should be controlled simultaneity by the chemical reactions rate and the dissipation rate of turbulent eddies. A hybrid particle turbulent transport controlled rate and soot oxidation rate model is present in this paper and Soot formation and oxidation processes have been modeled according to this model. A reasonable agreement of the measured and computed data of in-cylinder pressure, soot, and NO emissions for different engine operation conditions has been made. The precision of simulated soot concentration is improved compare with the commonly Hiroyasu—Nagel—Strickland (HNS) soot model.

Dual-Stage Turbocharger Matching and Boost Control Options

Diesel engines are gaining in popularity, penetrating even the luxury and sports vehicle segments that have traditionally been strongly favored gasoline engines as the performance and refinement of diesel engines have improved significantly in recent years. The introduction of sophisticated technologies such as common rail injection (CRI), advanced boosting systems such as variable geometry and multi-stage turbocharging, and exhaust gas after-treatment systems have renewed the interest in Diesel engines. Among the technical advancements of diesel engines, the multi-stage turbocharging is the key to achieve such high power density that is suitable for the luxury and sports vehicle applications. Single-stage turbocharging is limited to roughly 2.5 bar of boost pressure. In order to raise the boost pressure up to levels of 4 bar or so, another turbocharger must be connected in series further multiplying the pressure ratio. The dual-stage turbocharging, however, adds system complexity, and the matching of two turbochargers becomes very costly if it is to be done experimentally. This study presents a simulation-based methodology for dual-stage turbocharger matching through an iterative procedure predicting optimal configurations of compressors and turbines. A physics-based zero-dimensional Diesel engine system simulation with a dual-stage turbocharger is implemented in SIMULINK environment, allowing easy evaluation of different configurations and subsequent analysis of engine system performance. The simulation program is augmented with a turbocharger matching program and a turbomachinery scaling routine. The configurations considered in the study include a dual-stage turbocharging system with a bypass valve added to the high pressure turbine, and a system with a wastegate valve added to a low-pressure turbine. The systematic simulation study allows detailed analysis of the impact of each of the configurations on matching, boost characteristics and transient response. The configuration with the bypass valve across high pressure turbine showed better results in terms of both steady state engine torque and transient behavior.

EGR Oxidation and Catalytic Fuel Reforming for Diesel Engines

Exhaust gas recirculation (EGR) treatment techniques that include combustible substance oxidation, catalytic fuel reforming, and partial bypass-flow control have been experimentally investigated on a single cylinder diesel engine. Application tests are conducted to investigate the effects of the reformed gases on the diesel combustion characteristics and exhaust emissions. This research is aimed at stabilizing and expanding the limits of heavy EGR during steady and transient operations by enhancing the premixed combustion that may significantly alleviate problems with soot formation and cyclic variations. Additionally, the heavy treated EGR is applied to enable in-cylinder low temperature combustion. A preliminary investigation on the effects of water addition to the high temperature catalyst bed is also conducted. The potential of EGR reforming is also examined for possible generation of synthetic EGR (CO2) at low engine loads. The effectiveness of the treated EGR on engine emission and operating characteristics are therefore reported.

Blow-By Vapour Impact on Flexible Fuel Applications

The flexible fuel vehicles (FFV) are definitely a reality in Brazil. Most of the cars currently produced in Brazil are flexible fuel with fuel range from gasohol (E22) to hydrated ethanol (E100). All of these Brazilian applications are fuel sensor less. It means that the fuel inference is based on lambda sensor only. This fuel inference approach takes the intake air mass over the calculated mass of fuel delivered through the injectors in account to calculate the air fuel ratio (AFR) which points out the fuel blend in tank. One of the disadvantages of this inference strategy is that all the fuel considered for AFR calculation must come from injectors. Considering that the cold start and warm-up phases should operate in a richer mixture (choke operation), part of this fuel is delivered to the lubricant which is know as fuel dilution phenomenon in lubricant. According to the fuel properties and the lubricant temperature the diluted fuel evaporates and is mixed in intake air through blow-by system. This extra fuel source is considered in AFR calculation and can, in some conditions, distort the fuel blend inference. In order to prevent this blow-by vapour negative impact on flexible fuel inference logic a mathematical model was developed and calibrated for a 1.6 liter engine FFV application. Many test results with different blends and conditions are discussed. This study confirmed that this mathematical model presented positive results and some improvement opportunities were commented as well.

Low Temperature Combustion Using Nitrogen Enrichment to Mitigate NOx From Large Bore Gas-Fueled Engines

Low Temperature Combustion (LTC) is identified as one of the pathways to meet the mandatory ultra low NOx emissions levels set by regulatory agencies. This phenomenon can be realized by utilizing various advanced combustion control strategies. The present work discusses nitrogen enrichment using an Air Separation Membrane (ASM) as a better alternative to the mature Exhaust Gas Re-circulation (EGR) technique currently in use. A 70% NOx reduction was realized with a moderate 2% nitrogen enrichment while maintaining power density and simultaneously improving Fuel Conversion Efficiency (FCE). The maximum acceptable Nitrogen Enriched Air (NEA) in a single cylinder spark ignited natural gas engine was investigated in this paper. Any enrichment beyond this level degraded engine performance both in terms of power density and FCE, and unburned hydrocarbon (UHC) emissions. The effect of ignition timing was also studied with and without N2 enrichment. Finally, lean burn versus stoichiometric operation utilizing NEA was compared. Analysis showed that lean burn operation along with NEA is one of the effective pathways for realizing better FCE and lower NOx emissions.

Inline Pump Internal Flow Characterization for Optimized Diesel Injection

The injection process optimization plays a key role in diesel engine development activities, both for pollutant formation control and performance improvement. The present paper focuses on relatively small diesel units, equipped with fully mechanical injection systems; in detail, the considered system layout is based on the use of spring injectors; the amount of delivered fuel is controlled by the positioning of the pump plunger groove. The paper highlights the role of the inline pump and the influence of fuel characteristics on the system operation. By means of a three-dimensional numerical flow study, the behavior of pump fuel passages and delivery valve is simulated. Then, on the basis of the system features, a complete lumped/one-dimensional numerical model is realized, in which the discharge coefficients evaluated through the three-dimensional simulation are employed. Fuel injection rate and local pressure time histories are investigated, paying specific attention to the occurrence of the relevant phenomena in the system components. Obtained results are compared with experimental data.