Two-stroke engines are often used for their low cost, simplicity, and power density. However, these engines suffer efficiency penalties due to fuel short-circuiting. Increasing power density has previously been an area of focus for performance two-stroke engines — such as in dirt bikes. Smaller-displacement engines have also been used to power remote controlled cars, boats, and aircraft. These engines typically rely on gasoline or higher-octane liquid fuels. However, natural gas is an inherently knock-resistant fuel and small natural gas engines and generators could see increased market penetration. Power generators typically operate at a fixed frequency with varied load, which can take advantage of intake and exhaust system tuning. In addition, stationary engines may not be subject to size restrictions of optimal intake and exhaust systems. This paper examines methods to improve combustion stability, efficiency, and power density of a 29cc air-cooled two-stroke engine converted to operate on natural gas. Initial conversion showed significant penalties on delivery ratio, which lowered power density and efficiency. To overcome these issues a tuned intake pipe, two exhaust resonators, and a combustion dome were designed and tested. The engine was operated at 5400 RPM and fueling was adjusted to yield maximum brake-torque (MBT). All tests were conducted under wide-open throttle conditions. The intake and exhaust systems were designed based on Helmholtz resonance theory and empirical data. The engine utilized a two-piece cylinder head with removable combustion dome. The combustion dome was modified for optimal compression ratio while decreasing squish area and volume. With all designs incorporated, power increased from 0.22 kW to 1.07 kW — a factor of 4.86. Efficiency also increased from 7% to 12%. In addition to these performance gains, the coefficient of variation (COV) of indicated mean effective pressure (IMEP) decreased from just above 11% to less than 4%.

This paper is one of the first publications documenting the performance optimization of a port fuel injected, spark ignited, internal combustion engine for an unmanned aerial vehicle hybrid powertrain. An optimized hybrid powertrain enables long-duration quadcopter flight and provides vertical takeoff and landing capability for fixed-wing aircraft. This technology can improve flight times and lessen runway requirements, helping to solve two of the biggest problems faced by the booming UAV industry. The main activities contributing to this hybrid powertrain effort are the development of a small engine test facility, design of powertrain vehicle integration hardware, and the testing and characterization of both electric motor-generators and fuel-injected two-stroke engines. This paper investigates engine testing completed to date including experimental setup, testing procedures, data post processing, and results.

Two-stroke engines are capable of providing very high power density levels in a cost effective, easy-to-maintain package. They are, however, typically susceptible to higher levels of hydrocarbon emissions, lower durability, and a shorter lifecycle when compared to four-stroke engines. These detriments are easily overlooked in some military applications where power density is paramount, but most commercial two-stroke engines require specialized consumable lubricant. Typical military applications strive to minimize their logistics “trails,” which includes minimizing the variety of fluids they require. As a result, there has been very limited success in fielding small two-stroke engines for military use. As a preliminary study, MIL-PRF-2104K Single Common Powertrain Lubricant (SCPL, a four-stroke heavy diesel engine oil) was utilized as the consumable lubricant (in place of conventional two-stroke oil) in a liquid-cooled, semi-direct fuel injected, spark-ignition, two-stroke engine. Empirical data was collected to study the impact of the oil on deposit build-up, power, wear, combustion stability, and fuel conversion efficiency. Over 147 hours of operation were logged and analyzed. The performance of the engine on SCPL was consistent with conventional two-stroke oil and showed no degradation over the test duration. Brake specific fuel consumption was not negatively impacted with SCPL. Increased deposit build-up in the exhaust ports and on the spark plugs were the primary negative impacts of the SCPL oil. Spark plugs with hotter classifications and modification of the oiling rate resulted in a reduction of soot accumulation and spark plug fouling.

Recent in Two-stroke engine development for marine applications mainly deal with better and increased overall efficiency hereby reducing the CO2 emissions. Besides a mechanical design change the service conditions play a major role in enhanced efficiency. The so increased engine parameters with higher maximum pressure during combustion will inevitable increase the load on the bearings of the reciprocating parts. Since an increase in the bearing area is not possible due to the engine design parameters the object is to increase the specific load bearing capability of the bearing alloy while keeping the tribological benevolence. In particular, the fatigue properties and the strength of the used tin based alloys, commonly known as whitemetals or babbitts. This class of alloys stands out due to its emergency running capabilities, embedability and its high flexibility under edge load. Existing Al-based alloys like AlSn40 have improved fatigue properties but they fall behind on the named essential properties. Also, the dimensions of the AlSn40 bearings are limited due to the roll bonding process by which they are produced. On the other hand spin casting as standard production process of Babbitt bearings is limiting the alloying elements due to centrifugal segregation while solidification of the lining alloy. While tin based alloys are used in an environment of 90°C the homologue temperature is 0,7 which means that classic strengthening mechanisms like work-hardening and grain size effects are only shortly employable. Another restricting fact is the requirement of solid solubility for solid solution strengthening which also includes precipitation hardening. Hence there is a limited amount of elements dissolvable and not environmentally hazardous in tin this mechanism is already used to its maximum in the standard babbitts. In this paper a possible way to circumvent these limits will be presented. The use of high melting elements, compared to Sn, like Co, Ni, Mn, Al and Zn which are partly dissolvable in molten Sn, where used to improve the microstructure and therefor the overall performance of the bearing alloy. These trace elements serve as grain refinement for the primary precipitation of SbSn and CuSn during solidification. The high melting point of these elements anticipates relaxing processes in the alloy caused by diffusion at the high homologue temperature. Due to the smaller precipitations and the finer structure a better performance can be seen during tests. This leads in a higher strength while maintaining its ductility. Alternate bending tests as well as specific bearing test runs show a significant better fatigue, wear and embedability performance than standard alloys.

The objective of this research was to improve the scavenging process on a heavy-duty, two-cycle, medium speed, diesel engine (710 cubic inches of displacement per cylinder) under high load. It was desired to expel as much of the residual gases in the combustion chamber during the scavenging process, without having the fresh intake air escaping through the exhaust valves. This allowed a cooler intake charge with a higher oxygen concentration to be used. Having a cooler air intake charge was beneficial for emissions and performance. The power assembly configuration for the investigation was a two-cycle engine with intake ports around the bottom portion of the cylinder liner and four exhaust valves in the cylinder head. The main variable in this study was the axial location of every other intake port on the cylinder liner. When the axial location increased, this created pseudo longer intake timing. To keep the amount of trapped mass constant, the exhaust valves remained opened longer. The effectiveness of the scavenging process was estimated by the concentration of trapped oxygen (O 2 ), carbon dioxide (CO 2 ), and water vapor (H 2 O) along with the average temperature of the intake charge at the end of scavenging. The final part of the research explored each modified cylinder liner under full load, combustion operating conditions, while varying the injection timings. Soot, nitrogen oxides (NO x ) and fuel consumption were also examined. A proprietary, multi-dimensional, computational fluid dynamic (CFD) code was used for the modeling work. Sculptor™, a commercial software package, was used to morph the cylinder liner. The results showed that by increasing the axial location of the intake ports, a cooler intake charge with higher oxygen and lower carbon dioxide and water vapor concentrations resulted. This suggested more exhaust gases were expelled from the combustion chamber and replaced with fresh air. The rate of improvement slowed down after increasing the intake port axial location by 15.0 mm. In the next step, the compression and combustion process of the different port locations was studied. The cases with higher intake port axial locations were found to have lower pressure throughout the compression and power cycles. The temperature was lower for cases with elevated intake ports, until combustion started. The final part of the research studied the modified intake port cases under combustion simulation, while varying the injection timing. Next, the results showed an improvement in soot and fuel consumption with an increase in NO x for a given injection timing. However, while injection timing was varied, it was possible to improve NO x , soot, and fuel consumption simultaneously when compared to the baseline.

A limited amount of information exists on the effect of higher ethanol content fuel (greater than 10 vol%) for recreational vehicle engines. The possibility exists for misfueling of these vehicles, as ethanol content may increase at gas stations in the near future. Engine management systems in the recreational vehicle market are typically not equipped with feedback controls to adapt to the increased ethanol content. To address this concern and generate preliminary data related directly to the recreational industry, a study was conducted to evaluate the impact of E22 fuel on steady-state emissions and performance of two production snowmobiles. To fully analyze the impact of higher ethanol blends, cold-start, durability, and material compatibility tests should be performed, in conjunction with emissions and performance tests. While these additional tests were not performed as part of this study, there is a test program that is assessing all these factors on E15 fuel, which will be released in fall 2012. E0 fuel was used to establish baseline performance and emissions data. A 2009 four-stroke snowmobile with a 998cc, liquid-cooled, four-cylinder, intake port-fuel injected engine and a 2009 two-stroke snowmobile with a 599cc, liquid-cooled, two-cylinder, electronically controlled, crankcase-fuel injected engine were used for this study. Neither vehicle had any feedback air-fuel controls or after-treatment devices in the exhaust system. Power, fuel consumption, relevant engine temperatures, as well as, regulated exhaust emissions were recorded using the EPA 5-mode certification test cycle. The data showed no major impact on power output for either the four-stroke or two-stroke snowmobile. Brake specific fuel consumption varied with E22 as compared to E0. A reduction in CO emissions for both vehicles was observed for the E22 fuel. Both vehicles were factory calibrated rich of stoichiometric and hence, the addition of ethanol to the fuel effectively leaned out the air/fuel ratio and thus reduced the CO emissions. HC emissions were reduced for both the four-stroke and two-stroke engines, though certain test points of the two-stroke engine produced HC emissions that exceeded the analyzer measurement range (idle). Leaner operation reduced HC formation. Exhaust gas temperatures were observed to increase from 20°C – 50°C with E22 fuel, due to enleanment.

Generally, two-stroke engines have inherent advantage of higher swirl, as compared to four-stroke engines. Higher swirl helps with better mixing and atomization for mechanical and low pressure, electronic, unit injectors. With the introduction of higher injection pressure, the advantage of swirl reduced to a point in which it started to have a negative impact. In this work, the goal was to optimize a high pressure, common rail injection system for heavy-duty, two-cycle, medium speed, diesel engine (710 cubic inches of displacement per cylinder) under high load. A proprietary multi-dimensional computational fluid dynamic (CFD) code was used for the modeling work. Here, we optimized the new, skewed injection system, which takes advantage of swirl and further helps in atomization of spray. The spray in this study is introduced at a radial angle, which is aligned either along or against the swirl direction. The results showed significant improvement in combustion efficiency. Combustion efficiency is estimated as a decrease in fuel consumption and CO values. Emission parameters, such as nitrogen oxide and soot were also studied and showed significant reductions.

The objective of this research was to optimize an Electro-Motive Diesel (EMD) large-bore, two-cycle diesel engine (710 cubic inches of displacement per cylinder) at high load to minimize soot, nitrogen oxide (NOx) and fuel consumption. The variables considered were the number of spray-hole nozzles per injector, including spray angle and piston bowl geometry, for a range of injection pressures. Analytical simulations were conducted for a calibrated EMD 710 Tier 2 engine and a few of the top-performing cases were studied in detail. CONVERGE™, a commercially available, advanced combustion simulation software was used in this analysis. A surface deforming tool, Sculptor®, was used to obtain various piston bowl geometries. MiniTab® was utilized for statistical analysis. Results show that optimal combinations of injection variables and piston bowl shape exist to simultaneously reduce emissions and fuel consumption compared to Tier 2 EMD 710 engines. These configurations will be further tested in a single-cylinder test cell and presented later. This investigation shows the importance of bowl geometry and spray targeting on emissions and fuel consumption for large-bore, two-stroke engines with high power density.

When looking at the performance criteria of bearings for the application in Two-Stroke engines, properties like emergency running capabilities, embedability and the fatigue properties are vital to the performance of these engines. The typical approach is to use materials with a soft Tin matrix and hard intermetallic phases commonly known as “Babbitt” alloys. In a second, more recent approach, Aluminium alloys with elevated Tin content are more and more often chosen. Babbitts outmatch any bearing alloys by their outstanding tribological performance but have very limited mechanical properties. Oppositely, the Aluminium alloys have substantial higher strength but leak somehow with regard to emergency running properties. Whereas the poorer running properties of Aluminium Alloys can be overcome by using suitable running-in coatings, the strength increase of the Tin matrices of Babbitts is rigorously limited by the production technology of spin casting and the ban of hazardous alloying elements such as Cadmium. In order to satisfy the needs of engines manufacturers for a material which combines the advantages of Aluminium- and Tin-base alloys, a new approach which combines both metallurgical alloy development and process technology redesign is necessary. By a fundamental process analysis, the limiting effect with regard to alloy tuning of the existing production technology for steel-babbitt bearing shells will be shown in this paper. Further more, new process routings with the effect of enabling the production of Babbitt bearings with never practized alloy compositions with enhanced mechanical properties are presented.

This paper presents an investigation into the NO X reduction role played by the exhaust manifold of large-bore two stroke cycle engines by exploring the impact of the exhaust manifold design on turbocharger and engine operation. Exhaust manifold performance is defined as the ability of the exhaust manifold to: 1) optimize cylinder scavenging efficiency; and 2) minimize the pressure differential between the compressor discharge and the turbine inlet by exploiting the blow-down pressure pulses and minimizing the static pressure gradient along the exhaust manifold. Pressure pulses in the exhaust manifold have been identified as a plausible mechanism that hinders efficient cylinder scavenging and turbocharger operating range. While modifying the ports and manifold may not be cost effective, a complete understanding of and the ability to address the impact of these pressure waves on turbocharger performance and scavenging efficiency will lead to more reliable engine upgrade projects as the industry approaches the 0.5 g/bhp-hr engine. The research team chose “available energy,” or the amount of mechanical and thermal energy available to the turbocharger turbine for operation as the parameter for defining optimal exhaust manifold design parameters. This allowed the research team to: 1) investigate energy losses in the candidate Clark TLA-6 exhaust removal system on a component basis, and 2) translate the mitigation of these losses into expanded turbocharger operating range. The end point of the project was a set of exhaust manifold design guidelines aimed at maximizing turbocharger performance by way of the defined metrics, scavenging efficiency and waste-gate margin.

This paper presents an investigation into CO formation in large-bore two-stroke cycle (2SC) lean-burn engines. On March 5, 2009, the Environmental Protection Agency (EPA) proposed a new rule to addressing National Emission Standards for Hazardous Air Pollutants (HAP) for existing stationary reciprocating internal combustion engines. Specifically, the 2009 Proposed Rule identifies carbon monoxide (CO) as a surrogate for HAP and requires reductions in CO for 2SC lean-burn engines. This future promulgation has created the need for a comprehensive kinetic CO formation model. The CO model itself integrates equilibrium concentration values of CO with the CO concentration created later in the cycle from the dissociation of equilibrium CO 2 . The previously developed variable-geometry multi-cylinder Turbocharged-Reciprocating Engine Compressor Simulation (T-RECS) has been modified with a mechanism to model cycle-resolved CO formation using a calibrated kinetic reaction scheme. The simplified chemical kinetic CO reaction scheme has been tuned and validated with exhaust concentration data collected on a Cooper GMVC large-bore two-stroke cycle engine, and directly relates the impact of engine operating conditions and in-cylinder geometry.

NOx emissions are a major cause of ozone formation. Several technologies to mitigate NOx in internal combustion engines have been developed, both in-cylinder and aftertreatment. Some of these newer technologies are being implemented on new engines, but older engines, especially large diesel engines, have few options to reduce these emissions substantially. The most common method of NOx reduction is retarding the start of injection timing but this has a penalty in fuel economy. A program has been undertaken on an EMD 645E two-stroke diesel engine to combine a simple mechanical system with both retarded and variable start of injection — to mitigate NOx — with variable valve timing to offset the fuel economy penalty. Simulation modeling and on-engine experimentation have been carried out to quantify the extent of the NOx reduction with the impact on fuel economy.

In an internal combustion engine, both the two-stroke and the four-stroke variety, the ejection of exhaust gases from the cylinder is termed as blowdown. In a two stroke engine, during blowdown the in-cylinder gases which are at a pressure higher than the exhaust manifold pressure rush out of the exhaust port into the manifold, resulting in a high pressure wave propagating through the manifold. In a multi-cylinder engine where the exhaust ports of each cylinder are connected to the manifold, the high pressure pulses in the manifold will impact the in-cylinder performance of the cylinders downstream of the first cylinder. This impact was investigated using the Turbocharger Reciprocating Engine Computer Simulation (T-RECS), a zero-dimensional cycle simulation tool developed at the National Gas Machinery Laboratory (NGML). The manifold pressure distributions were generated using the NGML developed Virtual Pipeline Simulation Tool (VPST), which is a one-dimensional pipe flow simulation software package. The analysis assumed that at any given instant the exhaust port of only one cylinder is open while the exhaust ports of all other cylinders remain closed. The results showed that the in-cylinder pressure distributions could become significantly altered under the influence of the exhaust manifold pressure pulses. The knowledge gained would help in utilizing the pressure pulses more effectively in tuning the exhaust manifold.

Homogeneous-charge, compression-ignition (HCCI) combustion is triggered by spontaneous ignition in diluted homogeneous mixtures and has a gradual trend thanks to suitable solutions. It is considered a very effective way to reduce engine pollutant emissions, however only experimental prototypes have been based on this concept, except for a few small two-stroke engines. HCCI combustion is feasible with fuels both for S.I. and for C.I. engines, but currently it does not cover the whole engine operating field, thus the engine must be built to operate also as a conventional engine. In order to obtain a gradual combustion and not a simultaneous reaction (as it would be in spontaneously ignited homogeneous mixture), lean mixture is used and appropriate solutions, as Exhaust Gas Recirculation (EGR), are necessary. However, the admission of exhaust gas into the cylinder goes to detriment of engine maximum mean effective pressure. This paper concerns a preliminary study of an innovative concept to control HCCI combustion in Diesel-fuelled engines, apart from exhaust gas presence, the function of which is limited to NOx emission control. The main purpose of the research is the obtaining of Diesel HCCI combustion also with high mean effective pressures rendering the combustion behaviour more controllable as well. The concept consists in forming a pre-compressed homogenous charge outside the cylinder and in gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the flow rate transfer and high pressure gradients, typical of common HCCI combustion, can be limited as well. A first analysis has been done, considering a cylinder filled with a perfectly stirred mixture of air and diesel fuel through a transfer duct, only to test the validity of the concept, regardless of which effective solution will be adopted. Both Two and Four Stroke operations have been considered to realize the concept. Results in terms of pressure, heat release rate, temperature and emission production have pointed out the validity of the concept. Especially the Two Stroke solution produces more soot than the conventional Diesel, pointing out that the air-fuel mixing is probably not optimized. Regarding NOx emissions, both the proposed solutions give better results than the conventional Diesel engine.

The natural gas transmission industry integrates turbochargers into the engine system to strategically increase airflow for the purpose of decreasing pollutant emissions, such as Nitrogen Oxide (NO X ). Regulations are expected to be tightened in the coming years, forcing transmission companies to look past turbochargers for compliance. The solution to further decreasing emissions lies not in further retrofit, but focusing on the physics of the current system. The flow rate physics of the intake and exhaust manifolds impede equal distribution of air from the turbocharger to each cylinder. Imbalance in airflow creates a discontinuity in the trapped equivalence ratio from cylinder to cylinder. The trapped equivalence ratio is directly proportional to NO X production and a function of the fuel flow rate, airflow rate, and, in two-stroke cycle engines, the scavenging efficiency. Only when these three characteristics are balanced cylinder to cylinder will the combustion and the NO X production in each cylinder be equal. The engine NO X production will be disproportionately high if even one cylinder operates less lean relative to the other cylinders. Balancing the NO X production between cylinders can lower the overall NO X production of the engine. This paper reports on an investigation into the transient, compressible flow physics that impact the trapped equivalence ratio. A comprehensive, variable geometry, multi-cylinder Turbocharger-Reciprocating Engine Computer Simulation (T-RECS) has been developed to illustrate the effect of airflow imbalance on an engine. A new model, the Charge Air Integrated Manifold Engine Numerical Simulation (CAIMENS), is a manifold flow model coupled with the T-RECS engine processor that uses an integrated set of fundamental principles to determine the crank angle-resolved pressure, temperature, burned and unburned mass fractions, and gas exchange rates for the cylinder. CAIMENS has the ability to show the transient impact of one cylinder firing on each successive cylinder. The pulsation model also describes the impact of manifold pressure drop on in-cylinder peak pressure and the pressure wave introduced to the intake manifold by uncovering the intake ports. CAIMENS provides the information necessary to quantify the impact of airflow imbalance, and allows for the visualization of the engine system before and after airflow correction. The model shows that not only does the manifold pressure drop have a significant impact on the in-cylinder peak pressure, but it also has an impact on the pressure wave introduced to the intake manifold as the ports are opened. Also, each cylinder has a considerable impact on the airflow into each successive cylinder.

This paper describes the collection and analysis of discharge coefficients from the ports of large-bore two-stroke cycle engines. The literature includes some information on discharge coefficients from very small ports. The literature was found to not include data collected from very large ports, such is in Cooper, Clark, and Worthington two-stroke cycle engines. The methodology was to construct and then use a flow bench that was sized for large-bore engine cylinder liners. The flow bench is designed to experimentally determine the discharge coefficients of large bore engine ports. The discharge coefficients are an integral part of determining the air flow rate through an engine, and in modeling and predicting the airflow through an engine system. This information can be used by designers to better match turbochargers and aftercoolers to engines. Large bore engine cylinders are typically are 35–56 cm (14–22 in.) in diameter, and have power outputs ranging 745–3730 kW (1,000–6,000 hp). In general, the majority of these engines were built in the 1940–1950’s. The importance of predicting the airflow rate through these engines has become paramount due to increasingly stringent EPA emission regulations. The data shows that there is a vast difference between the discharge coefficients of the three primary engines used in the natural gas industry.

Emission controls in stationary gas engines have required significant modifications to the fuel injection and combustion processes. One approach has been the use of high-pressure fuel injection to improve fuel-air mixing. The objective of this study is to simulate numerically the injection of gaseous fuel at high pressure in a large-bore two-stroke engine. Existing combustion chamber geometry is modeled together with proposed valve geometry. The StarCD® fluid dynamics code is used for the simulations, using appropriate turbulence models. High-pressure injection of up to 500 psig methane into cylinder air initially at 25 psig is simulated with the valve opened instantaneously and piston position frozen at the 60 degrees ABDC position. Fuel flow rate across the valve throat varies with the instantaneous pressure but attains a steady state in approximately 22 ms. As expected with the throat shape and pressures, the flow becomes supersonic past the choked valve gap, but returns to a subsonic state upon deflection by a shroud that successfully directs the flow more centrally. This indicates the need for careful shroud design to direct the flow without significant deceleration. Pressures below 300 psig were not effective with the proposed valve geometry. A persistent re-circulation zone is observed immediately below the valve, where it does not help promote mixing.

In two-stroke engines replacement of carburetors with direct fuel injection systems greatly reduces engine emissions and fuel consumption by eliminating fuel short-circuiting. Air-blast direct fuel injection using a dedicated air pump has been successfully applied to both two- and four-stroke engines. In this study we re-examine the design of a low cost compression pressurized direct injection system. This system uses gases extracted from the combustion chamber during the compression stroke to supply pressure for the air blast injection, thus eliminating the air pump [1,2]. Gases, predominantly scavenging air, are transferred to a mixing cavity from the combustion chamber via a small (5mm diameter) solenoid poppet valve as the piston rises during the compression stroke. Proper functioning of the system requires careful optimization of the mixing cavity size and the blast valve timing to ensure adequate mixing cavity pressure and fuel atomization. To assist in the optimization of these design parameters a one-dimensional fluid dynamics model has been developed. Parameter sensitivity studies were carried out using the model to determine the optimum cavity size, blast valve timing, and fuel injection duration. These parameters were optimized over a wide range of engine speeds and throttle settings. Results show that a mixing cavity pressure of 500 kPa is attainable over the range of 1000 to 6000 rpm, from closed throttle to wide open throttle (WOT) without cavity pressurization encroaching into the ignition regime. Fuel maps and valve timings are presented and results are contrasted with the carbureted case, showing improved fuel efficiency and emissions for the direct injection system. These data will be used in the design of a physical demonstration engine.

Direct high-pressure liquid fuel injection is able to control the mixing process inside the cylinder for getting either stratified charge at partial loads or quasi-homogeneous conditions, as it is required at full load. This paper shows the development of this solution for small two-stoke engines, using multidimensional modelling. The aim is investigating how the design of scavenging ducts and combustion chamber influences charge stratification behaviour, taking into account fuel distribution and stratification stability varying engine load and speed.

In this paper, detailed computational study is presented which helps to understand and improve the fuel-air mixing in a new direct-mixture-injection two-stroke engine. This new air-assisted injection system-based two-stroke engine is being developed at the Indian Institute of Science, Bangalore over the past few years. It shows the potential to meet emission norms such as EURO-II and EURO-III and also deliver satisfactory performance. This work proposes a comprehensive strategy to study the air-fuel mixing process in this engine and shows that this strategy can be potentially used to improve the engine performance. A three-dimensional compressible flow code with standard k –ε turbulence model with wall functions is developed and used for this modeling. To account for the moving boundary or piston in the engine cylinder domain, a non-stationary and deforming grid is used in this region with stationary cells in the ports and connecting ducts. A flux conservation scheme is used in the domain interface to allow the in-cylinder moving mesh to slide past the fixed port meshes. The initial conditions for flow parameters are taken from the output of a three-dimensional scavenging simulation. The state of the inlet charge is obtained from a separate modeling of the air-assisted injection system of this engine. The simulation results show that a large, near-stoichiometric region is present at most operating conditions in the cylinder head plane. The state of the in-cylinder charge at the onset of ignition is studied leading to a good understanding of the mixing process. In addition, sensitivity of two critical parameters on the mixing and stratification is investigated. The suggested parameters substantially enhance the flammable proportion at the onset of combustion. The predicted P –θ from a combustion simulation supports this recommendation.