Abstract For uniflow scavenged two-stroke marine diesel engines, the main function of scavenging process is to replace the burned gas with fresh charge. The end state of scavenging process is integral to the subsequent compression and combustion, thereby affecting the engine’s fuel economy, power output and emissions. In this paper, a complete working cycle of a large marine diesel engine was simulated by using the 3D-CFD software CONVERGE. The model was validated by mesh sensitivity test and experiment data. Based on this calibrated model, the influences of swirl ratio and exhaust valve closing (EVC) timing on the scavenging process were investigated. The parameters evaluating the performance of scavenging process were introduced. The results show that, by adjusting the swirl orientation angle(SOA) from SOA=10° to SOA=30°, different swirl ratios are generated and have obvious differences in flow characteristics and scavenging performance.
Experimental Investigation of Orifice Design Effects on a Methane Fuelled Prechamber Gas Engine for Automotive Applications
Abstract Due to its molecular structure, methane provides several advantages as fuel for internal combustion engines. To cope with nitrogen oxide emissions high levels of excess air are beneficial, which on the other hand deteriorates the flammability and combustion duration of the mixture. One approach to meet these challenges and ensure a stable combustion process are fuelled prechambers. The flow and combustion processes within these prechambers are highly influenced by the position, orientation, number and overall cross-sectional area of the orifices connecting the prechamber and the main combustion chamber. In the present study, a water-cooled single cylinder test engine with a displacement volume of 0.5 l is equipped with a methane-fuelled prechamber. To evaluate influences of the aforementioned orifices several prechambers with variations of the orientation and number of nozzles are used under different operating conditions of engine speed and load.
Resonance Charging Applied to a Turbo Charged Gasoline Engine for Transient Behavior Enhancement at Low Engine Speed
Abstract Upcoming regulations and new technologies are challenging the internal combustion engine and increasing the pressure on car manufacturers to further reduce powertrain emissions. Indeed, RDE pushes engineering to keep low emissions not only at the bottom left of the engine map, but in the complete range of load and engine speeds. This means for gasoline engines that the strategy used to increase the low end torque and power by moving out of lambda one conditions is no longer sustainable. For instance scavenging, which helps to increase the enthalpy of the turbine at low engine speed cannot be applied and thus leads to a reduction in low-end torque. Similarly, enrichment to keep the exhaust temperature sustainable in the exhaust tract components cannot be applied any more. The proposed study aims to provide a solution to keep the low end torque while maintaining lambda at 1.
Abstract This work reports a CFD study on a 2-stroke (2-S) opposed piston high speed direct injection (HSDI) Diesel engine. The engine main features (bore, stroke, port timings, et cetera) are defined in a previous stage of the project, while the current analysis is focused on the assembly made up of scavenge ports, manifold and cylinder. The first step of the study consists in the construction of a parametric mesh on a simplified geometry. Two geometric parameters and three different operating conditions are considered. A CFD-3D simulation by using a customized version of the KIVA-4 code is performed on a set of 243 different cases, sweeping all the most interesting combinations of geometric parameters and operating conditions. The post-processing of this huge amount of data allow us to define the most effective geometric configuration, named baseline.
Low Volatility Fuel Cold Start Experience with a Stepped Piston UAV Engine to Address Single Fuel Objectives
Abstract This paper reports on the research and development challenges experienced from dynamometer testing of a spark ignition UAV engine operating on heavy fuel. The engine is a segregated scavenging two stroke engine with air charge delivery by means of integral stepped pistons overcoming durability issues of conventional crankcase scavenged engines. A key element of the experimental study builds upon performance development to address the need for repeatable cold start on low volatility fuel thereby eliminating gasoline from UAV theatres of deployment. Lubrication challenges normally associated with crankcase scavenged two stroke engines are avoided by the integrated re-circulatory lubrication system. The fuel explored in this study is kerosene JET A-1.
Characteristics of Abnormal Combustion in the Scavenging Zone for a Highly-Boosted Gasoline Direct Injection Engine
Abstract In order to improve low speed torques, turbocharged gasoline direct injection (TGDI) engines often employ scavenging with a help of variable valve timing (VVT) controlled by the cam phasers. Scavenging improves the compressor performance at low flows and boosts low-speed-end torques of the engines. Characteristics of the engine combustion in the scavenging zone were studied with a highly-boosted 1.5L TGDI engine experimentally. It was found that the scavenging zone was associated with the highest blowby rates on the engine map. The blowby recirculation was with heavy oil loading, causing considerable hydrocarbon fouling on the intake ports as well as on the stem and the back of the intake valves after the engine was operated in this zone for a certain period of time. The low-speed pre-ignition (LSPI) events observed in the engine tests fell mainly in the scavenging zone.
Numerical Analysis of the Steady-State Scavenging Flow Characteristics of a Two-Stroke Marine Engine
Abstract The scavenging process in two-stroke marine engines not only transports burnt gas out of the cylinder but also provides fresh air for the next cycle, thereby significantly affecting the engine performance. In order to enhance fuel-air mixing, the scavenging process usually generates swirling flow in uniflow-type scavenging engines. The scavenging stability directly determines the scavenging efficiency and even influences fuel-air mixing, combustion, and emission of the engine. In the present study, a computational fluid dynamics (CFD) analysis of the scavenging process in a steady-state scavenging flow test is conducted. A precession phenomenon is found in the high swirl model, and Proper Orthogonal Decomposition (POD) method is used to analyze the reason and the multi-scale characteristics of the precession phenomenon.
Analysis of the Effect of Intake Plenum Design on the Scavenging Process in a 2-Stroke Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) Engine
Abstract In this study, the effect of the intake plenum design on the scavenging process in a newly proposed 2-stroke Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine was studied in detail by three dimensional (3D) computational fluid dynamics (CFD) simulations. In the BUSDIG engine, the intake scavenge ports are integrated into the cylinder liner and their opening and closure are controlled by the movement of piston top while exhaust valves are placed in the cylinder head. In order to accommodate the optimized scavenge ports in the real engine application, the intake plenum with an inlet pipe and a scavenge chamber was designed and connected to the 12 evenly distributed scavenge ports in a single cylinder BUSDIG engine.
Evaluations of Scavenge Port Designs for a Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) Engine by 3D CFD Simulations
Abstract The 2-stroke engine has great potential for aggressive engine downsizing due to its double firing frequency which allows lower indicated mean effective pressure (IMEP) and peak in-cylinder pressure with the same output toque compared to the 4-stroke engine. With the aid of new engine technologies, e.g. direct injection, boost and variable valve trains, the drawbacks of traditional 2-stroke engine, e.g. low durability and high emissions, can be resolved in a Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine. Compared to the loop-flow or cross-flow engines, the BUSDIG engine, where intake ports are integrated to the cylinder liner and controlled by the movement of piston top while exhaust valves are placed in the cylinder head, can achieve excellent scavenging performance and be operated with high boost.
Investigation of the Gas Exchange (Scavenging) on a Single-Scroll Turbocharged Four Cylinder GDI Engine
Abstract For scavenging the combustion chamber during the gas exchange, a temporary positive pressure gradient between the intake and the exhaust is required. On a single-scroll turbocharged four cylinder engine, the positive pressure gradient is not realized by the spatial separation of the exhaust manifold (twin-scroll), but by the use of suitable short exhaust valve opening times. In order to avoid any influence of the following firing cylinder onto the ongoing scavenging process, the valve opening time has to be shorter than 180 °CA. Such a short valve opening time has both, a strong influence on the gas exchange at the low-end torque and at the maximum engine power. This paper analyzes a phenomenon, which occurs due to short exhaust valve opening durations and late valve timings: A repeated compression of the burned cylinder charge after the bottom dead center, referred to as “recompression” in this paper.
One of the major reason for lower efficiency and higher unburned hydrocarbon and carbon monoxide emission for two stroke engine is short circuit losses during the scavenging process. An attempt has been made in this study to understand and improve this phenomenon. A three dimensional transient CFD model is developed for a loop scavenged, Schnullar type, 70 cc two stroke engine. Three major processes, namely, blow down (expansion); scavenging and compression have been modelled. The model is validated with PIV measurement done in motoring mode. Model is also validated with experimental data for trapping efficiency with Watson method and for in-cylinder pressure during expansion, blow down and intake events. A good correlation is observed between experimental and simulation results. CFD model is used to quantify various parameters, such as, delivery ratio, trapping efficiency, scavenging efficiency, and amount of fresh mass short circuit at different load and speed points.
Piston Design Impact on the Scavenging and Combustion in an Opposed-Piston, Opposed-Cylinder (OPOC) Two-Stroke Engine
Abstract A comprehensive investigation on the impact of piston design on scavenging and combustion in an opposed- piston, opposed-cylinder (OPOC) two-stroke engine is carried out and presented in this paper. Two-stroke engines, in general, have superior power densities and brake thermal efficiencies. Compared with opposed-piston (OP) engines, the OPOC architecture comprises only one crankshaft instead of two, and all the forces generated on the piston go to this one crankshaft via a common bearing, thus making the engine structure inherently simple, lightweight, compact and efficient. Due to the piston motion of the OPOC engine, two opposing injectors were mounted at the center of the cylinder wall for each cylinder. This unique feature posed challenges on air entrainment for air/fuel mixing because of the inherent limited space for injection spreading angle near top-dead-center (TDC).
Particle Image Velocimetry Measurements of Swirl and Scavenging in a Large Marine Two-Stroke Diesel Engine
Abstract In-cylinder flow velocity measurements using particle image velocimetry (PIV) have been performed for the first time in a full-size marine Diesel engine. The engine was a four cylinder two-stroke engine with a bore diameter of 0.5 meter and a stroke of 2.2 meter. For such engines uniflow scavenging is used, with fresh air entering through angled ports at the bottom of the cylinder to generate a swirling flow and burnt gases exiting through a centrally located exhaust valve at the top. For efficient design of this process and for validation of CFD models it is essential to obtain an experimental characterization of the flow inside a fully operational engine. Optical access was obtained through a custom designed engine cover, fitted with a number of optical ports into which sapphire windows were mounted. Both the laser and camera used for PIV were mounted directly onto the engine in order to minimize effects of vibrations on optical alignment.
1-D Simulation Study of Divided Exhaust Period for a Highly Downsized Turbocharged SI Engine - Scavenge Valve Optimization
Fuel efficiency and torque performance are two major challenges for highly downsized turbocharged engines. However, the inherent characteristics of the turbocharged SI engine such as negative PMEP, knock sensitivity and poor transient performance significantly limit its maximum potential. Conventional ways of improving the problems above normally concentrate solely on the engine side or turbocharger side leaving the exhaust manifold in between ignored. This paper investigates this neglected area by highlighting a novel means of gas exchange process. Divided Exhaust Period (DEP) is an alternative way of accomplishing the gas exchange process in turbocharged engines. The DEP concept engine features two exhaust valves but with separated function. The blow-down valve acts like a traditional turbocharged exhaust valve to evacuate the first portion of the exhaust gas to the turbine.
Abstract In this paper, a new-type balanced opposed-piston folded-cranktrain (OPFC) two-stroke diesel engine is developed by Beijing Institute of Technology. OPFC has some potential advantages such as simple structure, good balance, compact, high power density and thermal efficiency. The structural feature of OPFC engine leads to the performance is different with the conventional engine. In order to study and verify the characteristics of this kind of engine, the folded-crank train dynamics, cylinders scavenging process and combustion process are investigated. The influence of parameters on the engine performance is investigated, includes the fuel injection timing, intake/exhaust port timing. In addition, the nozzle diameter is investigated as a main factor to affect the mixture and combustion process in the cylinder.
Effect of Piston Dynamic on the Working Processes of an Opposed-Piston Two-Stroke Folded-Cranktrain Engine
Abstract An opposed-piston two-stroke folded-cranktrain diesel engine was studied in this paper. In order to achieve asymmetric scavenging, asymmetric angle between two crank throws were designed. However asymmetric crank-throw angle has direct effect on the piston dynamic, which affects engine performance. This paper investigated the characteristics of the piston dynamic on an opposed-piston two-stroke folded-cranktrain diesel engine; effects of the asymmetric angle on the piston displacement, velocity and acceleration were analyzed; further researches were done to studied the effect of piston dynamic on the gas exchange performance and in-cylinder performance. The results show that, larger asymmetric angle is positive for the scavenging efficiency but negative for combustion.
Engine Scavenging Tuning for In-Field Product Expectations of a 45cc Stratified Two-Stroke Power Head
Because of todays new emissions legislation, a new 45cc Husqvarna trimmer/clearing saw power head was needed. When reducing emissions in a conventional two-stroke engine or a stratified scavenged engine, it is important that the tuning and basic scavenging characteristics of the standard engine are maintained. A dual charge intake system is necessary for the stratified engine but it also creates air fuel delivery issues compared to a standard two stroke engine. With increasing trapping efficiency more spent gases mixes with the fresh charge, creating less favorable combustion properties and thermal loading on the engine. On top of this the sequential stratified scavenging technology introduces a spatial inhomogeneous mix problem between scavenging fresh air, new mixture and spent gases. This all add sensitivity to long term stability due to deposits of carbon both in combustion chamber and exhaust duct, resulting in a change in engine parameters due to aging.
To effectively use Computational Fluid Dynamics (CFD) for engine emission development it is necessary to be able to simulate the scavenging flow in an engine. The CFD model for a stratified charged two-stroke engine is even more complex. This model have been tuned and finally validated with engine tests. A CFD model has been made of the Husqvarna 560XP two-stroke stratified charged chainsaw engine. The model contains piston, cylinder, inlet system ducting and exhaust silencer. The simulation runs with moving deforming mesh with all ports active. The airflow levels have been fine tuned with inlet restrictions similar to those in the air filter holder, which is not completely included in the present model. The results and behaviour of the CFD model has a very good match to the measured values of the finished product. This gives us confidence in the model and several aspects can now be studied that is virtually impossible to capture by other means.
The scavenging process is an integral part of any two-stroke internal combustion engine cycle whether it is spark ignited or compression ignited. The scavenging process is responsible for transporting the burned gases from the previous working stroke out of the combustion chamber to allow for the fresh charge or fresh air to enter for the next combustion/working stroke. This implies that the scavenging process is responsible for setting the initial condition for the combustion process, consequently affecting fuel economy, power output and emission of hazardous gases. Two-stroke diesel engines for marine propulsion are usually uniflow scavenged cross-head engines. In uniflow scavenged engines the scavenge air enters the cylinder via ports located near the bottom dead center and exits through an exhaust valve located in the cylinder head. The in cylinder flow is therefore concentrated in one direction which gives the method its name.
Highly downsized, Direct Injection (DI) engines benefit strongly from cylinder scavenging where possible, to reduce internal residuals thereby reducing the occurrence of knock. Some researchers also suggest that non-homogeneous distribution of internal residuals at high load could contribute to pre-ignition or ‘mega-knock’ with much higher pressure amplitude than that of common knock. For this reason, a computational study was conducted to assess the residual gas fraction and in-cylinder distribution, using the combustion geometry of the three cylinder, 1.2L MAHLE Downsizing engine, which has proven to be a very robust and reliable research tool into the effects of combustion effects under a number of different operating conditions. This study used a CFD model of the cylinder gas exchange. ES-ICE coupled with STAR-CD was employed for a moving mesh, transient in-cylinder simulation.
Modeling the Effect of Variable Cam Phasing on Volumetric Efficiency, Scavenging and Torque Generation
In a mean value engine model, the mass flow of air through the cylinders is, up to a constant factor, described as the product of the air density in the intake manifold, the engine speed, and the volumetric efficiency. The volumetric efficiency is traditionally modeled as a function of the engine speed and the pressure in the intake and exhaust manifolds, but for modern engines the model must also account for the effect of variable valve timings. The engine that is modeled here is equipped with variable cam phasing on both the intake and the exhaust valves. In order to reduce the complexity, we will only model the effect of the valve overlap, which is the number of crank angle degrees that both valves are open simultaneously. When the valve overlap is significant, there may be fresh air that flows directly through the exhaust valve, known as scavenging.
In a boat two-stroke two-cylinder engine, SC-port fuel injection of CNG was applied at running condition in comparison with the fuelling with a gas-mixer. Three methods of tests were employed; operation at a test-bench, at an anchored condition and on a running boat. In a lower engine speed, the beneficial effect of higher thermal efficiency was obtained, while in higher engine speed range especially at the running condition, it has the inverse effect of lower thermal efficiency. It is based on the limited range of lower injection rate of the fuel injectors, and thus the fuel injection rate of this type of fuel injectors has a key role of developing the technology of the SC-port injection.
Simulation of Scavenging Process, Internal Mixture Preparation, and Combustion of a Gasoline Direct Injection Two-Cylinder Two-Stroke Engine
The continuous improvement of the numerical methods together with the increase of computer power allows the simulation of more and more complex technical problems even for increasing calculation domains. In order to get effective and significant results for the two-stroke two-cylinder engine, the simulation of the complete geometry with both cylinders and the complete exhaust port is required. However, the simulation requires several revolutions until the gas dynamic inside the exhaust port achieves a steady state. Hence, the simulation of a two-cylinder two-stroke engine consumes a lot of calculation time; nevertheless it is still acceptable in the development process of a new engine. This paper covers the discussion of the simulation of a two-stroke two-cylinder high-performance engine using the commercial CFD Code Fluent 6.3.26. The used settings for the simulation, like the turbulence model, injection settings, combustion model and reduced reaction mechanism are presented.
Analysis of engine performances improvement by down sizing in relationship with super- and turbocharging, adapted scavenging and direct injection
The development of future internal combustion engines with high power density in correlation with drastically reduced fuel consumption / CO2 emission and pollutant emission requires both an improvement of the thermodynamic process stages such as scavenging, mixture formation and combustion as well as new strategies regarding the engine function fields. An advanced concept in this direction is the combination of down sizing with supercharging and turbocharging coupled in different configurations.
1D-3D Analysis of the Scavenging and Combustion Process in a Gasoline and Natural-Gas Fuelled Two-Stroke Engine
The paper presents a 1D-3D numerical model to simulate the scavenging and combustion processes in a small-size spark-ignition two-stroke engine. The engine is crankcase scavenged and can be operated with both gasoline and Natural Gas (NG). The analysis is performed with a modified version of the KIVA3V code, coupled to an in-house developed 1D model. A time-step based, two-way coupled procedure is fully described and validated against a reference test. Then, a 1D-3D simulation of the whole two-stroke engine is carried out in different operating conditions, for both gasoline and NG fuelling. Results are compared with experimental data including instantaneous pressure signals in the crankcase, in the cylinder and in the exhaust pipe. The procedure allows to characterize the scavenging process and quantify the fresh mixture short-circuiting, as well as to analyze the development of the NG combustion process for a diluted mixture, typically occurring in a two-stroke engine.
A CNG Two Stroke Cycle S.I. Engine Using Intermittent Low Pressure Fuel Injection from Scavenging Ports
Performance of a CNG (Compressed natural gas) two stroke cycle S.I. engine using intermittent low pressure fuel injection from scavenging ports is investigated experimentally. The test engine is a two cylinder, 398 cm3, two stroke cycle spark ignition engine. Gaseous fuel injectors are attached at the engine block, and a CNG is injected into the scavenging passage through a fuel injection pipe. The fuel injection pressure is set at 0.255 MPa, and the fuel is injected intermittently during the scavenging process. The length and tip geometry of the fuel injection pipe are varied, and the effect on the engine performance is investigated. Using the scavenging port fuel injection, the BSFC is reduced by 25 %, and the lean burn limit extends from λ = 1.2 to 1.46, at the maximum. The peak of the NOx emission shifts to leaner side, and the THC emission is reduced by 47 % at the maximum.
A detailed knowledge of the scavenging process becomes necessary during the development process, when the performance and in particular the emission output of two-stroke engines needs to be qualified and evaluated. This paper presents an experimental approach to describe the composition of the flow at the exhaust port by means of flow visualization and to quantify the relative changes of the scavenging losses imposed by design changes. The experimental set-up has been described in previous SAE papers and has been expanded by a transparent exhaust port, which gives optical access to the flow through the exhaust port. The gases in the cylinder are represented by differently colored water-based fluids. Typically, the burnt gas is clear and the fresh charge is colored to visualize its progress during the scavenging cycle and its distribution inside the cylinder.
R&D of a New G.D.I. 2-Stroke Engine with a Unidirectional Scavenging Flow and a Force-Feed Lubrication System
Preliminary bench tests have been carried out on an original-design 2-stroke single-cylinder prototype engine, which is equipped with an electronically controlled gasoline direct injection apparatus. The main design and operating features of the engine concern: a unidirectional airflow during the scavenging process (from the inlet ports near the BDC to overhead cam-actuated valves), an external air pump (a Roots volumetric type driven by an electric motor) and a force-feed lubrication system, like those usually exploited in mass-produced 4-stroke engines. Experimental bench tests were carried out under low load, intermediate rotational speed operating conditions. The performances were compared to those obtained from a commercial crankcase-scavenged 2-stroke engine using an indirect injection fuel feeding system. Encouraging results were obtained as far as fuel consumption and pollutant emissions are concerned.
In a cross-scavenged two-stroke engine, a piston with a deflector is often utilized to flow the fresh charge toward the cylinder head and away from the exhaust port. But flow-visualization studies have shown that the fresh charge flows toward the cylinder head even without the deflector. To find why the fresh charge flows toward the cylinder head, we used Fluent, a general purpose computational fluid dynamics (CFD) software, to simulate two- and three-dimensional flows in a cross-scavenged two-stroke engine. While the fresh charge entered from the scavenging port into the cylinder and flowed across the cylinder, we investigated the instantaneous fresh charge flow, velocity, and pressure distribution in the cylinder.
Effect of Fuel Injection Rate on the Performance of a 2-Stroke CNG Spark-Ignition Engine with Scavenging-Port Injection
The most serious problem in a 2-stroke spark-ignition engine is poor trapping of fresh charge. To solve this problem, a scavenging-port injection was applied, and a fuel injection pipe (FIP) was installed at the injector tip. In a previous study, it was shown that the BSFC and emission characteristics were drastically improved. In the present study, effect of increase in the fuel injection rate was investigated. It is shown that the BSFC and the THC emissions improved at high engine speeds, while they slightly deteriorate at low engine speeds. The increase in the fuel injection rate is effective particularly at high engine speeds, where the scavenging duration becomes shorter.