Comparison of Motored and Fired Velocities in a Two Stroke Engine

<div class="htmlview paragraph">The noise radiated from the tail pipe plays an important role in the total noise emitted for small two stroke engines. In this paper, an engine cycle and exhaust system model for a commercial 50 c.c., crankcase-scavenged type, single cylinder two stroke engine was developed to determine the noise emitted from the exhaust pipe.</div> <div class="htmlview paragraph">In modeling the engine cycle, a zero dimensional thermodynamic model was used to represent the compression and the expansion process in cylinder, and a two zone quasi dimensional model was used to represent the combustion process. As for the scavenging process, the perfect mixing model was used. In modeling the gas dynamics in the intake and the exhaust system, an unsteady one dimensional model was used, and the characteristic line method was employed to solve the partial differential equations.</div> <div class="htmlview paragraph">Seven different configurations of exhaust pipes were studied in this paper, including three straight pipes, two diverging-converging pipes, one pipe with an expansion chamber, and a real exhaust pipe. The test engine was run in both motoring and firing conditions. Pressure variations in the exhaust pipe and the total sound level and the one third octave frequency distributions outside the exhaust pipe were recorded.</div> <div class="htmlview paragraph">For the simple geometry pipes at motoring conditions, the calculated results of pressure variations inside exhaust pipe and the noise level outside exhaust pipe agree quite well with the measured data. However, for the complex geometry pipe at firing conditions, the calculated results are consistent with the experimental data at low engine speeds only. At high engine speeds, the model used in this paper can not predict the high frequency components of the pipe pressure, and the total sound levels predicted are underestimated.</div>

<div class="htmlview paragraph">The effects of the swirl level on the fresh air charge distribution is studied by using a multidimensional computer code for a uniflow type two-cycle diesel engine geometry. The flow is assumed to be axisymmetric. The inlet ports are simulated as an annular opening around the liner, and the exhaust valves are simulated as an annular slot in the cylinder head. Calculations are performed from exhaust valve opening to TDC for a large range of inlet port angles (10-30°). Results are presented in terms of velocity vector plots together with swirl velocity, and fresh charge contour plots.</div> <div class="htmlview paragraph">During the valves open period, the calculations indicate the presence of two vortices in the radial-axial flow plane as previously observed by Diwakar. The first one is near the liner wall (named as wall vortex) and the other is near the symmetry axis (named as the backflow vortex). The incoming fresh air tends to flow toward the cylinder head between these two vortices with reverse flows on the liner wall and near the symmetry axis. Increasing the swirl increases the strength and the region covered by the backflow vortex, redirects the upward fresh charge to a larger radius, and restricts the short-circuiting of the fresh air to the exhaust valves. This increases the trapping of fresh charge and the scavenging efficiency.</div> <div class="htmlview paragraph">Calculations from the exhaust valve closing to TDC show that: <ol class="list nostyle"> <li class="list-item"><span class="li-label">1)</span> <div class="htmlview paragraph">The size of the backflow vortex is reduced as the port is closed and diminishes immediately thereafter. The wall vortex is present up to 60° CA BTDC when the squish effects generated by the piston motion becomes dominant.</div></li> <li class="list-item"><span class="li-label">2)</span> <div class="htmlview paragraph">Fresh charge is not completely mixed with the residual gases even at TDC. There is about 11-17% difference in the fresh charge concentrations at the highest and lowest fresh charge locations at TDC. The inlet port angle (engine swirl level) seems to increase the magnitude of this stratification slightly, but more importantly, it changes the location of this charge stratification considerably. At low swirl levels (port angle 10°), the low fresh charge region is located near the mid-radius of the bowl, and the maximum fresh charge is in the clearance volume between the piston and the bowl. At high swirl levels (port angle 30°), the maximum fresh charge is compressed toward the bowl belly, and the low concentration region is both in the clearance volume and on the cylinder head near the axis. At medium swirl levels (port angle 20), the fresh charge covers the midsection of the bowl, while the residual gases are restricted only within the clearance between piston and engine head.</div></li> </ol></div> <div class="htmlview paragraph">For the diesel engine application and to enhance the mixing of the fuel spray with the fresh air, the residual gas-fresh air stratification pattern of the medium swirl case is better than both the high swirl and low swirl cases.</div> <div class="htmlview paragraph">In addition to the wall vortex (generated by flow separation above the port) and the Backflow Vortex (generated by the swirl velocity field), a new vortex pattern named as the Head Vortex is identified. The mechanism for the Head Vortex development is recognized as the stagnation of the fresh air jet on the cylinder wall rather than on the engine head. The presence of the head vortex changes the mixing rate between the residual gases and the fresh air considerably. The development of the concentration profiles to these final shapes is demonstrated. The effects of the engine swirl level on the mixing of the fresh charge with the residual gases are investigated in terms of the mixing rates and the maximum concentration difference within the flow field.</div>

<div class="htmlview paragraph">High exhaust gas recirculation (EGR) tolerance is always pursued not only for its advantages of the pumping loss reduction and fuel economy benefit, but also for stringent emission requirements by using conventional three-way catalytic converter (TWC) instead of costly NOx trap. How to keep fresh charge and EGR separated in the cylinder of a conventional four valve gasoline engine is a critical challenge. This work establishes advanced user subroutines and overall simulation strategies to model engine in-cylinder turbulent flow, temperature, pressure, and EGR concentration fields and to simulate EGR stratification process in a typical pent-roof gasoline engine cylinder during intake and compression strokes.</div> <div class="htmlview paragraph">A series of computational fluid dynamics (CFD) studies - combustion dome, ports, valve and valve mask configurations, EGR tolerances, swirl ratio, boundary and initial conditions, etc., have been conducted to relate the “corndog” flow structure and physical parameters - centrifugal force, heat transfer, swirl ratio, tumble flow, turbulent and burnt gas residual. Based on the CFD results of the four valve-port, pent-roof combustion dome system, the direction to reach the corndog flow structure should be: (1). Reduce the burnt gas residual; (2). Decrease the temperature difference of EGR and air to minimize their heat transfer and centrifugal force; (3). Reduce the turbulent intensity; (4). Design the combustion dome as flat as possible; (5). Avoid using masks; (6). Reduce tumble flow; (7). Keep certain level of swirl ratio.</div> <div class="htmlview paragraph">Another method to stratify EGR in cylinder, named “conjugate vortex EGR stratification” system, has been discovered by the CFD analysis. Two identical inversed helical ports, one intake and another exhaust, are combined together to create two strong inverse gas flow vortices in the cylinder. The double inverse vortices result in a stable separation of in-cylinder fresh charge and EGR flows induced from intake port and exhaust port during the entire intake stroke. A clear stratification interface between the two streams of flow was found and located in the cylinder center. The turbulent kinetic energy at the interface is quite low, indicating that the minimum mixing opportunity between the EGR and fresh charge occurs at this location.</div>

<div class="htmlview paragraph">The effects of tumble flow generated by various intake ports on the lean burn characteristics in a four-valve pentroof engine were investigated. Tumble intensities were tested for three intake ports of different entry angle; 25°, 20° and 15°. The flow characteristics in cylinder were measured under motored conditions with laser Doppler velocimetry. The lean burn performances were examined in an operating 4-cylinder engine. The combustion duration under lean mixture conditions were calculated by heat release analysis of the pressure trace. The flame developments in a single-cylinder optical engine were visualized with an image-intensified CCD camera to see the effect of different flow patterns with three inlet ports of strong and weak tumble.</div> <div class="htmlview paragraph">It was found that there is a correlation between the stronger tumble during induction and the higher convective velocity and turbulence levels at the time of ignition, which result in faster and stable combustion under lean mixture conditions. Images of the developing flame also revealed faster flame propagation in the stronger tumble port under lean mixture conditions, than that in weak tumble port. By comparing tumble intensities and combustion durations together with flame images, the design of intake port with the optimal entry angle was discussed.</div>

<div class="section abstract"><div class="htmlview paragraph">The subject of this paper is the reduction of the particle number emissions of a gasoline DI engine at high engine load (1.4 MPa IMEP). To reduce the particle number emissions, several parameters are investigated: the large scale charge motion (baseline configuration, tumble and swirl) can be varied at the single cylinder engine by using inlays in the intake port. The amount of residual gas can be influenced by the exhaust backpressure. By using a throttle valve, the exhaust backpressure can be set equal to the intake pressure and hence simulate a turbocharger's turbine in the exhaust system or the throttle valve can be wide open and thus simulate an engine using a supercharger.</div><div class="htmlview paragraph">Additionally, higher fuel injection pressure can help to enhance mixture formation and thus decrease particulate formation. Therefore, a solenoid injector with a maximum pressure of 30 MPa is used in this work. The fuel composition also has a strong influence on the combustion process and on particulate formation. In this work, alkylate fuel is used to reduce the particulate number emissions at high engine loads.</div><div class="htmlview paragraph">In-cylinder spectroscopic measurements and high-speed visualization techniques are applied in the combustion chamber of the engine and the results are combined with measurement data taken with an FTIR in the exhaust of the engine. A conventional exhaust gas analyzer is also employed and cylinder pressure indication supports thermodynamic analysis. This one-dimensional, three pressure analysis (TPA) is performed using GT-Power.</div></div>

<div class="htmlview paragraph">The implementation of the IFP-developped Compressed Air Assisted Fuel Injection process (named IAPAC) on a two-stroke engine allows the introduction of the fuel separately from the scavenging air in order to minimize fuel short-circuiting. The IAPAC process does not require an external air pump since the compressed air used to atomize the fuel is supplied, at no expense, by the crankcase. The premixed charge is delivered directly into the cylinder with a high spray quality and its stratification, for optimized combustion, is controlled by a valve. This process, therefore, provides the advantages of the direct injection but uses conventional low-pressure automotive type injection technology with commercially available gasoline injectors.</div> <div class="htmlview paragraph">In earlier work, we showed how the qualities of light weight, compactness, high specific power, high efficiency and low emissions make this concept particularly well-adapted for future automotive applications.</div> <div class="htmlview paragraph">Marine engine exhaust emissions from most conventional 2-stroke engines are excessive, and future legislation will no doubt take this fact into account. In this paper,we show how IAPAC technology provides an interesting solution to significantly reduce the pollutant emissions and improve the fuel economy of 2-stroke outboard engines.</div> <div class="htmlview paragraph">A 3-cylinder 1,2 L marine outboard IAPAC 2-Stroke engine has been developed and extensively tested. The results obtained point out that, with maintained quality of compactness and power output, this new engine gives fuel consumption reductions of more than 25 % and unburned hydrocarbon emission reductions of 70 to 90 %.</div> <div class="htmlview paragraph">Pollutant emissions are evaluated according to the ICOMIA procedure and the results clearly show how the IAPAC system is well adapted to take optimum advantage of the two-stroke cycle principle in marine outboard engines applications.</div>

<div class="htmlview paragraph">Four new 2-cylinder 4-stroke concepts are displayed as design and fitted in vehicles.</div> <div class="htmlview paragraph">These four different concepts comprise</div> <div class="htmlview paragraph"> <ul class="list disc"> <li class="list-item"><div class="htmlview paragraph">a Modular Concept V2- and W3-cylinder</div></li> <li class="list-item"><div class="htmlview paragraph">a MotoGP / Superbike concept with 2 and 3 cylinders,</div></li> <li class="list-item"><div class="htmlview paragraph">a narrow angle V-engine and</div></li> <li class="list-item"><div class="htmlview paragraph">a Building Block System Commuter CVT engine.</div></li> </ul> </div> <div class="htmlview paragraph">Each engine concept is designed to meet the different requirements of the four segments. Specific analysis and simulation concerning 1D thermodynamics, vehicle simulation and delivered performance and tractive force was done for each concept.</div> <div class="htmlview paragraph">The concepts are compared in the aspects of uniform rotation, inertia forces and moments, and the effect on performance by the pulse effects of the manifolded intake and exhaust systems.</div> <div class="htmlview paragraph">The Modular Concept contains an OHC engine with a wide range of displacements and commonality of many parts. Good versatility is obtained as the concepts can be applied for sport- or custom bikes. Also an advanced EMS with additional features is applied and a heated 3-way catalyst and air/fuel sensors are integrated to fulfill the most stringent emission regulations after the year 2006.</div> <div class="htmlview paragraph">The MotoGP / Superbike Concept has an advanced transmission position, which allows to design a very compact motorcycle size comparable to much smaller displacement bikes. The application of a pneumatic valve train allows for aggressive valve timing and high engine output. The concept is compared to engines with higher number of cylinders in terms of race competitiveness.</div> <div class="htmlview paragraph">The narrow-angle V-engine is a sportive and compact engine with newly developed mass balancing.</div> <div class="htmlview paragraph">The Commuter is a medium output design with a continuously variable transmission, featuring an extremely space-saving design</div>

<div class="section abstract"><div class="htmlview paragraph">A multi-position 4-stroke piston engine utilizing a novel rotary valve system was developed for handheld outdoor power equipment applications such as chainsaws, brush cutters, and string trimmers. The purpose of the project was to create a low-emission 4-stroke engine with 2-stroke performance levels including high RPM limits and power output. This was accomplished using a rotary valve system in lieu of the typical poppet valves of traditional 4-stroke engines. The prototype was then incorporated into a functional product for overall performance evaluation.</div><div class="htmlview paragraph">Three prototype 45cc rotary valve engines were developed and tested in both real-world usage and on laboratory engine dynamometers to measure power output and emissions levels. The rotary valve system provided the ability to achieve high RPM limits without the risk of valve float, delivered improved volumetric efficiency, and exhibited lower vibration and noise levels with improved power density over traditional 4-stroke valvetrains. Additionally, the total valvetrain part count and complexity was greatly reduced over poppet valve cylinder head designs.</div><div class="htmlview paragraph">This paper will discuss the goals of this exercise and the relative performance of the final prototypes.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The future potential of an opposed-piston two-stroke (OP2S) engine has attracted the attention of researchers worldwide as it offers a high thermal efficiency and power-to-weight ratio with a simple engine configuration. This engine can be used with low-carbon fuels and hydrogen to reduce greenhouse gas emissions. However, the two-stroke operation has always been limited by its low scavenging efficiency and short-circuit of fresh charge. The current work is focused on optimizing scavenging efficiency and short-circuit in a small 200 cc single-cylinder OP2S SI engine using 3-D computational fluid dynamic (CFD) simulations. The effect of four parameters, namely, area of intake ports, area of exhaust ports, and angular orientations of intake ports (swirl and tilt) on scavenging efficiency and short-circuit, has been assessed and optimized. A Latin-hypercube based Design of Experiments (DoE) methodology is used to sample the design space spanning over a range of four parameters. A response surface is generated using the Kriging method, and the geometry of intake and exhaust ports have been optimized for maximum scavenging efficiency and minimum short-circuit using a genetic algorithm on the response surface. The results show that the scavenging efficiency improves with the increase in exhaust port area, but it also increases the short circuit of fresh air. The Intake port swirl angle significantly impacts scavenging efficiency and short-circuit. The current optimization process achieved a scavenging efficiency of 85% (percentage of the total mass in the cylinder) and a short circuit of 12% (percentage of trapped fresh air). Apart from the geometric parameters, the effect of intake boost pressure and the engine speed on scavenging efficiency and short-circuit has also been evaluated.</div></div>

<div class="section abstract"><div class="htmlview paragraph">In this paper, a new method for the driving of the hydraulic free piston engine (HFPE) is proposed. Hydraulic differential drive achieves the compression stroke automatically rather than special recovery system, which has a great influence on the engine dynamic performance. The purpose of this paper is to solve the key operation and control problems for HFPE to commix fuel with air.</div><div class="htmlview paragraph">HFPE adopts two-stroke loop-scavenging and semi-direct injection. The semi-direct injection nozzle is located in the liner wall inside the main intake port, with the axes oriented towards the piston at the Bottom Dead Center (BDC). Different scavenging pressures and injection angles result in different impacts on the mixture of fuel and air in the cylinder. This study analyzes the changes of the combustion heat release rate by simulation. The result shows that the gasoline back-flow and short circuit loss is minimal, and the mixture of fuel and air is uniform, with the initial pressure of 0.13MPa and injection angle of 120°CA. Moreover, heat release of HFPE is enhanced, which complies with the requirements of the original design. The study can provide the theoretical basis for exploring the scavenging characteristics and operation mechanism of HFPE.</div></div>

<div class="htmlview paragraph"> </div> <div class="htmlview paragraph"> </div> <div class="htmlview paragraph">A computer simulation of a high performance two-stroke engine has been validated by a transient test method. The simulation included unsteady gas dynamics, together with detailed mechanical models for the reed valve, etc. The combustion model used Vibe coefficients derived directly from measured cylinder pressures. Cylinder scavenging characteristics which were measured on The Queen's University of Belfast (QUB) single-cycle rig were also incorporated in the simulation.</div> <div class="htmlview paragraph"> </div> <div class="htmlview paragraph">Validation of the model involved use of an inertial dynamometer and data acquisition system which has been developed at QUB. This dynamometer incorporates a flywheel which is directly coupled to the engine. During a test, the engine is accelerated, and the torque and power are calculated from the flywheel speed characteristics. Further, at pre-determined engine speed intervals, the cylinder and exhaust pressures may be recorded. This testing system provided a convenient and rapid experimental method to acquire data.</div> <div class="htmlview paragraph"> </div> <div class="htmlview paragraph">Measurement of power, pressures and temperatures during transient testing are discussed. Also presented are methods which reduce the errors arising from the slow response of thermocouples. The measured results are compared with the computer predictions for power and exhaust pressure. Excellent correlation was achieved.</div> <div class="htmlview paragraph"> </div>

<div class="section abstract"><div class="htmlview paragraph">Strict emission regulations and the need of higher efficiency of future dual fuel engines require an optimized combustion process. For getting a better understanding of the in-cylinder combustion process optical investigations represent a powerful tool. For medium speed dual fuel engines, optical investigations are pretty rare respectively not available. Especially the avoiding of knock events within the combustion process is a key development topic to realize high engine load and high engine efficiency.</div><div class="htmlview paragraph">For the investigations a fully flexible dual fuel test engine was used. The engine is operated with a natural gas / air cylinder charge which is ignited by a small micro pilot diesel injection within the gas mode.</div><div class="htmlview paragraph">Beside standard development measurement techniques like cylinder pressure measurement and exhaust gas emission measurement, optical measurement techniques were used to gain information about the in-cylinder combustion process of the pilot fuel combustion as well as the combustion of the natural gas / air cylinder charge. It is desirable to realize an entire visualization of the combustion within the combustion chamber under real engine conditions at full load. This means the need for a minimal invasive measurement technique with small modifications of the combustion chamber design and surface temperatures.</div><div class="htmlview paragraph">Due to these requirements a tomographic, fiber based measurement technique was used synchronal to a high speed visualization of the in-cylinder OH* chemiluminescence. The fiber based measurement technique uses 128 optical fibers in which every single fiber measures the luminescence of a special observation volume. The raw data of the single fibers is used to calculate, based on a tomographic algorithm, the in-cylinder light emission intensity distribution to visualize the combustion in a horizontal cutting plane over the whole bore diameter. To complete the information of the combustion by the third dimension, a synchronized, intensified high speed camera was used to record the OH* chemiluminescence. Both measurement techniques in combination give a far-reaching image of the in-cylinder combustion process.</div><div class="htmlview paragraph">To increase the understanding of the in-cylinder processes and to optimize the combustion process, typical combustion relevant parameters like pilot injection nozzle, pilot injection timing, pilot injection pressure and excess air ratio were varied. In this publication the basic combustion mechanisms are described for two different injection timings at full load. In addition the isolated pilot fuel combustion in pure air for an injection timing which gives a transient point between partly premixed and strongly premixed combustion is shown.</div><div class="htmlview paragraph">The measurement results show an ignition process which is strongly influenced by the reaction kinetic mechanisms. Especially for early injection timings a long ignition delay was measured which result in a well premixed pilot fuel within the natural gas / air mixture. Moreover, the flame propagation speed was calculated and a reaction kinetically controlled post flame combustion process was observed.</div><div class="htmlview paragraph">In addition a different mechanism for in-cylinder pressure oscillation, which usually is detected as engine knock, was shown which differs from the generally known mechanism in spark plug ignited passenger car engines.</div></div>

<div class="section abstract"><div class="htmlview paragraph">A quasi-dimensional dual fuel combustion model is proposed for a large 2-stroke marine engine. The introduced concept accounts for both diffusion combustion of the liquid pilot fuel and the flame front propagation throughout the gaseous premixed charge. For the pilot fuel case a common integral formulation defines the ignition delay whereas a time scale approach is incorporated for the combustion progress modeling. In order to capture spatial differences given by the scavenging process and the admission of the gaseous fuel, the cylinder volume is discretized into a number of zones. The laws of conservation are applied to calculate the thermodynamic conditions and the fuel concentration distribution. Subsequently, the ignition delay of the gaseous fuel-air mixture is determined by the use of tabulated kinetics and the ensuing oxidation is described by a flame velocity correlation. The development of particular sub-models is closely linked to corresponding 3D-CFD investigations whereas the resulting burn rates and pressure traces are validated against the experimental engine data. Furthermore, the model implementation into a commercial 1D simulation tool allows performing full cycle calculations and thus facilitates utilization for future engine development and optimization process.</div></div>

<div class="htmlview paragraph">The IAPAC® Direct fuel Injection (DI) system, developed by IFP, has already well proven its capability to reduce pollutants emissions and fuel consumption of 2-stroke engines for both 2-wheeler and marine outboard application. This crankcase Compressed Air Assisted Fuel Injection process allowing the introduction of the fuel separately from the scavenging air, minimizes the fuel short-circuiting and has shown its potential on various prototype demonstrators.</div> <div class="htmlview paragraph">This paper presents the development and pre-industrialization work performed to apply this concept to an SELVA Marine 2-cylinder 50 HP outboard 2-stroke engine. A standard carbureted engine has been converted to a IAPAC® prototype engine by mainly modifying the cylinder head. Then, this prototype engine has been calibrated, tested and optimized on the dyno test bench to comply with future emissions regulation while keeping similar power output than the reference carbureted engine. Transient operations, partially investigated on the dyno test bench, have been refined during the boat testing campaign.</div> <div class="htmlview paragraph">After this feasibility assessment study which has demonstrated more than 30% reduction in fuel consumption and the capability to comply with both IMEC Stage 2 and EPA 2006 emission regulations with sufficient margin, the pre-industrialization phase has been engaged by taking into account a better integration of the specific IAPAC® components.</div> <div class="htmlview paragraph">Final results of this IAPAC® pre-industrial D.I. 2-stroke engine are presented and also compared to its other engine counterpart technology showing its adequate application to face future stringent emissions regulations and environmental constraints while keeping its intrinsic advantages in terms of compactness, light weight and specific power.</div>

<div class="section abstract"><div class="htmlview paragraph">In recent years the automotive industry has been using an increasing number of high powered engines with fewer cylinders, with the goal to reduce weight and fuel consumption and hence to achieve lower CO2 emissions. In the following paper, an overview about the currently existing methods and products within the exhaust development is given which follow automotive lightweight trend.</div><div class="htmlview paragraph">Continuous innovations in new materials, structural design and manufacturing process as well as mastering the integration of the components and modules within the system with a thorough understanding and optimization of the system behavior is enabling the reduction of weight in exhaust system.</div><div class="htmlview paragraph">Another possibility to reduce the weight is the use of additional components such as valves. In the following, a discussion about the different types of valves is presented. These valves can be implemented within the exhaust system in order to bring a constraint in the system and consequently additional acoustic damping.</div><div class="htmlview paragraph">Due to engine downsizing, many premium vehicles lost their class-representing sound signature. An active system can be used in order to enhance the sound according to the customer demands. In addition to that, an active system can help reducing muffler volume.</div></div>