Browse Topic: Scavenging
The introduction of real driving emission measurements increases the need of improved transient engine behavior while keeping the emissions to a minimum. A possible way of enhancing the transient engine behavior is the targeted usage of scavenging. Scavenging is realized by an inlet- and exhaust-valve overlap. Fresh scavenging air flows directly from intake manifold through the cylinder into the exhaust manifold. Therefore, the mass flow at the turbine increases and causes a reduced turbo lag, which results in a more dynamic engine behavior. The unburned oxygen causes a decrease of the three-way catalyst (TWC) conversion rate. To keep the TWC operation close to stoichiometry, a rich combustion is performed. The rich combustion products (most notably carbon monoxide) mix in the exhaust manifold and react with oxygen so that the conversion rate of the TWC is ensured. In order to investigate the potential and risks of this engine operating strategy, a reliable 1D engine model is necessary. This work deals with the description of the most important aspects of the post-oxidation phenomenon and with the development of a 1D post-oxidation model, based on detailed 3D-CFD simulation results including a reaction mechanism. The 3D-CFD simulation permits a deep insight on the mixing effects inside the exhaust manifold (interaction of all four cylinders) and the resulting chemical reactions. The 1D post-oxidation model is capable of making a statement on the amount of scavenging air, which can be burned inside the exhaust manifold before reaching the TWC. The modelling approach relies on the mixing effect inside the manifold and on a chemical conversion of the emissions.
The efficiency and emission potential of pre-chamber combustion in a Miller cycle light duty gasoline engine operated under part load was evaluated. Several pre-chamber designs that examine the engine performance tradeoffs with nozzle diameter, pre-chamber volume, number of nozzles, and pre-chamber fuel enrichment were investigated for both excess air and cooled external EGR dilution strategies. The introduction of pre-chamber jet ignition was observed to significantly reduce the main-chamber combustion duration while reducing cyclic variability under dilute conditions, benefiting from the long-reach ignition jets and enhanced turbulence. However, the pre-chamber design that provided the fastest combustion led to reduced brake efficiency primarily due to increased wall heat loss. Maintaining the total nozzle area while increasing the number of nozzles was identified as a means to minimize the additional heat loss and maintain fast burn rates. In addition, fuel enrichment within the pre-chamber was observed to extend the lean limit while greatly reducing engine-out NOx emission dependent on specific pre-chamber geometry. Specifically, for the cooled EGR calibration strategy, the engine’s dilution tolerance was determined to be primarily affected by the scavenging performance of the passive pre-chamber. For all designs evaluated, the inferior scavenging and increased heat loss associated with passive pre-chamber resulted in similar efficiency when compared to a well-designed spark-ignition engine operating under part-load conditions and with cooled EGR dilution.
A 2-stroke boosted uniflow scavenged direct injection gasoline (BUSDIG) engine was researched and developed at Brunel University London to achieve higher power-to-mass ratio and thermal efficiency. In the BUSDIG engine concept, the intake scavenge ports are integrated to the cylinder liner and controlled by the movement of piston top while exhaust valves are placed in the cylinder head. Systematic studies on scavenging ports, intake plenum, piston design, valve opening profiles and fuel injection strategies have been performed to investigate and optimise the scavenging performance and in-cylinder fuel/air mixing process for optimised combustion process. In order to achieve superior power performance with higher thermal efficiency, the evaluation and optimisation of the boost system for a 1.0 L 2-cylinder 2-stroke BUSDIG engine were performed in this study using one dimensional (1D) engine simulations. The results show that the engine exhaust valve opening (EVO) timing and exhaust duration (ED) are key parameters affecting the engine performance with the single-stage turbocharging (T). By using an earlier EVO timing of 80 0CA and a longer ED of 140 0CA, a maximum brake power of 130.7 kW could be achieved at 3200 rpm and peak torque output of 488 N*m at 1600 rpm. Simulations were also performed to evaluate the engine performance with combined boost systems with a supercharger upstream the turbocharger (S-T) and a turbocharger upstream the supercharger (T-S). The results indicate that the combined boost systems increase both engine power and torque compared to the single-stage turbocharging system. In particular, the peak brake power and torque of the 1.0 L BUSDIG engine could reach 143.7 kW at 4000 rpm and 492 N*m at 800 rpm with the S-T setup.
Two-stroke opposed piston engines (2sOPEs) have great potential for industrial applications due to their simple design, technology and high efficiency, particularly with a turbocharging system. The paper presents possibilities for altering 2sOPE working parameters by changing geometrical parameters and boosting parameters. Obtaining higher engine efficiency is realised by altering the crank phase shift of the exhaust piston in relation to the transfer piston. It has been assumed that only the piston of the exhaust cylinder changes its position relative to the piston in the cylinder with transfer ports. Modifying the scavenging process by changing pistons’ position through connecting with two crankshafts enables asymmetrical scavenging timing. Closing the exhaust ports before the compression process and extending the time allotted to empty exhaust gases from the cylinder provides greater engine work, and a high boost ratio increases engine power. This type of engine was recently recommended for power plant stations. The paper includes mathematical modelling of thermodynamic parameters of 2sOPE and full analysis of engine work with scavenging and combustion processes for different timing phases. This endeavour is based on the geometry of the compression ignition Leyland L60 engine and uses the author’s own 0-1-D computer program and considering unsteady gas flow and computational fluid dynamics (CFD) modelling. Simulation tests indicate a high scavenging efficiency, good penetration of injected fuel and fast combustion process. The work contains figures of pressure, temperature traces and emissions of the main chemical species in exhaust gases, with comparisons of engine works for different timing phases. Applying a non-symmetrical scavenging process in a 2sOPE by closing exhaust ports earlier increases working parameters. Despite a shorter expansion time, the expansion work of this engine is larger when the exhaust port is opened earlier rather than later. A 2sOPE with earlier opening of exhaust ports decreases nitric oxide emission due to a lower combustion temperature. This study is a contribution for the future realisation of such processes in power plant engines with different fuelling systems
A computational fluid dynamics study of the scavenging process in a large two-stroke marine engine is presented in this work. Scavenging which is one of the key processes in the two-stroke marine engines, has a direct effect on fuel economy and emissions. This process is responsible for fresh air delivery, removing the combustion products from the cylinder, cooling the combustion chamber surfaces and providing a swirling flow for better air-fuel mixing. Therefore, having a better understanding of this process and the associated flow pattern is crucial. This is not achievable solely by experimental tests for large engines during engine operation due to the difficulties of measuring the flow field inside the cylinder. In this study, the axial and tangential velocities are compared and validated with the experimental results obtained from Particle Image Velocimetry (PIV) tests [1]. The simulations are conducted using both Unsteady Reynolds Averaged Navier Stokes (URANS) and Large Eddy Simulation (LES) turbulence models. We observe in general, there is a good agreement between the numerical and experimental results. The flow inside the cylinder is studied in different locations related to the bottom of the scavenging ports during the period with open exhaust valve. Moreover, the replacement of combustion products with fresh scavenge air is analysed. The effective flow angle is calculated for the air flow through the scavenging ports. It is found that the effective flow angle is different from the geometrical angle of the ports (20°). Results illustrate better performance of LES, especially in the prediction of the tangential velocity which is crucial for the simulation of an accurate swirl and air-fuel mixing inside the marine engines. LES predicts a uniform profile for the tangential velocity at the top of cylinder which is consistent with the experimental results while URANS predicts a solid body rotation.
Two-stroke engines have to face the problems of insufficient charge for short intake time and the loss of intake air caused by long valve overlap. In order to promote the power of a two-stroke poppet valve diesel engine, measures are taken to help optimize intake port structure. In this work, the scavenging and combustion processes of three common types of intake ports including horizontal intake port (HIP), combined swirl intake port (CSIP) and reversed tumble intake port (RTIP) were studied and their characteristics are summarized based on three-dimensional simulation. Results show that the RTIP has better performance in scavenging process for larger intake air trapped in the cylinder. Its scavenging efficiency reaches 84.7%, which is 1.7% higher than the HIP and the trapping ratio of the RTIP reaches 72.3% due to less short-circuiting loss, 11.2% higher than the HIP. The RTIP also behaves better in mixture formation and combustion performance with higher air utilization and superior heat release as well as work capacity. The CSIP has the advantage of higher thermal efficiency but have to face the challenge of high maximum pressure rise rate as the HIP. For the CSIP, the introduction of helical intake manifold makes little difference on this two-stroke poppet valve diesel engine.
Low-speed two-stroke dual-fuel engines has been paid more attention due to the energy efficiency design index and Tier III emissions limitations issued by International Marine Organization. Although the dual-fuel engines have strong merits on emissions reduction, which can reach the IMO Tier III without aftertreatment, the power output is much lower than that of diesel engines. Therefore, the dual-fuel engine is also needed to improve continuously. However, the mixing and combustion processes in the engine have not been fully understood. In this study, a 3D-CFD model of the dual-fuel engine was established using CONVERGE to explore the mixing and combustion processes. Locally embedding fine grids are considered at scavenging ports, natural gas injection ports, pre-chamber. The model was validated by experimental in-cylinder pressure. Then, the flow motion, mixing of natural gas and air, flow in pre-chamber, torch and combustion in main-chamber were analyzed based on swirl variation, flow velocity distribution, equivalence ratio distribution and torch propagation.
In this research, a novel methodology for the post-oxidation in a turbocharged spark ignition (SI) engine is proposed and investigated that can improve the emissions along with the reduction in turbo-lag. In this research, both simulation and experimental activities are performed. The 1-D simulation model was used for the identification of efficient scavenging. Thereafter, experimental validation tests for modeling and post oxidation were conducted on a 4-cylinder turbocharged SI engine. From the results, it was revealed that efficient scavenging and post-oxidation can be obtained at lower speed and higher load. The enthalpy in exhaust manifold increased due to the post-oxidation reaction which in turn increased the temperature and pressure of the exhaust gases and hence emissions reduced. Also, due to the increased enthalpy at turbine upstream, the turbocharger speed increased and as a consequence, reduction in the turbo-lag was observed. It was also noted that the post-oxidation is limited at higher load and overlap in an inline 4-cylinder engine due to the strong scavenging which increased the cooling effect in in-cylinder and exhaust manifold due to excess air.
Modern injection systems are characterized by low cost, light weight and diversified components based on a mature technology. In addition, the constant growth of computational resources allows an in-depth understanding and control of the injection process. In this scenario, increasing interest is presently being paid to understand if an application of such technologies to small two-stroke engines could lead to a return to popularity in place of the more widespread use of the four-stroke engine. Indeed, the possibility of achieving a drastic reduction of both specific fuel consumption and pollutant emissions would completely reverse the future prospect of the two-stroke engine. The authors in previous studies developed a low pressure direct injection (LPDI) system for a 300 cm3 two-stroke engine that was ensuring a performance consistent with a standard four-stroke engine of similar size. The main drawbacks of the system were the large time required for delivering the fuel and the incomplete vaporization in some working conditions, due to the large size of the injected droplets. In this study, the use of a single high pressure injector with an operating pressure of 100 bar was analyzed. An optimization study was carried out in order to identify the best injector configuration for the GDI system. The results of the preliminary 3-D CFD study are here reported. The effect of the injector positioning and injection timing on the spray vaporization, mixture homogenization and fuel short-circuit was evaluated at different engine operating points. The results will show that also in case of a high pressure injection the best performance can be obtained when a suitable interaction between the liquid jet of fuel and the flow of scavenging air is ensured, as well as with the appropriate choice of the injection timing.
This paper examines the energy pathways of a 29cc air-cooled two-stroke engine operating on natural gas with different exhaust geometries. The engine was operated at wide-open-throttle at a constant speed of 5400 RPM with ignition adjusted to yield maximum brake torque while the fueling was adjusted to examine both rich and lean combustion. The exhaust configurations examined included an off-the-shelf (OTS) model and two other custom models designed on Helmholtz resonance theory. The custom designs included both single and multi-cone features. Out of the three exhaust systems tested, the model with maximum trapping efficiency showed a higher overall efficiency due to lower fuel short-circuiting and heat transfer. The heat transfer rate was shown to be 10% lower on the new designs relative to OTS model. The fuel slip rate was in the range of 20-30% with custom designs showing 15% higher fuel slip rates on average, whereas the exhaust thermal energy was in the range of 12-18% of total input fuel energy and found to be 20% lower on custom designs relative to OTS model. In addition to energy pathways, various exhausts impacted the scavenging efficiency and trapped mass during the gas exchange process. Even though the delivery ratio and trapping efficiency varied between the new exhaust designs, they showed a similar increase in volumetric efficiency relative to the OTS model - 60% on average.
The paper presents a numerical investigation, aimed to explore the potential of 2-stroke Diesel engines, able to meet Euro VI requirements, for application to medium size commercial vehicles (power rate: 80 kW at 2600 rpm, max. torque 420 Nm from 1200 to 1400 rpm). The study is based on experimental performance of a highly developed 4-stroke engine. Two different designs are considered: Loop and Uniflow scavenging, the latter obtained through an opposed piston configuration. In both cases, no poppet valves are used, and the lubrication is provided by a 4-stroke-like oil sump. The study started with the development of a 4-stroke EURO VI engine, on the basis of a previous EURO IV version. A prototype of the new engine (named 430) was built and tested. The second phase of the study consisted in the comparison to the 2-stroke configurations, considering the same performance and emissions targets, as well as the same constraints Engine outputs are calculated by using GT-Power models: while for the 4-stroke unit these results are fully supported by experimental data, the 2-strokes are just “paper” engines. However, the CFD-1D modeling was supported by other detailed numerical simulations, including both scavenging and combustion analyses. The two stroke concepts analyzed in the paper appear to yield several advantages, in comparison to their 4-stroke counterpart: reduced fuel consumption, cleaner combustion conditions, more compact dimensions, higher flexibility of the EGR control. On the other hand, they require a strong effort for the development of a specific combustion system.
With the introduction of CO2 emissions legislation in Europe and many countries, there has been extensive research on developing high efficiency gasoline engines by means of the downsizing technology. Under this approach the engine operation is shifted towards higher load regions where pumping and friction losses have a reduced effect, so improved efficiency is achieved with smaller displacement engines. However, to ensure the same full load performance of larger engines the charge density needs to be increased, which raises concerns about abnormal combustion and excessive in-cylinder pressure. In order to overcome these drawbacks a four-valve direct injection gasoline engine was modified to operate in the two-stroke cycle. Hence, the same torque achieved in an equivalent four-stroke engine could be obtained with one half of the mean effective pressure. A wet sump was employed to avoid the inherent lubrication and durability issues of conventional two-stroke engines, and the scavenging process was ensured via external boosting. The adoption of direct fuel injection removed the problem of fuel short-circuiting present in mixture scavenged engines. Several loads were tested at 800 rpm and 1600 rpm and the overall engine performance was presented. Gaseous and smoke emissions were measured and examined, as well as an analysis of the spark ignition combustion process. The results demonstrated that very high torque at low engine speeds could be obtained at relatively low in-cylinder pressures and reasonable fuel consumption results.
CNG direct injection is a promising technology to promote the acceptance of natural gas engines. Among the beneficial properties of CNG, like reduced pollutants and CO2 emissions, the direct injection contributes to a higher volumetric efficiency and thus to a better driveability, one of the most limiting drawbacks of today’s CNG vehicles. But such a combustion concept increases the demands on the injection system and mixture formation. Among other things it requires a much higher flow rate at low injection pressure. This can be only provided by an outward-opening nozzle due to its large cross-section. Nevertheless its hollow cone jet with a specific propagation behavior leads to an adverse fuel-air distribution especially at higher loads under scavenging conditions. This paper covers numerical and experimental analysis of CNG direct injection to understand its mixture formation. For this purpose experimental investigations were carried out by the Robert Bosch GmbH using a two-cylinder SI engine at a high load operating point with high scavenging degree. To understand the mixture phenomena the test-bench activities were supported by numerical simulations with the 3D-CFD-tool QuickSim at the FKFS. The experiments included various injection timings and valve overlaps. Additionally, the tests were performed with two different nozzle concepts (outward- and inward-opening injector) to identify the influence of the jet shape on the fuel-air distribution. The simulations also contained these parameters and particularly considered the jet development and flow field in the combustion chamber and the intake port. The test-bench investigations revealed a close dependence of the mixture formation on the injection timing and jet characteristic during scavenging operation. The associated numerical studies resulted in a good agreement with the engine performance and led to a conclusive interpretation of the observed phenomena.
To achieve more stringent exhaust emission regulations will face more and more daunting challenges nowadays. It needs more new technologies to improve the IC engine performance but needing higher costs in order to meet Euro 6 and EPA standards in USA. Recently the opposed-piston engine (OPE) has been treated as the promising product to meet these new regulations but relatively lower costing. Although two-stroke OPE owning inherent thermal efficiency and power density advantages, the inefficient scavenge efficiency appears to become the main obstacle to enhance combustion efficiency whilst reducing exhaust gas emission. For the improvement of scavenge efficiency the transient gas exchange simulation was carried out for multiple Cases here, including two intake port configurations at various back pressures in exhaust system and two port timings. The effects of exhausting back pressure, porting timing and intake port layout on scavenging and trapped air mass in cylinder all were investigated by transient CFD simulation including blow-down and scavenging. The calculated results showed that for Case02 that intake port entrance orientation with a right tilt angle referred to baseline, the scavenge efficiency is relatively higher than one with left tilt angle in Case01 for different exhausting back pressures, also for trapped fresh air mass. The turbulence kinetic energy is extremely sensitive to back pressure in exhaust chamber. The investigation was also found that the port timing is also quite important for scavenging process and pump losses, and there will be a little increase of scavenge efficiency by about 2-3% via adjusting port timing for both Cases, and the net trapped masses are approximate to each other but with less fresh air leakage for new port timing. Apparently, the exhaust back pressure will impose much more considerable influence on the entire scavenging performance than the port timing, as using uniflow scavenge mode in a two-stroke opposed piston diesel engine.
Interest in 2-stroke engines has been recently renewed by several prototypes, developed for the automotive and/or the aircraft field. Loop scavenging, with piston controlled ports is particularly attractive, but the configurations successfully developed in the past for motorbike racing (in particular, the 125cc unit displacement, crankcase pump engines), are not suitable for automotive applications. Therefore, new criteria are necessary to address the scavenging system design of the new generation of 2-stroke automobile/aircraft engines. The paper reviews the transfer ports optimization of a loop scavenged 2-stroke cylinder, whose main parameters were defined in a previous study. The optimization has been carried by means of a parametric grid, considering 3 parameters (2 tilt angles, and the focus distance), and 3 different engine speeds (2000-3000-4000 rpm, assuming a Diesel engine). A set of scavenging CFD-3d simulations have been performed by using a customized version of KIVA-3V. The numerical approach was experimentally calibrated in a previous project (see appendix 1) The simulations results are presented by means of maps showing the influence of the geometrical parameters on the main scavenging coefficients. Finally, a refined mesh has been constructed for the optimum configuration found in the previous parametric analysis, and a set of multi-cycle simulations have been performed. The results demonstrated the very good efficiency of the scavenging process, close to a perfect displacement for delivery ratio up to 1.5, or for residuals fraction higher than 50%
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. By means of a new energetic analysis (available technical work capacity) the energetic contribution of the recompression to the boost pressure generation has been examined and is presented in this paper. Furthermore two different variable exhaust valve train systems in combination with a part-scroll-separation exhaust manifold are compared in this paper. The aim is to reduce fuel consumption at the nominal power. The two exhaust valve train systems increase the valve opening duration by either a two step system or by a system with the ability to offset the valve timing. It is shown in simulation results how both systems in combination with a prolonged part-scroll-separation in the exhaust manifold reach a potential to reduce fuel consumption up to 10 %.
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. 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.
Air charge calibration of turbocharged SI gasoline engines with both variable inlet valve lift and variable inlet and exhaust valve opening angle has to be very accurate and needs a high number of measurements. In particular, the modeling of the transition area from unthrottled, inlet valve controlled resp. throttled mode to turbocharged mode, suffers from small number of measurements (e.g. when applying Design of Experiments (DoE)). This is due to the strong impact of residual gas respectively scavenging dominating locally in this area. In this article, a virtual residual gas sensor in order to enable black-box-modeling of the air charge is presented. The sensor is a multilayer perceptron artificial neural network. Amongst others, the physically calculated air mass is used as training data for the artificial neural network. The air mass calculation is performed by taking into account valve timing, effective valve cross-sectional area and low-pressure indication at intake and exhaust manifold. It can be shown that by applying the virtual sensor, a global black-box-model of the air charge can be built. Furthermore, the sensor enables to reduce the required number of measurements by DoE and at the same time to maintain good modeling results. The global air charge model can be used to derive virtual measurements for the air charge calibration.
With Advanced PFI, Bosch has demonstrated that gasoline port-fuel injection is becoming significantly more energy-efficient by means of innovative system development. Advanced PFI combines fuel pressure increase, twin injection, PFI scavenging, and open valve injection. The use of Advanced PFI makes it possible to reduce consumption by 12%, with a corresponding decrease in CO2 emissions. The higher compression in the part-load range alone accounts for 2% of the reduction. The remaining 10% come from downsizing with turbocharging and PFI scavenging. At the same time, Advanced PFI allows a reduction in hydrocarbons (HC) emissions. Thanks to more homogenous air-fuel mixture formation and reduced manifold wall fuel condensation, HC emissions fall by 20% in the test cycle. And Advanced PFI also permits an increase in specific engine power output, with PFI scavenging achieving significantly higher low-end torque.
In this paper, a new-type balanced opposed-piston two-stroke (OP2S) gasoline direct injection (GDI) engine is developed by Beijing Institute of Technology. OP2S-GDI engine has some potential advantages such as simple structure, good balance, compact, high power density and thermal efficiency. The structural feature of OP2S-GDI engine leads to the performance difference compared with conventional engines. In order to study and verify the characteristics of this kind of engine, the dynamics characteristics and design scheme of opposed crank-connecting rod mechanism, in-cylinder scavenging process, mixture formation and combustion process are investigated. The influence of parameters on engine performance is investigated, including opposed-piston motion phase difference, intake and exhaust port timing, injection and ignition timing. In addition, the arrangement form of opposed crank-connecting rod mechanism is investigated as a main factor to affect the integral forced status and system balance for OP2S-GDI engine.
In a turbocharged engine, preserving the maximum amount of exhaust pulse energy for turbine operation will result in improved low end torque and engine transient response. However, the exhaust flow entering the turbine is highly unsteady, and the presence of the turbine as a restriction in the exhaust flow results in a higher pressure at the cylinder exhaust ports and consequently poor scavenging. This leads to an increase in the amount of residual gas in the combustion chamber, compared to the naturally-aspirated equivalent, thereby increasing the tendency for engine knock. If the level of residual gas can be reduced and controlled, it should enable the engine to operate at a higher compression ratio, improving its thermal efficiency. This paper presents a method of turbocharger matching for reducing residual gas content in a turbocharged engine. The turbine is first scaled to a larger size as a preliminary step towards reducing back pressure and thus the residual gas concentration in-cylinder. However a larger turbine causes a torque deficit at low engine speeds. So in a following step, pulse separation is used. In optimal pulse separation, the gas exchange process in one cylinder is completely unimpeded by pressure pulses emanating from other cylinders, thereby preserving the exhaust pulse energy entering the turbine. A pulse-divided exhaust manifold enables this by isolating the manifold runners emanating from certain cylinder groups, even as far as the junction with the turbine housing. This combination of appropriate turbine sizing and pulse-divided exhaust manifold design is applied to a Proton 1.6-litre CamPro CFE turbocharged gasoline engine model. The use of a pulse-divided exhaust manifold allows the turbine to be increased in size by 2.5 times (on a mass flow rate basis) while maintaining the same torque and power performance. As a consequence, lower back pressure and improved scavenging reduces the residual concentration by up to 43%, while the brake specific fuel consumption improves by approx. 1%, before any modification to the compression ratio is made.
The paper compares two different design concepts for a range extender engine rated at 30 kW at 4500 rpm. The first project is a conventional 4-Stroke SI engine, 2-cylinder, 2-valve, equipped with port fuel injection. The second is a new type of 2-Stroke loop scavenged SI engine, featuring a direct gasoline injection and a patented rotary valve for enhancing the induction and scavenging processes. Both power units have been virtually designed with the help of CFD simulation. Moreover, for the 2-Stroke engine, a prototype has been also built and tested at the dynamometer bench, allowing the authors to make a reliable theoretical comparison with the well assessed 4-Stroke unit. Even if the optimized design of each one of the two engines is similar to that of existing prototypes, the paper is not intended to be a benchmarking, but a general study, aimed to define the fundamental project guidelines and compare different solutions under the same conditions, including the unavoidable arbitrary hypotheses. The main results of the comparison may be summarized as follows: the 2-Stroke engine is more compact and light (−38% of frontal area, 35 vs. 50 kg); its fuel efficiency is slightly better, and further improvements are possible running on stratified charge; the reduction of NOx in the 2-S catalyst may not be complete, due to the unavoidable air short-circuit.
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