Conventional Two-stroke Engines Research Articles

Low emission stepped piston engine for marine outboard motor application

Separating the scavenging process from the lubrication methodology in conventional two-stroke engines is an effective method of reducing emissions. Segregation of scavenging by use of stepped pistons is one such method providing significant improvements in engine durability, and reduced lubricant consumption, together with reduced production cost. The study discusses the approaches used together with those applied to conventional engines. This paper focusses on a marine outboard motor application and cross-comparisons with a comparable four-stroke outboard motor are drawn within the study. One-dimensional computational fluid dynamics models of the engine types have allowed performance and emissions simulations. Where possible, data from experimental dynamometer testing is compared with the theoretical models. A reasonable level of correlation in terms of performance has been observed. Two-stroke cycle engines are able to demonstrate reduced oxides of nitrogen emissions when compared with conventional four-stroke engines. Models of the stepped piston engine have confirmed the lower NOx emission evident with operation on the two-stroke cycle.

Study of Hybrid Propulsion Systems for Lower Emissions and Fuel Saving on Merchant Ship during Voyage

The hybrid propulsion system is a vehicle propulsion system with two or more sources of propulsion such as diesel, batteries, and other renewable energy. The use of hybrid propulsion systems is not new, and they have been adopted worldwide. There are many applications for hybrid propulsion systems, from powering land-based vehicles to marine vessels. With the success of hybrid propulsion systems, many large vessels still rely on two-stroke engines for power generation. In this paper, the benefits and feasibility of using battery systems or hybridizing existing conventional two-stroke engines onboard large intercontinental vessels is studied and simulated. The merchant ship, a bulk carrier with 82,000 deadweight tonnage capacity, is used. The model vessel travels along a simulated route of up to 4000 nautical miles using weather routing software to generate data for estimating emissions with the help of widely adopted empirical formulas. A reduction of up to 3.4 tonnes of emissions and fuel consumption of not more than 1 tonne per kWh from peak shaving and boosting applications for a 12-day voyage were observed.

A Study of the Scavenging Process in a Two-stroke Free Piston Linear Engine using CFD

Unlike the conventional two-stroke engine, a two-stroke free piston linear engine can change compression ratio due to free motion of a piston in a cylinder. The free motion of the piston directly affects the gas exchange process or scavenging process of the free piston engine which is key to improving the engine performance. In this study, a numerical simulation using computational fluid dynamics (CFD) is conducted to examine the scavenging process taking place in a two-stroke free piston linear engine. The motion of the free piston is built first based on a dynamic model to define the motion profiles of the piston, and afterward, the piston’s motion profiles are imported into a commercial CFD software (Ansys Fluent v.14) to simulate the scavenging process. The simulation results in piston motion and in-cylinder pressure are compared with experimental results to validate the presented simulation. To provide information for this study, some key parameters such as operating frequency of the piston, exhaust port distance, and inlet pressure are changed to find out their effects on the trapping efficiency. The simulation results show that the trapping efficiency has a maximum value of 83% at optimal operating frequency f=33Hz. Besides, by increasing exhaust port distance and reducing inlet pressure, the trapping efficiency is increased.

The two-stroke poppet valve engine. Part 2: Numerical investigations of intake and exhaust flow behaviour

A two-stroke poppet valve engine is developed to overcome the common problems in conventional two-stroke engine designs. However, replacing piston control port with poppet valve will resulted different flow behaviour. This paper presents the model and simulation result of three-dimensional (3D) port flow investigation of a two-stroke poppet valve engine. The objective of the investigation is to conduct a numerical investigation on port flow performance of two-stroke poppet valve engine and compare the results obtained from the experimental investigation. The model is to be used for the future numerical study of the engine. The volume flow rate results have been compared with the results obtained experimentally as presented in first part of this paper. The model has shown good agreement in terms of the flow rate at initial and final valve lifts but reduced by about 50% during half-lift region.

The two-stroke poppet valve engine. Part 1: Intake and exhaust ports flow experimental assessments

A two-stroke poppet valve engine is developed to overcome the common problems in conventional two-stroke engine designs. However, replacing piston control port with poppet valve will resulted different flow behaviour. This paper is looking at experimental assessment on a two-stroke poppet valve engine configuration to investigate the port flow performance. The aims are to evaluate the intake and exhaust coefficient of discharge and assess the twostroke capability of the cylinder head. The results has shown comparable coefficient of discharge values as production engine for the intake while the exhaust has higher values which is favourable for the two-stroke cycle operation.

Single-cylinder 125 cc Stepped-piston Engine for Mobility and Portable Power Generation Applications

Two-stroke engines are far simpler than four-stroke engines from a physical perspective. For a given brake output, two-stroke engines are lighter, easier to work on, and provide higher power-to-weight ratio than four-stroke engines, making them suitable for small platform applications. However, conventional two-stroke engines have a reputation for generating smoke and unburned fuel, meaning they may not meet many emissions regulations, now enforced around the world. Thus, for many decades two-stroke engines have not been not favored, giving way to four-stroke engines for dominant applications, especially for mobile power-generation purposes. In the quest to improve the potential of such an engine, a group of researchers from the Automotive Development Centre (ADC), Universiti Teknologi Malaysia (UTM), has developed a 125 cc, air-cooled stepped-piston engine to demonstrate the higher power-to-weight ratio feature, apart from overcoming emission reduction. The engine is designed to mitigate the problem of mixture short circuiting, which is the major hindrance to combustion efficiency. To this end, they have incorporated a three-port stratification strategy into the engine. This paper provides an overview related to the earlier work done to integrate the necessary features and highlights some of the performance features of this unique engine design.

Optimisation of boosting strategy for controlled auto-ignition combustion in a four-valve camless gasoline direct injection engine running in two-stroke cycle

Due to their ability to simultaneously reduce fuel consumption and NOx emissions, controlled autoignition or homogeneous charge compression ignition combustion processes have been extensively researched over the last decade and adopted on prototype gasoline engines. Despite the fact that they were initially achieved on conventional two-stroke ported gasoline engines, there have been much fewer studies carried out on controlled autoignition combustion in two-stroke engines because of the inherent problems associated with the conventional two-stroke engine with intake and exhaust ports. In the meantime, engine downsizing has been actively researched and developed as an effective means to improve the vehicle’s fuel economy by operating the engine at higher load and more efficient regions and by reducing the number of cylinders. But the aggressive engine downsizing of the current four-stroke gasoline engine is limited by the knocking combustion and high peak cylinder pressure. As an alternative approach to engine downsizing, the boosted two-stroke cycle in a poppet valve engine is proposed and researched. It has been shown that the controlled autoignition combustion in the two-stroke cycle could be readily achieved and it led to significant reduction in carbon monoxide and unburned hydrocarbon emissions than the spark ignition combustion. In this study, extensive engine experiments have been performed to determine the optimum boosting for minimum fuel consumption in a single-cylinder gasoline direct injection camless engine operating in the two-stroke cycle. In order to minimise the air short-circuiting rate, the intake and exhaust valve timings were adjusted. Lean boost was applied to the engine operation, which was found to extend the range of controlled autoignition combustion, result in higher combustion and thermal efficiencies and significantly lower carbon monoxide and hydrocarbon emissions. By means of the cycle-resolved in-cylinder measurements and heat release analysis, the improvement in combustion and thermal efficiencies was attributed to the improved in-cylinder mixture, optimised autoignition and combustion phases.

Design Considerations of an Airborne Free Piston Compressor

The effect of the atmospheric conditions on the performance of a spark ignition free piston engine has been investigated. A detailed computer code has been developed to simulate the unit cycle and was used to evaluate the effect of its geometrical proportions and the ambient conditions on the cycle performances. The model includes a semi-empirical model for the scavenging process, calculations of the flows into and out of the cylinders, empirical expressions for the combustion process and a practical approach for the heat transfer. The model has been calibrated by using experimental results of a conventional two-stroke engine having identical cylinder and port assemblies. It was concluded that the thermodynamic efficiency of the unit is strongly dependent on the receiver to the atmospheric pressure ratio, the compression ratio of the compressor, the compressor aspect ratio and the ambient pressure. It was shown that the thermodynamic efficiency varies insignificantly with the ambient temperature.