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The development and optimization of two stroke engines, especially the development of internal mixture preparation and the combustion process, require effective and reliable simulation in order to shorten the development time and to reduce prototype and test bench costs. CFD (Computational Fluid Dynamics) is a state of the art tool to optimize and visualize the fluid processes, e.g. scavenging, in-cylinder charge motion, spray formation, mixture preparation or combustion. The drawback of full 3D CFD simulation is the required time for grid generation and calculation of the model, especially for the simulations in the early development phase or in the concept phase as the available time for simulation is limited. Additionally, two stroke specific models e.g. for the reed valve, are not available in commercial 3D CFD codes. In previous investigations [SAE 2005-32-0099] the strategies and the requirements for a predictive simulation have been discussed.

For the development of high-performance 2-stroke engines with internal mixture preparation it is essential to know about the interaction between charge motion and injection spray. With no prototypes available conceptual investigations can only render such information by using 3D CFD simulation. In this way an optimization of mixture preparation and charge motion can be achieved by varying the transfer and boost ports. To allow for the influence of these modifications on the mass balance (volumetric and trapping efficiency), the entire system of the loop-scavenged two-stroke engine has to be investigated. The state of the art calculation domain for 2-stroke 3D CFD simulation is bounded at the inlet of the crankcase (reed valve) and sometimes also at the outlet of the cylinders. The reasons lie in the so far not sufficiently reproducible components (e.g. reed valve) as well as in the reduction of calculation time.

The two stroke engine has a wide range of application, especially in the field of recreational vehicles, handheld products and small two-wheelers. This is due to the advantages of the two stroke working principle: high power density, low weight, and low costs. In order to reduce the system-inherent disadvantages of the loop-scavenged two stroke engine developments using latest methods are necessary. One of these methods is the CFD simulation of the scavenging process, the high pressure cycle and the injection process. Reliable predictive simulation in the early development phase of a new engine is required to shorten the development time and to reduce prototype and test bench costs. In previous investigations (1) [SAE 2005-32-0099; JSAE 20056552] the strategies for the simulation and the requirements for a predictive simulation were discussed. Finally a new methodology which bases on the combination of 3-dimensional (3D) and 0/1-dimensional (0/1D) CFD simulation was presented.

The wide variety of applications of the loop scavenged 2-stroke engine is based on 3 advantages which emerge from the 2-stroke working principle: the high power density, the low weight, and the low production costs. An important aim of research activities in the field of 2-stroke engines is to optimize these advantages while minimizing the known disadvantages of high emissions and fuel consumption. Important tasks of the research work within the development process are the prediction of power and emissions of engine concepts and the simulation with special regard to the scavenging process and the high pressure cycle. In this area of research two state of the art simulation approaches exist. The first one is a detailed simulation of the scavenging and combustion process which is necessary to understand and optimize the fundamentals of the 2-stroke engine.

The aim of the paper is to define a concept which has the potential to achieve and exceed the fuel economy target of 100 km/litre. Several existing vehicles based on different engine and vehicle concepts have been analysed for a benchmark study of the present best vehicles on the Indian 2-wheeler market. Additional to this study, more detailed investigations have been made to evaluate the potential of each individual loss in the drive train from the combustion over friction to the drive train, chassis and tyre specification. Out of these results four different engine and vehicle concepts have been synthesised and indicate how a 100 km/ltr in steady state driving could be achieved.

The market for handheld engines such as chainsaws, brush-cutters and for 2-wheeler applications like scooters, mopeds and minimum transportation vehicles shows a strong tendency towards 4-stroke application due to environmental and image reasons. The ranking of the priorities like Weight - Cost - Performance in these engines differs and beside this the fulfillment of the legal requirements for pollutant emission, noise and the expected fuel economy lead to a different approach for the layout of the cylinder-heads showing different optimum solutions dependant on the casting technology, whether pressure die cast or with sand cores. The variations include the number of valves from two to four and the arrangement of the valves and spark plugs. The porting and combustion chamber layout, including data for flow rate and charge movement concepts, are stipulated for the cylinder bore of 41 mm.