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Stratified concepts offer a reduction of the scavenging losses of 2-stroke engines. STIHL has developed a stratified reed valve engine to comply with Phase2 emissions, introduced into the market in the new STIHL Cutquik™ TS 700. The engine with a displacement of 98,5 cm3 offers performance and product specifications that exceed its conventional predecessor. The results of the engine optimization process including flow simulation, concept evaluation and combustion process development are presented. The paper focuses on the design specifications of the air head carburetor and the specific layout of the reed valve scavenging process. Further, a cyclone type air filter system is presented. The engine today meets the FEL 72 g/kWh limit. The concept offers further potential to improve trapping and volumetric efficiency, enabling a power and torque increase as well as emission control for the future.

When developing effective exhaust emission reduction measures, a better understanding of the complex working cycle in crankcase scavenged two-stroke gasoline engines. However, in a two-stroke gasoline engine detailed measurement and analysis of combustion data requires significantly more effort, when compared to a lower speed four-stroke engine. Particularly demanding are the requirements regarding the high speed (>10,000 rpm) which inevitably goes along with heavy vibrations and high temperatures of the air cooled cylinders. Another major challenge to the measuring equipment is the increased cleaning demand of the optical sensor surface due to the two-stroke gasoline mixture. In addition, the measuring equipment has to be adapted to the small size engines. Therefore, only a fiber optical approach can deliver insight into the cylinder for analyzing the combustion performance.

The quality of mixture formation and the combustion process is of significant importance for reducing the hydrocarbon emissions of small two stroke engines. The scope of this work was to investigate the mixture conditions after the exhaust closes and after the end of combustion depending on various engine operating points. For this experimental investigation a Gas Sampling Valve (GSV) was combined with a flame ionisation detector (FID) and a CO2-analyser. Using this technique, it was possible to measure the hydrocarbon concentration after end of combustion. Furthermore the local residual gas concentration after exhaust closes was determined. To allow for a comparison of the experimental results with calculations with CFD codes, in cylinder pressure measurement and exhaust gas measurements are done additionally.

This paper reviews the current and new engine technologies that are suitable to be implemented in light-weight outdoor power equipment that is mostly powered by two-stroke engines. It gives insight in the concerns associated with the improvement of known technologies and highlights trends for future engine developments. Among others, the paper covers ways for the conventional scavenging improvement, catalyst development, low pressure mixture injection, direct fuel injection, stratified scavenging and also four-stroke technology. These concepts are assessed with respect to performance, cost, size and weight. Special emphasis is laid on the research and development of a catalyst concept for high-performance two-strokes and direct fuel injection. The catalyst is one of the easiest and most effective ways for emission reduction. However, measures have to be found to overcome the severe thermal difficulties.

This paper presents the results and conclusions from a conceptual design study for a stratified charging concept. The stratified charging engine has a displacement of 64 cm3 and a four-port design with reed valves at two ports to control the bypass air. During the intake stroke of the engine, bypass air is sucked into the transfer ports to purge the air-fuel mixture present in the ports from the previous stroke back into the crankcase. This fills the transfer ports near the exhaust with pure air at the beginning of the scavenging stroke. During scavenging, the bypass air is ejected in the cylinder, shielding the exhaust against the fresh mixture coming from the ports on the intake side. This allows the hydrocarbon emissions to be reduced by over 40 %. The conceptual design study is based on an analysis of the mixture short-circuiting mechanisms and CFD investigations of the scavenging flow with charge stratification.

Secondary air injection after cold start gives two effects for reduced exhaust emissions: An exothermic reaction at the hot exhaust valves occurs, which increases the temperature of the exhaust gas. It gives sufficient air to the catalyst during the cold start fuel enrichment that is necessary to prevent driveability problems. Handicaps for the wide use of air injection include space constraints, weight and price. An electrical air blower was choosen to best satisfy all these requirements. The development steps are described. The result is a three stage radialblower with extremly high revolutions of about 18000 rpm. The system configuration and the outcome are demonstrated on the new C-Class of Mercedes-Benz. The results show emission reductions higher than 50 %, while also satisfying the development goals of noise, volume, weight and cost requirements.

To achieve low emission levels, the handicap of the TWC is its light-off characteristic. It achieves a maximum hot efficiency of nearly 100 %, but this requires a temperature in the range of 300 to 450 °C. To improve this time lag after cold start, the TWC needs additional help to reach the targets of future low (LEV) or ultra low emission (ULEV) levels. This paper describes the work on additional devices to reach the ULEV-levels such as: Electrical Heated Catalyst (EHC). Burner Heated Catalyst (BHC) Hydrocarbon Trap (HCT) as external device. Adsorber Coated Substrate (ACS) in the usual converter box. The comparison of these systems was done with a concept car. The low mileage exhaust results demonstrated the principal suitability of all these devices, but there is still much work to be done to meet the ULEV levels with the guaranted durability. The advantages and disadvantages of the systems are discussed, including estimated weight and cost.

This paper describes the exhaust emission control system designed for the new Mercedes-Benz sports car generation 300 SL and 500 SL, Remarkable features of the catalytic converters are maximum environmental compatibility, i. e. minimum of emission and noise under all driving conditions. By reduced back-pressure, they also contribute greatly to the well known requirements of a sports car: high performance and high torque over the entire engine-speed-range. Apart from an effective exhaust gas aftertreatment, great efforts were made to prevent the formation of pollutants under all driving conditions. This work is done by intelligent motor management consisting of digital spark control, mechanical-electronic injection, variable valve timing and EGR.