Search Results

Analysis of pressure traces from within the cylinder of IC engines is a long established technique, particularly in automotive applications. This approach allows burn rate data to be calculated from the shape of the pressure traces, providing direct combustion information to development engineers. With the proliferation of high-powered and low-cost computers, recording of pressure traces and analysis to give burn rates are now becoming standard measurements. However, this is still a complex technique, which is very open to error and prone to misinterpretation of results. This is particularly relevant for two-stroke engines where cyclic variations can be high and traces can be difficult to analyze. This paper considers the standard techniques available for pressure trace analysis, highlighting the areas for problems and outlining good practice for reliable and accurate measurement.

The two-stroke engine, by its nature is very dependent on the unsteady gas dynamics within an exhaust system. This is demonstrated by the tuning effects on two-stroke engines, which have been well documented. In consideration of current emissions legislation, a two-stroke engine can be fitted with a catalytic converter for the outboard, utility or automotive markets. The catalytic substrate represents a major obstruction to the flow of exhaust gas, which hinders the progression of the main exhausted pulse, and in turn effects the scavenging of the cylinder and ultimately the performance of the engine. Within this investigation, a 400 cc direct injection two-stroke engine was used with various catalysts positioned at different distances from the exhaust manifold. Comparison tests were performed between a fully lit off catalyst and a non-operational bare substrate.

Due to increasingly stringent emissions legislation the four-stroke engine is beginning to replace the two-stroke engine for motorcycle and scooter applications over 50cc. However, because of its comparatively poor performance, the four-stroke unit is not replacing the two-stroke for moped applications which are restricted to 50cc. To meet forthcoming European legislation the two-stroke moped engine requires an exhaust catalyst which presents considerable durability problems when applied to this type of engine. This would not be the case with a four-stroke unit, so if its performance could be improved it would be an attractive alternative. This paper illustrates the difficulties facing four-stroke engines of this size, the improvements required, the benefits (and problems) of a multi-valve approach and possible means of improving performance.

With the legislative demands increasing on recreational vehicles and utility engined applications, the two-stroke engine is facing increasing pressure to meet these requirements. One method of achieving the required reduction is via the introduction of a catalytic converter. The catalytic converter not only has to deal with the characteristically higher CO and HC concentration, but also any oil which is added to lubricate the engine. In a conventional two-stroke engine with a total loss lubrication system, the oil is either scavenged straight out the exhaust port or is entrained, involved in combustion and is later exhausted. This oil can have a significant effect on the performance of the catalyst. To investigate the oiling effect, three catalytic converters were aged using a 400cm3 DI two-stroke engine. A finite level of oil was added to the inlet air of the engine to lubricate the internal workings. The oil flow rate is independent of the engine speed and load.

Washcoat sintering and substrate meltdown have traditionally been the principle deactivating mechanisms of catalysts fitted to two-stroke engines. The reduction of the excessively high HC and CO levels responsible for these effects has therefore been the focus of considerable research which has led to the introduction of direct in-cylinder fuel injection to some larger versions of this engine. However, much less attention has been paid to the effects of oil and its additives on the performance and durability of the two-stroke catalyst. The quantity of oil emitted to the exhaust system of the majority of two-stroke engines is much greater than in four-stroke engines of comparable output due to the total loss lubrication system employed. The fundamental design of the two-stroke also permits some of this oil to ‘short-circuit’ to the exhaust in a neat or unburned form.

The most common mode of deactivation suffered by catalysts fitted to two-stroke engines has traditionally been thermal degradation, or even meltdown, of the washcoat and substrate. The high temperatures experienced by these catalysts are caused by excessively high concentrations of HC and CO in the exhaust gas which are, in turn, caused by a rich AFR and the loss of neat fuel to the exhaust during the scavenging period. The effects of catalyst poisoning due to additives in the oil is often regarded as a secondary, or even negligible, deactivating mechanism in two-stroke catalysts and has therefore received little attention. However, with the introduction of direct in-cylinder fuel injection to some larger versions of this engine, the quantities of HC escaping to the exhaust can be reduced to levels similar to those found on four-stroke gasoline engines.

The emissions produced from a simple carburetted crankcase scavenged two-stroke cycle engine primarily arise due to losses of fresh charge from the exhaust port during the scavenging process. These losses lead to inferior fuel consumption and a negative impact on the environment. Pressure on exhaust emissions and fuel consumption has reduced the number of applications of the two-stroke cycle engine over the years, however the attributes of simplicity, high power density and potential low manufacturing costs have ensured its continuing use for mopeds and motorcycles, small outboard engines and small utility engines. Even these last bastions of the simple two-stroke engine are being challenged by the four stroke alternative as emissions legislation becomes tighter and is newly formulated for many categories of engines. A simple solution is described which reduces short circuit and scavenge losses in a cost effective way.

A study of fuel injection during the open cycle of a small spark ignition two-stroke cycle engine has been carried out. A manually controlled electronic fuel injection system has been used as this appears to have many advantages over mechanically controlled equipment. Various injection locations were considered and injection timing and air/fuel ratio varied at each position to determine optimum power and bsfc requirements. The results presented are compared with baseline carburation and with fuel injection into the intake to assess the potential improvements gained from each injector location.

The advantages of high power to density ratio and low manufacturing costs of a two-stroke engine compared to a four-stroke unit make it currently the most widely used engine type for 50cc displacement 2-wheelers. This dominance is threatened by increasingly severe exhaust emissions legislation, forcing manufactures to develop their two-stroke engines to comply with the legislation. This paper describes a simple solution to reduce these harmful emissions in a cost effective manner, for a scooter application. The method of stratified scavenging is achieved by delivering the fuel into the rear transfer passage from a remote mechanical fuel metering device, operated by intake manifold pressure. Air only is delivered into the cylinder from the remaining transfer passages which are directed towards the rear transfer port, thus impeding the fuel from reaching the exhaust during the scavenging process.

This paper describes an experimental investigation into the effect of bore/stroke ratio on a simple two-stroke engine. This was achieved with a special purpose engine of modular design. The engine allowed four combinations of bore and stroke to be contrived to yield a common swept volume of 400 cm3 with bore/stroke ratios of: 0.8, 1.0, 1.2 and 1.4. Other factors that might affect engine performance were standardised: the exhaust, intake and ignition systems were common, the combustion chamber designs were similar, scavenge characteristics were similar, port timings and time-areas were kept the same, and cylinder and crankcase compression ratios were also kept the same. The most important conclusions were: Engine power was greatest with the compromise bore/stroke ratio of 1.0 or 1.2. Combustion efficiency tended to decrease with increasing bore/stroke ratio. Mechanical efficiency tended to increase with increasing bore/stroke ratio.