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Technical Paper

Correlation of an Alternative Method for the Prediction of Engine Performance Characteristics with Measured Data

This paper presents confirmation of the accuracy of prediction of an engine simulation model. The experimental data used to compare with the output of the simulation model are from a single cylinder four-stroke cycle engine and from a single-cylinder two-stroke cycle engine; both engines are naturally aspirated and use spark- ignition. In addition, for the two-stroke cycle engine, the experimental data includes two cylinders with different scavenging characteristics which induce variations of performance characteristics of up to 20%. The fundamentals of the theoretical approach have been presented before to SAE (1)* and this paper extends that theory by providing a detailed discussion on the inclusion of measured scavenging characteristics to enable the simulation model to predict the mechanism of the in-cylinder gas exchange process.
Technical Paper

Reduction of Fuel Consumption and Emissions for a Small Capacity Two-Stroke Cycle Engine

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.
Technical Paper

Application of Direct Air-Assisted Fuel Injection to a SI Cross-Scavenged Two-Stroke Engine

A 500 cc single cylinder two-stroke engine employing cross scavenging and direct air-assisted gasoline injection is described. Preliminary engine test results are presented for 3000 rpm full load and 1600 rpm part load operating conditions. The effects of fuel injection timing on full and part load brake specific fuel consumption and exhaust emissions are examined.
Technical Paper

Prediction of Two-Cycle Engine Performance Characteristics

Previous papers published by the author have described unsteady gas flow through a naturally aspirated two-cycle engine and the most recent of these publications details a theoretical modelling of the gas exchange or scavenge process for the cylinder of this type of power unit. This results in the ability to predict the trapped charge state, mass, and purity characteristics. With such information it becomes sensible to apply a closed cycle thermodynamic analysis to it and to further predict directly power, torque, and fuel consumption characteristics. This paper describes such a simple closed cycle analysis and compares the theoretical results of power, mean effective pressure, specific fuel consumption, and cylinder pressure diagrams with corresponding measured data from two engines.
Technical Paper

The Unsteady Gas Exchange Characteristics of a Two-Cycle Engine

The theoretical modelling of the scavenge process for a naturally aspirated two-cycle engine is described and employed in conjunction with an unsteady gas dynamic analysis of flow in the engine ducting. Programmed for a digital computer, the results of this theoretical study are shown in relation to a 250 cm3 engine with values of predicted charging efficiency, scavenging efficiency, and delivery ratio given as a function of engine speed. These are compared with measured values of scavenging efficiency and the usual performance characteristics of power, mean effective pressure, delivery ratio, and specific fuel consumption. Also compared are the measured and predicted pressure diagrams taken in the cylinder, the crankcase, and the exhaust and inlet ducts. The design of a somewhat unique cylinder gas sampling valve of the mechanical type is described and its usage discussed both theoretically and practically.
Technical Paper

Further Tests on Reducing Fuel Consumption with a Carburetted Two-Stroke Cycle Engine

This paper describes a unique and uncomplicated method of stratified-charging a two-stroke cycle engine which assists in reducing the short-circuited loss of fuel during scavenging. Performance characteristics as presented were acquired from tests conducted on a 400 cm3 naturally aspirated, single cylinder, spark ignition two-stroke engine with carburettor control of gasoline fuel, the design and construction of the engine also being done at The Queen's University of Belfast. Using a tuned exhaust pipe, the engine produces a peak power of 16 kW at 5000 rev/min and has a minimum brake specific fuel consumption of 0.275 kg/kWh. Moreover, for the tests presented at full and quarter throttle openings, virtually all of the brake specific fuel consumption values are below 0.36 kg/kWh. Most of the performance characteristics shown at various engine speeds are as a function of air/fuel ratio. This paper is a continuation of that presented as SAE 830093.
Technical Paper

Reduction of Fuel Consumption of a Spark-Ignition Two-Stroke Cycle Engine

The paper describes and lists the performance characteristics of a 400 cm3 single-cylinder two-stroke cycle engine with natural-aspiration, spark-ignition and carburetter control of gasoline fuel. The engine features an uncomplicated and unique system of stratified-charging which helps reduce the short-circuited loss of fuel during scavenging. With an untuned exhaust system the engine produces a peak power of 13 kW at 5500 rev/min and a brake specific fuel consumption which has a minimum of 0.265 kg/kWh but, more importantly, virtually the entire speed and load range is below 0.34 kg/kWh (0.55 lb/hp. hr). All performance characteristics at several throttle openings are presented at various engine speeds as a function of air/fuel ratio.
Technical Paper

An Experimental Comparison of Loop and Cross Scavenging of the Two-Stroke Cycle Engine

In a previous paper (6)* SAE 850178, the authors pointed out that the single-cycle gas simulation rig which they had developed would prove to be an invaluable experimental tool for the development of two-stroke cycle engine cylinders to attain better scavenging and trapping efficiency of the fresh charge. This paper reports on the use of that now proven experimental technique to examine one of the longest running, and hitherto unresolved, discussions in the field of small two-stroke cycle engines: is loop-scavenging really superior to cross-scavenging? All of the cross-scavenging tests in the paper are compared to tests conducted on loop-scavenged cylinders of the same basic geometry and which were reported previously to SAE. The main conclusion from the experimental investigation is that cross-scavenging is superior to loop-scavenging at low or modest scavenge ratios but is inferior at high scavenge ratios.
Technical Paper

Single Cycle Gas Testing Method for Two-Stroke Engine Scavenging

This paper presents a single-cycle gas simulation of the scavenging process in a two-stroke cycle engine. The apparatus used is described in the most detailed fashion and the experimental procedure is covered completely. On the apparatus is placed some eleven differing cylinders of a Yamaha 250 motorcycle engine and the scavenging efficiency - scavenge ratio characteristics of each determined experimentally. The results of these experiments are compared with the known performance characteristics of the same eleven cylinders which were obtained under firing conditions for variations of power, torque, air-flow, fuel consumption and scavenging efficiency at several speeds and throttle positions. The correlation, between the ranking of the several cylinders determined on the scavenging simulation apparatus with the performance characteristics obtained under firing conditions, is very good.
Technical Paper

Computational Fluid Dynamics Applied to Two-Stroke Engine Scavenging

A three dimensional computational fluid dynamics program is used to simulate theoretically the scavenging process in the loop-scavenged two-stroke cycle engine. The theoretical calculation uses the k - ε turbulence model and all calculations are confined to the in-cylinder region. The calculation geometry is oriented towards five actual engine cylinders which have been tested under firing conditions for the normal performance characteristics of power, torque, and specific fuel consumption. The same five engine cylinders have also been experimentally tested on a single-cycle gas testing rig for their scavenging efficiency - scavenge ratio characteristics. The ranking of the cylinders in order of merit in terms of scavenging efficiency by both the rig and the theoretical calculations is shown to be in good agreement with the evidence provided by the actual firing engine test results.
Technical Paper

A New Piston Design for a Cross-Scavenged Two-Stroke Cycle Engine with Improved Scavenging and Combustion Characteristics

This paper describes a unique design of deflector piston for a cross-scavenged two-stroke cycle engine which incorporates the advantages of good scavenging, rapid combustion and reduced thermal loading on the piston. Test results are presented to confirm this statement from two small capacity outboard marine engines and comparisons are made between the experimental test results from the modified and standard power units; of significance is the reduced fuel consumption rate of the modified engines in both cases. A high bmep 400 cm3 single cylinder engine is designed, constructed and tested so as to determine the extent of deflector burning under conditions of high thermal loading. On all three engines the ignition timing for best power is shown to be in the 21-24° btdc region, by comparison with 32-38° btdc conventionally. The spark plug seat temperatures are reduced to 150 C maximum at peak power by comparison with 250-280 °C normally.
Technical Paper


At the 1999 SETC meeting, a paper presented a simple, tuned and silenced exhaust system for a two-stroke engine which theoretically reduced both noise and exhaust emissions and increased engine power and fuel efficiency. In this paper that design concept is applied to a small 56 cc industrial engine and experimentally shown to deliver the projected behaviour which was predicted in that earlier publication. Experimental test results are presented for power output, fuel consumption, and exhaust emissions to illustrate these statements. An accurate engine simulation software package (VIRTUAL 2-STROKE) is employed to model the entire two-stroke engine and to demonstrate not only its effectiveness as a design tool in this area but also that it can accurately predict the above-mentioned performance and emission characteristics.