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Technical Paper

Unsteady Gas Flow through High-Specific-Output 4-Stroke Cycle Engines

The performance characteristics of naturally aspirated 4-stroke cycle engines are influenced by the through-flow or exchange of fresh charge for exhaust gas during the valve overlap period. During this gas exchange period the influence of unsteady wave effects in both inlet and exhaust systems are most important. Pressure-time histories were measured at various tract locations for four inlet/exhaust configurations to demonstrate the effects of wave action on performance. The good correlation shown between measured and predicted pressure-time histories suggested that the theoretical technique may be used in further design analyses with a high degree of confidence.
Technical Paper

Unsteady Gas Flow Through Reed Valve Induction Systems

Previous publications from The Queen's University of Belfast have described the unsteady gas flow through a naturally aspirated two-cycle engine and the most recent of these have detailed the scavenge process, the combustion model and muffler design. It is thus now possible to predict the unsteady gas flow behaviour through and the performance and noise characteristics in this type of engine with a good degree of accuracy. This paper describes a mathematical model which has been formulated to simulate the action of the two-cycle engine fitted with a reed valve due to the unsteady gas dynamic behaviour in the inlet tract and makes comparisons with measurements. A complete simulation on the computer of a two-cycle engine fitted with a reed intake valve is thus now possible.
Technical Paper

Unsteady Flow Effects in Exhaust Systems of Naturally Aspirated, Crankcase Compression Two-Cycle Internal Combustion Engines

This paper attempts to illustrate some of the reflection characteristics of exhaust systems, suitable for piston ported, crankcase compression, naturally aspirated two-cycle engines. In particular, the application is even narrower, being concerned principally with those engines of the spark ignition, gasoline burning type where a high bmep is desirable. The two principal exhaust systems considered are the diffuser and the expansion chamber. Both are analyzed experimentally and theoretically and presented as measured and digitally computed pressure-time diagrams in simulated and actual engine exhaust systems. These are compared and discussed.
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

The Pressure-Time History in the Exhaust System of a High-Speed Reciprocating Internal Combustion Engine

Measurement of pressure-time histories in the exhaust system of a naturally aspirated internal combustion engine poses some difficult instrumentation problems. This paper describes an experimental and theoretical approach in tackling this research. The exhaust system is simulated by pulses of compressed air at a frequency of up to 4000 pulses/minute, that is, a 1 cyl 4 stroke cycle engine running at 8000 rpm. The pressure-time histories are calculated by digital computer in terms of the cylinder, exhaust valve, and pipe friction characteristics and compared with the experimental pressure-transducer records at various positions in the exhaust system.
Technical Paper

Studying Scavenge Flow in a Two-Stroke Cycle Engine

The study of scavenge flow in two-cycle engines is of great importance in the development of that type of internal combustion engine and has been extensively covered by numerous researchers over the last half -century. Alfred Jante in SAE paper 680468 suggested an indirect and comparative test for the assessment of scavenge flow which he, and others, have shown to be both a simple and extremely relevant technique. The acquisition and reduction of data for this experimental method proved to be laborious and time consuming, and it is the purpose of this paper to show that it is possible to eliminate these tedious aspects by automation of both data recording and processing. This is described and examples of its usage are given.
Technical Paper

Sound Pressure Levels Generated by Internal Combustion Engine Exhaust Systems

A computer program has been developed which predicts the sound pressure level and the frequency spectrum produced by simple engine exhaust systems. The program utilizes unsteady flow gas dynamic theory to predict the pressure-time history in the exhaust system and the velocity-time history at the open end of the system. Acoustic theory is then used to predict the sound pressure levels and frequency spectrum in free space. The work was carried out on a twin-cylinder four-cycle engine, but the theory can be applied to any internal combustion engine.
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

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

Reducing Exhaust Hydrocarbon Emissions from a Small Low Cost Two-Stroke Engine

An experimental and theoretical investigation to minimise the hydrocarbon emissions from a 25 cm3 two-stroke engine with finger transfer ports is described. Finger ports have the side of each passage closest to the cylinder axis open to the cylinder bore making it possible to produce high-pressure die castings with the simplest of dies. Cylinders utilising this type of porting are believed to have inferior scavenging characteristics compared to those using closed or cup-handle porting. The effects of cylinder scavenging characteristics and port optimisation on engine performance were examined using a computer simulation. It is concluded that there is potential for a 70% reduction in exhaust hydrocarbon emissions through scavenging efficiency improvements and port optimisation, provided the cylinder scavenging can be developed to match that of the best existing unconventional crossflow scavenged designs.
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.
Technical Paper

Predicting the Performance Characteristics of Two-Cycle Engines Fitted with Reed Induction Valves

Earlier papers by the principal author in conjunction with others have described the prediction of noise and performance characteristics of two-cycle spark-ignition crankcase compression engines. These calculations are performed on a digital computer and are shown to simulate accurately the unsteady gas flow and thermodynamic processes in such power units. The engines described previously had induction control by the piston or with a disc valve. In this paper the work is extended to engines fitted with reed valves controlling intake air flow and examples illustrating the effectiveness of such calculations are presented. In particular, a single-cylinder industrial engine is employed to show clearly the effects of changing such parameters as reed petal thickness, stop-plate radii and numbers of reed petals on the performance characteristics.
Technical Paper

Observations on the Design and Operation of Pulsejet Engines as Derived from an Experimental and Theoretical Investigation

This paper describes some of the experimental and theoretical work carried out at the Queen's University, Belfast in connection with a pulsejet project. It starts from the earliest stages of trying to achieve a working reed valved engine and continues to the present where valve less pulsejets have been designed with the aid of a simulation program. Suggestions are made regarding the manner in which various parameters such as duct and intake geometry, orientation and flight speed can affect performance. It suggests four main criteria which must be fulfilled for any valveless pulsejet to operate successfully and discusses methods by which these can be achieved.
Technical Paper

Non-Isentropic Analysis of Varying Area Flow in Engine Ducting

In two previous papers to this Society (1, 2)* an ‘alternative’ method was presented for the prediction of the unsteady gas flow behaviour through a reciprocating internal combustion engine. The computational procedures led further to the prediction of the overall performance characteristics of the power unit, be it operating on a two- or a four-stroke cycle. Correlation with measurements was given to illustrate its effectiveness and accuracy. In the ducts of such engines there are inevitably sectional changes of area which are either gradual or sudden. A tapered pipe is typical of a gradual area change whereas a throttle or a turbocharger nozzle represents a sudden area change. In those previous papers it was indicated that a fuller explanation, of the theoretical procedures required to predict accurately the unsteady gas flow in such duct sections would be given in a later paper to this Society; this is that necessary publication.
Technical Paper

Noise Produced by Unsteady Exhaust Efflux from an Internal Combustion Engine

From a theoretical analysis of the unsteady efflux from the open end of a simulated reciprocating internal combustion engine exhaust system a prediction of overall and one-third octave sound pressure levels in space, due to this gas flow, is produced. The predictions are compared with measured levels and show a high degree of correlation.
Technical Paper

Motored and Steady Flow Boundary Conditions Applied to the Prediction of Scavenging Flow in a Loop Scavenged Two-Stroke Cycle Engine

The application of in-cylinder multi-dimensional modelling to the scavenging process within the cylinder of a two-stroke cycle engine requires a prior knowledge of the flow entering that cylinder. Without this information, assumptions must be made which limit the accuracy of the theoretical simulation. This paper describes laser doppler anemometry measurements of transfer port efflux flow for a two-port loop scavenged test cylinder motored at 200 rev/min. The cylinder was externally blown to ensure scavenge flow into the cylinder over the entire transfer port open period. The test results indicate that the flow does not enter the cylinder in the port design direction, but varies as a function of port height during both port opening and closing. Comparison of motoring results with those obtained under steady flow testing of the same cylinder, shows adequate correlation, thereby justifying the use of steady flow information for dynamic simulation.
Technical Paper


The paper discusses the application of maps of measured discharge coefficients for poppet valves, cylinder ports, and in-pipe throttles within a theoretical simulation of the unsteady gas flow through an internal combustion engine. The maps provided cover both inflow and outflow at the discontinuity being discussed and are displayed as contour maps of the discharge coefficient as some function of the geometrical flow area of that discontinuity and of the pressure ratio across it up to a maximum value of 2.0. An engine simulation package is used for both a four-stroke and a two-stroke engine to determine the typical pressure ratio and area ratio characteristics which pertain at all such discontinuities at representative engine speed and load conditions.
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

Further Studies of Noise Characteristics of Internal Combustion Engine Exhaust Systems

This paper describes a theoretical and experimental investigation of the noise characteristics of some basic internal combustion engine exhaust systems. On the basis of a one-dimensional analysis of the unsteady internal flow, the treatment is extended to consider the noise radiated by the efflux of gas from the atmospheric termination of the tail pipe. Using a rotary valve exhaust simulator, experimental pressure-time histories and one-third octave noise spectrograms were obtained. These are compared with those calculated.
Technical Paper

Further Developments in Scavenging Analysis for Two-Cycle Engines

In 1968 Professor Alfred Jante published an SAE paper detailing a method of assessing the scavenging behaviour of a two-cycle engine. It was a simple technique involving motoring the engine and measuring the (cylinder head removed) velocity contours at the cylinder head level using pitot tubes. It attracted wide attention in industry, but with varying degrees of acceptance and results. This paper attempts to establish in a logical manner and with a considerable’ volume of experimental data that the method proposed by Jante has real relevance, but to obtain acceptable accuracy in terms of predicting good and bad scavenging for particular engine cylinders the results have to be analysed rather more carefully and completely than the approach adopted by Jante.