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Technical Paper

A Direct Evaluation of the Exhaust Lead of a Two-Stroke-Cycle Diesel Engine

The outflow of the products of combustion from a two-stroke cycle diesel engine cylinder is examined and the numerical analysis used hitherto supplanted by a direct integration method. It is found that the equations of outflow result in a nonlinear differential equation capable of direct solution, thereby giving rapid computation of the exhaust lead, one of the most important single criteria of the overall engine design.
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

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

Computer Simulation of the Performance of a 1.9 Litre Direct Injection Diesel Engine

Recent environmental legislation to reduce emissions and improve efficiency means that there is a real need for improved thermodynamic performance models for the simulation of direct-injection, turbocharged diesel engines, which are becoming increasingly popular in the automotive sector. An accurate engine performance simulation software package (VIRTUAL 4-STROKE) is employed to model a benchmark automotive 1.9-litre Turbocharged Direct Injection (TDI) diesel engine. The accuracy of this model is scrutinised against actual test results from the engine. This validation includes comparisons of engine performance characteristics and also instantaneous gas dynamic and thermodynamic behaviour in the engine cylinders, turbocharger and ducting. It is seen that there is excellent agreement in all of these areas.
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

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

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

Initial Development of a Two-Stroke Cycle Diesel Engine for Automotive Applications

A three cylinder two-stroke cycle diesel engine is proposed for automotive use. The engine is of the simple loop or cross-scavenging type with a crosshead seal and under piston scavenging pump. This paper records the initial investigations of this concept using a purpose built single cylinder engine. Results from different combustion systems are presented together with tests with the same engine when using an external air supply. Measurements from a parallel investigation using a laser doppler anemometer to measure air swirl motion within one of the chambers are also presented.
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

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

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

Design and Initial Development of a High Specific Output 500 CC Single-Cylinder, Two-Stroke, Racing Motorcycle Engine

This paper describes the initial investigation and design of a lightweight racing motorcycle with a single-cylinder 2-stroke engine, capable of producing 60 bhp. The data discussed here pertain to the gas dynamic and mechanical parts and functions of the cycle. Designs of the various components are described and reports of tests on road and test beds verify the viability of this concept of a high specific output and large displacement cylinder for a lightweight, air-cooled motorcycle engine.
Technical Paper

Unsteady Flow in the Induction System of a Reciprocating Internal Combustion Engine

Pressure-time variations are recorded in the intake pipe and crankcase of a motored, crankcase compression, piston ported, loop scavenged two stroke cycle engine over a range of engine speeds from 2000-7000 rpm, for several intake pipe lengths and different inlet port timings. These pressure-time histories are presented together with the results of theoretical calculations, which include unsteady flow in the induction tract. Predicted delivery ratio trends are compared with measured values over the range of engine speeds and inlet tract lengths for different inlet port timings.
Technical Paper

The Development of a High Speed Dynamometer and Preliminary Results Obtained from a C.A.V.01 Turbine

Modern turbocharged diesel engines employ exhaust driven turboblowers operating at high speeds up to 100,000 rpm. The performance assessment of such units demands precise and controllable power absorption and torque measurements at these very high rotational speeds. Additionally the parameters, speed, mass flow, static and dynamic pressures and temperatures must be measured. The turbine power absorption and torque measutement present unique problems. The remaining parameters may present some difficulties but generally the problems are not so great. The design of a high speed dynamometer and the development problems encountered are described. The dynamometer has been used to establihs the performance characteristics of a C. A. V. 01 turbocharger and these are reported.
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

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

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

A More Complete Analysis of Unsteady Gas Flow Through a High-Specific-Output Two-Cycle Engine

The performance characteristics of a naturally aspirated two-cycle engine can be predicted with an unsteady gas dynamics analysis of flow through the crankcase and cylinder; such an analysis provides values of volumetric efficiency and trapping pressure at any given engine speed. The predictions of the volumetric efficiency and trapping pressure are compared with experimental values from a high-specific-output engine and further amplified with theoretical/experimental comparisons of pressure-time histories taken in the exhaust, transfer, and inlet systems at several engine speeds. The theoretical derivation of unsteady gas dynamic cylinder to pipe boundary conditions is presented so that they become both economical of computer time and mathematically stable.
Technical Paper

Design of Exhaust Systems for V-Twin Motorcycle Engines to Meet Silencing and Performance Criteria

This paper reports on the use of mathematical modelling by the GPB method of pressure wave propagation through finite systems, for the design of prototype exhaust systems and silencers for a Harley-Davidson motorcycle. The motorcycle engine is the classic 1340 cm3 45° V-twin power unit. The design objectives were to gain mid-range power and torque without loss of performance at either end of the speed range and to design silencers which would enhance the performance and the noise image of the machine. The Queen's University of Belfast (QUB) (3)* employed their unsteady gas flow modelling techniques to the design of the system and its silencers to complement a new camshaft design from Crane Cams. The results of the use of these computer based design techniques are reported as performance characteristics of power and torque for the new design by comparison with the stock system.