The Advanced Techniques in Two-Stroke Engine Design

INTRODUCTION TO THE TWO-STROKE ENGINE 1 1.0 Introduction to the two-stroke cycle engine 1 1.1 The fundamental method of operation of a simple two-stroke engine 6. 1.1, with the various phases of the The simple two-stroke engine is shown in Fig.

Methods of scavenging the cylinder .1 Loop scavenging

Uniflow scavenging

Uniflow Scavenging has long been held to be the most efficient method of scavenging the two-stroke engine. Indeed, the most efficient prime movers ever made are the low-speed marine diesels of the uniflow scavenged two-stroke variety with thermal efficiencies in excess of 50%.

Scavenging without employing the crankcase as an air pump The essential element of the original Clerk invention, or perhaps more properh

One of the successful compression ignition engine designs of this type is the Detroit Diesel engine shown in Plate 1.7. This can be either a positive displacement blower of the Roots type, or a centrifugal blower driven from the crankshaft.

Valving and porting control of the exhaust, scavenge and inlet processes The simplest method of allowing fresh charge access into, and exhaust gas

• Poppet valves
• Disc valves
• Reed valves
• Port timing events

Where the inlet port is similarly controlled by the piston, in this case the bottom edge of the piston skirt, is sketched in Fig. For engines with the inlet port controlled by a disc valve, the asymmetrical nature of the port timing is evident from both Figs.

Engine and porting geometry

In the case of the first programs introduced below, they are to be found as ProgList. Almost all of the programs are written in, and are intended to be used in, the interpreted QuickBASIC mode.

GCR=(SV+CV)/CV (1.4.4) Theoretically, the actual compression process occurs after the exhaust port is

• Computer program, Prog.1.1, PISTON POSITION
• Computer program, Prog.1.2, LOOP ENGINE DRAW
• Scavenge ratio and delivery ratio
• Scavenging efficiency and purity
• Trapping efficiency
• Charging efficiency
• Air-to-fuel ratio
• Cylinder trapping conditions
• The thermodynamic cycle for the two-stroke engine
• the theoretical value of ThE is readily calculated as 0.541, the considerable disparity between fundamental theory and experimentation becomes apparent, for
• The concept of mean effective pressure

However, if the engine is a multi-cylinder device, it is the total swept volume of the engine which is under consideration. By the same logic, the pumping work required in the crankcase is the cyclic integral of the pressure- volume diagram in the crankcase.

Power and torque and fuel consumption

Laboratory testing of two-stroke engines

• Laboratory testing for power, torque, mean effective pressure and specific fuel consumption

This is called the Brake Mean Effective Pressure, BMEP, and is calculated from a manipulation of Eq. BMEP=BPR/(SV*RPS) (1.6.6) It is obvious that the brake power output and the brake mean effective pressure.

There is little point in writing at length on the subject of engine testing and of the correction of the measured performance characteristics to standard reference pressure and temperature conditions, for these are covered in the many standards and codes already referred to. The reaction torque on the casing, which is exactly equal to the engine torque, is measured on a lever of length, L, from the center-line of the dynamometer as a force, F.

IMEP=BMEP+FMEP+PMEP (1.6.9)

Laboratory testing for exhaust emissions from two-stroke engines There have been quite a few technical contributions in this area (1.7)

The brake specific air consumption, BSAC, in kg/kWh units, is 6.3 The brake specific emission values printed are in g/kWh units. The brake specific hydrocarbon value by NDIR system, BSHC, is 7.0 The Trapping Efficiency, TE , as Z, is 64.5.

Trapping efficiency from exhaust gas analysis

The brake specific carbon monoxide value, BSCO, is 7.7 The brake specific nitrogen oxide value, BSNOX, is 1.6 The brake specific carbon dioxide value, BSC02, is 692.5 The brake specific oxygen value, BS02, is 518.3. A further complication arises for the comparison of HC values recorded by NDIR and FID instrumentation.

Potential power output of two-stroke engines

The use of this analytical technique, to measure- ments taken in a two-stroke engine under firing conditions, is described by Blair and Kenny(1.23); they provide further data on the in-cylinder conditions at the same time using the experimental device shown in Plate 3.3.

BPR=BMEP*SV*RPS (1.7.1) From experimental work for various types of two-stroke engines the potential

Introduction to the Two-Stroke Engine

• Influence of piston speed on the engine rate of rotation

PS=2*ST*RPS (1.7.2) As one can vary the bore and stroke for any design within a number of cylinders,

BS=BO/ST (1.7.3) The total swept volume of the engine can now be written as

Influence of engine type on power output

Swann, "An Experimental Comparison of Loop and Cross Scavenging of the Two-Stroke Cycle Engine." SAE Intnl. Blair, "Correlation of Theory and Experiment for Scavenging Flow in Two-Stroke Cycle Engines," Society of Automotive Engineers International Off- Highway & Powerplant Congress, Milwaukee, WI, September SAE Paper No.

Introduction

• Acoustic pressure waves and their propagation velocity
• Finite amplitude waves

2.1(a) and (b), the undisturbed pressure and temperature in the pipe ahead of the pressure wave is Po and To, respectively. From this it is clear that the propagation of the expansion wave is slower than the reference acoustic velocity but the air particles move somewhat faster.

Motion of oppositely moving pressure waves in a pipe

• Reflection of pressure waves
• Reflection of a pressure wave at a closed end in a pipe
• Reflection of pressure waves in pipes at a cylinder boundary This situation is fundamental to all unsteady gas flow generated in the intake or

The one certain fact available, physically speaking, is that the superposition particle velocity is zero in the plane of the closed end, as is shown in Fig. 2.5(b), the first logical assumption which can be made is that the superposition pressure in the plane of the open end is the atmospheric pressure.

Fig. 2.7 has lines of constant AK expressed as

Reflection of pressure waves in a pipe at a sudden area change It is quite common to find a sudden area change within a pipe or duct attached

2.9, and it can be seen that the pipe area can contract or expand at the junction. He assumed that the superposition pressure at the plane of the junction was the same in both pipes at the instant of superposition.

Gas Flow Through Two-Stroke Engines

• Computational methods for unsteady gas flow
• Riemann variable calculation of flow in a pipe (a) 3 and 6 characteristics

The new values of 3 and 6 at each mesh point at the end of the tjme step are shown in Fig. c) Reflection of characteristics at the pipe ends. For instance, assume that the right-hand end of the pipe is a plain open end to the atmosphere.

• Illustration of unsteady gas flow into and out of a cylinder
• Simulation of exhaust outflow with Prog.2.1, EXHAUST
• Unsteady gas flow and the two-stroke engine
• Introduction
• Fundamental theory

The exhaust pulse is past its peak amplitude and the front of the suction reflection has just arrived at the exhaust port. In the remainder of the pipe the particle velocity is observed to be accelerating towards the port.

CYLINDER, VOLUME SV

INFLOW

EXHAUST

• Perfect mixing scavenging
• Combinations of perfect mixing and perfect displacement scavenging Benson and Brandham(3.2) suggested a two-part model for the scavenging
• Inclusion of short-circuiting of scavenge air flow in theoretical models In the book by Benson(1.4), the theory for the Benson-Brandham two-part
• Experimentation in scavenging flow
• Absolute test methods for the determination of scavenging efficiency To overcome the disadvantages posed by the Jante test method, it is preferable
• Comparison of loop, cross and uniflow scavenging
• Scavenging the Two-Stroke Engine
• Comparison of experiment and theory of scavenging flow

The gases in the cylinder and the crankcase are at atmospheric temperature at the commencement of the process. This is particularly important for the theoretical modeller of scavenging flow who has previously been relying on models of the Benson-Brandham type.

CYLINDER, VOLUME V

Computational fluid dynamics

It will be observed that the order of accuracy of the calculation, over the entire SR spectrum, is very high indeed. The reason for this is that the measured data for the variance of the flow from the port.

Scavenging the Two-Stroke Engine design direction acquired by Smyth(3.17) and Kenny(3.31) was applied to the CFD

• Scavenge port design
• Conventional cross scavenging
• QUB type cross scavenging
• Loop scavenging
• Introduction
• The spark-ignition process .1 Initiation of ignition
• Air-fuel mixture limits for flammability
• Effect of scavenging efficiency on flammability
• Detonation or abnormal combustion
• Homogeneous and stratified combustion
• Heat released by spark-ignition combustion .1 The combustion chamber

The vital importance of the deflection ratio in the design of the piston crown and the porting of a cross scavenged engine is illustrated by Fig. Sher, "Prediction of the Gas Exchange Performance in a Two-Stroke Cycle Engine," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850086.

Combustion in Two-Stroke Engines

• Heat release from a two-stroke loop scavenged engine

The combustion chamber employed for these tests is of the "hemisphere" type shown in Fig. The analysis of the cylinder pressure trace for the heat release rate is shown in Fig.

TE*CAL*BSFC*BMEP*SV (4.2.15)

A one-dimensional model of flame propagation

This is possible by the use of a combustion model in conjunction with a computational fluid dynamics model of the gas flow behavior within the chamber. For example, should any one surface or side of the combustion bowl be hotter than another, the calculation will predict the heat transfer in this microscopic manner giving new values and directions for the motion of the cylinder charge; this will affect the resulting combustion behavior.

Squish behavior in two-stroke engines

This form of combustion calculation is preferred over any of those models previously discussed, as the combustion process is now being theoretically examined at the correct level.

The vigorous squish action in a QUB type cross scavenged engine at the end of the compression stroke

• A simple theoretical analysis of squish velocity

Thus, the values of cylinder pressure and temperature are PC 1 and TC1, respectively, at the commencement of the time step, dt, which will give an incremental piston movement, dH, an incremental crank angle movement, dCQ, and a change of cylinder volume, dV. Therefore, the compression process is assumed to be isentropic, with air as the working fluid, and the trapping conditions are assumed to be at the reference pressure and temperature of 1 atm and 20aC.

SPR=PS2/PB2 (4.4.1) At this point, consider that a gas flow process takes place within the time step so

• Evaluation of squish velocity by computer
• Design of combustion chambers to include squish effects
• BORE 7 0 STROKE 7 0
• STROKE 70
• Design of combustion chambers with the required clearance volume Perhaps the most important factor in geometrical design, as far as a combustion

MS2=Mtr*VS2/VC2 (4.4.2) During the course of the compression analysis, the mass in the squish band was. If the value is higher than that, the mass trapped in the end zones of the squish band may be sufficiently large and, with the.

NOTATION FOR CHAPTER 4

By this means, the squished flow does not attach to the piston crown, but can raise the combustion efficiency by directly entering the majority of the clearance volume. Indeed, some of the best designs have been those which are almost quiescent in this regard.

NAME SYMBOL SI UNIT OTHER UNITS Air-Fuel Ratio

As shown in the sketch, the designer should dish the piston inside the squish band and proportion the remainder of the clearance volume at roughly 20% in the piston and 80% in the head. Should such a chamber be tried experimentally and fail to provide an instant improvement to the engine performance characteristics, the designer should always remember that the scav- enging behavior of the engine is also being altered by this modification.

Novak, "The Prediction of Ignition Delay and Combustion Intervals for a Homogeneous Charge, Spark Ignition Engine," Society of Automotive Engineers, International Congress and Exposition, Detroit, Michigan, February, 1978, SAE Paper No. Pinfold, "Heat Transfer in the Cylinder of a Motored Reciprocating Engine," Society of Automotive Engineers, International Congress and Exposition, Detroit, Michigan, February, 1980, SAE Paper No.

Computer Modelling of Engines

• Introduction
• Structure of a computer model
• Physical geometry required for an engine model
• The open cycle model within the computer programs
• Computer Modelling of Engines for exhaust gas,
• The closed cycle model within the computer programs
• Computer Modelling of Engines The use of a simple heat release model of combustion can also be used for
• The simulation of the scavenge process within the engine model In Chapter 3 on scavenging flow, the fundamental impression which the reader

The composition of a model of the unsteady gas flow in the inlet, transfer and exhaust ducts of the engine. The structure of the model for the open cycle behavior in the cylinder and crankcase is almost exactly as detailed in Sect.

5.1.7) The value of CGtl is the superposition particle velocity at the first mesh point in

• Computer Modelling of Engines A similar situation would exist for the calculation of the instantaneous value of
• Deducing the overall performance characteristics
• Computer Modelling of Engines
• The chainsaw engine at full throttle
• Analysis of data for a Husqvarna motorcycle engine using Prog.5.2 In a previous paper(3.7), Blair and Ashe presented experimental and theoretical
• Computer Modelling o
• Single-cylinder high specific output two-stroke engines
• EC TDC 1500 DELIVERY RATI0=0.829

5.9, where a suction reflection from the open pipe end into the exhaust box arrives back at the exhaust port and assists with the scavenging of the cylinder. This is due to the presence of the suction reflection from the diffuser being at the exhaust port for the duration of its opening.

SYMBOL AF

In this case, as shown in Plate 5.2 for this very engine, it is necessary to have some path length available for the exhaust pulse between the relevant cylinders so that the phasing of the cross-charging process is optimized. Scavenge Ratio by volume Scavenging Efficiency Trapped Air-Fuel Ratio Trapped Compression Ratio Trapping Efficiency Area.

SI UNITS

This implies that the usable speed range would not be as wide as that of a three-cylinder, or V6, two-stroke engine. The short extension exhaust pipe, leading from the branched junction to the four cylinders of each bank of the Vee into the generous volume of exhaust box above the gearcase of the outboard, is clearly visible in the photograph.

OTHER UNITS

Heywood, "Develpoment and Evaluation of a Friction Model for Spark-Ignition Engines," SAE International Automotive of a Friction Model for Spark-Ignition Engines," SAE International Automotive Congress, Detroit, Michigan, February, 1989, SAE Paper No. Mikulic, "Thermodynamic Analysis and Optimization of Two-Stroke Gasoline Engines," SAE International Analysis and Optimization of Two-Stroke Gasoline Engines," SAE International Automotive Congress, Detroit, Michigan, February, 1989, SAE Paper No.

Empirical Assistance for the Designer

• Introduction
• Design of engine porting to meet a given performance characteristic The opening part of this section will discuss the relationship between port areas
• Specific time areas of ports in two-stroke engines
• The use of the empirical approach in the design process
• The acquisition of the basic engine dimensions
• The width criteria for the porting
• shows the result of a few moments of time spent at the computer screen, confirming that the total port width selected at 60 mm is mechanically acceptable
• regarding the orientation of the main transfer ports
• The port timing criteria for the engine
• The selection of the exhaust system dimensions
• Empirical Assistance for the Desigm angle. The exhaust pulse is seen to leave the exhaust port and to propaga
• Empirical Assistance for the Designer pressure wave action. The slower rate of area expansion of the first stage of the

Note that the maximum area of the inlet port and the exhaust port are about equal. -6.1.11, consider the case of the two engines studied in Chapter 5, the chainsaw engine and the Grand Prix engine.

DEP5=0.6*EXD

The next important criterion is to proportion the tail pipe exit area as a function of the exhaust port area so that the pipe will empty in a satisfactory manner before the arrival of the next exhaust pulse. With this knowledge of the exhaust port flow area, equivalent to a diameter, EXD, the empirical calculation describes this as the ideal exhaust pipe initial diameter.

DEP5=0.7*EXD

The use of specific time area information in reed valve design From the experimental and theoretical work at QUB on the behavior of reed

This implies that all of the data listed as parameters for the reed valve block and petal in Fig. 6.13 has to be assembled as input data to run that engine model. Within the paper there are more extensive descriptions of the specifications of the GRP and carbon-fiber composite materials actually used as the reed petals.

INTA=(BMEP+1.528)/774

• The design process programmed into a package, Prog.6.4
• Concluding remarks on reed valve design
• The empirical design of disc valves for two-stroke engines
• Specific time area analysis of disc valve systems
• A computer solution for disc valve design, Prog.6.5
• Empirical Assistance for the Designer The minimum radius, RMIN, is determined by the geometry of the engine
• Concluding remarks

The final step is to ensure that the reed petal will lift sufficiently during the pumping action of the crankcase. CARB=V[4*DPA*VKexp/xi] (6.4.10) The value of the expansion coefficient, Kexp, is very similar to that used for the.

Reduction of Fuel Consumption and Exhaust Emissions

Introduction

• Some fundamentals regarding combustion and emissions
• Homogeneous and stratified combustion and charging

For example, consider the situation where the air-fuel ratio is 20% lean of the stoichiometric value, i.e., there is excess air present during the combustion process. the air-fuel ratio is noted as the marker of the relationship of that combustion process to the stoichiometric, or ideal.

The simple two-stroke engine

Many of the simplest engines use lubricant mixed with the gasoline as the means of engine component oiling. To satisfy many of the design needs outlined above, the use of a computer based simulation of the engine is ideal.

Reduction of Fuel Consumption and Exhaust Emissions 7.2.1 Typical performance characteristics of simple engines

• Measured performance data from a QUB 400 research engine The first set of data to be presented is from the QUB 400 single-cylinder research
• Typical performance maps for simple two-stroke engines
• Optimizing the emissions and fuel economy of the simple two-stroke engine
• The effect of scavenging behavior
• The effect of air-fuel ratio
• The effect of exhaust port timing and area
• Conclusions regarding the simple two-stroke engine
• The more complex two-stroke engine
• The stratified charging and homogeneous combustion engine This type of engine has been tested in various forms and by several research

7.2, the problems inherent in the design of the simple two-stroke engine are introduced and typical performance characteristics are presented. These are very early days in the development of this form of the two-stroke cycle engine.

1500 2000 2500 JOOO 3500 4000 4500

The bulk of the engine is increased somewhat over that of a conventional two- stroke engine, particularly in terms of engine height. The engine is of the two-piston type with the cylinder axes at 90", and the geometrical shape has led.

• Direct in-cylinder fuel injection
• Air-blast injection of fuel into the cylinder
• Concluding comments
• Introduction
• Noise
• Transmission of sound
• Intensity and loudness of sound
• Loudness when there are several sources of sound
• The obvious ones are the intake and the exhaust system, where the presence of gas pressure waves has been discussed at length in Chapter 2. As these propagate
• Silencing the exhaust and inlet system of the two-stroke engine In the matter of silencing, the two-stroke engine has some advantages and
• The engine operates at double the frequency of creation of gas pressure waves and humans dislike exposure to higher-frequency noise
• The ports in a two-stroke engine open faster than the poppet valves of the four- stroke engine and so the pressure wave fronts are steeper, thereby creating more
• Many two-stroke engines are used in applications calling for an engine with low bulk and weight, thereby further reducing the space available for muffling and
• Two-stroke engines use ball, roller, and needle roller element bearings and these tend to be noisy by comparison with pressure-fed hydrodynamic bearings
• The engine with the tuned exhaust pipe produces a high specific output, but this is achieved by choking the final outlet diameter, thereby simplifying the design
• The crankcase pump induces air by pumping with a low compression ratio and, as this reduces the maximum values encountered in the air intake particle
• The peak combustion pressures are lower in the equivalent two-stroke cycle engine, so the noise spectrum induced by that lesser combustion pressure is reduced
• The theoretical work of Coates(8.3)
• Future work for the prediction of silencer behavior
• Theory based on acoustics for silencer attenuation characteristics The behavior of silencers as treated by the science of acoustics is to be found in

It is possible that this type of engine could be configured as in Fig 1 6 with an automotive type crankshaft, yet be self-starting with a turbocharger attached .the exhaust port. It is sufficient to remark at this juncture that:. a) The intent of a diffusing silencer is to absorb all noise at frequencies other than those at which the box will resonate.

Q=A/FR (8.5.1.2) As the frequency of the gas-borne noise arriving into the diffusing silencer

Reduction of Noise Emission from Two-Stroke Engines

• The side-resonant type of exhaust silencer
• Clearly this is an impractical result, but it does give credence to the view that such a silencer has a considerable attenuation level in the region of the natural

There is also no evidence from the theoretical solution of the reason for the attenuation in the measured spectrum of the fundamental pulsation frequency of 133 Hz, as commented on in Sect. The fundamental behavior of this type of silencer is to absorb a relatively narrow band of sound frequency by the resonance of the side cavity at its natural frequency.

Reduction of Noise Emission from Two-Stroke Engines To demonstrate the use of this computer design program, and to determine if the

• The absorption type of exhaust silencer
• Positioning an absorption silencer segment

Clearly this is an impractical result, but it does give credence to the view that such a silencer has a considerable attenuation level in the region of the natural such a silencer has a considerable attenuation level in the region of the natural frequency of the side-resonant cavity and connecting passage. To help the reader to use these acoustic equations for design purposes, a simple computer program is included with this book in the Computer Program Appendix, Prog.8.2, "SIDE-RESONANT SILENCER." This is listed as ProgList.8.2.