Two Stroke Engine Porting, Head Mods and Pipes

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Click on image for the presentation: SET-UP TWO STROKE is the universal software for optimal tuning of all 2-stroke engines. It lets you quickly to optimize carburetion, ignition timing, spark and transmission ratio, when change, the weather conditions, the compression ratio, the height of squish, the squish band, the exhaust opening angle and the gasoline used. 2T Exhaust Calc is a Freeware program created to design a 2 Stroke engine performance exhaust system generally referred to as an 'Expansion Chamber' The software equations used are derived from the work of Professor Gordon P Blair who carried out the most comprehensive scientic study of 2 stroke engines ever undertaken and published by SAE.

Two Stroke Tuning Advice From The experts at Bimotion

This Article Courtesy Of BIMOTION © 2007

This section describes the basic function and the most important processes in the 2-stroke cycle a tuner need to consider in general and with the Bimotion tuning software in particular.

The high performance 2-stroke engine is a pulse resonance engine which means that the operation in general and the scavenging in particular is not only dependent upon the pulses created from piston pumping but also from combustion properties. This is an important process to understand and is the reason why not only the cylinder decides the tuning degree but also the exhaust pipe, cylinder head and ignition. The charging efficiency is dependent on the pulse energy created from the cylinder pressure at exhaust port opening.

The process can be described with these steps:

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1. Ignition and piston work
2. Cylinder head
3. Exhaust port opening
4. Exhaust chamber

Ignition and piston work.

At this point the fuel property will decide the heat release, at which time the fuel will produce work. A well designed fuel is an important factor to race engines. The heat release time from combustion is mainly decided from the fuel properties, air/fuel ratio and cylinder head design. The fuel ratio of H/C (hydrogen/carbon) in the CnHm molecules varies, n and m takes certain values for different fuels. During the combustion, the molecules break down step by step with different heat release at each step. 50% of the available heat is usually released within 5° -10° ATDC (After Top Dead Center).

The picture shows the relationship to crank angle and crank moment. As a schedule example, if fuelblend 1 releases most of the energy in a very short time after TDC with a high pressure peak and blend 2 in a longer period of time with a moderate pressure peak then the moment on the crank will be different over the time for the two cases.

(Pressure in MPa creates an additional piston force)

• Fuel blend 1
• P1 = 10 MPa
• L1 = 5mm (5-10 deg ATDC)
• P2 = 1 MPa
• L2 = 20mm (60 deg ATDC)
• M1 = 10*5 = 50
• M2 = 1*20 = 20
• Fuel blend 2
• P1 = 8 MPa
• L1 = 5mm (5-10 deg ATDC)
• P2 = 2 MPa
• L2 = 20mm (60 deg ATDC)
• M1 = 8*5 = 40
• M2 = 2*20 = 40

The resulting power output will be an integral of crank moment over time. If the pressure on the piston varies as in the picture at a certain rpm, the area below the curve could represent the work. (A1 and A2)
Note that A1 could be equal to A2 !
A high pressure peak for a short time might produce less work than a low peak for a long time at a certain rpm. This means that a low rpm engine will produce less power with the fast burning fuel and visa versa. The engine stress and detonation risk would also increase with the higher peak.

The different heat release properties will suite different engine characteristics and should be orchestred with the actual exhaust port height, which decides the opening pressure, i.e. the charge pressure to the exhaust pipe !

Fuel blend 1 will produce a lower pipe charge pressure than fuel blend 2 with the same exhaust port. This could mean that the first choice is less sensitive to exhaust pipe changes and more sensitive to cylinder head geometry at a certain rpm.

Cylinder Head

The two heat release curves from above could also represent 2 different types of cylinder head geometries with the same fuel. The first curve could use a high efficient squish band, which speeds up the combustion by adding more kinetic (velocity dependent) energy to the gas mixture. That will also transfer more heat into the head wall surface, which will be a measured as a power loss in the end through the cooling system. Why?
If you move your hand quickly in warm water or blow on your skin in a hot sauna you will feel that the skin get warmer due to heat transfer from velocity. In the Bimotion Advanced Head program, this factor is called the Area/Volume factor. If a geometry change would give a higher factor with the same squish velocity, then we could expect more heat loss due to the increased area. The actual value don't need to be focused, but the changes to different head shapes are interesting to observe, especially if cooling is a critical factor in the original configuration.

With high performance engines kinetic energy is needed and with short compression times (due to high rpm and short connecting rods) the heat transfer will not necessarily be too high to the cooling system. (Adiabatic compression). A head designed to a high tuning degree will work best with a high speed engine, and a moderate tuned engine will feel a great improvement with a moderate tuned head instead of an head without a tuned squish band at all. As you probably already noticed, the head geometry will be somewhat dependent on the fuel type for a critical tuned GP engine and needs a lot of testing.

The mechanism of a squish band is to push the gas as close to the spark as possible at the ignition phase. During the squish, the gas will also increase the vaporizing of the fuel and add kinetic energy which increases burn efficiency. This is one of the reasons why the ignition timing needs to be reduced with higher crank speed in a 2-stroke engine. The charging from the exhaust pipe is another.

Usually, squish velocities of 25-50 m/s are the upper limit dependent on design, materials, cooling, fuel, etc. Too high squish velocity will transfer too much heat from the gas to the surrounding metal and will make the gas self detonate due to the energy increase. The fuel usually burn with this mentioned speed and it will serve as a good limit for the squish velocity. Detonation is an explosive behavior with reaction velocities in the region of 6000 m/s. A bad designed squish band will cause such detonations which will destroy the surrounding metal and sometimes hammer the piston, making it plastic deformed over a big area and size due to the expansion.

When the gas at the squish band is moving into the center, it will have to increase its velocity due to the fact that the area is decreasing. The red length is shorter than the blue length in the picture and the gas must pass these gates.

To optimize the squish behavior we need to have a constant squish velocity over the squish band. This is achieved by tapering the squish band height with the corresponding area ratio, so that A is the Squish Gap and B is the reduced height found as Y(C4) in the coordinate table of the program. This height reduction also reduces the inefficient burned volume. The blue line shows the mathematical correct squish band shape. We can see that a strait line will approximate the shape perfect over the squish band width.

The squish taper angle is not constant, it increases with increased squish gap A. The taper angle is tangent with the piston edge at B.

The Head geometry gets more important with high squish velocities, the surrounding surface needs to be protected from the heat transfer. This can be achieved by different thermal barriers (surface treatment) and with the gas itself. The secret key is to avoid the hot gas to transfer heat. This is why we like sharp inside corners from multistage heads. The two pictures below shows the difference.

In the upper picture there is no barrier to the head surface, more heat transfer to expect due to high gas velocity next to the head surface.

In the lower picture, the small gas pocket above the red line forms a natural barrier to heat transfer with low gas velocity next to the surface.

The same principal can be used to minimize heat transfer at the squish band. The Yamaha TZ-head (right picture) uses both these ideas. The edges are marked.

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The final conclusion from this discussion is that the squish band at all times will leak heat and transform useful piston work to kinetic gas energy, a power loss source we need to get as much out of the fuel as possible at combustion. If the squish band gets over dimensioned then there will be more energy loss but no improved combustion.

Exhaust port opening

Many people talk about a port area as a target value and indirect refers to a header area with an area factor for the port. That approach is a completely waist since port area don't give any information about port shape. Time area is still the standard unit for 2-stroke ports since at least 1971. An area change far up on the port will give a completely different change than on the bottom of the port. It would also affect the pipe different. The time area distribution is simply different over the port height because the port is open different periods of time during the stroke.
In the 'Bimotion Advanced Port & Pipe' program we can see this distribution in a chart. (Se pictures below.) We should then be more careful with the machining precision as the curve height increases since that part of the port is open for a longer time.

A square port gives an increasing time area distribution with port height.

The distribution curve will also show the effect of auxiliary exhaust ports which adds blowdown time area.We need a certain blowdown timearea to equalize the cylinder pressure to the crank case when the transfer ports opens. If this not works, we get blow back to the crank case and that will drop the power rapidly.

Bluestacks mac os download. Auxiliary exhaust ports adds time area which can be seen in the distribution chart.

The same port but without auxiliary ports.

Now, the blowdown timearea is not just a figure to match, it is dependent on how strong pulses the pipe deliver. If the pipes tuning degree is high (strong pulses), then we get away with less blowdown time area !
And the transfer ports don't need so much time for scavenging since the pipe suction wave is strong, pulling out the gas from the crank case. We could then have wide and low transfer ports. With low ports we can go for higher rpm without getting blow back into the transfers.
Why do we need to take all this in consideration for the exhaust port design? The answer is that it is not only a time area target value, the whole system needs to be investigated since there is an interaction, a balance between pressures as the pipe is affecting even the reed valve and the carburator at BDC!!!
The normal procedure for race engine design would be to keep the exhaust port as low and wide as possible and to match the blowdown target (depending on pipe). The exhaust blowdown target is a statistical value for the tuning degree decided by the bmep target (braked mean efficient pressure). The port duration should not exceed the recommended value.

Expansion Chamber

When the exhaust port shape is decided from time area targets etc. we can dimension the exhaust pipe header diameter. This is critical to the pipes blowdown efficiency and the pressure build up. We need to pressurize the pipe with a strong and not too short pulse and the length will interact with the pulse resistance to create the necessary pulse shape. This means that it is not always better to use a diverging header (L1), sometimes a diverging header may result in that the cylinder exhaust evacuation is too rapid and causes the pipe to 'loose the breath' during the stroke. It will gain power at high rpm but lose in the lower range, mostly okay for race engines though. The phenomena needs to be viewed in a simulator to be fully understood.

To simplify some we say that the first diffusor (L2) acts in the lower rpm range and the last diffusor (L4) in the upper range. The rear baffle (L6,L7) angle decides the top end rpm range and power 'hit'. Steeper angles will increase the pulse strength and decrease the pulse length, i.e. shorten the rpm range at which the usable power is produced. The internal length between the diffusors will also decide the power production characteristics, so if the first diffusor is relative long then it will gain power in the lower rpm range etc. A pipe that is pressurized with an early opened port (long duration) will be able to retain the pressure through the steep angles and deliver a strong suction pulse back at BDC. With strong pulses engaged we don't need too large/high transfer ports. The pipe will then help and pull out the gas from the crankcase, even manage to open the reed valve and pull more air through the engine. When the transfer ports are closed, a second returning wave in the pipe pushes back the fresh gas that was spilled out into the header, in the remaining blowdown window that is. The charging pressure is often in the region of two atmospheres (bar) and far over that.

This is why the pressurization of the pipe needs to be well investigated together with the exhaust port, we simply cannot fit any pipe to an engine. It have to fit the exhaust port and even the transfer ports too!!

Finally, the stinger need to be large enough to let the engine breathe. A small stinger diameter (and long stingers) will increase the internal pressure and power but also make the engine run hotter. The length becomes critical to pulse resonance over about 9000 rpm. At that point the plunging effect at the stinger end will interact with the pipes internal pressure and help to lower the evacuating pressure at piston BDC. However, for high rpm engines, the stinger in general needs to be smaller than on low rpm engines. High frequency pulses have shorter wave length and will fit a smaller pipe better. But since the high rpm engine also needs to breathe more this can often even out.

More to come..

This Article Courtesy Of

MOTA and the new MOTA-X. You can ask questions anytime by email .. email us ...
The new MOTA version 6.30 for Windows® is now available.
The world's leading 2-stroke engine simulation software available ready-to-run on your PC, at only AUD$265
( Australian Dollars ) including GST and delivery within Australia ... or exported to most countries.

For a limited time MOTA-X v1.0 will be included with the purchase of MOTA version 6.3 at no additional cost.

MOTA-X contains all the features found in MOTA plus expansion chamber optimisation code.

The optimiser searches automatically for the expansion chamber dimensions that give the maximum power or torque for your engine over a speed range that you input. It runs the tried and tested MOTA simulator many times, altering the expansion chamber dimensions intelligently between simulations, to produce an optimal expansion chamber design. This expansion chamber is not the product of simple correlations as for other products that abound on the web, but an 'automatic' design created by MOTA which is a full wave action 2-stroke engine simulator.

Because the optimisation process performed by MOTA_X is inherently slow, the use of facilities found in multi core processors is exploited in the code. This means that to run MOTA-X, you will need a post 2010 computer which is preferably a desktop, driven by an intel i7 4-core processor. Processors with a lower number of cores will run the MOTA-X expansion chamber optimiser, as will the latest i5 duo core processors, but at greatly increased execution times. An optimisation on an i7 4-core processor based desktop takes typically 15 minutes, but can range in time between 5 minutes and 30 minutes. A laptop with the same specifications as a desktop will take typically 2-3 times longer to run the same optimisation.

MOTA v6.3 does not require such high end machines to run, and, as always, will run on nearly any computer made after 2000. However, MOTA 6.3 does not have the expansion chamber optimisation capability provided by MOTA-X.

We also have a simple 2-stroke expansion chamber design program. This will get you started by providing, for your engine, the dimensions of an expansion chamber with a 2-stage or 3-stage diffuser. Free download here . Of course the dimensions so provided could be inserted in a MOTA engine data file and MOTA-X then used to provide an optimum expansion chamber design for your engine over your choice of speed range.
The earlier MOTA version 6.10 was an evolution of the v6.00 that includes some fabulous new tools.
Power/Torque Curves Cursor Bar
The display of Power/Torque curves has been enhanced by the inclusion of a vertical cursor bar which extends over the entire height of the plotting area and whose position can be controlled by the mouse. Where the bar intersects each curve a horizontal cursor is drawn and, to the right of the plotting area, the corresponding power and torque values and the engine speed are displayed.

Additions to the Expansion Chamber Construction Utilities
The Expansion Chamber Construction utilities have been extended considerably and are now accessed under a separate item on the Main MOTA Menu. A sub-menu offers the two selections 'Constructing the Development Pattern of a Cone' and 'Printing the Development Pattern of a Cone'. It is the options provided under the second selection which have been added to MOTA. You can now print the development pattern of a cone and this may extend over several A4 pages. Of particular note, you can produce the pattern of a cone having either end or both ends angled to the cone axis. A set of explanatory diagrams with text can be displayed. You may also define a single straight cone and print the patterns of each of the pieces which, when welded together, will provide an equivalent bend section. The number of pieces and the overall bend angle are entered through the keyboard. A MOTA engine data file may also be accessed and the pattern of each section of the expansion chamber printed. Alternatively, any one section may be selected and patterns suitable for the construction of an equivalent bend section printed.
We now have a simple 2 stroke expansion chamber design program. Free download here

This program is not a part of MOTA, but it has been put together by the same engineers as a starting point for those wishing to begin from scratch. It calculates the dimensions for both double and triple stage diffuser expansion chambers from a few basic engine dimensions. The information used in the program's calculations was taken from the books ‘The Basic Design of the Two Stroke Engine’ and the book ‘Design and Simulation of Two Stroke Engines’; both books are written by Professor G.P. Blair of Queens University Belfast, and published by the Society of Automotive Engineers. You are well advised to read at least one of the books mentioned above, since they contain the author’s academic lifetime of knowledge on the two-stroke engine.

There are several coefficients used in the design of the expansion chamber – these are a function of the engine’s state of tune. Those used in this program have been chosen for petrol engines, and are in the range 50cc up to about 500cc per cylinder. It is doubtful these formulae would work on small capacity glowplug engines, since the exhaust gas temperature is much lower, and the engine speed is much higher.

How it all began ....

MOTA® is the brainchild of Dr Julian Van Leersum, mathematics graduate from Monash University in Melbourne, Victoria. He is of Dutch/Swiss parentage, but now permanently resident in Australia.
With the successful MOTA® software, Dr Van Leersum has managed to combine his professional interests in computing & mathematics with his enthusiasm for karting and motorcycle racing.
'It occurred to me that most home tuners cannot afford the expense of hiring time on a dynamometer to check the viability of adjustments or special parts' said Julian, 'yet many people these days have a home computer which could easily run a suitably designed tuning programme.'
'Although there is really no shortage of books on the subject of 2-stroke tuning and preparation, I knew that an active software programme would be able to offer so much more.' So, this is how MOTA® was conceived.

MOTAis an engine simulation program suitable for everyone from the enthusiast to the university researcher - from the beginner racer to the professional tuner. See testimonials. You can test your own engine and then re-test and compare your modifications - or build your ultimate engine right on the screen. Millions of fluid and thermo-dynamic calculations are made by MOTA representing the conditions inside your two-stroke engine throughout it's complete operating cycle.
No previous knowledge of the two-stroke cycle engine is required - just input the required data and then run MOTA to set in motion this powerful process. Test your own theories on porting and exhaust pipe design; explore the limits of various intake methods; or just look for the highest power output from your own engine.
MOTA will accept a single-cylinder design, which will also cater for many multi's where 2, 3 or more cylinders of the same basic layout are repeated. Easy-to-operate, accurate and hours of fascinating results to enjoy! Excellent graphics you can analyse and compare. MOTA's 'Two-stroke Dyno' will give you and your PC the equivalent of many experts knowledge.
MOTAand it's related set of programs have been developed to simulate the performance of high-output single cylinder two-stroke engines. It will simulate one of the cylinders of a multi-cylinder two-stroke engine provided that the cylinders are identical in layout and dimensions and each cylinder has a seperate exhaust and induction system. It allows simulation of engines with reed-valve, rotary-valve and piston port timed induction systems. Simulation of engines with either a box-silencer or an expansion chamber is also possible.

Because MOTA solves the equations describing the conservation of fluid and thermodynamic properties throughout an engine, it requires specification of the full engine geometry. This is accomplished through a menu driven environment, which prompts you for the required dimensions which are easily entered via the keyboard.

The output from MOTA is provided in two forms; a file, which summarises the engine geometry and performance, and a graphical interface which allows you to plot the various performance variables. The output file and these plots can be printed if you have a printer connected to your computer.

How MOTA works for you

The strength in MOTA lies not so much in it's ability to predict accurately the performance of an engine, but in it's ability to allow evaluation of two different engine configurations. For example, if you have the MOTA produced power curve for an engine, and you want to see if modifying one of the exhaust pipe dimensions will increase or decrease power, then you can change the particular dimension and re-run the MOTA simulator. Comparison of the new power curve with the old will allow you to determine whether or not the change will be worth making.

2 Stroke Tuning Software

Where is MOTA different to other computer programs and tuning manuals - and what can it do?
MOTA operates in the 'real world' and it will:-
a) accept most any dimension and variations of dimensions.
b) operate on anyone's exhaust pipe theory by accepting almost all shapes.
c) operate within the limitations set under certain racing regulations.
d) accept that the data you input is from an engine that is running already, and will output it's predicted performance as a base for you to work from.
e) accept alteration to one engine dimension at a time - either real or imagined, and produce all of the new
outputs with the new dimension.
f) accept that ignition timing can be varied, and allows you to input such changes - possible or not, you can make the alteration to see what would happen.
g) does not ask complex technical questions - data inputs are easily measured and entered on the screen via the keyboard.
h) accept almost limitless variations, so you can test ideas beyond any current theories or practises.

So how does this help you?
Most everyone will have an actual engine that they wish to play with, or make perform better. MOTA can do that easily and quickly. It does not trouble you with the in-depth why's and wherefore's of two-stroke engine design theory - you are not trying to build an engine from scratch. You can do one alteration, or many, and MOTA works with that.

System Requirements for MOTA v6.30

• Windows compatible computer with a minimum of 20MB RAM, VGA colour display and 40MB of free hard disk space.
• Windows® compatible mouse and keyboard required.
• Printer is optional - only needed to produce hard copies of output graphics and files.
• PC to be loaded with a Windows® 7 , 8 or 10 ( 32 or 64 bit ) operating system.
• minimum 2 USB ports.
• USB hardware security lock ( dongle ) included in program package

Special features include

1. Suitable for Kart, Motorcycle, chainsaw, personal watercraft, Model Aero and most similar engines.
2. Accepts most expansion chamber designs.
3. Methanol or petrol fuels accepted.
4. Will test almost any reed valve material.
5. Ignition timing can be varied, and 'curves' accepted.
6. Integrated, box type or separate muffler designs accepted.
   

2 Stroke Tuning Expansion Chambers

Price: AUD $265 Australian Dollars including postage within Australia .. or exported including airmail post to any country.

Details for purchasing upgrades ...... The current version of MOTA is v6.30
MOTA version 6.30 for Windows® is now available , should you wish to upgrade.
The cost is AUD$195 if upgrading from versions 4.xx or 5.xx or 6.xx -- plus postage.
Your program number must be quoted when ordering -- it is written on both your dongle and distribution disk(s) or USB.
The earlier version distribution disks are not required to be returned when ordering an upgrade.
Purchasing MOTA
If you want an order form to purchase the MOTA software , or an upgrade, we have two available.
In .pdf format ..... pdf here or an .xls file to email us .... excel here

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