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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 theory will be updated continuously with more details and headlines.
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:
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.
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 right 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 picture below right , 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.
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.
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.
4. 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…
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