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 Making the Power-3

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Trevor Amos

Number of posts : 885
Registration date : 2010-08-13

PostSubject: Making the Power-3   Thu Nov 17, 2016 6:26 am

The Compression/Expansion Ratio.
This maybe be an unfamiliar expression to some of you, but is one which describes a particular aspect of two-stroke engine gas dynamics that has only recently become a small but important factor in race engine design that has consequences for the effectiveness of exhaust pipe resonance. The significance of this ratio is revealed during the comparatively short crank rotation from the effective completion of combustion to the opening of the exhaust port, a duration of perhaps 30 or so degrees. Its impact however is felt later, when the exhaust pipe is trying to make power for us.
No matter which direction the piston is travelling, as long as there is combustion there will be a pressure rise in the cylinder, but any pressure rise before TDC creates negative work so will oppose and slow down upward piston motion, and we certainly don`t want that. Combating this is easy enough by simply retarding the timing point of ignition, but that causes a major headache for an engine with a non-retarding system such as the large capacity Bantams are forced to use? Running at the base of the power band and low rpm there is little cylinder filling. The ignition timing on avgas could be 30*BTDC allowing plenty of time to try and burn the mixture, at low dynamic comp ratio, that is in the cylinder. But you won`t get too far at peak torque rpm with a static timing of 30* on an air-cooled 186 Bantam; huge compromises are forced upon these engines by this Catch 22 scenario!

A large expansion ratio, from a high compression ratio, before exhaust port opening takes away energy from the exhaust gas, energy that is so crucial to the beneficial phases of pipe resonance. However, this will mean the gasses flowing out through the exhaust are cooler and the quickest and easiest way to cool and slowdown hot gas, is to allow it to expand. So it follows that if the expanded exhaust gas is cooler at the point of exhaust port opening there will be less heat transfer to the barrel and the exhaust duct so the engine runs cooler: in other words, a higher compression ratio yields a cooler cylinder barrel! That may be seen as a good thing for we all want cooler running engines, but the compromise here is that the potential for the pipe to further assist in boosting engine power has been lost!

A low secondary compression/expansion ratio retains more exhaust gas energy for the pipe to exploit in “supercharging” the cylinder so that the next “bang” becomes bigger, and the bang after that and so on!
As gas energy levels are mainly effected by ignition timing and compression ratio, a lower compression ratio before TDC means a lower expansion ratio, hence lower temperature drop after TDC, and the average pressure between TDC and exhaust port opening will be higher. This means that more energy is retained for the pipe to do good things with and help make more horsepower. Another and rather fortunate consequence of lowering compression, or to be more precise increasing the cylinder volume above the piston prior to exhaust port closure, is that the over-scavenged mixture sitting in the duct and header has a much easier time when being stuffed back into the cylinder by the returning plugging pulse. If then that action is made easier, there must be every chance that even more mixture can be returned to the cylinder for the same time period and pressure ratio.

Some of this same thinking can be applied to mixture exchange from crankcase to cylinder via the transfer ducts. During transfer of mixture case pressure drops; the larger the case volume the smaller that pressure drop will be. That means the average pressure will be higher and by definition the density of that mixture will be higher, a denser mixture means more mixture mass is transferred to potentially make more combustion energy for the next cycle! Even if the flow volume was compromised by lack of transfer port time/area and poor, basic, duct flow efficiency, higher mixture mass still helps.

All two-stroke engines share one common, fundamental feature: the infinite atmosphere at the carburettor bell mouth and at the exit of the exhaust tailpipe. Within the oscillating nature of engine breathing cycles, these extremities are interconnected by the transfer ducts. The implications for and significance of achieving high coefficients of flow within the various ducts inside the engine becomes very obvious, with the biggest single impediment to any level of efficient flow being; turbulence! Given enough turbulence an aeroplane can fall from the sky; worth thinking about?


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