Frank Skilbeck Posted January 10, 2009 Share Posted January 10, 2009 Just wanted to correct something in Brain Which's column where he states the complete weight of fuel in a tank is the force pushing the fuel out. This is incorrect the force pushing the fuel out is the height difference between the fuel level and the spray bar plus the difference between the pressure in the fuel tank (above the liquid and the pressure at the spray bar.Also he states its the fact that exhaust gases are hot that causes the increase in pressure in the exhaust, in which case if you block off the exhaust outlet and turn the rngine over you should see no pressure increase. If you didn't heat the air (by combustion) and extract the heat to give power but applied power to the prop shaft then you would have a compressor.And the lower pressure at the spray bar is due to the velocity of the air going through the carburator, as per Bernoulli an increase in velocity gives a reduction in pressure and it's this that creates a lower pressure and draws the fuel in via the needle valve.The recommendations re the tank position etc are fine, but for the wrong reasons. Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted January 11, 2009 Share Posted January 11, 2009 Also, the paragraph on valve overlap is incorrect in that it completely ignores the depression created in the exhaust port (by gas column inertia and the effects of pressure wave action) and its influence on cylinder filling and scavenging.The problem is that these articles, although interesting, contain technical information written for a magazine rather than a technical publication and are perhaps not proof read for technical accuracy. PS. Also, it used to be said (probably still is) that it is advisable to avoid the use of the word ‘weight’ except when buying groceries from a shop.(Weight being a force exerted on a mass by gravitational acceleration). Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted January 12, 2009 Share Posted January 12, 2009 I'm not the only one whose teeth were gnashing then! The whole discussion on syphoning, how carbs draw fuel and the effect of heat in the exhaust was based on entirely false premises - for instance the detail that even if there WAS any pressure developed due to heat-soak in the exhaust system it would just result in warm air expanding out through the conveniently-positioned gaping hole at the back (the one the exhauust comes out of!).The syphoning is simply the result of the fuel jet being below the boddy of the fuel - if the pipe is full the fuel will syphon out (as demostrated by petrol thieves the world over!). There are many "cures", the simplest being to have a part of the fuel line that goes above the level of the top of the tank coupled to a routine practice of tipping the aircraft nose-up to drain the fuel lines back into the tank (if the fuel pipe is empty and the fuel line is higher than the fuel level then it won't syphon. Brian was also way out on the valve overlap thing. The valve overlap uses the energy in the exiting exhaust gas to help pull the fresh charge in, whilst also using the momentum of the incoming charge (which was moving quickly until the inlet valve shut) to create pressure behind the injet valve so that the gases enter more quickly (and at higher pressure) when the valve opens. For 4-strokes to run *at all* at speeds much abpove 5000 rpm needs an overlap of over 40 degrees, and much larger overlaps are used in higher-performance engines. Stick to machining advice, Brian! PDR Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted January 13, 2009 Share Posted January 13, 2009 Peter, thanks for the advice. But perhaps I was less than clear….The exhaust pressure wave travels at high speed and can reach the speed of sound (speed of sound varies with temperature and pressure) until it reaches the end of the pipe. Here it is reflected back up the pipe becoming a low pressure wave and augments the extractor action (provided it reaches the port at the correct time) through the open exhaust valve. This also enables the inlet valve to open several degrees before TDC. The ‘ramming’ effect of the high-speed inlet charge on overlap is somewhat limited due to the short distance from valve seat to carburettor air intake. At mid stroke the induction charge is at high speed and will be so until the commencement of the compression stroke. As the piston is travelling up the cylinder before the inlet valve closes the amount of ‘new’ charge taken becomes less than the swept volume of the cylinder and therefore volumetric efficiency is low. Large overlap does indeed allow for higher RPM but all factors need to be in harmony for an engine to benefit from large overlap, even then obtaining a reliable idle can be a problem because wave frequency is practically constant. PS. Was my previous post edited, as I seem to recall making a less than obvious joking link to Frederick Camm (brother of Sydney) and his famous comics? Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted January 13, 2009 Share Posted January 13, 2009 Brian,I am mortified with embarrassment - my remark about sticking to machining was addressed to Brian Wynch, not yourself!I'm familiar with the pressure pulse effects, all the more so having spent ten years in FAI pylon racing where a good understanding of pressure-wave supercharging (ie tuned pipes) was part of the winning requirements. I agree with everything you say, but would add that in the mid 80s we did manage to cure a number of nasty characteristics, and provide a small power boost, on an Enya 91 4-stroke by putting a long tract on the inlet and an anti-reversionary mismatch on the exhaust - this brought some moderation to what was otherwise a rather "hot" cam for the inetded application. Anyway, apols for the ambiguity!PDR Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted January 13, 2009 Share Posted January 13, 2009 Brian Parker wrote (see)Peter, thanks for the advice. But perhaps I was less than clear….The exhaust pressure wave travels at high speed and can reach the speed of sound (speed of sound varies with temperature and pressure) until it reaches the end of the pipe. Here it is reflected back up the pipe becoming a low pressure wave and augments the extractor action (provided it reaches the port at the correct time) through the open exhaust valve. This also enables the inlet valve to open several degrees before TDC. The ‘ramming’ effect of the high-speed inlet charge on overlap is somewhat limited due to the short distance from valve seat to carburettor air intake. At mid stroke the induction charge is at high speed and will be so until the commencement of the compression stroke. As the piston is travelling up the cylinder before the inlet valve closes the amount of ‘new’ charge taken becomes less than the swept volume of the cylinder and therefore volumetric efficiency is low. Large overlap does indeed allow for higher RPM but all factors need to be in harmony for an engine to benefit from large overlap, even then obtaining a reliable idle can be a problem because wave frequency is practically constant. PS. Was my previous post edited, as I seem to recall making a less than obvious joking link to Frederick Camm (brother of Sydney) and his famous comics?Not by me. Quote Link to comment Share on other sites More sharing options...
Brian Parker Posted January 14, 2009 Share Posted January 14, 2009 Peter, A case of ambiguity and to many Brians. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted January 17, 2009 Author Share Posted January 17, 2009 Same effect with the inlet, varying the lenght can at certain harmonics give a small "supercharging" effect. Some of the high power fourstroke engines use variable length inlet tracts to maximise this effect at different rpm. Takes me back to my 3rd yr engineering project (long while ago ) which was to look at effect of varying inlet tract length on a two stroke engine (MZ250 ) and plot the harmonics etc. After the test I checked the original air box and sure enough the inlet duct was the same length as the one that I had shown gave the max benefit. Get the wrong length and you will lower the pressure at the inlet port/valve just as it is opening having a negative effect. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted January 17, 2009 Author Share Posted January 17, 2009 Same effect with the inlet, varying the lenght can at certain harmonics give a small "supercharging" effect. Some of the high power fourstroke engines use variable length inlet tracts to maximise this effect at different rpm. Takes me back to my 3rd yr engineering project (long while ago ) which was to look at effect of varying inlet tract length on a two stroke engine (MZ250 ) and plot the harmonics etc. After the test I checked the original air box and sure enough the inlet duct was the same length as the one that I had shown gave the max benefit. Get the wrong length and you will lower the pressure at the inlet port/valve just as it is opening having a negative effect. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted January 17, 2009 Author Share Posted January 17, 2009 Is it me or are there some problems posting on the new site. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted January 17, 2009 Share Posted January 17, 2009 well we do have a few bugs still, but actually posting /or starting a thread is fine AFAIK Frank - what specifically is the prob ? Quote Link to comment Share on other sites More sharing options...
Pete Rieden Posted January 18, 2009 Share Posted January 18, 2009 Takes me back to my 3rd yr engineering project (long while ago ) which was to look at effect of varying inlet tract length on a two stroke engine (MZ250 ) and plot the harmonics etc. After the test I checked the original air box and sure enough the inlet duct was the same length as the one that I had shown gave the max benefit. Get the wrong length and you will lower the pressure at the inlet port/valve just as it is opening having a negative effect. Now that has just given me a serious attack of deja vu! Frank - were you ever in the woking club, or others in the surry/hants area? I have a feeling we may have met back in the late 70s or early 80s PDR Quote Link to comment Share on other sites More sharing options...
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