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Motorcycle Forum / General / Sportbikes / January 2006Semi on-topic: Race fuel/engine designs
For a slow AMS winter Sunday. Put on your thinking caps folks & don't be lazy and not even attempt to answer this semi-on topic question. Of these two very powerful race fuel formulas, which do you think produces more power and is thriftier by a considerable margin? Hint: The 1st is used in a "SI" "type" engine while the second is used on the same engine, converted to utilize a "CI" cylinder head. 1) Fuel # 1 for the SI design: 40% Methanol 30% synthetic/Caster oil blend. 30% Nitro-Methane 2) Fuel #2 for a CI design: 33% Either 30% synthetic/caster oil blend 33% Kerosene 4% Amyl Nitrate Note Compression ratios are ~14:1 for engine #1 and 14 to 18:1 for engine # 2. Otherwise they are identical except for their two stroke cylinder heads. Also engine #2 has the ability to vary its compression ratio externally. Finally both have a simple slide or barrel carburetor and engine #2 has no fuel injection rather simple fuel/oil mixed crankcase scavenged piston pumping operation with a rotary fuel induction valve, a common transfer port and an 80 degree slit and muffler/expansion chamber for the exhaust port. Fuel is also pressurized by the muffler in a non-float carb but rather like simple Fi. Also the aforementioned carb is a bit different than the crankcase pumper type diaphragm carbs used in weed eaters. Additionally: Both engine operate at or near stoichiometric air/fuel ratios and as I mention previously, both can be intake air throttled. PS: extra points for a reasonable understanding (explanation) of how these two engine function and their typical applications. Also, This is obviously not for you liberal arts types like Peter or AB-in "spirit anyway". Bob Nixon 01 Sprint ST "RED" 52K Chandler,AZ http://bigrex.net/pictures >For a slow AMS winter Sunday. Put on your thinking caps folks & don't be >lazy and not even attempt to answer this semi-on topic question. Stop playing with model airplane engines Bob. > For a slow AMS winter Sunday. Put on your thinking caps > folks & don't be [quoted text clipped - 57 lines] > "spirit > anyway". The CI engine will have higher peak horsepower because of the shorter intake path, higher intake pressure from velocity stack pulses, .....which you don't have with SI configuration (with the SI roughly 2 or 3 to 1 atmosphere max as a function of net crankcase displacement under piston stroked area), The CI motor will have a net higher compression ratio if the head is configured to accomdate it. (less displacement capacity wasted in re expansion from the top of the stroke). SI as I understand it here means scavanged intake from the crankcases, piston down stroke creates intake pressure... the crankcase can be and often is packed to increase this pressure. Here we are talking about two stroke motors (4 strokes can be built with CI or SI intakes also but are not comon) The GE locomotive diesels are two strokes, but with poppet valve exhausted cyl heads (sleeve SI intakes, but with blower powered crankcase pressure...) The CI engine would likely have a longer stroke than the SI motor.. so you see the slower burning kerosene added to slow the burn and mitigate on the pre-ignition issues, this provides more complete combustion and higher fuel efficiency for the CI motor.. With the CI engine at 18 to one might not need a timed ignition, providing closer to ideal ignition timing in contrast to an SI engine at 14 to 1 which would need a timed ignition, the timing per force not ideal at all speeds. CI means cylinder intake, intake pressured by velocity stacks and possibly a partial hit from exhaust gases (even though those are short on oxygen... the addition of amyl nitrate compensating. (extra oxygen in the net fuel/ air mix). The CI engine will be less tractable (narrow power band) than the SI engine since the SI engine's intake is pressurized mechanically by the piston, and not reliant on a tuned intake path shock wave that the CI motor is.. Peak power and fuel efficiency goes to the CI engine. Tractable power to the SI engine as I see it. Fuel efficiency goes to the CI motor. Phil Scott > Bob Nixon > 01 Sprint ST "RED" 52K > Chandler,AZ > http://bigrex.net/pictures >> For a slow AMS winter Sunday. Put on your thinking caps >> folks & don't be [quoted text clipped - 23 lines] >> engine # 2. Otherwise they are identical except for their >> two stroke [...] >The CI motor will have a net higher compression ratio if the >head is configured to accomdate it. (less displacement >capacity wasted in re expansion from the top of the stroke). > >SI as I understand it here means scavanged intake from the ^^^^^^^^^^^^^^^^^ Spark ignition or glow plug in this instance. Glow plugs work at this scale for two reasons or 1) small size vs burn time=low detonation factor & 2) Nitro-methane fuel has very high octane >120+ RON. >crankcases, piston down stroke creates intake pressure... the >crankcase can be and often is packed to increase this >pressure. > >Here we are talking about two stroke motors (4 strokes can be >built with CI or SI intakes also but are not comon) ^^^^^^^^^^^^^^^ Actually both two and four stroke Gas and Diesel engines are common. >The GE ^^^^^^^ Not GE; they're 4 stroke train engines but EMF (General Motors) still use two stroke Diesel train engines. >locomotive diesels are two strokes, but with poppet valve >exhausted cyl heads (sleeve SI intakes, but with blower >powered crankcase pressure...) That's correct as are their smaller 53, 71,91 & 149 series but the now obsolete 51 series was totally ported. The number stands for cubic inch per cylinder or 6-71 inline 6=426 cubic inches=7 liters and so on. Detroit Diesel bought out GM Reman-2 strokes. Reman used to make the following: 1-51, 2-51, 3-51,4-51, 3-53,-4-53,-6V53, 8V-53,3-71,4-71,6-71,6V-71,8V-71,12V-71,6V-92,8V92,12V-92,12V-149,16V-149,20V-149 The train engines are an order of magnitude larger and run at only 800RPM, while the smaller (truck, generator & boat) engines run at either 1500-2100RPM or 2800RPM in the smaller 53 series. BTW, most of the modern versions = to exceed 1HP/cubic inch. Yes even that 20 cylinder 20V-149 engine has ~3000HP out of 2980Cu-inches Around 1960 these engines went to 4 exhaust valves instead of just two and as you said, use a ring of ports at the bottom of the cylinder for scavenging with a roots blower but I believe the new EMF Diesels start with external air pressure and have done away with the supercharger and use ONLY a more efficient turbo-charger to scavenge the intake in and exhaust out in 180 degrees of the two stroke rotation. Two strokes have only a 180 degree compression & power strokes versus a four stroke's ~240+ degrees. That alone is the main reason why you can't really get twice as much power from a NA two-stroke engine. It's more like 500HP per/liter vs. 300HP/liter from an FI four stroke car engines vs. the zenith of the two FIM era, These figure are for naturally aspired engines of course. Actually, there's A two stroke ducked fan model airplane engine that produces an advertised 980HP/liter which exceeds either a top fuel dragster or those old turbo-charged F1 1.5 liter BMW car engines and the little two stroke is naturally aspirated!. Anyway, not to bore ya-all to death but just some stuff on piston engines I find enteresting. [...] >Fuel efficiency goes to the CI motor. Good try Phil! However CI=Compression Ignition=Diesel and SI=Glow plug ignition (not really spark) using a fairly mild blend of nitro-methane 10-30% max nitro compared to Dragsters @ 80% Nitro-20% methonol. The Diesel ONLY works carburated or in stoichiometric mode as opposed to direct injection "lean statified operation" because the fuel contains so much volitile either (very low flash point) to get the combustion process going at fairly low compression heat levels. If you think about the smallness these tiny engines, it's clear that heat loss to metal, regardless of compression ratio and subsequent heating would be prohibitive. Hence, we have nothing really different but a different cylinder head (with contra-piston to vary CR) instead of a glo-plug. The reason the Diesel has more power is two fold. 1) higher CR & 2) higher BTU fuel (kerosine 2X) and a third componant of cooler, more efficient operation of the CI head version, Bob Nixon 01 Sprint ST "RED" 52K Chandler,AZ http://bigrex.net/pictures > For a slow AMS winter Sunday. Put on your thinking caps folks & don't be > lazy and not even attempt to answer this semi-on topic question. [quoted text clipped - 41 lines] > Chandler,AZ > http://bigrex.net/pictures God damn it Bob, did you go digging around in my old flight box and dig up a couple of old .049 glow engines and my old McCoy .19 diesel?  SignatureGopher Greg '77 CB750K Stock '78 CB750K AHRMA '00 ZG1000 Stock '96 Ducati 900SS Former track bike '01 GSXR750 Current race bike **********pull 'mychain' to reply*********** ("I've abandoned the idea of trying to appear a normal, pleasant person. I had to accept myself as I was, even if no one else could accept me. For the rest of my life I would continue to say precisely the wrong thing, touch people in the raw and be generally unpopular. I had a natural gift for it" W. F. Temple) |
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