supertib
Posts: 3662
Joined: 3/30/2004
From: , MB, CANADA
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quote:
ORIGINAL: SAVAGEJIM
I thought through how nitro engines work and I realiaze I will have to edit alot of what I said prior. After a more thorough thinking about the physical positions of nitro engine parts, I came to the conclusion that nitro engines will benefit very little from super charging.
I made a rudimentary degree wheel with a round card piece and marked off special angles in 15degree increments. Supertib, I concede to you this: when all the ports are open simultaneously, I discovered big blocks with stock timing profiles will present very little openings.
Anyways, lets discuss why nitro engines cannot benefit much from super charging. You say that nitro engines have two phases when taking fuel-air into the engine. We will look at both phases, especially the second phase. We will assume the 16PSI charge you mentioned previously. We will assume that induction port in the crank is NEVER open when the sleeve ports are open.
Before any of these 2 phases, the air pressure in the card is increased by the forced induction unit, say to 16PSI. See the first illustration below.
Phase 1: The crank induction port opens and allows the fuel-air charge to enter the block. The piston is still up and the sleeve ports are fully closed. The fuel-air charge retains its 16PSI pressure. See the second illustration below.
Phase 2: This is where the charge will lose its pressure! Again, the crank induction window is still fully closed. As the piston goes down, the the exhaust opens first. Then, the Schneurle (lateral intake ports) and open and the boost finally opens too. ALL SLEEVE PORTS ARE OPEN! The fuel-air charge in the case still at 16PSI moves into the cylinder, of course the fluid flow of the charge is pushed away from the exhaust, but since the charge has more energy in the form of the 16PSI pressure, the charge expends alot of this energy (maybe about 3, 4, or maybe even 5PSI depending on the ambient pressure) by pushing some (maybe alot!) of the fuel-air charge out the open exhaust unburnt. This is where the aditional pressure is lost as the 16PSI pressure tries to balace itself with the ambient pressure since the charge is no longer in a fully enclosed system. Even with the crank port fully closed, you cannot stop this excess discharge. See the 3rd illustration below.
Worse yet, the energy the engine epended to turn the super charger is pretty much wasted.
Lets review. The super charger increases (boosts) the pressure in the carb neck; this is a fully enclosed sub-system. No pressure loss here. (The forced induction unit itself stops the air from coming bak out it and the close crank window prevents the charge from entering the engine).
Phase 1: The crankshaft allows the charge to enter the cas; again no pressure loss here because the charge is still full enclosed (the crank closes behind it and the piston has all sleve ports closed.
Phase 2: Pressure loss! Even with the crank induction port fully closed! As all sleeve ports (intake and exhaust ) are open simultaneiosly, no solid object stops the charge from flowing out. The charge will flow from the crank into the cylinder, out the exhaust port, and out the stinger into the ambiant suroundings without any sort of solid obstruction.
okay Jim your starting to understand..... but your still missing a few things.... first thing is that you have to quit looking at PSI as power, PSI means nothing..instead of looking at PSI look at air volume and even better yet mixture density... The supercharger will increase the density of the charge in the crank case, plain and simple, it will allow a much more tourough crankcase filling...resulting in more mixture being forced into the chamber....
Second thing is you have to realise the mixture from the crankcase is forced into the combustion chamber by pressure, the more pressure in the crank case the more mixture enters the combustion chamber..The piston traveling down after combustion is what creates the presure required to force the mixture from the crankcase up into the combustion chamber..... THis pressure is required for a 2 stroke to operate..it is perfectly normal to have blow thru on a 2 stroke, and in fact it is desired and leads to making more power....
The tuned pipe of a 2 stroke operates by returning that blow thru back into the chamber, which allows a 2 stroke to operate up and beyond 100% volumetric efficiency.... And the timing of the tuned pipe will not be afected by volume of mixture pushed into the header, but rather by the timing of the return pulse, more mixture in the header, the more extra mixture is forced back into the combustion chamber.....which means more power. A tuned pipes effect is based off timing, and as I say having a denser blowthru mixture will only increase the tuned pipes effectiveness..
Another thing to consider is these motors currently operate under 14.7 PSI, the only thing adding PSI is going to do is increase the volume and density of the mixture stored in the crank case and allow the motor to run much cvloser to 100% volumetric efficency, and possiblt beyond, as these 2 strokes do not operate anywhere near 100% volumetric efficiency, and the higher the RPm the harder and harder it is for the motor to fuly charge the crankcase..... As at 10 00 rpm the motor has 14.7 psi of force pushing the air into the engine, but at 40 000 rpm they still only have that 14.7 psi, So their efficiency begins to drastically drop off, as it cannot fully fill the crankcase between strokes at higher RPMS.... The supercharger is going to help the engine properly charge the crankcase between strokes, as it provides a larger volume of air available to the engine.... A nitro engines limiting factor for making power is its ability to pump air, actualy this is true for any internal combustion engine.... Porting a nitro engine increases its ability to draw air into the motor, as you cannot make more power without air..... SO supercharging allows you to increase the volume of air, nothing more and nothing less.... think of things in relation to air volume not PSI... PSI is just a by-product of higher air volume
ANother anlogy to look at is operate one of these engines at 20 000 feet above sea level, and you will see the engine makes very little power and alot less RPM, as it now has much less air pressure to work with, the motors ability is now limited to the volume of air available to the engine..
then take this same motor and operate is at sea level, and all of a sudden the motor makes tons of power, and much more RPM, as now it has much more air available to the engine, everything works better, the motor take smore charge between strokes and the tuning effect of the tuned pipe makes a more dramatic increase, as well as the motor scavenges alot more efficiently...consider the supercharger as the same as operating at seal level but one step further, all it does is gives the engine more air(oxygen) to burn fuel with, and the key to making HP is pumping more air thru the engine...
Anyways I am rushing so forgive any spelling mistakes...
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seeing is believing !
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All Forums >> RC Cars, Buggies, Trucks, Tanks and more >> RC Car Engines >> Car Nitro & Gas Engines >> RE: were to find a turbo or supercharger kit 
RE: were to find a turbo or supercharger kit - 
) before allowing it to pass up through the transfer ports into the cylinder. The only rider here is that because this much pressure in the crankcase now allows a much faster flow rate through the ports then some alteration to port timings could be needed to prevent charge loss out the exhaust due to a much higher pressure differential (the exhaust being basically at ambient pressure). 
