RE: Hi Wavemaker
SuperChargers are nice but not on Nitro, no, I'm not speaking from personal experience but rather from having read all the post and comment made regarding a similar question.
OK...here is a thread (LONG) that will explain everything and help you save your money.
Found it using the Search function.
Dan
his is a long read so be prepared
edit made at - 6/30/2007 11:30:51 AM by myself, adding a new section about diesel two stroke engines.
I have been seeing a lot of these threads asking about superchargers around lately, so i thought i'd make this explaining why most people will say superchargers don't work on our little nitro engines, and considering the search function on RCU is far from the greatest, or people forget about doing a search altogether i figure a thread here would give some answers.
First lets look at what a supercharger does, well it's very simply just a small fan powered by a belt coming off of the crank shaft of an engine, the fan is in a tube the go's into the cylinders, when the fan spins it pushes air into the tube and makes a small amount of boost pressure (boost pressure is how much more pressure above the 14.7 psi of the atmosphere we live in) so say someone says the boost pressure is 5 psi (btw thats pounds per square inch), the actual pressure is 19.7 psi.
Now, when you add that extra air into a cylinder it means there is more oxygen, and the more oxygen there is in a given space the more fuel you can burn in the same space (the ratio for this is about 1:9 in weight with 20% nitro fuel, so if you have 9 grams of air, you can burn 1 gram of nitro fuel with that air) so if you add 5psi of air pressure into an engine you can burn roughly 25% more fuel in that space.
Okay so thats superchargers out of the way, now let's look at the two stroke engine we all love to drive and fly around.
The heart of 90% of the engines out there is the piston and cylinder, just about every engine you will find has both of these, every single engine has an output shaft, in an engine with a piston it's the crankshaft.
With a two stroke engine the piston and cylinder perform a lot of jobs, the main job is to make power, when our fuel burns in the cylinder, it makes pressure that forces the piston down, when it forces the piston down it forces the crankshaft to turn by the use of a connecting rod, or commonly known as a con-rod.
The other job it performs is it is the valves, so the piston controls when the burnt fuel (exhaust or exhaust gases) gos out of then engine, and when new, unburnt air and fuel gos into the engine, and this is done by holes in the cylinder.
I will take this time to explain about the crankshaft here, because otherwise you may get a little confuzed by this next section, the crankshaft in a nitro engine is hollow, so the middle of the crankshaft is kind of like a piece of pipe, in the side of the crankshaft there is a hole, this hole is what controls the air and fuel getting into the engine.
We'll start from when the piston is at BDC (that's bottom dead center) at this point a new lot of fuel and air is in the cylinder (i will explain how this happens shortly) as the piston start's heading back up it covers up the holes or "ports" in the cylinder, the first port to be fully closed is the intake port, the one that lets the new fuel and air into the cylinder, as soon as this port is closed off a vacuum (like sucking a pringles box to your bottom lip) is made in the crankcase (this is where the con-rod and crankshaft are) at the same moment the hole i was talking about in the crankshaft lines up with the carburetor (or carbie), as the piston travels up it suck the new air and fuel into the crankcase.
When the piston reaches the top of the cylinder (TDC or top dead center) the hole in the crankshaft passes by and seals the crankcase, right at this point the air and fuel ignite and force the piston down, because the new air and fuel is in the crankcase and the piston is making the space in that crankcase smaller, it compresses it.
The piston comes down just a little bit more and it starts to uncover one of the hole in the cylinder, this hole or port is the exhaust port, because the pressure in the cylinder is still very high all of that burnt fuel wants to go somewhere, so it go's out of the exhaust port, and in going out of the exhaust port it gets all of the air inside the engine moving very fast.
The piston gos down a little bit further and it uncovers another port, this one is the intake port, this port is linked to the crankcase by a little channel in the side of the block, we know that the air in the crankcase is now compressed and the exhaust gases are moving very fast out of the exhaust port, both of these together suck the new air and fuel into the cylinder, this is called scavenging, the old burnt fuel and air helps to pull the new air into the cylinder.
Now we know that because the exhaust port was the first port uncovered that it must be higher in the cylinder then the intake, so if it's higher in the cylinder the piston is going to cover it up after the intake port is already completely closed, so the flow of new air and fuel has stopped coming into the cylinder, but the exhaust port is still open slightly until the piston covers that up to.
Then that just repeats very quickly, unto 40,000 times every minute, thats 660 times every second.
So, we know how a supercharger works, and we know how our little nitro engines work, so whats the problem? why does everyone say that the supercharger doesnt work? it makes more air and fuel go into the engine right so it must be a good thing? well no, not really, i'll explain why
I explained just above how the exhaust port in the engine is open longer then the intake port, so what happens when we put more pressure into the cylinder?
The supercharger spins and forces more air and fuel into the crankcase, so theres more pressure in there, when the intake port is uncovers, instead of being sucked into the cylinder, it blows itself into the cylinder, and because both the intake and exhaust port are open it go's strait through the cylinder and out of the exhaust port, so you loose most of your extra air and fuel there, then when the pistons travels back up and closes off the intake port, the exhaust port is still open! you have this pressure in there and theres only one exit, the exhaust port, so that's where it go's, strait out of the exhaust pipe, and you end up with just as much fuel and air in the cylinder as you would have has without a supercharger.
Then there is the one last problem of a supercharger, it's driven off of the crankshaft of the engine, so the crankshaft has to turn not only itself now, but also this heavy fast spinning fan, and that makes it very hard for the engine to make power if it's using it to drive this thing on the top of the engine so you loose some power there instead of gaining it!.
now i have been receiving a few pm's on the subject of diesel two stroke engines, so i thought it would be wise to add this section explaining the use the supercharger on these engines.
there is one exception to the basic rule of supercharged two stroke engines, and this is diesel engines, as most of us know, a diesel engine does not have a spark plug, and when the fuel is getting used, it is not injected in with the are, the reason for this is diesel engines are compression ignition engines, let me explain.
in a normal gasoline engines, weather it be two for four stroke, it has a spark plug, and the fuel is mixed with the air that is coming into the engine, on the intake stroke the fuel AND air get drawn into the cylinder, it is then compressed, and when the piston thes to the top of the stroke, it is ignited, then exhausted.
in a diesel engine things happen a little differently, instead of the fuel and air being drawn into the engine on the intake stroke, only the air is, now a diesel engine engine has a tremendously compression ratio, anywhere from 17:1 up to 30:1 in comparison to a gasoline engines 10:1, and as we all know when air get compressed, it gets hot.
now the air in the cylinder of the diesel engine is getting compressed, and because of the compression ratio it get hot, very very hot, over 700 degrees F, in a diesel engine there is no spark plug, but instead there is a fuel injector, when the piston gets to the top dead center, the fuel injector injects a small amount of diesel into the cylinder, and because the air in the cylinder is very hot from the high compression ratio, rgw fuel instantly starts to burn, giving you the power from a diesel engine.
now because the fuel is not mixed with the air like in a gasoline, or nitro engine, this means that if the air was routed through the crankcase to get the air into the cylinder, there would be no lubrication for the crankshaft and con rod, so instead the crankcase in a two stroke diesel engine is filled with oil, just like your car engine, but then this leave us with a problem.
two stroke engines need to have some pressure to get the air into the cylinder, and not this cant be done by the piston in that crankcase because the crankcase is full of oil, so what is the solution? a high volume low pressure supercharger such as a root type supercharger, this gives the intake air on a two stroke diesel just enough pressure to clear the cylinder of the old. burnt fuel and replace it with fresh air.
then the piston compresses the air to about 30:1 compression ratio, and at the top of the piston stroke, the diesel is injected, just as explained above, so as you can see, when you here about a two stroke diesel engine with a supercharger, or superturbocharger (yes that is the proper name of a turbo) that it is not to make it a super powerful engine, it is simply to perform the role of the crankcase in a gasoline, or nitro engine of getting the air into the cylinder.
Hopefully this is has helped answer a few questions, and wasn't to boring to read, any pm me with any complaints for suggestions.
< Message edited by ttoks -- 6/30/2007 1:06:03 AM >
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