speedfreak1234

I was wondering why the ABC construction of most nitro engines is preferred over piston rings used in petrol and diesel engines. I know that ABC has i very short break in period compared to ringed but the life span of a ABC stile engine is a lot shorter than ringed. Also with ringed any loss of compression can in most cases be fixed by changing the ring for a new one. Im thinking that it could posibly because of the high rpms of a nitro engine the rings might not be up to it. Could someone please shed some light on this for me.

Thanks.

Wide Open

im not sure about this, but maybe one factor is the difficulty of making rings so small? plus i think they might fly off at the high rpm these engines reach like you said.

speedfreak1234

Yeh and size could also be an issue or perhaps ABC keeps the cost down because of less parts.

thanks for the input

Wide Open

yeah, theyre already expensive engines as is. for the price of an lrp .28 spec 3 i could get an O.S. .50 SX and still have money to spare!

RCtruckRacer

I'd rather replace a piston/sleeve every 6-10 gallons than rings every gallon.

dustin7837

My Feedback: (1)

The rings would definately wear down quickly at high RPM.
Plus, what is the compression ratio of these RC engines??? Anyone know?
I would imagine it would be higher than your average car or tuck at 8:1 - 11:1
Maybe the engines need a metal to metal contact with very little give to maintain high compression.

Also, with a 2 stroke, the crankcase isn't filled with oil, so if there is a small leak from the crankcase into the chamber on the downstroke, it doesn't matter.

Another thing is that a piston to sleeve contact will last longer than rings, im sure. But for a piston-sleeve to fit properly, it has to be very well engineered and VERY precise.

with vehicles with multiple cyllinders plus a cooling system, alternator, ETC. THe block has a lot more metal in it for everything to mount to and it will expand and contract a lot in high/low heat situations. The rings are kinda springy to account for this, whereas an RC engine definately has some expansion, I don't think it is to the same degree as a multiple cyllinder engine.

Just a few things that came to mind when I thought about it.
Maybe waaaay offf.

Wide Open

actually, in all ringed engines ive run, they outlasted the abc ones due to less contact of the sleeve

dustin7837

My Feedback: (1)

true, but there is a BIG RPM difference, and If anyone knows, I would be interested in the compression ratio of these engines.

I wonder If I could rig up some fittings to hook a compression gage up to this thing...

Wide Open

you can actually buy compression gauges that fit right into the glow plug hole. ive seen them at the hobby shop

JFACKY

wide open, the gauge doesn't tell you the cr, just the actual pressure in the cylinder.

If you know the bore, stroke and the chamber volume. You can figure it out yourself.
the formula for cr is pi x radius squared x stroke divided by the chamber volume. to keep it simple I'll use a small block chevy bore and stroke. 4" x 3.48. chamber volume of 58cc we'll need to convert cc to inches. x.061. is 3.588"

2" radius sq.=4. x 3.14=12.56"
x stroke 3.48= 43.7088
43.7088 divided by 3.588 = cr is 12.18:1

speedfreak1234

Thanks everyone for your help.

there are as im sure most of us know nitro engines with ringed pistons, dont these opreate at a simular RPM range as an ABC engine?

also i think that ringed engines last longer than abc also is because of the pinch at the top of an abc sleave will be very high friction to make a seal and as wide open said above their is less contact

JFACKY

rpms are governed more by the displacement of an engine than by anything else. the smaller the displacement the less rotational mass there is and so the higher they can turn. before stress pulls the pieces apart. also, there is an absolute limit as to how fast a piston can run up and down it's bore. again, the lighter the parts the faster they can move. so yes ringed and abc engines, given everything else is the same, will turn at approximately the same rpms.

HTH

Jim

snowbl!nd

The Thunder Tiger .70 engine found in the EK4 monstor truck is a ringed 2-stroke engine. It peaks out at about 20,000 RPM and ibelieve the rest of the engine is of ABC construction.

It's probably due to the greater discplacement and lower peak revs of the engine as to why it is ringed. Not tomention the .70 was initially designed as a heli engine...

JFACKY

OOOPS

JFACKY

like I just said .70 is a lot larger than the other engines most talked about here. .12-.18 will turn about 35,000-40,000 rpms. .21-.30 about 35,000-30,000. and so on.

HTH

Jim
was suppose to be an edit not a Quote

Wide Open

haha, my os .50 only tops out at 9,300 rpm. but it still has a LOT of pull

JFACKY

Yes because the larger the displacement the heavyer the mass. the more mass you have the torque it stores.

HTH

Jim

TGreg24

ok now this isnt truely on topic but it deals with two stroke engines.

since alot of ppl are in search of the fastest car, why dont some car companiany (ford honda porche) make a full blown perpuse built speed car, like all carbonfiber and shuch. but heres the kicker

could you imagine a 457ci 12 cylinder TWO STROKE!!! the rpms would be increadible, it probably wouldnt launch that great but imagine that top speed.

dustin7837

My Feedback: (1)

LOL. That would be something to see, but the RPM's still wouldn't be that much higher due to the necesary weight of the pistons, con rods, and crank to be able to handle that much power. Plus 2 strokes are waaay less fuel efficient than four strokes.

There are some high revving 2 strokes out there too though. Look at some of the crotch rockets... a lot of them can go to 16000 RPM.

And some 4 stroke car engines can hit good RPMs too. I've got a 4 cyllinder Quad carburetted 2.0L racing engine that I can get up to 12000 with.

JFACKY

Some experimental F1 engines I have seen don't use poppet valves any more. that means no camshafts, lifters, push rods, rocker arms, springs and retainers. they used rotary valve type camshafts to move the air into the combustion chamber. these engines turn up to 20-25,000 rpm. they eliminate a lot of static weight by not using the extra parts that most all valve trains currently use. there is a situation that occurs when the weight and inertia of the moving valve parts overcomes the springs ability to keep it under control. it's called float. when this happens the valves don't open or close they stay neutral untill the piston comes into contact with it.(closes it for the spring.)[:'(] usually resulting in other parts saying no way pal. It's the valve train that is the biggest factor when it come to rpm limitations.

dustin7837

My Feedback: (1)

Wow, thats cool. I'm gonna hafta read up on that.
I mean, twin overhead cam engines eliminate the pushrods and rockers, but you still have cams and lifters and springs, plus some engines like the ecotec no have variable valve timing for incresed power and mileage.

I would imagine losing the cams would decrease the rotating mass of the engine was well?

Still not quite clear on how they get away with no springs, but I'm gonna do some googling.

Thanks for the info!

JFACKY

the rotary valve cams are on the tops of the heads and have passages (grooves) cut into the sides of them. The intake is mounted to the side of the head.The grooves, as they rotate open they connect from the intake side directly to the combustion chamber. as the cam continues to rotate. another groove opens from the combustion chamber to the exhaust port. The grooves are cut in such a way that they open and close at the proper time in the combustion cycle.

HTH

Jim

OH yea, just thought about this. check out the rcv eniges web site. here's the link , click on model air plane engines. they're using rotary valves. but since it's only one cylinder, it's a much simpler desgin.

http://www.rcvengines.com/