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Newbie questions - piston rings and reed valves?


m1tch_87

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Hi all,

 

Just some really newbie questions, I am looking into various tech used on other 2 stroke engines and seeing why they aren't used on the smaller nitro engines, my question is around piston rings and reed valves.

 

Even on the smaller 2 stroke engines such as on the mini moto engines there is a single piston ring (with in indexing mark so keep it from tearing up the intake ports!), I know that these engines are much smaller but was wondering if anyone had tried to fit a small piston ring to these engines at all? I am guessing these engines just run on the very tight tolerances between piston and liner?

 

The other thing I have noticed is that there aren't any reed valves or 1 way valves after the carb, when the piston is on the down stroke compressing the fuel and air mix the pressure would also be pushed out the carb - much like the intake charge going out the exhaust however this is bounced back into the engine. On the mini moto engines there are options to run 'boost bottles' which is a small sealed bottle to the side between the carb and engine which then catches the fuel mix being ejected out the engine and then richens the mix on the intake stroke. The other thing on mini motos is the use of reed valves in the intake - basically a 1 way valve made from spring steel or carbon fibre which would again stop any intake charge escaping - to ensure that the crankcase remains sealed when the mix is being compressed in the crankcase.

 

I was thinking of perhaps looking to see if I can run a small 1 way valve between the carb and engine to see if that helps seal the engine and improve the flow of the intake charge from the crankcase to piston top.

 

Just wondering if anyone had tried either of those or indeed if there were any high end engines which have them as standard, I am aware that the engines are really small but the fact that they are small would mean the gains might be higher even if there is only a small tweak to the engine. 

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The manufacturer's of these engines spend a fortune on devlopment. Im affraid to say it but I think you will just waste time and money with what you suggest.  These engines are very simple and rev high, the fewer moving components the better. Reed valves as small as they need to be would never cope with the rpm and heat. If you want more power simply up the nitro content and lean the mixture. Sure the engine will wear out faster but you will have more power. Until you have owned one of these engines and physically handled the components you will struggle to understand how highly stressed they are and how basic they need to be.

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On 2/6/2016 at 3:38 PM, spannermonkey said:

The manufacturer's of these engines spend a fortune on devlopment. Im affraid to say it but I think you will just waste time and money with what you suggest.  These engines are very simple and rev high, the fewer moving components the better. Reed valves as small as they need to be would never cope with the rpm and heat. If you want more power simply up the nitro content and lean the mixture. Sure the engine will wear out faster but you will have more power. Until you have owned one of these engines and physically handled the components you will struggle to understand how highly stressed they are and how basic they need to be.

 

Just had a research again into the various tuning options on the 2 stroke (still a learning curve) - our nitro engines use a rotary valve down the centre of the crankshaft which works better for higher RPM use vs the reed valve. I was still getting used to the engine design for the nitro engines, it now makes sense why people also port the crankshaft as its basically the inlet manifold for the engine. Once the crank goes past a certain point the fuel air mix is cut off from the carb and the engine is sealed ready for the piston to compress on the down stroke.

 

I am guessing that the width of this groove in the camshaft under the carb would be vital to the engine performance - guessing if this was too wide the intake would be open when the piston starts to compress the crankcase charge. I am also thinking that it would only be the part at the very end of the crankshaft under the carb that would be the effective part - this might mean that opening up the groove slightly after the carb might give additional volume meaning additional fuel and air into the crank case - although would need to be careful with any material removed!

 

This will also mean that the seal between the end of the crankshaft needs to be perfect to ensure a gas tight fit - will check the tolerances when I pull the engine apart once my engine arrives (its a partly used engine so need to inspect everything internally).

 

I also now understand why they don't have piston rings as the piston is tapered inside the liner which is pinched at the top - might see if I can get a dial indicator to measure the bore at different points. I would surmise that the base of the piston is a tight fit with the liner with the top part of the piston being a tight fit for the very top of the liner which is pinched in slightly? This would allow the piston to tightly seal at the top TDC but the base of the piston to seal the crankcase between TDC to BDC compressing the intake charge.

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You are correct in the top of the sleeve is pinched to seal the piston, this is why break in is so important as the two need to wear into each other. The skirt is not so tight and uses the oil fuel mix to seal as do front crank bearings. If they are dry they will leak air but as soon as they lube up if they are in good condition. You will need some very accurate measuring equipment to check tolerances etc on these engines. Most of us do it by feel and sight. There really is little gain to be had from "blueprinting" these engines unlike 1:1engines.

wtnmyI.jpg

If you ignore the fact the bearing failed on this picture you can see the crank port and sleeve ports. The 8mm socket gives some idea of scale. The portion of piston skirt missing should have a small cut out in to seal over the back plate at BDC.

6U4aDc.jpg

This is not the best picture but shows a totally worn out piston and sleeve, you can see the piston crown pushes out past the top of the sleeve.

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Mitch...these little engines have a maximum rpm that is way too high for either reeds or rings, neither would be able to keep up.....the reeds would not open/close fast enough and the rings would suffer from "flutter" ( whereby the ring will hammer the groove). So rotary valve and a tapered liner are used. High rpm is used to increase power as the size ( cubic capacity) is small the engine develops little torque...by increasing max rpm you get more power. Rotary valve engines have static opening/closing points for the inlet timing....on our little engines these points can be modified by altering the crank "window"....if you alter the bore of the crank you alter flow speed and crankcase volume. I did a thread on measuring timing somewhere, it has some links to various information...will see if I can find it.

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Thanks for that, I know that for some 2 stroke engines that do run at constant higher RPM usually have a slightly higher amount of 2 stroke oil due to the fact that the oil isn't in contact with the moving parts as long at higher RPM.

 

What is the usual % of 2 stroke oil in the blends? Would it be worth me adding a tiny bit of low ash 2 stroke oil when just buzzing around practicing to protect the liner etc.

 

Anthoop - thank you for your explanation, I am still getting my head around these great little engines, my blata B1 2 stroke engine was 37cc but only revved to 14,500 rpm making around 8bhp vs the .32 engine revving much much higher and making just over half the power which is very impressive.

 

I have seen that there are some silicone inserts you can put at the end of the crankshaft to help the intake charge move down the crankshaft - think they are usually the slightly larger aircraft engines though.

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The oil in nitro fuel is not 2 stroke oil. Its a personal choice on oils, some prefer castor oil blend some fully synthetic blends others a mixture of the two. Castor oil is good as its far better at lubricating vital internals. Anthoop is the man in the know about oil in fuels, I stick to a blend Im happy with and bash hard. Fuels are very personal choice in this hobby and I strongly belive there is no right or wrong fuel just different fuels for different uses and then some genuinely bad fuels. Stick to branded fuel and you wont go far wrong. Byron, Tornado ( if you can find it) and optifuel have all served me well in the past.

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On ‎08‎/‎02‎/‎2016 at 6:20 PM, m1tch_87 said:

Thanks for that, I know that for some 2 stroke engines that do run at constant higher RPM usually have a slightly higher amount of 2 stroke oil due to the fact that the oil isn't in contact with the moving parts as long at higher RPM.

 

What is the usual % of 2 stroke oil in the blends? Would it be worth me adding a tiny bit of low ash 2 stroke oil when just buzzing around practicing to protect the liner etc.

 

Anthoop - thank you for your explanation, I am still getting my head around these great little engines, my blata B1 2 stroke engine was 37cc but only revved to 14,500 rpm making around 8bhp vs the .32 engine revving much much higher and making just over half the power which is very impressive.

 

I have seen that there are some silicone inserts you can put at the end of the crankshaft to help the intake charge move down the crankshaft - think they are usually the slightly larger aircraft engines though.

As said before these little "nitro" engines are very different to a mini moto engine but they do both use the same principals...for these little engines in cars then the percentage of oil in the fuel is down to the type of oil used....personally I will always want some castor oil ( because it will protect the engine when it runs slightly lean)....I want synthetic oil because it is thinner and my engine will still be lubricated but will produce more power than running ( stodgy) castor oil only...The type and mix of oils along with how much oil is totally down to your experience....I would never recommend a new commer to run 9% oil of any type/mix. The *silicone inserts* into the crrank are not inserts....some cranks have a void ( and you can even drill a void and fill it)....but this is something that I would say is useless, by drilling out the centre of the crank you make it lighter...but the weight you remove is in/near the center....not much change....look up "silicone filling", "silicone ramping".

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