fyrstormer

iTrader: (1)

ARO?

What's wrong with running car fuel in this engine? It's a ringed engine with a steel ring, I'd assumed car fuel would be fine.

RCTecher12

iTrader: (1)

These 4-stroke airplane engines are designed to use airplane fuel. Car fuel is ~10% oil. Airplane fuel is ~18-20% oil. There’s a reason they need the extra oil. The bottom end (everything below the piston!) only gets lubrication from what blows by the piston ring. That’s it. The rocker box doesn’t get any oil unless the valve guides are worn or you put oil there. The cam box/housing - because of its location, gets virtually no oil from normal operation. You will also find that a castor synthetic blended fuel does far better than a synthetic only fuel.

And FWIW - OS uses hard steel liners and aluminum pistons with meehanite rings. Saito uses aluminum cylinders with chrome plating with a meehanite ringed aluminum piston. Enya uses nitrided cylinder liners and meehanite ringed aluminum pistons. Steel rings aren’t very common in these small engines. You’ll find steel rings in some of the bigger gasoline hobby engines, but fine grained low tension meehanite rings are undoubtedly the best rings for model engines.

timjs

Strictly due to the adjustability of the Buku.
From a torque holding point of view, the heaviest shoes are required. When coupled with heavy springs where you can choose the engagement point, you get a clutch that engages in a very small rpm window. The "slip zone" of the rpm range is very small. Basically, its on or off which is preferable here.
With my shoes (standard aluminum shoes with tungsten weights pressed in) its by lucky chance that .9mm springs engage at the right time. .8 springs drag at idle (about 6k rpm) and .7s are like having no springs at all. Compare that to engaging at 15k+ rpm with standard 2g shoes in a 2 stroke application.

The heavy spring thing is meant to be interpretted this way:
Two clutches...
One has light shoes, and light springs.. engaging at lets say 7krpm in a 4 stroke application.

The other set has really heavy shoes, so by design must have really heavy springs to ALSO achieve the same 7krpm engagement point.

How will they compare if they engage at the same rpm? The lighter combo will slip like mad under load and wear out very fast. Lets say it isnt full locked until 15,000rpm.
The heavy set will engage and lock in fast. Probably within a couple thousand rpm of engagement. So it lasts much longer, and transfers more torque.
Some people might think "well, dont the springs counter the extra weight, making them behave the same?" No, because spring tension is relatively constant, while the outward force applied by the shoes on the bell is exponential. Exponential might be the wrong term but you get the idea.

NitroVein

Yes, if you need the same stall speed but with heavier shoes then I get it, and you might be right that they have less clamping force in relation to the shoes when they are out.
I change the weight of the shoes a lot, change springs far less, at least on 3 shoe type.

fyrstormer

iTrader: (1)

Lubrication of a 4-stroke airplane engine does seem to be problematic. A normal 4-stroke engine would have an oil reservoir at the bottom, with a finger sticking off the bottom of the conrod to dip into the oil reservoir and churn-up a spray of oil. In lieu of that, crankcase ventilation seems like it would be helpful for encouraging blow-by oil to seep down the sides of the piston and into the crankcase a little faster. Although, since the crankcase isn't being washed by a continuous spray of methanol, presumably what little oil makes it down there also stays in-place longer.

Having said that, 2-stroke airplane engines don't actually get extra oil, even though airplane fuel has twice as much oil in it. The reason airplane fuel has 20% oil is because an airplane carb is tuned much leaner, so the engine can maintain proper operating temperature while it's being blasted with tons of cool air from the propeller. With such a lean fuel mixture, the engine would oil-starve if the fuel's oil content were down around 12% like car fuel is. Conversely, the reason car fuel has such low oil content is because the carburetor is tuned much richer, so some of the fuel can be used as evaporative coolant -- it never gets burned, just blown out the exhaust and takes engine heat with it. A car engine running fuel with 20% oil would choke on oil very quickly unless it were tuned dangerously lean.

If you compared a 2-stroke airplane engine and car engine of equal displacement, both properly tuned for their respective uses, you would find the oil usage between the two engines in microliters per combustion cycle is very nearly identical -- but the airplane engine would blow a lot less unburned fuel out its exhaust, because it's tuned so much leaner. The myth that airplane engines require more lubrication is supported by their being slathered in oil after every flight, whereas nitro cars and trucks aren't. The reason for that is because nitro airplanes run HUGE engines compared to nitro cars and trucks, and they run their engines at high RPM almost constantly, so their overall oil usage is much higher. Also, the propeller churns the exhaust and buffets it all over the fuselage of the airplane.

If I'm reading the definition of Meehanite correctly, it's just "engineering-grade" cast iron with a predictable crystalline structure. There is no way the ring in my FS-40S-CX could be cast iron, because cast iron of any type is too brittle to make springy parts like piston rings. It has to be some kind of spring-steel, though it may still be manufactured by the Meehanite Metal Corporation.

frewster

iTrader: (1)

You're kidding, right? There are forms of cast iron that are quite malleable. At least look this stuff up before grasping at straws.

Edit: Come on, man. This is literally the first result for "cast iron engine rings": I see nothing has changed since we last talked years ago.

timjs

I'm also going to have to disagree with you here.
The air fuel ratio is NOT used to counter the over-cooling of the prop. Airplane engines have almost no fin area or heat capacity compared to car engines. THAT is what keeps them at an acceptable temperature.

RCTecher12

iTrader: (1)

Right. Airplane engines are subjected to load continuously whereas car engines are not. This is why they need extra oil, mainly.

Aircraft engines are not immune to overheating even though they have air blown across them constantly. You can burn an engine up just as quickly in a plane as you can in a car.

The previous comment about airplane engines’ carbs tuned differently than car engines isn’t true at all. All carburetors are designed to give what I call fair fuel mixture across all throttle settings. This is not an easy task when only two needles are typically employed, but it can be done. A midrange adjustment makes tuning much easier as different fuels can require this. You will likely find that an engine designed for 25% Nitro to be burbly in the midrange when low nitro fuel is used - say 5-10% instead of 25%. Changing the head clearance and in some cases modifying the combustion chamber is necessary.

And yes, Cast iron - or more specifically - Meehanite is used because it’s better than steel in almost all categories. As long as the ring isn’t overheated, an iron ring will outperform (maintain compression seal) a steel ring. Brass and bronze has been known to be used as piston ring material too, but compared to iron, nothing else really stacks up in model engines. Automotive engines are a completely different animal.

And for what it’s worth - the methanol contributes to about 60% if not more of an engines cooling - in two strokes. Also the incomplete burn methanol fuels are known for helps to carry some heat away. See what temp your engine runs at when set 1/2-1 then richer than the peak setting and see how much difference there is in engine heat produced.

timjs

Alright so,
I have put several gallons through my Losi 8ight-T with the O.S. .62v engine. It's rock solid and the only real problem I have is that on longer high load runs (like full speed through a large field or doing donuts) the spur gears would overheat and melt. The transmission I used is from Ofna for their LD3 car and the spur gears are like 2.5mm thick cheap plastic. So, to address this I got a couple generic M1 chinese gears off ebay and machined them to fit the Ofna hubs. Will test it out soon. There's plenty of 4 stroke goodness in the pipeline and Ill try to keep this thread updated. upload images

Broloff28

timjs

Skip the stock carb completely. Use the 3.3 carb. The idle screw will likely need to be all the way in to idle. The HSN will be the only adjustment since the LSN will be completely pulled from the seat.
HOWEVER there is some strange conicidence of science or magic that makes the Traxxas carb work perfect for 4 strokes Ive seen it used on.
As for a heavy flywheel... have you already cut the crankshaft??

Broloff28

fyrstormer

iTrader: (1)

I chose poor wording for my previous post. Airplane engines aren't tuned absolutely lean, they're tuned relatively leaner than car engines. Airplane engines are tuned much closer to an ideal air:fuel ratio than car engines are -- something they can get away with because of the adequate cooling provided by the propeller. Car engines are tuned richer than ideal so the excess fuel (which is mostly alcohol and evaporates very easily) can absorb heat from the engine internals, evaporate, and carry that heat away through the exhaust. (and truck engines are tuned even richer than car engines, to account for the lower speeds, increased drag, and faster heat buildup caused by running off-road.) The large heatsinks attached to car engines are not enough to prevent the engines from overheating; they mostly just prevent premature detonation of the air:fuel mixture during the compression stroke.

Anyway, my point remains: regardless of why airplane engines use a leaner fuel mixture than car engines do, it is nonetheless true that they do run leaner, and so they require a higher percentage of oil mixed into the fuel to provide adequate lubrication. A higher percentage of oil + a leaner fuel mixture = an equivalent volume of oil entering the engine. So running an airplane engine with the carburetor tuned richer for use in a car, with high-oil-content airplane fuel, will cause the engine to be excessively lubricated. Granted, no engine ever wore-out from having too much oil in it, but there's nothing to be gained from it either -- and it may even cause higher running temperatures since there's less alcohol in the fuel to remove heat through evaporation.

The loading of the engine is not a significant factor in determining how much lubrication is required; a heavily-loaded engine may require thicker oil to avoid viscosity breakdown at the higher temperature caused by the heavy load, but it won't require more oil.

timjs

No, there is no relatively leaner anything here.
Where are you getting this information? Have you specifically measured air fuel ratios on various airplane and car engines? Stuck an oxygen sensor in the mufflers?
Or are you assuming that airplane engines use higher oil concentrations so that must be because they run leaner ratios? If that were the case (which it is not) manufacturers would design engines to run "normal" ratios and spec a lesser oil content.
The statement that airplane engines run leaner is completely unsubstantiated.

NitroVein

Airplane engines (2 strokes with ball bearings) doesn't need much oil at all to stay safe, it's just something that lives on from really old days.
Whole flying clubs here that run 5% oil, some take it a bit safer and run 10% especially if they run 4 strokes as well.
Though there is a difference in how much heat a car engine will generate depending on surface (for instance), you can't offset all of that by drowning it in fuel.