Dave Wave

Why do the small model engines use so much oil? Why will my string trimmer run on 50:1 gas, when my .40 need methanol with 15% oil?

With all of the advanced materials out there, one would assume they could build something that was more efficent and did not slobber all over the plane.

Thanks!

-Dave

Sport_Pilot

Gasoline has some viscosity and light oils in it from the start.  Methanol is much thinner and has no oil.  So that gets you to about 10 to 12% oil right there.  Then there is no needle bearings in the connecting rod of our engines, as there is with your string trimmer, so that requires another 5 to 10 % of oil.  Size matters as well larger SuperTigre engines require only 9% oil per the manual.  Their size is closer to the engine of your string trimmer.

beppeVRCS

Dave, oil is used to lubricate parts and to absorb some of the heat of the combustion. The smaller the engines, the greater the amount of oil. In a COX .049 turning 18K you can use 35% of Nitro, but use at least 25% of good, Castor oil. In a bigger engine, say a Supertigre 2000 (1.20cu.in.) 12% oild can be enough. Sometimes there is extra oil in the fuel because we modelers want to squeeze every possible RPM from our engines, and the result can be a way too lean mixture. I little extra oil can take care of that...
Beppe

earlwb

Basically the smaller the engine, the more oil it needs. The really small engines need 20% or more oil in the fuel.
The other consideration is small engines with a iron piston and steel cylinder. They need more oil as well.
Then if the engine uses a bushed crankshaft then it also needs more oil than a engine with a ball bearing supported crankshaft.
The final clincher on needing more oil is that the connecting rod on the small engines do not have needle bearings on both ends. Thus they need more oil to keep it lubricated. Otherwise the rod fails really fast on you.

The larger the displacement or bigger 2 stroke engines need less and less oil as the size of them go up. But if they don't have needle bearings on the rod (as well as the crankshaft having bearings too), then they need more oil. The larger engines like this can get away with 10% to 12% oil content in the fuel. But with lots of ball bearings and needle bearings they can reduce the oil content a lot more of course.

The other trick that the larger 2 stroke utility engines use is the cylinder is a special type with a ceramic or high silicon (maybe some iron too) in it so it is extremely hard. There is no steel sleeve or chrome or nickle plating on the cylinder. Thus with a high silicon content aluminum piston they can reduce the amount of oil needed for the cylinder/piston fit too.
The larger 2 stroke engines like you used to see in motorcycles years ago, used oil injection too. They used a oil pump to directly inject some oil into the rod bearings and crankshaft bearings so they could reduce the amount of oil needed by the engine.

The oil actually imparts a almost negligible cooling effect inside of a glow engine.  But it does reduce heat by reducing friction though. The methanol in the glow fuel has a much more powerful effect on cooling the glow engine, whereas gasoline has very very little cooling effect on a gasoline engine. That is mainly due to the higher volume of methanol that the glow engine consumes versus the much lower volume of gasoline being consumed by a engine. Thus the methanol gets to suck up a lot of heat during its vaporization process inside of the engine.

pe reivers

well spoken

MetallicaJunkie

what percent of oil is 50:1? and what ratio of oil to gas is 15%

1QwkSport2.5r

In 1 gallon, a 50:1 ratio is 2.56oz of oil, or 2% total content. 15% is 23.7oz per gallon.
Couldn't help it. Had to post it

turbo.gst

My Feedback: (8)

Here is a neat site for oil ratio's:

http://www.csgnetwork.com/oilfuelcalc.html

jeffie8696

The oil also helps seal the compression. Iwas taught in mechanics college that oil also helps spread the heat around in an engine to facilitate cooling.

Hobbsy

My Feedback: (102)

Also, the compression and combustion forces are spread over larger areas in the larger engines.

Dave Wave

If gas is the better fuel, both thermally and lubrication-wise....why no small engines that run on gas/oil?

I would assume fire safety and spark ignition required.

-dave

Hobbsy

My Feedback: (102)

How about a MERCO .61? It turns a Graupner 12x6 at about 8,600. , sorry can't load picture.

BarracudaHockey

My Feedback: (11)

We run gas engines all the time on gas/oil mixture (50:1 actually) but yes, they require an ignition.

Sport_Pilot

Because methanol with nitro provide more power (not more efficient)which becomes more critical with smaller models.

earlwb

Small gasoline spark ignition engines were available many many years ago. They had .12 cubic inch size gasoline engines even. But they used tricky ignition points, a condensor, low voltage coil and a battery pack to run them with. That added a lot of weight to a airplane. Plus most of the engines were for free flight and didn't run for more than one minute at the most mostly 15 to 30 seconds was the norm. So when control line flying became all the rage, the old spark ignition gasoline engines did not fare well at all. Most of them overheated from the longer engine runs and they had problems with reliability and the weight of course. It was a hassle trying to get your old finicky spark ignition engine to run. Many guys would show up and spend the day trying to get their engine to start much less try to fly it.

The first big solution was the model diesel engine or compression ignition engine. In 1947 the famous Drone engine came out and it almost completely took over the control line flying market at the time. Micron was another model diesel engine that was popular too. These engines were simple in that they didn't use spark ignition, or glow plugs even. the Drones were even more simple using fixed compression so you didn't even have to twiddle with the compression on it either. They quickly obsoleted the unreliable gasoline spark ignition engines. Plus they developed much more power for their displacement than a gasoline engine did. Guys would show up and fly all day, only having to stop to refuel the planes.

But in 1948 the first mass produced glow plug engine that used methanol for its fuel came out, the Fox .35. It quickly supplanted the diesel engines and from then on the glow engine became the defacto standard for model airplanes, cars and boats. The glow engine's simplicity of use, light weight, and great power to weight advantage won. By 1950 everyone had pretty much switched to glow engines over spark ignition or the compression ignition engines. So since 1950 the glow engine has ruled the roost, so to speak.

Large gasoline spark ignition engines started to become popular years ago with the advent of giant scale planes. A methanol powered glow engine starts using a prodigious amount of methanol in comparison to a gasoline engine. So the spark ignition engine enjoyed a comeback when giant scale became popular again. Up until the early 1980's the largest glow engines you could get was the OS Max .80 and the Fox .78. Some of us also used to convert trimmer engines to glow back then to fly larger planes too. It has only been in the last couple of years that smaller gasoline engines have started coming out. The smallest production gas spark ignition engine you can buy at this time is a 9cc (.55 cu in) size engine. There are some .15cc, 17cc and 20cc gas engines out too. So the gasoline engines are enjoying a come back of sorts, as the costs of glow fuel goes up. But of course, if you use FAI glow fuel with no nitromethane in it the glow fuel is quite reasonable in cost.

1QwkSport2.5r

I'd like to point out that even small engines run on low oil content. It's common for RC car engines to run on as little as 8% oil. 10-12% is the norm with racers running 8%. Of course these engines don't tend to last as long but high oil content makes them very difficult to tune. Most of these engines have a bronze bushed big end of the conrod but bare aluminum small end, and most car conrods are knife edged.

pe reivers

Car engines run at full power a very small amount of the time. Mostly they collect fuel and extra oil for the next short power burst. Oil content as low as 2% is feasible, depending on the track the engine runs on.

earlwb

Yes you are correct about the low oil content in RC car engine fuel. The RC car engine is unique in that the car doesn't run at full throttle for the entire time it is on the track. The vast majority of its time is spent at 1/4 to 1/2 throttle. The car only goes full throttle for short bursts down a short straightaway. So what happens is the oil tends to accumulate at low throttle and gets blown out during the full throttle run down the straight, and then it repeats itself again for each lap. So they can get away with less oil as they don't run at WOT much. But the poor little car engines tend to turn some really high RPMs for short times and tend to ingest dust and dirt as well causing them to wear out faster too.

I was recently looking at the RC road car track for the World's Championship races that are going to be stage in Europe and even though they have a super nice looking track, it has a short straight on it too. They have lots of turns but no long straightaway.

But a model airplane engine tends to be run at full throttle for most all of the time it is in the air. Few modellers fly around at lower throttle settings. So I think people tend to be really hard on their model airplane engines in respect to car engines.

Now I have seen guys destroy their car engines in short order at local parks, etc. Where they make long full throttle runs and short turn arounds and long WOT runs again. The engines usually sieze up on them after they do that a few times, if that much. But if you ran fuel with more oil in it for those situations, the car engines do just fine. I know, as I did this to myself once too and forgot to change fuels while messing around at the park one day..

pe reivers

To illustrate the high loads an RC engine is subjected to:
I had a nice ST engine for UC flight. I made a soft iron Dykes ring for it that ran forever and a day in the hard steel cylinder and on the rich 4-2 rithm. I then started out in RC flight and put that engine in my trainer. After two days, the ring was toast. A new ring did not last longer.

mike31

My Feedback: (67)

Try running your string trimmer at 13000 or more RPM's without the required oil for the small 2 strokes.

sps3172

This is all good info, but I had to step in here and add a few things, per my direct experience with racing 1/8th nitro on-road cars......

- RC car engines (at least those used for on-road racing) spend A LOT of time at FULL THROTTLE. I think most racers would agree that on a nicely prepared on-road track, the driver is using full throttle at the exit of most corners. Keep in mind, however, that full throttle doesn't equal full RPM. I think this may just be another way of looking at the info presented, already....not really a correction.

- R/C car engines should NOT save up fuel and oil while at low throttle, and then burn it during the WOT phases of the lap. Most poorly tuned engines (not just at the car track) seem to be too rich on the bottom end....too lean on the top end. when it comes to RC car engines, too lean on the bottom will NEVER hurt an engine.....too lean on the top will ALWAYS hurt an engine.

- R/C cars race for fixed periods of time, with fixed amounts of fuel. It's always a tradeoff between power, and sufficient fuel economy. Those who have experimented with oil content will note that as you reduce the oil, you have to close the needles to maintain the same fuel/air ratio. That being said, 'extra' oil just takes up space in the fuel tank, so if a car racer can 'get away' with less oil content, they have effectively increased the size of the fuel tank.


Lastly, in a glow ignition model engine, oil is one of the MAIN ways that you can manipulate the timing of the ignition event. More oil......equals less timing advance. Case in point.....model helicopters run the engine at full RPM, partial throttle, while in hover. This would be considered a lightly loaded condition. The extra oil that's become popular in helicopter fuels (last I used was a whopping 24%) is there mainly to retard the ignition timing and keep the engine running smoothly while in this lightly loaded condition. Although it doesn't speak to the differences in oil content among 'nitro' fuels, I think that need to control ignition timing is the key reason that you see a disparity in oil content between your 'average' glow ignition engine vs. a spark ignition engine (i.e. more oil in model fuel).





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Sport_Pilot

I don't think its how much it is at full throttle as much as how long.  RC car engines run only a few seconds at a time before throttled down, a model aircraft engine might run minutes at a time before being throttled down. 

airraptor

My Feedback: (66)

Lastly, in a glow ignition model engine, oil is one of the MAIN ways that you can manipulate the timing of the ignition event.[/b] More oil......equals less timing advance. Case in point.....model helicopters run the engine at full RPM, partial throttle, while in hover. This would be considered a lightly loaded condition. The extra oil that's become popular in helicopter fuels (last I used was a whopping 24%) is there mainly to retard the ignition timing and keep the engine running smoothly while in this lightly loaded condition. Although it doesn't speak to the differences in oil content among 'nitro' fuels, I think that need to control ignition timing is the key reason that you see a disparity in oil content between your 'average' glow ignition engine vs. a spark ignition engine (i.e. more oil in model fuel).





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[/quote]

yeah i dont belive this to be true. so with what your saying i can change the timing in a glow two stroke by changing the oil %???????????

one thing some have hit on and some havent is the volume of fuel/air going thru the engine. all engines require a certain amount oil to keep everything lubed up.

that heli engine running at full rpm with no load has the carb opened up very little. little "volume" of
(fuel/oil)/air needed to maintain that high rpm. with this you will need more oil to keep it oiled. when the Heli engine is under a big load and the carb is wide open it will be deliverying a larger "volume of (fuel/oil)/air to the engine. now it has more oil than it needs so the power is down from where it should be. hum so what did the heli guys do lol add more nitro. i can take those heli engine run a plane prop on them and make more power with 15% nitro 16% oil over the 30% nitro 22% oil on the same prop.


the car engines are reving very high with the carb wide open. 2-3 times the rpm of most plane engines. these cars can have a fuel burn of 2 ounces a minute or more. what this means again is a larger "volume" of (fuel/oil)/air in to the engine.

my point is the oil % should be based on the "volume" oil running thru the engine.

1. a 15 size car engine turning 30,000 rpm with 12% oil in the fuel
2. a 15 size plane engine turning 15,000 with 18% oil in the fuel

Which engine has more oil going thru the engine.

larger engines most of the time need less oil. gas engines even less with roller bearings on the rods. large engine have a high "volume" of oil going thru the engine.

A little 1/2A engine has a very small volume of oil going thru the engine with steel pistons and non bushed rods. they need lot of oil and most need 100% castor.

I run most of my 25-60 size engines on 16% oil. i run 90-and bigger on 14% car fuel with just a touch of extra oil to bring up around 14.5-15% this will lower the nitro % now just a little but less oil and its just a bit stronger than running 16-18% oil.

now this is just my opinion but i think it holds water.

earlwb

Well, for how many minutes does the car engine run at 30,000 RPMs with 12% oil?  I don't  believe that they run them that fast for more than a few seconds at the most.  Usually a Off road car/truck might approach that for a split second when airborne in a jump, but usually the driver has cut the throttle as the car goes airborne. The Road race cars usually don't get more than a few seconds at that RPM as by then they have run out of straightaway and have to slow back down by then.  Then they are zipping through the turns at much lower RPMs.

blw

My Feedback: (3)

Hovering probably requires the most power when flying helicopters. That is not a light load on an engine.