Gas, oil, and glow plugs?

[cwkelley75]

I'm pondering the idea of designing a 1 cylinder 4 stroke engine that's based on a briggs and stratton motor. My question is if I use a glow plug and use nitro (rc fuel), do I need to run oil in the motor? Or does that defeat 4 stroke idea. Or do I have to use a plug and some coil system and run gas? Once I solve this problem, I'm going to start doing my CAD models. I feel pretty confident in drawing the design and machining it, the ignition is my main concern.

Thanks,
C.W.
Grizzly G4000 & Clausing 8525

 

[Jasonb]

A two stroke glow motor draws the fuel/air mix through the crankcase so the oil in the fuel lubricates all the moving parts.

If you are using a carb with inlet valve then via the head to the combustion chamber then the bottom of the engine won,t see any lub. So you would still need to have some form of lubrication to the bottom end

J

 

[MachineTom]

The small 4 cycle aircraft engines use the blowby of the piston and rings to lube the crankcase bearings. These engines use BB except on the wrist pin and rod end. Oil in 4 stroke fuel is about 5% as I recall. the blowby is then vented by the crankcase, through a small orfice to maintain positive pressure in the case, so the lube travels out the prop end as well.

 

[user 10585]

5% oil content is a bit too low for a 4 stroke model engine. More like 14-18% is usual. Nitro from 10-20%. Some engines call for 20/20. 20% nitro, 20% oil with balance methanol. Castor oil and synthetic (Klotz) blend is also a good idea.

 

[trumpy81]

GDay All,

Most commercial model four stroke fuel comes with oil already mixed in. I'm not sure of the percentages, but a little research should turn up the info on that.

Different manufacturers will offer different percentages of Nitro & Oil, depending on the intended application etc...

If you decide to go with pump gas, then a mix of 50:1 with two stroke oil is a good place to start. You can 'water' it down a bit if the oil fouls the plug too much.

If you stick with methanol, then mix with about 5 to 10% Castrol M. Again, the ratio can be adjusted to prevent plug fouling whilst still providing cylinder and more importantly, valve lubrication.

Blow by past the rings should be enough to lubricate the bottom end but make sure you oil the bottom end BEFORE the first run, and after any period of sitting.

HTH

 

[deverett]

In the dim far distant past the favoured recipe for model engine fuel was equal parts of Ether, Methanol and Castor oil. To hot up the fuel a bit, Amyl Nitrate could be added, but the problem was that the engines of those days weren't designed for 'hot' fuel and would consequently wear out faster.

Even without the Amyl Nitrate, the planes still crashed with alarming regularity, so the ordinary fuel was quite 'hot' enough for the majority!

Dave
The Emerald Isle

 

[Jasonb]

Dave, thats the diesel mix which won't be much good in a glow motor.

J

 

[123RWO]

Compression ratio for a glow ignition motor must be at least 8:1 or it will not fire.

RWO

 

[peatrich]

Hi
I fly model aircraft using four stroke glow engines (bought, not home made).
Good quality engines run well on 20% fully synthetic oil, 15% nitromethane and the remaining 65% in methanol. Carburettors for Japanese, O.S. make, four stroke glow engines are set to take the above percentages from the factory.

The nitro offers easier starting, a good idle and easier throttling, while the synthetic oil prevents the problem of discolouration and smell that is a product of using castor based oil.

Fuel with 20% synthetic oil and a nitro content of between 5% and 30% is readily available from RC flying shops in most countries as it is recommended for most well made Japanese glow engines.

It is crucial not to run the engines too lean and to run them in with periods of very rich running and moderately rich running for an hour or so until they will instantly throttle up to full revs and maintain high revs without hardening off. Bear in mind they are air cooled, need the correct size propeller, firmly attached with a second locking nut, being four stroke and don't take kindly to long periods of use on the ground. They will also kick up debris and loose material close to them, particularly behind the propeller. They should also be firmly attached to something heavy to prevent them moving around when operating.

A four stroke .70 glow engine with a 13" prop will easily develop 7500 revs, pull an 8 pound model into the sky and is a lethal weapon if not handled correctly - good luck!

The glow plug needs a 1.2volt battery for starting.
kind regards
Peatrich

 

[seagar]

Hi Peatrich,as an aero modeller for too many years to remember,I agree with you 100%.Yours is an excelent answer to an often asked question.

Regars,Ian (seagar)