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Model 4 Stroke lubrication

Why not a "proper" system??

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Simon Chaddock29/04/2008 16:12:00
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Hi all

As a mechanical engineer it does seem a bit odd to me that aero modellers run their 4 strokes (unlike the rest of the world!) on a fuel/oil (2 stroke) mixture. Diluting lubricating oil in methanol (or pertrol) does not improve its properties. On a 2 stroke you have little option as the whole crankcase is part of the induction system but on 4 stroke everything below the piston (and above the cylinder head) is separate so why don't manufacturers arrange for a proper lubrication system?

I am sure the engines would last longer, possibly be more powerful (as all the fuel would be burnt), certainly use less fuel and I also suspect the spectre of wrecking an engine by lean running would diminish as well. No self respecting full size aero engine would consider anything less than positive lubrication to its bearings so why not ours? It can and has been done with surprisingly little extra complication.

Am I missing something? 

Tim Mackey29/04/2008 16:46:00
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Cost and weight ?

I assume a sump and pump system would be required, together with oilways etc....but its a good talking point!

Eric Bray29/04/2008 17:30:00
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Having a sump is great, but how many of us mount our engines sidewinder or inverted? The sump whould then become the cylinder, with a moving bottom (the piston!)

Then, if you recycle the oil, you need a filter, pump, recirc pipework and a scavanged sump vent, which means more weight, and more potential leaks. 

I assume the engines are designed on the KISS principle. 

Brian Parker29/04/2008 18:54:00
538 forum posts

Yes a dry sump/scavenge system with anti-airlocking valves would be needed to cope with inverted flight. Then there is the increased heat to dissipate.

Could even cost more than its weight in Lipos.

Tim Mackey29/04/2008 18:58:00
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Pete Norris29/04/2008 19:09:00
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 i have  enough  of   about  lub and  heat   lol  just want   to get one to  right  would  be  nice
Eric Bray29/04/2008 20:38:00
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Is that last missing a word or two?  just want one to  - what? - right?

What does it do? What does it not do? What engine/fuel/prop/plug, what in, etc. 

00129/04/2008 20:51:00
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I think the cost of a precision oil pump, tank, piping, filter would increase the price prohibitively disregarding the fact that there is no-one in the cockpit to monitor the oil pressure gauge! Why complicate things when it works so well?

Although Honda are making a petrol 4 stroke engine of 25cc. and 32 cc. for garden machinery I am surprised that nobody, as far as I know has used them yet for model aircraft.

Simon Chaddock30/04/2008 12:11:00
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Hi

It did say it could be done.

My dad built a lightweight 5cc petrol 4 stroke and it just needed an oil tank and two neoprene connecting tubes. The actual additional components would only add marginal cost and weight.

Yes the crank and camshaft were on ball bearings so only needed splash lubrication but the big end was positively lubricated using the crankcase as a vacuum pump to suck oil directly from the tank into the bearing and then to blow the excess back into the oil tank. Effectively a dry sump engine it could run either way up, used 20/50 oil and 97 octane petrol. It could and did run continuously for long periods on the weakest possible mixture, a rather important attribute for a engine intended to break the model plane endurance record. It was featured in both the Model Engineer and Aero Modeller at the time but unfortunately I do know its current whereabouts although I do have some engineering drawings.

http://www.modelflying.co.uk/sites/3/images/member_albums/31957/5cc1.jpg

Brian Parker30/04/2008 13:50:00
538 forum posts

Very interesting, have eaten humble pie! Although you did have this up your sleeve

As far as I can make out the drawing shows a rotary valve to control the oil supply.

Has this method been used by others?

Can you supply any other details of the of the design, particularly regarding the valve? This design probably favours a long stroke engine for efficient oil distribution. I assume there would be residual oil in the lower crankcase, did this cause a problem?

Again very ineresting, (Now need a 'spares or repair' 4 stroke from e-bay for conversion!)

Simon Chaddock30/04/2008 16:45:00
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Hi Brian

I do wish I still had this engine. It was just about an equal bore/stoke and reasonably powerful. During testing it was also run as a glow on a methanol/nitro mix & produced about 25% more power but used nearly twice as much fuel.

Yes, there was a "banjo" disk at the back of the crank case that was rotated by the crank pin. Oil entered at the centre through a non return valve and was fed out to the hollow big end that had cross drilled oil holes. From there it sprayed about the crank case, the rotating crank web ensuring it did not pool anywhere until it was blown back to the tank through the outlet non return valve. Like all dry sump aero engines it was a good idea to hand crank it a few times to ensure any excess oil was cleared before it was started.

You will notice that it had a float carburettor as it was never intended for negative G aerobatics. As tested it could run at full power nearly 8 hours on 4 pints of petrol giving an estimated flight endurance of over 15. By the end of testing and development the engine had run for some 30 hours and was considered only just run in but unfortunately my dads next project (a 15cc V8!) rather took over. 

Brian Parker30/04/2008 17:53:00
538 forum posts

Excellent.

Do you still have the V8?  What size prop would it swing?

Simon Chaddock30/04/2008 22:13:00
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Hi

The V8 was a quarter scale model of the 1.5 litre BRM grand prix racing engine - no prop!

Brian Parker01/05/2008 10:14:00
538 forum posts
As piloted by Graham Hill.
Eric Bray01/05/2008 21:43:00
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Well, they now call F1 drivers - pilots, don't know why, because up until this season, they 'flew' computers that they sat in!
Boots16/06/2008 19:19:00
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This brings forth something i dont understand with model 4st engines . How does the oil get to the rocker shaft and tappets?
Simon Chaddock19/06/2008 00:38:00
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When using a petrol/ oil mixture in a 4 stroke the answer is - it doesn't. I suppose the valve stems do get some lubrication but that's about it. Its amazing these engines run as long as they do if you think of all the trouble they go to lubricate the cam/rockers/tappets on automotive engines!

At least with the 5cc the oil mist in the crank case can get up the pushrod tubes to the rockers. 

peter hetherington16/07/2008 19:55:00
9 forum posts

hi im completely new to 4 strokes and have just bought off ebay a really good os 52s mk2, it really disturbs me to think there is no lubrication to the pushrods and rockers, so please someone put my mind at rest.

 .............Pete

Bugbear16/07/2008 20:46:00
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Crankcase pressure plus inertia plus surface tension (creep).
Simon Chaddock16/07/2008 22:36:00
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Don't worry too muchPete.

It only takes a minute amount of oil to lubricate things adequately and the movement of the push rods and rockers is small and lightly loaded when compared to the main and big end bearings. Compared to full size, the total hours a model engine will be run is relatively small. As long as you don't damage it in a crash a good 4 stroke should out last a model several times over. The positive lubrication on the 5cc is a luxury as the engine was intended to run reliably for 24 hours non stop.

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