How do you tune 4 stroke engines.

[Craig Spence]

Hi all,

 

I was wondering if you could advise me on how to tune a four stroke engine. I have been in the hobby for over four years now and have no problems tuning two strokes.

 

I would greatly appreciate someone telling me how to tune a four stroke properly (in lamens terms please, I dont know every engine part).

 

Also could someone recomend a good onboard glow starter (Its not called this, it is something which connects to your glow plug and keeps it going in flight, a onboard power supply is needed).

 

Ill tell you the set up and can someone advise me.

 

Basically I will be useing two SC52FS inverted in a Brian Taylor Mosquito. So I really need something as a back up should one of the engines want to quit the onboard starter should stop it. I really do not want to have one engine fail.

 

Cheers all, any advice, feedback appreciated.

[Martin Harris - Moderator]

There's no major difference in principle - earlier ones tended more towards air bleed carbs but most have the same 2 needle type as the average modern 2 strokes.

 

It's important to use a good 4 stroke plug as there's quite a gap between firing strokes while it blows, sucks and squeezes giving the element a chance to cool. Although some will say you need to tune with a tacho, I think that it's quite OK to do it by ear if you've got a good one. The usual principle is of tuning to max revs and then richening until a discernable drop in rpm is detected - a good secondary cceck is that there shouldn't be any significant change in note when doing a nose up check - don't hold it nose up and flick the throttle open for 1/2 a second as so many do - let the rpm settle while level - THEN - go nose up and there should be little or no change in pitch when held for several seconds.

 

Many people will say that you shouldn't need on board glows but, particularly on a scale twin, I'd go along with the idea. It allows you to start without external wires to tangle and extra holes in cowlings. It also means that the low end can run reliably a little richer than ideal which gives a small extra margin against unintended overleaning from partial blockages, bubbles, G loads etc.  I've always found the South Herts Models Intelligent Glow Systems work well

 

Now the thorny subject  of tuning twins:

People seem to spend hours messing around with tachos trying to get the engines within 10 rpm.  The important thing is to run in both engines until they're happy and tune them to run and throttle perfectly.  Once this is done and with the linkages set up as identically as possible (servo arm lengths, angles etc.) check the rpm output and richen the faster one to drop the rpm to atch the slower one.  I don't bother using the tacho for this - if you hear the "wonderful" twin beat it actually means the engines are out of synch! This occurs when the engines are very close but the beats disappear when the engines are perfectly matched.  Having them matched reduces vibration so is kinder to the airframe and might reduce fuel foaming but the important thing is that they are tuned to run reliably.

 

With a Mossie the golden rule must be, lose one engine, kill the other or kill the model.

 

Treat one engine out as a dead stick - shut the throttle and land ahead under control unless there is comfortable height to do any turns.

Edited By Martin Harris on 19/10/2010 20:53:26

[Craig Spence]

Hi Martin,

 

wow what a reply, many thanks for the help I really appreciate it.

 

The heads up on tuning is relieving and I will be getting the onboard glow systems, as you say I really dont want one engine to cut.

 

As you say about Tachos ive never bothered myself, but after reading this I might lol!, it sounds very complicated. I will take a note of what you have told me though and try to apply it.

 

Before I do though I will tell you what my original plan was, I was going to run both engines in on one of my fun fly models, the idea being that both engines are tuned and have at least 10 flights through them before putting in the mosquito. I was thinking this would insure reliability and boost my confidence.

Once I had done this I was going to put them in the Mossie and try to harmonise them.

At that point I would apply the info you have given me to get them right. 

Please tell me if this is a good or bad idea.

 

"With a Mossie the golden rule must be, lose one engine, kill the other or kill the model."

 "Treat one engine out as a dead stick - shut the throttle and land ahead under control unless there is comfortable height to do any turns."

 

Its funny you have said this as I have had more advice on this with the build im doing than anything else lol!, one fella who has the 81" version of mine sais when he losses one engine he throttles up and applies rudder and aileron to bring it in, but also agrees you should cut the other engine when you can.

I saw a horrible video of a real on crashing when an engine went and it puts things into perspective.

 

Rest assured I will be cutting the running engine lol!.

 

Cant wait to try these tips, thanks alot.

[Martin Harris - Moderator]

Yes, an excellent way to put fuel through the new engines in an ideal environment and check them out.  Plenty of power and heat variations if you fly a range of manoeuvres...

 

Once you're thoroughly familiar with twin handling and particularly with your specific model it is probably practical to fly on one engine but I've seen very experienced pilots fall foul of turning into the dead engine and of course, even the real thing tragically caught out the very experienced pilot of the BAe Mosquito as you've seen.

 

 

Edited By Martin Harris on 19/10/2010 22:18:23

[Biggles' Elder Brother - Moderator]

Totally agree with what Martin says here except for one thing. I do use a tacho to tune four strokes and find it useful. You can do it "by ear" but personally I find the "sweet spot" is not as discernable on a 4 stroke as it is on a 2 stroke.

 

The tacho helps to establish both the tick over and the top end. The method is exactly as Martin describes, but you find the max revs with the tacho then back off about 500rpm or so.

 

One great advantage I have found with using a tacho is that when the engine is new and you're gradually bringing it up to max revs you'll more than likely be doing this at the field during the models early flights. If your luck is anything like mine you'll find that the clubmate next to you in the pits will have his 8 foot span Yak with its 100cc petrol engine and he'll be testing his smoke system at full revs! Result you can't hear your engine - but the tacho still works!

 

Never flown a twin (a pleasure still to come) but I think Martin's advice about using the beat frequency to synchronise them makes perfect sense (afterall its how they do the full size!) rather than use the tacho. BUT...you can use the tacho to establish which is the faster engine and to double check that the beat frequency method has given you a sound result.

 

BEB 

[Martin Harris - Moderator]

Posted by Biggles' Elder Brother on 19/10/2010 22:23:10:

 

Never flown a twin (a pleasure still to come) but I think Martin's advice about using the beat frequency to synchronise them makes perfect sense (afterall its how they do the full size!) rather than use the tacho. BUT...you can use the tacho to establish which is the faster engine and to double check that the beat frequency method has given you a sound result.

 

BEB 

 I didn't put that bit very well BEB - I meant that after establishing which was the faster engine with a tacho I don't bother using the tacho for the actual balancing...but you're quite correct as well!

[Craig Spence]

Hi Fella's,

 

your response's have been great. Following advice from Martin I emailed the shop you recommended, my problem is I need a port in my receiver for the glow system as there are none spare, is there a way around this?.

 

Ill write my set up so you have an idea of what im on about.

 

My receiver ports to servo's.

 

Rudder port - rudder servo.

elevator port - elevator servo.

aileron port -  aileron servo's.

flap port - flap servo's.

gear port - undercarriage servo's.

throttle port -  throttle servo's.

auxillary port - power box 12 switch (Battery)

battery port - power box 12 switch (Battery)

 

As you can see every port is taken, where will I plug my onboard glow system?, also I will be useing a power box 12 switch which is a battey backer, so I will be useing two batteries and if one fails it will switch to the other.

Will/ should I use another battery for the glow system?.

Would you just use a y - harness for the glow system off of the throttle?.

 

Cheers fella's.

[Biggles' Elder Brother - Moderator]

Sorry Craig - not used an on-board glow so no real experience

 

BEB

[Martin Harris - Moderator]

Using an AGS in mode A you plug it into the throttle channel and the throttle servo (or servos via a Y lead in your case) into the connector on the AGS.

 

See their handbook for full details...

[Craig Spence]

Hi BEB,

 

no worries for not haveing experience in this field, I appreciate the help you have given.

 

Hi Martin,

 

thanks for clearing that up for me, I think im going to go with the system and hopefully it all works out well.

 

A few more things though fella's if you dont mind.

 

As I have said before, I will be useing two batteries through a battery backer (powerbox 12 switch) will I still need another for the glow system?

 

Will haveing the engines inverted really effect its performance?, ive seen many planes with inveerted engines and there fine. Also I flew my first plane on an inverted engine and it run better and longer than it did when it run the right way round.

 

Last of all, ive been looking for a nice fuel dot filler for a while but am not haveing much luck, its more for size really and looks. Ive found some that look great but are too big for my model (35mm), I could get away with 20mm, thats what I would like.

 

Any info appreciated like always, cheers all.

 

 

[Biggles' Elder Brother - Moderator]

Dubro do nice small fuel dot - I got mne from either SMC or Slough Models - its certainly less than 20mm in diameter.

 

I've had quite a few inverted engines and have never had any real problem. I have an inverted OS90 in my Chipmunk at present - and so far, so good. I think inverted engines sometimes get a bad reputation with beginners over priming them and so making them difficult to start. If you prime the engine simply by spinning it over with the electric starter I find all is well.

 

I've never had a problem with throttle pick-up on an inverted engine either, even after prolonged idling.

 

All that and I've never used a plug with a heater bar or an on-board glow!

 

But seriously, I can understand your caution - I certainly wouldn't fancy a one engined landing on a Mossie!

 

BEB

[Biggles' Elder Brother - Moderator]

Hi Craig,

 

The Du-Bro item I use is the "Kwik Fill" valve listed here. (Make sure you buy the standard size glow one - not the large model version or the petrol version!

 

BEB

Edited By Biggles' Elder Brother on 21/10/2010 22:21:42

Edited By Biggles' Elder Brother on 21/10/2010 22:23:14

[Martin Harris - Moderator]

I don't know the Powerbox 12 but normally you would need a separate glow battery. BEB has answered everything else...

[Tim Mackey]

Yep,most of the simpler onboard glow u nits are powered by a separate large capacity single nickel cell.

[Ed Darter]

Like the one I built from the kits that Tim used to do 4000mAH Puffin HiFlo NiMH cell. Shortly to get its first airing in my Spitfire.

The only problem you will get with having the onboard glow on a Y lead is that you can't turn it off independently, so as soon as the model is live, so is the glow. Got mine on the 9th channel and operated by a switch on the TX, model can be live but no power going to the glow until I flick that switch.

Ed

[Craig Spence]

Hi all,

 

I have a SC52FS here what is the second nipple on the engine for?, theres one on the carb and one under the shaft.

 

Help please.

 

Cheers.

[r6dan]

Hi Craig.one thing that seems to have been overlooked is internal work...

I have a fair amount of experience in tuning car and bike engines,getting the fueling correct is right but if it is not being burnt effectively you are still missing out.

If you take any engine cylinder head to bits you will see inside there are casting marks and imperfections on it,if these are all polished off with a dremel type tool it makes a difference,also  it all of the air intake and combustion chambers are polished and smoothed it can also help.

[ken anderson.]

hello  craig--i'm late joining your response's...word of warning-dont lean a 4st out to the extreme....for when they are and they miss a beat a full wack---it'll throw off the prop/spinner....and boy do they throw them...lock nut's help-but they still throw them...

 

          ken anderson...ne....1.

[Biggles' Elder Brother - Moderator]

Posted by Craig Spence on 05/11/2010 02:30:58:

Hi all,

 

I have a SC52FS here what is the second nipple on the engine for?, theres one on the carb and one under the shaft.

 

Help please.

 

Cheers.

 Hi Craig,

 

4 strokes pass oil and unburnt fuel into the crankcase. If there wasn't a nipple there then the crankcase would become pressurized - not a helpful thing! So that nipple lets the "goo" out. Usual practice is to connect a length of fuel tube to it and run it to outside the cowl - otherwise the inside of your cowl will get all the muck sprayed all over it!

 

As a point of info, most later OS 4 strokes have a pipe that connects this nipple back up to the rocker box - therefore burning off this waste - much is obviously much neater. And the latest OS engines even have this pipe internal to the casing - very nice.

 

BEB

[Craig Spence]

Hi Guys,

 

thanks for the advise and help there.

 

Hi BEB,

 

cheers for the explanation as to what the nipple is for, ill have to get another length of fuel tube.

 

Cheers.

[Craig Spence]

Oh and one more thing, for tuneing I will run two tanks through it rich at tick over then another tank at different throttle setting and lean it out.

 

Any problems with this?, hopefully be doing it soon.

 

Cheers.

[ken anderson.]

hello craig-if your 4st's are ABC...which i think they will be -i wouldn't do as you say ...2 tank's through rich at tickover?...abc engine's can have their  innard's damaged by...running too rich--- check out you maker's instruction's and follow them...

 

          ken anderson   ne...1.

[Biggles' Elder Brother - Moderator]

Ken is spot on ( as always!)

 

Engines differ as to the best running in proceedure - so you really do have to consult the manufacturer's recommendations and do it their way for their engine.

 

Some recommend gradual breaking in - as you describe, others recommend bursts of full throttle with cooling down periods in between, yet thers recommend constant full throttle but running alternatively rich and "on song" for 5 sec intervals etc. Read up - the manual will inform you best.

 

BEB

[Steve Houghton 1]

I recently purchased an ASP61FS and followed the breaking in instructions to the letter, which was to run it for about 10 mins really rich, so it is really four stroke banging at about a ¼ throttle. Then the next ten minute run was to run rich then lean it out for a couple of mins, then back to rich at about ½ throttle and so on. The last ten minute run was the same but at ¾ throttle and opening it out to full throttle.

 

I would always follow the manufacturers break in instructions in case there were any issues later on as an incorrectly broken in engine may validate the warrantly, and its been my experience that many manufacturers will back out of warranty issues if they possibly can, so don't give then a reason to.

[Craig Spence]

Hi all,

 

thanks again for the positive feedback. I do and have read the instructions, however they are not really that clear and always refere to other text.

It does say in the instructions that it should be run very rich indeed (saying this it sais that very rich is 5 clicks out from closed, i thought this would be verging on lean).

It also sais to make the high speed needle adjustments when the engine is belting at full throttle.

 

So I think what I will do is this (please correct me if I am wrong), run one tank through on a rich setting with the carb 1/4 open, then repeat for another tank.

I cant see this being a problem as it will be getting plenty of oil at this stage.

Then the next tank I will open the throttle up and make the adjustments to the high speed needle, I will only leave it open to do the adjustments and then return to idle/ tick over, opening up every now and then for short bursts.

Then I will leave it to run on tick over.

If I have any problems when opening the throttle such as the engine cutting out or searching which I cannot rectify with the high speed needle I will adjust the lower end as required.

 

Please tell me if im goin wrong here.

 

Cheers all.