Author Topic: Intake timing question. (Read 3156 times)

Lauri Malila · « **on:** September 23, 2010, 09:37:53 AM »

Hi.

Just thought about asking in here before starting the work..

Is there any "rules of thumb" for the intake timing of a stunt engine? What would be a good average for a starting point (total opening, before & after TDC)? What do changes to either direction cause, and changes in total opening?

From Moscow, Lauri

Edit, found the "?"

Tim Wescott · « **Reply #1 on:** September 23, 2010, 10:11:20 AM »

No answers, but suggestions:

http://www.modelenginenews.org has a bunch of engine building articles, some of which have intake timing figures. They're aimed at the amateur, but they'll give starting points for a mild sport run kind of engine.

Have you looked at a bunch of different engines & measured their intake opening?

You can think of air as a freight train -- it's hard to get going, and hard to stop. So the faster the engine needs to go the earlier the intake can open, and the later it can close. This will be to the detriment of starting, of course (every aspect of engine design can be footnoted with "this will be to the detriment of XXX, of course").

There's no one good answer. If you're looking for a 4-2-4 run then you'll need a different engine from one designed for a good piped run. There's probably more -- I don't know!

If you're making engines, make a test engine or two. Either make an engine with a rear drum valve and a rotateable housing, or make a front-valve engine that lets you rotate the whole front with respect to the cylinder. This will let you test advancing and delaying the intake timing, and if you make the rear drum valve one you can change the intake timing completely without touching the rest of the workings of the engine.

sleepy gomez · « **Reply #2 on:** September 23, 2010, 01:07:11 PM »

I have filled the crankshaft port with J-B Weld on several engines. Then re cut it to what I want. Easy to do and re do to get the desired results. You can turn a tiger into a pussycat this way. J-B Weld seems to be permanent untill you grind it out. Be sure to clean the crankshaft port with any brand of brake cleaner that comes in a RED CAN. The one in a green can (environmentally friendly?) is very flammable and I'm not sure it cleans as well. The one in the red cans will put out a fire!

Dave Adamisin · « **Reply #3 on:** September 23, 2010, 07:19:12 PM »

Non piped - Good starting point: 45abdc to 45atdc (180 total). Total should be in the low 180's.

RandySmith · « **Reply #4 on:** September 23, 2010, 09:08:12 PM »

Quote from: Lauri Malila on September 23, 2010, 09:37:53 AM

Hi.

Just thought about asking in here before starting the work..

Is there any "rules of thumb" for the intake timing of a stunt engine (cannot find the bloody questionmark from this Russian keyboard) What would be a good average for a starting point (total opening, before & after TDC) (questionmark) What do changes to either direction cause, and changes in total opening

From Moscow, Lauri

Hi Lauri

Lots of answers if the question is asked the way you stated it, You really need to to narrow it down , by stating what and how ...average RPMs, Prop sizes you want to run , what muffling system etc
The "average" start point will vary accordingly

Regards
Randy

Tim Wescott · « **Reply #5 on:** September 23, 2010, 11:21:39 PM »

Come to think of it, are there any good books on designing two-stroke engines that are really applicable to model airplane practice? I assume that the two biggest resources would be university texts to assuage even the most lusty thirst for charts, graphs, and equations, and motorcycle hop-up books that would lean heavily about how to modify porting for even more speed -- but not much on how to get a solid stunt run.

So having something that gives a good discussion of how the various bits interplay without making me go back to school and finally take those darn thermo classes that I managed to slip out of would be a cool thing.

Lauri Malila · « **Reply #6 on:** September 24, 2010, 06:43:01 AM »

Hi again.

Thanks for answers, and sorry for an unprecise question.
I am building a new backplate for the MB77 engine. It is a rear intake engine with drum valve (actually, the valve that we use at the moment is adopted from an F2C engine). So it will be very easy to experiment with the timing. The engine has a long stroke (29mm if I remember right, and 24mm bore and with a ringed piston. Untill now we have learned that the engine likes to run at 7000 to 8000rpm and it has lots of power to spin props up to 15" dia, pitch around 5,5".
I will use a simple expansion chamber muffler. I tested the Retro Discovery silencer but it seems to be too restrictive. I am also planning to make an absorption type muffler with steel wool or similar inside, that way I would get the minimum flow resistance and still a reasonably effective silencer. Maybe. Л

RandySmith · « **Reply #7 on:** September 24, 2010, 01:14:02 PM »

Quote from: Lauri Malila on September 24, 2010, 06:43:01 AM

Hi again.

Thanks for answers, and sorry for an unprecise question.
I am building a new backplate for the MB77 engine. It is a rear intake engine with drum valve (actually, the valve that we use at the moment is adopted from an F2C engine). So it will be very easy to experiment with the timing. The engine has a long stroke (29mm if I remember right, and 24mm bore and with a ringed piston. Untill now we have learned that the engine likes to run at 7000 to 8000rpm and it has lots of power to spin props up to 15" dia, pitch around 5,5".
I will use a simple expansion chamber muffler. I tested the Retro Discovery silencer but it seems to be too restrictive. I am also planning to make an absorption type muffler with steel wool or similar inside, that way I would get the minimum flow resistance and still a reasonably effective silencer. Maybe. Л

Hi Lauri

I would do as Dave had suggested and use 180 degree crank. This is the same start point I would make when I was making prototype engines, and with the size props and RPMs you are looking at I doubt you will need anything more open or more retarded

Regards
Randy

Lauri Malila · « **Reply #8 on:** September 25, 2010, 07:48:58 AM »

Yep, I'll start with that. Thanks. L

Brian Hampton · « **Reply #9 on:** September 25, 2010, 09:17:33 AM »

I've checked the crank timings on most of my engines and this is what I've found for those considered to be stunt engines.

ST G51, 43/40
Stalker PRO61RE, 37/42
Enya 40/45 models 6001/6002, 37/50
Enya 60-IIIB, 40/55
OS Max-H 35, 42/46
OS Max-H 40S, 40/34
OS Max-S 35, 45/40
Fox 35, 45/40
Merco 35, 43/43
Merco 61, 40/42
Moki M5, 45/30
OS 40/46VF, 37/55
Irvine 40RLS, 50/50

W.D. Roland · « **Reply #10 on:** October 05, 2010, 09:58:39 PM »

Good stuff Brian
Do you have same for exhaust duration?

All of Dads and my notes have disappeared in the last 35 years???

Brian Hampton · « **Reply #11 on:** October 06, 2010, 10:05:44 PM »

Quote from: W.D. Roland on October 05, 2010, 09:58:39 PM

Do you have same for exhaust duration?

Yep, sure do so here's the exhaust and transfer timings given as the opening point BBDC. The Mercos and Fox are Desaxe but I only noted both figures for the Merco 61 because when I checked the figures for the smaller Merco and the Fox I didn't think the figures would be of any interest, my bad but for them it was only about 1 degree although I seem to remember the Fox and Merco had the offset in different directions. Maybe I'll check them again one day. For the big Merco the first figure is the opening point.

Super Tigre G51 (stunt version), 70/60
Stalker PRO61RE (early gold head 4-2 version), 65/58
Enya 45 model 6001, 66/58
Enya 45 model 6002, 70/60
Enya 40 model 6002, 70/58
Enya 60-IIIB, 70/60
OS Max-H 35, 64/50
OS Max-H 40S, 70/58
OS Max-S 35, 63/56
Fox 35, 67/55
Merco 35, 65/55
Merco 61, (66.5/68)/(55.5/57)
Moki M5, 70/60
OS 40/46VF (slight variations between 2 of the 40's), one is 70/60 and the other is 71/57, the 46 is 71/60
Irvine 40RLS, 67.5/57.5/47.5 (third figure is boost port)

I forgot about the earlier Enyas in my previous list so just to round things out with exhaust, transfer and crank timings;
Enya 35 model 5224 #1, 60/50 (crank 33/43)
Enya 35 model 5224 #2, 65/55 (crank 31/45)
Enya 35BB model 5224, 64/52 (crank 35/50)

Tim Wescott · « **Reply #12 on:** October 06, 2010, 10:10:00 PM »

Quote from: Brian Hampton on September 25, 2010, 09:17:33 AM

I've checked the crank timings on most of my engines and this is what I've found for those considered to be stunt engines.

ST G51, 43/40

What's the nomenclature you're using? 43 degrees before BDC open and 40 degrees before TDC close? 43 before BDC open and 40 after BDC close? Neither seems right, but I hardly have a well developed intuition for these things.

W.D. Roland · « **Reply #13 on:** October 07, 2010, 12:48:53 PM »

Thanks Brian
Will see what can be done with those numbers.

I think the offset Brian mentioned is that the CL of piston(wrist pin sometimes) is offset from crank CL to better use the 'leverage' on power stroke.
Exhaust duration can be a total number as 1/2 is on one side of BDC and 1/2 on the other.(at zero offset)

Possibly add Brian's numbers together to get duration?

ENYA .45 6001- 66+58=124deg duration.

I remember numbers for duration in combat and rat engines as being over 150 deg duration so the 124deg duration on 6001 enya .45 would be believable.
Will have to check the engine(in airplane now)one day.

Brian?

Brian Hampton · « **Reply #14 on:** October 07, 2010, 08:33:28 PM »

Just to clear things up with the timings, for port timings I just use the point where the port (both exhaust and transfer) first crack open BBDC (before bottom dead centre) because unless it's a Desaxe set up the timings are symmetrical around BDC. For brevity where I've shown two figures together, like 66/58 for the Enya 45, the 66 refers to the exhaust port while the 58 is when the transfer port opens. This gives an easy way to see what the blowdown figure is which in this case is 8 degrees. Some like to use total duration, which just means doubling each figure, so the Enya would then be 132/116 but I consider the opening point to be the more important one. And just to really clear things up

I give the exhaust figure first because it's the first port to open on the power stroke.

With crankshaft timings, the first figure is when the port opens which always occurs sometime after BDC as the piston is moving up to create a depression in the crankcase. This can't actually happen until the transfer port closes but for various reasons they always begin to open a little before the transfer port closes. In the case of that Enya for instance the crank port opens at 37 degrees ABDC (after bottom dead centre). The second figure for the Enya, 50, is the degrees after top dead centre (ATDC) when the port finally closes as the piston is on the way down during the power stroke. Although the piston is on the way down and supposedly beginning to compress the fresh mixture in the crankcase it allows for the inertia of the incoming mass of air in the same way that valve timing in a 4 stroke acts the same way. So briefly (loud cheers from everyone

with the Enya crank timings given as 37/50 this just means the port opens 37 degrees ABDC and closes 50 degrees ATDC. The total open time is therefore more than 180 degrees at 203 degrees.

What surprised me the most when I started checking various engines was that, although cylinder port timings don't vary all that much, the crank timings are all over the place. One engine of interest is the Irvine 40RLS with crank timings of 50/50 giving a total duration of 180 degrees. Because it's got a removeable front end like the 40VF, if the front end is turned 90 degrees to the left so the venturi points to the side then it'll have identical timing if run in reverse. A 40VF could also run in reverse but not quite as well as normal because the timings would then be slightly off.

W.D. Roland · « **Reply #15 on:** October 07, 2010, 10:01:36 PM »

[[[I've shown two figures together, like 66/58 for the Enya 45, the 66 refers to the exhaust port while the 58 is when the transfer port opens]]]

Got it Brian.
I like that method THANKS!
I will do my readings this way from now on.

[[[With crankshaft timings, the first figure is when the port opens which always occurs sometime after BDC as the piston is moving up to create a depression in the crankcase. This can't actually happen until the transfer port closes but for various reasons they always begin to open a little before the transfer port closes. ]]]

This due to draft through as the end of transfer takes place and some ram tuning effects/ inertia(pressure rise in venture area after crank port closes)

[[[the crank timings are all over the place.]]]
This is where I understand the fox .35s get its 2/4.

As soon as I can make time I will print out your info and set up spread sheet from that info.
To this as time allows will add Combat and rat/ speed engine specs.

NOW next: do you have crank case total(Max) volume and crank case compression ratios for these engines?!?
If you do I will drive down and give you a hug!!!

How far is Australia from New Orleans?

Brian Hampton · « **Reply #16 on:** October 08, 2010, 06:05:13 AM »

Quote from: W.D. Roland on October 07, 2010, 10:01:36 PM

NOW next: do you have crank case total(Max) volume and crank case compression ratios for these engines?!?

For the Enya 45, total crankcase volume measures at 22.8cc with the piston at TDC and reduces by the 7.5cc swept volume at BDC to 15.3cc so crankcase compression is 1.49:1.

I'm kidding! I'm kidding!! No hugs necessary!

Dave Adamisin · « **Reply #17 on:** October 08, 2010, 11:06:55 AM »

Timing numbers are fun but port areas are just as important and explain the differences between the engine timings in most cases. May I respectfully recommend "the two stroke tuners handbook" by Gordon Jennings. Start reading around page 85.....

Try this link: http://www.vintagesleds.com/library/manuals/misc/Two-stroke%20Tuner's%20Handbook.pdf

W.D. Roland · « **Reply #18 on:** October 08, 2010, 02:37:40 PM »

Quote from: Brian Hampton on October 08, 2010, 06:05:13 AM

For the Enya 45, total crankcase volume measures at 22.8cc with the piston at TDC and reduces by the 7.5cc swept volume at BDC to 15.3cc so crankcase compression is 1.49:1.

I'm kidding! I'm kidding!! No hugs necessary!

Whew!
That was close!
Was having trouble locating a gas station between South Africa and Australia!

Thanks Dave
Just at a glance looks motorcycle or small outboard related.
Plenty to read this winter!

Dave Adamisin · « **Reply #19 on:** October 08, 2010, 03:45:06 PM »

Quote from: W.D. Roland on October 08, 2010, 02:37:40 PM

Whew!
That was close!
Was having trouble locating a gas station between South Africa and Australia!

Thanks Dave
Just at a glance looks motorcycle or small outboard related.
Plenty to read this winter!

Gordon J was as good a two stroke tuner as there was. Luckily we play with the same two strokes.... Actually it's the fundamentals of his discussion time/angle/area that are relevant. Building a (stunt) model airplane data base of angle area is at hand with the data in the thread and some time with a caliper.. A "wide" event can be the same as a "long" one....

W.D. Roland · « **Reply #20 on:** October 08, 2010, 04:43:15 PM »

Unless the gas velocity drops to low....I think....

My Fox has rabies....

Dave Adamisin · « **Reply #21 on:** October 08, 2010, 06:42:41 PM »

Quote from: W.D. Roland on October 08, 2010, 04:43:15 PM

Unless the gas velocity drops to low....I think....

My Fox has rabies....

Actually because it's a time function and the vol eff is the same, the velocity is the same. Wider means lower too which means less time.. Within reasonable l/d ratios of course.

Author Topic: Intake timing question. (Read 3156 times)

Dave Adamisin

Dave Adamisin

Dave Adamisin

Dave Adamisin