Compression Ratios

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Compression Ratios

This topic has 10 replies, 6 voices, and was last updated 7 May 2013 at 20:52 by Clive Farrar.

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6 May 2013 at 16:20 #2376
Sub Mandrel
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@submandrel
Whats best for glow engines.
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6 May 2013 at 16:20 #118541
Sub Mandrel
Participant
@submandrel
I've just measured the internal volume of a second head for my little 4-stroke engine. A sytri8nge of soapy water give 0.9cc.

The swept volume is a calculated 5.51cc, so the total volume is 6.41cc and the compression ration will be 7.12:1

I understand that the 'ideal' compression ratio for glow engines is 8:1, is this close enough for what is planned to be a low revving engine with a fairly weighty 'marine-type' flywheel. It is a 'square' engine of 0.756 bore and 0.750" stroke.

Other question, tappets are silver steel tempered to deep brown running in bronze guides. Cams and camshaft are one piece silver steel, what colour should I temper them to?

Thanks

Neil
6 May 2013 at 16:29 #118543
JasonB
Moderator
@jasonb
Should be OK, for comparrison the Bobcat is 6:1.

Assume you are using spark ignition not glow
6 May 2013 at 16:50 #118544
Sub Mandrel
Participant
@submandrel
HI Jason,

No it will be glow, because I don't want to spoil the apperance with a huge battery pack/coil almost as big as the engine again! I can hide a nicad battery and fit it with a dummy 'spark lead'.

I have some Model Technics GX5 (5% nitro 20% castor oil) which will hopefuly be OK (it will smell good!) I recall I was sold this as an excellent all-round fuel for 'sports flying' that is gentle on engines and easy to strat (they lied about that bit).

I am bewildered by the choice here especially as most of the choices of fuel have five or six different versions!

Neil
6 May 2013 at 19:32 #118559
mike T
Participant
@miket56243
Hello Niel,

The compression ratio for a model engine depends on which fuel you intend to use. For spark ingnition and petrol; a compression ratio of 6:1 will give good results with a "mild" engine. For a methonol burner; the compression ratio needs to be about 8:1 especially if you are using glow ignition. The engine will probably run on a lower compression ratio but you may need to keep the battery connected to keep the glow plug alive with a lower compression ratio.

As for how much nitro in the fuel, that depends somewhat on engine capacity. Larger engines will run well on little or no nitro. Your 5cc engine may require more than 5% nitro. As with everything to do with model engines, give it a try and see how it works. If it works, then you have found the answer. if it does not work, then you will need to experiment some more.

Mike
6 May 2013 at 20:10 #118564
Clive Farrar
Participant
@clivefarrar90441
I think you will find that glows are at least 8:1 and very often nearer 12:1

I 5% nitro is a good sport fuel The GX you mention is 100% castor . Many model fliers avoid this now due to the patina that builds up on the engine VERY quickly and the glaze that can bild up inside the engine.

There are now good synthetic oil fuels available that give a cleaner burn and residue and yet still smell good.

Model Technics Formula Irinve is on such at 5% nitro, 16 % synthetic , 2 % castor.

Thats what i will go back to once i finish the last 3 gallons of home brew I made from some free methanol.

Regards Clive
6 May 2013 at 20:29 #118566
Sub Mandrel
Participant
@submandrel
Hi Folks,

I made such a complete has of finishing off the head, and as it sounds like No. 2 still had too big a cavitty I'm remaking it again, but doing the fins last this time!

The new head will give a ratio of 9.4:1, and if that's too much it will be a few moments with a ball ended mill to add a little more space.

I can be a bit more confident now, although with my track record on starting glow engines, this could be a long haul!

The main challenge left will be making a carburettor.

Neil
7 May 2013 at 00:02 #118582
Roderick Jenkins
Participant
@roderickjenkins93242
Neil,

I've got a book by Peter Chinn with some commercial 4-stroke glow engine reviews from the 1980s. Most of the engines are in the range 7 – 8.5 : 1.

Hope this helps,

Rod
7 May 2013 at 07:47 #118587
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Neil ,

There is an opportunity to do some real engineering here – make an experimental cylinder head fitted with a contra piston and run series of tests to see for certain what ideal compression ratio is .

If space on head a bit congested you can use two or more small contra pistons instead of one big one .

Couple of notes :

Achieved compression ratio won't be quite the same as volumetric compression ratio .

Some simple sums to estimate the top temperature achieved during compression at different compression ratios would be instructive .

Real Diesels in both model and full sizes are sometimes fitted with compression limiting devices for starting purposes .

Regards ,

Michael Williams .
7 May 2013 at 20:52 #118682
Clive Farrar
Participant
@clivefarrar90441
I hve just remembered that you need to pick the glow plug carfully as well as they have differnt temperature ratings Hot plugs for engines that run cool and cool plugs for engines that run hot.

They can also alter the comression ratio slightly ie. if you fit a longer reach fourstroke plug it will up the ratio slightly. and yes they work perfectly ok in 2 strokes

Good starting point for plugs is an OS no 8 ( 2 stroke ) or OS F ( four stroke )

A good idea might be to find a local flier who has several different types you can try out before you shell out a £5 per plug of your own.

Regards Clive
7 May 2013 at 20:52 #118693
Clive Farrar
Participant
@clivefarrar90441
I hve just remembered that you need to pick the glow plug carfully as well as they have differnt temperature ratings Hot plugs for engines that run cool and cool plugs for engines that run hot.

They can also alter the comression ratio slightly ie. if you fit a longer reach fourstroke plug it will up the ratio slightly. and yes they work perfectly ok in 2 strokes

Good starting point for plugs is an OS no 8 ( 2 stroke ) or OS F ( four stroke )

A good idea might be to find a local flier who has several different types you can try out before you shell out a £5 per plug of your own.

Regards Clive
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