Reducing engine compression ratio

[Generatorgus]

I'm thinking I would like to reduce the compression ratio on my Wyvern engine.
My original thought was to simply dish the top of the piston a bit, but the engine recently had a serious crash. The timing wheel for the Hall sensor worked loose and the engine came to a violent halt when it fired somewhere near the bottom or lower part of the compression stoke while I was starting it.
The crank shaft is/was a build up with Locite connections backed up by pinned shafts, one of the shafts spun and sheared the pin. After a failed rebuild I put the old crankshaft next to my "jar of shame" and started work on a new one.
Back to the compression ratio thing.
It occurred to me that I could possibly accomplish lowering the ratio by shortening the 2" stoke, which would reduce the volume of the piston travel, but that lead me question if that would actually be better than dishing or shortening the piston a little.
I'm sure there would be some advantage/disadvantage to either.
I'm not at all worried about speed or power, I just want a nice smooth running engine.
Dishing the piston would be an easier fix if I don't like the affect.
I probably just answered my own question but my curiosity still wants to know.

GUS

Just want to add that the failed rebuild was caused by a very slight bend in the piece of steel rod I chose for the main shaft, which didn't become evident until the piece went in the lathe.

 

[barnesrickw]

Here is an article on increasing the compression ratio on two cycle engines. It offers insights of the advantages that may shed light on the opposite.

http://vintagesleds.com/library/manuals/misc/jennings/higher_compression.pdf



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[barnesrickw]

Here is an article on increasing the compression ratio on two cycle engines. It offers insights of the advantages that may shed light on the opposite.

http://vintagesleds.com/library/manuals/misc/jennings/higher_compression.pdf



Sent from my iPad using Model Engines

Wrote that before looking at what type of engine you had, sorry.


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[stevehuckss396]

spacer between the head and cylinder
Shorter piston
reduce stroke
Cam timing

There are many ways to skin that cat.

 

[cfellows]

spacer between the head and cylinder
Shorter piston
reduce stroke
Cam timing

There are many ways to skin that cat.

You could also shorten the connecting rod, but that's probably a bit extreme. Doesn't seem like dishing the piston would reduce the CR much, but maybe you aren't after much. I would think a spacer between head and cylinder would be the easiest to do (without actually looking at the engine ).

Chuck

 

[cfellows]

Oops, just looked at a picture of the Wyvern. That side shaft with all the cam and valve paraphernalia at the top end would seem to obviate the possibility of adding a spacer between the cylinder and head, or at least move it to the bottom of the list of possibilities.

Chuck

 

[bobsymack]

As you are making a new crankshaft it would be easy to reduce the compression ratio with a shorter stroke but you would need to be careful as you are also increasing the clearance volume.
Vince

 

[Generatorgus]

Thanks for the answers fellas, there were a couple there I didn't think of.
Still not sure of advantage or disadvantage thing.
But it's almost a moot point right now, as I built most of the new crank yesterday.
When I built the engine I was concerned about getting as much compression as possible because I had been bothered by poor compression on earlier projects. It is a little hard to figure the ratio accurately because of the shape of the head, and yes I am proficient with geometry(but hate trig. and calc.).
I made the piston longer than called for by the drawings. The piston is already out of the engine and I'll operate on it. There is enough room above the ring to take some height from. I specialize in do-overs and try agains.

GUS

 

[velocette]

Hi Gus
"It is a little hard to figure the ratio accurately because of the shape of the head"
you can easily measure the volume of the combustion space.
T.D.C. valves closed spark plug out a burette filled with a known quantity of light oil and pour it into the combustion space and measure the remaining oil and subtract from original quantity. Eureka!!!! the ancient greeks already have done it

Eric

 

[Swifty]

As compression ratio is the ratio between the volume at bottom dead centre and the volume at top dead centre, I think that dishing the top of the piston is not the way to go, as you are increasing the volume at bottom dead centre which won't help.

Paul.

 

[velocette]

As compression ratio is the ratio between the volume at bottom dead centre and the volume at top dead centre, I think that dishing the top of the piston is not the way to go, as you are increasing the volume at bottom dead centre which won't help.

Paul.

Hi
The swept volume is bore area X stroke length
Divided by combustion chamber Volume = Compression ratio

Top of the piston shape makes no difference to the compression ratio it is the increase or decrease in the combustion chamber volume

Eric

 

[Swifty]

Hi
The swept volume is bore area X stroke length
Divided by combustion chamber Volume = Compression ratio

Top of the piston shape makes no difference to the compression ratio it is the increase or decrease in the combustion chamber volume

Eric

compression ratio
noun
noun: compression ratio; plural noun: compression ratios
1.
the ratio of the maximum to minimum volume in the cylinder of an internal-combustion engine.

With all due respect, but isn't that what I said in my post?

I feel that the swept volume is not the way to go as it's not taking into consideration the additional volume that's left at TDC.

Paul.

 

[Ripcrow]

Always thought compression ratio was a term only. The volume of air under compression becomes pressure which is a exponential of the original volume,sorry if I confused you here having trouble putting this to words, to raise compression you shave the head effectively decreasing volume, clear as mud I know hopefully someone can decipher my thoughts better

 

[Ripcrow]

Yeh I have got to disagree I believe that dishing the piston will decrease the compression due to increasing the space as the piston reaches top dead centre,same as lowering the head increases compression due to less room for the air to fill

 

[Gedeon Spilett]

This is not a compression ratio anyway, this is a volumetric ratio. In addition, compression depends on temperature at TDC and BDC and tight or leaky valves and rings etc.
modifiing piston height above gudgeon pin is IMO the simplest way to change this volumetric ratio.

 

[Rustkolector]

Gus,
The Wyvern is a slow speed engine, and assuming a smooth running engine is your goal, then reducing the C/R to about 3.5-4.0:1 is in order. Shaving the piston crown is the easiest way to get there. It is not the most important design factor, however, in getting a slow speed smooth running engine. In my opinion there are a number of other factors. The first item needed, but often overlooked, is a good sealing combustion chamber. Check your valves with a vacuum gauge. Keep piston ring side clearance to a bare minimum, etc. Next is a good healthy and hot spark. This is often overlooked, but can make a big difference. Not all sparks are equal. Then heavier rim mass on the flywheels. Next would be lowering the C/R, and then you need valve timing with no overlap and slightly shorter intake duration than normal. A very small bore carburetor should also be used and will normally require "tweaking" with even small speed change. With the above features, a 1" bore engine can run nicely at 375-450 RPM. Larger bore engine will run even slower.

Jeff

 

[mu38&Bg#]

Dishing the piston will reduce compression ratio, no different than using a thicker head gasket. Anything you do to increase volume at TDC will increase it proportionally at BDC, except changing the stroke. If you relieved the inside of the piston to the crown thickness the plans show, there isn't much room to "dish" so the effect will be small. If you can leave a 1-1.5mm thick crown and top land go for it.


If you want to calculate an accurate volume you'd have to measure appropriately. To me, combustion chamber volume when discussing compression ratio is actual volume at TDC, not just the bowl in the head. I don't think the goal here is to achieve a certain compression ratio, but simply to reduce it.

http://www.roversd1.nl/sd1web/capacity2.html

 

[velocette]

Hi to all
Thanks for the varied and sometimes bizarre answers to problem of a lumpy running engine all theory and lessons in grammar aside.
Rustkolector & dieselpilot have got back to basics and provided very helpful replies that anyone building running models can relate to.

Eric

 

[Generatorgus]

I've read all of the replies with great interest, it kind of helps to dust the attic once and awhile. I thank all who posted for their input.

In the long run I gathered that some controversy seems to remain, so I'll proceed to dish the top of the piston which was my primary thought. If it helps I'll be happy, if it worsens matters, a new piston will be in order. I have the material on hand and it will be much simpler the having to make a new crankshaft, which I spent the last couple of days doing.

The compression of the engine is excellent, no improvement needed. Seems I must have learned at least something from all the time I've spent gleaning this wonderful forum and I appreciate all who pass their knowledge and experience along to others. I'm in my mid 60s and model machining has opened an entirely new experience for me.

Rustkolector, I've been reading a lot of somewhat conflicting posts on valve timing. This is the first engine I've built that doesn't have an atmospheric intake valve. So far I've had the timing set with a fair amount of overlap, I'll try messing with the timing before I alter the piston.
What about the exhaust timing?
You've also got me thinking about the carb.

Dieselpilot, The piston is longer above the pin than specified, partially because I was concerned about the extra head gasket I had to use because of the way I built the head, I had no castings to work with.
Taking some material out of the top should present no problem, although it needs to be accomplished by dishing because the top o-ring is fairly close to the top of the piston.

It may be awhile before I'm done fooling around with the Wyvern. I started reworking some things on it as I ran into a hold up with the engine I'm currently building.
But when I get the Wyvern where I want it I'll be sure to post the results.

GUS

 

[Rustkolector]

Gus,
Those features I mentioned are primarily for slow speed engines. That's what I like to build. The exhaust valve crankshaft duration is about 215 degrees closing at TDC or a few degrees after. Intake duration should be about 170-180 degrees duration opening about 10+ degrees ATDC. Not much longer than an atmospheric intake valve.That said, no one change generally makes a significant operational difference in an engine unless you are correcting a design or workmanship problem.

A slow speed engine would normally have plenty of time to breathe easily under load so a small carb throat is essential to atomize the fuel. Same holds true for propane. However, you will find you will have the throttle almost closed when running at, or near, no load, so the intake manifold vacuum will be quite high. This makes adequate filling of the cylinder on the intake stroke difficult requiring a really hot spark with a wide gap to fire it reliably. I usually clip off the horizontal portion of a Rimfire Viper plug ground strap. Ignition timing at TDC or a little later whichever works best. Higher compression doesn't help the latter situation. It only increases the minimum capable speed the engine can run.

Jeff