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HomeRC & PowerAircraftHelicopterEngines Plugs Mufflers Fuel › see "these" Glow engines running on WD
08-15-2017 02:07 AM  15 months agoPost 21
wjvail

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Meridian, Mississippi

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Wouldn't the mechanical drawing/specs provide this info....or at least the dimensions to calculate it?
Well of course the drawings very likely have some estimate of combustion chamber volume. PAW or DDD might be able to provide a design compression ratio. Someone more versed on model Diesels likely could quote a number. If truth were told, I don't run Diesels that much. More than many but not enough to be considered a resident expert.

I often like to measure these things myself and from that perspective, actual compression ratio is not something I can measure. Had I ever actually measured a diesel's compression ratio, I might remember it.

I might help to note that the construction of many model diesels is not like the helicopter engines we all know. I'm thinking of the PAW engine in the video I posted. What appears to be case fins and part of the cylinder, is really just a clamp holding the steel cylinder down. You can remove what appears to be the 4 head bolts but all that comes off will be the aluminium fins. The steel cylinder is still slipped into the lower case. You can then pull the steel cylinder, along with its contra piston, from the lower case but by doing so, you will loose all ability to measure combustion volume.

The long and short of it is, without a glow plug hole, there is no access to the combustion chamber.

"Well, Nothing bad can happen now."

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08-15-2017 04:27 AM  15 months agoPost 22
Gearhead

rrMaster

Vt

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all you need to know is the swept volume and combustion volume

here, this tells it better than I can say it in short,
If we have an engine, at BDC that has a swept volume of 900cc in the cylinder and a combustion chamber volume of 100cc, then the total volume is reduced to the 100cc of the combustion chamber at TDC this would be a compression ratio of 1000:100, or reducing it fractionally, a compression ratio of 10:1

Jim
Buzz Buzz Buzz

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08-15-2017 04:39 AM  15 months agoPost 23
wjvail

rrKey Veteran

Meridian, Mississippi

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all you need to know is the swept volume and combustion volume
Well yea. OK. I knew that. I got a college degree in mathematics.

Swept volume. When do you want to start measuring compression? Bottom dead center or when the ports close? What you suggested would be typical for a four stroke but meaningless in a two stroke. What is the swept volume after the ports close? With enough time, effort, and some measuring equipment, this is calculable.

The hard part is finding the combustion volume of a Diesel. How do you propose I do that? (Never mind that it is adjustable - while the engine is running.)

"Well, Nothing bad can happen now."

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10-06-2017 06:51 PM  13 months agoPost 24
ChrisW

rrApprentice

Bonsall, CA

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Here's an interesting article relevant to this thread...

http://www.davisdieseldevelopment.c...helicopters.htm

and video here...
http://www.davisdieseldevelopment.com/videos.htm

CWcopter.com

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