Semper Fly
New member
I'm new to diesels and I don't have a clue what the Decompression Release does.
Can anyone clue me in?
Can anyone clue me in?
Diesel Newbie
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DavidNPro
Silver Member
It is used to prevent the engine from creating pressure while it is turning over. In turn, it allows the engine to spin over much more easily and prevents it from starting. It is often used to allow the engine to build oil pressure before starting.
For example, you would pull the compression release, turn over the tractor until it has oil pressure, release the compression release and then start that tractor.
deepNdirt
Veteran Member
Welcome to TBN..... You're sure to find answers to all your questions here,
to try and answer the one about "decompression" it can be used for a couple of things,
1- as the weather gets colder in some parts of the country the oil in the engine can thicken, causing the engine not to be turn over as easily as would be in warm conditions, the decompression lever can be used to spin the engine over more freely,
also helps to pre-warm the cylinders by creating rapid friction from the piston & rings,
I'll let the other members elaborate further
Last edited:
Gary Sweat
Platinum Member
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I've found that it's about the only way I can start mine if the temperature gets below about 50 degrees. I don't have glow plugs. Sometimes I have to pull it a couple of times. I would think it would help extend the life of the starter as well??
OP
Semper Fly
New member
Thanks for the replies guys. I couldn't ask for a better response and thanks for clearing that up for me.
California
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- An hour north of San Francisco
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- Yanmar YM240 Yanmar YM186D
And I'll add my 2 cents worth (mostly repeating what was already said):
These things have higher compression than a gas engine and heavy flywheels. It is gentler on the starter to bring the flywheel up to full speed, if it doesn't hit a compression stroke immediately.
And after the flywheel is spinning freely at maximum speed and you drop the compression release, the flywheel's inertia assists the starting motor in overcoming the resistance of compression. So that first compression stroke is accomplished at higher rpm. In cold weather with a chilled battery this inertia may make the difference between start on the first stroke/give up and try again.
2) For these rice-paddy tractors I wonder if the compression release is also intended to get the water out of the cylinders without damage after the operator underestimated the depth he was driving into.
These things have higher compression than a gas engine and heavy flywheels. It is gentler on the starter to bring the flywheel up to full speed, if it doesn't hit a compression stroke immediately.
And after the flywheel is spinning freely at maximum speed and you drop the compression release, the flywheel's inertia assists the starting motor in overcoming the resistance of compression. So that first compression stroke is accomplished at higher rpm. In cold weather with a chilled battery this inertia may make the difference between start on the first stroke/give up and try again.
2) For these rice-paddy tractors I wonder if the compression release is also intended to get the water out of the cylinders without damage after the operator underestimated the depth he was driving into.
normde2001
Elite Member
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It's a manual version of the automatic compression release on a lawn mower engine.
stormpetrel
Gold Member
Welcome Semper! What tractor do you have?
I have a follow up question: can the engine start at all while the compression release is engaged, or does it start after it's dis-engaged? It's the compression that causes combustion, yes? I don't see how the engine can fire with NO compression. Thoughts?
I have a follow up question: can the engine start at all while the compression release is engaged, or does it start after it's dis-engaged? It's the compression that causes combustion, yes? I don't see how the engine can fire with NO compression. Thoughts?
California
Super Star Member
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- An hour north of San Francisco
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- Yanmar YM240 Yanmar YM186D
It holds the valves open so the engine spins freely with no compression. You have to let go of the lever/cable/whatever to allow compression, and therefor starting.
For an example go to my page referenced from my sig, below, scroll to the bottom, and listen to the sound file.
Edit:
Here's a direct link to that sound/vid.
For an example go to my page referenced from my sig, below, scroll to the bottom, and listen to the sound file.
Edit:
Here's a direct link to that sound/vid.
(If you're at work turn it up LOUD!)
Gary Sweat
Platinum Member
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I'm no diesel mechanic but from observing my tractor, it is apparently letting the fuel sucked in on the intake stroke just go out the exhaust. Lots of nice gray/bluish smoke till you let go of the release. I've actually gotten in the habit of using it all the time to start the engine. Hope that is not leading to problems down the road
