Compression Relief Valve

[joerocker]

I understand wanting to get oil up into everything as soon as possible.

Isn't it the ACT of the engine turning without oil that wears things out faster regardless of whether it's actually firing? What is the REAL difference between a "dry" engine simply turning over by the starter and one turning over running in idle?

Is it significant enough to worry about?

Also, I always worry about "spring loaded" mechanical "things" wearing out with use. Meaning, will the compression release springs weaken and eventually won't be able to "keep compression"? In a sense, the spring wearing out from use and being so weak as to be constantly "compression released"?

 

[greg_g]

Perhaps you don't understand that the starter motor turns the flywheel, which turns the crank shaft, which untimately operates the oil pump. The act of cranking with the compression release open is a method by which you AVOID your "dry engine" scenario.

There are no "compression release springs" to wear out. When you engage compression release, a solid metal rod rotates solid metal fingers that push down on the valve stems - keeping them open against the valve springs.

Using the compression release is a factor in making your engine components last longer, NOT wear them out sooner.

//greg//

 

[joerocker]

I realize that turning the engine with the starter with EITHER the decompression OR letting it start and idle BOTH start off with a "dry" engine. The ONLY difference is that there is either hot gasses in the combustion chamber or there isn't. I was just wondering HOW MUCH REAL DIFFERENCE there is between simply spinning the engine with it decompressed or having it actually start and idle. Is there REALLY that much difference in engine wear between them?

Am I explaining that well?

Both ways start with a "dry" engine. One turns without any "bang" the other a mild idle "bang". It doesn't take long to build up oil pressure and until that happens BOTH methods are turning a "dry" engine. I imagine that a decompressed engine has a smaller "load" due to no "bang" and there is less pressure on some "dry" moving parts initially, but is it enough to really matter?

As for the starter...is it better to simply start it with compression and the associated stresses right off the bat, or is it REALLY better to have it turn easily and then HIT IT with a "braking" force when you release the decompression and actually start the engine. I'd figure it wouldn't like the SLAM it gets when you turn off the decompression.

Lastly, there must be SOME kinds of springs in the decompression system. My decompression lever is TURNED for decompression. There is quite a bit of force on it, spring tension I'd imagine. My fear is that the springs "wear" as they're used and eventually weaken to the point where they can't hold enough compression for the engine to perform optimally. Am I right here? There must be springs keeping tension on something to allow compression. Every time you crank that lever and compress those springs for decompression, I imagine you lose some of the springs tension.

Just curious...

 

[3RRL]

You are right in your assumption Joe.
As soon as the starter moter turns the flywheel, everything else starts moving simultaneously. The crankshaft, pistons start moving, the oil pump...everything.

The decompression valve lets the starter motor work with less initial resistance on it from compression in the cylinders. The oil pump does not move or start pumping without the other stuff turning.

The decompression valve uses a rod that holds the valves open so no compression occurs in the cylinders. So technically, you are correct about the valve springs being squished to open the valves (manually). But that happens a million times anyway as you motor runs.

 

[greg_g]

Joe, you're making problems where none exist. The tension you feel on your compression release handle is a RETURN spring. All it does is return the lever to the "home" position when you let go. In the process, the fingers lift off the valve stems - permitting the rocker arms to take over.

Just twisting the ignition key and cranking till it's running is a DRY START. Using the compression release to spin the engine long enough to observe oil pressure is NOT a dry start. There are no "bangs" - no hot gasses - until AFTER the oil has started to rise.

If your fears were actually founded in fact, decompression systems wouldn't exist in the first place.

//greg//

 

[MrFixit]

Just wondering... is the starter motor turning the oil pump (Engine) at a speed lower than idle when using compression release?

Does cold oil pump quicker with a faster spinning pump than it does with a slow spinning pump?

Would a engine that is actually running provide oil to moving parts faster than an engine that is moving slow while compression release is activated?

Tim

 

[joerocker]

Thank you for the replies...

Rob,

I didn't relize that the tension I was feeling when turning the decompression handle was simply the valve springs being held open. I wasn't really sure how the decompression release system worked...I was thinking it was some valve in the cylinder head that was spring loaded to stay closed and we were opening it to allow pressure to blow off. I must say, the way it REALLY works is more elegant!

And it seems obvious that there would be LESS strain on a "spinning" engine with no "load" due to no "banging"...my question was: "Is it enough to matter?" In real life will it matter after thousands of hours? Or, is it like a smoker, every minute you run it without adequate oil pressure takes an hour or more off the engines life? And what about Tims question? Makes sense to me that en engine turning at 600-1000RPM would build up and distribute oil faster than a slow spin fron the starter.

Now, how about the starter...is it better to "load it up" at the beginning or SLAM it while it's already turning when you release the decompression? Anyone with "real life" experience in different starting techniques?

Greg,

I always enjoy your responses but I see it differently...I look at it as my "NOT creating a problem where none exists". I'm wondering if it REALLY makes a difference doing this decompression thing. Whether it REALLY reduces wear on the bearing surfaces and engine life having the starter to spin it up decompressed until oil pressure pumps up. ALL car/gas engines I know of simply start up "dry" and they last a long long time.

As always you guys are a wealth of information and I learn something new every day! Please don't take my questions/comments wrong. I'm simply curious...

 

[greg_g]

</font><font color="blue" class="small">( Just wondering... is the starter motor turning the oil pump (Engine) at a speed lower than idle when using compression release?
Does cold oil pump quicker with a faster spinning pump than it does with a slow spinning pump?
Would a engine that is actually running provide oil to moving parts faster than an engine that is moving slow while compression release is activated? )</font>

In the face of physical laws Tim, I think the answers are self evident - and moot. Of primary concern to me is that I see oil pressure developing BEFORE combustion actually occurs. That - and the fact that there's considerably less wear and tear on both battery and starter - are the reasons I ALWAYS cold start with compression released.

I really don't understand this chicken little phenomenon here. Like I said before; a compression release wouldn't be installed in the first place, if any of these issues had merit.

Compression release is not limited to Chinese tractors. I suggest you take a glance HERE. You guys might be riding/driving vehicles with automatic compression release, and not even know it.
//greg//

 

[bluechip]

The big advantage is less load on the starter. since the chinese do not use a gear reduction starter as most "more modern" tractor engines it takes a lot more amps to get things turning. So by using the CR you let the starter start the oil pump (yes the pistons are sliding up and down the cylinders but there is not any real load on the bearings in the crank as the pistons are not compressing anything). once teh oil galleys are full, releasing the CR provides compression and boom, you are going (very little shock to the stater, these things have BIG heavy flywheels to tak that load). Another advantage is that unless you hold off the fuel cutoff, you are also pumping diesel so you are more likely to have enough spraying by the time you drop the hammer on the Compression release. I will admit that I only use it when we are trying to start a tractor that has been sitting and the battery is down, but over the life of a tractor I think you will replace fewer starters and batteries if you got in the habit of using it.

 

[Paddy]

With regards to the sudden loading on the starter after you release the decompression lever. It called angular momentom. If you don't use the decopression, you fight compression and getting every moving. A good way to look at this is bump starting a moter cycle. With the clutch depressed, there is very little load. When you quickly let out the clutch, momenton is on your side and it starts. Now do this with just pushing the bike in gear! You can start it this way but I reccomend you have an extra guy or two. This "extra guy or two" is the strain you are placing on the starter.