Huh. My 2GM20, circa mid-80s, has 2 levers (one/cylinder), easily accessible at the top of the engine.
Runaway diesel
- Thread starter Captain Larry-DH
- Start date
Just keep in mind that something easily accessible when the engine is off isn’t necessarily as easy or safe when it’s running a few thousand RPMs. At that point you probably want the shutoff option furthest from the giant pieces of metal trying to destroy themselves. The air intake generally fits that bill.
I feel like I'm playing devil's advocate on this post, and its not intentionally challenging, but maybe to provoke a bunch of discussion and inputs.
Isn't the block of a diesel enough to contain the really big stuff?
I guess I think the connecting rod could pop p off the top, but a crank shaft wouldn't go through the block. Maybe down the bottom and through the oil pan? If I am right, and I mean IF I am right, then I'd be more worried about it driving down through the bottom and sinking the boat.
It could work in an emergency but would be likely to cause damage to the valve train. In the manual, it states that they compression release is never to be used to kill the engine. It is a starting and maintenance aid. When the engine is not turning over, there is no compression in the cylinders so the linkage only takes the pressure of the valve springs. Once the engine is turning over, you have the added pressure of the 20:1 compression which greatly increases the pressure required to engage the release. When the engine is running, you have the huge addition of the exploding fuel on the power stroke.
Interesting question. I don’t think I’ve heard of a boat sinking from a rod or other part going through the pan and hull. I have heard of boats sinking, or at least burning to the waterline, from runaway associated fire. I’d guess the top of the engine near the valve cover would be the most likely place for shrapnel, but that’s just a hunch and not particularly informed.
When engines blow up, I've never seen anything coming out of the bottom, I've seen things come out the sides (rare) but usually only the top end. External flying debris I've seen are parts of valves, push rods and the broken engine pieces they punched through. Internally there can be a lot of damage. If you look at engine design, it makes sense.
dj
That would work, with probable engine damage, but I haven't seen an engine since the 90's with a decompression lever.
Mark
Strange. The levers (one / cylinder) are intended as a starting assist. Why discontinue a good idea ?
Only they would know why. Most likely $$$$$$$$$$$.
Maybe. My old Volvo MD7a had a compression lever and a crank to hand start it. (I never tried it)
Perhaps engine design has relegated the decompression lever to archives of diesel history.
Decompression levers were only used on engines that were designed to be hand cranked for starting as far as I'm aware.
The biggest engine I've ever seen them in was an old Volvo - MD series but I don't remember if it was three or four cylinders. You would flip the decompression levers on all cylinders and then get the big flywheel spinning then get the first cylinder firing. That then allowed you to get the other cylinders firing.. They were actually pretty easy to start. These engines didn't have starter motors, they had what was called a "dynostart" IIRC. It was an genernator that would also work as an electric motor to turn the engine over. That was easier than hand cranking but hand cranking was not difficult, you just had to get that flywheel spinning fast enough.
While there may be some $'s reason, I think it's more likely there was not felt a need. You don't need decompression levers unless you are going to hand crank one of these engines to start.
dj
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When running in a larger diesel motor (ie in the shop - to break in the camshaft and achieve some ring seating) there is a rule: Keep a Co2 extinguisher at hand. If governor failure runaway or other - Co2 kills the motor instantly - every time. Runaways do happen occasionally and if you have not seen one - it is terrifying.
It is not unusual for (very) large engines to have manual or automatic decompression to aid starting.
Standard 3GM series Yanmar engines have a decompression lever for each cylinder.
They also were fitted with a handcrank . Hard work on a 27 HP engine!
Yanmar now have an automatic valve decompression system fitted to their small stationary diesel engines.
I suspect that this system is also fitted to the 27 to 57 HP marine engines but can't find a source for that suspicion.
I have a small (ancient) YSE engine that I crankstart once a month.(except in winter).
This gives me an upper body work out, helps me maintain my technique (each engine has its own peculiarities) and lets me know that the engine is in good condition.
Easy starting is one of the best indicators of an engines condition.
As for removing the decompression system to save money.
That is not how quality Japanese companies operate.
Gary
Standard 3GM series Yanmar engines have a decompression lever for each cylinder.
They also were fitted with a handcrank . Hard work on a 27 HP engine!
Yanmar now have an automatic valve decompression system fitted to their small stationary diesel engines.
I suspect that this system is also fitted to the 27 to 57 HP marine engines but can't find a source for that suspicion.
I have a small (ancient) YSE engine that I crankstart once a month.(except in winter).
This gives me an upper body work out, helps me maintain my technique (each engine has its own peculiarities) and lets me know that the engine is in good condition.
Easy starting is one of the best indicators of an engines condition.
As for removing the decompression system to save money.
That is not how quality Japanese companies operate.
Gary
I have a little baby single-cylinder Yanmar 1GM, which is only about 7 hp. It has a decompression lever, and I also have a hand crank for it. Darned if I can start it with that hand crank, though. Checking into it online, it appears that I'm not the only one.
I should point out that starting it normally, the engine fires right up.
I always found the Yanmar hand crack engines quite difficult to start, but the old MD series Volvos actually quite easy. I used to fire mine up by hand cranking all the time until I replaced the dynostart. There were a few other hand crank engines that also worked well (Id have to think about which ones, I had friends with them, I didn't own them) But I never found the Yanmars easy to hand crank.
There was a time when the "serious" blue water sailors always said you needed an engine that you can hand start in case you found yourself without battery power.
dj