vyv_cox
Well-known member
Just an attempt to reason why Bukh tell us not to do it. The decompression facility is to assist turning the engine over by hand. What is your reason for the instruction not to do it?
1983 Bukh 20 hp. Indirect injection
- Thread starter dylanwinter
- Start date
Just an attempt to reason why Bukh tell us not to do it. The decompression facility is to assist turning the engine over by hand. What is your reason for the instruction not to do it?
James_Calvert
Well-known member
Mine (1985 and one of the last batch of 20s) doesn't have a thermostart. It does take a little while to fire up when very cold. I think that's normal...
dom
Well-known member
Nice to see you back Dylan!
ghostlymoron
Well-known member
Would it be prudent to carry a spare exhaust elbow?
James_Calvert
Well-known member
The difficult bit in replacing it is in unscrewing the old one off the exhaust flange. Big pipewrenches, a bench vice and heat maybe needed. Ideally you'd have a spare made up complete with a spare flange and gasket, then it's an easy DIY job. Alternatively get a spare, fit it at leisure, and you're good for another 5 years.
LittleSister
Well-known member
I wasn't aware Bukh tell us not to do it. Where is that? (I have checked and can find no such advice in either the current DV36 manual, nor an ancient DV10 manual I have.)
My reason for querying the aversion to stopping an engine with the decompressor is that I thought based on my experience, albeit indirect, is that it is unlikely to be a problem, and thought that might be of interest to the forum as a potentially less hazardous means of means of stopping the engine if the solenoid stop fails than trying to block the air inlet. It is what I would do in that situation.
Seumask suggested, and had used, the decompressor to stop. Vic said he thought it was 'generally' not recommended, but didn't know why. You made a tentative suggestion why it might be. I was just throwing my experience into the pot.
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vyv_cox
Well-known member
I cannot find the Bukh advice at present but I do recall reading it. Googling the topic of stopping diesel engines using the decompression lever finds lots of hits, all saying never do it. Here is one of them:
Module 2, Engines
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CP Fitters Homepage / ► Engines / ► Resources / ► How a Valve Looses Heat
How a Valve Looses Heat
How a valve looses its heatHeat is transferred in 3 ways:
Conduction
Convection
Radiation
In a running engine, valves get very hot, and unless we remove this excess heat, the valve will quickly overheat and burn. The valve land and valve face will be damaged, and the gas-tight seal between the land and the valve seat will be ruined.
In a running engine, a valve looses about 5% of its heat through radiation, and out the valve port.
The valve also looses about 25% of its heat through conduction from the valve stem, through the valve guide, and on to the cooling jacket. The reason this figure is not higher is that the valve stem must be a sliding fit in the valve guide, so contact between them is not at so high.
The bulk of heat (75%) is lost through conduction between the valve land and the valve seat. Unlike contact between the stem and the guide, here, contact is very good, as the valve is held in place by a strong valve spring.
If, for any reason, there is not good contact between the valve face and the valve seat, the valve will tend to overheat, and can be burned and damaged, thus destroying the gas-tight seal.
This is why it is so important never to use a decompression lever to stop a diesel engine-it causes the valves to be burned.
Last modified: Thursday, 29 January 2009, 12:22 PM
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chewi
Active member
I'm confused by that explanation.
What difference would it make stopping by the solenoid? Wouldn't the valves still stop in an arbitrary position just as they would by decompression? They don't burn out that way.
LittleSister
Well-known member
Thanks for that, Vyv. I remain sceptical, though.
Once you lift the valve with the decompressor, there's no further compression, and there's no further firing. So, from where would the heat come to 'cause the valves to be burned'?
We are, in any case, talking about an emergency stop for the engine. Doing this once, or even a few times, ain't gonna burn out the valves.
Once you lift the valve with the decompressor, there's no further compression, and there's no further firing. So, from where would the heat come to 'cause the valves to be burned'?
We are, in any case, talking about an emergency stop for the engine. Doing this once, or even a few times, ain't gonna burn out the valves.
vyv_cox
Well-known member
Me too. Doesn't seem logical.
PaulRainbow
Well-known member
Sounds like nonsense to me Vyv.
Champagne Murphy
Active member
If we took the logic to its full limit we would all carry spare alternators, HP pumps, exhaust tubing, gasket sets, water pumps etc etc. Given the cost of Bukh parts I don’t think I could afford it, even if I had the space!
As I said, I’ve only replaced it once in 7-8 years, a spare might have rotted away in a damp bilge by now!
VicS
Well-known member
My thoughts are that when used as a starting aid you operate the decompressor to slightly open stationary valves or at least to prevent slow moving valves from closing completely during cranking.
however if you use the decompressors to stop a running engine they come ito contact with rapidly moving valves and in the few moments it takes for the engine to come to rest they will have been hammered a number of times by the closing valves.
rotrax
Well-known member
Last year I stripped and rebuilt the Bukh DV20 fitted to our Hartley 32 in Wellington, NZ. It had not had a working stop solenoid in six years to my knowlege, and by the look of the innards of the old one, it had burned our long before that. The decompressors had been used to stop the engine since it failed.
The tops of the rocker arms and matching surface where the decomp lever acts were badly worn. As only the decomp lever acts on the rockers there, perhaps that is why Bukh dont reccomend it.
I managed, with a bit of ingenuity and common sense, to adapt the bit of stop solenoid to be cable operated.
Works a treat and cost just time and odd bits of scrap.
LittleSister
Well-known member
I conclude from that there is no problem in using the decompressor to 'emergency stop' a Bukh, which is what we were discussing.
It is strictly true that Bukh 'don't recommend' using the decompressor to stop the engine: but only because they don't mention the decompressor at all in relation to stopping. (The manual just says turn the key to stop!) Nobody has been able to point to anything written by Bukh recommending the decompressor isn't used to stop the engine.
stevie69p
Active member
Pretty certain I read in some Bukh literature that it is acceptable to use the decompressors in an emergency to stop the engine, but not to do it on a regular basis.
When operated, they act directly on the top of the rockers to hold the valves open, which I would assume then leads to the pushrods clattering up and down with no rocker surface to bear on to.
When operated, they act directly on the top of the rockers to hold the valves open, which I would assume then leads to the pushrods clattering up and down with no rocker surface to bear on to.
Champagne Murphy
Active member
Could the pushrods jump off their mountings at high speed? Less likely at the lower speeds brought about by the starter motor.
VicS
Well-known member
FWIW
Yanmar say it is because fuel accumulating in the cylinders could make restarting difficult !
Yanmar say it is because fuel accumulating in the cylinders could make restarting difficult !
jwilson
Well-known member
I had a boat with a single-cylinder volvo 7 hp that was fitted with separate throttle and gear levers. Fully back on the throttle stopped the engine. As it was far too easy to accidentally stop instead of just going to tickover the previous owner had adjusted the throttle cable/lever so it would not actually stop the engine. So always decompressed to stop. He'd been doing it for years and I did too for many years.
dylanwinter
Active member
I always carry a spare engine
