1983 Bukh 20 hp. Indirect injection

[sgr143]

When I suggested that in post #39 I was told I was talking rubbish.

Me again - the one whose anecdote about a burnt-out solenoid and stopping the engine by blocking the air intake kicked off this discussion.

The "emergency stop "procedure I used was the recommended one on the RYA diesel engine course I went on.

Of the various explanations that everyone's chewed over, I think the one about not creating very large clearances in the valve train when the engine is at operating revs is probably the most tenable. My black museum of knackered Triumph engine parts (1961 6T) has a fine set of pushrods with cracked and splayed ends caused by my letting the valve clearances go way out; the engine still ran but the innards looked rather a mess when I checked! So I think you'd get away with stopping the engine using the decompressors for quite a while, but in the long term it might risk some damage to the valve train.

Dylan - don't be put off by Bukhs - they are reassuringly solid and simple engines. But it sounds like the boat has enough other stuff that needs attention as to not be worth your while?

Steve

 

[sgr143]

I actually changed mine of similiar vintage for a second hand BUKH 24, this is basically the same engine but with a re-designed cylinder head. This means that the bottom end of the engine is the same as the Bukh 20 and therefore a direct slot-in replacement

Is this just a higher compression ratio head, then? If the bottom end is unchanged the extra 4hp must come from somewhere...

An interesting thought though - swapping a new head in for a bit more grunt from the engine. More trouble than it's worth, I think, unless the current head goes spong for some reason, but useful to know all the same.

 

[AntarcticPilot]

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
◄ Previous activityNext activity ►
You are here
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
You are currently using guest access (Login)

Just to add that there are diesel engines fitted to boats where the ONLY means of stopping them is the decompressor. This was the case for the Sabb 6-8 HP fitted to Halcyon 27s, one of which my Dad owned from new. However, I suspect that they make Sabbs by starting with a solid block of iron and carving the engine out of it, and I know from first hand experience that they survive the most heavy-handed maintenance techniques imaginable (we relocated a pushrod by hitting it with a hammer...)

 

[stevie69p]

Is this just a higher compression ration head, then? If the bottom end is unchanged the extra 4hp must come from somewhere...

An interesting thought though - swapping a new head in for a bit more grunt from the engine. More trouble than it's worth, I think, unless the current head goes spong for some reason, but useful to know all the same.

It's now direct injection, and the max RPM has been increased which will be where the higher output comes from.

 

[Heckler]

I wonder how’s much horse power is lost by the hug flywheel on the Bukh, anyone know?
I replaced my DV20 3 years ago as it was a bit underpowered for my boat. It never missed a beat, always started eventually, clean no smoke and fairly economical for an old style engine. When I sold it, it had over 5000hrs on the clock with no work other than exhaust elbows and that luciferian stop solenoid, one alternator and a misdiagnosed starter motor.
I remember at the time I had a dilemma of what will I replace it with.

No energy lost, the flywheel "stores" it. Explained elsewhere.

 

[Heckler]

When I suggested that in post #39 I was told I was talking rubbish.

Typical!

 

[Heckler]

Me again - the one whose anecdote about a burnt-out solenoid and stopping the engine by blocking the air intake kicked off this discussion.

The "emergency stop "procedure I used was the recommended one on the RYA diesel engine course I went on.

Of the various explanations that everyone's chewed over, I think the one about not creating very large clearances in the valve train when the engine is at operating revs is probably the most tenable. My black museum of knackered Triumph engine parts (1961 6T) has a fine set of pushrods with cracked and splayed ends caused by my letting the valve clearances go way out; the engine still ran but the innards looked rather a mess when I checked! So I think you'd get away with stopping the engine using the decompressors for quite a while, but in the long term it might risk some damage to the valve train.

Dylan - don't be put off by Bukhs - they are reassuringly solid and simple engines. But it sounds like the boat has enough other stuff that needs attention as to not be worth your while?

Steve

Got a 6t for sale?

 

[sgr143]

Got a 6t for sale?

No, but if you want a starter kit to build one yourself, I have some knackered push rods, a piston with a big hole in the top of it, a cast iron head with 2 bent valves, a con rod missing its small end, a couple of cylinder blocks with fins missing and a very unbalanced crankshaft that might not be for a 6T anyway. And more! Collectors' items!

 

[VicS]

.................................. Collectors' items!

 

[LittleSister]

When I suggested that in post #39 I was told I was talking rubbish.

Nobody said that, Vyv.

 

[Rafiki]

Is this just a higher compression ratio head, then? If the bottom end is unchanged the extra 4hp must come from somewhere...

An interesting thought though - swapping a new head in for a bit more grunt from the engine. More trouble than it's worth, I think, unless the current head goes spong for some reason, but useful to know all the same.

I didn't suggest just changing the head. The point is that the same basic engine has been in production for maybe 40 years, Plenty of 2nd hand ones and plenty of spares available. This gives many options if you ever need to change the engine as you can buy new, buy 2nd hand and spend the winter working on it in your garage ( done that) or buy a hardly used 2nd hand so-called lifeboat engine and do a direct replacement. There is a good market for old Bukh's - providing it starts and runs you can get £500 for it, which goes along way towards the fitting ( not the buying) costs of a replacement Bukh.

 

[LittleSister]

I do not know the detailed differences that give the higher output of later versions, but the DV series (now sold as 'lifeboat engines', and they continue to be used for that purpose) twin cylinder Bukh was originally(?) 20 hp, but is now sold in 24hp, 29hp and, with a turbo, 32hp versions.

The twin is, as far as I can see, basically a doubled up single. The single was originally(?) 8hp, then 10hp, and is no longer in production. The DV36hp triple came later (1979 onwards) than the singles and twins, and though similar seems a somewhat different engine, and is still sold as a 36hp.

Bukh now produce three other ranges of engines, too, quite different to the DV series and going all the way from 24hp to 800hp.

 

[VicS]

I do not know the detailed differences that give the higher output of later versions, but the DV series (now sold as 'lifeboat engines', and they continue to be used for that purpose) .........................................

Designed for lifeboats apparently

"The classic BUKH DV RME range is designed and constructed for free fall, partially enclosed or open lifeboats, is available with several starting options and is SOLAS approved".​

 

[vyv_cox]

Designed for lifeboats apparently

"The classic BUKH DV RME range is designed and constructed for free fall, partially enclosed or open lifeboats, is available with several starting options and is SOLAS approved".​

Quite a long time ago the Shell catalogue listed an API CA oil for lifeboat engines. I think the only CA I have ever seen.