FOR THE EXPERTS !

The engine is a 14.8L V8 diesel ! Compression ratio 16.7. Why is it raising a compression pressure of 400+psi indicting a compression ratio of 27 : 1 on all but 1 cylinder thought to have a head gasket issue ? No the cylinders are not full of fuel, oil, or water. The valve gear is in good order. New pistons and liners 1500 Hrs ago/ full rebuild ! Open to any ideas, including those that at first glance may seem silly ! Noel.

Adiabatic heating of the air increases the pressure over that caused by the simple volume change.

Wrong pistons? I had to do a rebuild on an engine in Malta many moons ago the engine was available as naturally aspirated and turbocharged variations, major difference between the two was the pistons.

Paul.

assuming this has suddenly happened & it was ok after its rebuild then has it hydrauliced & bent a conrod, also does it use swirl flaps in the inlet manifold, does it have hydraulic adjusting cam followers, if it has pressed on cam lobes has one moved, is it breathing if so could be a ring, more info is really needed.

As said, adiabatic compression, meaning little or no heat loss over the compression stroke. This is why your bicycle pump heats up. 400 psi cranking pressure is quite normal for a diesel engine. Without this effect the compressed air wouldn't heat up, the fuel wouldn't ignite and the engine wouldn't run. The "opposite" of adiabatic compression is isothermal (slow) compression where the air charge remains at ambient temperature and this would give a peak pressure of 16.7atm. not allowing for leakage.

Edited By Clive Brown 1 on 06/02/2022 21:22:52

And the compressed air is hot enough to ignite cold oil sprayed into it! This gas law calculator suggests the temperature of a litre of air at 20°C that's been compressed 16:1 to 400psi will rise to 4700°C. Rudolf Diesel used the gas laws to design his engine.

Dave

Gerry, I must admit not making a habit of checking compressions on engines of this size, not least because it needs special tools that are not normally to hand. I had to make up a dummy injector and gauge to suit this engine and had not expected much above 300psi on a worn engine. These engines are fitted with ether injection as standard and on a temp of 5C will need it. Paul, one thought is that wrong pistons have been fitted but turbo pistons are lower compression than a normaly aspirated ones, so it would have to be pistons from a different engine type. Kevin, it ran OK after the rebuild but suffered a failed starter motor, new one was fitted but then the engine hydrauliced, full of water ! hence the compression test to find the cylinder. No swirl flaps, hydraulic followers or pressed on cam lobes. The high compression pressures indicate valves and rings in fair order. I only get called in when they can't fix things and often after others have messed with things. The engine is almost 50 years old, weighs about 1ton and each head over 100Kg and 2 engines are crammed in a 14' X 8' X 6' space. Clive and Dave, I understand the principles of compression ignition and how Rudolf Diesel started out as a refrigeration engineer but understanding thermo dynamics realised that increasing the compression ratio of the existing gas engines would vastly improve their thermal efficiency.

Questioning of others involved in the rebuild suggest that the block faces MAY have been skimmed. Things like liner protusion, valve and injector height Etc were not understood by some of thoses involved in said rebuild.

Where can I purchase a new magic wand - I may need it ! Noel.

Olivander's in Diagon Alley. Time to walk the dog

Edited By duncan webster on 07/02/2022 00:12:23

Adiabatic calculator:

**LINK**

The theoretical pressure for adiabatic volume reduction of 16.7:1 is in the region of 800 PSI.

Cummins 903 in a marine installation? Don't know it but I would be thinking about easy to fix ways that the water could get in the cylinder - on other engines I have had badly seated injectors and failed manifold gaskets cause this, both a lot less work than pulling a cylinder head. A leakdown test would probably tell you more.

Dear Adrian, your almost right, it is a 903 and a sort of marine application check out WASH MONSTER. ! It's not a normal marine instalation. Injector sleeve/pocket is a possibility. The inlet manifold is dry and the watercooled exhaust is remote from the head but will be pressure tested.. Noel

I hate fixing faults when someone else has been 'at it'! It's so easy for semi-skilled over-confident enthusiasts to introduce bizarre new problems. Debugging gets seriously complicated when nothing can be trusted because Bo-Bo Bodger 'helped'.

As a semi-skilled over-confident enthusiast myself, I try to keep good notes and photographs of what I've done. A magic wand would be better, but being able to review what was done often reveals silly mistakes. My memory's not good enough to recall more than a few changes.

Dave

Noel, being a well advanced engine of its day are different thickness head gaskets available relevant to piston protrusion above the block, you are probably aware this is standard practice today, I also am not convinced you have a problem with 400psi but clearly the odd cylinder is in trouble,can you measure the amount of valve travel before contacting the piston crown at tdc for good & bad cylinder this vould prove conrod length is correct before major work, I am fairly certain I can remember others struggling with the engines in that creature around 25/30 years ago.

Noel said it ran for 1500 hours then it hydrauliced now compression is wrong on one piston. its head off time possible more i can't see an easy fix

Agreed. Even a hydraulic lock at cranking speed with no other pots firing might well be sufficient to bend a rod. A compressed air check on that cylinder may aid with diagnosis if anything else is at fault.

1500 hours does not particularly indicate a ‘worn’ engine for an industrial design.

Are they wet or dry liners (just interested)?

If this was the case then the inlet manifold should have also been done as the heads lower and move closer together. On car engines depending on the type of manifold this normally gives you oil leaks from the lifter valley, or a lean cylinder. It's possible the engine could have sucked in coolant. but then the colour of the 'water' in no1 would have told that story. + the lack of rust.

There is nothing to exclude a chunk of valve dropping as well as the obvious bent conrod or pushrod. Is no1 low or zero compression?

Did the coolant get blown out of the rad / catch tank first? Is there water in the oil?

Hi Noel!

Looks like "pull it apart" time. Could be quite a "monster" job, I suspect.

It could be leaky valves on that one cylinder Distorted / burned seats? ) even a bent valve stem, preventing the valve from seating properly. Piston to tdc, remove valve spring and cotters, wrap wire around collet groove. (No point in having to fish for a valve that has fallen into the cylinder! VERY nearly did once on a 8.8 litre V8. Was lucky and realised my mistake just before the stem left the guide! ) Piston down bore to check of the valve moves freely in the guide over all the travel.

Having hydrauliced it could be a bent rod, or broken piston rings.

I suppose the injector is OK, and has not been dribbling, to burn the piston? I've seen a piston look like it had woodworm in the ring grooves after a dribbling / hosing injector let b the liquid fuel start burning in the grooves..

Keep us [posted on what you find

Best of luck!

Howard

!5900 mhours is probably less than a quarter life for that sort of engine.a number

For 18months after the rebuild the engine (Both) of them smoked. I stood by as others did the rebuild and I could not understand the injection system - the tolerance was 1thou, it was all set up with the proper gear. The method used a DTI. De rated pumps, different spray angles, different injector nozzles all to no avail - cost a fortune. Then one day a man turns up and resets the injectors and the smoke is gone - I was right, on a 50yearold engine the only way to set the injectors is with a torque wrench, all the manual info is duff, dti and 1thou Etc. Little bits of wear on the cam, followers, push rods, rockers and shaft all add up to more than 1 thou Not a good start ! The way these engines have been used the problem could be almost anything ! The head gaskets come in 1 thickness ! On engines this size the inlets are 2 manifolds joined by a flexible tube ! On reflection the 400psi is not something to worry about, I had given little thought to adiabatic compression ! I have already found a problem with the injector rocker and these engines were known to drop exhaust valve heads.

On a previous breakdown, one knew it was not good when one finds TWO exhaust valve heads in the manifold ! !!! Piston, liner, little end, oil pump all smashed to bits in the sump, gudgeon pin wedged across a crank throw, 2 LARGE holes in the crank case, even the back plate was broken. Certainly the best wrecked engine I've seen !

The exhaust manifolds have been pressure tested, the electric hoist has been rigged and a blow down test will be done tomorrow. The valve gear seems OK on the low cylinder,The compression was about 150psi, so I'm thinking head gasket blown into a waterway. I will keep you posted. Noel.

Noel, my first move in this situation would be to remove the injectors

and pressure test the cooling system, looking for leaks or a crack in

the injector tubes. In my experience this is the most common cause

of a hydraulic lock in 903's. And yes I've always set the injectors in

903's with a torque wrench, Are they turbocharged?

Dave

Hi Dave, No ! Non turbo ! The water in the works May be what stopped it, I should point out that the water issue happened overnight, not whilst running. It may come down to a full strip/rebuild. There may be more than 1 problem. There are several possible causes, of which injector sleeves are one, cracked head, the gasket, pin holed liner, or liner seals. The owner looking at costs and not understanding engines wants the work done insitu not withstanding the limited space and the weights involved. Testing the cooling system, their staff have already disconnected the pipe work Etc so a pressure test will have to be done on a surface plate. Ah well, thanks Noel.