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Author Topic: 18/1 Bump  (Read 3777 times)

mcreeferson

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18/1 Bump
« on: June 21, 2016, 06:54:32 PM »
Hey all,

I'm slowly moving things along on the 18/1 project, and am heavily leaning on 38ac's excellent step-by-step on the WOK for these indian engines to try an get things right.

I've come to a bit of an impasse regarding setting bump clearance though...

If you happen to have a D.!. head and aluminum  piston with the chamber built into it then all bets are off. You will get a different answer as to the proper bump from whomever you ask. We set the bump in the with a compression gauge and every one of them is different, our experience here is not large enough to say that bump "x" works best in them.

I am hoping you guys may be able to shed a little light on this subject for me, as to be honest, I don't even know where to start. Bump clearance and it's effects on an engine are a total mystery to me, perhaps I could get a little lesson on theory there so I at least know what I am looking to accomplish (other that avoiding having it tear itself apart by allowing any interference of course)...

Is it just a question of achieving the proper compression ratio?

Hugh Conway

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Re: 18/1 Bump
« Reply #1 on: June 21, 2016, 11:06:33 PM »
@ MacReeferson
For starters, I have no experience with an 18/1, never even saw one. I just recently had a big lesson in bump clearance myself. Definitely does affect compression ratio. I had been running my 6/1 with way too much bump. On 1000hr decoke, I adjusted to proper spec for a non-COV 6/1. Bump is adjusted by altering the thickness of the cylinder base gaskets. You install more than necessary (a guess) and after torquing down and checking bump, the appropriate thickness of gaskets are removed to bring the bump into spec. Really very easy once you know it should be done.
Made all the difference in ease of starting, especially in colder weather. I also have a Dursley 6/1 that I set up properly, and it is also a very easy starter. Different spec. though, as it has a COV.
I have no idea of what's proper for an 18/1, but will caution that too small of a clearance can cause problems with valve/piston interference.
Someone here will be in a better position to advise on the actual bump spec for your engine, but I can say for sure that too much will give you low compression and too little can cause valves and pistons to be in the same place at the same time.
So, yes, it is all about compression ratio.
Not very much help, but something!
Cheers,
Hugh
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buickanddeere

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Re: 18/1 Bump
« Reply #2 on: June 22, 2016, 01:56:47 AM »
The ideal piston to head clearance is a close as possible without the piston actually touching the head when hot .

Thob

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Re: 18/1 Bump
« Reply #3 on: June 22, 2016, 02:23:06 AM »
Short answer:
Yes, bump clearance is mainly concerning compression ratio.  Assuming you have enough clearance that the piston does NOT hit the head or valves when the engine is cold or hot, then the only concern is compression ratio.

Long answer:

The compression ratio is the ratio of the volume of the cylinder, head, piston, etc at bottom dead center (BDC) to the volume at top dead center (TDC).  Increasing the bump clearance will add a small amount of volume to both BDC and TDC volumes, it can be calculated knowing the diameter of the cylinder and the amount of change in bump clearance.  The area of the cylinder is "pi R squared", the change in volume is the change in bump clearance multiplied by the area.

That's the easy part - the hard part is knowing what volume remains in the head, combustion chamber, etc.  One way is to disassemble the engine and measure the volumes with a liquid such as oil. 

The volume at BDC can be estimated by using the stroke of the engine multiplied by cylinder area, which is added to the volume at TDC.

But who wants to do all the math anyway?  That's why a compression gauge is needed.  Decreasing the bump clearance will increase the compression pressure, increasing bump clearance will decrease compression pressure.  All you need is to know what is the proper compression pressure and enough patience to put in the required shims to make it work.  And enough muscle to crank it over consistently while measuring compression pressure...
 
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dieselspanner

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Re: 18/1 Bump
« Reply #4 on: June 22, 2016, 09:43:08 AM »
Nice answer Thob!

The only thing I would say is that a few minuets with the calculator on your laptop or mobile phone and the measuring jug the Mrs has in the kitchen will get you in the ball park straight off. The thing has to come to bits to add or subtract the gaskets under the cylinder anyway. It also suits my 'Bush Engineer' mindset!

Do the calcs in centimetres and the sums are fairly easy, I'm also too tight to buy a decent compression test kit!
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38ac

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Re: 18/1 Bump
« Reply #5 on: June 22, 2016, 02:14:41 PM »
In layman's terms what you are after is enough compression to start the engine from cold but no so much that it is hard on the engine's internals during operation.  How does bump affect that?  As Thob has explained the amount of air squashing (or drawing in the case in the case of the intake stroke)  the engine can preform is fixed by it's bore and stroke. This is known as the displacement of the engine  or cubic inches, or cubic liters if your from that part of the world. There some other factors to consider but we are going to assume 100% efficiency for simplicity of discussion.  Our example engine here is going to displace 100 cubic inches.  If that 100 C.I. is compressed into an area that is 50 cubic inches at TDC then our engine will have exactly 2-1 compression ratio. If it is displaced into 5 C.I. area then it is 20-1 compression ration.  When you make the bump smaller you cause the engine to compress the same amount of air into a smaller space and then you have a  higher compression ratio. A problem with diesels  is that it takes  much more compression to start from cold than it does to run well when warm. Higher compression causes higher stress on the engines internals. This is why Lister went to all the trouble and cost to place the change over valve in the CS type. You then had about 19-1 compression for ease of starting and about 15-1 for running. 

To your engine and question
The Lister CS and all of it's variations are pretty lightly built for a compression ignition engine. When Lister got rid with the COV on the 8-1 they settled upon 17-1 as  good  compromise for starting and running. Their data  states that should equate to 560 PSI  Since I make no claim to being an engineer I simply use the same data when I build a D.I. engine.  I have seen several variations in the D.I. pistons and your 18/1 has a larger bore and stroke than the 6 and 8/1 D.I. engines I have built so even if I gave you the bump specs from those builds they would be no better than a wild guess.   To do this right you need a compression gauge and an adapter machined to use it and set the compression at around 600 PSI +- 50  there just isnt any way around it.  Is it possible to guess it? yes but that will take lots of time and patience. You place plenty of gaskets under the cylinder and remove them one at a time until the is enough compression to start the engine.  You must of course be sure the injector is bleed and working properly.  When right the engine will start on first compression down to about  20 degrees F.
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mcreeferson

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Re: 18/1 Bump
« Reply #6 on: June 22, 2016, 03:40:13 PM »
Alright, that's pretty simple, I think I can manage that easy enough.

Thanks for the rundown everyone.