Author Topic: High compression vs low compression  (Read 7703 times)

Stan

  • Guest
High compression vs low compression
« on: January 27, 2008, 04:23:34 AM »
Here's a very clear description of the difference in running a Lister 6/1 or 3.5/1 in high and low compression.  Ken is a very knowledgeable guy in the UK when it comes to Listers.

http://tech.groups.yahoo.com/group/Lister_CSOG/message/6128

Stan
ps...hopefully you will be able to read the message if you are not a member of the Lister CSOG, you just won't be able to reply. (I think)

listerdiesel

  • Guest
Re: High compression vs low compression
« Reply #1 on: January 27, 2008, 09:32:10 AM »
It was permissible to run the engines on high compression at light loadings, otherwise the combustion was not good and a lot of carbon/oil deposits formed.

The later higher output engines used the same conrod bearings etc., although an ali piston was fitted and the valve timing charged to help with starting without compression change-over.

Quite a few of these engines spent their entire life working on light loads with the higher CR with no problem, and I know of a couple that ran permanently at high CR because the valve was broken....

Peter

Stan

  • Guest
Re: High compression vs low compression
« Reply #2 on: January 27, 2008, 05:59:53 PM »
That makes sense, the physics of it would dictate that higher loadings would equate to more wear on the con rod bearings, as would higher compression.  With both higher loadings and higher compression you might be quadrupling the loading.
Stan

mobile_bob

  • Administrator
  • Hero Member
  • *****
  • Posts: 2940
    • View Profile
Re: High compression vs low compression
« Reply #3 on: January 27, 2008, 06:26:02 PM »
couple that with the rather poor top oil feed design
and it is no small wonder that the brgs got beat to hell
under heavy loading and high compression.

personally i think all the pieces to the puzzle are there,
makes me wonder why they stayed with the top fed oil holes ?

simplicity?

bob g
otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

Stan

  • Guest
Re: High compression vs low compression
« Reply #4 on: January 27, 2008, 06:33:53 PM »
I wonder if it had anything to do with the wear on the sml con rod bushing allowing it to turn in its housing.  When I pulled it on mine, I found that the bushing had turned almost a 1/4" and just about closed the oil feed hole.  This is a bad thing.  I put some stuff on it when I pressed the new one (can't remember exactly what it was called but it was designed for just this purpose)  to keep it from turning.  With the heavy flywheels, startup and speed up no matter how slight would cause more wear on the con rod bearings.
Stan

lendusaquid

  • Sr. Member
  • ****
  • Posts: 353
    • View Profile
Re: High compression vs low compression
« Reply #5 on: January 27, 2008, 06:38:42 PM »
Am i right in saying that higher compression reduces knock because thats what seems to happen on my engine. As soon as i unscrew the cov the knock gets bad .And then when i try to get rid of the knock by retarding the ignition i get lots of smoke.

Doug

  • Hero Member
  • *****
  • Posts: 3386
  • Why don't pictures ever work for me?
    • View Profile
    • Doug's Petteroid Stuff
Re: High compression vs low compression
« Reply #6 on: January 27, 2008, 06:44:49 PM »
Quote Bob:
"couple that with the rather poor top oil feed design
and it is no small wonder that the brgs got beat to hell
under heavy loading and high compression.

personally i think all the pieces to the puzzle are there,
makes me wonder why they stayed with the top fed oil holes ?

simplicity?"


No one has brought up the oil pump feeding the crank pin on the Gm 90.
Does this mean it can tollerate a higher compression ratio?
Does a direct injection 6/1 hammer the bearings harder than Lister?
It's a Good Life, If You Don't Weaken

Stan

  • Guest
Re: High compression vs low compression
« Reply #7 on: January 27, 2008, 06:45:08 PM »
Are you unscrewing it all the way, hard!  And, are you loading down the engine or is it lightly loaded?
Stan
« Last Edit: January 27, 2008, 06:46:56 PM by Stan »

lendusaquid

  • Sr. Member
  • ****
  • Posts: 353
    • View Profile
Re: High compression vs low compression
« Reply #8 on: January 27, 2008, 06:50:56 PM »
O yes, full in or full out. I also thought it run nicer with a plug,but thats been removed because of worry about to high compression.

oliver90owner

  • Hero Member
  • *****
  • Posts: 861
    • View Profile
Re: High compression vs low compression
« Reply #9 on: January 27, 2008, 06:59:50 PM »
Stan,

Remember these engines were designed and built to a specification at that time.  

Common parts were obviously a design criteria (or is it -um) and so was the engine speed.

Now if you wanted to increase the loading on a splash-lubed big end bearing and keep longevity, you would be out of luck with the bearing materials and sizes they were working with.  So it was simple physics or basic design calculations to work out the bearing loadings at given rpm.

If that size engine had been designed for 1500rpm the power could have been x2 1/2 and the loading would have been approx the same but the rod lube would not have been ideal and the flywheels would have needed a different design.  They could, of course, have used 3 inch crank bearings etc etc etc... but they didn't.  So we are left to use the engine within the design parameters distributed by the experts - the designers, builders and suppliers (Listers).  Piston wear at 1500rpm would much exceed their longevity calculations, of course!  They simply got everything designed right.  They did their job and made a good reliable long-lived simple engine which could be maintained adequately even by the owners of the day.

Soooo, the engine was rated for 650 rpm and 6 HP eventually, after they found it was not stressed at 5HP.  The change-over-valve (COV) was simply a means to get the engine started in cold conditions, but it was OK to leave it 'engaged' if bearing loading was kept within design limits (ie lower power limit) and that option had the benefits descibed earlier.  You are simply not able to keep the bearing loadings within the limits of the design if operating at high power with the COV is screwed in.

As Peter points out an aluminium piston was used later - this would allow higher piston speeds and more power for a given bearing loading.  

Bearing materials have improved since 1930, as have oils and fuels.  Most engines now produced are high revving and of relatively short life span.  They are designed for a life-time cost better than this old Lister design.  But they may not be able to cope with waste oil and lard as fuels like these old engine designs.  Modern engines are designed 'just' to hold together, by computers.  Any surplus strength or metal is designed out.  Competing designs have to be competitive and price + lifetime running costs in the 'write-down' period are the accountants' criteria, not how long it might last.

Regards, RAB

buickanddeere

  • Hero Member
  • *****
  • Posts: 807
    • View Profile
rifle bore rod for pressure lube???Re: High compression vs low compression
« Reply #10 on: January 31, 2008, 10:14:43 PM »
 To pressure lube the connecting rod's small end.
  Has anybody ever chucked a rod in a lathe and rifle bored it from the large end to piston pin end?  .177 or .204 drills for making rifle barrels are economical, readily availiable and of good quality.   

mike90045

  • Mendocino Metro
  • Hero Member
  • *****
  • Posts: 1594
  • Mmmm BBQ
    • View Profile
    • Mikes Solar PV page
Re: High compression vs low compression
« Reply #11 on: January 31, 2008, 10:29:11 PM »
Quote
  Has anybody ever chucked a rod in a lathe and rifle bored it from the large end to piston pin end?  .177 or .204 drills for making rifle barrels are economical, readily availiable and of good quality.

So ... how would this work ?   I've always wondered how the splash lube got oil up to the piston pin.   Lots of splashing ?

Why would a drilled hole help - is the crankshaft drilled & pressurized  ?

bitsnpieces1

  • Full Member
  • ***
  • Posts: 247
    • View Profile
Re: High compression vs low compression
« Reply #12 on: February 01, 2008, 12:40:33 AM »
     If the big end had pressurized oil feed, the rifle drilling would allow the oil to move to the small end and lube it. 

  Thinking:  1) You use the higher compression to start because a higher compression produces a higher temp in the combustion volume, then drop back to lower compression to run.  2) Running under higher loading maintains a higher combustion volume temp. than lower compression.  3) Running at higher compression under light loading or lower compression at high loading keeps carbonization down.  4) I see that a lot of the smaller high speed diesels are running at ratios not much over 10:1 (say 11:1). 
   It would seem the determining factor is the actual temp. in the combustion chamber.  The higher the temp the less the carbon.  I think most of us would agree that any engine running under a full load gets a lot hotter then one running at a gentle idle.  So the real thing to aim for is to maintain the higher temp without letting it get too high [melt the piston]. 
Lister Petter AC1, Listeroid 12/1, Briggs & Stratton ZZ, various US Mil. surplus engines. Crosley (American) 4cyl marine engine(26hp).

Doug

  • Hero Member
  • *****
  • Posts: 3386
  • Why don't pictures ever work for me?
    • View Profile
    • Doug's Petteroid Stuff
Re: rifle bore rod for pressure lube???Re: High compression vs low compression
« Reply #13 on: February 01, 2008, 03:35:31 AM »
To pressure lube the connecting rod's small end.
  Has anybody ever chucked a rod in a lathe and rifle bored it from the large end to piston pin end?  .177 or .204 drills for making rifle barrels are economical, readily availiable and of good quality.   
.

Gm 90's have a cross drilled crank and the Piston pump derived from a Petter.
not sure whats in the other other DI clones.
It's a Good Life, If You Don't Weaken