Piston ring design and sulphated ash

[cmpbyr01]

Why do American engines require a certain sulphated ash rating is it because of the design of the top ring? Why do European engines require a low ash rating? European top ring position is lower on the piston than American pistons. Heat dissapation I know also plays a major role in terms of where that top ring is positioned. Japenese design is also different in that the ring is positioned somewhere between where an American and European ring would sit.

My understanding was also that the piston rings for the american engines, was different in design to the european design and japanese design. Where the european and japanese design has a flattened ring, the american design is angled to allow for ash to collect. Is this for lubrication purposes, and if so how do the american designs prevent bore polish from occuring. Some comments on this issue would be welcomed.

In short what is the significance of the sulphated ash rating in engine lubricants with regard to different engine designs. Some info on this matter would be appreciated.

cmpbyr01 (Rookie)

 

[NickB]

I've only built a few engines, but i've noticed that the top ring has a square edge on both the japanese and north amerian engines i've worked on. The second ring is usually tapered out at the bottom.

Japan seems to have less stringent emissions control regulations than the exact same engine produced for north american usage. (Specicially a few hondas, and the mitsubishi 4G63 turbo motors) Placing the top ring closer to the top of the piston crown will reduce he amount of trapped hydrocarbons during operation, which will reduce emissions a little. It also means the top ring land is a little weaker, and better materials and processes are required to maintain the same strength with less metal above the ring.

Most European cars end up detuned when they hit the orth american market, i'm assuming this is also due to emissions regulations. Using the 4G63 as an example again, it's very easy to make a ton of power from those little 2.0 engines. The mitsubishi EVO 7 makes over 320 hp, but the same motor in north america has never been sold making more than 205. The Audi TT is another example, it lost a bunch of power by the time it got here.

Yeah, i know i said nothing regarding ash content (I have NO idea about that) but i think my comments are relevant to the topic. (at least a little, LOL)

 

[Chumley]

I'm no tribologist either, but the word "allow" might be better than the word "require" in terms of sulphated ash content for oil American automotive engines. Most American air cooled aircraft engines specify ashless dispersants for their oil. Cylinder head and upper cylinder wall temperatures typically run higher in air cooled engines leading to higher oil temperatures so that might be part of the reason. The same occurs in many NA and turbo charged waterboxer European engines which are usually smaller in displacement but much higher on the hp/inch3 ladder as they come from the factory when set up for European use. The oil temperatures typically run higher since normal driving speeds on the autobahns are typically much higher. Those Germans love running things hot and design many of their "family" cars to run at wide open throttle all day long!

Please kick me in the head and straighten me out if I'm all wet on this!

Chumley

 

[romke]

the sulphated ash indicates a certain amount of metallic additives present in the oil. These additives are used for various purposes: keeping the engine clean, preventing wear of heavily loaded components like camshafts and as an anti-oxidant.

The amount you can put in in gasoline engine oils is somewhat limited: too much might lead to deposits that might cause knocking under certain driving conditions because they tend to glow. Therefore the amount of sulphated ash in gasoline engines oils is limited, although some more is allowed in European then in American engines.

The reason for this is the differences in design: European engines tend to be much smaller then US engines and do normally operate under higher loads and speeds. Therefore in Europe we need more anti-wear to keep the camshafts in shape and also more anti-oxidant because temperatures tend to be higher.

In dieselengines the differences are still more marked: most European engine builders require a high sulphated ash oil to keep their pistons clean and the piston rings free moving in their grooves. US designs tend to be different: larger clearances and a cutback piston design. If you use a European type of high sulphated ash oil, there may be a tendency to build up coal in the clearances and on the tapered off topland that, due to the tilting action of the piston against the liner, is compressed to a very hard substance that actually can promote wear (borepolishing).

Therefore American designs usually prefer a low ash diesel oil. The idea is that although the clearances are taken up by deposits, these are broken down again due to shockwaves occurring due to the combustion proces. Since these deposits are not that hard (because of the low ash oil) an equilibrium is reached after a certain amount of time where a good balance is struck between relative cleanliness and sufficient gas pressure behind the pistonrings to prevent oil consumption.

Most European long drain diesel engine oils are highly unsuitable for heavy duty American diesels, American heavy duty diesel oils may be used in European designs, but because of inferior engine cleanliness capability should only be used with relatively short drain intervals.

 

[NCD13]

Sulphated ash helps to extend the valve recession. Especially in gas engine or diesel engine because of no liqiud phase flow through the intake valves . Implies no liquid film to absorb the mechincal impact between valves and its seats.

The reason behind ash requiremen is valve design.
For example , Waukesha gas engine used to require 1% ash in the early stage and then drop to 0.3% nowadays ( after valve & seat design has been improved ) , while MWM engine requires zero ash.

Hope this help.