coolant formulation tech questions 4

[curtis74]

At my fab shop I deal mostly with the driveline engineering. By default I almost always just dump in a 50/50 mix of green coolant from a bulk 55-gallon barrel and ship off the car, but after recently having purchased a diesel F250 and a diesel Mercedes, things have me curious.

The F250 requires a special anti-cavitation additive to preven water jacket corrosion. Can someone clear me up on that? Why would the diesel be concerned about cavitation corrosion and not a gas engine?

I am currently flushing the Mercedes and I can't get a straight answer on which coolant I can use. Mercedes of course wants you to buy their $25 coolant, so of course they recommend it. Most Mercedes owners (no offense) don't know anything technical about their cars so I haven't found anyone Merc forums that can intelligently advise me either.

So, my second question is a little broader: Can someone give me a quick rundown of some coolant formulations and which ones are best suited for certain metals/plastics? For instance, my Mercedes has an aluminum/plastic radiator, iron block/Al heads, and probably some brass fittings here and there. I'm hoping to reach an understanding of why certain coolants are spec'd for certain applications so I can make wise decisions on my own... instead of listening to a Mercedes dealer's opinion.

 

[ivymike]

diesel has higher firing pressure and thus more energetic vibration of the cylinder wall.

 

[curtis74]

So its more of a sonically-induced cavitation?

I've been using a special Cat coolant designed for their deisels in the F250. It is supposed to have enough anti-cavitation additive for 300k miles. That is, of course in a large diesel with five times the coolant capacity of mine, but I think I'm safe for a while.

Should I be concerned with that in the Benz?

 

[IceStationZebra]

I agree with Ivymike - The higher cylinder pressures and rapid pressure increase (which also make the classic knocking sound) cause the cylinders to vibrate much more than in a gas engine. When the cylinders move in ever so slightly the pressure in the coolant right next to the cylinders momentarily drops allowing small bubbles of gas to form (I'm not sure if it is localized boiling or air coming out of solution). When the cylinders move out ever so slightly the bubbles pop or implode creating very high pressures in a very tiny areas which can erode the cylinders and eventually go all the way through.

Per another site "the concentration of air bubbles increases when cooling system pressure is low or when the system leaks. Also, increased vibration amplifies the quantity of air bubbles. Vibration multiplies when the engine is run cold, because of increased piston-to-cylinder clearance. Vibration also multiplies when the engine is lugged." You could also add that vibration goes up when the engine power is "enhanced" through chipping or turbo upgrades.

If I remember correctly it is more of a problem on engines that run wet sleeves. It could also be a problem on a parent bore engine with thin cylinders. Basically the more the bores flex the worse the problem will be.

FYI - Some of the old International tractors ran a "coolant filter" which was in reality a time release coolant additive to prevent/limit cavitation.

ISZ

 

[romke]

basically cavitation in a diesel engine is the result of cilinder liners vibrating and the subsequent occurrence of implosions of air bubbles trapped in the coolant near the cilinderwall. the bubbles are formed due to rapid pressure drops near the cilinder walls due to vibration and implode again when there are pressure peaks that make them collapse/go into solution again. due to the rapid changes in pressure around the vibrating cilinderwall there may be small parts of iron actually be ripped of the cilinderwall leaving virgin material that is very prone to corrosion. if he proces goes on, very characteristic pin hole size cavities are formed in the cilinderwall that may over time go fully through. the pinholes are usually formed in those spots where the cilinderwall vibrates the most, usually about half way from the top and near the underside where the cilinderwall is held with kind of rubber seal.

it is important to understand that cavitation of cilinderwalls has two components: vibration and the building of "virgin" surfaces that are very prone to corrosive attack. both these items indicate where the solution should be sought. apart from using better linermaterial, giving the liner a surface treatment or using better seals at the bottom to prevent vibration itself, good anticorrosion properties of the coolant and perhaps a "damping influence" of the coolant on the "impact" of the implosions are used. for the latter emulsions of water and oil are used (about 2% of oil in water), in the past even higher concentrations were used in combination with oilcompatible seals at the bottom end of the cilinderwalls.

a good anticorrosion additive in the coolant may prevent the forming of cavitation holes or at least retard it, but basically it is a design problem that the engine builder did not fully solve.

the international tractors mentioned where a nice demonstration of that: the engines in the tractors used simple cast iron cilinders that showed cavitation occasionally that to a certain extend was solved with the extra "filter", the same engines mounted in shovels used cast iron cilinders that where chromed at the outside and had no problems at all...

to prevent cavitation the best you can do is thus use a coolant that has excellent anticorrosion capability for cast iron.

the formulation that is best for cast iron is usually not as good with other materials, especially aluminium.

most us manufactured coolants are geared towards protection of cast iron liners and work not as good with aluminium. that usually is no problem, since us made equipment usually is somewhat "beefier" then european equipment.

the european situation is different. cavitation with cast iron liners is no problem anymore, but aluminium, solder, and copper are a problem, since material thickness usually is quite thin. that has led to quite different formulations, that cater for those specific material combinations. another problem is the waterquality that may vary enormously, and therefore usually "complete" coolants are used: antifreeze, water and anticorrosion additves in one package. therefore it is advisable to use the prescribed type of coolant in those cases, since the standard us type coolant or the prolonged use variety may not necessary give full protection.

 

[curtis74]

Excellent tech, folks, I appreciate it. For now on the Benz I think I'll just pony up the $25 for their coolant.

Any word on the different formulations and how they affect the different metals so I know which ones to choose in the future? Edumacate me

 

[Bribyk]

I think most good coolants (Prestone or equivalent) are fairly well formulated to deal with corrosion and material differences. The diesel (and some large natural gas engines with weak blocks) cavitation issues are the only coolant issues I've come across in a while. I'm sure there are more qualified people on this forum that may differ.

 

[romke]

a lot of info about coolants can be found here:

http://www.eetcorp.com/antifreeze/antifreeze-faq.htm#q1

 

[curtis74]

Excellent. Thank you for the helpful information.

 

[IceStationZebra]

Good info Romke. Have a star on me. ISZ

 

[4xWriter]

Another source of cavitation is localized boiling due to thin cylinder walls and insufficient coolant flow and/or system pressure. Combined with all the other factors noted above, the Navistar (Ford) 7.3L IDI (basically a bored 6.9L) suffered from this. With relatively thin bores, the problem could get serious quickly. These were not sleeved engines, by the way.

Jim Allen
Keeping the Good Old Days of Four-Wheeling Alive