OIL CONSUMPTION 2

[FredRosse]

A question: Oil consumption in IC engine practice is very low, but with steam as a working fluid, oil consumption is prohibitively high. Why is this so?

With a conventional IC engine converted to steam, any significant oil in the exhaust is very bad, however the oil consumption normally associated with IC engine practice would be acceptable.

The engines are conventional trunk piston type, with conventional IC engine rings, operating with peak steam inlet pressures of 150 to 250 PSIA (10 to 17 BarA), with atmospheric exhaust pressure. Steam inlet temperatures range from about 350F-550F (175C-290C), exhaust at 212F-250F (100C-128C). These engines typically are rated for a few thousand RPM in IC service, 2-20HP (1.5 - 10 kW), but operate at slower speed in steam service (500 - 1500 RPM).

Accumulation of condensate in the crankcase oil is avoided by keeping the oil temperature above 212F (100C), with the crankcase vented to atmosphere. Therefore any steam leaking into the crankcase is maintained as a gas, not liquid.

Experience with several engines shows oil getting into the steam exhaust, which is a few orders of magnitude more than typical for IC engine service. The valve arrangement does not allow any oil leakage into the exhaust. All of the oil getting into the exhaust must come thru the piston/rings/cylinder path.

Thanks in advance for any technical explanation here.

 

[GregLocock]

Chemically water is known as the universal solvent. In this case you are steam cleaning the oil off the cylinder walls every stroke.

Cheers

Greg Locock


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[tbuelna]

The only reason you would be getting excessive lube oil past the piston is because the oil control ring is not functioning properly. Many IC engines are not designed with materials that can handle exposure to steam, so the rings and bore surface may be suffering corrosion.

 

[ivymike]

Actually, oil consumption in IC engines is quite a bit more complicated than the response above would lead you to believe - all three rings play important roles in oil control, and the "oil" ring is just the first step (it takes the big puddles off the wall on the downstroke).

Corrosion or abnormal wear of the ring/cylinder surfaces are definitely suspects, depending on the amount of time the engine has been running (if the increased oil consumption is instantaneous, then you can probably rule this out as the root cause..but if it increased after a while then this seems likely). You may be polishing the cylinder wall, which would normally increase oil consumption quite a bit. You may also have rubbed the ring face profiles right off of the compression ring(s). Have you inspected the cylinder bores and ring faces for damage?

Do you have anything you can share about how in-cylinder pressure varies vs. crank angle over a cycle?
Do you know what the cylinder wall temperature profile looks like in this steam conversion configuration?

 

[romke]

one other reason for the difference experienced may be the cylinder pressure during the down (working) stroke. that pressure should push the piston rings sufficiently against the cylinder bore and if it is lower it might be not high enough for sealing sufficiently or maybe allow ring flutter, both leading to higher oil consumption.

 

[gruntguru]

The rings are designed to leave a specified thickness of oil on the cylinder wall. If in-cylinder conditions are removing that layer, correctly operating rings will pass additional oil to reform the film. It is likely the oil is being "steam cleaned" from the cylinder walls. Steam engines typically admit cylinder lubricant with the steam and seperate it in the condenser for reuse.

Viscosity can be up to ISO 1000 compared to automotive engine oils at ISO 50-200.

Engineering is the art of creating things you need, from things you can get.

 

[FredRosse]

I think the steam/condensate washing of the cylinder walls is the answer here. Steam cylinders almost always suffer from water condensation during steam admission, with water re-evaporation on the exhaust cycle. Thanks to all who contributed here.

We have tried new and properly broken-in (IC Service) ring-piston-cylinder combinations, iron cylinders, aluminum pistons, and various IC engine ring materials. The arrangements have been tested with virtually no corrosion inside the cylinder. Also tried chrome cylinder with bronze rings, iron rings, alloy rings, same problem persists.

The pressure vs stroke conditions are very similar to IC engine practice; full steam pressure admitted for about 1/2 of stroke, expanding to about 1/3 of inlet pressure before the exhaust valve opens for atmospheric exhaust.

Cylinder temperature runs at 250F-300F, not significantly different from IC engine practice.

The traditional method of dealing with the problem is to inject oil into the admission steam, and allow oil in the exhaust steam. In condensing (closed cycle systems) the oil must be captured with both mist eliminators and condensate oil separators, both of which are troublesome for long term operation.

 

[ivymike]

can you keep the cylinder wall above 100C?

 

[ivymike]

(or whatever temperature corresponds to little/no condensation under the inlet conditions)

 

[gruntguru]

Perhaps a dry-lube solution? Chrome bore, aluminium piston with teflon buttons, gapless teflon ring(s) with viton o-ring expander. Modify crankcase to minimise oil delivery and splash to cylinder.

Engineering is the art of creating things you need, from things you can get.

 

[FredRosse]

How long might the Teflon on chrome cylinder be expected to last? I thought this solution would have a very short life.

 

[gruntguru]

I have only used that sealing concept in a low-cycle, pneumatic cylinder application. Hopefully someone here can estimate the life expectancy.

Perhaps it is possible to oil lubricate such a system while designing it to wipe most of the oil from the bore on the downstroke?

Engineering is the art of creating things you need, from things you can get.