Am I nuts for thinking of diesel fuel as engine lubricant? 1

[RodRico]

Guys (and Gals if any),

I'm in final design of the fuel and oil distribution system for the 50cc prototype of my engine, and it occurred to me I could cut a lot of complexity if I used the fuel (Diesel #2) for the lubricant. This takes some explanation.

The main bearings are sealed and pre-lubricated 6201-2 single row deep groove ball bearings, so I'm not talking about lubricating those with diesel fuel. I'm using the rotor itself as a centrifugal fuel and oil pump. The fuel injectors spray into the intake manifold and are unit type (each having its own mechanical pump with stroke adjusted to meter fuel), so I don't need a lot of fuel pressure. The engine "pauses" for a good deal of time during scavenge/intake, and I'm pumping oil between the rings during that time to lubricate them and cool the piston faces. I also spray a small amount of oil on the cam faces during operation. The rotor is used as a centrifugal pump to circulate the fuel and oil through the engine, and these fluids also run near the cylinder walls to aid in cooling. Residual oil from sprays and leakage collects in known locations due to centripetal force and is fed back into the return line.

I would like a good deal of flow between the rings and across the cylinder walls, so I'd like to have large cross sectional area in the oil passages. Space within the 50cc engine is limited, and distributing both fuel and oil results in less cross sectional area in the passages than if I route only one fluid. The fact that two-strokes typically mix oil in their fuel got me wondering about lubricity of diesel fuel (which is more like oil than gasoline is).

I did a bit of research and found that Ultra Low Sulfur Diesel (ULSD) has poor lubricity, but most manufacturers add lubricants to compensate for those lost in ULSD processing. I also found information indicating bio-diesel has much greater lubricity than ULSD, and bio-diesel blends are often used in place of other additives to restore lubricity to ULSD. I also found a paper comparing the lubricity of different diesel additives (Link). Of interest to this discussion is the fact that they also tested 15W-40 engine oil (product #13) that had been used for 5,000 miles and concluded 12 of the tested lubricity additives performed better (while 6 performed worse).

I think I've convinced myself I can use the diesel #2 fuel as the lubricant insofar as I require addition of lubricity enhancers which, for a 50cc engine, will have very little impact on fuel cost.

Am I nuts? I'd appreciate any comments or suggestions.

Rod

 

[Gary_321]

I found this, but I am not able to access the report (Auto-ignition of lubricating oil working at high pressures in a compressor for an air conditioner). I don't know the relevance of the content to the original question, but the title looks promising.

 

[RodRico]

TugboatEng,

The bearing is sliding in one direction. Only the contact point moves.

Brian,

Your comments on the precision of fuel pumps are salient; I was just speaking to my mechanical engineer son about how I will fab my injectors. They inject into the intake manifold and don't require the extreme pressure of a diesel injector, but great precision will still be required. We came up with three possible solutions. In order of preference, they are: 1) make a master cylinder and injector nozzle from tool steel, make the injector piston and pin out of soft steel, then lap the piston and pin into the master cylinder and nozzle; 2) make the master parts as described in the first option, make injector components of aluminum, soften the aluminum by heating, then compress the aluminum parts into the master components to shape them; 3) for the prototype, make the parts from steel and mill a groove in the piston, coat the cylinder with oil, wrap a segment of solder around the piston in the groove, then heat them to melt the solder which will expand against the cylinder wall leaving a small gap due to the oil film (thanks to Home Model Engine Machinist for this one).

All,

You, in conjunction with my consultant and mechanical engineer son, have convinced me not to attempt use of diesel fuel as the primary lubricant, I'm now incorporating separate oil and fuel distribution systems. Only after everything is working well will I conduct wear and reliability testing using a common fluid in both systems.

The discussion has been very helpful and illuminating. Thank you all for helping me think through this!

Rod

 

[RodRico]

jgKRI,

True dat. I was thinking of the fuel pump components, but the loads likely aren't applied in the same fashion as my cam.

Gary,

Thanks. I'm going to see if I can get a copy of that paper.

Rod

 

[tbuelna]

Rod,

Here's a couple references you might find helpful regarding the issues noted in my post above:
Lambda Ratio and Fluid Film Thickness
Flash Temperature and Dynamic Fluid Film Thickness (section 7)

If you want to learn more about fluid film lubrication in general (with some help from your engineering consultant/son) here's a excellent public source technical reference on the subject.

 

[RodRico]

tbuelna,

I am an experienced engineer and have little problem following tribology texts. I used a good number of them when deciding to use the tilting pad approach. What I was missing were the qualities of diesel relative to lubrication (which I now know are covered in the fuel specifications thanks to PJGD). Nonetheless, I'm far from an expert and always appreciate good reference materials. The links you provided are in my reference library now, and will be invaluable in the next pass of analysis in support of critical experiments.

Thank you for helping me through this one!

Rod