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

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

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

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.


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

Not completely nuts, but not far off. Nevertheless, diesel fuel as sold at any retail outlet has to comply with ASTM D975 in the US or EN590 in the EU and one of the requirements is a lubricity standard which was demanded by the fuel injection system manufacturers. This is measured by the HFRR (high frequency reciprocating rig) test, and D975 allows a 520 micron wear scar while EN590 has a tighter standard of 460 micron wear scar. Thus you can be assured (pretty much) of a minimum level of lubricity in any normal diesel fuel from a reputable supplier. Note that virtually all light duty and even some heavy duty common rail high pressure pumps are fuel lubricated, i.e. the cam and tappet survive the >2000 bar loading on the plunger at speed and for the life of the vehicle, typically while running in fuel. This requires a good class of steel with good heat treatment and good surface finish.

Beyond that, Toyota Research have recognized the above reality with respect to fuel injection and have given serious consideration to lubricating their diesel engines with diesel fuel, so that there is just one tank that serves both the fuel system and the engine lubrication system, with the latter being recirculated and filtered many times before it eventually gets consumed in the engine. Outlandish though this may sound, they have several patents in this area, of which US7461637-B2 is an example.


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


Thank you for your *very* informative response! I hadn't thought to look at the fuel specifications to nail down lubricity sans additives. In the prototype, every surface I need to lubricate will be made of heat treated and highly polished Maraging 350. I chose it because it machines like soft steel when annealed (33-37 HRC) then becomes very hard (50-54 HRC) and suffers very little dimensional change during heat treatment. In production, those surfaces will likely be something cheaper (TBD).

I'm no off to read the patent. Hopefully it's not so broad as to preclude me using fuel as lubricant!

EDIT: Using the fuel to lubricate rings may be a problem; some amount in the ring crevice will ignite and burn rich (yielding hydrocarbons and soot). More thought required.


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

Compare to existing 2-Stroke engines, the ones that use the gasoline + 2-stroke oil mixture to lubricate and fuel.

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

Something to consider. Is your engine going to be sold worldwide?

If you run the engine using "good" diesel and everything is OK, will it run using the "worst" diesel? Can you get hold of some really poor quality diesel to test your engine?

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

Precisely. That's what started me down this train of thought. Traditional two-strokes are not known for their low emissions, however, so I'm still concerned that liquid fuel (not atomized) above the top ring will combust yielding a rich mixture, lots of unburnt hydrocarbons, and smoke. I'm still researching precisely what happens with oil on the cylinder walls and in the ring crevice when using *normal* oil. I know the *normal* oil sometimes causes pre-ignition problems in heavily boosted fours (google "Low Speed Premature Ignition" or "LSPI"). What I can't find is what's going on with the oil during the *planned* combustion event. Maybe it lights off to some extent as well, but there just isn't enough of it present to cause a huge spike in emissions. Research continues.

Chevron/Phillips sells reference fuels for gasoline, diesel, and aviation fuels (Link). Their diesel reference fuels and their cetanes are "High" (52.45), "Low" (43.47), "T-30" (77.1), and "U-30" (21.9). The last two, T-30 and U-30, are mixed to create whatever cetane value is desired. US pump cetane is typically 40-44 and Europe's is 49-51. I will do most of my testing using Shell diesel from the station near my home. I will only test operation with specific cetane values after everything seems to be working.


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

There's a lot more to diesel fuels than just the cetane numbers.. I think Gary is talking about what fuels your engine might be subjected to in, for example, a third world country where the quality of the fuel is low due not only to suboptimal cetane values, but also impurities and/or contaminants in the fuel, and unknown or zero additive packages.

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

Quote (jgKRI)

There's a lot more to diesel fuels than just the cetane numbers.. I think Gary is talking about what fuels your engine might be subjected to in, for example, a third world country where the quality of the fuel is low due not only to suboptimal cetane values, but also impurities and/or contaminants in the fuel, and unknown or zero additive packages.

That's exactly what I was getting at. I use vegetable oil instead of diesel fuel in my van and use 10% diesel as an engine flush during an oil change. I know someone who puts his used engine oil in the diesel tank, but I've not done that - yet! Goodness only knows what rubbish fuels are used worldwide.

I'd image that Rod's engine is made to similar tolerances as an injection pump (and that's lubricated by diesel), so I'm pretty sure that the engine could be made to run on US or EU diesel. Perhaps, if Rod can test using the testing fuels in his link, he can specify Chevron diesel, and that would cover any warranty problems. It may be that stating diesel must be to US or EU standards may be good enough.

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

I tried the used motor oil recently. I ran about 10-15% used motor oil. It ran fine, idled more smoothly, actually. No difference in starting down to 40 degrees F.

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

It was at one time standard practice in some diesel generating plants to dump the used lube oil into the fuel supply.
Some fish boats and small freighters also dumped the used lube oil in the fuel tanks.
With large storage tanks the concentration was quite low.
The energy of a diesel fuel is closely related to the specific gravity.
The used lube oil gave a slight advantage in lubrication of the injector pump and a slight increase in fuel economy due to the higher specific gravity.
This also solved the problem of disposal of the used oil.
Moving forward to better injectors and better filters; Ten or fifteen years ago I had breakfast with the vice president and chief engineer of the local Caterpillar dealership. The conversation got around to disposal of used oil, and problems the dealer was having disposing of the lube oil from a number of large generators on wet lease.
I was told that with the newer finer injectors and the attendant better quality filters, that used lube oil would quickly clog the fuel filters and any saving in fuel cost would be more than offset by the increased cost of replacing clogged fuel filters.

"Why not the best?"
Jimmy Carter

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

The injector tip hasn't changed much in the last 40 years (little improvements like reduced sac volume and optimized spray patters are all I have seem advertised), neither have the filter ratings. I see the bigger problem is that engines are certified to meet emissions standards on a specific fuel. If you start pouring additives into your tank, you're essentially invalidating your emissions certificate. Add a DPF or SCR to your exhaust and some of the anti-wear additives that don't get combusted may cause clogging in that region.

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

I would think when using waste lube oil that you would want to use a good centrifuge filtration system to remove any solids, as well as many banks of filters. I'm curious about using it on a non constant running engine, I would be concerned about acids from the lube oil etching injection equipment.
As far as the use of the fuel for lube you will not have much for extreme pressure lube using the fuel oil, you may have to do the same as the small 2 stroke engines and use an oil mix.

Okay how can I patent the rollers that you will be needing for use on your cam track to handle the use of diesel fuel as the lube?

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


I ran this idea by my consultant who has participated in design of several well known engines. I even expanded the idea to use off-the-shelf canola oil which burns as well as diesel and has far better lubricity. He said it can be made to work, but asked "do you really want to introduce even a little more risk into the prototype?" That's a good point. I have deferred pursuing this until after I have everything working in the traditional way.

I have no idea how to specify an oil other than to reference a specification. I'll specify the ASTM and EU specifications or, if I end up allowing vegetable oil, I'll reference B100 specifications rather than "clean unused canola oil from your local supermarket."

The experiences everyone cites in putting vegetable oil and engine oil in the fuel tank is informative. It tells me there's a wide range of possibilities. I will have to figure out how to design for these alternate fuels, however. This reminds me of something else my consultant said, "do you really have time pursue this properly right now? You can always run the necessary experiments, study wear patterns, and so forth after you have the basic engine working." He's right. I'm going to design for traditional fuel and engine oil *then* explore these alternatives on the bench.

The concerns over filtering lubricant before applying it to injectors are well founded; I hadn't thought about that. I may be able to mitigate the clogged filter issue by exploiting two key facts about my design: I use the rotor as a centrifugal oil pump which could be exploited to separate contaminants; and all the metals in the lube oil will be ferrous and can be drawn out using magnets.


P.S. enginesrus, I'm not using roller bearings as cam followers... too much mass, too much cost, and unable to handle the loads at the required surface speeds. I'm using hydrodynamic tilting pad followers (part of my invention in the patent). I honestly don't know how well these are going to work. Hopefully, their friction will be equal to or less than piston slap (which my design lacks). The very first hardware builds will support experiments and measurements of follower performance, cooling via the centrifugal fan, injector spray patters, etc. Though I have analysis of all these characteristics, I won't be confident until these experiments are complete. If I can't get good results in the experiments, my project stops.

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

Tilting pad bearings are for flat thrust bearings. Round shafts already have the right profile to create a hydrodynamic bearing.

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

Referencing a known spec is the correct approach.

At the surface speeds and pressures you're talking about, I don't know enough to say for sure that fuel-as-lubricant WILL NOT work, but I have some doubts.

Yes, things like injectors and pumps are often lubricated by diesel, but they also deal with much lower temperatures, and surface speeds/pressures, than a lot of your rotating parts will have to deal with- the cams and followers especially. The additive package matters.

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


The outer cams of my engine do serve much like a thrust bearing. The curvature on the tilting pad is equal to the minimum radius of the cam profile which is significantly larger than the shaft diameter required for the maximum load/deflection.


The load on each cam surface is about 1600 psi (about the same as seen in common rail fuel systems), and I can easily reduce them by increasing the contact width of the cam surfaces. I believe the high surface speeds are actually a good thing as they keep the cams in hydrodynamic mode from idle to max RPM. The desire to reduce loads is what led me to having so many cylinders (which does in turn yield a bit more total friction and heat loss).

I am very interested in how these cams behave when operated at speed using oil or fuel as the lubricant. It will be one of my first experiments, so measurements will be forthcoming early in the build process.


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

What about the surface the pad tilts on? A bearing that rocks back and forth is much harder to lubricate than one that slides in a constant direction.

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

Regarding injection pumps lubricated by diesel fuel:

The fuel has to be finely filtered before going in. (~ 5 microns)

Plungers and cylinders in diesel injection pumps need to have extremely close fits which implies extremely tight tolerances (microns) and very smooth surface finishes. Manufacturing these parts requires specialized equipment that the average machine shop doesn't have (and which the equipment that you discussed elsewhere for manufacturing this, is IN NO WAY capable of!). In addition, most designs are such that there is intentional known leakage past the pressure side into the backside of the plungers, and that backside is simply the incoming fuel cavity, so such leakage doesn't really cause a problem aside from having to know with some accuracy "how much" leakage so that it can be taken into account in the fuel delivery calibration. Also on common-rail injector pumps, leakage past the plunger doesn't affect the injection quantity as long as it is not so excessive that it causes the pumping rate to fail to keep up to demand. If you are trying to seal a combustion space, you don't have that luxury.

The whole injector pump has fuel circulating through it, which keeps the whole lot at pretty close to the same temperature, which means plungers and cylinders aren't significantly affected by thermal expansion. Not so in a combustion engine.

Even with that, there are certain injector pumps out there which are known to have a failure rate rather higher than most users would desire.

It's pretty likely that your design will want a lubricant of lower viscosity than your average motor oil, but I do think it will want something intended to be used as a "lubricant" as opposed to something intended to be used as fuel but which secondarily has to have lubricity because of the type of equipment that uses it (diesel fuel injection systems).

Bear in mind that in a normal piston engine with normal piston rings, the sealing is accomplished between the piston ring and the cylinder with a thin film of oil retained in the "crosshatch" surface finish of the cylinders, and the gas pressure above the piston works its way into the piston ring groove and forces the ring out against the cylinder wall; many high quality pistons have special passages in the pistons to facilitate getting this gas pressure in behind the piston ring to make it seal better. The oil has to be high enough viscosity to stick in those microscopic grooves in the crosshatch.

Two-stroke engines relying on oil dissolved in the gasoline rely on the gasoline mostly evaporating and leaving just enough oil on the cylinder walls to lubricate the piston rings. These engines always use rolling-element bearings in the crank and big-ends of the rods, which are okay with sparse lubrication. We've discussed the emissions consequences of piston-porting in your other thread already. Even with this, these types of engines always have a short life compared to a halfway decently designed four-stroke with pressure lubrication and no piston ports.

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

If you intend to use diesel fuel as a lubricant for metal surfaces having sliding/rolling contact, and want to maintain better than boundary conditions at the contact interface during operation to prevent wear, then check your dynamic oil film properties. The biggest issue will likely be maintaining sufficient thickness within the oil film to prevent degrading into boundary conditions. Pay particular attention to the flash temperature rise within the dynamic oil film. You may find achieving sufficient dynamic oil film thickness to prevent scuffing/abrasive wear, when using diesel fuel as a lubricant, to be rather difficult.

Good luck to you.

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

1600 psi of physical contact pressure != 1600 psi of fluid pressure...

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


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


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).


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!


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


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


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


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


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!


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