Diesel fuel does not have characteristics that make it a good lubricant. Diesel injection pumps commonly use the fuel as a lubricant, but that's because there's no choice in the matter, not because it's a good thing to be doing. While it's true that engine oils have been trending towards lower viscosity, there's more to what makes a good lubricant than just the viscosity.

It also isn't a good heat transfer medium. Water has a higher specific heat. Water has a higher heat of evaporation, which helps if you have a local hot spot (water will locally boil at the hot spot then condense when the resulting small steam bubble gets out into the main flow stream).

I think even if you were to somehow make an engine function using diesel fuel as its lubricant, the inevitable contamination with carbon particles from combustion leakage would have to be filtered out before getting to the high-pressure diesel fuel injection system (which is extremely sensitive to contamination), which means you will still have a filter to deal with. Diesel fuel injection apparatus doesn't like contamination by water, either ... and that's a combustion byproduct.

And ... I don't see the motivation for doing this. There are two vehicles with internal combustion engines in the driveway right now with >200,000 km between them. Neither one of them has needed the cooling system touched aside from maybe the odd top-up of the level in the tank. Coolant pumps are a common weak point in general, but your proposal would still need one.

Until you get up into the realm of commercial vehicles, diesel doesnt make economic sense as a fuel so you're proposing trying to lower <$1500 in maintenance costs by burning multiple thousands.

As mentioned diesel is a lousy cooling medium, and has this nasty habit of growing fungii in small spaces like those that comprise most of the cooling and lubrication system. Modern ULSD also has less lubricity than water, so using it as a lubricant would be rather difficult.

Toyota already have this covered. They have several patents for doing exactly that with respect to using diesel fuel to lubricate the engine, but not as a cooling fluid. One example of the Toyota patents is: https://patents.google.com/patent/US7461637B2/en

With respect to diesel high pressure pumps, they don't have to be fuel lubricated; several are oil lubricated. However, the lubricity of diesel fuel is closely controlled and monitored in EN590 in Europe and ASTM D975 in the US (which is a slightly laxer standard than the EN spec), and the hertzian loadings in a fuel lubricated diesel pump are far higher than most engine bearings will see.

PJGD

How many R+D $ are expected to be invested in this idea when the UK is set to ban ALL diesel and petrol engines in less than 20 years and everyone else promoting EV and hybrid engines.

Coolant-free engines are common. Air cooled VW, small engines, etc. Using fuel to carry away heat might result in cooked fuel. Better to stick to air.

Lubricant-free (sort of) engines are common in the form of two-strokes.

If you can point to an air-cooled, two-stroke diesel engine, it might be otherwise coolant-free and lubricant-free (sort of).

I'm not sure anyone would want one. Combination of everyone's least favorite characteristics.

Quote (hydrae)

No, most engine fires are ignited from a leak in the coolant loop, not the fuel.

I'm curious as to the background behind this statement.
Just about all the (fixed speed, off-highway diesels) incidents I've been involved in as far as engine fires are concerned generally consist of fuel leaking or spraying onto the turbo, rather than a coolant leak.

Freddy
Discussions with fire fighters
plus a little of my own experience
I have seen only 2 engine fires both were leaky coolant hoses spraying onto exhaust system

My engine fire experience has been lubricating oil spraying onto turbochargers. The lube oil is piped directly to them. The flash point of lube oil is also slightly lower than diesel fuel.

Nearly every engine fire I've seen has been electrical, doesn't matter what fluids are inside.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

The possibility of coolant being the combustible agent in a fire intrigued me, and so I went looking for other information.
I found the attached journal article where some tests were done for an automotive application.

I would note though, that in my experience much effort has been made to ensure that diesel as a fuel is kept below its flash point so as to consider it a combustible fuel rather than a flammable one. I would expect that using it to transfer heat from an engine to a radiator or other cooling equipment would likely result in it being circulated around the proposed cooling loop at a temperature exceeding its flash point.
Incidentally, I believe that there was a discussion elsewhere here on the temperature of diesel when circulated in a common rail fuel supply loop, apparently in that application despite being highly pressurised its temperature rise isn't as high as expected either.

• https://files.engineering.com/getfile.aspx?folder=13e4e837-fd8a-4bdc-9010-b

Any machine can be a smoke machine if you use it wrong enough.

Most all large truck and even small truck and diesel vehicle fires are caused by the emission control devices, I'm betting those fires spew more crap than it would with out the devices.
If you want to get rid of the cooling system and the lube system, do like the old stationary engine days and use friction less bearings, the other catch is the use of low rpms.

Quote (Most all large truck and even small truck and diesel vehicle fires are caused by the emission control devices,)

Do you have a citation for this assertion?

jack vines

I considered doing precisely as you suggest and ran the idea across my highly experienced engine design consultant. He shot it down for one insurmountable problem above and beyond the fuel's poor lubricity (which can be improved using bio-diesel blends) and heat capacity (which can be accommodated in design): the "lubricant" will ignite. Remember that diesels compress air to the fuel's autoignition temperature and spray the fuel into the hot air with precise control of timing. If there were diesel fuel collected on the rings and cylinder wall, it would ignite with uncontrolled timing and produce destructive detonation. Even highly prepared lubricating oils have this problem, called Low Speed Pre-Ignition or LSPI, in modern turbo-fours. Oil manufacturers are still working the problem to the best of my knowledge. In the meantime, many engine makers have specified oils they have tested and certified for use with their engines, and users should be careful to use those specific oils until such time as industry wide standards addressing the issue are adopted. In the end, I decided to use engine oil for lubrication and cooling. While its heat capacity is lower, eliminating a fluid is attractive, and I determined engine oil will be adequate for my engine's cooling needs.

Rod,
Thanks for the hole poke, exactly what I was looking for.

Also thanks for admitting to having the same crazy idea and even for moving forward with what should be possible. I look forward to seeing how the lubricant as coolant plays out.

Hydrae

Packardv8, a good search will find much on the topic. Here is from a quick few seconds of search, https://www.researchgate.net/publication/264182933...
Just check out many of the various truck fires on places like YouTube. Of course when it comes to things where information is suppressed its a real battle to find the info.
Another find. https://www.axi-international.com/dpf-diesel-parti...

hydrae,

The heat capacity of oil is about half that of water (approximately 2 and 4 kJ/kg/K respectively), so I have to move twice as much oil around the liners as water to move a given amount of heat from each cylinder. Toward that end, I have eight coolant channels radially distributed around the active portion of the liner (and complete coolant surround of the liner regions where the pistons "park" with their associated ports fully open).

The engine is a rotating cylinder radial in which the spinning rotor serves as a centrifugal oil pump. I use roller bearings throughout, which means I have no need for pressure, and the entire oil loop thus operates with high flow volume as required for cooling.

I don't view this approach as terribly high risk, but will verify its operation during the critical experiment phase.

Rod

Using diesel fuel as a coolant has been done in the past. The dutch railways in the fifties employed a small shunting locomotive that nearly idled continuously and never actually got to it's most optimal operating temperature. The problem was finally "solved" by changing the coolant hoses to diesel compatible types and using diesel as the coolant. The temperature of the diesel never went higher then 50 degrees C ( https://www.youtube.com/watch?v=HvOxh1PHuR8 ). Lubricating oil usually plays a major role in engine cooling for those parts that cannot be reached by a standard fluid cooling system: piston under crown, connecting rod and crankshaft bearings.

However, the efficiency of water based cooling systems is far higher and over the years they have reached a remarkable degree of robustness, reliability and adaptability to changes in engine load, requiring quite a bit of fine tuning in design to cope with the continuously changing cooling requirements.

Diesel fuel as a lubricant is not a very good idea. Normal operating temperatures of the lubricant in a engine will vary with load and in some parts of the engine will well be above 140 degrees C and higher. Apart from the danger that might arise relating to the flammability of the fluid, it will also lead to a unacceptable loss of viscosity to the extend that no longer a sufficiently thick lubricating film may be formed.

In some types of diesel injection equipment the diesel fuel is also used as a lubricant. The standard specifications for diesel fuel ascertain that that is possible - the fuel must comply with a wear test. Bear in mind though that quite a lot more fuel circulates through those pimps then is required for injection in the combustion chamber - the fluid used as lubricant will have a rather low temperature, usually lower then it's flash point.

Thus exchanging the standard lubricant and standard coolant by diesel fuel does not seem such a good idea. The diesel fuel would have to meet two types of additional requirements (as a lubricant and as a coolant) and that most likely would be detrimental to its original requirements as a more or less clean burning self igniting fuel - killing the environmental improvements over the last decade along the way.