Is diesel oil compressible ?

[minh-thanh]

Hi All !
I am working with fuel pump, ball valve
I have a question : Is diesel oil compressible ?
Thanks !

 

[TonyM]

Yes but definitely not enough for you to worry about. Research for bulk modulus of diesel.

 

[minh-thanh]

" bulk modulus of diesel. "
Sometimes just changing the question results in a completely different outcome
TonyM !
Thank you very much .

 

[Lloyd-ss]

Wow, just when you think you have a handle on something, you find out there is a "gotcha." OK, so a stress-strain ratio for fluids.... I was totally unaware of this. But 1% volume change at 3000psi is a bit esoteric. Doesn't temperature have a much greater affect on volume?

A question along the same line, but maybe for another thread .... types of "fuels" for a model diesel engine, including cetane boosters???? Another rabbit hole.

 

[petertha]

Using this example, raising the temperature of 100 litres of (undefined) oil by 20 deg C, increases the volume by 1.4 litre (1.4%) using thermal expansion coefficient of 0.0007/deg-C

https://www.engineeringtoolbox.com/volumetric-temperature-expansion-d_315.html
other fluids here
https://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html

 

[minh-thanh]

Now I don't care about compression nor thermal expansion
Thank you very much !
One more question:
I don't know how to describe it - Like pouring oil into a cylinder and using a piston to create negative pressure, does the volume of oil increase?

 

[ccolby]

For all practical intents and purposes, fuel oil is not compressible, just like virtually all liquids (by definition). When you draw a vacuum on diesel fuel trapped in a cylinder, it will instantly boil, forming an oil vapor bubble. The result may seem like "expandibility" (sp?), but it is not.

 

[Peter Twissell]

Minh-thanh, the short answer is yes.
The liquid had the same stress strain behaviour as you reduce pressure, so if you are able to pull a vacuum, you have reduced stress on the liquid by 1 atmosphere, or about 0.1 mPa and the liquid will exhibit strain accordingly.
As above, the magnitude of this strain will be so small as to be negligible for practical purposes, but evaporation and cavitation are very real problems for a fast acting pump.
This is the why you may need to pressurise the supply to an injector pump.
Pete.

 

[minh-thanh]

For all practical intents and purposes, fuel oil is not compressible, just like virtually all liquids (by definition). When you draw a vacuum on diesel fuel trapped in a cylinder, it will instantly boil, forming an oil vapor bubble. The result may seem like "expandibility" (sp?), but it is not.

Minh-thanh, the short answer is yes.
The liquid had the same stress strain behaviour as you reduce pressure, so if you are able to pull a vacuum, you have reduced stress on the liquid by 1 atmosphere, or about 0.1 mPa and the liquid will exhibit strain accordingly.
As above, the magnitude of this strain will be so small as to be negligible for practical purposes, but evaporation and cavitation are very real problems for a fast acting pump.
This is the why you may need to pressurise the supply to an injector pump.
Pete.

Hi Ccolby , Pete !
Thank you !
- " it will instantly boil, forming an oil vapor bubble "
- " evaporation and cavitation are very real problems for a fast acting pump. "
That seems to be the problem I've encountered, but I'm not sure
The image below is 2 previous setups with my fuel pump

In case A., when the plunger reaches the TDC there is still quite a bit of oil left in the cylinder and it doesn't seem to be creating much pressure - but I'm not sure because if I want to determine 90 to 100% accuracy then all tolerances and settings must be exactly the same in both cases

 

[Peter Twissell]

Another potential issue is the spring force on the inlet valve of the pump.
That force, divided by the active area of the valve, is an additional pressure which must be overcome to allow fuel to enter.
For a fast pump, the spring force needs to be large enough to close the valve quickly. This means there will be a significant pressure requirement to open the valve.
An automotive electric fuel pump would definitely help.
Pete.

 

[peterl95124]

it seems in both cases your plunger goes past the inlet hole, which is bad, on the return stroke the oil will boil (bubble) until the inlet is uncovered but the bubbles may not completely collapse right away, and these bubbles will make it seem like the oil is compressible when (at the scale of measurements we're dealing with) it is not. this is just an opinion not a tested fact, I'm also building a diesel (Finn Hanson type) and collaborating with Pirmin Johannes, but haven't gotten to the pump and injector, just working on the large parts first and waiting to see what the rest of you diesel builders come up with !!!

 

[Lloyd-ss]

Having the plunger go past the inlet hole is not an uncommon practice in such plunger pumps. If the travel actual allowable distance of the inlet check ball is very small, the return spring only serves to "suggest " to the ball that it needs to seal on the compression stroke. I have tried some commercial pumps and nozzles and found that their seemingly optimum performance only occurs within a certain travel distance and speed range. And that distance must be biased a certain amount before and after the inlet hole. Outside of those parameters, sometimes it just doesn't work at all.
As Minh Thanh has suggested in previous threads, and I have found this to be true also, there can be a lot of leakage inside a diy injector and pump, plunger clearance, leakage past check valves, refill rate, etc, etc, are all variables that are almost impossible to determine and quantify. If you do the stoichiometric calculations for diesel in a 58cc cylinder you get approximately 5mm^3 of diesel per combustion. So, going from lean to rich, the actual injection volume might need to vary from 1mm^3 to 6mm^3. If the plunger is only 2mm in diameter (3.16mm^2), the effective piston stroke only has to be from about .3mm to 2mm. I certainly haven't made a pump that is that efficient volumetrically, and I haven't heard of anyone doing it. The strokes of most pumps, just going on calculations, will pump many times the required volume of diesel. But in reality, they do not.
It seems to be a test of trial and error to get a plunger diameter and stroke that actually works within the necessary requirements of the engine.

Even though I haven't posted on my build thread lately, the number of "almost there" designs is significant. Here is one of the latest in that category. This is a unit injector with pump and nozzle combined into a single unit.
Lloyd

 

[minh-thanh]

Hi Peter !

it seems in both cases your plunger goes past the inlet hole, which is bad, on the return stroke the oil will boil (bubble) until the inlet is uncovered but the bubbles may not completely collapse right away, and these bubbles will make it seem like the oil is compressible when (at the scale of measurements we're dealing with) it is not.

For case B, It doesn't matter much - at least I feel it and it works fine with my engine.

this is just an opinion not a tested fact, I'm also building a diesel (Finn Hanson type) and collaborating with Pirmin Johannes, but haven't gotten to the pump and injector, just working on the large parts first and waiting to see what the rest of you diesel builders come up with !!!

Here's the discussion - any comments are welcome, to clear up some of the reasons I was suspicious, and it should help others as well.
This is the image I changed when I was machining the fuel pump and ball valve block - this change helped me eliminate possible causes
And there's another reason: I really hate drilling into pump cylinders

 

[peterl95124]

Lloyd, when I make for example a valve and valve guide/seat, I make some extra because not all of them turn out to be usable (I've built a compression tester to test them while I'm lapping them in), which makes me wonder, how many of your "almost there designs" were unworkable designs or just unlucky machining (eg after lapping a valve and guide/seat combo a certain number of times and it still doesn't seal I throw the pair out, but don't consider the design the problem, just the machining for the one particular valve or seat). Peter.

 

[Lloyd-ss]

Lloyd, (edit by Lloyd) ...........which makes me wonder, how many of your "almost there designs" were unworkable designs or just unlucky machining............ but don't consider the design the problem, just the machining for the one particular valve or seat). Peter.

Peter, yes, I agree with you whole-heartedly, but won't be so polite on myself. A lot of it is just poor execution on my part. Trying to squeeze both the pump and nozzle into one package exposes unexpected weaknesses, and usually, it just leaks somewhere. So, a minor design change to add a little more thread depth here, or little more spacing there, or a little more precision somewhere. I feel like the basic design is sound, but I am now getting ready to re-make the main body with a few minor tweaks to solve a couple of "issues." But honestly, each failure teaches me something new, even though sometimes it is just,"I was afraid that was going to happen."
Lloyd

 

[minh-thanh]

Hi All !
I did a little test today with the fuel pump that used to run with my engine and the result was :

For all practical intents and purposes, fuel oil is not compressible, just like virtually all liquids (by definition). When you draw a vacuum on diesel fuel trapped in a cylinder, it will instantly boil, forming an oil vapor bubble. The result may seem like "expandibility" (sp?), but it is not.

Minh-thanh, the short answer is yes.
The liquid had the same stress strain behaviour as you reduce pressure, so if you are able to pull a vacuum, you have reduced stress on the liquid by 1 atmosphere, or about 0.1 mPa and the liquid will exhibit strain accordingly.
As above, the magnitude of this strain will be so small as to be negligible for practical purposes, but evaporation and cavitation are very real problems for a fast acting pump.
This is the why you may need to pressurise the supply to an injector pump.
Pete.

Exactly !

Hi Ccolby , Pete !
Thank you vey much !!

 

[peterl95124]

Peter, yes, I agree with you whole-heartedly, but won't be so polite on myself. A lot of it is just poor execution on my part. Trying to squeeze both the pump and nozzle into one package exposes unexpected weaknesses, and usually, it just leaks somewhere. So, a minor design change to add a little more thread depth here, or little more spacing there, or a little more precision somewhere. I feel like the basic design is sound, but I am now getting ready to re-make the main body with a few minor tweaks to solve a couple of "issues." But honestly, each failure teaches me something new, even though sometimes it is just,"I was afraid that was going to happen."
Lloyd

Lloyd, I can definitely relate to the problem of deciding the pitch of threads and how long those threads should be and how much material needs to be behind the threads !!! another note, I've given up on mechanical seals and instead use loctite 542 thread sealant (its medium strength and I've never had a problem disassembling it, if I ever do then I'll try their 577 which is even lower strength sealant). Anyway, if/when I get my Hansen Diesel to work I'll be sharing the design (ditto Pirmin Johannes with whom I'm somewhat collaborating), I hope you're like minded, as Hansen himself (may be a bit of a recluse and) doesn't want to share much. Peter.

Minh-Thanh, hope you don't mind our digression on your blog, I appreciate your design change to not have any holes drilled into your pump cylinder, definitely something for the rest of us to think about as we contemplate our own pumps. Also you've probably already said what material you're using for cylinder and plunger, but I'd start with hardened steel for both and polished to mirror finish, and the plunger length should be at least 5x its diameter maybe more (yours looks plenty long), and I like your design where it appears that the plunger and cylinder are separate from the housing and can be replaced if they don't seem to be adequate (IE like the way I make more valve and guide/seat pairs than I'll need and cherry pick the ones that work) , your thoughts ? Peter.

 

[minh-thanh]

Hi Peter !
Cylinder : steel , plunger : Drill rod

Polished to mirror finish :

Close, but I'm not sure, because it's too small for me to see clearly inside,

The plunger length should be at least 5x its diameter maybe more :

When I make cylinders and plungers, I really don't care about that
I made the cylinder for the test 12mm long, it was fine and when making the pump for the engine I wanted it to be longer so I made the cylinder 17mm
Why ? Because I want the leakage to be the lowest , no leakage better . and it ensures the pressure as well as the amount of oil...
I always want to make it the best I can so I don't have to worry about it

 

[Lloyd-ss]

Minh Thanh, Is that table for the two materials rubbing/sliding against each other?

 

[HMEL]

Now I don't care about compression nor thermal expansion
Thank you very much !
One more question:
I don't know how to describe it - Like pouring oil into a cylinder and using a piston to create negative pressure, does the volume of oil increase?

View attachment 143134

No, it is a matter of what is called a state property. When the pressure is reduced with a piston withdrawing from the cylinder the volume increases and pressure is reduced forming gases. I say gases because diesel fuel is a composite of several hydrocarbons. The trick is to keep constant pressure on the injection system usually with a primary pump. These gases can vapor lock systems. So you either design the system to inject the fuel into the cylinder so that the delivery injector is empty when it withdraws or you add a booster pump ahead of the injector system. One of the reasons you try never to run a diesel without fuel because it will draw in air and then it has to be bled and primed. A slightly different problem with same effect. If heat is involved the viscosity of the oil is reduced which makes it more sensitive to flashing. There was a modeler on youtube from Norway or Germany in which he described the work on diesels and his injectors. He would talk about his injectors but not release the design for some reason. I will see if I can find the video but it has been some time since I viewed it.
HMEL