Is diesel oil compressible ?

[minh-thanh]

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

Hi Lloyd -ss !
It's a material combination table for cylinder and piston but I think it is extendable for shaft and bearing,,,

 

[HMEL]

Here is one of the better explanation of the design of an injector. You have to listen to what this man says as he is reluctant to share actual design calculations. But there is no reason what he has done can not be duplicated. Most importantly he describes why he is doing what he is doing.

 

[minh-thanh]

Hi HMEL !

I have built a diesel engine and now I am trying to build a 4 cyl . diesel engine

Diesel engine : Kromhout Gardner 4LW 1940

With injectors and fuel pump I am not too worried about it, this topic I want to know more about basic fuel properties, that helps me understand more as well as clear some of my doubts with the pump. fuel
But anyway Thanks for the video link.

 

[Roger B]

A few thoughts based on my experiences.

The compressibility of diesel fuel is not a problem in model injection systems but trapped air/gas is. An air bubble of similar size to the injection volume will completely stop the injection. The complete system needs to be designed to minimise the volume of fuel and allow all the air to be bled out. Closing the inlet port with the plunger as in full size practice removes the inlet valve from the problem and defines the start of injection accurately. A ball inlet valve tends to trap air and the closing and hence the start of injection is not well defined.

The diesel injection timing seems quite critical. 5° will significantly affect the running. Petrol injection is no problem, there is a 180° (or more) injection window.

I have not had a problem with diesel fuel evaporating in the inlet port with a head of around 1/2m. Petrol needs a pressure of 0.8bar/10psi to stop it vaporising.

Leakage of the injection plunger is a problem with the tolerances we can reasonably attain. I made some output tests with my diesel injection system. The injection pressure was around 150bar.

The output drop off with speed tends to cause stalling as I haven’t installed a governor yet. The plunger is a pin gauge, already hardened and lapped to high accuracy. The pump bore is carefully reamed (reaming is not that easy :-( ) and then lapped with an Acro needle lap and 1 micron diamond paste until the plunger just goes in.

 

[minh-thanh]

Hi Rger B !
Thanks for the info .

 

[Lloyd-ss]

Roger, thank you for sharing your testing documentation. Is the system gravity fed? If it is, would a low pressure booster pump timed with the high pressure pump possibly reduce what appears to be a cut-off at about 4mm^3 per injection? I am assuming that the higher output volume per rack position at the higher rpm is because the internal leakage in the pump becomes less of a factor?
Nice linear increase in pump output.
Good work. Thank you.

 

[minh-thanh]

Closing the inlet port with the plunger as in full size practice removes the inlet valve from the problem and defines the start of injection accurately. A ball inlet valve tends to trap air and the closing and hence the start of injection is not well defined.

Closing the inlet port with the plunger
It's the best choice
But with my engine the plunger stroke is too small - 1mm on average, and if I adjust the plunger stroke to about 0.5mm the effective stroke of the plunger becomes "Crazy" - with a 10 cc engine
I think it's best to use both ball valves for input and output - at least for the engine I'm building because it's about 9 cc.
There is another solution: make the cylinder even smaller, somewhere around 1 or 1.5mm. And it became : crazier."

 

[Roger B]

Roger, thank you for sharing your testing documentation. Is the system gravity fed? If it is, would a low pressure booster pump timed with the high pressure pump possibly reduce what appears to be a cut-off at about 4mm^3 per injection? I am assuming that the higher output volume per rack position at the higher rpm is because the internal leakage in the pump becomes less of a factor?
Nice linear increase in pump output.
Good work. Thank you.

The 4mm3 limit is the full mechanical pumping stroke of around 1.5mm. It is designed to be 2mm but for various reasons and tolerances including my timing adjuster it is a little reduced but is still more than sufficient for the 2mm3 maximum for a 20cc engine. The variation in output with speed is indeed due to the effect of leakage.

I have a small diaphragm feed pump on my petrol injected twin which is phased with the inlet of the injection pump. The horizontal engine uses an electrically driven gear pump and a relief valve to maintain the pressure on the injection pump inlet.

 

[Roger B]

Some information on ball vaves that might be useful:

https://www.precisionballs.com/ball_valve.php
http://ibls.org/mediawiki/index.php?title=Design,_Care_and_Feeding_of_Check_Valves

 

[minh-thanh]

Hi Roger B !
A useful information !
Thanks for sharing
Did you use that site's lapping tool ?
If so, is it durable ?

 

[Roger B]

Yes I have a 3mm ball lap. It was not cheap but hopefully improves the consistency. I have probably lapped 10 seatings so far, I reworked all my existing pumps.