IDI injection pressure mods.

[Blownoiler]

IDI injection pop-off pressure
I'm trying to work out the best pop-off pressure for the pintle injectors in my 3.1 litre Isuzu IDI TD. There doesn't seem to be a lot of info around on what happens inside the chamber when pop-off pressures are increased/decreased in an IDI, so I thought that I'd bring the subject up here to see if anyone has any insight on this type of mod.
There is a complication that I should bring up now, and that is the fuel being utilized is used cooking oil which doesn't atomise as well as what diesel fuel does, and doesn't burn as completely either.
The Isuzu factory workshop manual gives the standard pop pressure of 2133 psi for this engine (4gj2T), yet other Isuzu engines that have the same injection pump have higher pop pressures, indicating that the pump is capable of handling a reasonable pressure increase without throwing in the towel. The 4JB1T engine which runs the same pump has a 2631 psi injector pop pressure, and interestingly the malaysion/argentinian model of the same engine has a 3128 pop pressure listed, probably for emmision purposes I'm thinking.
Given that the factory offered a warranty on these engines that ulilize these higher pressures I'm confident that no major pump problems will arise from a substantial increase in my engines operating injector pop pressure.
I'm still tring to understand what changes occur involving the ignition/burn characteristics when the pop pressure is changed from the standard setting , and am thinking that the shot of fuel could benefit from being injected at higher pressure for the better atomisation, which theoretically should increase the burn rate and result in less unburnt fuel, which we see as soot/smoke.
Increasing the pop pressure of the injectors should theoretically retard the start of injection, yet the increased atomisation of the fuel along with the faster rate of the injection (due to the higher injection pressure) should lead to the combustion chamber peak pressure being reached earlier , which would have the effect of advancing the injection timing. So, what is going to happen regarding the time it takes to reach the chamber peak pressure with much higher injection pressures?
Also, I would expect better fuel efficiency with a higher injection pressure setting, yet have heard people claim that they got better mileage from lowering injector pop pressure, .....so, If I increase my idi's injector pop pressure from the current 2133 psi to 3128 psi what can I expect in terms of efficiency?

 

[Leiffi]

Increasing pop pressure doesnt increase injection pressure.

 

[Blownoiler]

Increasing pop pressure doesnt increase injection pressure.

Then why would my injectors spray an un-atomised stream of diesel when set to open at 1200 psi, yet spray a well atomised fog of diesel when set at 2200 psi?...what else could be causing the fogging of the diesel if your statement is true?

 

[Leiffi]

Then why would my injectors spray an un-atomised stream of diesel when set to open at 1200 psi, yet spray a well atomised fog of diesel when set at 2200 psi?...what else could be causing the fogging of the diesel if your statement is true?

How did you test it ? With hand pump ? I think your pump makes something like 10000 psi injection pressure. Pop pressure only prevents injector dripping after injection is over.

 

[Blownoiler]

How did you test it ? With hand pump ? I think your pump makes something like 10000 psi injection pressure. Pop pressure only prevents injector dripping after injection is over.

There are plenty of utube video's showing different spray characteristics at various pop pressures, a higher injector pop pressure gives a more finely atomised spray of fuel. The start of injection happens when the injector pintle pops off its seat, and the pop pressure is adjustable. A higher pop pressure usually ensures better atomisation, and a faster injection of any particular fuel quantity due to the higher fuel pressure.

 

[Leiffi]

There are plenty of utube video's showing different spray characteristics at various pop pressures, a higher injector pop pressure gives a more finely atomised spray of fuel. The start of injection happens when the injector pintle pops off its seat, and the pop pressure is adjustable. A higher pop pressure usually ensures better atomisation, and a faster injection of any particular fuel quantity due to the higher fuel pressure.

Better atomisation at the start and end of injection, makes nothing to peak injection pressure but lowers injection quantity. There is injection systems without needle so zero pop pressure and they atomise fine.

 

[Blownoiler]

Better atomisation at the start and end of injection, makes nothing to peak injection pressure but lowers injection quantity. There is injection systems without needle so zero pop pressure and they atomise fine.

Is English your second language? I'm not sure what you are trying to explain!

 

[crackerman]

Hydraulics is hydraulics. Pressure needed to overcome a restriction will set minimum psi in a system.
Aka 1200 psi pop in one injector means injection pressures are 1200psi
and 2200psi in another...
Yes hand pump may put out 10k psi, but since it pops at 2200, it cannot be higher than 2200 unless enough pump is present to out flow the injector.

 

[Leiffi]

Hydraulics is hydraulics. Pressure needed to overcome a restriction will set minimum psi in a system.
Aka 1200 psi pop in one injector means injection pressures are 1200psi
and 2200psi in another...
Yes hand pump may put out 10k psi, but since it pops at 2200, it cannot be higher than 2200 unless enough pump is present to out flow the injector.

Injection pump has enough flow to outflow injector. Injection pressure is always much higher than pop pressure. If you studied a little bit there would be no need to argue.

 

[Leiffi]

Is English your second language? I'm not sure what you are trying to explain!

Yes, you want that in finnish? What I'm trying to say is that pop pressure has nothing to do with injection pressure except at the very beginning and the end of injection. You can call that starting pressure of the injection, meaning its never lower than pop pressure but rises quickly much higher than that.

 

[Blownoiler]

Hydraulics is hydraulics. Pressure needed to overcome a restriction will set minimum psi in a system.
Yes hand pump may put out 10k psi, but since it pops at 2200, it cannot be higher than 2200 unless enough pump is present to out flow the injector.

You can call that starting pressure of the injection, meaning its never lower than pop pressure but rises quickly much higher than that.

So basically once the injectors pop pressure is reached flow will start through the injector, and if the pump has a decent plunger diameter (and lift rate) then pressure will increase through the injection event until the spill valve allows the pressurized fuel to escape through the spill port, at which point the check valve at the pump head will snap shut which allows the injector to retract back onto its seat. I think that we are agreeing on this!

 

[Blownoiler]

Have you guys looked at the flow rate of the check valve on the pump head yet? The restrictive looking internal design could slow (or limit) total flow a tad. No doubt internal pump flex would be the result of too much resistance to flow.

 

[Leiffi]

So basically once the injectors pop pressure is reached flow will start through the injector, and if the pump has a decent plunger diameter (and lift rate) then pressure will increase through the injection event until the spill valve allows the pressurized fuel to escape through the spill port, at which point the check valve at the pump head will snap shut which allows the injector to retract back onto its seat. I think that we are agreeing on this!

Yes, that's about right.