Effects of diesel timing

Aspergers

Member
Joined
Aug 14, 2010
Location
UK, South Coast
TDI
Citroen Xantia
I'm trying to learn more about diesel combustion theory to help me get the best tune out of my Bosch VE IP. A friend has promised to lend me a good book on the subject but I want to cut right to the heart of the matter - timing. Specifically the effects of too much advance (or retard) on an IDI engine.

This specific query is born out of a post I read on this forum suggesting the application of 12v to the timing solenoid for more power (which is ECU controlled) but I think it might have been tongue in cheek as I suspect that too much advance at lower RPMs would be unwise and detrimental to the health of my engine? Perhaps a load/rpm based switch would work?

If someone could point me towards some (free) good reading or even describe the effects of diesel timing in this thread, I would be most grateful.
 

Vince Waldon

Top Post Dawg
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Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
Late model IDI engines (like VW's AAZ) have an ECM... Engine Control Module... an analog control system that like the TDI's ECU makes some timing decisions, including when to apply power to the timing advance defeat solenoid bolted to the front of pumps on these engines.



OP, in terms of reading... your best bet is to hunt down the Bosch Yellow Jacket for distributor-type injection pumps... it is pretty inexpensive and probably have the most through "real" info on how Bosch designed these pumps, timing, etc:

http://www.bentleypublishers.com/bo...sports/bosch-ti-dis-type-diesel-fuel-inj.html
 
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Aspergers

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Citroen Xantia
Hello good souls and thanks for your replies,

I have a PSA XUD9T engine, it uses the Bosch AS3 to completely control the timing while the rest of the fuelling is mechanical.

The combustion theory I'm looking for is cause and effect of timing on a diesel engine. I have read the Bosch book on VE pumps and it has no information on diesel timing events, just how the pump works.

I want to understand the combustion process itself - perhaps there is a "...for Dummies" copy that would suit me? :eek:
 

Aspergers

Member
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Location
UK, South Coast
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Citroen Xantia
I guess I'm asking too much, I apologise.
Perhaps if I could break it down and ask, for instance, what are the effects of too much advance on an IDI diesel engine using SVO on otherwise stock TD pump/injectors?
Ditto for too much retard.
And if it's not too much to ask, how can I determine the best timing for power? I guess a dyno would be invaluable but cannot afford that at this time.
 

jjblbi

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lbi, nj
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2014 Passat SEL TDI
While I don't profess to be a diesel injection timing wizard I'd recommend you think of injection timing somewhat like spark timing/advance on a gas engine. If the event happens too early (advance) the piston is still travelling up in the bore and the flame front will meet piston crown on the up stroke fighting the inertia of the reciprocating mass. Too late (retard) and the combustion energy is chasing a piston crown that is already moving away from it.

As rpms rise so does piston speed and more advance is required to time the flame front to provide the optimal push as the piston travels down in it's stroke. There are additional factors (temperature, load, fuel quality, etc) but that's the general concept of internal combustion engines. John
 

Aspergers

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Thanks for your input John. That analogy I understand. I'm more interested in tuning for best power and how to recognise too much advance/retard - ie how the engine behaves and how I can "read" the signs.
 

robnitro

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Jan 19, 2004
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NYC area, NY
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2001 Jetta TDI GLS silver
I think one person said to do a 60-80 run in 3rd gear, change timing and do it again, see if time gets better or not. Or, if you have access to a dyno, do the runs, changing the timing. If you find an ideal max timing at xxxx rpm, please do let us know!
 

Aspergers

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Hi Rob, I've got a plan to work out best acceleration, thanks but that won't help me understand the effects of timing in general.
For instance, while I'm wanting best power, I'd also like to know why, at part-load, my engine knocks - is it too too retarded or advanced? - while full load she's sweet sounding. I'd even like to know best timing for cold idle, fast idle etc.
 

jjblbi

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lbi, nj
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In general less advance (retarded timing) favors low speed operation while more advance favors higher engine speeds; think piston speed relative to injection event. The manufacturers recommendated timing is a compromise on both based on intended engine operating speeds. If you intend to run your engine mostly outside their parameters adjust accordingly.

It soounds like you timing is advanced if knocking at low speeds and running well at higher speeds. I'd recommend you set timing stock, try it for a week or two (maybe more ~500 miles), then try the same with a slight advance, then slight retard. Try to keep all other parameters the same during these trial periods. You should feel the performance and mileage differences and then can "tune" to your liking. John
 

dillenger1

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Location
cleveland,ohio
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2015 gsw tdi
Ive never enjoyed factory timing, as apposed to advanced.Advanced timing will gain mpg's,power,probably smooth out a tired injection pump and injectors.
 

manual_tranny

Smyth Performance- Intern
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Jan 5, 2010
Location
New Bedford, MA
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2001 Golf @182K; 2000 Jetta @290K
Hi Rob, I've got a plan to work out best acceleration, thanks but that won't help me understand the effects of timing in general.
For instance, while I'm wanting best power, I'd also like to know why, at part-load, my engine knocks - is it too too retarded or advanced? - while full load she's sweet sounding. I'd even like to know best timing for cold idle, fast idle etc.
Diesel engines are supposed to "knock". They are knocking whenever they are running.

Perhaps you are thinking this sound is bad because of gasoline engines where this knocking sound indicates that the fuel has been combusted (too early) by too much compression. In a diesel engine, the fuel ignites purely from the heat the engine generates from compression.

Personally, I prefer the 'ping' or 'knock' sound that a diesel engine will make when timing is mechanically advanced a little bit.
 

manual_tranny

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You have to make a compromise when you choose timing. Too far advanced, and some might say that the increase in cylinder pressure right after ignition might cause damage. However, increasing the start of combustion gives the fuel more time to burn. This can make the car more efficient- 2-3mpg better. (So far, those testing advanced timing on ALH engines have had little trouble, however attempt at your own risk...)

There will be a "sweet spot" where your timing and injection quantity will match the maximum amount of boost your car is tuned for and burn the fuel most efficiently to produce the most power possible. I would guess that this spot will be only slightly advanced from the 'center of the graph' in vag-com.

I am not a tuner and I don't know enough about the subject to tell you that anything here is actually factual, I am merely passing on the unverified information I believe that I remember reading here on tdiclub various places. It is easy to get in trouble repeating information only to find out that it was incorrect in the first place, so take my words with a grain of salt.
 
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manual_tranny

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I don't know about your specific VE pump, but on the ALH our injectors actually have three separate pulses of fuel. First, a small pre-ignition fuel pulse is injected into a small piston bowl where it has a chance to start a good little fire. Once that fire is going, the injector spits out a bigger shot of diesel, which really gets the fire roaring. At this point the piston is really starting to move- but there is a little more oxygen left in there and things are really hot, so the injector spits a little more fuel, and viola, good power and economy!

The newest "CR" engines have 5-pulse injectors!
 

Aspergers

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Citroen Xantia
According to my diagnostic machine, the dynamic timing peaks at 14 degrees at high rpm at full load. Does this sound right for an IDI diesel or is it measuring pump angle which rotates at half engine speed, therefore engine timing is actually 28 degrees at full advance or have I got that wrong?

Either way, they both seem a little conservative so I'd like to experiment with advancing the timing. Unfortunately, it's ECU controlled via a Pulse Width Modulation through a solenoid valve. I can inject my own signal but am unsure on how to proceed regards how much the engine can take before I break something. A forum member, in another post I've read suggests applying 12v to give full advance but I'm skeptical this could cause problems at anything but high rpms?
 

Aspergers

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Still desperately trying to engage someone with good diesel combustion knowledge...

I experimented with adding resistance to the ECT output, making the ECU think the engine is still cold, thus advancing timing accordingly. Sure enough, at low rpm the advance was showing around 10 degrees (still not sure if that's Pump Angle or Crank Angle) whereas it'll normally be almost half that and higher rpm timing appears unaffected - peaking at 14 degrees. I was hoping to notice a slight power increase but if anything, it feels flatter.

Comments please?
 

hendomatic

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Kansas City
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The pump on my 98 AHU now has 230K miles on it and i believe it is getting time for a rebuilt.
This is a heavily modded engine with PP220's and a GT2056 turbo.
Some days, this car starts and runs totally awesome with gobs of power and almost no smoke. Other days, it starts all clattery and stays that way with a mushy go pedal and high egts and smoke. I feel like the timing gets stuck over advanced, but vag logs dont seem to support the notion. A boost leak or exhaust leak pre-turbo also would seem probable, but everything checks out.
Temperature and whether the AC is on in the summer do seem to play a role, but not the root cause.
Lately its been more constantly clattery and smokey which leads me to believe that my VE pump is starting to fail.
Sound familiar to anyone?
 

Bob_Fout

Oil Wanker
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Sep 5, 2004
Location
Indiana
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2003 Jetta - Alaska Green (sold) / 2015 GTI 2.0T
Still desperately trying to engage someone with good diesel combustion knowledge...

I experimented with adding resistance to the ECT output, making the ECU think the engine is still cold, thus advancing timing accordingly. Sure enough, at low rpm the advance was showing around 10 degrees (still not sure if that's Pump Angle or Crank Angle) whereas it'll normally be almost half that and higher rpm timing appears unaffected - peaking at 14 degrees. I was hoping to notice a slight power increase but if anything, it feels flatter.

Comments please?
In talking with various people, if found out more timing is not always better. I suspect there is a point of diminishing return with it.
 

Aspergers

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Citroen Xantia
In talking with various people, if found out more timing is not always better. I suspect there is a point of diminishing return with it.
I have learnt that with some engines, too much advance and they'll run backwards :eek: but I guess that requires a special sort of valve timing, mine would just probably die.

I get the impression that my 14degrees at full advance (assumed pump angle, therefore 28degrees crank angle?) is somewhat conservative anyway.

Someone (can't recall who but has a good rep AFAICT) on this forum in another post stated that forcing the timing to full advance by applying (via a switch for instance) 12v direct to the timing solenoid would increase power and I'd like to try it but I'm unsure how that might affect lower rpms thus when to activate said switch, what to watch for etc.
 

Aidan

Veteran Member
Joined
Feb 5, 2004
Location
Midlands, UK
Try http://www.vwdiesel.net/forum/

There is some stuff on there regarding your last sentence. It was something like timing was advanced at higher revs for more power, but retarded at lower revs and idle for lower NOX emissions, something like that. Maybe noise too.

Its definitely all in there somewhere.
 

SamuraiSam

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Jan 8, 2009
Location
Anacortes, WA
TDI
None (yet)
The information you are looking for can be found at a course from EFI University.
I have not taken their diesel course but did the equivalent gas course, well worth the money.

If you are already technically competent you could gleam some information from a Bosch bible.
 

Aspergers

Member
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Location
UK, South Coast
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Citroen Xantia
Try http://www.vwdiesel.net/forum/

There is some stuff on there regarding your last sentence. It was something like timing was advanced at higher revs for more power, but retarded at lower revs and idle for lower NOX emissions, something like that. Maybe noise too.

Its definitely all in there somewhere.
You could be right but I'm not having much joy finding it specifically although I did find more interesting reading on the pump itself and results of other people's endeavours. Certainly doesn't seem to be much help here :confused:

The information you are looking for can be found at a course from EFI University.
I have not taken their diesel course but did the equivalent gas course, well worth the money.

If you are already technically competent you could gleam some information from a Bosch bible.

I'm not sure how to take this but thanks I guess? :confused:
 
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