[Dougal]

Has anyone else noticed a drop in boost with advanced timing?

I'm trying to work out if it's actually happening or if it's my imagination playing tricks on me.

 

[LincTex]

From what others have posted here, it is possible.

If advancing timing lowers your EGTs, you will probably also see a little less boost. Others also report getting less low rpm torque with advanced timing.

 

[Dougal]

I advanced my pump (inline, so rotated the whole thing) about 1 deg, which should be 2 deg of advance at the crank.
My EGT's at 100km/h cruise dropped about 30 deg C but I'm not sure yet if fuel economy has changed.
I've backed the fuel down to stop smoke (both aneroid and full load) and can still chirp the tyres from standstill, however it's not quite as smooth down low as it was.

I'm working on what I call the "Crossbones" timing method. Named after the guy on 4x4wire who first told me about it. Using a glowplug as a cylinderhead temp gauge. Shoot for the timing that gives the lowest temp (lowest voltage readout) at cruise. I figure this might not be the lowest EGT as overly advanced timing will lose more heat to the block.
I'm using a K type thermocouple reader (handheld) which is currently reading about 42 deg C at 100km/h cruise. The number itself isn't accurate at all, but using a thermocouple reader gives me a better resolution than using a voltmeter (we're talking a few millivolts).

I'll try to back the timing off a little and see what reading I get. But the nuts holding the pump are a complete pain to get to so it's a job I tend to put off. To get one of them requires alternating two spanners, between them there's just enough offset in the 12pt hex to get the next one working.

 

[Carl Donica]

Most definitely Dougal. I had a 4mpg drop in economy that coinsided with ULSD. This is one of the tricks that I used to recover. Advanced mine up to
2mm. Not sure about economy at this setting but big drop in egt. However, lost about half of my boost pressure, (8lbs), so I backed off quickly. Also large drop in power at low rpm. Ended up at about 1mm or so. A little egt help and a little better economy at this setting but still good bottom power.
Carl

 

[Jcrow]

Where did you put the thermocouple?

 

[Dougal]

Where did you put the thermocouple?

I'm using a glowplug as a thermocouple, stock on my Isuzu 4BD1T, but not on a cummins. They work both as a heater and a thermocouple. Close to Type B apparently for 12v ones, mine is 24v.

Carl, how did your fuel economy change going from 2mm advance to 1mm?

 

[scout4bta]

Has anyone else noticed a drop in boost with advanced timing?

I'm trying to work out if it's actually happening or if it's my imagination playing tricks on me.

Losing boost is one of the things that happens when you advance the timing, along with a decrease in EGT's, a decrease in Black and white smoke, but it also increases cylinder pressures and fuel mileage.

 

[Carl Donica]

Sorry Dougal, no info on that because when it caused a ton of boost loss and more important, a big loss of low end power, I moved it back to about 1+mm. My recently calibrated Hind-end dyno couldnt percieve much low rpm power loss if any at the current 1+mm setting.
Carl

 

[Dougal]

Losing boost is one of the things that happens when you advance the timing, along with a decrease in EGT's, a decrease in Black and white smoke, but it also increases cylinder pressures and fuel mileage.

There's a point though where fuel mileage will again decrease as timing is advanced too far. This is the point I'm trying to find (and avoid). Hopefully I can find someone else who's been there and done that.

At the moment the loss of low end boost is enough of a problem that dangerous EGT's can be reached too easily.
In a way that's very strange as the higher EGT's should bring on the boost, but the timing advance seems to have changed the whole EGT/RPM/Boost relationship in a way that doesn't entirely make sense to me and is impacting on drivability.

Looks like the battery is dying in my handheld thermocouple reader too.

 

[JimmieD]

I wonder if this is a very simple relationship. On a gasser as you advance spark you get quicker/easier starting, but the power band moves up slightly with each degree of advancing. Retarding the spark contributes to more bottom end power and a loss of top end. At a certain point the spark is advanced too far, causing detonation/pre-ignition.

Possibly when advancing the pump too far a person is approaching or arriving at detonation, causing an increase in combustion temperatures but not concurrent power increase? Very hard to hear detonation in a diesel power cycle.

Interesting that water/meth equipped trucks can take some pretty wild advance, as the W/M douses some of the combustion fire, cools mixture and adds power through ignited methanol.

 

[Dougal]

I wonder if this is a very simple relationship. On a gasser as you advance spark you get quicker/easier starting, but the power band moves up slightly with each degree of advancing. Retarding the spark contributes to more bottom end power and a loss of top end. At a certain point the spark is advanced too far, causing detonation/pre-ignition.

Possibly when advancing the pump too far a person is approaching or arriving at detonation, causing an increase in combustion temperatures but not concurrent power increase? Very hard to hear detonation in a diesel power cycle.

Interesting that water/meth equipped trucks can take some pretty wild advance, as the W/M douses some of the combustion fire, cools mixture and adds power through ignited methanol.

I believe that part of it is very similar to a petrol engine. Except for the detonation and the added complication of turbocharging.

The whole idea is to get the peak combustion pressure occuring at the crank angle where it generates the most torque. With diesel burning speed not changing a whole lot with engine rpm and maybe the injection pump advance not quite advancing enough to compensate entirely, leaving the final pump timing to set the best operating point.

Detonation on a diesel can't happen as there's no fuel present before the injection. But what can happen is peak pressure occuring too early so the charge cools through the cylinder walls and head before it can exert it's max torque on the crank.
You also have the negative work of the piston working to compress a gas mixture which is burning and trying to expand sooner than usual.

Which brings me back to the question "how do we know where on the best efficiency point we currently are"?
I'm thinking cylinder head temperature is a very good clue, but it only works as a comparison (change timing, see if it's better or worse).

 

[Dougal]

I found this excellent research paper which covers the subject.
http://www.jstage.jst.go.jp/article/jsmeb/48/4/648/_pdf

Even for those who will find some of it way too technical, it's worth a look through. They made a two piece piston (steel and quartz) and placed a high-speed camera looking up through the quartz window at the injector and valves while the fuel burns. Very impressive stuff.

They mention a 5 deg injection retardation effect having a 5% penalty on fuel consumption and show the effect of different boost and injection timing on emissions and efficiency.

 

[66gaza]

Dougal the pics of injection/combustion are interesting but would make more sense to me if the bottom line TDC 1-30 was enstead 30-1 TDC. As it looks like injection started at 1* TDC or is that at 1* after TDC?

Gaza

 

[IHRAcer]

I wonder if this is a very simple relationship. On a gasser as you advance spark you get quicker/easier starting, but the power band moves up slightly with each degree of advancing. Retarding the spark contributes to more bottom end power and a loss of top end. At a certain point the spark is advanced too far, causing detonation/pre-ignition.

Actually, you have it exactly backwards concerning gas engine performance. I am still on the steep end of the learning curve when it comes to diesel performance, but I have drag raced gasoline burning vehicles for some 15 years and know a thing or two about how to make them run.

Advancing the initial timing does make for a better starting gas engine, but the engine's ability to rev is reduced slightly due to higher cylinder pressures (due to the advanced timing) at higher rpm. Advancing the timing on a gas engine effectively increases cylinder pressures at lower rpms, making for a torquier engine and moving the powerband down the rpm scale. Conversely, retarding the timing on a gas engine makes for a soggier bottom end, but allows for a more free revving engine at high rpms because it doesn't have to fight so hard against its own cylinder pressures to rev. In drag racing, you can have the best of both worlds with an electronic ignition box with a multi-step retard. These allow you to run as much timing as you can get away with at lower rpms to increase your starting line torque, and then as your engine begins to accelerate, rpm activated switches retard your ignition timing as the rpms climb to allow the engine to rev higher than it could otherwise and avoid detonation.

 

[Dougal]

Dougal the pics of injection/combustion are interesting but would make more sense to me if the bottom line TDC 1-30 was enstead 30-1 TDC. As it looks like injection started at 1* TDC or is that at 1* after TDC?

Gaza

I believe they were injecting after TDC. It does sound strange, but it's a very low revving diesel they were using (1000rpm in that shot).