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Old 20-01-14 Thread Starter
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Injection Timing: how late can it be?

Hello everyone, I was wondering how to improve engine power/efficiency by tuning the injection timing and found out that some studies claim the best power can be obtained if peak cylinder pressure happens arround 15 ATDC.

Looking at SOI/duration maps, injection ends before TDC, so pcp will be much earlier.
How long do you people recon that fuel takes to start burning after injection? I've seen claims that high cetane (55) diesel will start burning in about 0.4ms on a modern diesel. If this is correct, why is fuel not being injected much later?

For example at 4500rpm, soi (for my 2.4 200hp engine) is 28.5 BTDC, won't diesel start burning so soon (arround 18 BTDC maybe?) that it will make a lot of negative work on the piston? Wouldn't a SOI of arround 10 BTDC mean combustion started arround TDC and improve performance? (much less negative work?)

Does anyone run with this kind of late injection?

Thanks!
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Originally Posted by exar1711 View Post
Hello everyone, I was wondering how to improve engine power/efficiency by tuning the injection timing and found out that some studies claim the best power can be obtained if peak cylinder pressure happens arround 15 ATDC.

Looking at SOI/duration maps, injection ends before TDC, so pcp will be much earlier.
How long do you people recon that fuel takes to start burning after injection? I've seen claims that high cetane (55) diesel will start burning in about 0.4ms on a modern diesel. If this is correct, why is fuel not being injected much later?

For example at 4500rpm, soi (for my 2.4 200hp engine) is 28.5 BTDC, won't diesel start burning so soon (arround 18 BTDC maybe?) that it will make a lot of negative work on the piston? Wouldn't a SOI of arround 10 BTDC mean combustion started arround TDC and improve performance? (much less negative work?)

Does anyone run with this kind of late injection?

Thanks!
Me and Tribesman did some tests with different SOI numbers. At 4000 rpm we tried 25, 24, 22 and 21 for the same IQ.
The only measurable difference it makes is EGT. The earlier, the lower the EGT.

And ofcourse, higher SOI makes higher pcp which we can't measure.

But in terms of power, you won't find any suprises unless the EOI becomes too much after TDC. Then power will drop and smoke/ EGT goes up.
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Old 20-01-14 Thread Starter
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PCP is supposed to be lower with late SOI, not higher. EGT of course increases and is higher at late injection.

What duration are you using for that SOI values?
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PCP is supposed to be lower with late SOI, not higher. EGT of course increases and is higher at late injection.

What duration are you using for that SOI values?
With higher SOI, I mean higher value before TDC.

Duration for those values was, if I recall right, around 22 degrees @ 4000 rpm.
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Old 21-01-14 Thread Starter
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This is strange, why would SOI 25 and 21 be the same? If actual burn starts about 10 after injection starts you would get:
SOI 25: Start of burn at 15
SOI 21: Start of burn at 11

This means that negative torque should be reduced a lot.

Also, for either 25 or 21 SOI, this means EOI is before/at TDC. Wouldn't this mean only residual burn would be done ATDC? And if so, how would it even work. The fact that the engine doesn't run "backwards" tells me this is not what's happening inside the cylinder, but I can't figure out where it is wrong.
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Old 21-01-14
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Have another look at the graphs on the conrod length thread i posted off diesel rk.
The burn process carries on a long way down the bore, but of coarse gets lets less as it goes but it gives good diffences of end of injection position , pcp, and heat release( combustion burn)
All we can really do is make a change and observe and record results.
With data from zbrsk and jsjtd we have plenty of data and feel of where eoi is best for max hp and min egt , rather safest high egt.
What and where are you looking to improve ?
Max hp , max mpg , less eng noise ?
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Old 21-01-14 Thread Starter
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Focus is on HP, noise at idle and low loads, but this should be achieved at the same time. Diesel knock comes from the sudden explosion at the begining of the burn. This happens because fuel is injected to soon and won't burn immediatly, when it does a large ammount explodes, but if you inject later on, it will start burning faster and won't cause so much spike in the cylinder pressure, making less diesel noise.

I think idealy the burn should start after TDC, but how close can it actualy get from ideal conditions?
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I produced spreadsheets using the soi, pressure and duration tables to determine eoi point and assumed that ignition of the fuel would be effectively instantaneous. I adjusted soi values to get an eoi point of around 5 deg atdc as high up the rev range as I could although I could not achieve that even with 30 deg btdc at 4000+ at max fueling due to my duration lengths. I found the torque went up noticeably over the earlier soi I had before but the power high up was not significantly affected.
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Old 24-01-14
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lets look at it another way...

diesels can't go much beyond 6000 RPM efficiently because the fuel burns too slowly, this should give you a clue of how long it takes to burn the mixture. Petrols can go to 20.000 rpm for race engines.

At 20.000 RPM, 1 Revolution takes 0.003 seconds, half a revolution 0.0015 seconds so it's safe to assume the burn can complete in that time.

for a diesel 0.01 seconds seems to be about the limit, so try with ending the injection in your model at least 10 milliseconds before the exhaust valves open
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Old 25-01-14
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Quote:
Originally Posted by Cuore_Sportivo_155 View Post
lets look at it another way...

diesels can't go much beyond 6000 RPM efficiently because the fuel burns too slowly, this should give you a clue of how long it takes to burn the mixture. Petrols can go to 20.000 rpm for race engines.

At 20.000 RPM, 1 Revolution takes 0.003 seconds, half a revolution 0.0015 seconds so it's safe to assume the burn can complete in that time.

for a diesel 0.01 seconds seems to be about the limit, so try with ending the injection in your model at least 10 milliseconds before the exhaust valves open
The problem isn't that Diesel burns too slow, but that injection, mixing and ignition can take place in a limited time window.
With a gasser you have a much more wider window to do the injection and mixing, because ignition is independent of those.
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that is true, but ignition does take more time regardless
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I can rev mine effectively to 5700rpm without problems but due to the duration of injection I can inject much fuel because of the window. With the ducati injectors I'd be able to push effective power right upto that point. I can make 200 wheel power at 5250rpm with a 43.6/56 compressor currently.
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Old 25-01-14
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how much smaller are your current injectors compared to the DC?
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Old 26-01-14
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There is 20% more flow from the Ducati ones I believe I read here.
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Old 26-01-14
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Yes im sure it was equal to 3 degs less duration at the top end.
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Quote:
Originally Posted by nutron View Post
I produced spreadsheets using the soi, pressure and duration tables to determine eoi point and assumed that ignition of the fuel would be effectively instantaneous. I adjusted soi values to get an eoi point of around 5 deg atdc as high up the rev range as I could although I could not achieve that even with 30 deg btdc at 4000+ at max fueling due to my duration lengths. I found the torque went up noticeably over the earlier soi I had before but the power high up was not significantly affected.
Ignition is not instantaneous, it takes specially long when boost is very high, it will take some time after you start injection before it actually burns. I had thought about 0.4ms, but this is maybe not long enough. But even so maybe you could even inject later, have you tried later EOI?
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Old 27-01-14
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Im sure he will correct me if wrong but i think he was injecting 120mm3 with std injectors and without going too early with soi, its had to be pretty late eoi to be able to get 120mm3 in there.
As joel found out later eoi past tdc at max hp max revs can only be balanced with emp and egt temps as this seems to go up drastically (100c) for a few degrees.
At lower load and lower revs later eoi is desirable
The oe designers spent a lot of time and money working it all out, emissions leading power and mpg though so, people can tweek log test and learn.
There are a few good (long)threads on ecuconnections on econemy, power ,soi but tdi related.
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Old 27-01-14 Thread Starter
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I know the engineers spent a lot of time, but their goals aren't the same as some of us. Stock car has dpf, egr, precat and so on. Also we may not care about emissions and mpg, at least not at full throttle.
I know egt increases the later you inject, but you probably can get the same power with less iq if you inject it later (up to a point) of course.
If tests show that peak cylinder pressure happens shortly after beginning of injection and that ideal location of peak cylinder pressure for max power is 15 ATDC, theoretically you would need to inject very late. It doesn't look that this would be reasonable in real life, but has anyone tried EOI arround 10 ATDC or later?
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Problem is that the mixture is not homogeneous, this means that even if burning begins very shortly after injection, you will still have to wait that the air and diesel mixes up properly. This depends greatly on cylinder swirl and so on.

It is far from simple equation.

But the rule of thumb says that anything that you inject after TDC @4000rpm will mostly heat up the turbo. 5C ATDC is the latest point where you can expect at least some power gain. Anything after this is only smoke and heat.
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Yes of course, first burn is the most explosive one and it is when pressure is highest, but after that burn continues more slowly.
There isn't much difference in cylinder conditions at 5 ATDC or 10 ATDC, because of it's geometry it has hardly moved in height between 5 and 10, so it will burn as nicely at 10 too. Perhaps it is possible to inject less fuel later and get similar performance.

If I ever get my car on the dyno I will make a stock IQ + SOI versus stock IQ + EOI 5/10 ATDC to see if there is any noticeable difference.
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Old 27-01-14
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Arnt the manyfacturers already retarding soi to reduce nox and soot, so if no interest in emisions advancing the soi seems logical for hp and mpg.
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Old 27-01-14 Thread Starter
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I don't think that's correct. If you inject later you will get "more" soot, more egt, worse CO emissions, but more power. If you inject sooner you won't actually get much less soot, the difference is that more soot will stay inside the engine, mostly on the oil, and will over time make the engine very dirty. Earlier injection will release more NOX, create higher cylinder pressure and more negative work.
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Old 27-01-14
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Ignition is not instantaneous, it takes specially long when boost is very high, it will take some time after you start injection before it actually burns. I had thought about 0.4ms, but this is maybe not long enough. But even so maybe you could even inject later, have you tried later EOI?
With higher boost you get more heat (more air compressed) and so shorter time between SOI and ignition...
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Old 27-01-14 Thread Starter
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Heat improves ignition delay, but boost does not because of the pressure increase makes it difficult for fuel to "mix" and heat up. I think that this effect is bigger than the temperature effect and so boost makes it worse. This requires validation though.
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Old 28-01-14
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Quote:
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It doesn't look that this would be reasonable in real life, but has anyone tried EOI arround 10 ATDC or later?
Well I have tested 4 ATDC at 4000 rpm with 87 mm3, standard injectors at the dyno.
It made high EGT's, quite some smoke and 210 HP.

When we advanced SOI with 4 degrees, power was still the same but EGT dropped with more than 50 degrees.
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