Timing Effects

[ISX]

Once you get to about +75 HP (and above) injector then the MPG are neutral again from stock so now the duration is too short and atomization is poor. I'm not saying this is the case with everyone just saying general rule here...

It's about the same on a HPCR. 50's provide an increase, 90's no change, and 90+ a decrease... again as a generalization.

So the OEM injectors are too small. Actually it's kinda interesting. I don't know how big the 24V injector is but I would assume mine would be smaller. I get great mileage so it's weird how you guys going to a bigger injector get better mileage when the OEM was already bigger than mine to begin with. It's things like that that drive me nuts. I know the engines are slightly different but stuff just never adds up and unless we buy a million dollar machine to see exactly what happens then I will never be satisfied We can assume stuff all day though the clues might get us closer to the real truth. I just wish there were more clues, but I am getting sooo close to finding the truth to it all.

[bbraden]

Taken off the Cummins Forum (NOT MY WORK!) This is actually a tuning "how-to" for the Quadzilla Adrenaline:

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Pump Stretch.

This will make the biggest difference in overall power and smoke. This is the amount of time the injector is actually opened and spraying fuel into the cylinder. I have this set pretty low on the 1000 tune that I sent you. The COMP file is set to 1800. More than 1800 creates more torque and a lot more smoke but, actually makes less power on the upper end based on all the trucks I have had on the dyno. This is where the COMP file is set to. Feel free to run it to 2400 anytime you want lots of low end power and smoke but, realize you are giving power up on the top.

This does not mean you will get this amount of fuel all the time. There is a map, in percentages based off of this number as the maximum. Beyond that the map is also scaled by the TPS. So even if the map commands 100% but, you are at 50% throttle you will be getting 50% of the max stretch. If the map calls for 25% and you are at 50% TPS you will be getting 12.5% of the max stretch.

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..... This makes me question just exactly what capabilities the VP has. If it can vary how "long" the injector is open... could it make it such that the injection event is lower pressure (and lower volume) across the span of the event, but make the event so long that you get the same amount of volume in the end? This could possibly create an ignition event that is started way too early (causing timing knock), yet continue long enough into the power stroke that it's wasting fuel.

From what I understand, one hot and fast event also creates NOx. The cooler you can keep the event, the more fuel you can give it, creating the least amount of NOx. If the event was lengthened out so that it never got extremely hot at one particular point, you could possibly make the same amount of power with more fuel, but with less NOx. Couple that with timing that's too advanced, and you have less fuel efficient combustion (with less NOx) at lower RPM and load levels.

I guess it works like this in my head -

12v - pop set at 260 bar, pressure in injector lines exceed 260 bar and opens the pintle at say... 14 degrees BTDC. Injector remains open and injection pressure continues upward until a max of 1000 bar at say... 6 deg BTDC. Pressure spike begins decreasing at 6 deg BTDC, decreasing below 260 bar by TDC, closing the pintle.

24v - pop set at 310 bar, pressure in injector lines exceed 310 bar and opens the pintle at say... 24 degrees BTDC (timing knock). Injector remains open and injection pressure continues much more slowly upward until max of 500 bar at say... TDC. Pressure spike begins decreasing at TDC, decreasing below 310 bar by say... 24 deg ATDC, closing the pintle and ending the event (but burning much more inefficiently).

If the VP44 can control pressure at any given time in the injection event, it should be able to accomplish such a thing. Just because the VP is capable of more MAX injection pressure than the P7100, doesn't mean that it necessarily uses that pressure all the time.

[ISX]

From what I have seen the VP can only vary duration. Duration limits quantity and thus fueling. I don't think it has the capability to limit pressure, I don't think the CR trucks can even do that. RPM would dictate pressure since it would be pumping faster but on a CR the pressure is always the same in the rail so it would always be the same out of the injector. Not 100% on all this but 99.9% sure.

[bbraden]

Is the pressure actually always the same in the rail of a CR? I thought that it fluctuated quite a bit with fueling requirements? Where it should be low at idle and higher at RPM's.

[ISX]

Well that would be interesting. I don't know much about the CR I guess, didn't think it was able to control pressures. If you search for dynamic common rail pressures you come up with all sorts of things.

[AH64ID]

Pressure is always varying in the CR world, it's 100% controlled by the ECM (okay, not 100% if your demanding more fuel than you can move, or modified and flow more at idle).The ECM moves what is called the FCA (fuel control actuator) which meters the fuel into the CP3, and thus the output pressure. Most tuners (not all) will modify this in one way or another. I personally would like to smooth it, yet maintain stock peak pressure for reliability and longevity. On a CR you have 100% (same 100%) control over timing/pressure/duration as none of it is pure mechanical, and there are no pop pressures. Here is an example of one CR pressure map.

[bbraden]

Well... I think that the VP44 can do something very similarly to the CR by varying max injection pressure... but I don't have any evidence to back that up.

[Mopar1973Man]

I'm 99% sure that how all them do it (VE, P7100, VP44)... More pressure = more duration of spray.CR has the ability to command length by signaling the injectors. But also the rail pressure is a factor.

[bbraden]

Then I guess the next logical question is - how does the VP, or any of the pumps for that matter, vary injection pressure if pump displacement is fixed?

[AH64ID]

CP3 does it by metering the fuel into the pump with the FCA.

[AH64ID]

Bringing this back up.

I have been playing with my timing quite a bit lately, and here is the map I am running now. Without giving too many details away I idle at around 7° ADC, and peak at 20° BDC.

I also found this, trying to find more info. But this goes to show that timing is not why VP trucks are so noisy.

"Once not too many years ago someone posted a VP44 timing map on TDR (and promptly took it down). If memory serves it was 12.7° at idle and 25.7° at 3500 RPM."

[bbraden]

Well, I haven't revisited this in a while, but it's way past time to stop back by.

I've been reading a lot about one of my other hobbies - VW aircooled engines. I built my own dune buggy a few years back and have helped work on numerous others. About a year ago, I converted mine to run on propane like many rock crawlers do, and ever since then I have been on a quest to make it run on the least amount of fuel possible. In doing so, I've read and read about how to make a VW engine more efficient and have stumbled on lots of good information about timing. Granted, this is on a gasoline engine, but the theories are similar. Truthfully, it seems that as a whole, the gasoline tuner crowd has a much better understanding of timing than we do.

As it turns out, timing BTDC is BAD. Ya'll may all understand this already, but it's really sunk into my head now. I've said this before and stood by it, but now I have a little better comprehension.

In theory, the perfect engine would never have any combustion happening BTDC. Any pressure built BTDC is wasted as negative work against the piston. However, with our physically limited fuels, we have to start injecting BTDC in order to have peak pressures ATDC when the piston can actually make use of the fuel. On the same note, when we want to start increasing fuel input, it must be injected somewhere, so we advance timing and lengthen the event so that we can squeeze more fuel into the combustion chamber during the same event. Given too much timing, we are once again performing negative work on the piston.

The only way to make a diesel more efficient is to perform more work ATDC without carrying over any of the event into the exhaust stroke. Decrease the amount of time it takes to make a complete burn in the power stroke, and you begin to see better efficiency. We do so by increasing cylinder pressure (boost). The higher the pressure, the faster the flame front. We can atomize the diesel more, making smaller droplets and a faster flame front. We can introduce swirl into the chamber (whether by valve structure, or piston/head design) which brings the fire around more quickly. We can decrease the viscosity of diesel to make it separate and atomize quicker and into smaller droplets. Particulate matter slows the combustion process, so we can evacuate the cylinder more quickly during the exhaust stroke to ensure all PM is out of the cylinder (difficult for the those EGR guys). NOS increases burn speed by increasing oxygen content, creating faster flame front. All of these things have a beneficial effect on timing, by making the fuel burn more quickly. Quicker burn = less timing required = more POSITIVE work on the piston = better fuel economy. Of course, as RPM increases, more timing will be required because the flame speed won't change, but the piston speed increases. This will require extra timing BTDC to continue building highest pressure ATDC.

As the Cummins injection systems have continued improving and seeing higher and higher injection pressures, it becomes obvious that the newer (and higher the pressure) systems will NOT require as much timing advance because the flame speed is fast enough that the event can be over with before the end of the power stroke. BUT... we worked backwards by lengthening the single event out into three actual events for "noise control" or "pollution control" as is the case in the CR. In-cylinder EGR camshafts also work backwards to our goal by not evacuating all PM during the exhaust stroke. My thoughts are, if you were to get rid of all the extra events, in-cylinder EGR, and run the highest pressure possible for fastest flame speed and retard timing as close as you could to exert the most work on the piston ATDC, you would have an extremely efficient CR engine.

Maybe everyone here knew all this, but it seems as if there is a LOT of hocus pocus about timing and nobody really understands the negative effect it can actually have.

[AH64ID]

Just to clarify, peak pressure before TDC is bad, timing before TDC isn't bad. Peak pressure should occur about 15° ADC, per some older P7100 Cummins info, for best performance/economy. Things like fuel pressure, rpm, cylinder temp, and boost all effect the rate of ignition. But nearly all fuel starts injecting prior to TDC. Doing some reading a few months back on EFI Live stuff they have a timing calculator that uses piston speed, duration, and rail pressure to let you decide the timing you want. You put in how much fuel you want injected before and after TDC and it gives you the timing. So if want 50% of the fuel before and 50% after it would tell you what timing to run. Just becuase 50% of the fuel injects before TDC doesn't mean you will get peak pressure prior to TDC. To meet emissions nearly all of the fuel is injected ATDC on the 04.5-07 motor, but there is a small rpm/load band where ALL of the fuel is injected prior to TDC.

[bbraden]

Absolutely. Peak pressure BTDC is not good at all. The CR injection system should theoretically be able to inject just about anywhere to get peak pressure wherever needed. Low boost levels + high RPM (where you would normally be during a cruising situation) should have the highest amount of advance, which is probably the majority of injection BTDC.

[AH64ID]

Take a look at stock timing on a 04.5-07 CR. It's VERY retarded. This is done 100% for emissions, not for optimal timing.

Now look at this... It's more optimized for performance/economy, which means a higher cylinder pressure and worse emissions, but uses less fuel. It's not what I am running, but not too far from it.

[bbraden]

How interesting... I wonder where the extra injection events fall on those maps?

[AH64ID]

No, that is the main event timing only.

[bbraden]

As most of ya'll know, Crazy Carl "Turbolver" put a supercharger on his P-pumped 24v. I've followed it very closely with great curiousity but haven't seen any talk about the timing. I read where he bumped timing from 15 degrees to 24 degrees, but he never reported back with results. How would a supercharger affect timing, and how could you maximize efficiency using one?My thoughts are: since extra boost (especially down low) increases flame speed of diesel, you might actually be able to retard timing and maintain similar EGT's and increase torque. Am I way off?

[CSM]

Let me put my mechanical engineer and Internal Combustion Engine hat on. Good posts.

Timing before TDC isn't bad. Its required, unless the enigine RPM is extraordinarily low, like old Fairbanks Morse 1900s diesel low.

You have to consider that when the piston is coming up on the compression stroke (on an otto cycle) and the spark ignites the mixture, the mixture takes time to burn. The piston face doesn't see pressure until the pressure wave/flame front makes its way down to the upcoming piston.

You want advanced timing so that the piston is relatively close to TDC when the majority of the fuel is burned. This isn't usually possible, so we time it so the pressure wave hits the piston face near TDC. Otherwise, you have a "knocking" engine.

Same concept with the valves. You should have the exhaust valve open during the power stroke, as you want the pressure wave from the open exhaust valve to hit the piston face right about BDC. Timing is what makes a good engine.

Flame speed increases at higher pressures... AKA more boost or compression. Necessitating less advance at higher boost/same RPM for many engines. Piston design can also effect flame speed.

However, at higher RPM, piston speed goes up and you need more advance as well.

The other thing to consider, more with the common rails than our VP or P7100 poppet injector trucks... Is that a gasoline/air mixture will burn very quickly. A diesel mixture burns slow. You will hit an RPM limit around 5,000 RPM where the engine will not deliver any additional power for most diesels. This is because the fuel can't burn fast enough to completely burn by the time the stroke is over... Even if you injected at BDC. However, this can be remedied somewhat, by decreasing the droplet size (diesel stays in drops even when shot from an injector) by increasing the injector pressure. Diesel fuel burns in droplets, radially inward. So the burn rate is a function of the surface area of the droplets. Small droplets, means more surface area for the same volume of fuel. If you really ramp injector pressure, and get the droplet size way down... you can get the effective RPM of diesels up to around, 10,000. This will also make the actual burn time a bit quicker and more efficient. This, in theory, is why a common rail SHOULD be the most efficient diesel out there, being able to control pressures and timing to perfection. Unfortunately, very efficient diesels tend to have a NOx problem, and they usually band aid it with EGR and other tidbits.

So, bbraden, you are somewhat right. In a very slow engine, you will have peak efficiency by only igniting at TDC. However, this won't work AT ANY appreciable efficiency (if it even runs!) in most engines that run at over a few hundred RPM. The big fuel oil diesels in ships are a good example of this, they are super efficient, but they are very low horsepower per lb, due to their low RPM.

In a higher RPM engine, you can time ignition BTDC so that the pressure wave hits the piston face near TDC. And honestly, its different for each piston, at each throttle setting, at each load, etc. So, we have to guestimate it and get as close as possible. Hence the modern engines having a knock sensor and having the engine change the timing curve to stay on the ragged edge of knocking.

And lets not even get into the horsepower vs. torque argument.

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Bringing this back up.

I have been playing with my timing quite a bit lately, and here is the map I am running now. Without giving too many details away I idle at around 7° ADC, and peak at 20° BDC.

I also found this, trying to find more info. But this goes to show that timing is not why VP trucks are so noisy.

"Once not too many years ago someone posted a VP44 timing map on TDR (and promptly took it down). If memory serves it was 12.7° at idle and 25.7° at 3500 RPM."

I am so jealous of your Smarty and Common Rail.

[bbraden]

CSM, I agree 100% with your post. I didn't know that diesel burned that much slower than gasoline, even when artificially pressurized (turbo or super boost). Many VW gas engines run 30-32 degrees total advance when cruising under part throttle conditions. If we can increase the burn speed - whether by compression, smaller droplets, or more swirl - we can pull out more of the timing and create a more efficient engine.

I was thinking about the difference in piston/head design between the 12v and 24v. I know that both engines have a more or less flat top piston with a combustion bowl built in, but would the injector direction cause a big difference? I know that the 12v injector sits at an angle... much like the plugs do on a fast burn head.

I know with gasoline engines, head designs like the "fast burn" and Vortec heads lend themselves to better fuel economy. The smaller the intial combustion area, the higher pressure this little area sees. In the fast burn heads, the spark plug is directed in such a fashion as to be right in the middle of the smaller initial combustion area, and the valves are situated in such a way as to enhance swirl. Ideally, the combustion chamber is a wedge shape with the exhaust valve sitting at the smallest part of the wedge, so that exhaust is funneled out of the chamber more efficiently during the exhaust stroke. All of these things make for a more efficient combustion design that allows you to pull more timing out, but doesn't create knock.

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Or maybe... it's as simple as combustion pressure.

A 12v has 17.5:1 CR as far as I know. At 5 psi boost > 19.7 PSIA * 17.5 = 344.5 PSIA

24v non-HO has 16.3:1 CR. At 5 psi boost > 19.7 PSIA *16.3 = 321.11 PSIA.

I'm sure this would have an effect on flame speed, and thus efficiency. If the 12v has higher compression, it should theoretically require less total advance - but that may be made up by the smaller droplet size on a 24v.