Mike D

My statement was more in general terms, I have no direct knowledge of how high his egt's were.

I've always thought timing was a matter of when, not how much... so I don't get the "less" fuel argument. I'm sure there's more to it, but I don't build performance boxes for a living.

Maybe you could give us a more in depth understanding, Quad?

I have no dog in this fight, just giving my personal preferences.

qzilla

I am not sure I can spit it out exactly in words (I am more of a draw it up kind of person) but, I will give it a go.

Keep in mind when I talk about this that anything can be used to the point of no return.

When you advance the timing you do start the injection earlier like you mentioned. Doing this allows the combustion process to obvisouly start sooner. The combustion process is what moves the internals of the motor. The longer the combustion process has to burn the injected fuel, the more effective the comsbution process is before the exhaust valves open. No there are situations where you can have too little fuel or way too much fuel, and you just cannot burn it all or you burn it too quickly. In the case of these motors you never have too little as we are typically waiting on more air to burn the fuel we have which is what causes the black puffs of smoke.

So on a stock type truck if you advance the timing you can somewhat compensate for the lack of air by allowing a longer combustion period. This is why products that have a timing only setting allow for trucks to run with little to no smoke in most cases.

Just by advancing timing you do not get a shorter fuel pulse width. The way it really goes down is that under power the engine is either trying to either maintain or gain RPM. If you effectively increase the power stroke a motor will gain and maintain rpm easier.

There is a point of no return. You can over advance the motor and you can also over retard the motor. Ideally 50% of the combustion takes place before TDC (top dead center) and 50% takes place after TDC. That will never happen but, that is the ultimate in efficiency.

Not really sure I got out what I am trying to say but I will try to summarize.

By increasing efficiency a certain amount of fuel will allow the motor to gain more revs with advanced timing than with retarded timing to a point. From experience about 6-8 deg advanced over stock is fantastic. This would be in the 28deg overall range.

All this goes out the window if you are running at WOT or something like that though.

NoSeeUm

I also live at a higher altitude.....

Thanks Quad, I seem to lean more toward Mike's and Hohn's opinion concerning towing and timing. To be honest, it could be simply "Common Knowledge" that came from who knows where aligning itself with "Internet Myth" that also comes from who knows where. But I some how picked up the notion that advanced timing does two things other than just make more power.

The two things I am talking about are increased cylinder temperature in conjunction with lower EGT. What the actual cylinder temperature is, is kind of an unknown for me. One would think that it is some where above the measured EGT. How much above is a great question I have, so I typically try to tow leaning towards less added timing vice more added timing.

Any of this make sense to you?

FWIW I run your XZT+ exclusively on timing + fuel. I rely on it to make dynamic timing changes. I also have done a mod that switches out the IAT and fools a steady state signal of 105F once the truck warms up, which so far seems to really be the schpickity ding ding for how the trucks engine sounds and it has eliminated any timing stutter problems I had in cold weather. The engine runs much quieter, while still making good power. I also have an advantage because my twins manage EGT's very well if I in the right gear and I seldom go over 1000F for any extended length of time.

Jim

HOHN

I quote Quad here not because I'm speaking to him or that this is directed to him in particular at all-- he simply raises some really good points. I'd like to comment on them, but this is more just me sorta "thinking out loud"-- and I could be completely wrong in my take on this or the weight I assign to different factors.

What is "ideal" timing?

Ideal timing is a myth, because it only exists for ONE combination of variables-- compression ratio, injector atomization, AFR, engine load, etc etc-- the list of variables is very long.

So what we'll see is a lot of factors working against each other. For example, a larger injector (higher cup flow) might increase the rate of discharge for a given flow rate, causing a shorter injector duty cycle for a given rate-- which would seem to speed along the combustion event and require less timing advance. However, if an injector has poorer atomization as the result of that higher cup flow, it may tend to burn slower, which might require more timing advance.

Advanced timing is a necessary evil because we haven't yet figured out a way to inject fuel @ TDC and burn it all by the time the exhaust valve opens.

The penalty of advanced timing is negative work-- we have a rising piston attempting to compress expanding gases, which tends to produce forces opposing the power we are making.

Merely providing more time for the gases to combust is not necessarily a good thing.

The goal of injection timing is managing the point of peak cylinder pressure relative to piston motion and valve events.

It's not necessarily ideal that we burn 50/50 on either side of TDC. The goal is that the point of peak cylinder pressure occurs at some defined optimal point, which varies based on valve event timing, fuel type, compression ratio, injector flow, AFR, etc.

Consider a cloud of expanding combustion gases under three diffent points of piston motion. When the piston is rising, the gases work against it-- this is bad. When the piston is at TDC, it can do no work because there is no leverage on the crankshaft (try loosing a lugnut while standing on the wrench positioned vertically).

So we want to have the gases expand upon a descending piston so that they can be converted to work. Unfortunately, the peak efficiency will be reduced dramatically because peak cylinder pressure is very low when the gases are chasing a moving target.

In almost all cases, you want the crankshaft to be ~17º ATDC when the point of peak combustion pressure occurs. Before this point, much of the cylinder pressure is doing negative work and after this point, we are losing potential energy to the rapidly expansion combustion space.



So what is the effect of elevation in relation to timing? One would first be tempted to think that richer AFR=more timing=less oxygen to go around.

But it's not that simple. For one thing, the cylinder pressure and temp at the time of injection initiation will be lower in thin air. This is because the compression of the rising piston is capturing less air mass in the thinner air (which the turbo helps, but cannot completely redeem). It would seem to make sense that timing should be retarded to the point where the combustion space is less (i.e. closer to TDC) so that cylinder pressure/temp is higher when injection is initiated. Remember, it's a lot easier to keep a fire going than to start it!

Of course, at higher elevations, the "negative work" effect of advanced timing is reduced because the rate of cylinder pressure rise will be slower (less oxygen to speed along combustion).


So I think the VA was causing more smoke in the thin air not JUST because of extra fuel-- but because of the additional timing advance.

I think the stock ECM will allow a good bit of timing retard at high load and high boost, and if the VA is overriding this and forcing more advance even under these conditions, then higher smoke levels seem to be plausible enough to me.


Finally, a little combustion event analogy. Consider the combustion of gunpowder in a cartridge as an event in time. What if it took 2-3 seconds for the gunpowder to burn? You wouldn't have an explosion, you'd have a fire. The peak pressure in the barrel would be very low, and the bullet might not even leave the barrel-- the gases might just leak past the bullet.

Now what happens if you get a bullet with the wrong powder that burns far too fast? You might lose a hand as the barrel disintegrates into shards of steel.

The point is that power is inversely related to time. The more time a combustion event takes, the less potential power you have. (hence the RPM component of hp=tq*rpm/5252). Which is more powerful-- popping a baloon or letting the helium leak out overnight?

We want to maintain high average cylinder pressure, and we want it to be on the side of TDC where it will do work for us. The best balance is to have peak cyl pressure ~17º ATDC, but this can vary with engine geometry.

JMO (see sig)

Anyone else wanna think out loud?

PaulDaisy

This is interesting. I would guess that the more combustion happens before TDC, the more resistance that cylinder is producing, basically working against the rotation of the engine. From what I understand about ICE operation, the idea of advancing the timing is to allow full combustion by increasing duration and combustion efficiency with heat resulting from the compression from advanced injection to TDC. This increased combustion efficiency comes at the expense of resistance because the gases expand the moment the fuel starts burning, so any time before TDC is working against the rotation. So, advancing timing is a compromise - this is why over-advancing the engine causes bad effects from pinging on gas engines to stalling and everywhere in between. As long as all fuel burns completely on the expansion stroke the 100% utilization is achieved, and the more of it burns right at TDC at the top 30% of the expansion stroke the better, I would think. Of course this is all just speculation on my part, we all know that advancing timing creatively is a way to boost efficiency but again, at the expense of extra wear and tear if it is over advanced. (Edit: Hohn seems to have covered it, although I think the power vs time analogy he used is too rough to directly apply in this case).

Now, the sequence of events:
Advanced timing -> Better combustion -> More power output -> ECM trying to maintain RPM under load with more power -> Lowers injection pulse width -> Injects less fuel -> Possibly lower EGTs
makes sense to me.

NoSeeUm

Somewhere along the line I picked up the tidbit that the maximum cylinder pressure should occur at 10-14% ATDC.

Jim

Mike D

Another thought that I had was with regards to nozzle angle and efficient burn rate from the fuel, which I think Hohn touched on in his post.

We've all heard about advancing timing and running a thinner washer to reduce smoke while running the Marine 370 injector. Mainly this was to compensate for the bowl difference between the Marine piston bowl to the stock piston bowl. Advanced timing while running the 370 injector caused the spray pattern to go outside the bowl. Running the thinner washer put the injector tip further down in the key hole/bowl to help keep the spray pattern inside with advanced timing.

How much does 7 degree advance effect a particular injector with varying spray angles from stock to ??? I have no actual data to say spray patterns are changed over stock but it's been suggested.

PaulDaisy

Sometimes I wish that a diesel engine designer joined the thread and answered the questions with precise knowledge and experience from thousands of hours of experiments. But then, that would take all the fun out of trying these different things and figuring it out

qzilla

That would be super duper handy but, it would have to be the one that designed this particular engine. For example we run up to say 30 deg advance on these motors while we might run nearly 40deg advance on something like a Duramax!

Like the our engineers always joke.....sure wish this thing came with an instruction manual!!

SWC

What started out as a good write up by PaulDaisy on towing in the mountains has turned into an excellent discussion on how our Cummins 5.9 works.
Thanks to all of you who know so much more then me for your opinions and civil discussion!!!
When I have more time I will read this more thoroughly.
Keep the information coming.
Stan

NoSeeUm

Another thing to realize about timing is that once the crank passes 90* the rotational energy imparted to the crankshaft is dramatically reduced. Even though the piston itself is still providing downward force at essentially the same level for the entire stroke.

Think about riding a pedal bike and where the maximum effort is realized. It is on the top half of the pedal stroke. To reach a maximum cylinder pressure at 10-14* ATDC injection can not start at TDC due to the fixed delay in the start of combustion.

I do not understand the mechamism of timing stutter, but it sure feels like a miss on gas engine. So obviously, it must be something like no or very little combustion taking place. Possibly even reverse thrust on the piston? Any more ideas?

Jim

HOHN

It stands to reason imo that advanced timing needs a little more spray angle (relative to horizontal).

Earlier timing will mean the piston is lower in the bore at the point of injection, which means the engine geometry would seem to dictate a different spray angle.

Bowl geometry matters, also. A higher compression engine (like an HO vs an SO 24V) would seem to tolerate early injection better, either through higher piston position (i.e. taller compression height of piston) or through a shallower bowl.

jh

hopak

I know having a pyro would be a good idea; but as a practical matter do I need to worry about high EGTs with the VA v5? I generally shift out of OD or into 3rd or 2ed and keep my rpms at around 2000 when climbing in the Colorado
And since I'm not really interested in power per se would a Quadzilla Max Mileage give me lower EGTs and just as good if not better mileage ?

PaulDaisy

Are you towing? If no then probably not. If yes, I can say from my own experience that I could either keep going in high gear or downshift, and the EGT would be the guidance I would need. Which I did not have, and wanted a lot. With the auto I think all you can do is watch if your RPMs are too high and back down to lower EGTs, and tranny temperature is more important there so that you don't burn up the clutches. On an empty truck (well, with a bed full of something other than lead, say some 1.5k lbs in bed) I would not worry about EGTs: the truck flies up any highway pass when empty, in almost any gear, at or near 2000 rpm. On a winding pass you can't speed much anyway, so you are in a safe range there.

MikeyM

Darn good info; thanks Paul for starting the thread (got your PM, thanks) and to all who have added great info. Paul, if you're not running a boost elbow or j-hook or similar, I would venture to say you could probably get a few more psi of efficent boost (obviously giving cooler EGTs and more power with your added fuel) before you start to pump hot air. I agree - you should get a pyro; mainly to tell me how to build my truck

Seriously, though - glad those 1.6s are working well for you.

Mike.

PS - I still had NO idea that a V5 added fuel. I thought it was timing only.. learn something new every day.