Diesel Injection Timing theory and practices

[CamTom]

Hey there!

Thought I'd share my limited experiences with ignition timing in the cruise cells (which appears contrary to what I've been reading, but makes sense in my head) and ask some theory/experience questions as I move into the higher load parts of the map.

So I started off reading a fair amount of folks talking about bumping timing up slightly in the cruise cells to get better fuel efficiency. Granted, most of the info I've read is from folks tuning Cummins motors, but that's all I could find.

So I tried that and didn't get any better economy. Then I saw a thread where JaegerWrenching posted his map and he had LESS timing than stock at low loads, so I gave that a shot. I picked up some economy and the driveability got a little nicer with the timing between -4 and -1 in my cruise cells.

This made sense to me because if we're targeting ~12-14 degrees ATDC for peak cylinder pressure, and we've already "preheated" the airmass with a couple of pilot shots, I can't imagine ignition delay (ID) being any more than ~10-15 degrees at those rpms. I read an article that found calculated ID at 25% load to be around 1 +/-0.05ms at 1800rpm using a bunch of different calculating methods. They were using with a slightly smaller bore/stroke but a higher compression ratio and no pilot injections. At the bottom end (0.95ms) at 1800rpm is about 10 degrees and the top end of that time range (15ms) at 1800rpm is about 16 degrees of ID. So my findings made pretty good sense to me.

For example, I'm currently running -2.5 degrees at 1800rpm (~74mph in my truck) and 369lbft of requested torque. I think that should put my peak cylinder pressure at ~13.2 degrees ATDC. Maybe.

I think it makes real good sense at lower loads, but here's where I get doubtful. As the saying goes, all models are wrong and some are useful...

They continued the calculations out to 100% load at 1800 and 2100rpm and found ID to be around 0.5ms (~5.4 and ~6.3 degrees respectively).

Does that mean I should be looking at -7 degrees of timing at 2100rpm and WOT? The stock map has 5.2 degrees there. I think the model breaks down at the edges.

The study did note that advancing the start of injection lowered cylinder temps/pressure at TDC, which increased ID. I guess that's what's probably happening here.

Does anyone else have any experiences with better fuel economy at lower load at reduced timing from stock? I'm starting to move out to the 443 and 516lbft requested torque cells at 1800rpm (I'm currently near stock but smoothed, around 4.5 degrees) and am calculating that timing still wants to be around -2.5 degrees , but I'm wondering if that's far enough out on the model to where the math will start to be less useful. How's everyone else looking in that range?

My truck does have dual fuelers, but I'm not sure how much of a difference that makes at cruise other than being a horsepower/fuel suck. I haven't seen where I can log rail pressure yet.

[Powerstrokecustoms]

on a 6Liter you can actually pick up better power and mileage just in injection pw and not even have to mess with timing.

[CamTom]

Interesting! I think I'd like to play with that - do you know of a way to modify that on a 6.7?

[JaegerWrenching]

Hey there!

Thought I'd share my limited experiences with ignition timing in the cruise cells (which appears contrary to what I've been reading, but makes sense in my head) and ask some theory/experience questions as I move into the higher load parts of the map.

So I started off reading a fair amount of folks talking about bumping timing up slightly in the cruise cells to get better fuel efficiency. Granted, most of the info I've read is from folks tuning Cummins motors, but that's all I could find.

So I tried that and didn't get any better economy. Then I saw a thread where JaegerWrenching posted his map and he had LESS timing than stock at low loads, so I gave that a shot. I picked up some economy and the driveability got a little nicer with the timing between -4 and -1 in my cruise cells.

This made sense to me because if we're targeting ~12-14 degrees ATDC for peak cylinder pressure, and we've already "preheated" the airmass with a couple of pilot shots, I can't imagine ignition delay (ID) being any more than ~10-15 degrees at those rpms. I read an article that found calculated ID at 25% load to be around 1 +/-0.05ms at 1800rpm using a bunch of different calculating methods. They were using with a slightly smaller bore/stroke but a higher compression ratio and no pilot injections. At the bottom end (0.95ms) at 1800rpm is about 10 degrees and the top end of that time range (15ms) at 1800rpm is about 16 degrees of ID. So my findings made pretty good sense to me.

For example, I'm currently running -2.5 degrees at 1800rpm (~74mph in my truck) and 369lbft of requested torque. I think that should put my peak cylinder pressure at ~13.2 degrees ATDC. Maybe.

I think it makes real good sense at lower loads, but here's where I get doubtful. As the saying goes, all models are wrong and some are useful...

They continued the calculations out to 100% load at 1800 and 2100rpm and found ID to be around 0.5ms (~5.4 and ~6.3 degrees respectively).

Does that mean I should be looking at -7 degrees of timing at 2100rpm and WOT? The stock map has 5.2 degrees there. I think the model breaks down at the edges.

The study did note that advancing the start of injection lowered cylinder temps/pressure at TDC, which increased ID. I guess that's what's probably happening here.

Does anyone else have any experiences with better fuel economy at lower load at reduced timing from stock? I'm starting to move out to the 443 and 516lbft requested torque cells at 1800rpm (I'm currently near stock but smoothed, around 4.5 degrees) and am calculating that timing still wants to be around -2.5 degrees , but I'm wondering if that's far enough out on the model to where the math will start to be less useful. How's everyone else looking in that range?

My truck does have dual fuelers, but I'm not sure how much of a difference that makes at cruise other than being a horsepower/fuel suck. I haven't seen where I can log rail pressure yet.

Your findings are correct and the reasons are exactly as you think. The other thing to think about is how your cam vs rpm effects in cylinder swirl, and how it gets more or less efficient.

[CamTom]

Weird, I had two other posts from last night that I must've accidentally deleted while trying to change the "?" into apostrophes.

Your findings are correct and the reasons are exactly as you think. The other thing to think about is how your cam vs rpm effects in cylinder swirl, and how it gets more or less efficient.

I hadn't really thought about swirl as a factor for injecting fuel. I've got some more reading to do.

I also hadn't really thought about containing the injection in the piston bowl for higher load/higher rpm. That'll definitely require timing advance.

I just set up a couple of new logging tables to record FRP and PW. I'll do some driving on them and a little math before I start messing around too much in the higher load areas. For the power stroke, how many degrees ATDC would you consider too late for injecting fuel?

[CamTom]

Ok, is there a trick to logging fuel rail pressure and or pulsewidth? I was able to log fuel mass (mg), and I guess I can try and convert that to mm3 to try and back into the table?

[jetskier]

There is a PID for commanded and actual fuel rail pressure. I don't think there's one for pulse width directly (at least one that's available in VCM Scanner for my 2013 6.7L). There should be Total Injection Quantity Cyl 1 to 8 in mg.

As for your timing observations, I noticed the same. I have the 2015+ style turbo (2019 actually). I had to update a lot of parameters using a 2016 base calibration for reference. I ended up averaging the main timing between 2013 an the 2016 in the 1200-2800RPM range and a lot of it went negative. Turbo spools quicker, EGT's cooler when towing even with running torque based fuel closer 120-140 in the higher commanded torque areas. I don't have the newer pump or injectors to go higher there. Seems to run better than the tune base I started with (not stock, reputable tuner).

[CamTom]

There is a PID for commanded and actual fuel rail pressure. I don't think there's one for pulse width directly (at least one that's available in VCM Scanner for my 2013 6.7L). There should be Total Injection Quantity Cyl 1 to 8 in mg.

As for your timing observations, I noticed the same. I have the 2015+ style turbo (2019 actually). I had to update a lot of parameters using a 2016 base calibration for reference. I ended up averaging the main timing between 2013 an the 2016 in the 1200-2800RPM range and a lot of it went negative. Turbo spools quicker, EGT's cooler when towing even with running torque based fuel closer 120-140 in the higher commanded torque areas. I don't have the newer pump or injectors to go higher there. Seems to run better than the tune base I started with (not stock, reputable tuner).

Found it earlier this afternoon, thanks!

Did some logging... Interesting findings.

So at 2100rpm and 664lbft of requested torque, my timing is currently at 6.8 degrees BTDC. I logged FRP and Total Injection Quantity Cyl 1, then multiplied the mg by .835 (a factor I found on a European diesel tuner site). It showed at that load cell the truck commands the injector open for 1016.8 microseconds, which comes out to 12.8 degrees at that rpm. Assuming a .6 millisecond ignition delay, I should have timing set at -0.2 degrees. Does that seem right? If requested torque scales with actual load, that's only around 64% load, so it seems like it'd be ok.