Effects of advancing timing...Interesting!

[ENafziger]

At work a couple days ago I was running one of our instrumented dyno engines, and had a few hours to play around. I can't reveal the engine make (protected by privacy agreement with the supplier), but it is an inline 6-cylinder, turbocharged and intercooled diesel... and is of the same basic configuration as the engines that we have in our 3rd Gen trucks. Anyway, with the ever-growing selection of electronic boxes to add to our trucks (including TST's timing advance feature now), I took the opportunity to see what effect changing the timing advance would have on some other parameters...



The test was conducted at 1800 rpm, with the engine being fed by conditioned air at 79° F and 54. 8% relative humidity. Fuel was 2007 certification fuel, and the fueling duration was held constant at 25°.



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Sorry for the small size, but I think it's still readable.



The yellow band indicates the point at which peak torque occured (12°).

The blue band indicates the point of best "bsfc" (fuel efficiency) occuring at 19° advance.

The pink band at 9° advance yielded the lowest peak in-cylinder pressure rise.



I know there are a lot of differences between our engines based on the pressure boxes, timing boxes, turbos, intake and exhaust mods, etc. that we've chosen, but for a representative case like this, here are some general observations:



At this engine speed, advancing the timing increases the torque output until about 12°, after which it tapers off. Based on "bsfc", fuel economy gains can be seen by advancing the timing to a range between 19° and 21°. Note however, that the "peak pressure rise" is nearly doubled here compared to more moderate timings. The "peak pressure rise" is how fast the pressure is increasing in the cylinder during a combustion event. The units are pressure rise (in bar) per degree of crank revolution. Lower values here tend towards engine longevity, while values approaching 8 - 10 bar/deg can cause severe knocking. "BSFC" is brake specific fuel consumption, and is basically a rating of how much fuel the engine is using to produce a given horsepower (lower values equivilate to better fuel efficiency).



Also interesting is that as timing was advanced, the turbo temperature, boost pressure, and fuel injection pressure all decreased.

 

[betterthanstock]

What advance was the engine designed to run at? Are these numbers the absolute timing, from the TDC, or the amount advanced in addition to the OEM timing?

Does this engine have pilot injection, and if so, is it the pilot or the main event that you are displaying?

 

[hemic67]

Very interesting chart. Good questions Betterthan. Look forward to the answers!

 

[DieselFiend]

So to put this in a practical perspective, how much timing advance does an edge juice provide?

 

[ENafziger]

The engine "design" points have not been locked down yet, as the engine is not slated for production for a couple years yet. Three main categories of operating conditions exist for the engine... one for performance, one for economy, and one for emissions. The performance and economy points fall within the range listed here, but the emission-tuned category actually involves retarding the engine to -6° advance or more... of course at the expense of economy and power.



All timings are absolute with respect to TDC. This is not additional advance added to the OEM. The first run performed was with the injection event starting at TDC, and continueing until 25° after TDC. For the last run, the injector opened 25° before TDC, and closed at TDC. This engine was not instrumented with a needle lift sensor on the injector, so the timings listed in the chart represent "commanded" injection timings; because of hydraulic delay inherent in the system, the actual beginning of injection may be occuring 1° - 2° after the commanded.



The OEM issued engine map will likely include a pre-pilot, a pilot, a main, and a close-coupled post injection event. For the tests listed here, all injection events were turned off except for the main; thus, the fueling duration of 25° indicates a fueling duration of 25° on the main event.

 

[Msilbernagel]

Wonderful information, thank you!!



In your experience with engines like ours , what effect does a pilot injection and/or post-main injection events have on bsfc? I expect the extra events have some profound affect upon peak-pressure (and resultant noise). .



And, once again, we are teased with the "Holy Grail" - the dream of turning off (especially) the post events... :{ One only wonders what kind of mileage and power you could get with an engine tuned to optimize those two... :-laf



Mark

The engine "design" points have not been locked down yet, as the engine is not slated for production for a couple years yet. Three main categories of operating conditions exist for the engine... one for performance, one for economy, and one for emissions. The performance and economy points fall within the range listed here, but the emission-tuned category actually involves retarding the engine to -6° advance or more... of course at the expense of economy and power.



All timings are absolute with respect to TDC. This is not additional advance added to the OEM. The first run performed was with the injection event starting at TDC, and continueing until 25° after TDC. For the last run, the injector opened 25° before TDC, and closed at TDC. This engine was not instrumented with a needle lift sensor on the injector, so the timings listed in the chart represent "commanded" injection timings; because of hydraulic delay inherent in the system, the actual beginning of injection may be occuring 1° - 2° after the commanded.



The OEM issued engine map will likely include a pre-pilot, a pilot, a main, and a close-coupled post injection event. For the tests listed here, all injection events were turned off except for the main; thus, the fueling duration of 25° indicates a fueling duration of 25° on the main event.

 

[betterthanstock]

Also interesting is that as timing was advanced, the turbo temperature, boost pressure, and fuel injection pressure all decreased.

I can understand about the exhaust temp and boost decreasing, but what told the fuel pressure to decrease? Is it programmed based on boost pressure?

What would it look like if fuel pressure stayed constant? A faster rise in power/torque (and in cylinder press/temp) as you advance it, and a changed BSFC, seem logical.

 

[ENafziger]

what told the fuel pressure to decrease?

Good question! The design of this particular fuel system builds pressure on a cycle-to-cycle basis... thus the farther into the cycle you are, the more pressure has built up. This is definately not optimum performance wise, but it's cheaper to manufacture. My guess is that at 25° advance, the fuel pressure simply is still "building up". In fact, there are two separate solenoids or valves for each injector... a needle lift valve and a spill control valve. The spill control is used to prevent the pressure from building too high when injecting at retarded timings.



With a little more time, I could have made up for the pressure building phenomenon by playing games with the needle lift and spill control valves, and could have held the injection pressure constant as well. If I get a chance again soon, I may try do this...

what effect does a pilot injection and/or post-main injection events have on bsfc

Pilot injections and pre-pilot can lend efficiency benefits. Post injection (as far as I know) does nothing for fuel efficiency... it's primarily used for emission control. As an interesting note, there comes a point in high pressure injection that the benefits obtained from the higher pressure don't outweigh the extra energy needed to achieve that pressure... it becomes a law of diminishing returns.



In the upcoming months as we progress with the project, I may be able to share some more data... as long as confidentiality is maintained for the OEM!