Diesel Injection Timing - What Factors Determine Timing?

[Wotname]

Help me understand what factors influence the designed timing of mechanical injection pumps? Yes, I know that you use the timing that is stated in the manual however why / how did the designer choose the correct timing. I'm trying to improve my limited knowledge of diesel engine theory.

What sparked this enquiry was when I noticed the timing of a Yanmar YSE8 is 10 degrees yet the timing of a YSM8 is 25 degrees. There is almost no difference between these two engines w.r.t. piston, bore, valves, hp etc except for the injector pump. They are quite different. I think the camshaft is the same; if so, then it is the injector pump that is responsible for the difference and perhaps the lobe that actuates it? How so?

Looking at some small Yanmar engines, I found this...

1&2GMxx 15degrees
3GMxx 18 degrees
2QM20 25 degrees
3QM30 28 degrees
3HM 18 degrees
3HM35 21 degrees

While there is some variation, it isn't as great as the YSE / YSM variation noted above.

Educate me please

[model 10]

[QUOTE=Wotnamee; if so, then it is the injector pump that is responsible for the difference and perhaps the lobe that actuates it? How so?


Is there a lobe somewhere in the high pressure pump? Whatever, the engine with more advance and the different pump has a longer path for fuel to reach the injector. The engines with only a small difference in timing, have slightly different needs for injector timing.

[Lepke]

Some of the difference in timing is the engine design. Some have pre-combustion chambers and others directly inject. Now timing is altered in some engines to get a cleaner burn. As the piston travels up the cylinder, the air gets very thick, about 20x thicker near the top. Firing closer to the top takes an injector pump capable of very high pressure and more expensive. One that injects earlier doesn't need as much pressure. As the piston travels, the compressing of the air causes the temp to rise above diesel ignition temp. An engine timed 28° BTDC probably has the fuel mixing with the air before ignition temp is reached. One injecting near the top is firing instantly.
It doesn't matter how long the path is in injector pump engines. The injector is closed until proper pressure is reached in the tube, then the injector opens, the engine fires. Think of the tube containing little blocks of fuel working their way to the injector each cycle. An injector tube might contain 10-20 firings.

[a64pilot]

Horse power required from the engine is one parameter.
Another is fuel injector and operating pressure, a small injector operating at lower pressures will have to have a longer dwell time to inject the same amount of fuel as a larger one.
When you go to design a new engine, you go to the parts shelf and pick available parts that will work, this is known as the potato head system of manufacturing, and it has many advantages, parts availability and cost to name a few as well as development and warranty risk is lower, so often parts aren’t ideal, but they work, so you have to adjust things like timing to dial them in.
However likely the biggest variation is RPM, seeing as how the flame front progresses at the same speed, but an engine at higher RPM has less time to reach peak cylinder pressure, higher RPM’s require greater lead in timing.
The reason a Diesel is primarily a lower RPM engine has to do with both the inability to adjust ignition timing and the amount of time available to put the required amount to fuel into the combustion chamber.
To some extent common rail lessens both of these problems, the stupid high pressure of common rail means you can get a significant amount of fuel in in very little time, and being electrically fired, timing adjustment is easy.
Size and type also play into it, is it direct or indirect injected? Big cylinders require more lead as the combustion chamber is larger.
Noise, vibration and harshness is another reason to dial back timing, at peak efficiency and power, the thing could be really noisy and vibrate, so they dial it back so the engine isn’t so obnoxious.
It may even be cooling, this particular engine may not have good cooling, so that at peak power and efficiency, it may overheat, so dial back the timing to cool things down, there are many, many reasons.

Then of course emissions throws a whole nother huge problem into the mix, you will have to accept less than ideal timing to meet emissions.


One important thing to remember is that a warmed up Diesel, the fuel begins to burn the moment it becomes present into the combustion chamber, it is not like a spark ignition motor that you can let the fuel accumulate and then start ignition when you want by firing the plug.
In both engines, there is no explosion, it is a smooth, but fast burn, the explosion is a myth, unless it detonates.
An explosion is known as detonation, and can really only occur in a spark ignition engine, that is when as opposed to there being one ignition point and a flame front spread through the combustion chamber, the fuel air mix completely and instantly “detonates” or burns all at once, pressures go stupid high as does temps, and you often end up with a dime sized hole in your piston.

[model 10]

Wotname said that the only difference he could see in the 2 engines was the fuel pump. The difference in timing has to be in the fuel circuit. One pump needs to fire sooner to work right.

[a64pilot]

No, fluids are incompressible, only difference in a long fuel circuit would be friction, and of course it would have to be stupid long for that.
Just like on an outboard engine where the difference between a 8 and a 9.9 is the carb, often the biggest difference between similar displacement, but different power Diesels is the timing.

Now, don’t go advancing the timing to boost power, cause you may break something, there may be more to that pump difference, the slope that pressure builds in the pump may be much steeper etc.

[model 10]

I don't think the length of the plumbing has anything to do with it. It's how the pump internals work. If it had anything to do with length, each cylinder would need to be different. In order to generate a pulse in the fuel line at the correct time, one pump needs much more advance.

[hellosailor]

Along what a64 said, the injection timing probably has been optimized for a specific rpm in that specific engine. As the rpm changes, the optimum instant to inject the fuel, and the position of the piston versus the creation of the flame front, are going to change. So unless you've got an engine computer that is adjusting the injection timing to match the speed, the load, the ambient air temperature/humidity....you pick one point, one set of "most likely" conditions for the engine and optimize your injection timing and everything else around that one point.
And there are apparently some considerations of valve overlap and how that affects combustion gas flow and similar semi-mystical proprietary combustion science companies sometimes reconsider and base changes on.
Which incidentally turns a stable reliable simple engine into yet another rats nest of computers and sensors and vulnerabilities, but that's how they squeeze performance out of the new engines.
If you are brave, you can of course try varying the timing a tad up and down, depending on your own installation and use. If you tend to cruise it at 1800 rpm, or 2800 rpm, you may find a timing number that's better at one or the other. (Burns cleaner, uses less fuel, affects cylinder temperature, etc.)
When and if engineers were involved in building the engine, they usually know what they are up to, but they also don't always tell you what *their* goal was. It might not match yours.

[NoTies]

Originally Posted by Lepke

Some of the difference in timing is the engine design. Some have pre-combustion chambers and others directly inject. Now timing is altered in some engines to get a cleaner burn. As the piston travels up the cylinder, the air gets very thick, about 20x thicker near the top. Firing closer to the top takes an injector pump capable of very high pressure and more expensive. One that injects earlier doesn't need as much pressure. As the piston travels, the compressing of the air causes the temp to rise above diesel ignition temp. An engine timed 28° BTDC probably has the fuel mixing with the air before ignition temp is reached. One injecting near the top is firing instantly.
It doesn't matter how long the path is in injector pump engines. The injector is closed until proper pressure is reached in the tube, then the injector opens, the engine fires. Think of the tube containing little blocks of fuel working their way to the injector each cycle. An injector tube might contain 10-20 firings.

All injector HP pumps are capable of very high pressure. This high pressure is required to fully atomise the fuel, not to overcome the compression pressure.

[nhschneider]

It's a decent question but wholly irrelevant. The engineer who wrote the specifications had very compelling data to specify the timing parameter of each engine and we assume he (or she) did so correctly. The engineer could probably tell you specifics of why one engine is different from the other and, as others have said, it could be any one or multiple reason(s). Regardless, I would suggest that the timing be set to the specification for that engine while you continue to ponder the great mysteries of life including huge discrepancies in fuel injection timing.

Fair winds and calm seas.

[a64pilot]

Originally Posted by NoTies

All injector HP pumps are capable of very high pressure. This high pressure is required to fully atomise the fuel, not to overcome the compression pressure.

I don’t think that is what he was saying.
I think he was saying higher pressure can flow more fuel in a shorter time interval than lower pressure.
However if your injecting fuel into a vessel at 300 PSIG, then you need 300 PSI higher pressure to achieve the same results than if your injecting at atmospheric, I have never considered that before myself.

[NoTies]

"Firing closer to the top takes an injector pump capable of very high pressure and more expensive. One that injects earlier doesn't need as much pressure." Pretty unambiguous really.

[Wotname]

Thanks everyone for the replies!!!
Lots to read and digest so I won't be making any comments just yet, I have to understand first what you have written.

But (there is always a but), allow me to address this one post.

Originally Posted by nhschneider

It's a decent question but wholly irrelevant. The engineer who wrote the specifications had very compelling data to specify the timing parameter of each engine and we assume he (or she) did so correctly. The engineer could probably tell you specifics of why one engine is different from the other and, as others have said, it could be any one or multiple reason(s). Regardless, I would suggest that the timing be set to the specification for that engine while you continue to ponder the great mysteries of life including huge discrepancies in fuel injection timing.

Fair winds and calm seas.

I concur with all of this except the relevance aspect. It is highly relevant if one wants to understand the finer points of engine design. To repeat, I only thought about this while rebuilding an old YSE8 engine; the slightly later model is a YSM8 and the vast majority of the parts are identical but the timing is radically different 10 vs 25 degrees. Why is it so I asked. When I didn't fully understand, I have asked the experts - CF members . The major difference between the two engines is the injector pump so either the pumps are quite different and / or the actuating lobe is (or something else, yet undiscovered). Capacity and horsepower is the same!

[atoll]

when i rebuilt my YSB 8 i tried very hard to set it up at 12 degrees,but found it ran fine no matter what combination of shims i put in.

probably the best test would be to run it under load with multiple shims at about 20 degrees then remove shims untill the exhaust runs with less smoke as a sign of less unburnt fuel and better combustion so as to find the sweet spot,bench testing is not really going show optimum combustion angle when under no load.

[a64pilot]

As an old motor head who has hopped up nearly everything imaginable and blown up quite a few engines. I can tell you there is absolutely nothing to gain by messing with the timing on a little stock Diesel, and quite possibly something to lose.
There is every trick in the world like running an engine hard, then very quickly adjusting the valves to zero lash, theory begin you get the absolute max lift and duration that way, lash is there only to allow slop so it doesn’t get too tight when hot.
All those tricks added together don’t mean squat, and you can harm things.
Set everything as close of spec as you can and enjoy it, don’t go messing with it.