Diesel Injection Timing - What Factors Determine Timing?

[jimbunyard]

Originally Posted by 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

Don't know how 'educational' this will be; always seems to me that the more I think I know the more I realize I don't...

But anyway, here's something to think about.

The whole range of difference, 15 degrees, only accounts for 1/24th or about 4% of the compression/power stroke; as others have noted the engine will likely run if you're anywhere close to the specified timing. Your ears and nose could be the best tools to determine the ideal setting for your application.

I was trying to find if the valve cam and pump can were the same for both engines, could only find p/ns on the YSE, and the dealer didn't get back to me on the YSM. If the p/ns are the same, then so are the parts, if not, then they're different.

My guess would be that the pump cams are different and the valve cams are the same.

As you say, the main difference seems to be the pumps. On the 'E' pump the plunger appears to provide a uniform amount of fuel at the same pressure on every stroke, while the amount of fuel supplied to the injector nozzle is modulated by a needle that regulates the position of a ball on a seat. Seems a bit crude, but if it works and is simple...

The 'M' pump is more conventional, in that a rack operates a pinion that turns a grooved plunger that injects a specific amount of fuel relative to the position of the plunger groove and the barrel (bushing) as defined by the governor/throttle position. This might seem more elegant, and probably provides more precise control of the fuel, but those extra moving parts are subject to wear, and sticking and binding from varnish and less than clean fuel.

This difference in the fuel pumps could certainly be the cause for the difference in timing.

On a small engine like these I'd think simpler is better, especially if the end result is the same...

Regarding the actual timing, there are several criteria that have to be met; efficiency, power, longevity, emissions, ease of starting. Generally, for a single-injection-per-stroke engine, the specified timing is likely to be something of a compromise; try and get it close to what it's supposed to be, but you won't hurt anything if you're not exactly 'right on the money'.

For comparison, modern common rail, electronically controlled, solenoid-actuated injectors inject as many as 5 times per stroke, with variation in quantity and duration.


As an example, we had a 6 cyl Volvo T/A engine that we bought, supposedly rebuilt, that started OK, but that always smoked and overheated easily. When rebuilding/updating it in an effort to resolve those issues, I tore it down without noting any specifics about the timing; gears are always marked from the factory. Or so I assumed.

Turns out there were some marks, but they appeared 'homemade'. Not having anything else to go by (the Volvo manual was mute), I reassembled the engine using those marks, set the timing as specified by Volvo with a dial indicator, fired up the engine on blocks. Virtually no change, with all new cylinder kits, rebuilt injectors, fuel pump, and rebuilt head.

Since the only thing I was unsure about was the gear train timing, that was the logical place to start. Without going into details. I finally determined the pump drive gear was 4-6 teeth off (couldn't be sure which). Set it at 5, put the front cover back on, timed the pump to factory spec and tried again.

Started instantly with no glow plugs, didn't smoke at all. Put it in the boat and wound up timing it 'by ear', setting it for the best combination of hole shot and top speed.

[Compass790]

Originally Posted by Wotname

Compass, are you referring to the valve cam lobes or the bolt on injector pump actuating cam lobe?

The camshaft lobe. Think YSB has same pump as yours.

[Compass790]

Whoops sorry i didnt answer yr question properly. I only measured the v/v cam lobes not the pump cam lobe when rebuilding engine
Aiming at the wrong target. Will measure the two different engine pump cam lobes soon. Lucky you questioned me.thanks

[Wotname]

Originally Posted by Compass790

Whoops sorry i didnt answer yr question properly. I only measured the v/v cam lobes not the pump cam lobe when rebuilding engine
Aiming at the wrong target. Will measure the two different engine pump cam lobes soon. Lucky you questioned me.thanks

Thanks.

FWIW, a couple of pics of the YSE8 IP lobe.

[Diesel Bill]

Hello all, While I haven't been able to be on the forum as often as I would like these days, I had to get in on this subject.
( working on the boat and home repairs) I just saw this today. The one thing that pops into my noggin right away is the many differences between old style mechanical injected and the newer electronically controlled diesels. It seems that reducing emissions is the key factor nowadays. Remember the V.W. scandal. Older mechanically injected engines were pretty tolerant of fiddling with timing and would still run O.K. if timing was off a bit.
The newer electronically controlled engines can be chipped and double stacked to change injection timing and fuel delivery way beyond whats possible on the older mechanical injected engines and return to stock for smooth idle.
I have a 96 2500 Ram with the older 12 valve Cummins engine. Iv'e shimmed governor springs for 700 RPM increase and adjusted rack movement for max travel for faster beginning of fuel injection. Some other slight mods to increase power.
My friend has a newer 24 valve engine in his truck. All he did is add a couple of aftermarket controllers to his and turn it into a H.P. and torque monster. His truck makes approximately double the power of mine. Yet we still get the same M.P.G.as each other.
Someone on here earlier mentioned NOX. That's oxides of nitrogen. Better known as smog. Dialing back on timing reduces temps so less NOX created. Remember the factory hotrods from late 60's and early 70's. The glorious horsepower wars with all the big 3 manufacturers participating? It was NOX that killed off the eary muscle cars by gov. decree. 400 plus horsepower engines knocked down to 180 H.P. Lower compression and retarded spark timing killed all the big engines then the fuel embargoes buried them.
That's when I switched over to diesel engines thinking they will never put black boxes on them. Ha Ha, the joke was on me. So I was dragged kicking and screaming into the world of electrons and algorithms. I still can rebuild Holly's and Q-jets in my sleep but the computers can do things electronically that I can't do mechanically. Altered fuel injection timing is only a part of it. Now you can have your cake and eat it too.
Much more to say but I forget what it was. Off to the bathroom again. Every one have a great day.

[atoll]

Originally Posted by Wotname

Good to know!

Which injection pump does the YSB have? The YSE Yanmar one (simpler one) or the Bosch YSM one?

i think it is the same as the YSE,pretty sure it is not a bosch

[vic008]

If management asked for a 540hp engine, can the engine engineers design thaat on paper, or do they end up with something 50hp either side , that they then need to tweak?

[Shawn67]

Originally Posted by jimbunyard

The whole range of difference, 15 degrees, only accounts for 1/24th or about 4% of the compression/power stroke; as others have noted the engine will likely run if you're anywhere close to the specified timing. Your ears and nose could be the best tools to determine the ideal setting for your application.

When I swapped out the pump on my MD7A I initially had timing off by 1 gear and didn't originally realize it. Engine still ran but I had the pump turned all the way in one direction to get it to run. After correcting the gear alignment I ended up timing it based on ease of starting and amount of smoke. Turned out the easiest to start location also produced the less smoke so I called it good. Of course this is on a slow NA 13hp engine that typically ran at about 1600rpm. On a much higher strung engine this probably wouldn't be enough.

Shawn

[Shawn67]

Originally Posted by Diesel Bill

It seems that reducing emissions is the key factor nowadays. Remember the V.W. scandal. Older mechanically injected engines were pretty tolerant of fiddling with timing and would still run O.K. if timing was off a bit.
The newer electronically controlled engines can be chipped and double stacked to change injection timing and fuel delivery way beyond whats possible on the older mechanical injected engines and return to stock for smooth idle.

Common rail diesel certainly opens up a whole new world and the torque they can produce is way beyond what a mechanical system can do. For those that don't know common rail diesels don't just have one injector pulse per cycle, they can have many. Instead of dumping all the fuel into the cylinder at once they spread it out over time to keep the burn happening longer to keep cylinder pressure high even as the piston is moving down and increasing volume. That is where their crazy torque comes from.

Totally know about the VW scandal. I bought my 6 speed TDI 2 weeks before the news broke. My car already had diesel exhaust fluid injection and VW was just *dumb* about all of that with that engine. My car has had both stages of the emission repair done and it drives better than it did before the repair. More power down low and MPG hasn't declined. I think VW was just trying to get away with putting cheaper, undersized emissions equipment into their cars. The NOX catalyst was replaced as one part of the repair. For the earlier engines they have to add a full DEF system to get them compliant.

Shawn

[Compass790]

Originally Posted by Wotname

Thanks.

FWIW, a couple of pics of the YSE8 IP lobe.

Yea Wotname, YSE8 or B injection pump cam lobe measured 41.5mm. The YSM8 is 46mm as jim Bunyard guessed. Doesn't answer your question but just putting the record straight.