I am trying to understand what makes a diesel engine start easily when cold, and I can't get it right when it comes to injection timing.

After a bit of reading, it appears that P7100 pumps will do better cold starts with their stock timing. Too much advance is said to make for difficult cold starts. The explanation is that in order for the fuel to light up, it should be injected close to TDC when the compression heat is maximum. Stock timing being 12.5-13.5 degrees BEFORE TDC, therefore more advance means farther from TDC and more difficult to light the fuel. Seems logical.

Now, VE pumps are known for starting easier in cold temps than Ppumps, and the reason behind it is the KSB feature which provides more ADVANCED timing when cold...?????

So, is advanced timing BETTER or WORSE for cold starts???

Despite VE's KSB being mechanically different from a P7100's static timing, in the end I believe that they produce exactly the same result, that is an injector starts spraying fuel at some precise time... it's just the link between that timing and cold starting that's not clear to me.

I'm not wanting to be **** bout this, just want to really understand what is going on. I'm going to be a professional diesel mechanic soon, and you can never be good at what you do unless you KNOW what you're doing.

 

That's an interesting observation. There can be many reasons for this. I don't know what the reason is but consider these variables.

1) The iron quenches the heat pretty fast and at 13 degrees the piston is slowing down a lot maybe additional compression is offset by quenching. Is the VE retarded more then 13 degrees and needs advance due to TDC quenching? Does the ppump continue to exhibit better cold starting with even less advance? I don't know.

2) The ppump probably makes more line pressure and better atomization so there is less ignition delay from the start of injection to the start of combustion. The VE pumps do create slightly more combustion knock indicating increased ignition delay or advanced timing or both. In the ppump era, high line pressures and fine atomization were being used to control emissions. VE uses timing to control emissions more effectively and probably does not need the line pressure. The expense to buy the ppump to make the line pressure was over $1,200 per pump, I was told buy my Cummins Engineering friend.

3) How does an advanced VE cold start compared to a ppump, with stock settings? Which is better stock?

4) Is the compression and cam identical on the 2 engines?

 

I'm no engine builder, but the way I think of it is that when using the starter to turn the motor over, its moving pretty slow, a few hundred RPMs. Even with high compression, if the fuel is sprayed too far away from peak cylinder pressure/temp it won't light as good. If you get that motor spinning at a few thousand RPM, that injector popping (or beginning to pop) further away from peak cyilinder pressure will work in your favor, as it's not an electrical circuit like a spark plug (very fast) but a definite duration of fuel squirting into hot air, that takes a certain amount of time to combust...I don't know if that makes sense to anyone else but thats how it goes in my mind.

 

The VE pump will always start easier than a P Pump due to dynamic timing advance and full fuel at start up. It was due to EMISSIONS that the p pump was designed to start without full fuel etc. Because of this a stock P pump smokes less at start up.

There is more to it than just the KSB/timing advance.

 

What you guys say makes a lot of sense to me. First, I get that the fuel needs to be sprayed close to the time of max pressure/temp in order to light up. Second, because the VE delivers full fuel at start-up. I was discussing this subject with my future boss, (a seasoned diesel mech) and he said that providing full fuel at start-up (sometimes called ''excess fuel starting'') helps to raise the effective compression ratio by occupying a part of the cylinder's volume with the fuel.
At one time I owned a 1978 International 125E tracked loader with a DT239 engine that used ''excess fuel'' (no glow plugs or heater of any kind) at start-up. It always started with a cloud of black smoke. It sure wasn't very emissions-friendly... but effective. The injection pump was a Bosch VA, the VE's predecessor IIRC.