AlpineRAM

Hauling: On a 24V there's no electronics box I'm aware of that will only change timing. They'll all change fueling and especially low boost fueling (as the AFC on the P-pump does).

Too much timing for a certain engine speed/boost situation will lead to the following: Fuel is injected into an environment that will not ignite the fuel, the small droplets will collide and stick together to bigger droplets before the situation is hot and pressurized enough to ignite the fuel. If you put all of the injetion before the appropriate situation you will have an engine that's running very rough and will sound much like a pinging gasser. If for example the first half of the injection amount is too early to ignite but the second half is just in time the flame from the second half will ignite the fuel from the first half. Now it depends on the droplet size the first half has collected to, and the rpm(You need more time to burn bigger droplets- square to cube relation roughly) . The droplet size is also depending on load and rpm, because the speed of the fuel through the holes in the injector is determined by the factors mentioned before. To get the total compression ratio and the temperature you'll have to figure some boost into the equation, intake air temp and intercooler efficiency. The higher the boost and the higher the air temp on the intake the earlier (in degrees engine rotation) the fuel will find an environment hot and dense enough to ignite. The trick is to start the injection of the fuel as early as possible so that it will ignite and to inject just as much as there is air to be consumed.

AlpineRAM




HTH

AlpineRAM

Haulin_in_Dixie

Ok, I believe I have it now, at lower rpm of max torque, it has lower boost, and causes an incomplete burn because of it. As the rpm increases and boost goes up, there is more compression and heat to cause a complete burn. This would also be why the advanced timing gives a bit more noise at cruise with very light throttle.

Thanks guys, just trying to get it straight in my mind.

AlpineRAM

It's even gonna smoke more and have worse economy under light throttle with too much timing.
HTH

AlpineRAM

HOHN

Good explanation, Alpine.

The trick is injection fuel as soon as the cylinder conditions will support complete combustion.

This is what makes timing on a diesel so much more critical (and delicate) then on a gas engine. On a gas engine, excessive timing will be loud, and cause pinging and such-- BUT THE FUEL WILL BURN. That's because the engine isn't counting on compression for combustion. When the plug fires, it will burn. With the gasser, you just set timing to where max cylinder pressure occurs at roughly 15 degrees ATDC.

But with the diesel, overadvanced timing means the fuel won't burn-- it will smolder and puddle, as Alpine described.

Now, there's another wrinkle to add to the mix-- TURBOCHARGING!

The presence of boost (and how much of it there is) changes the point at which the cylinder is "ready" for the fuel. The more boost, the earlier cylinder temps hit the point where diesel will combust instead of just oxidize (which is SOOT).

So as cylinder pressure comes up (from boost), the earlier you CAN begin the injection event. But you also want to RETARD timing as boost comes up to reduce the peak cylinder pressure spike that can take out headgaskets.

Anyway, you can see that timing is such an important thing-- and that it REALLY needs to be variable to account for different cylinder conditions-- pressures, temps, quantities of air, etc.

It also reveals just how LIMITING a dumb pump like the P-pump is in that it cannot vary the timing.

jlh

AlpineRAM

Hehe, now let's add some EPA requirement to the mix: We want low peak pressures and temps to get NOx down, we want early timing and abundant air to get soot and carbon monoxyde down and we want early timing, high compression, low egts to get economy up. Stir well with the requirements mentioned before, add some limits like head gasket stability, need for a broad usable torque curve, and parts and processes that should be affordable in mass production and voila! That's why there are some 2 or three (or maybe even more ) technicians at work developing on the diesel engines of each supplier.
Now let's add that a P-Pump can only regulate the amount by retarding the end of the injection event (the start is fixed to a certain crankshaft position), the elasticity of the inj. lines as a retarding factor that increases with rpm and the inertia of the injectors etc, the variations of the swirl in the combustion chamber over the rpm, boost, backpressure variables, variations on initial droplet size, variations in fuel quality, cetane rating, viscosity and temperature while being injected

AlpineRAM

RowJ

The followup to that is...delayed turbo spoolup, reduced combustion produces less pressure to drive the turbine. Creates a viscious circle! Have heard this happening with the Hot Rod VP44's.

RJ