Oil & Fuel Diesel Timing

[Mark Page]

I've never thoroughly understood how a diesel engine times it's ignition. I know that the heat to ignite the fuel air mixture comes from the high compression ratio super heating the air from the intake stroke. When the piston reaches TDC (or so) the injectors spray the fuel into the combustion chamber and the hot air ignites the fuel forcing the piston down on the power the stroke. How does the injector know when to inject? What times it? Does it run off the camshaft? On a mechanical system what drives the injector pump, the crank? Why does the fuel have to be injected at such high pressure? How hot does the air have to be to ignite the fuel?
I'm sure I've got some of this mixed up.

 

[dodge man]

I can't answer all your questions, but I can answer some.

To further confuse you, some diesels are indirect injection. They inject not in the combustion chamber, but near the intake valve. A lot of diesels also have governors that control the injection pump, think tractors, generators etc.

Its not just the timing, like you would think on a gas engine, as much the quantity of fuel that is also controlled. On mechanical injection, its controlled by the pump and governor, its kind of preprogrammed and can't really change on the fly. This is common on older diesels and todays smaller diesels. A lot of newer diesels have a computer that controls the injection process, and there can be more than one injection in the combustion process.

Hope this helps some. I am also fairly new to diesels and you can't really compare them to gas engines, they are very different in terms of the combustion and injection process.

 

[bill50cal]

I've never thoroughly understood how a diesel engine times it's ignition. I know that the heat to ignite the fuel air mixture comes from the high compression ratio super heating the air from the intake stroke. When the piston reaches TDC (or so) the injectors spray the fuel into the combustion chamber and the hot air ignites the fuel forcing the piston down on the power the stroke. How does the injector know when to inject? What times it? Does it run off the camshaft? On a mechanical system what drives the injector pump, the crank? Why does the fuel have to be injected at such high pressure? How hot does the air have to be to ignite the fuel?
I'm sure I've got some of this mixed up.

the ignition starts when the fuel is introduced to the combustion chamber.mechanical pumps are driven a half crank speed like the camshaft. mechanical pumps operate like a camshaft to move plungers ( think lifters on valves ) and force fuel to the injectors at high pressure to make them open and as it is timed and operates like a camshaft that is how it is at the precise time also an advance mechanism is also incorporated in the pump to move the timing just like an old gas engine ign. distributor. the governor and throttle control work together to control engine speed by how much fuel is put into the cylinder to set the engine speed. the high pressure is needed to atomize the fuel so it will ignite, ever seen how you can drown out a fire with a large splash of fuel by suffocation? the higher the fuel pressure the more complete and cleaner the combustion because of better atomization. most of the older mechanical systems run in the 2200-2500 PSI range while the newer common rail runs over 20,000 PSI and with computer controls fires the injector multiple times.

 

[Mark Page]

Dodge Man.
If I understand you the fuel is injected at the bottom of the intake stroke, not at the top of compression stroke.
MP

 

[Chilly807]

Mark,

On a mechanical injection system, the injection pump is driven by the engine gear train, or in some cases part of the engine camshaft. The pressure is so high because the injectors fire either directly into the combustion chamber, or a pre-combustion chamber. Each element of the injection pump (one per engine cylinder) is timed to each other by design, and the pump itself is timed to the engine crankshaft to ensure fuel delivery at the correct time.

Typically the fuel injection begins at about 20-30 degrees before TDC on the compression stroke. There is a short time delay, or lag, before the fuel begins to burn. It will usually ignite and begin to build pressure a couple of degrees after TDC, and will continue to burn for most of the power stroke, delivering a sustained push on top of the piston.

Fuel timing may be variable either mechanically or electronically, normally variable speed engines have variable timing, although this isn't always the case.

The air-fuel ratio varies widely depending on how much fuel is being injected, unlike a gas engine that stays relatively constant at about 15 parts air to 1 part fuel.

The diesel has no butterfly valve in the air intake, it can have as much air as it can either suck during the intake stroke, or as much as a supercharger can stuff in while the intake valve(s) are open. The governor controls the fuel injection pump so it only injects enough fuel to maintain the speed the governor has been set for, either as a constant speed engine, or variable speed as in most tractors.

A simple governor has a set of flyweights that rotate as the engine runs. When the engine slows down (as in a load being applied), the flyweights sense the drop in engine speed and act on the fuel rack (pump) to increase the amount of fuel delivery to bring the speed back to it's set point. If the load decreases and the engine speed increases, the governor senses that via the flyweights, and decreases the amount of fuel being delivered.

Unlike a gas engine, which is largely self-limiting due to the volume of air and fuel which it can take in, a diesel will accelerate very rapidly, therefore all diesels must have a governing device of some sort.

If you've not noticed before, try setting the hand throttle on a diesel tractor as you approach a hill. You don't have to move it at all to maintain the same speed as you go up the hill, unless you're overloaded in which case it won't matter what you do anyway. It may drop slightly, this is a phenomenon called speed droop and is inherent in all mechanical governors due to their design. Hydraulic or electronic governors can be set at a given speed and maintain it very accurately (often within one or two rpm) throughout the engine's rated load capacity. A tractor doesn't require that fine a speed control, so the cheaper-to-build mechanical governor is more than adequate.

A diesel starts with maximum fuel delivery in most cases, it's why you get that puff of black or grey smoke at each startup. If there is a fuel limiter involved, it usually limits fuel delivery at start-up to about 40-50% to avoid smoke and over fueling. Turbocharged engines usually have these to limit the amount of fuel injected until there's enough air boost to burn it effectively.

I hope this helps, if you have any questions feel free to ask.

Sean

 

[Mark Page]

Thanks Sean. Well put.
Mark

 

[workinallthetime]

to add to your confusion most modern diesel engines are capable of multiple events during one "cycle". this is why diesel engines have become so much more quiet in recent years. the isuzu diesel has injectors that are capable of 7 events during one cycle, they dont use them all at this point but may in the future. Most of whats on the road today are using 1-3 events such as 1 pilot injection to start the cumbustion in a the area of the cylinder desired and then a main injection for the "boom" and then a post injecetion to help evacuate the cylinder and help keep emisions down. the first you fellas probably heard of this was on the 01 gmc durimax, called "pilot injection". this engine uses a 32x or 64x (cant remember) crank signal to tell the ecm where the engine is so it can tell the ficm to deliver fuel to which cylinder. the fuel pump/high pressure pump runs off a gear in the front of the engine but is not timed like the old 6.5 which had to be set perfect.

 

[GaryCrowell]

An older mechanical diesel works quite differently than a newer common rail engine.

On a mechanical diesel like most tractor engines the injection pump is the heart of the engine. The injection pump is what controls the timing of the ignition. It's ran off of the crankshaft by a set of gears so it's timing is in coordination with the crankshaft. An injector is nothing more than a "pop off valve", it opens when the pressure in the line from the injection pump reaches a high enough level to cause it to fire, that pressure is typically about 4500 psi. At a preset time before TDC of the compression piston stroke the injection pump will send a pulse of fuel to the injector which will fire when the proper pressure is reached. The pressure is that high because it has to be able to flow and atomize the fuel in a short amount of time, lower pressure means less fuel flow ability and poorer atomization. On most older diesels you actually adjusted the timing of the injection by loosening a couple of bolts that hold the injection pump in place and turning the pump slightly, rotating it clockwise or counterclockwise to advance or ****** the timing just like you'd do with a distributor on a gas engine. On newer engines the timing is controlled electronically.

Newer common rail diesels (think of all the new pickup diesels) operate totally differently than the older diesels. These don't have an injection pump, they have a high pressure pump who's only purpose is to keep a manifold (the "common rail") pressurized with high pressure fuel, typically around 20,000 psi. The injectors are no longer just pop off valves but computerized solenoids that fire when an electrical signal is received from a computer. That's the reason an injector is so expensive on a common rail engine, it's much more sophisticated than the older mechanical injectors. By using an extremely high fuel pressure compared to the older engines, the common rail system can utilize multiple fuel injection events per each piston stroke. These multiple events can be timed to decrease the noise of the engine, boost power, and control emissions. To make these very fast injection events happen you have to have a lot of fuel pressure, the lower pressure of the older systems wouldn't be adequate to keep up.

So in other words, in the old systems the timing was all done by the injection pump, on the new diesels it's all by computer.