For info the nozzle holes are anything from about 0.002" up depending upon spec and application (Inches that is-- Sorry I am old fashioned Brit)
The object is to inject high pressure fuel into a refined atomized state into the combustion chamber (if direct injection) so that it mixes evenly within the swirl of incoming air and burns evenly. This contributes to the delay or lack of it within the combustion period of the three phases of injection taking place. The discharge pressure is carefully calculated to mix under the high combustion pressures present the cylinder without causing dense pocketing of fuel within the air charge or eroding the piston combustion chamber edges.
The delay period gives the diesel engine its distinctive knock.
Indirect diesel engines generally have a pre-chamber (like older MB 614 units) the injection is less atomized since a different form of combustion takes place. In fact the nozzle tip has a different needle and seat form.
As previous mentioned by previous contributors it is essential that the needle lifts and snaps back on its seat with an abrupt cut off of fuel.
If needle and nozzles seats are eroded or the seat is poor, internal spring is weak, internal injector pipe /component exfoliation build up in the sac, fuel will dribble after snap closing causing soot build up on the nozzle exterior and a smoky exhaust.
Obviously it is essential that a line pressure is maintained in the injector pipe of older diesel engines so that stored fuel in the pipe is ready to inject without any delay by loss of pressure by dribbling.
On many diesel injector pumps of the older style the delivery valve has a sharp edge on it seat shoulder to add to the instant cut off fuel.
Measured by pressure drop on an oscilloscope it looks very much like the raster wave on a petrol engine contact breaker ignition system.
Typical pressures are about 1.5 Atm for this type of engine.
Now out of fashion to comply with modern E standards and CI power systems are mostly common rail these days. However the smoke emission from the older engines was largely counted large particles of carbon soot which was less harmful to humans and not embedding themselves deeply into lung tissue.
On the other hand the common rail system has only short injector pipes say like a Sprinter or none at all in some applications. Pressures are generally speaking much higher and so are the injector nozzles which can have multi stage injector opening to reduce the noise from delay. The snap closing is conducted by electronic/hydraulic control action. Typically the engine rail pressure is anything between 300 atm at idle to 1500 atm full power again depending upon spec /application. Hence internal erosion and exfoliation can be a problem with these injection systems.
Although some manufacturers are dismissive about these types of engines having a direct effect upon human health; medical reports suggest nano particle exhaust deposits are being deeply embedded into lung tissue especially of the young and old. These are on the increase especially in city and urban environments and my daughter has also noted a distinct increase in tissue contamination of cadavers lungs dissected of late. Certainly cause for concern if you work on modern diesel engines like me.
Food for thought
Dennis
zedcars
