The following is a post of Alastair's from another site. It is very relevent to this thread. I hope you dont mind it being posted.
As has been described elsewhere, The first small Car Diesel-Engines were Indirect Injection, Meaning the fuel was sprayed by the Injector into a small Ante-Chamber known as a Pre-Chamber or Swirl-Chamber, normally contained within the Cylinder-Head. This chamber was connected to the main chamber by a tangentally arranged narrow passage or by small holes as in the case of the Mercedes Pre-Chamber type.
This was needed to be done, as a huge amount of Turbulence is needed to effectively mix the air and diesel-fuel spray in the tiny amount of time to allow high speed operation and make for a very Progressive Burn, rather than a sudden explosion. If the fuel didnt burn at a controlled and progressive rate, it was found not to be possible to run the engine at high speeds without major noise and smoke emissions.--Something tolerable with Trucks in the '30's but Not cars of the '70's and 80's when Diesel Cars started creeping on to the scene.
There are generally Two Types of 'Pre-Chamber' or Indirect Injection engine. The Mercedes System, where a small spherical chamber is connected to the main chamber by a blind ended tube with several small holes set at right-angles to the piston axis. Contained in this spherical chamber, there is a 'Hot-Spot', formed by a small rod with an enlarged central section. This is in direct line of 'squirt' from the injector and allowed to run at very high temperatures--Around glowing orange heat This promotes a good vapourisation and ignition of the fuel and a really well controlled progressive + quiet burn....
The other system, developed by the Combustion Engineers, Ricardo's (Yes, The same guys of the Infamous 'Report'), developed in the '30's and called the 'Ricardo Comet' design is used by nearly every other car engine maker using IDi technology. This as mentioned earlier is a semi-spherical chamber, with the Injector at the top, mounted at an angle as to Not direct its spray down the throat of the communicating passage from the main combustion chamber. This small passage is set at an angle to the sphere, so as air is compressed in the cylinder, it will be forced into the pre-chamber with some force and at just the right angle to cause the air-mass inside the chamber to swirl at very high velocity, just like a whirl-wind
The lower half of the sphere containing the tangental passage, is made from a special autenitic Stainless-Steel and deliberately allowed to run at very high temperatures, by reducing its contact area with the actual metal of the cylinder-head into which its pressed (Its outer cercumference has a recess in it, below the face of the head...)
This promotes excellent mixing and vapourisation of the injected fuel as well as a good progressive burn.
As can be seen, With the IDi system, There is MUCH less likelyhood of Unburned Fuel making it the distance involved to the cylinder-wall, without first vapourising and burning away.
Consider the Direct Injection type engine. Here, the Combustion chamber is formed in the top or 'Crown' of the piston. Usually it forms a bowl sometimes with a flat bottom sometimes with a raised bottom Often there is a small ridge of around 1mm around the top of the bowl, an attempt to contain the combustion 'charge' and promote turbulence during the last few mm of upward piston travel, causing 'Squish' of the air between the head and piston, forcing the air towards the centre of the chamber. The Inlet-Manifold and valve-tract is often very carefully designed to cause 'tumble' of the air as its drawn into the cylinder through the open valve, to promote as much Turbulence in the air-mass as possible.
There are NO hot-Spots or anything like that to assist in the vapourisation of the injected fuel, as well as the fact that the piston-bowl runs at a very much lower temperature than a Pre-Chamber or M.B 'Hot-Spot', as the piston is made of Autothermic Aluminium Alloy and cooled quite effectively by the cylinder-walls and the lube-oil from underneath, flung up from the crank-pin and connecting-rod...
Being a large open chamber, in comparison to the Swirl Chamber types, with nothing much between the cylinder-wall and the injector there is a Huge likelyhood even on the best designs and everything perfect that unburned and or partially burned or 'pyrolised' fuel will hit the cylinder-walls
Another factor in favour for us 'Veggies' of the Venerable IDi is the type of Injector used with the IDi system. Generally, this is a 'Pintle' type. meaning that its rather like a needle-valve, where the needle protrudes slightly from the hole, in the face of the nozzle and during normal operation the movement of the needle lifting will keep the hole and needle pintle fairly clean of coke (Carbon to us Mortals!)
The Di system generally uses a 'Hole' type injector Usually, up to eight tiny, and I MEAN TINY holes are arranged in the nozzle tip. these are Very short passages, their inner end is closed off by the nozzle needle when its not actually spraying fuel. They are kept as short as is mechanically possible to reduce the amount of fuel that will slowly boil off and vapourise due to the heat of combustion just after injection has finished, which would otherwise raise smoke and emission levels.
Consider the effect that even a minute amount of Carbon build-up would have on this type of injector nozzle...It would completely disrupt the correct operation of the nozzle, and instead of a very fine spray, one or two holes could literally produce a jet of un-atomised fuel, just like a water-pistol....
So,--Why should I care if unburned veggy hits the cylinder-walls of my pride and joy...?
Just Two Words....
RING-GUMMING.........
When exposed to heat and oxygen Veggy-Oil will start to break down. It starts to thicken and polymerise and as time goes on, it turns from a sticky brown 'goo' into a hard black Coke/Carbon material. Just take a look at the frying-pan in your favorite Greasy-Spoon!
Any unburned veggy or partially burned veggy hitting the cylinder-walls will find its way to the Piston-Ring grooves on the next upward stroke of the piston. This veggy will collect in the gap behind the rings and the small clearances either side of the ring and piston. As time heat and plenty of oxygen are available, it breaks down and sets solid, sticking the ring in its groove, so it can no longer follow the irregularities of the cylinder-wall and seal the piston, as it was intended.
This ring gumming is slow and progressive, The speed at which it happens is dependent on Many factors The main ones being the temperature of the combustion-chamber and the condition of the injector atomisation, but as it progresses, gets faster and faster, at a weirdly logarithmic rate...
This causes loss of compression pressure gradually over weeks/months of use. which contributes to the secondary effect....Often the slow and progressive slight power loss may not even be noticed, or blamed on other issues, such as blocking filters or the choice of veggy oil or even the weather (No one wants to admit even to themselves there's something nasty growing in the Crank-Case!)....
(A Diesel Engine relies on the fact that when air is compressed, it gets hot, just like the effect noticed when pumping up a bicycle tyre. It depends on this heat for the ignition of the fuel. If for some reason there is a loss of air, say, from leaky rings, the peak temperature attained by the air at the point where the piston is at Top Dead-Centre, A.K.A. Top of the cylinder, the point of maximum compression pressure, the air will be cooler than intended, thus there is a very real danger of incomplete combustion, or even a complete Misfire, where the veggy wont burn at all, Pale bluish or White-Smoke in exhaust and maybe an unsteady engine when idling)
Not all the veggy is burned Particularly when cold, so this just adds to the supply of 'Ring-Glue'
As it progresses, the amounts of unburned veggy in the combustion chamber/cylinder, increase where it will pass to the lubrication oil by the operation of the oil control rings scraping it off the bores. The Oil Scraper/Control rings, being the lowest on the piston are last to be affected, The Top ring, which is the main sealing ring and subject to the full force of combustion pressure is first to go...
(Just as well the Oil-Control ring is last to go, or the engine could have otherwise end up in a runaway condition where it burns all its lube-oil in a minute or two and hits revs the makers would never have dreamed possible...The danger to life when driving a vehicle when this happens is better not even imagined, You CANT SHUT IT DOWN!....although I dont need to imagine it...Its happened many years ago to me....)
This ring gumming is Progressive and Accelerating Process, Thats why its so Insidious!
Continued in next post