The purpose of these important items, is to supply a highly broken up, or atomised mist of fuel, at the precise rate and time required to achieve an efficient, quiet and smooth combustion of fuel.
It is important that the finest of sprays is employed, so as the fuel droplets being small, take the least amount of time to evaporate and form a vapour, and quickly are heated by the hot air mass within the combustion chamber, where combustion then initiates.
In common with most atomisers, this is achieved by forcing the fuel through a small orifice at extremely high pressures, the shape, size and opening pressure is carefully tailored to the demands of the combustion chamber and the use to which the engine is intended.
If the engine is to be reasonably quiet, the fuel delivery by the injector, needs to be slow and very well atomised at the very start of injection, so that the fuel will start to burn in the shortest possible time. Being that there is only a small amount of fuel present in the combustion chamber at this instant of time, means that the pressure rise is relatively slow, and combustion noise is low. The transition to the main delivery is fairly smooth, and progressive. All subsequent fuel supplied by the injector therefore enters an already burning mixture. This is achieved by the size and shape of the nozzle Pintle, in the IDI engine. These nozzles are known as Delay Pintle Nozzles, and are nearly always used with the Indirect Injection Engine, and have part numbers like, DN0S"D"299A.--The "D" in inverted commas, refers to Delay type nozzle.
"DN" "0" "S" D "299A"
"DN" referrs to Pintle type nozzle
"0" referrs to the spray angle, whether its defined as a number of degrees or not.
"S" refers to the size of nozzle body.
and "299A" refers to specific characteristics of that particular nozzle ( apart from this number its not possible to find the charaterictics of the nozzle from the makers)
Any letters before the "DN" refer to the maker, eg. B, C, K, R, V are all Lucas / Delphi maker codes, Bosch generally use just the basic number.
The Hole Type nozzle, used with Direct Injection engines, open very abruptly, so the delay has to be achieved in other ways, such as the design of the pump and high pressure system. Some DI injectors achieve the "delay phase" by employing two springs, One to open at a relatively low pressure, and allow the nozzle only to open a small amount, to deliver the initial fuel charge, and as the delivery phase progresses, a higher pressure is needed to open the nozzle further, which is controlled by the second spring. These types are found on older "first generation" DI car engines like the Perkins Prima used in Maestros and Montegos etc.
Its important to use the correct type number of nozzle with a particular engine, as its rate of fuel delivery, the delay and other factors have been chosen to optimise operation and give lowest emissions for that application.
Due to the makers not disclosing information on the specific characteristics of nozzles in their ranges, its not possible to choose a more suitable type for operation on Vegetable oils for a specific engine type or model, and the best that can be advised is to stick to the standard type, or a similar nozzle found in the SAME engine, used in a different application.
Increasing the opening pressures as done by many pioneers, has worked for them, but has caused other undesirable issues when tried on a variety of my own vehicles. I can find no basis where the increase of opening pressures will give a "finer spray"--The atomisation is dictated by the design of the nozzle to the greatest extent, and not an opening pressure increase of 10-20%, as is generally advised!
Increasing the opening pressures will effect the RATE of delivery, maybe give a quieter engine generally, however this will retard the effective point of injection and affect the torque delivered by the engine at low speeds, affecting drivability, more noticable with smaller engines. This can cause in some cases, smoke in the exhaust due to the now effectively retarded timing, particularly with the DI engine, which are more sensitive to timing inaccuracies.
Advancing the timing may remove the smoke from the exhaust, but the different torque curve from standard will remain.
Experiments in the reduction in opening pressure have given good results, on a variety of engines in starting from cold, drivability and emissions. probably due to Vegetable oils different viscosity properties.
Even the Pioneers of Vegetable Engine technology now recommend that the injector opening pressures are left at OEM recommendations, but they do supply alternative (Their own) nozzles....
Advancing the timing whether you leave the opening pressures as standard or not, helps with starting from cold, carbon and gum build-up at the nozzle face, and general smoothness of the engine as a whole. Injection Timing should be advanced around 3-4 degrees crank, but no more--doing so will cause possible overheating of the engine or nozzle and an increase in diesel knock.
The advancement in timing counteracts the increase in burn-time Vegetable oil and BioDiesel have Both fuels take longer to completely combust in an engine combustion chamber, which should be compensated for by timing advancement.
During fuel delivery, fuel does not flow in the normal way, At the high injection pressure pulses, the fuel is slightly compressible, and the fuel lines are slightly expandable. The fuel passes through the lines in the form of a pressure wave, expanding the high pressure line rather like the cartoon interpretation of water flowing through a fire hose. The speed of this pressure wave is dependant on the fuel itself, and the effective speed of sound in that fuel. Vegetable oils have different properties, most notably its viscosity, which appear to be slower than standard diesel fuel in this respect.