Hi folks,
Since a few days, i read a lot of document about diesel engines, combustion process and mosly on pre-chamber engine. I will post some quote of the documents i read. As stated, IDI has a better potential for combustion and fuel mixture process. The industry jumped on the DI engine because they didn't want to invested to solve the major problem on IDI engine, the heat loss of the pre-chamber. By beeing able to insulate the chamber, to keep the heat loss minimal, great gains can be acheived, better performance, less noise and better mileage.
IDI engines are fueled by self-cleaning, single-hole, pintle-type nozzles. The combustion process is too complex to be explained here, but my above-mentioned SAE technical paper No. 960058 describes it in more detail. In general, it is a two-step combustion process characterized by its speed and tolerance of fuel-system inconsistencies that allows operation of present automotive engines (such as the Mercedes IDI engine mentioned above) up to 5000 rpm. Combustion is faster and more complete than with DI systems, with more of the fuel being consumed even with lower amounts of air per cycle (lower A/F ratio) at the same smoke level. Since no swirl is required in the main chamber, high-efficiency directed intake ports can be used instead of the helical ports employed by DI engines, and more air is processed to provide higher volumetric efficiency with smoke-limited A/F ratios of less than 20:1. The combination of higher volumetric efficiency, reduced port-pumping losses, higher engine speed and higher combustion efficiency at lower A/F ratios produce higher power; typically, 10-15% more power at the shaft for similar-displacement engines. The indicated cylinder power is even higher, but two factors contribute to high thermal losses, which are detrimental to power output and fuel consumption. The first is the pumping losses in and out of the pre-combustion chamber and the second is the heat losses through the pre-combustion chamber walls. The technical world has concluded that these problems are unsolvable for small engines, and interest in pre-combustion chamber combustion has been lost, in spite of the fact that the overwhelming majority of pre-combustion chamber combustion characteristics are, for small passenger cars, far superior to those of the DI system. The Ricardo side pre-combustion chamber has remained unchallenged, except by some modifications that other researchers have performed including some work that I have done, as described in my U.S. Pat. No. 5,417,189, issued May 23, 1995 and my aforementioned SAE technical paper No. 960058. The only new application of a pre-combustion chamber system combined with four valves can be found in the new Mercedes-Benz DOHC family. Even so, the pre-combustion chamber and injector tip in this DOHC family differ very little from the 1927 Mercedes-Benz designs. Therefore, to continue enjoying all the benefits of pre-combustion chamber engines, while improving the fuel consumption profile, it is important, amongst other measures, to minimize the two main sources of losses; that is, pumping and thermal as exhibited by the current Ricardo and Mercedes designs. In reality, it is not required that they be eliminated completely. The reason being, as already explained, that the energy released by combustion is far higher than that of the DI system due to the more efficient burn. Therefore, the IDI system can tolerate some losses and still be competitive with DI; however, both sources of heavy losses must be reduced.
Other four-cycle, four-valve American engines from Cooper Energy Services, as well as from Caterpillar and Waukesha have also used pre-combustion chambers for many years, some as pure IDI diesels; others as spark-ignited gas engines. The latter are very popular in environments where low emissions are already closely regulated. With the trend towards the use of pre-combustion chambers, it has been predicted that newer, more efficient pre-combustion chamber designs will be required to minimize the pre-combustion chamber heat losses through heat transfer.
The need to keep the pre-combustion chamber as hot as possible has been acknowledged from the earliest use of the Ricardo "Comet" pre-combustion chamber in 1929. In the "Comet" pre-combustion chamber, the lower inserted portion of the pre-combustion chamber, called the "cup", is made of exotic heat-resistant material such as Nimonic and is designed to maintain an insulating air gap between its sidewalls and the cavity bored inside the head so as to reduce the heat losses. However, with the "Comet" pre-combustion chamber, the upper cavity is typically machined in the structure of the cylinder head and is prone to crack because of the high thermal gradient between the hot inside of the pre-combustion chamber walls and the cooler outside walls exposed to the cooling media compounded by the rates of firing pressure and maximum firing pressures as the fuel is ignited. To avoid this problem the design uses a water jet, typically drilled across the head, between the two valve ports (these engines typically being two-valve engines), both to cool the bridge between the valves and to impinge on the pre-combustion chamber's upper cavity. The upper-half of the pre-combustion chamber, therefore, not only suffers from the normal heat losses through its walls made of parent material exposed to the cooling jacket, but also has to cope with water being impinged upon it to avoid cracking the wall. In the process, it loses a very considerable amount of heat energy.
Some engines, made by Isuzu and others in Japan over fifteen years ago, upgraded the material of the pre-combustion chamber "cup" from Nimonic to ceramics, which has a far lower heat transfer coefficient; however, the top half of the pre-combustion chamber was not changed and still suffered high heat losses. Developments under my direction, using the lower pre-combustion chamber cup from Isuzu engines on an experimental Chrysler engine, proved that the engine not only reduced its fuel consumption by 4-5%, started faster, and produced less noise, but that it also burned faster and cleaner, allowing the injection timing to be retarded for reduced NO.sub.x, as well as hydrocarbons, particulates and smoke. Recognizing the fact that the main losses were still through the upper-half of the pre-combustion chamber ; a heat shield designed for disposition inside the upper pre-combustion chamber cavity. The heat shield is intended to minimize the high heat losses of the pre-combustion chamber at this location by increasing the total wall thickness and creating an insulating air gap between the shield and the parent-metal cavity. It has been calculated that such shield could improve the engine's fuel consumption another 7-8 percent and all the other combustion parameters as well, by reducing the heat losses.
According to the engineers, IDI engine have more potentiel than DI engine and i believe in this theory. It's a long process, time consuming, but i will invest some of my free time to try to improve and overcome the problem with heat loss in the pre-chamber. I already a couple of ideas in mind and im calling all diesel head to share idea.
IDI can kick DI ass badly :twisted:
Any suggestion, comments are welcome.
Marc/
