Quote:
Originally Posted by Piwoslaw
So all diesels produce sub-micron particulates, DPF-ed engines let out the same amount (or slightly more) as non-DPF-ed?
I read that the hotter your engine (or the hotter the exhaust) is running the less particulates it produces. Unsure whether the reduction is the same for all sizes, or whether only certain sizes are reduced. The price is more NOx.
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All diesels produce sub-micron particulates (SMP). DPF use tend to result in more. Also, higher pressure injectors produce more SMPs by virtue of the much smaller droplet size they produce. To be exact, modern high pressure injection systems produce only about 20% by mass particulate mater that is larger than a micron. The rest is sub micron. Oxygenated fuel additives can also increase the amount of SMPs as larger droplets are broken up into smaller ones.
It is not so much that heat reduces particulates but more so that good combustion results in more heat and less fuel is turned into particulate mass. In other words, the particles are smaller and less massive though numerically the same. But, as you pointed out, this results in increased NOx.
When a fuel droplet enters a sufficiently hot combustion chamber it's surface starts evaporating and mixing with oxygen. Once the mix reaches the lower flammability limit, it starts burning. The middle of the droplet, not exposed to the oxygen depolymerizes and releases it's hydrogens resulting in a complex mix of carbon forms. This is referred to as nucleation. These particles can combine into larger particles. This is accumulation mode.
As Oil Pan4 pointed out, the deletion of EGR can reap huge performance gains. EGR and more specifically, cooled EGR causes the fuel droplet to enter a combustion chamber with reduced oxygen resulting in a lazy evaporation and combustion rate around the fuel droplet. The burnt exhaust gases also provide a buffering heat capacity sucking energy from the combustion reactions. This all reduces peak temperatures and the formation of NOx but has the adverse effect of increasing the mass of particles as the fuel droplet is not as well vaporized before it enters nucleation and solidifies. High pressure injection reduces the droplet size to begin with but it just means there are more particles produced, thus the bias to SMPs. If you could do away with the EGR, the greater availability of oxygen causes less massive (smaller ) particles and more power is available.
Note that a properly operating EGR system will reduce the flow of exhaust to the intake stream as engine load is increased as less oxygen becomes available to an increased mass of fuel. At lambda=0.8 what is generally regarded as the threshold for smoke to appear, the conditions are similar to those under EGR. The droplet finds less oxygen to combine with and burn and consequently, the particle mass increases. Thus the term "Rolling Coal" was coined as the smoke becomes very noticeable. It is not that there is an excess of fuel per se, but more correctly, the fuel is finding it difficult to pair up with an oxygen. Thus, it more readily nucleates and accumulates into the thick smoke we have all seen. The deletion of EGR really helps performance in the load ranges below lambda=0.8. This is where most driving is done and thus you can really feel the difference in added performance and fuel economy.
The above is a gross simplification of the process but it should give you a feel for what is happening in a diesel engine's combustion chamber.