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I feel like the soot discussion got a bit dumbed down. Can we raise the bar a bit?
Soot is always produced in a diesel engines combustion cycle, if, as Greg Roles said, there is insufficient air. In the 'olden days', you could make any turbo diesel that was blowing smoke smoke less (and make more power) by increasing the airflow.
This is all well and good up to a point - however. As you increase the 'spare' oxygen in the cylinder, the nitrous oxide production also increases. This means that there is a trade off happenning - on the one hand you are balancing soot, and on the other nitrous oxides.
A diesel makes the most power per unit fuel when the air/fuel ratio is about 16:1. At this ratio, it will also be producing some soot (quite a bit actually!).
Once the air fuel ratio goes up past 25-30:1, the nitrous oxide emissions can really climb aswell (when the engine is making power - Nox emissions at low loads are not high enough to be a problem)
So, to tune a diesel engine (at maximum power output) well, you need to balance the air fuel ratio with the capabilities of the emissions system components in order to produce the most power that you can whilst not overproducing Nox. Nox is the one diesel emission that is very hard to do anything much about. This is why new diesels have dpfs at all - basically because the tunes need to be slightly air deficient so that the nox emissions are low, and hence the need to catch and burn the soot particles in the DPF.
What the heck does all this mean? Well, as the power output of the engine (for any given displacement) increases, the likely production of Nox also increases (because the engine is running more and more boost and higher combusion temperatures and pressures). So, you can still have a clean diesel engine without a dpf, but you cant make it super powerful. Likewise, you can have a super powerful diesel, but its hard to make it clean
Soot is always produced in a diesel engines combustion cycle, if, as Greg Roles said, there is insufficient air. In the 'olden days', you could make any turbo diesel that was blowing smoke smoke less (and make more power) by increasing the airflow.
This is all well and good up to a point - however. As you increase the 'spare' oxygen in the cylinder, the nitrous oxide production also increases. This means that there is a trade off happenning - on the one hand you are balancing soot, and on the other nitrous oxides.
A diesel makes the most power per unit fuel when the air/fuel ratio is about 16:1. At this ratio, it will also be producing some soot (quite a bit actually!).
Once the air fuel ratio goes up past 25-30:1, the nitrous oxide emissions can really climb aswell (when the engine is making power - Nox emissions at low loads are not high enough to be a problem)
So, to tune a diesel engine (at maximum power output) well, you need to balance the air fuel ratio with the capabilities of the emissions system components in order to produce the most power that you can whilst not overproducing Nox. Nox is the one diesel emission that is very hard to do anything much about. This is why new diesels have dpfs at all - basically because the tunes need to be slightly air deficient so that the nox emissions are low, and hence the need to catch and burn the soot particles in the DPF.
What the heck does all this mean? Well, as the power output of the engine (for any given displacement) increases, the likely production of Nox also increases (because the engine is running more and more boost and higher combusion temperatures and pressures). So, you can still have a clean diesel engine without a dpf, but you cant make it super powerful. Likewise, you can have a super powerful diesel, but its hard to make it clean
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