In CA, trucks are (were) exempt from smog checks. Hopefully law enforcement will send those caught rolling coal to a date with the state referee.

They most definitely are not exempt - I've had to do the same smog checks on my (smog compliant) diesel 2006 F-250, my 36' RV, and my current '06 light duty truck as I do on my minivan or sedan.

You're right, you'd need an F450 to hit the 14k lbs GVW, not sure why I thought they were exempt.

Statistically nobody goes out of they way to defeat emissions devices on comparable gas vehicles. Sure some people will neglect to replace stuff that fails over time but nobody is going looking for emissions delete stuff for gas cars and trucks whereas with diesels even new truck buyers who wouldn't consider touching anything consider "tuners"

Why is that?

EGR is bled pre-turbo - disabling EGR increases the amount of exhaust going to the turbine

It's possible to tune a diesel without making smoke - normally this is done by leaving the wastegate closed for longer to make more boost and then adding more fuel.

Any smoke is too much extra fuel.

Maybe a bit anectodal, but the reason is that diesel emission devices are more complex and more prone to failure or require more maintenance (perceived as annoyance by the user).

Here is more explanation: with the diesel emission devices, you are trying to control soot and NOx gasses. For example the EGR (exhaust gas recirculation) system reintroduces a bit of exhaust gas back to intake to reduce combustion temperature to reduce NOx gasses. However, this means that you are introducing sooty gas back to the intake, where it messes with other stuff like swirl flaps.

Swirl flaps are there to create a small tornado inside the cylinder for more efficient mixing and burning. But these are mechanical flaps and, in the best case, they stop working when they soot up. In the worst case, however, they go inside the cylinder and cause catastropic damage, requiring an engine rebuild/replacement.

Then another emission device for diesels is the particle filter. This is a very fine filter to catch soot exiting the exhaust. The engine puts a bit more fuel every now and then to burn them off properly. But the engine needs to run under a certain load and operating temperature for this operation to succeed, and if not, it gets permanently blocked and you need to visit a garage to force it or to have it cleaned/replaced. This happens if you only drive short distances and never let the engine to warm up, or if you idle for too long, or if you always drive in stop and go traffic. And sometimes, the filter collects ash instead of soot and that can't be burned off. This happens when you use incorrect sort of oil in a modern diesel car or if the fuel is bad. The only cure is cleaning or replacement of the filter.

Then you have another emission device: the ammonia. Newer cars spray a bit of ammonia (AdBlue) every now and then on the exhaust gasses to reduce NOx emmisions. This adds one more thing to refill every now and then and the car reduces power if you don't comply.

Now compare this to the gasoline car emission device: the catalyst. This one fails if you overheat it, or if it goes too old by age. The overheating happens if the combustion is too rich (too much fuel for the amount of air) for a long time but the engine controller prevents it anyway. Catalyst is there to burn any unburnt fuel and turn carbon monoxide into carbon dioxide.

Some newer cars also have EGR in them like a diesel car AFAIK, but I am not aware of the relevant failures of that bit.

Without a turbo there is only minimal power gains you can get from computer changes. Diesel engines almost all have a turbo, gas engines almost never to. In addition some of the emissions stuff on diesels does cut efficiency by a significant amount, there isn't much gain in a gas engine if you are willing to ignore emissions.

Don't confuse the above with zero gain from custom tuning a gas engine, just that the gains are not nearly as great as you can get with a diesel. If you are running a gas car on a track you can get measurable results from custom tuning - but it isn't nearly as noticeable in day to day driving.

> Diesel engines almost all have a turbo, gas engines almost never to

In USA are humungous engines (5.0l +) without a turbo In EU are mini engines (1.0l ~ 3.0l) and almost always with turbo. Is it better fuel economy?

For gas engines, but this is about diesel which have very different properties.

Even when talking about gas engines, if you are towing (this is about diesel where towing is more likely than normal) your turbo is running all the time anyway and there is no savings over a bigger turboless engine, and the bigger engine will last longer. The bigger engine can also run at lower rpms which brings up power. In the end for a lightly loaded gas engine a small turbo is more efficient.

Some people with sports cars bypass for performance

But a gas car with no cats, tuned for maximum power is not really comparable to a diesel truck rolling coal. One might leave soot on its own rear bumper, the other covers everything behind it in disgusting soot.

Diesel and gas have different emission profiles and the regulations don't get evenly applied. For example, harsher regulations on oxides of nitrogen will impact diesels more than gas. As someone else eluded to, "soot" may not be the best proxy for "emissions".

True, there are plenty of invisible emissions causing chronic air quality issues, but um an over-rich diesel exhaust in your vicinity is rather acute.

There are tons of exhaust pollutants that are not visible.

Those are all exarcebated by the incomplete pollution of trucks rolling coal far more.

To say they are "all exacerbated" isn't quite accurate. Air/fuel mixture affects the various emissions differently. For example, going from lean to stochiometric will increase NOx but decrease CO.

Lean/Stoichiometric is one thing, you still can (in theory) have perfect combustion on that part of the axis. Running very rich will increase both NOx and CO, for example.

It's a bit more complicated than just air-fuel ratio. The primary mechanism in NOx formation is combustion chamber temperature, while the primary mechanism in CO formation is incomplete combustion of HC. If you look at most emission maps, the NOx is decreasing rapidly while CO is relatively flat or increasing. My main point is that combustion is complicated and not as simple as "just do this thing to reduce all emissions". It's often a balancing act, which is why European regulations may be more favorable to diesels than American regulations and the opposite is true for gasoline.

I definitely agree that within reasonable combustion regimens it's a balancing act.

However, when you run your fueling extremely rich like someone that is rolling coal, NOx is going to be higher than in any reasonable tradeoff, because if the irreversible damage to the catalytic converter. It will be far worse than any conceivable lean air mixture.

Ah, ok, I'm tracking your perspective now, thanks for continuing the conversation. Yes, I wasn't really speaking to the damage to the catalytic converter. But I think you may have it reversed?

Under lean conditions, the Rh2O3 layer in the cat reacts with the Al2O3 to create an inactive oxide. This degrades the ability for NOx reduction.

Under very rich conditions, unburnt fuel will combust in the catalytic converter, drastically increasing the temperature and greatly speeding up oxidation, which degrades its effectiveness with regard to all pollutants, including NOx, and oftentimes pretty dramatically.

High temps in the catalytic converter can sinter the precious metals, reducing the surface area of catalytically active surfaces, but I believe the effect is much more pronounced in CO and HC activation. The conversion of NOx is much more constant.

Ah, I was under the impression this wasn't the case, so you're probably right. Do you have a source I could refer to?

Internal Combustion Engine Handbook by van Basshuysen and Schafer. My edition is rather old (2004) so there may a newer one with different info as diesel and emission tech has changed a bit in the last 20 years.

Thank you! I'll take a look and try to find a more up to date source if possible.

Also generally speaking the volume of the motors, diesels are usually larger displacement engines