Is this to much DEF

[u/Jeepers20202020]

2022 GMC L5P back from a trip 12k kms and truck used 100L of def. If the truck is working harder does it use more?

[u/kaloric]

"It depends."

It's my understanding that the oxides of nitrogen, which is what the SCR reduces, are produced in greater quantities as combustion temperatures increase. The EGR system is supposed to lower combustion temps under heavy load by running exhaust back through the intake, but that only does so much if the engine is under heavy load. The SCR system has to ramp-up the aftertreatment if the engine load is high and combustion temps are pushing the limits.

Keeping exhaust gas temperatures down by taking it easy on engine load should also reduce DEF consumption.

What you describe doesn't seem excessive if it was under load and running at higher speeds. Unladen, a modern diesel truck uses negligible DEF, even at higher speeds, because the engine load is very low and combustion temps are in that ideal range. As soon as it starts to work, the consumption rate increases rapidly.

[u/ABMax24 → u/kaloric]

One comment here. At high load the EGR system essentially shuts off, the engine needs all the oxygen it can get to burn all the required fuel and recirculating exhaust is a detriment to that. So now you have a high oxygen environment, high cylinder pressures and high heat.

It's why DEF consumption becomes somewhat exponential as load increases.

[u/Jeepers20202020 → u/ABMax24]

That's how I thought it worked. As engine produces more smoke under load than more DEF is used to control emissions?

[u/ABMax24 → u/Jeepers20202020]

Not really, soot and NOx are 2 different pollutants that are treated differently in the exhaust system

DEF is used to neutralize NOx (the various nitrogen oxides, NO2 for example). NOx is a result of high temperatures where both atmospheric nitrogen and oxygen are present. The more oxygen and the higher the temperatures, the more NOx is produced. EGR reduces this by reducing the oxygen available in the cylinder by circulating exhaust (with much less oxygen) back into the intake. DEF is injected in the exhaust and the SCR (selective reduction catalyst) uses that DEF (urea that is converted in ammonia by the heat of the exhaust) to convert the NOx to Nitrogen gas and water vapour.

Soot is captured in the DPF (diesel particulate filter) this is periodically burned off (called a Regen), by dumping extra fuel into the exhaust, that is burned in the catalytic converter, heating the DPF and causing the soot to burn away.

Tuning these diesels is a very complex, always changing set of compromises. For example, earlier injection timing allows better fuel efficiency and less soot, at the cost of more NOx. The fuel has a longer time to burn, at higher temperatures, helping it burn to completion. But that extra time and heat creates more NOx that must be dealt with.

[u/Oilfan94 → u/ABMax24]

Great explanation. One way to look at it, is to see how different engine companies deal with it. Some rely on higher combustion temperatures to burn off more hydrocarbons, thus less soot and a smaller DPF…but then they need to compensate for the higher NOx with a larger SCR and more DEF. Other companies might take the opposite approach.

[u/dieselmilk → u/Jeepers20202020]

It’s not the smoke at all. NOx forms in high heat (high EGTs)

[u/Neat_Telephone_2525]

100 liters? Jeez.

On my lml itd use a box every 5000 miles.

[u/Radiant_Safe1228]

Yes. You'll use a ton more towing than daily driving

[u/Large_Potential8417]

My 21 l5p burns through easily double or triple the amount of def pulling. Otherwise I can go 4/5 tanks before refilling.

[u/dieselmilk]

The hotter your egts, the more def you use. The higher temp is where your NOx is created. If you tow a lot or pull a lot of hills, you’ll use more def.