Thread: WoW: supermini class diesel cars produce more NOx than a 40 tonne truck!

Pat,
Grey Nomads towing caravans would account for absolutely zero consideration when it comes to worldwide motor vehicle policy.

NOx emission limits are set at a rate per km. It doesn't matter of you burn 2 or 20L to travel the distance the overall NOx will have to be below the limit.

The wash from all of this is that diesels WILL have to meet those targets in real world conditions.
The politicians in the cities of Europe want air that can be breathed.

s

NOx is formed by the combustion process when there is excess oxygen for the fuel to be burned.


big turbo diesel engines (especially those matched to autos) run a lot closer to the optimal stoiochoimetric ratio a lot more often than smaller engines. they also do sneaky stuff like turn off cylinders (which means no combustion in that cylinder so no nox) in a pattern while Idling so the cylinders that are getting fired fire harder so produce less nox, and when the vehicles braking the engine will often not be firing any cylinders.

and then they do the adblue thing.

As Dave has pointed out, small diesels can be made to the same emission standards as the trucks are - but it comes at a cost. No manufacturer is going to do that if everyone does not have to. And if they can meet the standards in test conditions that is all they are required to do.

As Dave also points out, the reason diesels usually produce more nitrogen oxides than petrol is that most of the time they operate very lean compared to petrol engines. Which is why they are more economical - and tend to have lower CO2 emissions.

John

Originally Posted by roverrescue

Pat did you read the linked article?
Real world trucks in the city are meeting euro6 NOx outputs of 0.08g/km
Small cars are emitting up to 40 times that.

This article has NOTHING to do with efficiency just NOx emission.

s

It has everything to do with efficiency,less fuel going in means less pollutants coming out. Pat

Originally Posted by PAT303

It has everything to do with efficiency,less fuel going in means less pollutants coming out. Pat

Not really. Hydrocarbon and especially carbon dioxide pollutants depend very largely on the amount of fuel burnt, but nitrogen oxides depend mainly on combustion temperature - which increases as combustion efficiency increases, and to a lesser extent on the amount of fuel burnt - really on the amount of air heated above a critical temperature. (Before any exhaust treatment of course)

John

Originally Posted by PAT303

It has everything to do with efficiency,less fuel going in means less pollutants coming out. Pat

waving the broad axe, and just working the simple perfect world scenarios for petrols yes for diesels no.

petrols always run the optimal fuel air ratio, too lean and they get hot, do internal damage and produce huge amounts of nox, too rich and they run poorly and produce huge quantities of HC and particulate matter. power control is achieved by varying the total mass of the fuel air going into the combustion chamber

Diesel run lean until they are at full power conditions where they run just a touch rich. The mass of air drawn in is constant and only the fuel changes the less power the engine is making the more nox its making.

turbo diesels run lean till they get the boost wound up then keep adding fuel until all the air is used up. the mass of air in the initial stages is constant until the turbo kicks in and then your doing the more air more fuel thing. trailing the throttle and you're making nox, not at full boost/power settings and most likely you're making nox.

Overfuelling a diesel at full load causes nox production to stop because theres no free oxygen left to hook up with nitrogen in the combustion chamber. Similarly going to full engine braking in a diesel means no combustion process so once the combustion chamber temp drops off theres no nox production, you're just pumping air.

its hugely more complicated that that but thats the basics and its easier to control pollution on larger diesels that operate at a constant (or near constant) load condition and dont have large rev ranges.. (most trucks have an 800-1200rpm operating rev range compared to something like 5000rpm for some small car diesels hell a tdi300s got something like 3000rpm worth of useable rev range)

one of the early generation (think mechanical injection) diesel engine setups that was one of the biggest contributors to NOX issues was generators running lightly loaded.

And just to complicate the question, nitrogen oxide emissions are only a problem in places where there are very high concentrations of cars together with weather conditions that lead to stagnant atmosphere conditions.
Compared to this, CO2 emissions are a concern anywhere on the planet.

John

I hear what you are saying about large diesels running at optimum loads. But the tests referred to in the article that is this thread were done on trucks trundling around London. I can't imagine a truck punting in traffic is operating at optimum loading. More so that other poke jiggery plus adblue is ensuring that heavy vehicles meet 0.08g/km ... I think that it is impressive that the truck companies have taken NOx emissions seriously but car companies have not....