Author Topic: 23W6 Field Measure/Recall  (Read 11883 times)

Offline dfsGTD

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Re: 23W6 Field Measure/Recall
« Reply #20 on: 26 February 2018, 14:47 »
 Excuse my ignorance but what I don't understand about the whole thing is that I fill up at the same diesel pump as drivers of other makes of car and get about the same mpg but my GTD is "polluting" more than non VW cars. Where does this extra Nox etc. come from?

Offline fredgroves

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Re: 23W6 Field Measure/Recall
« Reply #21 on: 26 February 2018, 15:34 »
Quote
Q. What is the difference in combustion between a diesel engine and a petrol/gasoline engine?
A. In a petrol/gasoline engine, a mix of fuel and air is injected into the chamber. This is compressed and then ignited by a spark plug.
In a diesel engine air is injected into the cylinder, and is compressed by around twice as much as in a petrol/gasoline engine. This compression generates heat, so that diesel fuel burns spontaneously when it is injected.

Q. Why do diesel engines produce more NOx than petrol engines?
A. Diesel engines operate at a higher temperature and pressure than petrol engines. These conditions favour the production of NOx gases. The quantity depends on the volume and duration of the hottest part of the flame.

Q. Why are diesel cars more fuel efficient than petrol/gasoline cars?
A. Diesel fuel produces more energy for a given volume (diesel has a lower calorific value, but a higher density than petrol/gasoline). Also the higher combustion temperature in a diesel engine makes it more efficient. Heat engines can generate more useful work if they operate at higher temperatures.

Q. How do you reduce NOx emissions from diesel engines?
A. By lowering the combustion temperature, typically by Exhaust Gas Recirculating (EGR). Some exhaust gas is cooled and injected back into the combustion chamber. There is less oxygen in the exhaust gas because some has been consumed by previous combustion, so there is not as much to feed the flame. The exhaust gas also has a higher heat capacity than air, so it takes longer to heat up.

Q. Are there any other consequences of using EGR?
A. Yes, there is a downside. As the combustion temperature drops, so does the power, and the fuel economy.

Q. How can you remove NOx from exhaust gases?
A. There are various techniques, depending on the applications, although a lot of effort goes into designing burners which reduce NOx emissions in the first place.

    Selective Catalytic Reduction (SCR) is the most common method in diesel vehicle exhausts, but it is expensive so isn’t use in small cheap vehicles. There are various proprietary blends of ammonia and urea which can be injected into the exhaust flow. These react with NOx gases over a catalyst, which turns them into harmless nitrogen and water.
    Selective Non-Catalytic Reduction (SNCR) – takes place in ducting where the temperature is about 1000°C (1800°F). Urea or ammonia is injected, and the NOx gases are reduced to nitrogen without the need for a catalyst.

Most larger oil burning cars use AdBlue which is using SNCR... the Golf doesn't. The Passat does...

But as you can see from the above, as fuel efficiency increases, NOx also increases. Fuel efficiency directly related to CO2 emissions.... so as you save fluffy kittens sat next to the A272, you also drown people in the Pacific Islands...

 


Current: Mk8 GTI DSG, Adelaides, DCC, HUD, HK, Winter Pack, Rear Camera.. Aka "HMS Weasel"

Gone: 2017 Mk7.5 GTD,manual, NavPro
Gone: 2014 Mk7 GTD, manual, NavPro, DCC