What do the MOT emissions signify?

Ian Anderson · Post by **Ian Anderson** » Sun Jun 21, 2009 11:11 am

Can someone Explain the Emission Readings?

2 days back I had the Dax through it's MOT inspection for the first time as it is now 3 years old.

It is a 1990 rover engine 3.9l running the Bosch "hotwire system" with lambda sensors in. The heads have been extensively worked to improve breathing and the whole engine balanced by John eales of Britihs Racing fame

Now the specifications it must meat on emissions are based on the engine year so must meet

Max % CO of 3.5 acheives0.31%
Max HC 0.12 acheives 927ppm

The engine has been idling for about 15 mins with fans cycling on and off before the test so she was HOT

So what do these readings say about engine tune - lean rich or OK?

Thanks for any advice

Cheers
Ian

Darkspeed · Post by **Darkspeed** » Sun Jun 21, 2009 1:19 pm

LOW CO figure means that the fuel mixture is getting burnt well with plenty of air available for complete combustion.

High HC figure means that you may have a plug/lead breaking down allowing unburnt fuel into exhaust.

Could be a tad on the weak side and the High HC meaning to weak to burn on one - ? blocked injector

kiwicar · Post by **kiwicar** » Sun Jun 21, 2009 3:56 pm

Hi Ian
Want an engineers understanding of the chemestry?
In my understanding the fuel is a load of hydrocarbon molacules as the name surgests basically hydrogen bound to carbon atoms roughly in an overall ratio of 1.6 H to 1C depending on mix in the fuel. Air's main components are Oxygen and nitrogen. right with the right theoretical mixture of air and fuel (about 14.7 to 1) you should get as an output heat and water and carbon dioxide (H2O and CO2). The production the CO2 ocours via the production of CO so the reaction is HCn plus 02n to plus H2O plus COn then more O2burns with this to H2On plus CO2n. if the reaction does not occour at a high enough temperature and presure then the reaction does not ocour compleetly and you get excess CO and O2 in the exhaust (mixture is right but ignition too late or not enough compression or exhaust valve opens too early). If this in turn is beacuse the mixture rich and then you get excess CO and HC in the exhaust.
If the burn is too hot you get the Nitrogen in the air combining with the oxygen this time you get carbon and oxides of nitrogen in the exhaust. if the mixture is too lean and the burn to hot (probably with detonation) then you will also get CO and oxides of nitrogen in the exhaust. (this time the CO2 degrades in the presence of the extra oxygen in the hot exhaust manifold to CO).
That is what I understand happens
someone will correct me I'm sure
Best regards
Mike

Ian Anderson · Post by **Ian Anderson** » Sun Jun 21, 2009 4:38 pm

Well I pulled the plugs and they all looked very si,ilar "coffee brown on the bnt electrode - about what I would expect - so no obvoius "one failing" but what a pain = have to remove my dip stick tube to get the 2nd plug out on left bank and the exhausts are trouble too - thank goodness for UJ on the socket!

Any other ideas?

Ian

Pocket rocket · Post by **Pocket rocket** » Sun Jun 21, 2009 7:06 pm

Just for the avoidance of doubt - your HC limit is 1200 ppm so your reading is well within the limits.

Given the state of your plugs, you have nothing to worry about

Darkspeed · Post by **Darkspeed** » Mon Jun 22, 2009 11:37 am

There are two area's to read on a plug.

In rough and ready terms.

The nose and electrode - the colour of which indicates that the plug is the correct heat range for the engine

The outer ring land on the thread - this indicates the state of combustion within the engine as it reflects the combustion chamber.

You can have an engine burning running very rich and with a perfect plug nose insulator colour but completely black outer land. You need to read the plug with the knowledge of the plug heat rating in mind.

Andrew

Ian Anderson · Post by **Ian Anderson** » Mon Jun 22, 2009 12:11 pm

Thanks for the info - all greatfully received

From a laymans point of view
If the limit is 3.5 I would expect to see a number between say 2.5 and 3.25 and accept this as "normal" but to get a reading about 1/10 to the max seems strange to me

On the other reading Max 1200 mine 927 - about right for an engne with 4000 miles on the clock (75% of max)

So what would the readings be on a standard 1990 Rover V8 running hotwire - would mine equate?

As I say all a huge learning curve to me

Cheers
Ian

Darkspeed · Post by **Darkspeed** » Mon Jun 22, 2009 1:14 pm

Have a read of this

Puma Racing Combustion

Saved me further tryping

Andrew

Ian Anderson · Post by **Ian Anderson** » Mon Jun 22, 2009 4:11 pm

Andrew

Many thanks - starting to make a bit more sense now

So it looks like it is lean from the reasoning (Makes sense too as the heads are ported and big valves on a standard fiel map (modded fiel map gives more power but a bugger around town at 1500 - 1900 rpm))

Odd "fluf" at idle could also mean it's lean (or poor electrics).

Perhaps I should just bite the bullet and get rid of this JE 102 cam that only really works over 2500 rpm

Ian