No curiosity kills the cat

I was manning the EEC stand at the recent Sandown Park MechanEx show, when a college tutor with six of his students asked me if I would show them the internal monolith of a catalytic converter and to explain to his students how it worked.

I did this and then proceeded to explain the importance of understanding the values from an emissions gas test. He interrupted me, explaining that he uses a simple rule of an MoT failure, stating: “If Lambda was between .99 and 1.01 and the CO and HC was just over the MoT limit, the problem was usually a faulty cat.”

I agreed that by fitting a new cat you could reduce the HC and CO to within the MoT limits, therefore resulting in a pass, but I then started to explain that his assumption regarding Lambda was misleading. I was left disappointed, as at this point he walked away, refusing to listen to further information!

Lambda calculation - The Brettschneider Equation, general principles and methods 
The Brettschneider equation is the de-facto standard method used to calculate the normalised air/fuel balance (Lambda) for domestic and international I&M inspection programs. It is taken from a paper written by Dr. Johannes Brettschneider, at Robert Bosch in 1979 and published in “Bosch technische Berichte”, Vol 6 (1979) N0. 4, Pgs 177-186.

In the paper, Dr. Brettschneider established a method to calculate Lambda (Balance of Oxygen to Fuel) by comparing the ratio of oxygen molecules to carbon and hydrogen molecules in the exhaust. The equation is a little complex, but is relatively easily calculated from the measured values of CO, CO2, unburned HC, and unconsumed O2 in the exhaust.As you see from the example on page 59, its a simple calculation!

As a result of the tutor’s theory I questioned several visitors using the same example, and was amazed at how many people had the same assumption regarding Lambda.

I know this can be misleading when looking at only the CO, HC and Lambda as required for an MoT report, but you need a four gas report to be able to make an in-depth emissions diagnosis, e.g. CO, CO2, HC, O2 and Lambda. These values are available from the majority of gas analysers on the market and can be printed on the MoT emissions report.

We all know that Lambda is a measurement of fuel/air (1 part fuel, 14.7 parts air). This measurement was calculated by Dr Johannes Brettschneider of Robert Bosch. Part of his paper is as follows:

CO @ .3, HC @ 203ppm and Lambda of 1.01, would show as a fail, as the CO and HC are just over the MoT limit of .2 & 200ppm respectively, however the Lambda reading is perfect.

Common mistake
A common error many technicians make when attempting to overcome the problem is changing the cat, as it has failed. Doing this could reduce the HC to between 100 > 160 PPM, and CO to <.2 which would now result in an MoT pass.

Unfortunately there is a fault and it needs correcting, otherwise it will result in the new catalytic converter failing again after just a few thousand miles.

How to overcome the problem
If you record all four gases in the example above, with the CO2 @ 14.2 and O2 @ .65 you would see that the O2 is too high. It should be <.2. The reason it is high is that air is entering the exhaust system between the front of the manifold and the first Lambda sensor.

Result
The Lambda sensor picks up the extra air, the ECU thinks that the mixture is lean and adds more fuel. Now there is extra fuel and air. If the fuel/air ratio is right, whatever the volumes it can still register a perfect Lambda.

Bad example
O2 @ .65, HC 203 PPM, CO2 @ 14.2, CO @ .3, Lambda of 1.01. Lambda is perfect. All other values are too high.


Good example
O2 @ .2, HC 0, CO2 @ 13.4, CO .1, Lambda of 1.00. All values are perfect. You can demonstrate this by going to www.smogsite.com/calculators. Scan down until you get to the Lambda calculator and enter some values.

As you increase the HC and the O2 you can still end up with a Lambda reading of 1.
The correct values are CO <.2 CO2> 13.5 HC <15 PPM O2 <.2 and all gas tests should be at 2500 rpm and at a temperature of 350 degrees.

I hope this explanation will show you that although Lambda is correct it does not mean that the air/fuel mixture is also right.

Emission the point

Why are the new vehicle emissions regulations getting tougher? 

I thought it was to reduce pollution. If it is a genuine commitment for all of the EU governments to reduce pollution, why are the EU MoT emission values so lax? In fact, why are there different regulations throughout the EU?

There are even different regulations within the UK – let’s take Northern Ireland as an example. Their emissions pass regulations are CO less than 3.5% and HC less than 1,200PPM. In England, Scotland and Wales the pass values are CO less than 0.2% and HC less than 200PPM.

The silly thing about this is that if you fail the MoT on emissions in these three countries, you could catch a ferry to Northern Ireland and pass. You can see with their limits, it is virtually impossible to fail! 

Also, their MoT certificate is valid all over the UK. In theory there should be a sliding scale of emission values within the MoT, for example, 3 to 5 year old cars, 5 to 10 year old, and over 10 years – with the toughest values for the newer vehicles. In my opinion, this should be a standard test throughout the EU.

False consciousness

Unfortunately, the general public look upon the MoT as a health check and if their car passes the MoT they believe it is in excellent working order. As we know this is not always correct.

If a vehicle only just fails its MoT on emissions, it is possible to replace the catalytic converter which could result in a pass.

This would appear to be the correct diagnosis, yet nine times out of ten it is not! There could be an overriding problem which needs rectifying immediately, otherwise the problem will reappear.

Fitting a new cat which is working at 100% efficiency, will just mask the problem. In time, the new cat will also fail and the chances are it would be detected in another twelve months at the following MoT. The easiest way to pinpoint the fault is to carry out a 4 gas test.

If the values exceed CO <0.2, CO2 >13.5, HC <15PPM, O2 <0.2, and a Lambda of 0.99 to 1.01, there will be a problem. Depending how the values exceed these parameters, will depend on how large the problem is, and by evaluating the 4 gases you can locate and rectify the fault, e.g. CO @ 9.48, CO2 @ 9.10, O2 @ 0.58, HC @ 533PPM, and a Lambda reading of 0.76.

This was a report given to me by a customer of R&J Doncaster. The aforementioned diagnosis points to a failure located at the intake manifold area, due to a lack of air accessing the combustion chamber, thus increasing the HC, which in turn has damaged the cat.

Know your enemy

Hydro Carbons are the greatest enemy of a catalytic converter. If they are greater than 40PPM, they will be damaging the cat, which will result in a deterioration of the coating and, in extreme cases, a meltdown of the monolith.

EEC is committed to an education and training programme within the industry to help technicians understand, evaluate and diagnose emission problems.

The introduction of type approval for catalytic converters has been a great leap forward and has created a more level playing field. We have invested heavily in our own wash coating facility in Denmead, thus ensuring we produce a top quality product for our home and export markets.