Wed 17 Feb 2010 20:07 What does the MOT lambda test mean? - tyro

My Berlingo has just gone through its MOT, and I was studying the emissions test data, trying to make sense of them. I can understand the CO and HC ppm levels (though am interested that while the CO levels are the same as last year, the HC level has jumped from 5 ppm to 9 ppm, and am not quite sure what to make of this.) However, I don't know what the lambda figure means. As last year, the figure is 1.01, which is within the limits of 0.97 and 1.03. But what should I make of the fact that it is 1.01 rather than 1.0? And what would I make of it if it was 0.99? {Probably more suited to Tech Matters ------> kick } Edited by Dynamic Dave on 18/02/2010 at 01:06 Reply | Report as offensive | Link

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Wed 17 Feb 2010 21:15 What does the MOT lambda test mean? - RichardW

Lambda is a calculated rather then directly measured value. It's used to demonstrate that the engine is running at near stoichiometric ratio, which the cat needs to operate properly. eg you could pass the other emissions if the engine was running lean, but then the cat would not be operating properly. Small variations in lambda are not to be worried about, can just vary with a slight change in exhaust temp. Reply | Report as offensive | Link

Thu 18 Feb 2010 08:13 What does the MOT lambda test mean? - tyro

I sort of guessed that it was something along those lines. But that doesn't really answer my question. What, for example, is the difference between an engine that gives a reading of 1.01 and one that gives a reading of 0.99? >>{Probably more suited to Tech Matters ------> kick } There are many times when I really don't know when to post something in technical or discussion. In this case, I put it in discussion because it was not car specific, and was not actually a problem. And because I suspect that a lot of knowledgeable people in the Back Room rarely visit Tech Matters. Reply | Report as offensive | Link

Thu 18 Feb 2010 19:31 What does the MOT lambda test mean? - lee123

in simpler terms its a sensor in the exhaust either in the manifold or precat it reads the emitions coming through and will tell the ecu if the engine is running rich or lean and will either put more or less fuel/air for a smooth running engine, is this sensor fails itll be a rough running engine and will fail its mot on emissions Reply | Report as offensive | Link

Sat 20 Feb 2010 15:07 What does the MOT lambda test mean? - tyro

OK, let's see if I've got this right. A value higher than 1.00 basically means that in combustion, the ratio of air to fuel used is above what it is supposed to be, and a value lower than 1.00 basically means that the air to fuel ratio is below what it is supposed to be. Hence a values of less that 1.00 indicate poorer fuel efficiency, while values of over 1.00 indicate better fuel efficiency - but are associated with higher temperatures, which could cause damage to the engine. Reply | Report as offensive | Link

Sat 20 Feb 2010 19:47 What does the MOT lambda test mean? - Number_Cruncher

There are two very different uses of the word lambda on motor cars. One is the lambda sensor in the exhaust, which provides feedback to the engine management unit and tweaks the fuelling to keep the engine close to stoichometric. The other use of lambda is the value reported by the gas tester during an MOT, which is where Tyro's question comes from. As RichardW has said, the MOT lambda value is a calculated value, obtained from the ratios of the gases measured by the MOT station's analyser - it has very little real correlation with what the car's lambda sensor is reading. HC is unburnt, or partially burnt hydrocarbons. In gas analysis, these hydrocarbons can come from two sources. First, the engine may be worn and passing oil into the combustion chamber, which cannot be fully burnt. Second, the engine may begin to misfire, or suffer from only partial combustion of the fuel. This unburnt fuel registers as HC on the gas analyser. In formulae written about engine combustion, the greek letter lambda is used to signify the excess air ratio, and the higher the number, the weaker the mixture. However, if you have a cylinder which is partially or fully misfiring, the gas analyser will detect a mixture of gases which *wrongly* suggest a weakening of the mixture. I think that in Tyro's case, the HC increase for 5 to 9 isn't massively significant in itself, but, I suspect that the pairing of HC and lambda numbers suggest a slight misfire which will probably be sorted out at the next service. Reply | Report as offensive | Link

Sat 20 Feb 2010 20:10 What does the MOT lambda test mean? - tyro

Thanks for that very full answer, NC. I'll see what happens at the next service. And I'll have to try to remember to report back at the next MOT! Reply | Report as offensive | Link