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Air fuel ratio sensor or Lambda on diesel engines

Practical Diesel Tuning

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First of all stop thinking like a petrol head if you want to run a diesel. AFR or Lambda on a diesel is a complete different things as on an engine with sparkplugs. Remember this! A diesel dos not run on a defined mixture in order to get combustion going and it dos not have an active ignition source like a sparkplug.

AFR or Lambda is great for indication but please stop being wise telling it’s numbers as a guideline even between same engine models as any chance to the fuel system can make a big difference on air fuel ratio even pointing you in the wrong direction. We seen loads of smoke and loads of oxygen left on the AFR readings on the same engine, same as running almost Lambda 1 and no smoke making massive power so EGT and smoke produced and the feel of power first and AFR or Lambda as last and yes if you still run EGR or some other throttle valve on the inlet reducing your air intake to the engine giving you problems yes AFR or Lambda will indicate this on low to part load. Some common sense is needed on that part as well.

Back to basic on Lambda reading. Have a look @ the links and there specifications.

Some of them only read from 10 to 20 AFR missing out your engine oxygen left on low to part load while other claim numbers even a Bosch LSU4.9 can even read.





Miss reading your lambda numbers. If your device or ECU give you a average AFR you can be fooled thinking there is plenty oxygen left but why? There is a thing called trapped air volume or leak air volume.

Trapped air volume. It’s simple where no combustion took place and will not take place for example air outside the piston dome or air trapped inside the exhaust manifold due to valve overlap during valve changeover. Same things goes for leak air volume where outside air is sucked in to any leak in the exhaust system after the exhaust pressure wave has passed the manifold pulling a vacuum behind it. On most small engines the exhaust manifold is one part. On bigger engines it can be a 2 or 3 part manifold with expansion arias that after some abuse by hi EGT start leaking a bit.

If you use a hi sample rate on AFR reading you will actually see oxygen spots in your data log and it’s common on older engines to have more trapped air volume as on the last generation commercial used diesel engines a 1.2 Lambda can be done and on race engine we build some are able to do close to lambda 1 @ low end power and 1.1 on hi end power no smoke running over 430Nm per 1000cc displacement. So how to get there? Well or the most part the engine manufacturer will provide a very good last generation engine due to emission standards and restriction on fuel consumption. This is the diesel industries @ it’s best and hard for an outsider to get to this quality standard as on tolerances and material used are insane on there specifications.

For older engines. Well change the shape of the combustion chamber mostly resulting is a lower CR and start playing with injection timing even taking the pilot injection out while running some boost and find out what will happen on engine performance drop in EGR and fuel consumption and a significant chance in trapped air if your able to read this.

Hi there,

Can I use my readings of total air quantity and total fuel quantity and calculate my AFR or would these values not be correct.

I know that t calculate the AFR you need to take both air and fuel in mg/r.

The way we calculate is with air mass for the most part using a compressor flowmap as using a MAF sensor that can read over 0,5Kg/sec air mass is hard to find. We have ways to test this on bigger turbo to get an outline of the compressor performance but would not try this on a VNT as it probably will not survive this.

We know what fuel amount is entering the engine so we know the theoretically AFR and are able to compare these with a o2 sensor reading and we are able on last generation engines to get this reading within 0,1 points accurate on AFR.

For calculation always use the same as for example if air mass is in Kg/sec than you need to calculate fuel also in kg/sec as well. I take diesel fuel @ 40 degrees C and that will make 0,8Kg/litter.

Calculating engine power on the save site take 4Kw hour per litter diesel fuel.

We usually reply within 12hrs (often sooner)

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