Niet aangemeldRX8NL.COM
Forum Hulp Zoek Registreren Aanmelden
Omhoog Onderwerp Techniek en prestaties / Techniek / Air Fuel Ratio / Lambda and engine operations
- Van Rasputin (SuperForumposter) [be] op 23-09-2004 13:31 Edited 24-09-2004 18:54
This thread to explain in a few lines the basic principles of spark ignited (SI) engine (= petrol or gasoline engine) fuel calibration, in order for you to better understand what's going on in your engine / PCM and the data I will share with you on the CAN OBD thread. And I'll be glad to answer any question (ook in het Nederlands wanneer het mogelijk is).

---------- A few definitions first :

- AFR, Air Fuel Ratio : it's the ratio between the mass of air and the mass of fuel in the air-fuel mixture. AFR = M(Air)/M(Fuel). Mainly used in the USA.

- Stoichiometric (in short : stoich) operation : operating mode where ther is complete combustion of the fuel, without excess or lack of air. Stoich AFR for 95 RON lead free fuel is around 14.7:1. Therefore 14.7 kg of air are theoretically needed to obtain a complete combustion of 1 kg of 95 RON fuel.

- Lambda (greek letter) : it's defined by the ratio between the actual measured AFR and the stoich AFR. Therefore, if an AFR of 14.7 is measured in the exhaust gas, Lambda = 1. Lambda is often used in Europe.

- Lambda > 1 , AFR > 14.7 => lean mixture, with an excess of oxygen (air)
- Lambda < 1 , AFR < 14.7 => rich mixture, with an excess of fuel.

----------Usual operating modes in SI engines :

- Lambda = 1 (stoich) : this operating mode is needed for maximum exhaust gas cleansing by a 3-way catalyst. Indeed, whern the mixture gently remains around stoich (Lambda = 1), the catalyst can simultaneously reduce (as in chemical reduction - taking off oxygen ions) Natrium Oxydes (NOx) and oxydise Carbone Monoxyde (CO) and unburnt Hydrocarbons (HC) for an optimal aftertreatment. This operating mode is the default mode for idle and when at part loads, when one cruises or drives calmly.

- Slightly lean mode : gives a combustion optimised for fuel economy. Back then when our cars had carburettors, they operated at this AFR in cruise mode and at part throttle. Today, because of the 3-way catalyst gas aftertreatment and emission regulations, this mode is not used any more. Only DI (direct injection) petrol engines run in lean burn mode. It's good for fuel economy but bad for NOx emissions.  They need a NOx trap that momentarily absorbs the NOx and reject them during stoich or rich mode  for aftertreatment in the 3-way catalyst. It is indeed very difficult to reduce (= take off oxygen ions) NOx when there is an excess of oxygen.

- Slightly rich mode : give maximum torque and therefore max power too. Good NOx reduction, however high levels of unburnt HC and CO in the exhaust gas. This operating mode is only used under heavy load, when the driver requests a high engine utput, e.g. under WOT (Wide Open Throttle) operations.

All for now, your questions and remarks are welcome.
- Van Leo (Rotary SuperSpammer) [nl] op 26-09-2004 14:55
In de afbeelding in je OBD CAN thread (http://forum.rx8nl.com/cgi-bin/mwf/topic_show.pl?tid=112#fp) zie je een AFR meter, met de groene waarde op 1.
De gewone AFR kan het niet zijn (met een schaal van 0 tot 1.5) , en gezien de ideale waarde "1" op de schaal gok ik dat hier de Lambda waarde wordt getoond. Echter, bij de ca. 2800 toeren die je daar ziet, lijkt de motor wel een erg arm mengsel te krijgen (bijna 1.4). Dat zou weer niet erg fijn zijn voor de stikstofoxyde uitstoot waar je over sprak. Kun je verklaren wat we zien op de schermafbeelding?
 
- Van Rasputin (SuperForumposter) [be] op 27-09-2004 08:20
AFR is lean probably because the car is under deceleration and the decel fuel cut-off is operating (cutting off fuel on overrun to save fuel). So no fuel means the lambda sensor rests on its leaner clip = highest lambda value the system can measure). There is no NOx production as there is in fact no combustion at all!

Cheers,

Fabrice
- Van Rasputin (SuperForumposter) [be] op 28-09-2004 09:22
Part II : Close Loop / Open Loop Fuelling.

A mentionned above, for optimal cleansing of the exhaust gases, it is important for the engine to run within 3% of the stoich fuelling (lambda = 1). To achieve this accuracy, the ECU must get a feedback measurement of where the actual AFR (or lambda) is in order to amend the fuelling. It is achieved by the use of an Exhaust Gas Oxygen (EGO) sensor. There are many sources on the internet axplaining how such a sensor works. But just remember this : its output voltage (or current, depending on the sensor technology) varies with the AFR (or lambda). Often, these EGO sensor are called "Lambda" sensors!

Under Close Loop operations, the signal from the EGO is received by the ECU (or PCM). If the signal tells the ECU the mixture is too rich, the ECU will slightly reduce the fuel injector's pulse width. If it's too lean, the ECU will provide an longer injection pulse. This calcualtion is done several times per second, with a high precision.
In the past, only "switching" EGO sensors were used. These "narrow band" sensor could only work around lambda = 1 and could not prodvide any useable information about the AFR outside this narrow AFR band. Therefore, engines would only run in Close Loop at lambda = 1, i.e. at idle and part throttle and cruise operations.
Once the driver requested max torque (Wide Open Throttle), the system would switch to Open Loop operation and the ECU would not receive any feedback of the AFR the engine ran at. Therefore, variations could be observed from one engine to another engine of the same specs due to difference in sensor and actuators, or due to ageing of the engine hardware.
Today, thanks to the use of "wide band" EGO sensors, the ECU can get a feedback signal from the sensor under a wide range of AFRs. The RX8 is fitted with such a sensor that enable an accurate control of the fuelling in almost all conditions, including in rich WOT operations. Other engines, like the Audi's direct injection FSI use a similar sensor to run in lean conditions, where accurate control of the fuelling is crucial!

It will be interesting to see, with the CAN OBD tool, where the Renesis engine runs in close loop and in open loop.
Omhoog Onderwerp Techniek en prestaties / Techniek / Air Fuel Ratio / Lambda and engine operations

Powered by mwForum 2.29.7 © 1999-2015 Markus Wichitill