Lambda Sensors......

[CHRIS CLARK]

Can anyone give me a clear and concise account of the exact workings of the said sensor.This is probably a Peter C or Rob W question!

 

TIA Chris

[Jason Plato]

Chris ,

Lambda sensors sit in the exhaust and constantly provide a measure of the air/feul ratio whilst the engine is running , this measure is fed back to the management system .

Obviously under varying throttle openings / accelaration this ratio is constantly changing , but under constant loads or at idle the management system uses this information to maintain a stoichometric mixture ratio ( optimum ) @ 14.7:1 ( I think ) .

This maintainance of an optimum mixture ratio is known as "cloosed loop running " .

When setting up or "mapping " an engine the information from the lambda sensor is used to set feul air ratio under various conditions , commonly set a little rich under load to ensure a degree of saftey and to ensure max power .

The problem we have with caterhams is that we only have a lambda sensor in the one cylinder exhaust so the settings for the other 3 - which may have slightly different requirements due to exhaust design , inlet tract length , combustion chamber shape design etc , are a matter of educated guesstimate .

 

WARNING : dont run your engine with a lambda sensor on Lead replacement feul as the additives will "contaminate" the sensor and effectivly kill it !! .

 

Dave J

 

 

[Tony Martyr]

The sensor measures the oxygen concentration in the exhaust gas flow (lambda is the ratio of the quantity of intake air over the theoretical air requirement to achieve complete burning of the fuel). If you take the unit out of the pipe all you see is a porous ceramic bulb that is covering two electrodes made of Zirconium Oxide (I think). At high temperatures the conductivity of these electodes change and generates galvanic voltage that varies with the oxygen level. The signal from many units is rather non-linear and pretty useless under the critical temperature. Newer units have intergral heaters (the Bosch unit on the Caterham k series is a 4 wire heated unit)

Standard emission tests include a minus 8 Centigrade cold start sequence to make sure the engine management strategy minimises the exhaust emission of unburnt fuel before the lambda sensor starts working. By some mechanisms unknown to me lead in petrol stops the sensor from working

[Rob Walker]

The Lambda sensor or O2 sensor produces a voltage signal that recognises the amount of unburnt oxygen in the exhaust. The Lambda sensor is essentially a bettery that generates its own voltage. When Hot at least 250 C the Zirconium Dioxide element in the sensor`s tip produces a voltage that varies according to the amount of oxygen in the exhaust compared to the ambient oxygen level in the outside air. The greater the difference the higher the sensor`s output voltage. Sensor`s range from 0.2 volts (lean) to 0.8 volts (rich). Perfectly balanced or "Stoichiometric" fuel mixture of 14.7 parts air to 1 part of fuel gives an average reading of around 0.45 volts.

 

Sensor`s output can be checked using a 10K ohm digital multimeter. * CAUTION * never use an ohmmeter on a Zirconium O2 sensor in an attempt to check the sensor because doing so will damage it.

 

Typical life of a Lambda sensor is thought to be about 30K to 50K miles in a road car. Tuned K`s running aftermarket EMS and crude maps could eat them in far less.

[Peter Carmichael]

When Hot at least 250 C the Zirconium Dioxide element in the sensor`s tip produces a voltage that varies according to the amount of oxygen in the exhaust compared to the ambient oxygen level in the outside air. The greater the difference the higher the sensor`s output voltage. Sensor`s range from 0.2 volts (lean) to 0.8 volts (rich). Perfectly balanced or "Stoichiometric" fuel mixture of 14.7 parts air to 1 part of fuel gives an average reading of around 0.45 volts.

---

 

Because of the abrupt change in output voltage, it is almost impossible to observe the lambda reading at the 0.45 volts mentioned. ECUs use a strategy of alternately reducing and increasing the fuelling to observe a clear reading of the low and high voltages. This is because a reading of 0.2 volts does not mean "lean". It means "lean or at stoich more or less". Just as 0.8 volts means "at stoich more or less or rich". The only way the ECU can know where the fuelling is, is to observe the change. The fuelling cycles to switch across the stoich change about once a second.

[CHRIS CLARK]

Thanks chaps.

 

The one I have is from a 1400 'K' and has 4 wires from it.Does that mean it has a pre heater? At present my 'K' Caterham (pre cat) does not use a sensor for its MEMs ECU. When it gets upgraded (sorry Peter), will it be better to fit this if only for initial mapping and then removed once set? (I believe this is your chosen approach Rob?).

 

On a similar tack, a friend has his Lambda set in the pipe just after the joins for the 4 into 1. I presume this is better for overall set up, but it looks untidy (in my opinion!)_as it is permanently attached. Would he do better to:- a)Tidy up the installation, b) Save the life of the sensor by removing it & blanking the hole? Would the sensor just need unplugging or what? Is the Closed Loop adjusted multi times a second or just at initial mapping?

 

Lots of question.gif but TIA; Chris

 

[Rob Walker]

Chris

AFAIK the sensor should be located at a common point as close to the engine as is possible as it works best when Hot. As you rightly say two of the four wires are for a heater, this ensures that the sensor can function in all conditions. My standard Caterham arrangement is less than ideal being mounted only in the fourth branch of the primaries. Most aftermarket EMS`s can run closed loop but are usually set up to run open loop if the engine is to run without a Cat. I think this is mainly as a result of the rather crude way in which the engine is mapped invarably set up rather rich for safety. Therefore if you intend to run open loop you may remove the sensor after mapping, it will last longer not being clogged up with soot.

[CHRIS CLARK]

Cheers Rob!

[Peter Carmichael]

Rob's right.

 

I had my exhaust made up with a Lambda boss in both the #4 primary and in the collector, so I could have it optimum for the mapping and move it back for closed loop.

 

I run open loop however.

 

The fuel economy on part throttle is shocking, which is down to how the mapping was achieved.

 

Watching Dave Walker doing the mapping, the Lambda was only an advisory input. He watched it closely in case it dropped into the lean region at any stage in the production of the base map, but he was more concerned with the power output at each site, trimming the fuel for maximum power (pointless fuel-wasting exercise at part throttle). I have since carried out some real road research to determine how much these part throttle sites could be leaned out using the Lambda probe.

 

The boss in the collector feels like a wasted effort now.

[CHRIS CLARK]

OK Peter, #4 primary only it is!