Air Fuel Ratio (from old site)

<TABLE border=0 cellSpacing=0 cellPadding=0 width="100%"><TBODY><TR vAlign=top><TD>Air Fuel Ratio

contributed by Ian Simpson
What it means and measuring it.


See Boards thread

It's on the old site and you have to log in there first. But as of typing this message only committee can do that as only they have unexpired memberships.

Basically, that thread says buy yourself a wideband Lambda kit and use that to measure you air fuel ratio.

And don't waste your money on a narrow band Lambda kit.

Thread from April 2004.




Ian S RTOC Profile https://www.rtoc.org/boards/images/bu...iend-small.gif





Air fuel ratio, what is it and why is it important ?

Article by Scoff, including quotes from Ian S and Mart, 2004.


What is AFR ?

AFR, or air fuel ratio is a measure of fuel entering an engine for a given amount of air. Its a ratio, so when you see somebody talking about a 12:1 AFR they are refering
to the air/fuel mixture having 12 parts air to 1 part fuel. AIR:FUEL. A lean AFR is a high percentage of air, a rich AFR is a low percentage of air.


Why is it important ?

AFR is critical for the proper and reliable operation of any internal combustion engine. It gets much more important when you are talking about forced induction,
so supercharging, or in our case turbo charging. At low engine loads (idle, cruise, over run) a poor AFR will not generally harm anything in the short term but instead
it might harm drivability and/or fuel consumption. The real problems can occur at full throttle (wide open throttle, or "WOT"). If an AFR is too lean you can cause a lot of
damage to your engine: blown head gaskets, melted pistons and cracked liners being the common failures. Too lean an AFR will induce detonation, leading to the afore
mentioned results.


How do I measure AFR ?

To properly measure AFR on our turbocharged engine's we need something called a wideband AFR meter. Common narrowband systems are not suitable because they do not have
the ability to read the ranges we need to see what our forced induction engine is doing. Narrow band are generally only suitable for normally aspirated motors, and then they are
generally only used in closed loop idle and cruise systems to keep emmisions and fuel economy fixed.
Wideband is typically able to read from about 9:1 AFR upto about 22:1 AFR, and so is ideal for tuning and monitoring forced induction engines.
Lots of companies offer wideband systems - tried and tested units include Innovate's LM-1 and LM-2 hand held units, Zeitronix units, AEM units and units and kits made
by Tech Edge. They all tend to use the same Bosch LSU-4 wideband exhaust probe, so accuracy of all units using this probe should be the same. The Bosch LSU-4 probe is
capable of quickly responding to changes in AFR, almost instant to the eye.


What should my AFR's be ?

The desired AFR varies according to engine load, RPM, tendency to knock (detonate) and many other things. But, it can be simplified, as follows:

Idle: AFR should be 14:1 or leaner (so 14 or more). Too lean and the engine will not idle smoothly. 13.9:1 equates to about 1.5% CO.
Sometimes a richer idle is required in some engines, typically those with lairy camshafts that have a lot of overlap. Creating a richer idle in these engine's can
smooth their idle out quite a lot.

Cruising: Normal driving, light throttle and cruising you should aim for around 14:1 again. 14.7:1 is where most factory cars are tuned to, but this is a compromise on
torque in order to obtain better fuel efficiency. 13.5:1 would be acceptable, much richer than that and you are wasting fuel. Much
richer than 12.5:1 and you are not going to be doing your piston rings and bores any favours either. At about 15:1 or leaner the engine will start to feel "flat" (a lack of
torque). At about 16:1 or leaner it will generally not run at all, or be very hesitant.

Full throttle (on boost): This is where you can cause the most damage if you are not carefull. If you had less than, say 20psi boost in your R5 GTT you should be aiming
for AFR's of 12:1. Beware, much leaner than that and you are asking for trouble. Much richer than that and you will be making less than optimal power. On higher boosted
engine's you'll want to go richer than 12:1, aim for 11.7:1.


Finally, I've bought a wideband kit, where do I mount the sensor ?

The sensor needs it's boss welding into the downpipe somewhere. Normally it should live at least 6" from the turbo outlet, so part way down the downpipe in an R5 GTT is fine.
The sensor should also be mounted such that it leaves the downpipe at some angle above horizontal, in other words the wire from the sensor should point roughly upwards, and definitely not downwards in order to prevent the possibility of moisture collecting inside and damaging parts of the sensor.

Care should be taken when fitting the sensor to not subject it to excessive shock, ie, from dropping on a hard floor or possibly hammering the exhaust with the sensor fitted, as this can break the ceramic heater within the sensor.


Ian S says:

Really there is little or no point in using the narrow band type with the 5GTT. I fitted and experimented with one and with a 1.3mm main jet on a standard car it was off the
top of the scale all the time. If I adjusted down the carb so the reading was on the scale the car was slower. And the idle was weak and rougher when set to a leaner mixture
according to the Lambda sensor, it was best a bit rich.
So i didn't bother to get another when I sold that one with the exhaust.
Wide band it has to be. They read a wider range of air fuel ratio so you can see how rich it is for best running.
As far as sensor location goes, do you have one with a heated element? If yes than at the bottom of the down pipe is OK as the heat is more stable there. The sensor needs to be
at about 600°C all the time. The heated ones stay at that but hot gas will take it higher so causing a big output error.
But the unheated ones need to be near the manifold or they dont work.
3 or 4 wire will be heated.


Mart says:

Wideband & narrowband refer to the actual lambda sensor being used. Narrowband are cheapo sensors that do the job good enough for ecus on modern cars; ie, they measure the fuelling around the stochiometric ratio (14.7:1 afr) so that when the afr starts going lean (+14.7:1) fuel gets dumped in, and vice versa when the afr goes -14.7:1. It's crude, but it works.

The problem is, is once you go a couple a units outside stochiometric the narrowband sensor can't differentiate between lean/mega lean and/or rich/mega rich conditions, so for tuning purposes they're not much good. When used with a display or a dvm, n/b sensors are ok for giving you a rough idea that the fuelling is leaning off (or richening), but won't give an accurate account of how much the fuelling is leaning off (or richening). This is where a wideband sensor comes into its own.

A wideband lambda sensor basically works the same as a n/b (in fact it uses a n/b sensor), but measures the afr's in more minute detail due to having a built-in O2 pump cell/heater arrangement (details are on the net if you wanna know more) which helps produce a faster response/sampling rate and more importantly, is more accurate over a wider range away from stochiometric, hence it now can differentiate between rich/mega rich, or lean equivalent, conditions.

The only downside to w/b sensors is that you need to use a controller (uego) to control the heater via feedback signals from the pump cell, thus it's like a tiny processor
working away, hence the cost of running a w/b setup is more than that of a n/b setup (the w/b probes are at least 4 times more expensive than std n/b's for starters), but then if you want tuning accuracy...







See also: https://www.rtoc.org/boards/showthread.php?t=6031 for details about a Tacho input fix for the Innovate LM-2.


</TD><TD width=15></TD><TD width=200>https://www.rtoc.org/articles/i/autof...14_150x112.jpg

A bit rich










</TD></TR></TBODY></TABLE>