7 replies to this topic

[Lukin]

Hey,
I know bugger all about engines, but I have a question regarding lambda. As far as I understand it, it is the normalised stoichiometric air/fuel ratio. In the last race we were getting pretty low values (arouns 0.75) which seems very rich. What are the implications of that. Without knowing a great deal about it, it seems that the more fuel you have to remove from the cylinders on each cycle, the slower it will gain RPM(I told you I'm bad with engines) and the worse the throttle response will be. I read somewhere that peak power is around 0.9. Can anyone shed some light on this please.
Cheers.

[McGuire]

More info please. In your application does the sensor provide feedback to the engine control system, or is it simply part of your data system? Also, what is the bias voltage of the sensor circuit?

[Lukin]

Hi McGuire, it's a part of our data system. I don't deal with the engines, I just figure I should know something about it. Sensor info can be found at:

http://www.motec.com...cts/plm/plm.htm

Hope that helps, if not I can get some more details. Cheers.

[Lukin]

Also http://www.moteceuro...mv?rsn=4&rpn=22

[Greg Locock]

Most engines are limited in power by the amount of air they can pump. So, in order to maximise the power utput, you try to use all the oxygen. There's some chemical theory or other (or common sense) tells you that if you want to maximise the use of one part of a binary reaction, you need an excess of the other part, in this case fuel. Whereas if you want to burn ALL the fuel, ie high efficiency, you'd need an excess of air.

Also you may be using excess fuel to cool the exhaust valve etc.

Also also you may be compromising the steady state lambda in order to improve pickup.

[Lukin]

Oh ok, cheers for that Greg.

[bobqzzi]

Is this a normally aspirated or turbocharged engine? A rough rule of thumb is that N/A engines make best power at around.87 (12.8:1), but as Greg pointed out extra fuel can be used to cool the exhaust valve.

Turbocharged cars are typically run richer as the extra gas is used to keep combustion temps under control. This will vary with many facors including boost level, cylinder pressure and fuel octane. We run our turbo street cars at about .79 at full boost on pump gas.

I believe the Motec allows running with the A/F under closed loop control, and this is an excellent feature for a race car. You can determine optimum A/F on the dyno then simply program the ratio you want and the lambda sensor will keep it there . This assumes an accurate wideband UEGO type sensor.

[McGuire]

The Motec system produces a true lambda value...if the readout shows a constant .75, the mixture is indeed rich, but how rich we can't say as that is the far limit of the sensor view with respect to 02 content. If the reading is .75 at WOT but varying elsewhere it may not be so far off, but if it seems locked down in that value region that indicates it's probably quite rich. Plug reading and other clues will fill in the picture.