Running rich /lean

I haven't got any links to hand but here's why...

For optimum combustion (the best bang from resources available), the mixture of air / fuel needs to be just right. The ideal mix is 14 parts air, to one part fuel. When the engine runs too rich, there is more than 1 part of fuel per 14 parts air, and some of the fuel ends up not getting burnt. This causes a lower temperature combustion and is less of a bang, producing less power. The unburnt fuel then enters the exhaust attacking the metals in the catalyst (on injected motors) and subsequently into the air (bad). Also, fuel in the exhaust pipe can ignite due to the high temperatures causing a loud bang (back fire) which can shag an exhaust system pretty quick.

Running lean is where there is too much air in the mix. This is bad for the opposite reason because when compressed, air heats up a fair bit, sometimes enough to cause pre-ignition (pinking or knock). If a piston is halfway up the cylinder when the combustion takes place, it strains the piston head, con rod, crank and supporting bearings which can f*ck an engine quicker than a quick thing. Also, the extra high temperature can cause pistons to melt or parts of the piston which can see it launch through the block.

HTH.

Rich:

1) Uses too much fuel and increases emissions. 2) Leads to lots of sooty deposits and carbon build up. 3) Fouls the spark plugs with soot eventually leading to misfires.

Lean:

1) Can result in misfiring under load and in adverse weather conditions. 2) Lean mixes INCREASE exhaust temperatures resulting in burned out exhaust valves at worst.

The ideal fuel/air ratio is 14.7 parts air to 1 part fuel by weight (not volume).Or also referered to as the stoichiometric limit. This applies to all petrol engines, regardless of whether they've got a carb, or are fuel injected.

If an engine runs rich, the following things can happen -

Lower combustion chamber temperatures Higher carbon buildup (which can lead to further problems if excessive) Bore washing can happen in extreme cases (lubricating oil is washed of cylinder walls, causing accelerated bore/piston ring wear) Excess fuel may not be burned (can cause back firing, can wash the catalyst substance off a catalytic convertor, and even burn in the cat causing the ceramic core to melt). Increased Hydrocarbons (unburnt fuel).

If an engine runs rich-

Higher combustion temperature, which can lead to - Pre-ignition (hot spot in the combustion chamber causes the fuel to be ignited before the plug sparks), which can lead to holed pistons (piston basically melts), piston ring damage (excess combustion force), and may even lead to major engine failure (typically con-rod failure) The higher temp can also lead to valves being burnt out (typically the exhaust valves)

No, the ideal fuel mix for the best burn is 14.0:1, but you do tend to get more nitrogen oxides at this level.

A Catalyst only works most efficiciently on the slightly richer 14.7:1, which is now what closed loop EFI with cats run at. Or try to run at.

Tim..

The message from "Tim \(remove obvious\)" contains these words:

14.7:1 is /leaner/ tha 14.0:1.

And in fact, without knowing the *exact* composition of the fuel, the isomer mix and additives you cannot pluck 14.7 or 14.0 or whatever out of the air. That's why HEGO sensors are used to find out if you are running lean or rich.

Hmmm...never had the 14.0:1 mentioned when I was at college...