Fuel Ratio

A lot is posted about air/fuel mixtures. The air/fuel ratio is the ratio of the mass of air to the mass of fuel:

AFR = (mass Air)/(mass Fuel)

Stoichiometric, for our purposes, is the ideal ratio where theoretically the combustion process is complete. But this isn't an ideal world and a car engine is not a perfect thing either. So typically we see that a richer mixture of around 12.8:1 produces the best results for power output.

Lambda is a factor of stoichiometric. At 14.7:1 Lambda is 1.0. If you are running at 0.85 Lambda you are running at 0.85 x 14.7 = 12.5:1.

You can estimate the AFR by measuring the CO concentration % of the exhaust gas and applying a roughing calculation:

AFR = 14.7 - (0.425 x CO%); or reworking it to find CO%:

CO%= 34.588 - (2.3529 x AFR)

Ratios lower than stoichiometric (14.7:1) are generally called rich and ratios higher than are called lean. For our purposes we will work the terms around the satisfactory fuel ratio (e.g. 12.8:1).

Generally the fuel ratio will have an effect on engine vitals such as temperatures and exhaust gases:

Satisfactory mixture: well obviously this is the one that gives us the best power, the best exhaust temperature, the best emissions, the best engine temperature, etc. Typically you can tell optimum by measuring the exhaust gas temperature (EGT) and the conjugate heat transfer (CHT) to the engine's water jacket. A good condition is both of them high. Expect about 720°C to 835°C EGT for a normally aspirated engine and about 900°C for a turbo or supercharged jobbie (these are measured on an exhaust runner pre turbo or pre primary collector, after the turbo the temperature will drop about 95°C and pre primary may rise about 65°C). Your water temp should be around the 93°C mark normally, but engines vary.

Tech Tip. To find your optimal EGT imperically, start with a rich mixture. Gradually lean the mixture while observing the power increase and the EGT rise. Unless the power starts dropping off, you will find a point where the EGT starts to fall; you stop leaning and make a small adjustment to enrich. Of course if you don't have a dyno it could be a bit tricky.

Rich mixture: when the fuel vapourises it absorbs heat from it's surroundings (latent heat of vapourisation) which results in a lowering of the surrounding temperature. So a rich mixture results in a cooler combustion temperature, cooler exhaust gas temperature, cooler engine temperature, lower Nox, higher CO and higher hydrocarbon emissions.

A lean mixture would logically have the opposite effect to a rich mixture, but there are a few traps for new players here. Being lean the flame will be slower, which normally results in a hotter engine, but if the mixture gets really lean the engine may get cooler. And the exhaust gas temperature will be cooler than the satisfactory mixture, not higher. You risk burnt and damaged exhaust valves if you run for prolonged periods with overly lean mixtures. Expect less power because of a lower and later peak combustion pressure.

If you are wondering where the economies come into play here is a little graphic that should be self explanatory:

Note: Measuring CO concentrations to indicate AFR is more accurate than O2. The CO level is a direct measure of the combustables in the exhaust gas. Oxygen measurements, while convenient, only approximate the AFR; they are handy however, when you want an indication of AFR above 14.7.

Forum Post

Quote Cwone:

Stoic. or a little leaner (14.7-16:1) is fine for anything from -30in.Hg to around -10in.Hg. Above that you start to drop it steadily until you're at about 12:1 at full boost, 11.5:1 if you're running more boost than standard or want to keep EGTs down. If you have a cat. converter, don't go much leaner than 14.7:1 or you'll start overheating the matrix (Cats work best at stoichiometric ratio). Over-run cut-off should be activated, it gets rid of those annoying pops and such on decel. (You'll notice the A/F display go fully lean on overrun when it's working properly).

As well as A/F, EGT is also very handy for tuning a turbo car, and is also a good gauge to have generally. Try and install it upstream of the turbo, it's not difficult to drill and tap the underside of the central runner and install a compression gland and temperature probe.

Don't forget that idle maps are for idle (ie. Less than 1200 rpm, or whatever RPM is set as the idle map cut-in point) Above that you're still in the main map.

As for temperature corrections, if it were a real ECU it would do the IAT correction for you (after you calibrated the sensor), and the coolant correction isn't critical, just richen it up enough so that it still runs nicely when the engine is stone cold. To get the IAT corrections right, you can just wait til a real cold day, pootle around slowly and check the ratios when cold, then hammer it, get the I/C nice and hot, and compare the ratios when the IAT is hot. Adjust your corrections so that the ratios are the same for both cold air and hot. DON'T go playing with the maps because you see the ratios all screwed up, you'll just start chasing your tail around when it's hot again, use the IAT correction.

The accelerator pump settings aren't tuned by A/F, you just adjust it down until it hesitates on snap throttle openings, then adjust it up until you feel it bog down a bit from over-fuelling on same, then set it at the mid-point between the two extremes. You may find this setting needs tweaking over time, as you test out the throttle response over other RPMs and loads.