Air-fuel ratios

Fuel can’t burn on its own: it needs oxygen, and the engine gets its oxygen from air. The air-fuel ratio, or AFR, is how much air there is in the cylinder for each bit of fuel, measured by weight.

For petrol, the chemistry works out at about 14.7:1, said “fourteen point seven to one”: to burn completely, the fuel needs 14.7 times its own weight in air. At that ratio, the oxygen in the air is exactly enough to burn all the fuel, with nothing left over. It’s called the stoichiometric ratio, from the Greek for “measuring the elements”.

Air is very light, so that’s a lot of it. As How an engine gets air explains, each litre of petrol needs about 9,000 litres of air.

Any other mixture has something left over:

• Rich means more fuel than the air can burn, a lower number such as 12:1. Some fuel goes out of the exhaust unburnt.
• Lean means more air than the fuel needs, a higher number such as 16:1. Spare oxygen goes out of the exhaust.

You’ll also see the mixture written as lambda (λ): the real ratio divided by the stoichiometric one. Lambda 1 is just right, below 1 is rich and above 1 is lean. Oxygen sensors and other engine sensors shows how the ECU measures it.

The demo runs one cylinder of the 2.0-litre engine at full throttle and 4,000 rpm. The throttle and revs stay fixed, so every fill holds the same 600 mg of air. The slider changes only how much fuel the injector adds to it. The graph shows the engine’s power in Mustard and its efficiency in Coral: how much of the fuel’s energy it turns into push.

• Start at Stoichiometric, 14.7:1. Each fill gets 40.8 mg of fuel, all of it burns and nothing is left in the exhaust. The engine makes 81 kW (109 bhp) and turns 37% of the fuel’s energy into push.
• Press Best power, 12.8:1. Now there’s 15% more fuel than the air can burn. Power rises to 84 kW (113 bhp), the most this much air can make, but 13% of the fuel goes out of the exhaust unburnt, shown as Orange dots.
• Press Rich, 11:1. Power is back down to 81 kW (109 bhp), the same as at 14.7:1, but the engine is using a third more fuel to make it, and a quarter of it goes to waste.
• Press Best economy, 16.2:1. All the fuel burns with air to spare, shown as Paper dots, so the engine gets the most from each drop: 38%. But there’s less fuel, so it makes less power, 76 kW (102 bhp).
• Slide past 17:1 into the Misfire zone. The mixture is now so thin that the spark can’t always light it. Power falls away fast, and the unburnt mixture goes straight out of the exhaust.

Why is best power rich? The amount of air is fixed, so the oxygen is what runs out first. A little extra fuel makes sure every bit of oxygen finds some fuel to burn with. The spare fuel also soaks up heat as it evaporates, which cools the charge so more fits in and it’s less likely to knock.

Why is best economy lean? With spare air, every drop of fuel finds oxygen, so none is wasted. Past about 16:1, though, the mixture burns too slowly and the gain is lost.

The model is simplified: real engines vary, and their limits depend on the design of the combustion chamber, the fuel and the temperature.

An engine never runs at one ratio. The ECU picks a target for each moment, and it’s a balance between power, economy, protecting the engine and keeping the exhaust clean. Typical targets for a petrol engine:

• Idling and cruising: 14.7:1. The catalytic converter can only clean up all three harmful gases when the mixture is right on the stoichiometric ratio. A little rich and it can’t clean up the carbon monoxide and unburnt fuel; a little lean and it can’t clean up the nitrogen oxides. So for nearly all everyday driving, the ECU holds the mixture there, checking with the oxygen sensor in the exhaust many times a second.
• Full throttle: about 12.5:1 to 13:1. For the most power, and because the spare fuel cools the pistons and valves when they’re working hardest. A slightly lean mixture burns hottest of all.
• Turbo engines on full boost: about 11.5:1 to 12.5:1. Richer still, to keep the pistons and the turbo cool and hold off knock. Newer engines built for the latest emissions tests stay closer to 14.7:1 even flat out, with the exhaust manifold cast into the cylinder head and cooled by the coolant instead.
• Cold starts: much richer. Cold fuel doesn’t evaporate well, and some of it sticks to the cold walls of the ports and cylinders. The ECU adds extra so that enough turns to vapour to burn, then cuts back as the engine warms up.
• Lifting off: no fuel at all. When you lift off the accelerator at speed, the ECU shuts the injectors off completely. The engine is turned by the wheels, which saves fuel.

Diesels work differently. They let in plenty of air all the time and control their power with the fuel, so they always run lean: from about 18:1 flat out to over 60:1 at idle. Add more fuel than the air can burn and the extra comes out as black smoke.

Lean-burn petrol engines tried to save fuel by running lean while cruising. Some 1990s Hondas ran at about 22:1, and some direct-injection engines kept a richer cloud of fuel round the plug in otherwise thin air, reaching 40:1 overall. Most were dropped because the catalytic converter can’t clean up nitrogen oxides in lean exhaust.

Other fuels need different ratios: diesel about 14.5:1, ethanol 9:1 and methanol 6.4:1. Pump petrol with 10% ethanol, E10, needs a little less air than pure petrol, about 14.1:1, but the ECU allows for this automatically.

The mixture decides what comes out of the tailpipe before the catalytic converter cleans it up:

• Rich leaves fuel without enough oxygen to finish burning. It comes out as carbon monoxide, a poisonous gas, and unburnt hydrocarbons, and very rich mixtures make soot.
• Slightly lean, around 15:1 to 16:1, burns hottest, and that heat makes the most nitrogen oxides, which cause smog and harm lungs.
• Very lean misfires, so unburnt hydrocarbons rise again.

That’s why the oxygen sensor and catalytic converter work as a team. Catalytic converters shows why the converter needs the mixture so precisely right.

The ECU measures the air coming in, sprays in fuel to match and then checks the result with the oxygen sensor. If the oxygen sensor shows the mixture is a little off, the ECU corrects it, called fuel trim. When something’s wrong, the trims run out of range and the engine management light comes on, often with a fault code for running lean (P0171) or rich (P0172).

Running lean usually means air is getting in that the ECU hasn’t measured, or fuel isn’t getting through:

• An air leak after the air-flow meter: a split intake hose, a perished vacuum pipe or a leaking intake gasket. Typical signs are a rough or hunting idle and a hiss from the engine bay.
• A dirty air-flow meter that reads less air than is really coming in.
• Not enough fuel: a weak fuel pump, a blocked fuel filter or clogged injectors. Signs are hesitation and flat spots when you accelerate, and misfires.

A lean engine under heavy load is the dangerous case. It runs hot and is more likely to knock, which can burn the valves or melt a hole in a piston.

Running rich usually means too much fuel or the ECU being told the wrong thing:

• A leaking injector or a faulty fuel pressure regulator.
• A faulty coolant temperature sensor that tells the ECU the engine is cold, so it keeps adding warm-up fuel. A thermostat stuck open does something similar.
• A slow or failed oxygen sensor.

Signs are a smell of fuel, poor economy, black soot in the tailpipe and on the spark plugs, and failing the emissions test on carbon monoxide. The unburnt fuel burns inside the catalytic converter instead, and can overheat and ruin it.

Almost every engine modification changes the air or the fuel, so the mixture is at the heart of tuning.

• Intake and exhaust changes. On engines with an air-flow meter, the ECU measures the extra air and adds fuel to match, and the fuel trims tidy up the rest. But at full throttle most ECUs follow their fuel map without checking the oxygen sensor, so a big change can leave the engine lean when it’s working hardest. This is one reason modified cars get a remap.
• More boost needs more fuel. Turning up a turbo pushes in more air, and if the fuel doesn’t keep up, the mixture goes lean. Bigger injectors and a higher-flow fuel pump often come with more boost.
• Wideband gauges. The oxygen sensor most cars come with only says whether the mixture is richer or leaner than 14.7:1. Tuners fit a wideband sensor and gauge, which reads the actual ratio, to set the mixture across the whole map.
• Typical tuning targets are about 12.5:1 to 13:1 at full throttle on a non-turbo engine and 11.5:1 to 12:1 on boost, but the right number depends on the engine and fuel. Tuning a road car lean to save fuel isn’t worth the risk to the engine.
• E85, a blend of mostly ethanol, resists knock very well, so it allows more boost or ignition advance. Its stoichiometric ratio is about 9.8:1, so the engine needs about 30% to 40% more fuel by volume, which usually means bigger injectors and a bigger pump.

Any change should be set up on a rolling road with a wideband sensor, watching the ratio at every load and speed.