Running Rich vs Running Lean

Running rich: not terrible
14.7:1. This is the ideal ratio of air to fuel in a combustion engine, but if we want more power, can’t we just add more fuel? Not really. Let’s go back a bit and talk about some high school chemistry. Stoichiometry is essentially the relationship between all components in a chemical reaction. And a stoichiometric ratio is when every element on one side of a reaction is also represented on the other side. So in an engine, we generally want 14.7 grams of air to 1 gram of fuel, which when very simplified, looks something like this. 25 O2 + 2 C8H16 = 16 CO2 + 18 H2O + energy (). If we have more fuel we consider that to be running rich, less fuel to be running lean. So why can’t we just add more fuel and make more power? Well that 14.7:1 stoichiometric ratio means that all the fuel in the combustion chamber is reacting with all the oxygen in the combustion chamber. Any more fuel is just a waste - without more oxygen, that extra fuel can’t combust and passes through the engine without igniting, which also means that fuel won’t create any more power. So we can’t get more power, but are there any other upsides or downsides? In passing through the engine, even though it’s not igniting, the fuel will cool the engine down as it absorbs heat from the combustion chamber. This means that the engine will be less prone to pre-ignition, but at a cost. The cost is lower fuel economy, because we’re using fuel at a faster rate, potential for damage to the catalytic converter, because we have unburnt fuel flowing into the catalytic converter, and some other minor issues. So generally, vehicles run a stoichiometric ratio most of the time, with exceptions at wide open throttle and acceleration, not for more power, but for better engine block cooling.

Runaning lean: worse idea
Running lean, meaning using less fuel than the ideal stoichiometric ratio, is, unlike running rich, awful for your engine. I talked previously about how gasoline cools an engine, but let’s take a step backwards for a bit. In modern vehicles, there’s 2 different ways to get fuel into the combustion chamber, port or direct injection. And either one follows the same general principles. Fuel needs to be atomized, emulsified, and vaporized before it combusts. Atomization is the process of creating a fine mist of fuel, and this is done by the injector. Emulsification is the process of mixing with the air, and this is also done by the injector combined with the intake airflow design. And vaporization is the process of turning the liquid drops of fuel into a gaseous state. And this vaporization is where the cooling effect takes place. The heat from the combustion chamber vaporizes the fuel and in doing so the heat is absorbed from the cylinder walls. The gas is essentially boiled, and like how you boil water with heat from your stove, boiling the gasoline absorbs heat as well. So let’s get back to the point of this video which is why running lean is dangerous. When we run lean we have less fuel going into the engine, which means less cooling occurs during this vaporization process. Yes, there’s less fuel combusting, but the cooling effect from the fuel has a greater effect than the heat generated from that same fuel, especially at higher temperatures. Because your cars cooling system is meant to work alongside this vaporization cooling process, without that cooling effect, your engine block will get hotter and hotter, until it starts to cause damage to your engine like warped head gaskets, or damage to your engine oil by burning the additives in it.