Originally Posted by
FB71
Actually, that's not correct. Stoiciometry is used to determine the amount of products and reactants that are produced or needed in a given reaction. Stoiciometric ratio of 14.7:1 refers to the resulting reduction of non-ethanol gasoline, oxygen, and nitrogen into water, carbon dioxide and nitrogen, under the Ideal Gas Law. Under the IGL of PV=nRT, where P is pressure, V is volume, n is the number of moles of energy, R is the constant, and T is temperature, the most complete release of energy, under perfect circumstances, results in no waste heat, and no waste compounds. This is the peak of combustion efficiency, and would yield the highest cylinder pressure. However, other factors must be included and compensated for. These include, but are not limited to; thermal losses via the components of the engine, resulting in inconsistent combustion temperatures, forming unwanted compounds such as HydroCarbons, Oxides of Nitrogen, and Carbon Monoxide. The physics of the intake runner and combustion chamber greatly influence charge homogenization and flame propagation and duration, which contribute to changes in combustion chemistry as well. To compensate for these inefficiencies, the charge A/F ratio is manipulated to reach an effective combustion ratio of 14.7:1, again, for non-ethanol fuel. As alcohol content increases, the ideal stoic ratio drops, with pure ethanol being around 9:1 and methanol being around 7:1. Alcohol raises the octane rating, reducing the occurrence of spark knock (taking the place of tetra ethyl lead or methyl tertiary butyl ether), as well as aiding in vaporization to improve charge homogenization. Under different loads, different ratios can be utilized to improve vehicle performance, but not necessarily improve combustion efficiency. Driving the charge somewhat lean can raise cylinder pressures slightly to maintain torque while reducing fuel consumption, at the expense of elevated combustion temps, creating oxides of nitrogen pollutants. The introduction of an inert gas (recycled exhaust gases) partially fill the cylinder during intake, reducing its volumetric efficiency. This allows a smaller fuel and air charge to expand into a larger volume, lowering its temperature, preventing the formation of the nitrogen compounds. Under high loads, the ratio can be driven rich, utilizing the excess fuel (the charge will run out of oxygen long before the fuel is fully catalyzed) to absorb some of the combustion heat via the Latent Heat of Vaporization principle. The charge needs to be slightly cooled because of the opportunity for spark knock. By slightly lowering the charge temps, the mixture is much less likely to auto-ignite. If the engine were able to operate under ideal conditions, a catalytic converter wouldn't even be required. The converter's job is to finish what was started in the combustion chamber, but wasn't fully completed due to the operating conditions of the engine. The ceramic substrate of the converter is coated with a wash of certain precious metals, which, with the added heat, ionize the hydrocarbon particles, and cause them to adhere to the substrate via static electricity. Once excess oxygen is introduced, and if converter temperatures are sufficient, the hydrocarbons catalyze with the oxygen particles, producing water and carbon dioxide. So, a vehicles fuel management system is intentionally designed to deviate from stoic under certain conditions. In fact, one of those conditions is to intentionally load the cat with HCs, and then intentionally starve it, forcing it to 'light off', initiating the catalytic reaction. So, maintaining stoic will actually starve a cat, rendering it useless.
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