Gasoline in Automobiles
When you think of petroleum products the first thing that comes to mind for most of us is gasoline. Gasoline is in the fraction that has a boiling point between 40-100 oC. It may contain over a thousand compounds within the fraction. The property we are all familiar with is the octane number. What this number equates to is the tendency of an engine to knock or make noise as it combusts in the engine, no knocks mean that the fuel is combusting in a controlled manner. The number tells you the percentage of isooctane (C 8H 1 8) to heptane (C 7H 1 6). Therefore gasoline with an 87 octane rating would have 87% isooctane and 13% heptane [5].
Combustion of gasoline is a reaction and it gives off products. Perfect combustion of a hydrocarbon with oxygen in the air would convert all the hydrogen in the fuel to water and all the carbon in the fuel to carbon dioxide. Nitrogen in the air would remain unaffected. The combustion process in a typical engine is not "perfect". Due to this nature of engines, side reactions happen with compounds other than oxygen and impurities in gasoline and there are several pollutants emitted.
In order to illustrate this to the students, have them build molecular models of methane, ethanol, and octane molecules and then "combust" them with oxygen. Doing these manipulations will help students determine the balanced chemical equations for these combustion reactions. This will also show how much CO 2 is produced when these fuels are burned. Using standard bond energies, you can than calculate and compare the amount of energy released during these combustion reactions.
Car exhaust is not a simple thing to analyze. Current fuels for vehicles are derived from crude oil which is produced primarily from the decay of marine organisms. This crude contains many elements. The most common elements being carbon, hydrogen, nitrogen, oxygen, and some sulfur. When the hydrocarbons and organic compounds containing nitrogen and sulfur are burned, the products are water, carbon dioxide and monoxide, oxides of nitrogen and oxides of sulfur. Many of the hydrocarbons pass through the internal combustion engine unconsumed and are released along with other exhaust fumes in the tailpipe. Energy efficiency of city driving falls below 15%, and the majority (approximately 80%) of cars are driven in cities. This equates to having only 1.5 gallons of usable fuel in a 10 gallon tank and the remaining 8.5 gallons are transformed into heat and pollutants [6].
Having students use Drager tubes to collect samples of exhaust will be a good way to start out an analysis of the pollutants. These tubes are placed into a piece of vinyl tubing and connected to a large syringe. The tube is place in the stream of exhaust of a vehicle and then the sample is taken. The tubes applicable to this are the carbon monoxide, carbon dioxide, and hydrocarbon gas-detector tubes. These tubes can be purchased through a scientific supply company. Having students collect samples from various types of vehicles: cars, vans, SUVs, trucks, as well of different types of fuels: gasoline, diesel, biodiesel blends, will make for an interesting data set.
Ideally you can ask the state to bring some emissions-testing equipment to your school and test vehicles that your students or teachers drive to school each day. This will make for more accurate data and most likely more parameters you can analyze. I was fortunate to acquire the data for this testing for a number of vehicles at a colleague's school that I can use with my students. This type of data manipulation makes these numbers seem more relevant to your students.
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