Gasoline burns rapidly, just short of an explosion

fuel-blending-process

Finally you get to gasoline, the subject everybody knows a little about. Gasoline is the product almost everybody buys and occasionally spills on their shoes. They know what yesterday’s gasoline price was because it’s posted on every other corner. They all understand that higher octane is better than lower octane, but they probably don’t know why. That’s all okay for most people because a competitive marketplace delivers gasoline grades that will work just fine in any car. Most people don’t need to understand much more—except you.

It used to be that understanding how to make gasoline was a fairly simple matter. However, since the 1980s, governments and institutions have learned how bad “old fashioned” gasoline can be for your health. Governments, especially in America, have successively put more restrictions on the content of gasoline, giving progressively bigger headaches to refiners. The best way to cover the accumulated complexities is with a historical tour, starting with what refiners think of as “the good old days” and environmentalists consider the “black past.” So this article  will start with gasoline in the 1960s and work its way to the 21st century, covering these questions:

  • What happens when gasoline is burned in a car engine?
  • What is octane, and the equally important property, vapor pressure?
  • Whatever happened to tetraethyl lead, and what did that do?
  • How does gasoline effect the environment?
  • How do refiners blend gasoline now to not affect the environment?
  • What effect does gasoline blending have on the way to run a refinery?

Gasoline Engines
The essential parts of a gasoline engine, at least for this discussion, are the gas tank, the fuel pump, fuel injection, the cylinders, the pistons, and the spark plugs. Engines without the last item (spark plugs) will be discussed later because they’re diesels.
You might say the whole motive process in a car starts at the gas tank when you fill it at a gas station. As you start the engine, the fuel pump sucks gasoline out of the tank and sends it to the fuel injection apparatus.
The purpose of fuel injection is to get the gasoline in the right state and in the right place to burn it, releasing the chemical energy. Gasoline, like other hydrocarbons, doesn’t burn in its liquid state. It has to be vaporized and mixed with oxygen to ignite. For example, when you throw charcoal lighter on the coals in your barbecue grill and toss in a match, the vapors ignite, not the liquid. That’s why some dummies can get away with squirting charcoal lighter directly on a lit fire without barbecuing themselves.
The liquid stream doesn’t ignite. Unless the charcoal lighter has been sitting in the sun or next to the grill and is warm, the stream doesn’t vaporize in the air, only when it hits the hot coals.
The fuel injection apparatus mixes the gasoline with air, pumps it up to a high pressure, then squirts it into the cylinder. The sudden release of pressure is enough to vaporize the mixture, filling the cylinder with a gasoline/air mixture, ripe for ignition. The nitrogen that’s also in the air just passes through the process, more or less unaffected. (Less is bad, as you’ll read below).

The downward movement of the piston works together with the fuel injection apparatus to suck the gasoline/air mixture into the cylinder. At the bottom of the stroke, the space in the cylinder reaches its maximum and is filled with the fuel. The fuel injection apparatus closes and in the next step, compression, the piston moves up the cylinder, compressing the vapor. When the piston reaches the top of the stroke at the point of ignition, the spark plug gives off a powerful spark, igniting the gasoline vapor. The gasoline burns rapidly, just short of an explosion. The gases expand and put huge pressure on the piston, forcing it down the cylinder in the power stroke. If the gasoline is formulated correctly, different molecules will burn at different times so that the combustion takes place over the whole length of the power stroke, smoothing out the motion. Power is transmitted to the crankshaft as the piston is forced down the cylinder in the power stroke. At the bottom of the power stroke, the exhaust valve at he top of the cylinder opens. Burnt fuel is pushed out in the exhaust stroke as the piston moves up the cylinder. At thetop of the stroke, the outlet valve closes, the injector opens up again to squirt the fuel in, and me process is ready to be repeated.
Note that each cycle requires two trips of the piston up and down the cylinder, which is not the reason why it’s called a four cycle engine. The four cycles are intake, compression, power, and exhaust, which aren’t really cycles, but nobody ever said automotive engineers were all that articulate.
Some small two cycle engines (lawnmowers, outboards, etc.) use gasoline. The big two cycle engines run on diesel fuel or the heavier residual fuel.

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обновлено: September 22, 2016 автором: dannik