Thursday, July 19, 2012

Gasoline and Driving Performance Explanation

Most country in this world very depend on fuel oil for their industrial or transportation activities. One of the most important fuel is Gasoline. Here is short hystory of Gasoline :

In May 1876, Nicolaus Otto built the first practical four-stroke-cycle internal combustion engine powered by a liquid fuel. By 1884, he concluded development of his engine with the invention of the first magneto ignition system for low-voltage ignition. The liquid fuel used by Otto became known as gasoline in the United States; elsewhere it may be known as gasolina, petrol, essence, or benzin (not to be confused with the chemical compound benzene).

Many people know about Gasoline just as a fuel engine or fuel car, only few people know and understand that Gasoline keep an amazing secret to be learn. And you know what ? here is the secret :

1. Because gasoline almost always performs well, drivers forget what a sophisticated product it is.   
    More thought would reveal a demanding set of performance expectations:
  • An engine that starts easily when cold, warms up rapidly, and runs smoothly under all conditions.
  • An engine that delivers adequate power without knocking.
  • A vehicle that provides good fuel economy and generates low emissions.
  • A gasoline that does not add to engine deposits or contaminate or corrode a vehicle’s fuel system.
2. Volatility

Driveability describes how an engine starts, warms up, and runs. It is the assessment of a vehicle’s response to the use of its accelerator relative to what a driver expects. Driveability problems include hard starting, backfiring, rough idling, poor throttle response, and stalling (at idle, under load, or when decelerating).The key gasoline characteristic for good driveability is volatility – a gasoline’s tendency to vaporize. Volatility is important because liquids and solids don’t burn; only vapors burn. When a liquid appears to be burning, actually it is the invisible vapor above its surface that is burning. This rule holds true in the combustion chamber of an engine; gasoline must be vaporized before it can burn. For winter weather, gasoline blenders formulate gasoline to vaporize easily. Gasoline that vaporizes easily allows a cold engine to start quickly and warm up smoothly. Warm-weather gasoline is blended to vaporize less easily to prevent engine vapor lock and other hot fuel handling problems and to control evaporative emissions that contribute to air pollution.

It is important to note that there is no single best volatility for gasoline. Volatility must be adjusted for the altitude and seasonal temperature of the location where the gasoline will be used. Later, this chapter will explain how gasoline specifications address this requirement. Three properties are used to measure gasoline volatility in the United States: vapor pressure, distillation profile, and vapor-liquid ratio. A fourth property, driveability index, is calculated from the distillation profile. Instead of a vapor-liquid ratio, a vapor lock index is used outside the U.S. to control hot fuel handling problems.

3. Vapour Pressure

With respect to gasoline, vapor pressure (VP) is the single most important property for cold-start and warm-up driveability. (Cold-start means that the engine is at ambient temperature, not that the ambient temperature is cold.) When gasoline vapor pressure is low, an engine may have to be cranked a long time before it starts. When vapor pressure is extremely low, an engine may not start at all. Engines with port fuel injection (see page 66) appear to start more readily with low vapor pressure fuel than do carbureted engines. Vapor pressure varies with the season; the normal range is 48.2 kPa to 103 kPa (7 psi to 15 psi). Higher values of vapor pressure generally result in better cold-start performance, but lower values are better to prevent vapor lock and other hot fuel handling problems.

4. Distillation profile

Gasoline is a mixture of hundreds of hydrocarbons, many of which have different boiling points. Thus gasoline boils, or distills, over a range of temperatures, unlike a pure compound; water, for instance, boils at a single temperature. A distillation profile, or distillation curve, is the set of increasing temperatures at which gasoline evaporates for a fixed series of increasing volume percentages (5 percent, 10 percent, 20 percent, 30 percent and so on) under specific conditions (see page 48). Alternatively, the profile may be the set of increasing evaporation volume percentages for a fixed series of increasing temperatures. Figure 1.1 shows the 2008 U.S. average distillation profiles of conventional summer and winter gasolines. A distillation profile is also shown for a summer reformulated gasoline (RFG) containing ethanol.

Various ranges of a distillation profile correlate with specific aspects of gasoline performance.
Front-end volatility is adjusted to provide:
  • Easy cold starting.
  • Easy hot starting.
  • Freedom from vapor lock or other hot fuel handling problems.
  • Low evaporation and running-loss emissions.
Midrange volatility is adjusted to provide:
  • Rapid warm-up and smooth running.
  • Good short-trip fuel economy.
  • Good power and acceleration.
  • Protection against carburetor icing and hot-stalling.
Tail-end volatility is adjusted to provide:
  • Good fuel economy after engine warm-up.
  • Freedom from engine deposits.
  • Minimal fuel dilution of crankcase oil.
  • Minimal volatile organic compound (VOC) exhaust emissions

5. Vapor Lock and Hot Fuel Handling Problems

Vapor lock and hot fuel handling problems occur when excessive gasoline vapor accumulates somewhere in the fuel system of a vehicle and reduces or interrupts the fuel supply to the engine. This may take place in the fuel pump, the fuel line, the carburetor, or the fuel injector. When the fuel supply is reduced, the air-fuel ratio becomes too fuel-lean (too much air for the amount of fuel), which may cause loss of power, surging, or back firing. When the fuel supply is interrupted, the engine stops and may be difficult to restart until the fuel system has cooled and the vapor has recondensed. After a hot soak (engine shutdown), it may be difficult to start the engine if too much vapor has formed in the fuel system. Overheated fuel or overly volatile fuel is the main cause of vapor lock. Fuel temperature depends on several factors: the ambient temperature, how hard the vehicle is working, how well the fuel system is isolated from the heat of the engine, and how effectively the fuel system is cooled.

6. Carburetor Icing

Carburetor icing occurs when intake air is chilled below the freezing point of water by evaporation of gasoline in the carburetor. Ice forms on the throttle blade and in the venturi and can interrupt carburetion, causing an engine to stall. Icing can be acute when the air is moist (70 percent or higher relative humidity) and the ambient temperature is between 2°C and 13°C (35°F and 55°F). These weather conditions are common during the fall, winter, and spring in many parts of the U.S., and they can last well into the summer in some coastal regions. Carburetor icing is not a problem when the intake air temperature is below freezing because the air is too dry. The extent of carburetor icing does not depend on weather alone. It also involves carburetor and vehicle design and the mechanical condition of the engine, in particular, those components that affect warm-up time, such as thermostats, automatic chokes, intake air heaters, and heat risers. Icing also involves gasoline volatility. The 70 percent evaporated temperature in the distillation profile is a good index  to measure the tendency of a gasoline to cause carburetor icing; the lower this temperature, the more severe the icing. Carburetor icing is not as big a problem as it used to be. For emission control reasons, most carbureted engines built since the late 1960s have been equipped with intake air-heating systems that generally eliminate carburetor icing. Today the problem is minimal because fuel-injected vehicles have replaced most carbureted vehicles.

Source : Chevron



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