Why is fuel used in airplanes different from those used in motor vehicles?

Aviation turbine fuels are used for powering jet and turbo-prop engine aircraft. Kerosene was used to fuel the first turbine engines. Kerosene-type fuel was chosen as having the best combination of properties.

As the primary function of aviation turbine fuel (jet fuel) is to power an aircraft, energy content and combustion quality are key fuel performance properties. Other significant performance properties are stability, lubricity, fluidity, volatility, non-corrosivity, and cleanliness. Besides providing a source of energy, fuel is also used as a hydraulic fluid in engine control systems and as a coolant for certain fuel system components.

However, compared to a kerosene-type fuel, other type fuels like used in motor vehicles were found to have operational disadvantages due to their higher volatility:

Greater losses due to evaporation at high altitudes.

Greater risk of fire during handling on the ground.

Crashes of planes fuelled with wide-cut fuel were less survivable.

Lighter (less dense) fuels, such as gasoline, have higher heating values on a weight basis: whereas heavier (more dense) fuels, like diesel, have higher heating values on a volume basis. Since space is at a premium in most aircraft, the amount of energy contained in a give quantity of fuel is important. A fuel with high volumetric energy content maximises the energy that can be stored in a fixed volume and thus provides the longest flight range.

There are currently two main grades of turbine fuel in use in civil commercial aviation: jet A-1 and jet A, both are kerosene type fuels. There is another grade of jet fuel, jet B which is a wide cut kerosene (a blend of gasoline and kerosene) but it is rarely used except in very cold climates.

Jet A-1 is a kerosene grade of fuel suitable for most turbine engine aircraft. It is produced to a stringent internationally agreed standard, has a flash point above 38 degrees centigrade (100 degrees Fahrenheit) and a freeze point maximum of minus 47 degrees Centigrade.

Jet A is a similar kerosene type of fuel, produced and normally only available in the U.S. It has the same flash point as Jet A-1 but a higher maximum freeze point (minus 40 degrees centigrade).

Jet B is a distillate covering the naphtha and kerosene fractions. It can be used as an alternative to jet A-1 but because it is more difficult to handle (higher flammability), there is only significant demand in very cold climates where its better cold weather performance is important.



How is the speed of a computer measured?

Two important factors that determine the speed of a computer are the amount of data that the Central Processing Unit can process in a given period of time and the CPU's clock speed.

The speed at which a CPU executes instructions is called the clock rate.

Every system contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components. The CPU requires a fixed number of clock ticks to execute each instruction.

The faster the clock, the more instructions the CPU can execute per second. Clock speeds are expressed in megahertz MHz or gigahertz GHz. Mega means million and hertz means times per second, 200 MHz is 200 million times per second (and 200 GHz is 200 billion times per second).

The internal architecture of a CPU has as much to do with a CPU's performance as the clock speed. One common architecture is parallel processing. For example, while an instruction is being executed, the next instruction can be fetched from memory and decoded.

Instruction Prefetching is another idea where the CPU fetches the next instruction beforehand and places it in a queue for the execution unit to use the same.

The overall speed of a computer is also affected by the speed and size of the instruction/data bus. The instruction/data bus is the pathway for data communications between the computer's CPU and the various components in the computer.

The computer's bus has a certain size or width called the data path which is measured in bits and the speed of the bus is measured in MHz.

The larger the bus width and/or the faster the bus speed, the more data that can travel on it in a given amount of time.

Another factor affecting the speed is the size of the primary memory and cache. Increasing the size of the primary memory will speed up the performance if you run several applications at the same time or work with large files and documents. Cache is a small amount (normally less than 1 MegaByte) of high-speed memory residing on or close to the CPU. Cache memory supplies the CPU with the most frequently requested data and instructions.

Finally, effective interfacing of Input-Output devices to the CPU also increases the speed. Systems today use direct memory access (DMA) hardware wherein I/O device acts as a master and transfers large number of data to/from memory without intervention by the CPU.



Why do doctors prescribe some medicines to be taken before and some after food?

When we take a medication, it is absorbed from various parts of our gut — some get absorbed in the stomach, some pass through the stomach into the intestines and get absorbed there.

The most important reason for timing a medication that is taken orally is to maximise its absorption so that more of the medication goes through the stomach into the blood.

Many medicines get absorbed better when food is not present and hence are taken on an empty stomach (an example is the hormone thyroxine which must be taken first thing in the morning). A few actually get absorbed better when food is present — an example of this is the antibiotic azithromycin. Some drugs are taken specifically with or after food because this may reduce the side effects of the drug on the stomach.

For example pain medications and certain antibiotics all can irritate the lining of the stomach and therefore are best taken with or after food. Some drugs work in the wall of the stomach to reduce the absorption of food and this is the desired therapeutic effect — an example of this is the anti-diabetic drug called acarbose. This drug must be taken with the first bite of food. Similarly other oral anti-diabetic drugs and the injection insulin are taken before food because that is when they need to act — just after you eat a meal. The long and short of it is that the timing of a drug has important effects on its absorption, action, potency and even side effects and it is a good idea not to leave the doctor's office without being sure when you should be taking your medicine.

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"Life is either a daring adventure or nothing."
Helen Keller