How can a plane fly? (I) – Introduction to Jet engines.

How can planes and spacecrafts fly? Many of them use engines which work with petroleum derivated fuels.

For the first decades of flight, aircraft engines used the same kind of fuel as cars. When airplane engines became more powerful, gasoline stopped of being the best choice. Tn the early 1900s, most gas had octane ratings of 87 or less. That was enough for a car to work, but airplane engines needed a higher octane grade. Fuel is rated according to its level of octane. The octane rating of gasoline allows us to tell how much the fuel can be compressed by the cylinders before it ignites. The higher the octane, the higher the compression it can bear before igniting. Higher octane levels allow engines to burn fuel more efficiently, rather than “knock,” which indicates potential damage. Merely increasing the octane wasn’t enough for efficient flight, though. A high-octaned gas has a low flashpoint-the temperature at which it can catch fire by an open flame. Gasoline’s flashpoint is around -1 degrees Celsius [°C]. Aviators wanted a fuel that would be safer.

At first, aviators used a mixture of kerosene and gasoline. It was called Jet Propellant 1 (JP-1), but a drawback was the way it smoked as it burned.

Aircraft then went to using Jet-A commercial jet fuel. It is kerosene-based and has a flashpoint of 49 °C (100 °F). It’s a high-quality fuel that includes an anti-freeze to prevent ice buildup inside fuel tanks. Jet A-1 is used by most turbine-powered aircraft. It’s quite similar, but it has a higher freezing point. (Only the military and outside of the US is antifreeze added to jet fuel). In the US there are absolutely no additives in Jet fuel, also it’s not made “from” kerosene. It’s close to the same distillation, but they are different fuels.

Airplanes can still use leaded gasoline, and octane ratings of 115 are seen in high-performance airplane engines. Jet engines, however, burn kerosene-based jet fuel. Just as automobile drivers are concerned with fuel efficiency and engine condition, airplane and jet pilots are concerned about using the best fuel for their type of aircraft engines.

One of the first attemps to build a jet engine was the motorjet, which was very rudimentary, heavy and low powered . At its heart we can find an ordinary piston engine which drives a compressor. The compressed air is driven into a combustion chamber, where fuel is injected and ignited. The high temperatures generated cause the gases in the chamber to expand and escape at high pressure, creating a thermal reactive force that provides a good thrust. This kind of jet engine was used in a few ancient planes like the Coanda-1910 or the Mikoyan-Gurevich I-250, a soviet fighter aircraft.

How does the engine of a car works? (II) – The Diesel cycle

The internal combustion engine is a device that obtains mechanical energy from the chemical energy from a fuel burning in a combustion charmer. The combustion is called internal because this combustion occurs within the camera, contrary to the steam machine.

In this new chapter we will focus on describing the working of an explosive combustion engine that works with  four-stroke diesel cycle. Its main feature is that they use less fuel than gasoline engines. For this reason, they are becoming more frequent in cars. There are also two-strokes diesel engines but those are use in large rail traction engines or marine propulsion engine.

The Diesel is the fuel that reacts with air in the chamber of the diesel engine. It is a fuel that can be obtained from the distillation of petroleum. The Diesel engine was invented by Rudolf Diesel in 1890.

A diesel engine operates on a thermodynamic process that consists of the following 4 stages:

• Compression, process 1-2: It is an isoentropic compression process (reversible and adiabatic), in short, no heat interaction with the outside. The piston rises from the bottom dead center compressing the air of the cylinder, raising the thermodynamic state of the fluid, increasing its pressure, temperature and decreasing its specific volume thanks to the adiabatic effect. The ideal process (isoentropic) is governed by the isoentropic ecuation P*v^k=cte, with k=Cp/Cv.
• Combustion:process 2-3: in this ideal process, the heat input Qin is simplified by an isobaric (at constant pressure) process, even so, diesel combustion is more complex: in the environment of the top dead (TDC) starts the fuel injection (in automobile engines is often used gasoleo or some other fuel sufficiently self- igniting). The injector sprays the fuel, which in contact whit the interior inside the cylinder it starts to evaporate. In the diesel engine combustion exists two reactions: The first reaction is a combustion that goes on fast before the fuel has had time to homogenize, usually occurs turbulently and imperfectly. Thereupon, on the fresh mass has not been yet burned occurs the second reaction more leisurely and perfect summarized in an isobaric process (the aforementioned). In the first reaction burns little fuel but occur most of the irreversibilities of the cycle. Thanks to the chemical energy liberated in this combustion, interpreted in this model like a heat that the thermodynamic fluid receives its expand.
• Expansion, process 3-4: Another isoentropic (adiabatic) expansion of the thermodynamic fluid is repited to recover the specific volume it had at first.  This expansion occurs due to the effect of raising the thermodynamic properties of gases in the combustion, which tend to push the piston from the TDC to the BDC, accordingly and just in this stage of the cycle work is done.
• Last stage, 4-1 process: This stage is an isochoric process (at constant volume) where the final pressure has the same value as the initial compression pressure. In fact, lacks any physical meaning and simply uses the ad hoc to close de ideal cycle.

This combustion causes an energy which pushes the piston down, consequently, the crankshaft flywheel will absorb the energy in the form of the kinetic energy and it will use to raise the piston, causing the expulsion of gases in the chamber through a valve, finishing the last stage.

These  4 movements cause  the longitudinal movement of the piston, which employed his lineal speed to cause the rotational movement of the cranckshaft.

These engines use gasoil because of its higher heaviness, which is better for the self-inflammation with the air. The indirect injection (when the reactions occur in a pre camera) reduces the disadvantages of a diesel engine (maintenance, price…) but increases the consumption, losing one of the principal advantages of this kind of devices.