Entropy Changes of an Ideal Gas
“How does the entropy of an ideal gas change with time?”
When an ideal gas undergoes a nonadiabatic process, it’s entropy is bound to change. However, how can we quantify such a change? Well, let’s use our engineering mindset to figure this out. One way would be to look at a thermodynamic property table, find the specific enthalpies for different temperatures, and then take the difference in values. Another way would be to plug in the equation delta s = C_v*ln(t2/t1) + R*ln(v2/v1) or delta s = C_p*ln(t2/t1) – R*ln(p2/p1).
The Ideal Diesel Cycle
“How can we analyze a diesel engine?”
For better or worse, diesel engines are one of the most utilized technologies across the world. And as engineers, it would be logical to analyze how they operate. When a diesel engine starts, it draws in a fluid at a constant pressure. Afterwards, this fluid will be compressed in an adiabatic manner in Stage 1. Then, more heat is added in Stage 2. The system will then spark and go through an adiabatic expansion in Stage 3. Heat is then removed from the system in Stage 4. When everything is completed, the fuel will be ejected. This cycle is known as the Ideal Diesel Cycle and is used by Mechanical Engineers around the world for energy analysis (as well as undergraduate engineering students to power their way through thermodynamics!)
“How can we improve upon the Bourdon tube design?”
Bourdon tubes are excellent for measuring pressure in a pipe. However, due to their C shaped geometry and oval cross-section, they can be difficult to manufacture. So how can we use our engineering mindset to solve this problem?
Well, luckily for us, there is a solution out there called an Anti-Bourdon Tube. An Anti-Bourdon tube has an initial circular cross-section and straight body. However, the hole is not centered in the center of the tube and is instead located more towards another side. When fluid is used to fill the device, the tube will expand and bend towards the thicker side. This bending motion can then be used to turn a gear to read the pressure. Anti-Bourdon Tubes are easier to fabricate than their regular counterparts, making them a good choice for pressure measurement projects.
“How can we measure a tube’s pressure without sensors?”
Most engineering systems built in the modern day use a great variety of sensors to achieve quick and easy measurement. However, how can we accomplish a pressure measurement in a tube using more old-fashioned methods? Well, let’s use our engineering mindset to learn more about a shrewd device known as a Bourdon tube.
Bourdon tubes work as follows. A hollow, oval-shaped tube will be wrapped in a “C” shape. As air moves into the tube, the profile will fill into a circular geometry, causing a contraction. When the end of the “C” moves downward, a link connected to a sector/pinion will move, turn a gear that turns a pressure dial which allows users to read the measurement of the total pressure.
In a way, Bourdon tubes are fundamentally like an inflatable glove. When air goes through, the hand will inflate and turn the dial.
“How do metals waste away with time?”
Metals are some of the most widely used materials in the world. However, nothing within the realm of physics lasts forever. If a metal is immersed in an atmosphere, then it will be surrounded by chemicals alien to its own. Chemical reactions are bound to occur, and over time this metal will decay and waste away in a process known as corrosion. Corrosion is a very important engineering factor, especially for public infrastructure. So much so that in 1998 alone the total annual direct cost of corrosion in the U.S. was around. $276 billion!
The Otto Cycle
“How can we describe the operation of a spark ignition engine?”
Spark ignition engines (also known as Internal Combustion Engines) were the backbone of 20th-century vehicles. And since us engineers love to describe things, how can we do so in a systematic manner? Well, to start, let’s analyze how several key variables change over time. To start, let’s draw in some air into the piston/cylinder under a constant pressure. Also, let’s label this process 0-1. Then, let’s move the piston such that an adiabatic compression takes place from the bottom dead centre (maximum height) to top dead centre (minimum height) in process 1-2. Afterwards, let’s represent the ignition as a constant volume heat transfer in process 2-3. This should soon cause an adiabatic expansion back to bottom dead center in process 3-4. Then, let’s complete the cycle with a constant pressure heat rejection in process 4-1. Afterwards, let’s reject the air at a constant pressure in the final process 1-0. This is known as the Otto cycle and is one of the most powerful tools for a Mechanical Engineer.
“How can we measure the velocity of a fluid without using any moving parts?”
Measuring the velocity of a fluid is one of the most useful things we can do. With this, we can find out how much mass is flowing within a system, and adjust all calculations accordingly. But since fluids lack any form of defined shaped, measuring their average velocity can be very difficult. So how can we use our engineering mindset to solve this problem? Well, to begin, let’s look at how pressure moves within a system. A fluid’s total pressure is made of up both static pressure (the default, inert pressure) and velocity pressure (the pressure associated with the momentum of the fluid).
Since it is rather simple to obtain the total and static pressures and use their values to find the final velocity, let’s build a machine to do exactly that. Since fluids move steadily through a pipe, let’s start with that. And since we want to find the total velocity of a fluid, let’s also put the fluid through the hole. Then let’s also have holes perpendicular to the main tube to measure the static pressure. Then let’s subtract the difference to get the velocity pressure, and divide by the fluid’s density to obtain the fluid velocity. This machine is known as a pilot tube and is used widely in airplanes to measure the airspeed and HVAC systems to find the refrigerant flow rate.