ROCKET ENGINES

TURBOJET

The turbojet employs the same principle as the rocket. It burns oxygen from the atmosphere instead of carrying a supply along.
Notice the similarities: Fuel continuously burns inside a combustion chamber just like the rocket. The expanding gasses escape out the nozzle generating thrust in the opposite direction.
Now the differences: On its way out the nozzle, some of the gas pressure is used to drive a turbine. A turbine is a series of rotors or fans connected to a single shaft. Between each pair of rotors is a stator -- something like a stationary fan. The stators realign the gas flow to most effectively direct it toward the blades of the next rotor.
At the front of the engine, the turbine shaft drives a compressor. The compressor works a lot like the turbine only in reverse. Its purpose is to draw air into the engine and pressurize it.
Turbojet engines are most efficient at high altitudes, where the thin air renders propellers almost useless.

TURBOPROP

The turboprop is similar to the turbojet, except that most of the nozzle gas pressure drives the turbine shaft -- by the time the gas gets past the turbine, there's very little pressure left to create thrust.
Instead, the shaft is geared to a propeller which creates the majority of the thrust. 'Jet' helicopters work the same way, except that their engines are connected to the main rotor shaft instead of a propeller.
Turboprops are more fuel efficient than turbojets at low altitudes, where the thicker air gives a propeller a lot more 'traction.' This makes them popular on planes used for short flights, where the time spent at low altitudes represents a greater percentage of the overall flight time

TURBOFAN

The turbofan is something like a compromise between a pure turbojet and a turboprop. It works like the turbojet, except that the turbine shaft also drives an external fan, usually located at the front of the engine.
The fan has more blades than a propeller and spins much faster. It also features a shroud around its perimeter, which helps to capture and focus the air flowing through it. These features enable the fan to generate some thrust at high altitudes, where a propeller would be ineffective.
Much of the thrust still comes from the exhaust jet, but the addition of the fan makes the engine more fuel efficient than a pure turbojet. Most modern jetliners now feature turbofan engines.

AUTOMOBILE ENGINES.ATIKSON ENGINE

The Atkinson engine is essentially an Otto-cycle engine with a different means of linking the piston to the crankshaft. It was originally designed to compete with the Otto engine, but without infringing on any of Otto's patents.
The clever arrangement of levers allows the Atkinson to cycle the piston through all four strokes in only one revolution of the main crankshaft, and allows the strokes to be different lengths -- the intake and exhaust strokes are longer than the compression and power strokes (In this illustration... see below).
This also obviates the need for a separate cam shaft. The intake (if used), exhaust, and ignition cams are located on the main crank shaft. My illustration shows only an exhaust cam.
Everything I know about the Atkinson engine came out of Building the ATIKSON ENGINE CYCLE This illustration draws heavily from that excellent book.
A sharp-eyed, well informed visitor to this site has just discovered an important error in one of the above statements, and in my illustration. In Atkinson's original engine, the power stroke was made longer than the intake stroke. Apparently, by allowing the mixture to expand to a larger volume than was drawn into the cylinder, more energy was extracted giving greater fuel efficiency than the Otto-cycle (everything else being equal). Thank yous go to George, for pointing this out; and John for the additional information.
My illustration draws from a book about building a model of the Atkinson engine. I may have my measurements wrong, or perhaps even the model designer may not have taken the stroke lengths so carefully into account. After all, he didn't have much to work with: "...Jim was able to build his engine using one simple drawing and a photo..."
In any case I still recommend the book, and still feel that I've correctly illustrated the fundamental operation of the Atkinson. I would love to hear any additional comments about this delightful engine.

piston and cylinder

SOMETHING ABOUT ENGINES

TWO STOKE

The two stroke engine employs the crankcase as well as the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston.
Intake. The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston. The illustrated engine features a poppet intake valve, however many engines use a rotary value incorporated into the crankshaft.
During the downward stroke the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke

Transfer/Exhaust. Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well.

Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).
Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.
Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes two stroke engines popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes. Unfortunately most two stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.

SOME MICROMOUSE DESIGN WHICH MAKES COMPACT

MY MICROMOUSE COMPETITION....

Micromouse is a miniature maze-solving robot that is capable of moving to any accessible point in a maze. Micromouse navigates using only infrared sensors and optical encoders. He is always aware of his position in the maze and how to return to the start. Micromouse’s sensors also allow him to avoid walls and keep track of how far he has traveled within every square of the maze.
In the APEC competition points are deducted when a robot touches a wall or is helped by a person. The walls of the maze are only 5cm tall. The maze is a grid of 18cm x 18cm unit squares. At every lattice point where four corners meet there will always be at least one wall touching. To comply with these regulations we placed all of the sensors near the ground and tried to make the smallest platform possible. We found that size was the greatest challenge in this project. When working on a small scale tiny changes in distance make a huge difference, and accurate steering is much more difficult. We learned that an error of 0.5cm in a turn in the beginning of the maze could be disastrous. To work with such small measurements from our sensors we discovered that redundancy was key. By constantly taking measurements from every sensor and making small corrections, it was possible to keep Micromouse on the correct path.

Integrated System

Micromouse is controlled by a Mavric-IIB board equipped with an Atmega128. We chose this board because of its multiple analog inputs, PWM outputs and practical layout. This is by far the best microcontroller board for a novice because it has extensive documentation and many software examples online. However it may have been overkill for Micromouse because of its large size, and many unused features, such as the RS232 chip and extra I/O ports.Micromouse is designed to always be aware of its current (and past) surroundings. This task is simplified by viewing the entire maze as a two dimensional coordinate system. The only variables we account for in Micromouse’s environment are the four walls immediately surrounding the square he is in. Micromouse uses infrared sensors to obtain and continuously update this information. The encoders allow for the Micromouse to track distance and know the coordinates of its current location in the maze. With only this knowledge, Micromouse is capable of solving the maze.

Mobile Platform

The Micromouse platform consists of three layers. The bottom layer is enclosed and houses only the servos and the power board, with some extra room for wires. The middle layer holds the batteries and the front IR sensor. The microcontroller is on the top layer allow repairs or changes to be made easily. Every layer has large holes for cables to travel through. Micromouse has two wheels on its central axis and casters in the front and back for balance. The center of gravity lies in the back so it does not wobble during deceleration. The entire platform fits into an 18cm x 18cm unit square that represents one square in the maze. This size constraint was the most difficult part of building the platform. A smaller Micromouse is preferable as it can negotiate corners easier and allow some room for error in the wall following algorithm. The small platform allowed for two IR sensors to be placed on both the front and the back of Micromouse facing outwards. This was convenient because we could know if the entire robot was contained within one square of the maze, and use the front and the back sensors to align with walls.

Actuation

Two servos are used to control the wheels of the Micromouse. Servos were chosen rather than stepper motors because they are smaller, lighter and easier to control. Although they are less accurate, optical encoders were sufficient to monitor distance and speed. Precise turning was achieved by using the front and back IR sensors to align with walls when the Micromouse came to a dead end. When the robot entered a square that had a wall on the far side, it was possible to reposition Micromouse by moving it the correct distance from the wall it was facing. This prevented him from falling behind as he solved the maze, which would lead to incorrect decisions.

A MAZE EXAMPLE

When Micromouse is very close to a wall, which happens quite often, it is impossible to know what side of the characteristic curve we are looking at as he moves quickly through the maze. We solved this problem by placing two sensors back to back on the front and back of Micromouse rather than on the sides of the platform. This ensured that the walls were on the more linear side of the curve.

BehaviorsMicromouse always begins its mission in a corner of the maze. The maze was constructed on a grid of 7” x7” squares, almost identical to those in the competition. When the reset button is pressed, it will begin to find its way to the center of the maze as quickly as possible. The Micromouse knows the center of the maze by its size; it will be four grid squares together with no walls in the center.

The robot is in the green square as wishes to move to the red square. The values in the lower right corner of the square are the cost of each square. The entire program only used about 1 K of RAM