The Four Stroke Engine

 

1. The Intake or Admission Stroke

During the intake or admission stroke, the piston moves downward as a charge of combustible fuel and air is admitted into the cylinder through the open intake valve. At the completion of this stroke the intake valve closes.

 

2. The Compression Stroke

During the compression stroke, the crankshaft continues to rotate, the piston is forced upward in the cylinder, and both intake and exhaust valves are closed. The movement of the piston upward compresses the fuel-air mixture.

3. Power or Expansion Stroke

As the piston approaches the top of its stroke within the cylinder, an electric spark jumps across the points of the spark plugs and ignites the compressed fuel-air mixture. This is the ignition event, or event No. 3. The intake and exhaust valves are closed.

Having been ignited, the fuel-air mixture burns. It expands as it burns and drives the piston downward. This causes the crankshaft to revolve. Since it is the only stroke and event that furnishes power to the crankshaft, it is usually called the power stroke, although it is sometimes called the expansion stroke for purposes of instruction. This is event No. 4. The intake and exhaust valves are closed.

4. The Exhaust or Scavenging Stroke

During the power or expansion stroke, the hot gases obtained by combustion exert tremendous pressure on the piston to force it to move downward, but near the end of the stroke this pressure is greatly reduced because of the expansion of the gases. At this stage, the exhaust valve opens as the crankshaft continues to revolve and the piston is again moved upward in the cylinder by the connecting rod. The burning gases remaining in the cylinder are forced out through the exhaust valve, hence this stroke is usually called the exhaust stroke, although it may be called the scavenging stroke for purposes of instruction.

This completes one engine cycle.

 

Jet Engines

Centuries ago in 100 A.D., Hero, a Greek philosopher and mathematician, demonstrated jet power in a machine called an "aeolipile." A heated, water filled steel ball with nozzles spun as steam escaped. Why? The principle behind this phenomenon was not fully understood until 1690 A.D. when Sir Isaac Newton in England formulated the principle of Hero's jet propulsion "aeolipile" in scientific terms. His Third Law of Motion stated: "Every action produces a reaction ... equal in force and opposite in direction."

The jet engine of today operates according to this same basic principle. Jet engines contain three common components: the compressor, the combustor, and the turbine. To this basic engine, other components may be added, including:

1. A nozzle to recover and direct the gas energy and possibly divert the thrust for vertical takeoff and landing as well as changing direction of aircraft flight.

2. An afterburneror augmentor, a long "tailpipe" behind the turbine into which additional fuel is sprayed and burned to provide additional thrust.

3. A thrust reverser, which blocks the gas rushing toward the rear of the engine, thus forcing the gases forward to provide additional braking of aircraft.

4. A fan in front of the compressor to increase thrust and reduce fuel consumption.

5. An additional turbine that can be utilized to drive a propeller or helicopter rotor.

The Turbojet Engine

The turbojet is the basic engine of the jet age. Air is drawn into the engine through the front intake. The compressor squeezes the air to many times normal atmospheric pressure and forces it into the combustor. Here, fuel is sprayed into the compressed air, is ignited and burned continuously like a blowtorch. The burning gases expand rapidly rearward and pass through the turbine. The turbine extracts energy from the expanding gases to drive the compressor, which intakes more air. After leaving the turbine, the hot gases exit at the rear of the engine, giving the aircraft its forward push ... action, reaction !

For additional thrust or power, an afterburner or augmentor can be added. Additional fuel is introduced into the hot exhaust and burned with a resultant increase of up to 50 percent in engine thrust by way of higher velocity and more push.

The Turbofan Engine

A high bypass turbofan engine

A turbofan engine is basically a turbojet to which a fan has been added. Large fans can be placed at either the front or rear of the engine to create high bypass ratios for subsonic flight. In the case of a front fan, the fan is driven by a second turbine, located behind the primary turbine that drives the main compressor. The fan causes more air to flow around (bypass) the engine. This produces greater thrust and reduces specific fuel consumption.

A low bypass turbofan engine

For supersonic flight, a low bypass fan is utilized, and an augmentor is added for additional thrust.

 

The Turboprop/Turboshaft Engine

A turboprop engine uses thrust to turn a propeller. As in a turbojet, hot gases flowing through the engine rotate a turbine wheel that drives the compressor. The gases then pass through another turbine, called a power turbine. This power turbine is coupled to the shaft, which drives the propeller through gear connections.

A turboshaft is similar to a turboprop engine, differing primarily in the function of the turbine shaft. Instead of driving a propeller, the turbine shaft is connected to a transmission system that drives helicopter rotor blades; electrical generators, compressors and pumps; and marine propulsion drives for naval vessels, cargo ships, high speed passenger ships, hydrofoils and other vessels.

 

The Ultra High Bypass Engine

A logical approach to improving fuel consumption is even higher bypass technology. Mechanical arrangements can vary. During the 1980s, GE developed the Unducted Fan UDF® engine which eliminated the need for a gearbox to drive a large fan. The jet exhaust drives two counter-rotating turbines that are directly coupled to the fan blades. These large span fan blades, made of composite materials, have variable pitch to provide the proper blade angle of attack to meet varying aircraft speed and power requirements. Powerplants such as the UDF® engine are capable of reducing specific fuel consumption another 20-30 percent below current subsonic turbofans.