The following material includes a general technical overview, as well as an in-depth discussion of the principles of operation of the G8-2 jet engine. The technical discussion is included for those who would like a more thorough understanding of the principles of operation of the G8-2 engine. It is not necessary, however, that one understand the physics involved in order to build the engine.The G8-2 jet operates differently than a turbojet, ramjet, or pulsejet. It is essentially a burner with a sonic synchronized intake. Classically, the G8-2 engine is a "pressure jet" engine. If you would like more technical information on this jet please contact us or purchase our informational CD.
How it Works
The G8-2 engine was designed around the fuel, liquid propane; primarily because of propane's "double energy" content. The energy available from propane comes from the compressive energy resulting from its low boiling temperature (-43.8 degrees F), as well as from the heat of combustion. By taking advantage of propane's natural compressive energy, the design of the engine is simplified. In addition, propane is safer than gasoline, readily available, easily stored and controlled, and it is inexpensive. Propane weighs 4.23 pounds per gallon, and releases 21,690 btu/lb when burned (oxidized).
Propane's compressive energy, the inherent pressure inside the fuel tank, delivers the liquid fuel to the engine, which eliminates the need for a fuel pump. Upon reaching the heat exchanger coil inside the combustion chamber, the liquid propane is vaporized and super-heated. After being converted to a hot gaseous state (about 1200 degrees F), it is then routed through the hot gas line to the nozzle, where it is injected into the first stage of the intake system at supersonic speed. The high velocity of the gas does the work of drawing in the surrounding air. When the charge enters the combustion chamber, enough air has been induced to provide the correct fuel-air ratio of 15:1 by weight (15 parts of air to 1 part of fuel). Prior to this point, the mixture is too rich to support combustion.
In order to operate correctly, the second stage duct must be sonically tuned to the third stage duct. The intake ducts are tuned to one-half wavelength in the second stage and one-quarter wavelength in the third stage. This permits the jet to operate statically at its rated thrust.
The effect of the tuned intake is similar to that of a tuned exhaust on a conventional reciprocating engine, but in reverse. With a conventional engine, a tuned exhaust is used to reduce back-pressure. With the G8-2 jet engine, the tuned intake system is used to increase pressure on the intake side. The effect is to overpower shock wave reversals within the intake system, which would otherwise inhibit the flow of the charge into the combustion chamber. Acting in conjunction with the kinetic energy of the incoming propane, the tuned intake helps maintain a stationary flame front at the entrance to the combustion chamber, which prevents the flame front from backing up into the intake system. Without a sonically tuned intake, the engine will produce very little static thrust.
When the air/fuel mixture reaches the combustion chamber it releases its second energy, the heat of combustion, which results in jet thrust. During dynamic operation (forward motion), thrust increases with speed due to the effects of ram air. Ram air also reduces specific fuel consumption (sfc), or the fuel consumed per unit of thrust. At 200-250 mph, for example, sfc drops to about one-third of the static value, and continues to decline with increased speed. A sealed intake scoop, however, is essential in order to gain the benefits of ram air induction.
The length of the tailpipe is such that complete combustion takes places inside the engine. If the tailpipe were shortened, a visible flame would appear, and jet thrust would decrease. The fishmouth cut in the tailpipe, and the small hole(s) in the side(s) of the third stage intake duct, damp shock wave reversals in the intake system. This reduces noise and increases thrust. The engine is throttled by opening or closing the liquid fuel supply valve. Ignition is provided by a model airplane spark plug mounted in the tailpipe.