Pulse Jet Engine- Jam Jar Jet
Author: Alvin Lee (forked from LabCoat)
I couldn’t find any source on the internet that provides an explanation about how the jam jar jet works so I came up with my own explanation…
The Jam Jar Jet is a simple construction. A suitable sized jar with a 1-1.5 cm hole drilled in the center of the cover. Fuel, usually methanol or ethanol, is poured into the jar and the jar is given a good shake to allow the fuel to evaporate and create a flammable mixture of fuel vapor. Then the jar is lit from the hole and the engine starts, with the flame oscillating back and forth, creating the characteristic murmur and interesting light show.
What creates the oscillation is something interesting.
First Stage- IgnitionIn the first stage when the jar is lit, the flame spreads and consumes most of the fuel air mixture inside the jar and this ignition causes much of the air to be pushed out with the flame front. The air exits the jar with some speed.
Second Stage- Decreasing pressure
Because the air exits the jar at quite a fast rate, and the air carries some momentum, it creates a partial vacuum inside the jar.(Due to the air’s inertia) Also, because the jar has a rather large surface area, the jar cools fast, contracting the air inside the jar. Both actions cause the pressure in the jar to drop.
Third Stage- ‘Strike back’
Since the existing flame front ‘rides’ on some residual fuel vapor and survives on the oxygen outside the jar, when air gets sucked into the jar, the flame also gets pulled into the jar. Meanwhile, the heat from the first combustion evaporates even more fuel, releasing high concentrations of fuel into the lower and middle part of the jar. As the flame front (which is moving with the air, rich in oxygen) meets the fuel vapor line, it starts another round of combustion since all 3 conditions for combustion is met (Heat- from flame front, Oxygen- from air being sucked in, Fuel- from fuel vapor line).
The process then repeats itself causing the flame to oscillate.
As the combustion progresses, the jar is ‘seasoned’ and the burning is stabilized.
Often, the jar may either refuse to light up or light up with a whoosh and then… nothing. Refusing to light up may be caused by too high a fuel concentration which the whoosh may be caused by too low a fuel concentration. It may also be caused by incorrect size of the hole being drilled.
[MJC InnoLog : Keep It Simple, Stupid]
11:23 PM