THE PULSE JET ENGINE the PULSE JET can be defined as a compressorless non steady flow jet engine.
The pulse jet and wave engine are closely related. Pulse jet is in effect a wave engine with a wave percompression
ratio of about 1.0. But the pulse jet by definition is without wave precompression of the combustion
charge. In the standard form the pulse jet consists of a shaped tube fitted with a check valve to stop
reverse flow in the tube. The air flowing into the tube through the valves is mixed with a continuous
spray of fuel or gas. The mixture is then ignited and a pressure rise is a product of the explosion.
The check valves close and the exhaust gases are forced out the tailpipe. The expansion wave generated
by the explosion creates a vacuum behind the check valves and the check valves open to let in a new charge
and the cycles repeated. The spark plug is only needed for starting. When the regular cycle is established
the fresh fuel and air charge is ignited by the hot gases of the previous explosion. In this explanation
the idea that the fresh charge is ignited by the reverse expansion wave is not included in this mechanism
as that is an attribute of wave engines.
THE VALVELESS PULSE JET ENGINE The function of the intake valves in the conventional pulse jet is
to prevent the reversal of flow at the inlet and therefore a negative momentum transport when the combustion
chamber pressure becomes higher than the free stream stagnation pressure. This same purpose can be achieved
without valves by proper utilization of the wave phenomenon in an inlet duct of adequate length. The
combustion chamber pressure required to stop the flow in the intake duct is the hammer pressure corresponding
to the initial conditions of this flow. This hammer pressure is generally higher than the free stream
stagnation pressure but may still be exceeded by the peak explosion pressure. Therefore the pressure
waves that are generated by each explosion may still produce a reversal flow in the intake duct. The
flow reversal can however be kept from extending all the way to the intake by making the inlet duct long
enough to permit each pressure waves to be weakened to hammer strength by the expansion wave which follows
and overtakes it. A pulse jet working on this principle is noteworthy that the only feature which distinguishes
the configuration of this duct from the conventional ram jet is the sharp change in cross-section at
the combustion chamber inlet. The sudden cross section change causes the collection of residual hot gasses
in the annular or dead water region behind the shoulder and the turbulent interpenetration of these gasses
with the interflowing fresh charge. The area of the contact surface and with it the instantaneous rate
of heat release are thus increased to the extent that for the equivalent pollytropic process becomes
appreciably lower than 0 the combustion takes place intermittently even though the fuel is injected continuously.
The distinction between ramjet and valveless pulse jet is actually debatable because of severe burner
pressure fluctuations have been known to be associated with spontaneous occurrence of flow oscillations
in ramjets under certain conditions. Flow oscillations in ramjets under certain conditions represent
beneficial modifications of the heating mode. There are indeed subsonic flight conditions under which
the performance of ramjets is favorably affected by such fluctuations. Ramjets which operate in this
manner are also sometimes referred to as valveless pulse jets despite their inability to operate statically
like the pulse jet proper. The mechanism of operation of these three types of ducts is essentially
the same. Side intake arrangements appear to be particularly effective in increasing the instantaneous
rate of heat release and therefore the thermal efficiency of the engine. The decay of the upstream propagating
pressure waves in the inlet duct can be hastened by the use of reservoir chambers. The reservoir chamber
inlet is often successfully used. Use of flow rectifiers such as passages offering less resistance to
the flow in one direction in the opposite direction has been attempted for the purpose of inhibiting
the reversal of the flow in the intake duct but none of these rectifiers has proven more effective than
the simple duct arrangement of adequate length. With low-frequency pulse jets the intake duct length
required to prevent spillage at the intake may be excessive. In such cases it is profitable to accept
some measure of spillage. The momentum of the reverse flow can be conserved for thrust by turning the
intake duct back 180 degrees as in several of the SNAPPYBOY models. The potential performance of the
valveless pulse jet engine is the same as that of the pulse jet with valves. The main advantages of the
valveless pulse jet are its mechanical simplicity, long service life,and in addition throttling and
wave energy losses can be made lower in the valveless then in the valved type pulse jet. The valveless
configuration lends itself better to external streamlining.
SNAPPYBOY DID NOT INVENT THE PULSEJET. MUCH OF THIS INFORMATION IS 100 YEARS OLD. SNAPPYBOY SNAPPYJETS
AND GLUEBABIES ARE STANDARD SNAPPYBOY HARDWARE ITEMS.
TO ORDER SEND TO: SNAPPYBOY 921 S
VAL VISTA 163 MESA AZ 85204 USA
