Fuel injection System

Final Design

• Pressure Vessel

• Valve Selection

• Control System

• Containment Structure

Pressure Vessel

             The pressure cylinder is where the pressurized Nitrogen and the JP-10 come into contact and the line pressure is developed.  The cylinder must be strong enough to contain the relatively high pressures inside without exceeding the failure stress of the material.  Since the nozzle designs require operational pressures up to and including 150psi the design was toleranced to a factor of safety of at least 2 for 200psi.  Given that this indicates a maximum operational hoop stress of approximately 5.51 MPa with an outer radius of 8 cm and a thickness of 2 cm.  This is well below the failure stress of steel piping, which averages to around 500 MPa. 

An alloy of steel should be used for numerous reasons.  First, the ability of steel to avoid fatigue failure means that the device can be used much more often without fear of a rupture when compared to a material such as aluminum.  Second, there are numerous reliable and cheap surface treatments that can be used on steel.  A surface treatment such as Chromium plating would be favorable for the inside of the vessel to avoid possible oxidation or other contamination of the fuel.  Finally, steel is readily available and is easily machined using standard processes. 

The finalized design of the pressure vessel consists of two main parts, the body and the lid.  The body can be made through welding a round plate with an outside diameter of 16 cm and a thickness of 2 cm onto a 24 cm length of steel pipe with an outer diameter of 16 cm and an inner diameter of 12 cm.  The non-welded end should be inset such that it has a 2 cm long, 1 cm deep lip.  This lip should be tapped to accept the matching threads to be placed on the lid.  Having a well machined threading fitting here helps seal the joint between the body and the lid so that a gaseous or fluid leak becomes unlikely.  A second seal can be accomplished by cutting a shallow ring into the non-threaded surfaces where the body and the lid meet.  Rubber o-rings can be fitted into these cuts such that when the lid is tightened down a leak proof seal is obtained.  These o-rings should have half of their body sitting above the upper part of the cut when installed into the cuts.  A drilled and tapped hole should be placed on the welded end of the body in a manner which allows an air line quick connect (such as a Coilhose Pneumatics Male-Threaded Six-Ball Coupler (A937B4M)).  This quick connect will allow a standard male fitting to inserted in order to fill the tank with the required pressure of Nitrogen.  When not being used for this purpose the line can be disconnected and the quick connect will provide a leak proof seal for both the Nitrogen and alternatively the JP-10. 

            The lid of the pressure vessel can be simply milled out of a 16 cm diameter round piece of 3 cm thick steel.  A cut 1 cm deep and 2 cm wide from the bottom edge is to be made along the bottom edge of the circle to match the similar notch in the body of the pressure vessel.  Threads corresponding to the tapped section of the body are also to be cut into this notched region.  At the center of the lid is a through hole that is drilled and tapped to accept ½” NPT piping and threads.   This hole is to be used to run the piping system through which the JP-10 will be pressurized and will flow through when the valve is opened.  Also through the lid should be a drilled and  tapped  hole that will accept the same type of air line quick connect as is present at the bottom of the body of the pressure vessel.  This second quick connect becomes important when the orientation of the pressure vessel needs to be flipped.

Pressure Vessel Sketch: (all units in cm)

Black represent the body of the pressure vessel, red lines outline the lid, blue outline the piping, and yellow lines represent quick connect air line fittings.

            Versatility is a key aspect to the success of this design, and the pressure vessel’s design reflects this.  There are three main orientations in which the design can be used during experimentation.  The fuel delivery system may be set up such that it sprays the fuel downward, sideways, or straight up.  Any one of these orientations may be used by the Air Force staff, while the design team would recommend that the downward and upward shots may provide the best results.  This is due to gravity’s influence on the spray pattern.  When shot upwards, spray droplets will eventually run out of momentum and stall in the air, allowing for a greater spread of fuel throughout the test chamber.  Likewise, a downward spray will allow for lower pressures to be used, resulting in larger droplets that fall like rain drops through the test cell.

In order to be able to facilitate these three orientations the pressure vessel has two main design aspects.  First, the air line quick connects on both the top and bottom of the vessel (with the lid securely tightened down) means that either the top or the bottom can receive the pressurized Nitrogen.  This is important since the pressure vessel can be flipped up to 180° in order to change from a downward to an upward spray.  Having the quick connects on both sides also keeps one from putting the Nitrogen up through the fuel which could cause the aeration of some JP-10 particles or the stagnation of Nitrogen bubbles into the JP-10. 

The second important design aspect is the threaded center hole in the lid of the pressure vessel.  This allows for piping to be inserted from both ends of the hole.  Coming out of the hold through the top of the lid would be a permanent pipe which leads to the control valve and then to the nozzle.  If the experiment calls for a downward or a sideways spray then this will not be changed with the lid being on the bottom of the pressure vessel pointing towards the ground.  For these spray orientations the quick connect mounted in the body of the pressure vessel would be used to fill the vessel with pressurized Nitrogen.  Should the experiment call for an upward shot a pipe with a length of 18 cm can be threaded into the bottom of the lid.  When pressurized the JP-10 will flow up through this pipe, through the permanent pipe and then through the valve to the nozzle.  In this scenario the quick connect in the lid of the pressure vessel will be utilized for Nitrogen pressurization.  For a sideways spray a 90° elbow will be used to direct the spray parallel to the ground.  Placing this 90° elbow immediately before the valve should produce results that would most closely relate to those of the same parameters that lack the elbow.

3-D Rendering, Set Up For :

An (1) upward spray (2) downward or sideways shot (3)  Different angle for an upward shot