P U L S E   J E T   E N G I N E   P E R F O R M A N C E

     My associate Robert Gray  and my old flying buddy Sonny Williams   and I have spent several months (retired as we are) doing considerable exploring trying to find some possible fresh useful  information on P-J engines.  We appreciate very much the assistance we have received from present and past jet flyers to give us a good  starting point in this project.  .   However, we wanted to see if there were some other sources of P-J info that may have not been explored fully, or at all,  as having possible  application for  C/L Jet Speed Flying.  We had explored thoroughly all of the Jet articles on Tom Wilk’s  CD of NASS Speed Times issues from 1983 to 2003 (CD $20).  Wilk's has two other CDs with jet information.  “CD #2 U.S. Speed”  and "Jet Speed Only" which are $10 each available at www.cpinternet.com/~tawilk36 .  They are truly a valuable source of Speed information.   We have reviewed info on full size and model size Valved Pulse Jets, Ram Jets, Valveless Pulse Jets and anything else that might have cool air coming in the front and hot air going out the back.  The Internet makes available information that would have been impossible to assemble in the past.  We have had considerable  communication via the forum at www.pulse-jets.com with “amateur pulse jet experimenter” who design and build their own engines or just plain love P-Js as we do.  .   
      .  We, as a group, have generally been modifiers of engines that we have had available.  First the DynaJet, then the extended Thomas head, some had equipment to custom make low port bore angle  extended heads, Hoyt/Davis/Raven head and Bailey pipe as built by BMS and now the Jet Bill designed  heads on the Bailey designed pipe.  These are the engines that we have applied Dremel tools, files , sandpaper and polish to supposedly make them a little more powerful.  But have we fully explored how they really work in order to get the most out of these noisy little beast?. 
     We would like to share with you some of our findings that  may be old “stuff” to most of you but there may be a “tid bit” or two that will spark your own thinking in a new direction or add something to what you already know.  We will start with the “jet master” himself, Franco Marcenaro whose pulse jet theory article first appeared in a 1983 issue of NASS’s Speed Times,  Vol. 2, No. 4, starting on page 13.  This article is on the previously mentioned CD.  It may also be downloaded from the Internet.  “Advanced Pulse Jet Theory” http://users.bigpond.net.au/pulsejet-pjaa/theory.html   This will get you started in what goes on in these engines and especially with the function of “waves” on the P-J cycle.

     Here are some other sites on the Internet to take a look at:
http://www.aardvark.co.nz/pjet/howtheywork.shtml   From www.aardvark.co.nz/pjet   Bruce Simpson,  a very basic and informative text and visual description of P-J cycle.  Also, home built P-J info and videos of P-Js running with sound!.
www.beck-technologies.com    Eric Beck  – he has a forum  – and much info on “home built” designs and pictures.  Very valuable. 
www.pulsejet.,amtjets.com     Netherlands engine design
www.precision-technologies.com     Troy Legner’s P-J engine
 www.pulse-jets.com    Forums on Valve and Valveless Pulse Jets.  Valuable info on both.   Also on the home page, there is a listing across the top of the page, a little hard to read, click on “Forum” and also click on “Pulsejet”.  On the latter you will find a lengthy paper by Fredrik Westburg “Inside The Pulse Jet Engine” and also a list of P-J engine drawings for download.  Check out the “Soviet FAA Class” drawings which are the Ivannikov World Record engine.  Note these postings relative to  C/L P-Js: “Pulse Jet Intake Air Velocity,” “C/L Speed Fuel Systems,” C/L Speed Engine Design,” and some comments about Franco’s article: “ Franco Marcenaro Comments.”. 
     Another source of valuable info is the  Russian P-J Engine  Book (1958 103 pages)  available from Rocket Science Books via e-bay.  The price seems to bounce around.  Best I have seen is $10.  The illustrations are worth the price of the book even if none of us are able to read Russian.  These guys had it “together” on P-J info as far back as the late 1950s and a small amount of this info made it into issues of the British Aeromodeller magazine thru the years.  That is where the text and drawings of Lipinsky’s 245mph record ship appeared in the 1972-73 ANNUAL.  There are also some valuable  points to be picked up on several   Forum posts titled: “Sport Jet Rules”, “Thrust”, “Line Rake Calculations”, “Go Fast on Alkie”, and the 4 part series on “Jet Speed History.”   At www.clspeed.com  Then in this order:   Speed Forum – C/L Speed Forum  – General Discussion  – then click #### msg.  to get the full list of topics  to come up. 

      COMBUSTION CHAMBER (CC)   What is going on in our P-Js?  Let’s start in the middle of the combustion cycle.  We have just had an explosion and exhaust gases are going out the pipe and the reed valves are starting to close.  We now  have negative pressure (vacuum) forming in the CC and this results in some of the  exhaust gases being “re-ingested” back up the pipe.  This is where the flare helps out when the ship is at rest or the engine is on test stand (static).   Some say the flare is not needed at high speed, but I have not heard a first hand report of anyone trying this.  Dale Kirn drilled perimeter holes around the flare but got no performance change.   It will obviously kill much static thrust and the ship may be slow getting off the ground or give you an obviously low reading on a test stand.  It was reported that Earl Bailey had a pipe flaring tool at one or more test sessions and “tweaked” this flare for optimum results.  Does anyone know if Bailey ever tried a longer CC and then backed down to the length that went in to production?  You would have to keep the pipe the same length to properly give this evaluation. 
       Back to the “ingestion” of exhaust gas.  This returning charge has at least two effects.  First, it reignites the air/fuel in the CC  for the next cycle.  This continuing ignition is not caused by the heat from the CC.   Next, this returning charge acts as  a compressor for fuel/air that is now coming in around the valves into the CC.  This would be slightly like, but not identical to,  the compression that takes place from the returning charge of fuel/air on a tuned pipe two stroke.  We may or may not have some wave effect on the P-J to assist in this compression.  It is believe that the portion of the returning charge that makes it back to the CC does not contain much if any  usable oxygen, but is just spent exhaust gases.   Some illustrations refer to the exit of the exhaust pipe as the “nozzle.”  It appears more likely that the tapered portion of the pipe is acting as the “nozzle” as you would have in other jet engines or rockets with out an “exhaust extension.”  The extension portion is, of course, a part of the overall length that the wave lengths are calculated on from the tip of the pipe to the back of the head.  It is of interest the different types of nozzles (taper) of pipes on small size P-Js.   Most appear to be dictated by fabrication methods or equipment.  Why the DynaJet designers  arrived at the extreme curve at the taper when previous P-Js (Buzz Bomb etc.) had very low angle tapers is still unknown.   The taper on “home builts” varies considerable. The Bailey F-J pipe makes sense with a way to stay within the  1.25 sq. in. area rule using the megaphone and larger exhaust pipe.  However, it brings into question as to why the Sport Jet  engine used the small pipe rather than the same large one, or at least a non tapered pipe with 1.25 sq. in.. area which would have been simpler to fabricate..       
      Now this may be a new thought for some of you.  Before the valves have time to close (they are not moving nearly as fast as the returning exhaust gases) there is spent combustion gas that goes forward  around these valve petals and into the intake.  There is an illustration of this in the Russian Jet book and there are others on the P-J Forum who are in agreement with this.   So it appears that at  the start of the intake cycle, the first gas to enter the CC is at least partially spent exhaust gas ...... not a fresh slug of fuel and air from the intake.   

       INTAKE:   There was an old expression back in the Hot Rod days: “It’s What’s Up Front That Counts.”   That has generally been the case with our P-Js.
 LENGTH;  We have found nothing to tell us why the 3 - 3-1/4" intake performs better.  The Russians used a cowl/tank/intake  about 5" long for their engines.  Maybe this is what it took to get their fuel capacity while staying within a minimum drag  pipe diameter.  There was a 5" long head and a Thomas head that went for less than $200  on e-bay several months back.   I wonder if the original owner from Ohio had ever tried it?  The “home built” or “P-J Experimenters” and Europeans don’t seem to “think out of the box” on head design and have continued to stay with basically copies of the DynaJet configuration. 
 PORT BORE ANGLE:   Jerry Thomas stuck with the old DynaJet 24 deg. Port angle.  The BMS Raven was 15 degrees and Jet Bill has settled on 16 degrees.  Same thought as above from “outsiders” applies here.  Most of the “P-J Experimenters”  drill the ports DynaJet style or use a separate back plate.  In either case, they make their own reed valves to cover some very big port openings.
 VENTURI or INTAKE BORE.  Most ‘home built” engines  are using venturi shaped intakes even if they have another way of getting fuel to the engine.  A number are using liquid propane as an easy way to get their engines to operate for their static stand experiments.  Others use petroleum and methanol fuels.  When pressure fuel systems were perfected in C/L Jets, it enabled the intake throats to be enlarged.  They seem to have settled at 1 1/4" I.D.  The only reasoning we see here is the old Hot Rod reasoning ....... “Bigger Is Better.”  Our discussion will give some further insight in this area.
 COWL/INTAKE CONTOURS:   Pull up this Internet site and review the text and illustrations.  http://Http://en.wikipedia.org/wiki/jet_engine   At that site, go to “Air Intakes” and scroll down to item 6.1.1 “Subsonic Inlets.”  We think this and additional information throw some light on why the “bull nose” or “blunt nose” cowl works.  It was evidently an accidental beneficial discovery that no one has fully understood.  This type cowl is a “one size fits all” solution to an intake situation that there is no perfect solution to short of a variable intake system.  We have air flowing through  the intake at speeds from zero, at time of closed valves, to possibly approaching 200 mph at peak intake velocity.  The more powerful leaf blowers have an outlet velocity of 200 - 230 mph. How would choking off your P-J engine with fingers over the intake compare with the leaf blower?  If you have some info on intake air velocity, please let us know.  Check out the “P-J Intake Velocity” post on www.pulse-jets.com for a discussion of this.   At rest (static)  there is no problem with the entering air with some of it even coming from back and around the cowl.    However, as the aircraft begins to move and the valves close, the air is forced to spill out and around the cowl until the valves open again.   Obviously the faster you go the greater the amount of spillage becomes and the leading edge curve becomes even more important.  The “bull nose” evidently helps the aerodynamic flow of this spillage by causing significantly less drag.   Notice the description of the  “Streamtube” which is only going to act perfectly at one instance in the pulse cycle.  The rest of the time we have various assorted conditions but it appears that in almost all conditions the outside airspeed is  going to be greater than the average air speed going into and through the intake.
         We have another problem with intake air flow.  This is referred to as the “Vena Contracta”effect.  (A description will follow) .  Dick Hall and I had discussed this phenomena in 1972 when he had observed fuel/air flowing back out of a straight bore two stroke  reciprocal engine intake.  We had discovered the same thing with a 196 mph record setting jet that had evidence of fuel/air flowing forward in the intake and out and around the cowl.  Later discussions with Dick and Don Monson in 1974  led us to the reduction of the intake diameter on the nitro burning 212 mph record setting Super Burp.  The other thing that can take place with an intake having improper curves is the stall factor on the intake entry.  I watched a movie at one of our model club meetings in the early 1970s that showed a wind tunnel test showing at certain stall conditions on an airfoil the boundary layer of air actually moved forward toward the leading edge.  As I said  before, a cross section through our jet intakes is an airfoil.  Both interior and exterior work together acting  like the top and bottom of a wing airfoil.   On the “bull nose” cowl, the air speed differential between top and bottom of this airfoil  is amplified even more by  the high  curve of the top and the  difference in air speed going in the intake and that going around the outside of the cowl.  Add to that the fraction of time that you have a “wall of still air” at the intake entry and  all of the “streamtube” having to divert out and around the cowl and you have a very strange unknown phenomena. 
DIFFUSER: The rear chamber of our intakes is a true diffuser.   The total surface area of the port bores prior to the valve surface is greater than the total area of the intake bore.  This causes a diffuser action with the air velocity being reduced and the pressure increasing in this rear portion of the head.  It is desirable to slow down the fuel/air charge so that the  “flame front”  does not just blow thru the CC. 

VENA CONTRACTA: I just  received a reply from a recent  enquiry I had made to Dick Hall, a retired Missile Engineer at Redstone Arsenal.  I quote: “You mentioned me telling you about fuel flowing back out the top of a straight bore venturi.  (1972 discussion, bp) I talked to a modeler friend who was the head of the aerodynamics section at the Arsenal.  There is a technical term for what was happening called “Vena Contracta.”  Latin for something like “entrance contracted.”  The best way to visualize what was happening is a bucket of water with a  hole in the bottom.  If the hole is just a plain round hole in sheet metal, the stream leaking out will be smaller than the hole diameter.  If the hole is flared with sufficient radius on the inside,  the stream of water can be as large as the bore of the hole.  Without the flare, the water flowing in from the periphery of the hole pushes toward the center and restricts the flow.  When air is involved instead of water and without an entrance radius, the air flowing in around the sharp entrance pushes toward the center and causes turbulence along the inside wall which can actually flow in the opposite direction along the wall.” This link will help show the effect through an orifice but does not cover possible turbulence along the wall of the flow path.  http://en.wikipedia.org/wiki/vena_contracta.com

We hope that this information will be food for thought and  give us all a better understanding of the operation of our beloved pulse jet engines.  It has been an enlightenment, to say the least, for Sonny, Robert and me.  Mike Langlois has probably been buried by all the e-mail copies as we have tried to keep him informed..  Do not hesitate to copy, forward, publish or do what ever you want to do with this information to give it the widest possible distribution.  We think it is of value for most jet flyers and others interested in the operation of the pulse jet engine.

Bill Pardue     
April 16, 2007
www.pardueatticus@aol.com