Propex vehicle in motion

Discussion in 'Mech Tech' started by Dazza, Dec 6, 2017.

  1. I've just driven the van the first couple of times since I've fitted the propex , both times it cut out when I got a little bit of speed up.
    I did look at the vehicle in motion bracket when I first fitted it as I couldn't see the principal behind it...not enough to pay the £30 for it anyway..

    So , now I know I need to do something , can anyone explain how the vehicle in motion bracket is supposed to work?
    At the moment my inlet and exhaust both go in opposite directions towards either side of the van.. The VIM bracket looks like it mounts them both side by side which does seem a bit 'wrong' to me...
  2. I use my propex on the move with no issues.....both pipes just exit facing backwards one either side of the existing "heater they are probably over a foot apart.
  3. The heater will fail if there is a large air pressure difference between inlet and outlet - either the combustion fan cannot blow air against pressure from the exhaust,or the flame will blow out with too much air.
    So the closer together the pipes are, the better , or at least at the same pressure in the airflow round the bus.

    If you put them opposite sides of the bus, then even a cross wind can blow the heater out.

    Provided the exhaust gas does not go straight back into the intake it will work. So spacing them apart underneath the bus should work but could go out if the exhaust gases collect underneath.

    So putting the exhaust pipe just behind the intake facing into empty space, on the side of the vehicle means the pressure difference is small even at speed, and providing you do not put an awning or something in the way, the hot exhaust gas should rise up out of the way of the air coming in.
    Molteni Mike and snotty like this.
  4. Had this few years ago and as above depends on exhaust outlet, we just cable tied it up and out of the way as it was just hanging before. Trial and error really
  5. I'll move them both to the opposite side to where we fit the awning and put them a foot or so apart facing toward the rear , I see your point about the pressure difference mike...ta
  6. They dont need to be so far apart and are best positioned exhausting towards the non sliding door side. There is a maximum length of inlet and exhaust total which i cant recall but may be circa 3.5m. Will see if i can find the info. Inlet to the front and exhaust to the rear approx. 4 inches apart or possibly less but im unsure as to how critical that is.
    If in doubt call propex. They will advise and wont try to convince you you need the bracket in my experience.
    Last edited: Dec 7, 2017
    snotty likes this.
  7. Divert the exhaust out the rear for higher performance.
  8. what do you mean?
    Fat_Brum likes this.
  9. I think this may be reference to Newton’s 3rd law of motion.

    “For every action there must be an equal and opposite reaction”

    Exhaust exiting at rear would add to the forward motion by the same amount.

    Fat_Brum likes this.
  10. Taking into account the effect of the intake duct and any losses within the ductwork (and also noting that the movement of combustion gases are fan assisted) then the overall effect would be equilibrium and there would be no propulsive contribution, however the requirement for electrical energy ultimately from the alternator and therefore the engine would result in more energy being required to propel the vehicle forwards. Therefore for the same energy input the output would be reduced therefore effectively slowing the vehicle rather than increasing its speed!
    nicktuft, fritt and snotty like this.
  11. If the heater can accelerate the air it can produce thrust...

    This is just a backfiring Propex .
    SeanOC likes this.
  12. Sod a Scoobie conversion, just strap one or two of those to the van!

    Sent from my iPhone using Tapatalk
    Razzyh likes this.
  13. but the thrust must come from accelerating air which needs to be taken in to the closed system therefore the two cancel each other out. if a jet engine took in air from the same side of the engine it took its intake from it wouldn't work would it.
  14. Don’t forget what Newton said about ‘equal’
    The forces do indeed cancel each other out but then ‘mass’ comes into the equation.
    The air going into a jet engine is compressed before combustion and has a greater mass as it is expelled and tries to push the engine forward.
    Can’t believe I’m quoting ‘o’ level physics I learned over 40 years ago. I knew it would come in handy one day.


    By the way, that’s how thrust reversers work on jet engines. They divert the air back out in the same direction it went in. Because the compressed exhaust gases have a greater mass - deceleration is induced
    Dicky likes this.
  15. Ok - poor comparison. However, energy comes into the equation both from the electrical power - covered above, and from the gas! (in the jet engine comparison the energy is from aviation fuel) the gas is outside the vehicle performance equation but the electricity isn't therefore the vehicle effectively is slowed rather than increased.
  16. I hope nobody is reading this thinking were actually serious btw!
  17. bernjb56

    bernjb56 Moderator

    I’m reading it thinking ‘Haven’t they got any work to do?’ ;)
    Bertie the Bus, Dicky and Terrordales like this.
  18. Terrordales

    Terrordales Nightshift Mod

    I think this is what happens when people retire.
    Sydney, bernjb56 and fritt like this.
  19. :lol: always a risk!
    bernjb56 likes this.
  20. @Dazza , has the nooo blooo Propex , this has flow sensors to ensure the balance between the inlet and exhaust gasses are something like equal . As said before, the unit will trip out if the pressures are fluctuating due to the van being driven .
    The older versions such as the 1600 compact and X1 don`t have this feature so are far easier to use whilst driving - i think the X1 was fitted to thousands of GPO delivery vans back in the day , before sensible cab heaters came along ....

    Dazza likes this.

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