CoggesRailway said:
Hello.
Could someone explain the pumps some of the nicer models tend to have. Is it injecting water constantly in lieu of venturi like injectors? If so it must be a very small amount to stop it cooling things down and also I suppose it needs to pump at a higher pressure than the boiler?? I only have a fill, run, stop, fill again set up. Sorry for dumb question.
No such thing as a dumb question ? OK there may be but this is not one of em!
Generally speaking, the sort of locomotive that has a waterpump and 'on board' boiler feedwater, is a locomotive that uses a continuous supply of fuel and a blast pipe to draw the fire. A gas locomotive has a finite supply in that one can fill the gas tank and then run until the gas supply is exhausted. A single fill of oil, gas and water and this sort of locomotive will run very well until the gas runs out and one has to start from scratch. A water gauge is not needed for this type of locomotive run in this way.
With coal firing (and some spirit fired locomotives) one has a blast pipe to draw the fire, several firetubes in the boiler thereby reducing the water capacity. Like a full size locomotive one needs therefore a continuous supply of water to keep the boiler fed. On a larger scale model one can use injectors. It IS possible in our size, but as you are probably aware, they can often require a bang with a wheelspanner on the prototype to get em working again and in our sort of size they are definitely problematical. What is done therefore is to provide an axle or a crosshead pump to supply water to the boiler whilst running. You have picked up on the overfilling problem and to cope with this there is a hand operated bypass valve to return water to the tank or tender when the gauge glass creeps up and the loco gets a bit waterbound. The water gauge is therefore needed to keep an eye on the water to ensure that a) one has enough and b) one does not have too much!
You will have noted that more and more nowadays, gas fired locomotives are supplied with a water gauge. Not necessary for single fill, but most locos can now be converted to have water added to the boiler during a run. It is then perfectly possible to stop briefly when the gas runs out to refill whilst remaining in steam. One can also stop once an hour to refill the oiler. This is a sort of halfway house if you like. A water gauge is also important if you have a whistle and like to use it! With experience and even without a water gauge one will get to know how much water is in the boiler by the way your gas fired loco behaves. For instance as the water level in the boiler drops, your loco will get a little more sprightly..
The gas fired locomotive does not have a blast pipe because the burner entrains its own air in the jet holder. This is what the two holes either side of the holder body are for. This is not the same as the full size or a coal fired locomotive in that the loco fire does not respond to the load put on it as a coal fired loco does. As a coal fired locomotive hits a gradient the engine is working harder and the blast will sharpen up which will in turn draw the fire and increase pressure.
The gas fired loco has many advantages. It is of course easier to use, but for me the most important one is that it burns fairly cool and one gets a nice show of steam exhaust at the chimney. One can always replicate the sound of the blast pipe (the chuff) by adding a chuff pipe to your model and you will hear it work as it pulls a heavy train up a gradient.
Hope this helps ? ask away if there is anything you are not clear on (I have not mentioned pot-boilers either gas or spirit).
