A couple things: temperature and pressure are so interdependent it does no good to seperate them. The question itself as expressed makes no sense.

A "spot fire" is an oil burner's idle setting.......and can be held steady provided the fuel, atomizing pressure and medium, burner and drafting are steady and good. It's akin to a small fire on one bit of the grate in a coal burner. There's no excuse for having to rush either one.

dave

We put a Tee in the oil feed line ahead of the firing valve and would use an overhead tank of diesel fuel to slowly bring the engine up to about 25-30PSI, then start heating the oil in the tender and when it was warm enough would cut over to No. 6.
You can keep a spot fire on the oil burners so that everything stays warm.

You can also use an oil furnace burner through the firedoor, or natural gas or propane, which in the case of test firing has the added advantage of keeping it clean in there.

dave

As others have posted a spot fire allows you to have a small flame in the firebox of an oil burner. There is a limit to how small you can go.

The burner uses steam or air to "atomize" the oil into a mist which is then ignited. The actual oil burners on steam locomotives are typically pretty crude affairs, they don't have nice fine nozzles like a fuel injector. If you drop below a certain pressure with the steam/air, the oil will drip down the front of the nozzle, which we call "drooling", forming a puddle on the ground as well as being a fire hazard. So you need some velocity to propel the oil clear of the burner lip.

Given that, you also need a certain minimum amount of oil flow to keep the fire burning. too little oil flow for the air flow you're using and you'll blow out the fire. So, there are limits on the low end of things. Exactly what those limits are depend on a variety of factors, oil viscosity and flammability, air temperature, firexbox temperature, whether you're using steam or air, and probably 10 more that I've overlooked. Suffice it to say, it varies, even on the same locomotive.

Once the firebox has warmed up a bit, you should be able to get a nice little torch burning. That fire is enough to very gently heat things up if the engine is cold, or if the engine is hot a good spot fire will maintain the pressure you already have. If the fireman is good and the oil is consistent, he/she will find a balance point and not have to make many adjustments. On some engines it can be a challenge to make a small enough fire to not build pressure, but again it varies due to many factors and also whether or not the boiler and firebox is still hot from a recent run.

It is quite possible to vary the amount of heat an oil burner puts out, and I'd say it's also much easier to make rapid changes, going from a hot fire on a stiff grade to a small spot fire in the station. Of course that could also be harder on the boiler.