As already explained, GE engines, along with most other four stroke cycle engines use turbo-supercharger to boost the inlet air pressures that are driven of the energy of the exhaust gasses. Under normal conditions, the turbo is not boosting until the load is such on the engine that the exhaust reaches a suitable temperature to spool up the turbo.
On two stroke cycle diesels such as EMD and F-M engines, a positive air charge must be maintained at all times in order to scavenge the exhaust out of the engine and to supply fresh air for combustion. On EMD engines just before the piston goes to bottom dead center the exhaust valves open up and relieves the pressure inside the cylinder. The pistons then reach near bottom dead center and uncover the intake ports and the fresh air forces the remaining exhaust gasses out of the valves, the exhaust valves close, then the air is slightly pressurized (only 3-5 pounds) and that is what is used for combustion. The mix comes in with higher horsepower EMD and the parasitic loss of horsepower by the blower. It takes horsepower to make horsepower. To get around this, EMD turbochargers are gear driven (again, all ready explained). At idle or low load conditions, the turbo is on the clutch providing positive air displacement for scavenging the exhaust. When the load conditions increase and suitable exhaust temps are achieved, the turbine compressor over spins on the clutch and free wheels driven entirely by exhaust temperature.
Since GE is a four stroke cycle engine, the air is drawn into the cylinders by vacuum-the effect of the intake valves open as the piston travels down the cylinder, then near the bottom of the stroke, the valves close and the air is compressed. Once again, under light loading or idle, the turbo isn't adding any boost until the engine is loaded. Now, you run the engine up to full RPM without a load ("governed no load") the engine will reach maximum RPM but the turbo will not boost as there isn't sufficient exhaust temperature to spool the turbo. Put the engine under full load ("governed load") then the exhaust temps will increase and spool the turbo up, usually, the turbo will start boosting around the sixth notch on a GE.
Now, in certain emergency standby power setups, (i.e., Nuke power plants, hospitals, etc.) the engines are required to start up, spool up and be at maximum horsepower in seconds. In these applications, generally, there is a large high pressure air line mounted to the exhaust manifold of a four cycle engine (such as GE's 7FD series and Alco 251s. During emergency start up, the compressed air force the turbo to spool up to full speed while the engine is started and coming on line. I have never heard of or seen any electrically driven positive displacement pump for huffing a medium speed diesel engine in locomotive service and I have over twenty years as a journeyman Machinist.
Now, back to the fire out the stack issue on GE engines.
Generally, one of four things has failed: Plugged turbo after-cooler, air manifold seals failed, hole in the air manifold, or a gang on injectors has failed electrically.
On EMD, one of three things has happened: Blower engine: Airbox fire, Turbo engine: plugged parts catcher, plugged after cooler. Generally, in the case of the turbo EMD, fire out the stack is detrimental to the turbo, and only lasts a few seconds before the turbo overheats and overspeed and destroys it's self.
I'll have my thirteen year old son read this to see if he understands what I posted. If he does, then I can be assured it will make sence to anybody else here.
Paul
"We are all here because we are not all there."