Allison 250

Air enters the engine through the compressor inlet and is compressed by six axial compressor stages and a single centrifugal stage. The compressed air is discharged through the scroll type diffuser into two external ducts which convey the air to the combustion section. Air enters the single combustion liner, located at the rear of the engine, through holes in the liner dome and skin. This air is mixed with fuel sprayed from the fuel nozzle to support combustion. Combustion gasses move out of the combustion liner to the first stage gas producer turbine nozzle.

The four stages of turbines are mounted between the combustion section and the power and accessories gearbox. The two stage gas producer turbine drives the compressor and accessories gear train. The two stage power turbine furnishes the output power of the engine through the power gear train. The expanded gas discharges downward through the twin ducts of the exhaust collector. A two stage helical gear set in the power and accessories gearbox reduces rotational speed from 33,290 RPM at the power turbine to 6016 RPM at the rear power takeoff drive spline. The power turbine gear train drives the power turbine tachometer generator, N2 overspeed governor, and the load the engine is turning. The compressor, fuel pump, gas producer tachometer generator, gas producer fuel control, and starter generator are drivern by the gas producer gear train. A spare drive is also furnished in this gear train. The two gas generator turbines and compressor spins at 51,120 RPM under full power.

Below are some pictures of actual engine internals. These are not from the engine in the boat, but are from other Allison 250 engines I have. Not the typical JE forged pistons, and Brodix cylinder heads!

This is the intake of the engine. It is also is the #1 bearing support of the engine, and bleeds hot compressed air through the hollow blades to anti-ice the compressor section. In a boat application, the anti-ice feature is not used.


	6 stage axial compressor with an 7th stage radial impeller. This compressor is responsible for generating the 6.2:1 Pressure Ratio at 3.0 lbs per second mass flow through engine.

Compressor case halves contain the 6 stages of stator blades. The 5th stage features an air blead that can modify the amount compressor map.

The compressor discharge air is routed through an diffuser, scroll, delivery tubes, and ends in this combustion chamber case. This is where the compressed air will then be mixed with fuel. The two ducts on the sides are the inlet, igniter / fuel nozzle is on the end of the dome, and the remaining two ports are for draining the combustor case in case there is an buildup of liquid fuel. The large center flange bolts up to the turbine assembly.

This is the combustion chamber. It's main purpose is to provide an enviroment for a steady combustion. The incoming compressed air is distributed along the skirts, holes, and passageways to mix with the incoming fuel. The air also cools the skin of the combustor by generating a thin film of boundary layer air along the inside surface. This shields the metals from the 1500 F degree flame. The center hole on top is where the single fuel nozzle sprays in the fuel, and the adjacent boss allows for the igniter (Spark plug) to ignite the fuel when the engine is first started.

The Allison 250 has 4 turbines as shown in the above cross section picture. After the combustor, expanding gasses flow through the #1 turbine nozzle, then #1 turbine. The gasses exit the #1 turbine, and enters #2 nozzle which then directs the gases to the #2 turbine. The #1 and #2 turbines drives the compressors only, and is refered to Gas Generator, or N1 section of engine. The #3 and #4 turbines is refered to the Power turbines, or N2. The power turbines is what does the work to propel the boat.

Here is the # 1 turbine nozzle shown on the far left, and corresponding # 1 turbine wheel to the right. The duty of the turbine nozzle is to direct the incoming gasses to the optimum angle to the turbine most efficently. There is an turbine nozzle to direct the incomming expanding gasses for each stage of turbines.


Closeup of #2 turbine wheel. Note the longer blades than the #1 turbine stage. The concentric rings in the hub are labyrinth seals.	Closeup of the #4 turbine wheel. This is the last turbine and is one of the 2 power turbines that actually powers the boat. Note the labyrinth seals machined into the intergal outer ring.