|19 foot Baja Sport I/O powered by an Allison 250-C18|
The Allison 250-C18 Gas Turbine Engine
The Allison 250-C18 Engine, also known by its military designation T63-A-700, is primarily a helicopter turboshaft engine, used to power models such as the well known Bell 206B Jet Ranger. The engine is a dual spool, free shaft turboshaft that produces a takeoff rating of 317 shaft horsepower, at an output shaft RPM of 6,016. Maximum continuous power is in the neighborhood of 270 shaft horsepower. Peak specific fuel consumption is .697 lb-shp/hr.
The 250-C18 is a fairly unique design. The compressor is of mixed flow design, featuring a six stage axial compressor followed by a single centrifugal stage. Maximum pressure ratio is 6.2:1 and maximum air mass flow is 3.0 lbs/sec, both occurring at a maximum compressor speed of 51,600 rpm. The compressor features a bleed air valve for anti-icing and to improve engine acceleration.
Air from the compressor flows outward, and into two diffuser tubes which travel toward the rear of the engine. The tubes empty into the reverse flow, single can combustor, which faces back toward the front of the engine. Fuel is sprayed into the combustor via the single fuel nozzle. The fuel and air is initially ignited via an igniter plug, but once the engine has reached self-sustaining rpm, the ignitor plug is no longer needed to sustain combustion.
The combustion gases are expanded first through a two stage axial gas producer turbine, which drives the compressor and the accessory gear train. Then, the gases expand through the two stage axial free power turbine, which drives the engine output reduction gearbox. The output gear train reduces the power turbine rpm from 35,000 rpm at the power turbine, to 6,016 rpm at the output shaft. Power can be taken off from the front or the rear of the engine. The output gearbox shares the same casing with the accessory gearbox. The accessory gearbox drives the oil pumps, the fuel pump, the starter/generator, the fuel controller, and the N1 tach generator. The output geartrain drives the power turbine governor, the torquemeter, and the N2 tach generator, as well as the engine load.
Power control is through a complex hydromechanical and pneumatic controller, which consists of the fuel controller and the N2 (power turbine) governor. The fuel controller can be further broken down into an N1 speed governor and an acceleration limiter. The N2 governor is an overspeed governor which prevents N2 from exceeding a pre-set limit. The fuel controller meters all fuel flow to the fuel nozzle through a single metering valve. The pressure differential across the metering valve is held at a constant, so fuel flow is metered by varying the orifice of the valve. The metering valve is actuated by a rod which is attached to two sets of bellows. One is referred to as the governor bellows, and the other is the acceleration bellows. The bellows are moved with compressed air which comes from the compressor air bleed. The compressed air to control the movement of those two bellows, and ultimately the metering valve, is itself metered by the N1 governor valve, the enrichment valve, the N2 governor valve, and a temperature sensing datum valve. These air valves are actuated through a complex set of linkages involving rotating flyweights, cams, springs, and levers. Moving the power lever, for example, repositions a cam which re-adjusts the spring tension that holds the flyweights, and thus an air valve, in the closed position.
The net result is precise governing over the speed and acceleration of N1, and overspeed governing of N2. The N2 governor takes precedence over the N1 governor, but is only active when the N2 speed reaches or attempts to exceed the pre-set limit. When this occurs, the N2 governor takes over for the fuel controller and limits N1 speed to prevent N2 from overspeeding. If N2 is operating below the governed limit, then the fuel controller acts as a topping and bottoming governor for N1. Moving the power lever to a certain position is essentially a request for an N1 rpm. The controller then responds by increasing or decreasing fuel flow, and then maintains the set speed until the power lever is moved or until the N2 reaches overspeed condition, at which point the N2 governor will take over.
In a helicopter, N2 speed is always 100%, and the power turbine governor and fuel controller will automatically alter N1 speed to maintain a constant rotor rpm. Power is modulated by changing N1 speed. However, on a boat, the 250 must act as a multi-speed engine, meaning that the output shaft must turn the propeller at all speeds, from fully stopped to full speed, around 3300 rpm at the prop. The free shaft design is perfectly suited to this role.
Click here to see stats on the Allison 250-C18 (T63-A-700)
Rolls Royce Allison 250-C18 Gas Turbine Shaft Engine
Finding the Right Propeller
All the horsepower in the world is useless if it can't be harnessed efficiently and effectively. The engine produces the mechanical energy to propel the boat forward, but it is the outdrive and propeller which must convert this mechanical energy into thrust to move the boat. The nature of this type of engine is such that it will only provide as much power as the propeller can absorb. Therefore, a propeller that is too small or doesn't have enough pitch will only allow part of the horsepower to be used. Conversely, if the prop is too large or has too much pitch, then the engine will struggle to spin it up to rated speed, where maximum horsepower is produced. To compensate, the N1 governor will add more fuel which will increase the turbine outlet temperature and torque levels, possibly beyond their limits, but the engine will never reach its rated power.
Prop size and pitch is like a gear ratio in a transmission. A large diameter prop with a coarse pitch is like a very high gear. If you can spin it up to rated speed, you will move very quickly indeed, but the engine has to work much harder to get it to that speed, if it can at all. A smaller diameter prop with a fine pitch is like a low gear. It is much easier for the engine to turn the prop at the maximum speed, but you don't move as far with every revolution.
As a general guideline, if N1 is at 100% but N2 can never reach 100%, then the prop has too much pitch and/or is too large. The engine torque can't overcome the prop load. On the other hand, if N2 easily reaches 100% but N1 remains somewhere below, that means that there is too little pitch and/or the prop is too small. There isn't enough load to keep N2 from overspeeding, so the N2 governor must reduce fuel flow which in turn keeps N1 rpm below its rated speed and output. The perfect match for the engine would allow both N1 and N2 to reach 100%.
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