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performance calculations for the Wright R-3350-18DA4 Turbo Compound aircraft engine

Curtiss Wright R-3350

Wright R-3350-18DA4 Turbo Compound air-cooled 18 -cylinder double row radial engine 1700 [hp](1267.7 KW)

gear-driven supercharger, constant power to height : 7400 [m] blower ratio : 8.67 [:1]

introduction : 1952 country : USA importance : ***

applications : DC-7/DC-7B

normal rating : 1700 [hp](1267.7 KW) at 2400 [rpm] at 7400 [m] above sea level

take-off power : 3250 [hp] (2423.5 [KW]) at 2800 [rpm]

reduction : 0.4375 , valvetrain : overhead valves, pushrod operated

weight engine(s) dry with reduction gear : 1500.0 [kg] = 1.18 [kg/KW]

General information :

In the late forties, Wright engineers from the R-3350 Cyclone 18 developed the Wright TC18 Turbo Compound engine. This engine had almost the same dimensions as its predecessor, but delivered 30% more power, 2390 KW and the s.f.c. was 20% lower at 243 [grams/KWh] at cruising power. This extra power was obtained by placing three power-recovery turbines (PRT) in the exhaust gases. This provided a net 405 KW of extra power. These turbines rotated at 13,000 to 16,000 rpm during the cruise flight and up to 20,000 rpm during takeoff. The power was delivered to the crankshaft using a fluid coupling and conical and straight gears. The turbines gave a higher back pressure in the exhaust gas channel, which gave a slight reduction in the total power. The shape of the turbines were such that this back pressure was minimal during noise flight. The turbine blades were exposed to very high temperatures, up to 800°C and therefore a separate circulation cooling was installed for each turbine. The Turbo Compound engine required a lot of technical maintenance and the time between two major maintenance sessions in 1953 was only 800 hours, (Mechanics tended to call the PRT Parts Recovery Turbines, since increased exhaust heat meant a return of the old habit of the engine destroying exhaust valves). 800 hrs was very little but this had already grown to 2000 hours in 1958 which was considered acceptable. But despite the operational problems, the turbo compound engine was the most economical piston engine ever on the market. In October 1949 it was tested for the first time and in March 1950 it went into production. By 1958, more than 12,000 copies had already been delivered.

fuel system : fuel type : octane grade no. 100 oil system :

engine starter type :

bore : 155.6 [mm] stroke : 160.2 [mm]

supercharger compression ratio : 2.60 [ ]

valve inlet area : 37.0 [cm^2] one inlet and one exhaust valve in cylinder head

gasspeed at inlet valve : 46.4 [m/s]

blower speed : 20808 [rpm] , mixture :13.0 :1

compression ratio: 6.85 :1

high octane fuel needed tp prevent detonation

stroke volume (Vs displacement): 54.862 [litre]

compression volume (Vc): 9.373 [litre]

total volume (Vt): 64.206 [litre]

overall engine diameter: 141.7 [cm]

calculated engine length: 119.50 [m]

specific power : 23.1 [kW/litre]

torque : 5044 [Nm]

engine weight/volume : 27.3 : [kg/litre]

average piston speed (Cm): 12.8 [m/s]

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intake manifold absolute pressure (MAP) at 7400 [m] altitude Pi (ata.) : 1.03 [kg/cm2]

(29.83 [inHg])

mean engine pressure (M.E.P.) at 7400 [m] altitude Pm : 9.71 [kg/cm2]

compression pressure at 7400 [m] altitude Pc: 11.41 [kg/cm2]

estimated combustion pressure at 7400 [m] Pe : 45.59 [kg/cm2]

exhaust pressure at 7400 [m] Pu : 4.04 [kg/cm^2 ]

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compression-start temperature at 7400 [m] Tic: 346 [°K] (73 [°C])

compression-end temperature at 7400 [m] Tc: 552 [°K] (278 [°C])

average engine wall temperature at 7400 [m] : 476 [K] (202 [°C])

caloric combustion temperature at 7400 [m] Tec: 2326 [°K] (2053 [°C])

polytroph combustion temperature at 7400 [m] Tep : 2204 [°K] (1930 [°C])

estimated combustion temperature at 7400 [m] Te (T4): 2237 [°K] (1964 [°C])

polytrope expansion-end temperature at 7400 [m] Tup: 1158 [°K] (884 [°C])

exhaust stroke end temperature at 7400 [m] Tu: 1118 [°K] (845 [°C])

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calculations for take-off/emergency power at sea level

average piston speed : 15.0 [m/s]

gasspeed at inlet valve : 54.1 [m/s]

intake pressure at sea level for Take-off Pi : 1.42 [kg/cm2] (41.24 [inHg])

max. intake pressure (full blower) at sea level for Take-off Pi : 2.69 [kg/cm2] (77.90

[inHg])

rich mixture 11.5 : 1 applied for cooling > carburettor adjustable in flight by the pilot

high combustion pressure > high loads on piston rod and crankcase

water injection for take-off

caloric combustion temperature at sea level Tec: 2130 [°K] (1857 [°C])

insufficient cooling, can run max 2 minutes at this power before overheating

additional 20% power given by three power-recovery turbines (PRT)

emergency/take off rating at 2800 [rpm] at sea level : 3245 [hp]

Wright R-3350 TC18 Turbo Compound Engine Operating Instructions

Thermal efficiency Nth : 0.382 [ ]

Mechanical efficiency Nm : 0.742 [ ]

Thermo-dynamic efficiency Ntd : 0.283 [ ]

design hours : 3288 [hr] time between overhaul : 3791 [hr]

dispersed engine heat by cooling air : 19558.78 [Kcal/minute/m2]

required cooling surface : 26.25 [m2]

weight cooling ribs : 81.13 [kg]

fuel consumption optimum mixture at 2400.00 [rpm] at 7400 [m]: 403.49 [kg/hr]

specific fuel consumption thermo-dynamic : 212 [gr/epk] = 284 [gr/kwh]

estimated specific fuel consumption (cruise power) at 7400 [m] : 250 [gr/kwh]

specific fuel consumption (volume*rpm at 1 atm MAP) at 2400 [rpm] : 293 [gr/kwh]

estimated specific oil consumption (cruise power) : 8 [gr/kwh]

Literature :

Aero engines – Bill Gunston page 200,201

Turbo Compounds (enginehistory.org)

Wright R-3350 Duplex-Cyclone - Wikipedia

DISCLAIMER Above calculations are based on published data, they must be

regarded as indication not as facts.

Calculated performance and weight may not correspond with actual weights

and performances and are assumptions for which no responsibility can be taken.

Calculations are as accurate as possible, they can be fine-tuned when more data

is available, you are welcome to give suggestions and additional information

so we can improve our program. For copyright on drawings/photographs/

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notes :