APPENDIX A
APPENDIX A THE NAPIER NOMAD AIRCRAFT DIESEL ENGINE During the early 1950s, D. Napier & Son, a British aviation engine firm, designed and developed the "Nomad, "14 an aircraft diesel engine with a 35 percent lower fuel consumption than con- temporary turboprop engines. The engine is horizontally opposed, 12 cylinders, and of 2-stroke loop scavenged design. The power plant consists of the reciprocating engine and a gas turbine com- bined to form a "compound" engine, whose operating cycle is devised for each component to make its maximum contribution to the overall results.
In the compound engine, the cycle of operations is shared between a 2-stroke diesel engine and a turbine that is powered by the exhaust gases expelled from the engine. The back pressure imposed by the turbine on the engine cylinders establishes the lower pressure level of the operating cycle in the cylinders.
This also determines the degree of supercharge necessary to pass the required quantity of air through the engine.
Engine Design and Construction The mechanical layout of the Napier Nomad is shown in Figure 61. An "infinitely variable gear" has been interposed in the gear system between the turbocompressor set and the engine. This permits a range of gear ratios to be selected for any crankshaft speed and is responsible for the higher power output and fuel efficiency at an altitude, which the Nomad exhibits compared with a fixed-gear-ratio engine.
The diesel engine has 12 cylinders in banks of six, hori- zontally opposed on a six-throw crankshaft, and the cylinder design is a simple 2-stroke type, using piston-controlled intake 12 CYLINDER TWO-STROKE DIESEL ENGINE VARIABLE GEAR
i_ i _-¸
II
PROPELLER SHAFT . _ .,, QUILL SHAFT: fURblNE AIR INTAKE AXIAL FLOW TURBINE COMPRESSOR AIR AT AIR AFTER AIR AFTER ENGINE INTAKE PRESSURE AXIAL COMPRESSION PISTON COMPRESSION EXHAUST GASES Figure 61. Diagram of Nomad Engine.
and exhaust ports. The arrangement of the cylinder is shown in Figure 62, which depicts the loop-scavenging gas flow and also the centrally located diesel fuel injector. The injection pumps, which are in blocks of six, are of normal "jerk-pump" design, specially developed to deal with the high outputs and speeds of operation required.
The pistons (Figure 63) have aluminum alloy bodies but are fitted with austenitic steel crowns designed to operate at 1100 to 1300F (593 to 704C) at full power. At this point, oil cooling is provided behind the piston rings to ensure minimum tempera- tures.
The wrist pin-connecting rod-crank pin bearing designs are quite innovative because of the unidirectional loading, which exists in a 2-stroke engine. Figure 64 shows that the connecting rod has half-bearings or "slipper" type construction at both the small and big ends with light straps as safety devices in the reverse-loading direction. A later development of this type of bearing is applied at the small end because the unidirectional (compressive) loading usually prevents separation of the bearing and journal, thereby preventing the formation of a substantive oil film between them. On the Nomad, this difficulty has been overcome in the manner illustrated in Figure 65.
The wrist pin bearing is divided lengthwise into three sec- tions comprised of two outer bearings "X" and a center bearing "Y". The outer bearings are coaxial with each other but their centers are displaced transversely from the centerline of the connecting rod. The axis of the center bearing is similarly dis- placed on the opposite side of the connecting rod axis. There- fore, the three bearings constitute two bearings eccentric to one another, equally spaced [0.035 inch (0.9 mm)] about the axis of the connecting rod. The operation of this arrangement can best be understood by studying the exaggerated diagrams, B, C, and D in Figure 65, which show the rocking motion of the connecting rod that unloads and loads the segmented bearing.
Figure 62. Arrangement of Engine Cylinder.
OpIG!T,_,_,E PACE BLACK AI',_',] V#H_TE F'HOiGGRAPH ?
-ORiGiNAL PRgE ,_.T- OF PO;L,R Qi:._,C.L _''_', Figure 63. Arrangement of Piston.
ORIGINAL PAGE BLACK A_D WHITE PHOTOGRAPH Figure 64. Arrangement of Connecting Rod.
!
ROTilON A D C Connecting-Rod Small-End Bearing.
7igure 65.
The axial compressor has 12 stages and operates at a maximum pressure ratio of 8.25/1 with an air mass flow of 13 ib/sec (5.9 kg/sec) and to extend operating range at the low-speed end, has adjustable inlet guide vanes. The adiabatic efficiency of the compressor at takeoff (sea-level static) is 85 percent and its peak efficiency is 87.5 percent.
The three-stage turbine is mounted coaxially with the com- pressor, and its blading is designed to extract maximum energy from the gases, rather than to obtain some jet thrust from the exhaust. The efficiency at takeoff (sea-level static) is 84 per- cent and 86 percent at the altitude cruise rating. Both compres- sor and turbine are connected into the rear gear casing by shafts having splines at each end to allow expansion effects.
The engine is equipped with an infinitely variable speed traction drive between the turbomachinery and the reciprocator.
This device, also known as a Beier, consists of a pair of conical members that are end-loaded together. The gear ratio is varied by sliding one cone over the other. Actual details are shown in Figure 66, where a "pack" of disks with narrow conical rims are mounted on a central shaft and spring loaded to trap between them a series of coned disks carried on each of three planetary shafts and arranged to swing about a fulcrum to obtain changes in speed ratio. This arrangement provides a constant mesh system whose speed ratio may be changed while running.
The variable-speed drive efficiency ranges from 65 to 92 percent, being the greatest at high power. Only 30 percent (1000 shp [746 kW]) of the total engine power is transmitted through that part of the system where the speed-varying device is fitted.
The arrangement of this complete system in the Nomad is shown in Figure 67.
A longitudinal section through the engine is shown in Figure 68, the layout following closely the diagrammatic arrangement of s f
cA A
Figure 66. Diagram Showing Operation of Variable Gear.
/ Figure 67. Variable Gear as Fitted to Engine.
I *,-I
Z G C) e_
CD
°,-I Figure 61. Figure 69 shows the engine cross section and clearly indicates the vertically divided magnesium crankcase and the aluminum cylinder blocks, which are bolted together by steel through bolts. It will be seen that dry liners are employed with the liners themselves being of chromium-copper material and chromium-plated in the bores. Figure 70 shows a cutaway drawing where the main features have been identified. A photograph from one side has been made (Figure 71) and shows that the engine is suspended from four pickup points. This arrangement is conveni- ent for maximum accessibility, particularly to the compressor and turbine.
Engine starting is by electric motor. Since the compression ratio in the engine cylinders is insufficient to give self- ignition under these conditions, spark plugs are fitted in each cylinder head and are activated by a high-energy system. These plugs are used only for starting.
Enqine Performance The following details apply to the Napier Nomad: 6 x 7.375 Bore x Stroke, in. (nun) (152 x 187) Number of Cylinders Rated Speed, rpm 2520 (12.7) Piston Speed, ft/min (m/sec) 3580 (1624) Net Dry Weight, ib (kg) Take-Off Performance at Sea-Level, Static 3155 (2339) Dry: BHP, ehp (ekW) 0.345 (207) BSFC, ib,/ehp-hr, (gm/ekW-hr) 3580 (2671) Water Injection: BHP, ehp (ekW) 0.336 (202) BSFC, lb/ehp-hr (gm/ekW-hr} 56__in.
i, Figure 69. Crosssection of Nomad Engine.
r_ Z '0 .,-4 p_ ORIGINAL PAOE IS OF POOR QUALITY Plus Auxiliary Combustion: BHP, ehp (ekW) 4100 (3059) BSFC, ib/ehp-hr 0.374(224) (gm/ekW-hr) *e = equivalent - includes reciprocator and turbine outputs plus jet thrust contribution.
The basic engine take-off performance includes 2745 bhp from the reciprocator and 390 hp, which is surplus from the turbine for a combined output of 3135 ehp.
The normal charge air temperature at takeoff is quite high [477F (247C)]. It is necessary to reduce this temperature if increased output is required. The use of aftercoolers would impose bulk, weight, and thermodynamic penalties on the system.
However, the charge temperature can be reduced most conven- iently, and density increased without energy loss by injecting water into the inlet manifolds. When this is done, a greater quantity of air can be packed into the engine cylinders and the power output still further increased. Using water injection in this way, the equivalent power of the engine is increased to 3580 hp (2671 kW).
Another possibility for increasing the maximum power output arises from the presence of excess air in the exhaust manifold between the engine and the turbine. At the expense of a slight increase in the turbine inlet temperature, a small quantity of additional fuel can be injected and burned at this point. This extra fuel would be burned at low expansion ratio and therefore at low efficiency, but the effect on the overall SFC is small. This system of combustion could be used in asso- ciation with water injection, and for a turbine inlet tempera- ture of 1377F (747C) the take-off power could be increased to 4100 hp (3059 kW). The take-off performances against altitude for the three conditions referred to are illustrated in Figure 72.
,!
i / 45OO | !
!
I i.c.AN.AT.OS_E_E 1 STATJC CONDITIONS | L / W .J o / / I- f A .¢ 30o0 _J J A- BASJC ENGINE INJECTION.
B, BASIC ENG(NE WITH WATER _ 2500 WITH WATER INJECTION C• BASIC ENGINE IE AND AUXUARY _5TION.
IL J 0-4( (n _ c u 0"3_ ..J
/
o.3zs _ bJ a.
0.30 •7_00 Ioooo m2soo I_ o_," S.L. 2500 5O00 ALTITUDE FT.
Figure 72. Takeoff Powers Against Altitude With Fuel Consumption.
The power split at which the engine is controlled over the speed range under sea-level static conditions is shown in Fig- ure 73. The shape of this curve is devised to maintain optimum thermal efficiency and altitude performance over as wide a range as possible and also to suit control requirement of the propeller. Also, on the same diagram are the corresponding diesel, turbine, and compressor powers and at 1500 rpm, the turbine power exactly balances that absorbed by the compressor.
At lower speeds, the power developed by the turbine is below that demanded by the compressor and this power deficiency has to be made up by the diesel engine. The engine rpm where bal- ance occurs between the turbine and compressor powers varies with altitude and forward speed, but over the useful cruising range, the power transmitted through the infinitely variable gear is relatively small.
AS seen in Figure 74, the shaft power at each engine crankshaft speed increases with altitude until a maximum is reached. The lower power ratings are able to be attained at almost 30,000 feet (9.14 km) before derating sets in. The rea- son for the power increase with altitude is that the infinitely variable gear set permits the turbomachinery to increase its speed with altitude to maintain the in-cylinder equivalence ratio. The reciprocator power is essentially constant (flat rated - refer to Figure 72) and the turbine power surplus is fed into the crankshaft to increase the overall equivalent shaft output. When the turbomachinery can no longer increase its speed due to thermal and rotational limitatlons to maintain charge density, the power lapse rate with increasing altitude is approximately 3 percent per thousand feet (0.3 km) takes over. The altitude cruising curves for both equivalent shaft horsepower and SFC are remarkable for their "flatness" across the range of altitude. In fact, due to the increased turbine thermodynamic performance and output, the BSFC decreases until the turbine speed limit is reached. The best fuel economy of 325C b,, _!
?._00 2 ooo _J o w IO00
,oo
z 0.., uz -J w CONS. L8/$. IH.P - HR: -'- S,,,,,,PEC.FUEL 0"30 tO _O I_' ) • D ' i600 leOO ZO00 ?..ZOO DIESEl. ENGINE R.P.M Figure 73. Sea-Level Statfc Interconnection Power Curve.
o_ 22(X_ LfJ
\
RECOMMENDED zooo-cRu,s( ,z$o \---__
\
300 KNGTS I I.C.A.N. ATMOSPHERE w 1200 --LOW 0tJTPUT 150( CRRM --_J;;_;_ ;;;am "_ _o_ f _ 0.40 " t ......
i 1 v) S.L. 5000 I0000 15000 ZOO(X) 25000 30000 ALTITUDE - F T.
Figure 74. Altitude Crulsing Curves with Fuel Consumptions.
0.326 ib per ehp/hr (195 gm ekW/hr) is achieved at the recom-
mended cruising rating at 22,250 ft (6.78 km) where a power of
2027 ehp (1512 kW) is developed. A complete heat balance for
this operating point is depicted in Figure 75 and from this it
can be seen that a brake thermal efficiency of 42 percent is
obtained, which is greater than that of any other engine (Circa
1954).
Summary The Napier Nomad Aircraft Diesel Engine was a highly effi- cient power producer, which came into being at an inopportune time. The combination of i0 to 20 cents per gallon fuel and the advent of large, relatively efficient turboprop engines were sufficient in 1955 to bring about the end of Nomad produc- tion. However, since 1973, fuel prices have increased to the $i to 2 dollar per gallon figure and the time for of the com- pound cycle engine for both airborne and ground power applica- tions may have come again.
FUEL. INPUT _.
100"1.
RJ_DIATION __.f | I I I _- ENGINE I_ _m. ............
| i | IN01CATtO I _ | .--el GAS GFNERATOR ,.-.,L _"_-,,.}e'_ t I I PRESSURE LOSS |_1 .... | I _ L 23% I Irfc_s_c_k I I_ [T_-_E _CT,ON
i,.,.ossEs o -- J i i _.-'_:,_
co,._,._ss,o_ _ _ ! i °'''/"
2s5% L_--.-4---Ik...."" I I I TuRe''e h II _i i_F,"SJ.
,.:_uo.o,,,____-i II \\I - ;
j0- l' '
I I1"_ P I SHAFT ,,,,<+,,_-r --_" // .- .,,,_ ,<>:,.
no.a'l.
i 2027 H.P 43.0% 1750 C.P,.P.M. 3OOKNOTS. ;12.250 FT P.!klililii Figure 75. Heat Balance Diagram.
APPENDIX B
APPENDIX B METRIC CONVERSION FACTORS TO: MULTIPLY BY: FROM: MULTIPLY BY: TO: FROM: kPa km 06214 mi bar 1.01972 kg/cm 2 km 3281 ft bar 14.50377 psi m 3.281 ft bar kPa m 39.37 in MPa 10.1972 kg/cm2 dm 3.937 in MPa 145.0377 psi mm 0.03937 in MPa 0.14504 psi m 3 35.31 ft 3 kPa N/cm2 9.912 m3 61.023 in3 kg/cm2 14.223 psi m 3 264.2 gallons kg/cm2 0.10197 kgm 0.0353 ft3 N-m ft-lb 0.73759 61.02 in3 N-m 7.2333 ft-lb
1 oro4 o kom
It3/XP 26.331 m2 10.76 It2 m3/kW
nS/Ih
16,0165 metric ton 2.205 Ib en3/kg HP/in3 0.02198 kg 2.2046 Ib kW/ 1.644 Ih/HP MN 224.8! 0 Ib kg/kW Ib/in 3 0.03613 kN 224.81 Ib kg/L BTU 3.9083 N 0.102 kg kcal BTU/Ib !.8 N 0.22481 Ib kcal/kg BTU/Ib -oF !
° K !.8 oR kcal/kg-° C Ib/HP-hr 6,OO164 ° C 1.8°C + 32 oF o/kWh kW 1.341 HP REFERENCES i. E.A. Will_s and W.T. Wintucky, "An Overview of NASA Inter- mittent Combustion Engine Research," AIAA-84-1393.
2. H. D. Wilsted, "Preliminary Survey of Possible Use of the Compound Adiabatic Diesel Engine For Helicopters," SAE820432.
3. H. Sammons and E. Chatterton, "Napier Nomad Aircraft Diesei Engine," SAE Transaction Vol. 63 1965.
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174, 1960, pp. 1044-1051.
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REFERENCES (Contd) 15.
E.E. Chatterton, "Compound Diesel Engines for Aircraft," Royal Aeronautical Society Journal, Vol. 58, No. 525, pp.
613-633.
16.
P.H. Schweitzer, Scavenging of 2-Stroke Cycle Diesel Engines, The MacMillan Co., N.Y., 1949.
17.
D.A. Richeson, et al., "Application of Air-to-Air Charge Cooling to the 2-Stroke Cycle Diesel Engine," SAE 850317.
18.
W.D. Annand and T.H. Ma, "Instantaneous Heat Transfer Rates to the Cylinder Heat Surface of a Small Compression Ignition Engine," Proc. Inst. Mech. Engrs., Vol. 185, 1970/1971.
19.
C.F. Taylor, "The Internal Combustion Engine in Theory and Practice," Vol. i, p. 441.
20.
R. Herschkron, et al., "Contingency Power Concepts for Heli- copter Turboshaft Engine," American Helicopter Society Sym- posium, 1984 pp. 597-608.
21.
P.H. Schweitzer, et al., "Fumigation Kills Smoke-Improves Diesel Performance," SAE Transactions Vol. 66, 1958, pp.
574-495.
22.
N.J. Beck, et al., "Direct Digital Control of Electronic Unit Injectors," SAE 840273.
23.
G. Sovran and E. Klomp, "Experimentally Determined Optimum Geometries for Rectilinear Diffusers with Rectangular, Coni- cal or Annular Cross Sections," Research Publication GMR- 511, Nov. 16, 1965.
24. N.A. Graham, "Bypass Lube Oil Filtration," SAE 860547.
25. J. Melchior and T. Andre-Talamon, "Hyperbar System of High Supercharging," SAE 740723.
26.
H.G. Braendal, "Modern Ring Design for High Output Engines," 10th CIMAC Conference, 1973, Washington, D.C., discussion by Kent Thurston.
27. J.C. Hallinan, "Development of the Caterpillar 3500 Series Engines," ASME 83-DGEP-2.
28. M. Fellberg, J.W. Huber, and J.W. Duerr, "The Development of Detroit Diesel Allison's New Generation Series 53 Engines," SAE 850259.
REFERENCES(Contd)
K. Okamura, et al., "Recent Developments of the Mitsubishi
29.
WZ High Speed Engine," 10th CIMAC Conference, 1973, Washington, D.C.
63, SAE-MEP-5, "Some Unusual Engines," p.
30. L.K. Setright, 1979.
31. W.R. Alexander, et al., "Improving Engine Durability via Filters and Lubricants," SAE 852125.
32. N.A. Graham, "Bypass Lube Oil Filtration," SAE 860547.
33. G.E. Thomas and R.M. Cuthbert, "Ingested Dust, Filters, and " SAE 680536 Diesel Engine Wear, F.A. Christiansen and P.I. Brown, "Military and Manufacturer 34.
Specification Oils - Their Evaluation and Significance," SAE 1962, Paper 573B.
J.E. Bush and A.L. London, "Cocktail Shaker Cooled Pistons 35.
and Valves," SAE Transactions, Vol. 74 1966 pp. 446-459 Discussion by J.M. Cherrie.
36. W.T. Lyn, "Optimization of Diesel Combustion Research," SAE 780942.
" Flight 30 April 1954, pp. 543-551 37. Alvon, "Napier Nomad, , oO r7 O0 m m U m a 5 z