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In the present paper (Part II) the engine, and more broadly the power plant, is considered in its relation to altitude. The discussion hinges mainly on the question of supercharge. After a brief reference to the various types of supercharger the author opens the discussion on an ideal basis, in which the supercharger is supposed to be 100 per cent, efficient, and the atmosphere is treated as adiabatic, as was at one time supposed. Then the “International Standard Atmosphere” is introduced as modifying the conditions, and after that the question of supercharger efficiency is dealt with. The author treats this on the basis of a law of adiabatic type but with a different index, and shows that this treatment gives an efficiency varying from approximately 60 to 70 per cent, as a function of altitude.
The difficulties connected with supercharge as affecting temperature, having been made clear, the subject of inter-cooling between compressor and engine is dealt with at length with the aid of thermodynamic diagrams. Then the propeller is discussed, and the case for variable pitch made clear.
Finally, the subject of surface cooling is dealt with, applied both to the engine and to the inter-cooler.
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- Research Article
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- Copyright © Royal Aeronautical Society 1937
References
1 The demand is for a constant torque; the increase in h.p. is due to the higher optimum speed.
2 There are two types of mechanical blower, both of which are considerably used, namely, the Rootes type, and that with a single rotor with vane. Between these it is not necessary here to make a distinction.
3 The combination of centrifugal and turbo action results in a more constant delivery than either separately.
4 Cf. §14 (last paragraph).
5 This is not the same thing as constant pressure. Temperature plays a part.
6 On the convention that 100 per cent, efficiency corresponds to adiabatic compression.
7 Or adiabatic (same thing), as at one time supposed or assumed.
8 By convention, sea-level normal temperature and pressure (”N.T.P.”) are taken 15°C. = 288 (abs.) and 1.01388 bars (= 760 m.m. mercury reduced to 0°C.) or (British) = 14.7 pounds per sq. inch. Density ρ (c.g.s.) dry air = .001226, saturated air = .001206, or taken (approx.) = 1/13 = .0771bs./cu.ft.
9 The relation of temperature to altitude may vary several degrees, representing some thousands of feet for any stated temperature.
10 In this paper abs. temp. deg. C. are indicated by an underline thus, 288.
11 Normal value at sea-level taken as unit.
12 This is the value assumed for the “perfect gas” in place of the value 1.408 commonly given for air.
13 The International Standard Atmosphere.
14 Mr. Oliver Thornycroft, writing from Messrs. Ricardo & Co., Shoreham, states that, ”The knock limit is also affected by pressure, and there may be differences in the degrees of response to this effect by different fuels. I am afraid this so far has not been fully examined.”
* For Kθ read isothermal. For Kϕ read isentropic, or adiabatic. Where figures are underlined temperature absolute is understood.
15 We are here treating the subject broadly; in the case of a petrol engine the evaporation of the fuel absorbs a part of the heat: in the Diesel this is, of course, not so. Also there is the heat balance to be considered as between loss by radiation and convection from the supercharger casing or exterior, and heat received by conduction from the engine when the two are designed as a combined unit; this cannot be treated generally, it is a matter for the test bench in any individual case.
16 No account is taken of the loss of volumetric efficiency which certain types of blower exhibit when slow running, then, over a certain range, the efficiency may rise with altitude.
17 There is a cumulative effect which should justify the form of expression adopted by the author.
18 A more exact figure is 1.98.
19 At all altitudes, see Fig. 19.
20 A group of such graphs is shown in a paper by E. W. Stedman as relating to different types of supercharge mechanism. The Engineering Journal, Aero. Section Report, December, 1936, p. 21.
21 This increased torque is available for climbing. If the condition of horizontal flight alone were in question the engine would not be called upon to develop more torque than that at ceiling level, namely, 1,000 pound ft., actually this climbing condition would, to some extent (depending upon the propeller characteristics), affect the relation between r.p.m. and flight velocity, and so in some degree invalidate the argument; it would, in effect, be an infringement of condition (b) above, i.e., the effective pitch of the propeller would be no longer constant. In the light of the above the actual torque will be somewhat less than given by the graphs, τ1,τ2 and τ3, for, when, without an actual change of pitch, the effective pitch becomes less, the consequent increase of engine speed tends to diminish the torque without material change in the torque thrust relation. When, on the contrary, at low altitudes the engine gains speed from an actual reduction in the propeller pitch, the thrust in relation to torque may be greatly increased.
22 As defined in Fig. 13A.
23 This is the region of the torque curve at which, when in testing with a prony brake, the running becomes unstable and the engine stalls.
24 R. & M. 94, 1912-13, p. 40. The practical application of this theory antidates this by many years.
25 According to the author's version of thermal dimensions, which is widely (but not universally) accepted.
26 This ignores the heat transference which would take place at zero velocity as due purely to conduction. This proves to be negligible in the case of air, but not so when water is concerned.
27 Proceedings, Institution of Automobile Engineers, Vol. X, p. 59, 1915. In this paper, p. 82, the expression V 3/108 is given as the h.p. expended per sq.ft. in overcoming skin-friction, representing the theoretical minimum work done in cooling. The expression from which this is derived is Coρ V 3/32.2x550, it would seem that in this case C0 was taken = .0023.
28 C.f. Journal Royal Aeronautical Society, February, 1937, p. 76.
29 1. C.H.U. = 1400 ft.pounds.
30 It is usually convenient to express the heat per second in terms of its mechanical equivalent in h.p. because the heat rejected is commonly expressed as a proportion of the engine output.
31 Justified by data at that time available.
32 We did not use the term Reynolds number in those days.
33 “The Part Played by Skin-Friction in Aeronautics,” Royal Aeronautical Society Journal, February, 1937.
34 In Fig. 16 a uniform linear scale has been adopted.
35 This is approximately identical with the value assumed in his I.A.E. paper, C.f. §26. It is doubtful whether it is of advantage’ to attempt to secure a low value of Co when dealing with the problem of cooling. Probably the reverse is the case.
36 Cooling by the boiling away of water (as in some of the earliest motor cars) is, of course, out of the question, as is also the direct cooling by water as applied in marine vessels and launches.
37 Proceedings, Institution of Automobile Engineers, Vol. X, p. 59, 1915.
38 In a water-cooled engine it may be taken equivalent = 2/3 b.h.p. Compare “The Flying Machine from an Engineering Standpoint,” footnote p. 63. “From tests made by the author, in the case of a Daimler (sleeve valve) engine 39.2 b.h.p., total jacket heat rejected = equivalent of 40 h.p. or cylinder and head only (not including exhaust port loss) equivalent = 27 h.p.” In an air-cooled engine a greater proportion of the heat loss is carried off by the lubricant and by conduction to the crank case
39 The fins, on this basis, pitched ¼in. apart would be ¾in. in height.
40 Where the surface temperature is frequently > 300°C.
41 Although at the moment our calculation relates to sea-level, that is not supposed to be the ceiling.
42 The surfaces are not always conformable to the flow and are not usually polished or highly finished.
43 It would clearly be wrong to take t 2 – t 1, as the temperature difference, because however great the plethora of air passing the hot surface its temperature must be raised to some extent. In practice the extent of the increase would be either measured or calculated for any particular case as demonstrated in §29. Here it is only possible to suggest a correction in general terms.
44 Or =0.2x30/12 = 0.5 (C.f. §31).
45 In Fig. 22 there would be a transfer loss between the compressor cylinder and the pressure reservoir which would not exist under actual conditions and may therefore be ignored.
46 The author has not thought fit on the present occasion to take into account the influence of combustion space volume as absorbing part of the compressed charge.
47 Messrs. Ricardo & Co. (1927), Limited, Bridge Works, Shoreham-by-Sea, Sussex.
48 The author finds that this graph may be expressed very closely by an empirical equation, C.R. = (3.5 x η 3) + 4.5, in which η is the proportion of iso-octane. Thus if η = 0.9, C.R. = (3.5 x 0.729) + 4.5 = 7.05 as given by graph. See caption.
49 It is a different matter if the fuel be rich in free hydrogen.
50 The author makes this statement on the authority of Messrs. Ricardo.
51 With acknowledgments to Messrs. Ricardo.
52 The distance the flame has to travel cannot in itself be accepted as an explanation because if this were so in a large engine it would be almost impossible to get rid of knock. For this reason the author is not entirely satisfied with the explanation given.
53 The River Severn is a famous example.