FANS WASTE POWER Says P. A. C. Brockington, A.M.I.Mech.E. F AN
Page 39
Page 40
Page 41
If you've noticed an error in this article please click here to report it so we can fix it.
efficiency under operating conditions is difficult to estimate by dynamometer tests, and for this reason many of the figures for fan horsepower, quoted from static trials, are suspect. After removing the fan drive when a unit is on bench test, the increase in b.h.p. developed at the brake may, for example, be 4-5 per cent. of engine output, whilst the power absorbed on the road may in practice amount to 10 per cent.
No fan has a consistent performance over the full working range, and air-flow obstruction decreases efficiency. A vehicle may, therefore, run for long periods at a road speed corresponding to a critically low fan efficiency, and the power loss would then be relatively high. Blanking the radiator in cold weather lowers efficiency, and a combination of unfavourable factors can result in a loss of as much as 15 per cent, of engine power.
Conserving Power The air velocity, at high road speeds, is generally sufficient to cool the radiator without fan assistance, and all the power absorbed by the fan is wasted. A scientifically designed fan and cowl, incorporating an automatic drive control to declutch the fan . at lower temperatures, would obviously be more costly to manufacture than the conventional system, but the gain in terms of useful power output and reduced fuel consumption would be substantial.
Some designs of automatic clutch, notably the eddy-current type, have the additional property of allowing slip between the driving element and fan when the engine is accelerated without load. This reduces the maximum loading and increases belt life. Tests made by the Ministry of Supply confirm that the horse-power absorbed by some fans is 10 per cent. or more, when they are installed in the vehicle. Cooling systems employing fans of aerofoil section and correctly proportioned cowlings have shown that the maximum power loss under the most critical conditions should not exceed 5 per cent.
Some German manufacturers have fitted clutch-controlled fans as standard equipment, and reports from America suggest that the eddycurrent clutch will soon be developed for general use. At home, the greater efficiency of the cooling systems fitted to underfloor-engined chassis has aroused interest; being remote from the flexibly mounted engine, the fan may be close-cowled and its efficiency improved. .These developments indicate that the importance of fan design is recognized, but as yet no system has been introduced which approaches the ideal. The technicalities of fan design are closely related to the study of aerodynamic flow, eddy disturbances and • similar factors. A short summary of the performance factors illustrates the more serious results of poor design. For the air flow across the blades of a fan to be uniform from the hub to the blade tips, the blade angle or the chord, or both, must be reduced as the blade radius increases. The standard type of sheet-metal fan has blades of constant width, and optimum flow is usually obtained only at the mean diameter of the blades. Inside this diameter the loads are heavier and on the outside they . are lighter. Consequently, the pattern of air velocities across the surface of the radiator is irregular and power is wasted in imparting a radial velocity to the air.
Under criticalconditions an oscillating flow, or surge, will _result from this phenomenon, and the flow at the centre will be reversed. If such pulsation occurs within the working range, cooling is adversely affected and fan efficiency is reduced. Should a modification be subsequently made to prevent overheating, the engine may be over-cooled when operating in other parts of the range.
Even if engine flexibility can be discounted, the inherent inaccuracies of the sheet-metal fan require a clearance of at least 1 in., and the efficiency is rarely greater than 50 per cent. A cast fan of aerofoil section should run with an efficiency of approximately 80 per cent., given that it is finished with sufficient accuracy to operate with a nominal clearance in a well-designed cowl.
The performance of an aerofoil fan could also be unstable within a comparatively limited range of conditions, but its characteristics may be modified to preclude instability in the normal working range. Flexible mounting of the blades in rubber has been suggested as a modification to prevent metal fatigue, but this might make the blades unstable.
Measuring Air Flow The road speed at which air velocity approaches the pumping capacity of the fan is about 35-40 m.p.h., and thereafter the fan horsepower increases with the cube of the speed. The percentage power loss therefore increases as the speed is approached at which the air flow is sufficient to allow the performance of the fan to be reduced, or its use to be discontinued entirely. Moreover, at temperatures below about 50 degrees
F., the fan absorbs a relatively greater power, and although the under-bonnet temperature does not long remain below this figure after the engine has started, it requires considerably greater power before the running temperature is reached.
The clutch-controlled fan of thc oil engine fitted to the Bussing underfloor-engined chassis is of particular interest. The cooling system is n6 raised in pressure to 1.25 atmospheres and has a special compensating tank mounted above the radiator level. The sheet-metal fan, which operates in a cowl, is declutched electrically against the action of a spring when the radiator water temperature drops below 70 degrees C. (158 degrees F.) and is re-engaged at a temperature of 75 degrees C. (167 degrees F.). A warning lamp on the instrument panel indicates when the fan ceases to operate.
A thermostat in the radiator energizes an electrical relay, which automatically engages and disengages the fan. An overriding manual control switch is fitted on the instrument panel, so that the fan may be engaged at any t^mperature.
A blanking flap, controlled by the driver, enables the degree of cooling to be regulated to meet seasonal changes in temperature. A remote action thermostat system can be fitted to delay actuation of the warning lamp, and presumably the relay, until a temperature of 90 degrees C. (190 degrees F.) has been reached.
Fan control is also a feature of the new M.A.N. rear-engined bus chassis with a forward-mounted radiator. The manufacturer says that a fan is not needed under normal conditions.
Cooling-air circulation is greatly improved by installing the radiator away from the power unit. The unobstructed air passages reduce pressure losses, heat radiaton from the engine has no effect, and consequently the air current at a reasonable road speed on the level is sufficient to cool the radiator without a fan. Both the fan and fan drive cease to turn when the clutch is disengaged, and the saving is given as 4 13.11.p. Fuel consumption is noticeably improved.
The sheet-metal fan is eowled to increase its efficiency. The clutch is haused in the transfer box and forms a coupling between a train of gears and the long propeller shaft, which drives the fan through a belt at the forward end. It is actuated by compressed air and controlled by a pushbutton on the instrument panel. A warning lamp indicates when the temperature of the coolant exceeds a certain figure, and fades out when the driver has engaged the fan clutch.
If the temperature later drops below 60 degrees C. (140 degrees F.), the circuit of a second warning lamp is closed and the driver switches off the fan. An automatic control and electric transmission from the engine are in course of development.
An eddy-current coupling controls fan output by varying the slip in accordance with air-flow requirements. For this reason it has the additional advantage that the drive also slips when the engine accelerates freely, thus preventing overloading of the fan belt.
In a typical system, the clutch windings are energized by the battery or generator, via a thermostatically compressed carbon pack, the resistance of which is reduced with increase in pressure. The saving in power at high speeds may reach 80 per cent., and the circuit can be wired to the generator system to provide increased rotational speed at low road speeds. Matched fan and coupling characterstics can be employed to raise the overall efficiency over the full range, and the size of the radiator can consequently be reduced.
Blade Pitch
As an alternative to the use of a clutch, a fan may be fitted with a thermostatically controlled gear to vary the blade angle. The pitch is altered by changes in temperature and at low temperatures the blades are feathered to reduce the fan output to zero. The speed is the same as that of a conventional fan, but bearing wear is reduced and the system has much to commend it.
The characteristics of the eddycurrent coupling and variable-pitch systems are similar, in that fan output is varied progressively to snit temperature conditions. Cutting-in and cutting-out of the fan are avoided. The operating mechanism of the variable blades increases the weight of the fan, and the load on the belt is, therefore, correspondingly higher when the engine is accelerated with out load. The number of blades is also limited because of the space re quirements of the variable-pitch mechanism.
Overall cooling efficiency may be raised by under-bonnet cooling, or by exposing the underfloor engine to the windstream. In a bonneted engine, an improvement in cooling which is effected by increasing air space and circulation may also raise fan efficiency.
In every case the gain is multiple, in that heat dissipation from every part of the unit is improved. Under a given set of conditions, water and oil temperatures are lower, heat radiation from the exhaust is ,reduced, and the accessories may benefit. The possibility of using a cast fan with line clearances, when the fan and radiator are embodied in •a . unit, represents an additional advantage of the underfloor engine.
The system employed on Daimler Freeline chassis provides a good example of underfloor-engine practice in this country. The radiator is cooled by a five-bladed fan of aerofoil section supplied by The Airscrew Co. and IicWood, Ltd. It operates with a cowl clearance of in.
When tested in conjunction with a Gardner 6LW engine,. the power absorbed at the maximum-power speed of 1,700 r.p.m. was 1.4 b.h.p. The ambient temperature in the test house was 90 degrees F. and the fan was pumping 3,500 cubic ft. of air
per minute, with a blade angle of 2.'; degrees.
When operating on the road under normal conditions, the power absorbed •by the fan would undoubtedly be higher, but the benchtest results indicate the remarkable gain which scientific fan design can offer.
Alternative Cowling
Flexible mounting has been mentioned as the engine . characteristic which prevents the adoption of close cowling for an engine-mounted fan. Whilst cowling within the limits allowed raises fan efficiency to a certain extent, the difficulty of renewing the fan belt is increased. As an alternative to the metal cowl, a canvas gaiter may be employed which fits between an engine shroud and the radiator.
This improves efficiency without materially reducing fan-belt accessibility. Moreover, the clearance between the fan and the shroud cannot vary as the engine flexes. The use of such a gaiter is exemplified by the Dodge-Unipower light four-wheeldrive chassis built for oilfields exploration work in the tropics. Driving a medium-size drill for many hours each day, the vehicle is static and therefore, there is no movement of air through the tube stock of the radiator other than that produced by the fan.