P ARADOXICALLY, it may well be that the impetus given to
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design progress by the motorways will be of greater benefit to operators who continue to run vehicles on normal routes than to those who specialize in motorway services.
According to some operators, cruising speeds of 70 m.p.h. for coaches and speeds of 55 m.p.h. and more for goods vehicles will be common on the London-Birmingham motorway, which will be opened on Monday. Tests have already shown that speeds of this order will increase fuel consumption by about half and necessitate the use of special tyres. Over a period, average running costs could be doubled.
Vehicle manufacturers and operators have yet to determine a "critical sustained speed" above which running costs increase at a much higher rate than the gain in average speed on uncongested level roads. Any yardstick of critical speed is, however, arbitrary and in every case must relate to a particular type of vehicle carrying a particular type of load.
Obviously, the higher the gross laden weight of a vehicle operating within its rated capacity and the lower its normal speed, the lower will be the cost of increasing its average speed a given percentage by improving its rated performance. Any higher average to be derived from running on the motorways will be more nearly applic able to operation on normal longdistance routes, particularly on night trunk services, than a proportional improvement relating to a lighter vehicle with a much higher maximum speed.
Whereas an analysis of the improvement in rated performance required to increase the cruising speed of a vehicle from, say, 55 m.p.h. to 65 m.p.h. would be of interest mainly to operators regularly using the motorways, data showing the factors involved in a proportionate advance in the speed of a maximum-load vehicle from 35 m.p.h. to 41 m.p.h. could be important to the majority of operators running sixand eight-wheelers.
A simple yardstick of critical speed is that at which wind resistance attains value equal to rolling resistance. Rolling resistance increases approximately in proportion to speed, but wind resistance increases as the square of the speed. Thus, for example, raising vehicle
speed from 20 m.p.h. to 40 m.p.h. will approximately double the rolling resistance, whilst wind resistance will increase four times.
Contrasts show the relative significance of these variables. When a heavy vehicle is travelling at a walking pace up a steep gradient on full throttle, wind resistance is insignificant, and a rise in peak engine power will provide a corresponding increase in miles per hour.
If a private car or light commercial vehicle is driven at 80 m.p.h. on the level, the horse-power absorbed by windage losses is a high percentage of the total power required. In such a case rolling resistance is often ignored in making a rough assessment of resistance factors, and a comparison with power absorbed at 60 m.p.h. would show that the resistance was increased by approximately 1.8.
Power on Hills
The power required to climb gradients, in excess of that necessary to run at the same speed on the level, must, therefore, be considered in addition to wind and rolling resistance when assessing vehicle performance on any route other than one over flat country.
How much does an increase in speed cost? The answer may well be: Power for high speed is expensive; power for obtaining greater speed on gradients is cheap, and power for increasing m.p.h. on the level up to the critical speed is normally a worthwhile economy.
It is, however, necessary to emphasize that the last claim completely ignores the heavy cost of higher speed in terms of the life of tyres and chassis gear, and in practice it is applicable only to speeds up to a given maximum. This is probably in the region of 50-55 m.p.h.
A detailed analysis of the problem will be possible only after practical erience. It will then be essential onsider the form in which power is eloped, in addition to maximum ver, gear ratios in relation to nges in frontal area, and the value (chicle momentum when approach a gradient relative to wind stance.
-hese additional factors will be It with in a later article; they will w the extreme importance of lying the most suitable formula for -elating maximum engine power,
K imurn torque in the normal m.p.h. ge, gear ratios, load, wind resist e and rolling resistance.
Guessing Wind Resistance •
Vhilst rolling resistance on a typical d surface can be calculated fairly urately, wind resistance must be :ssed by intelligent guesswork, ed on the known performance of vehicle carrying a full load. inges in the shape of a load, as well its frontal area, can cause large iations in wind resislance. and in case of high resistance the reduct in speed may in practice be iter than the theoretical loss ause the final-drive ratio is not able.
f a 24-ton-gross eight-wheeler were lited with a speed of 35 m.p.h. in gear when the power unit was eloping a maximum 125 b.h.p., this ild imply that the load was exees:ly bulky, or that the chassis was ipped with a high van body. The icle would be restricted by the ernor to a speed of 35 m.p.h. when rying a low load because a higher ✓ ratio was not available.
r the engine were developing its power of 125 b.h.p. at 35 m.p.h., engine-output equivalent of rolling stance on the level would be und 54 h.p., and wind resistance aid absorb about 65 b.h.p., giving a LI of 119 b.h.p. The difference aieen the power required and the :d b.h.p. of 125 represents a transsion loss of about 5 per cent.
Transmission Loss
)espite the relatively low speed of vehicle, wind resistance exceeds ing resistance. and the vehicle will operating above its critical speed !n carrying a high load. If it were ided to increase maximum speed to rn.p.h., it would be necessary to e the output to about 190 b.h.p., ch would provide a total traction of 181.
If this, 75 h.p. would be absorbed rolling resistance, whilst wind stance would account for 106 h.p. peed of 55 m.p.h. would require an me developing 272 b.h.p., and in case the wind resistance equivalent of 160 h.p. would compare with a rolling resistance equivalent of 98.
These figures provide greater interest if they are matched against those derived from the hypothetical performance of an articulated six-wheeler of 20 tons gross weight, and capable of attaining a speed of 45 m.p.h. when the engine is developing its peak rated output of 110 b.h.p.
Although nominally the two vehicles are not dissimilar with regard to the load-speed factors involved, and equivalent frontal areas would create a similar wind resistance, the performance quoted for the articulated vehicle indicates that windage losses are unusually low. It should be emphasized that the performance of the sixwheeler represents an extreme case, in that the windage losses are appreciably reduced compared with the power which would normally he absorbed by resistance to a vehicle with a concentrated flat load.
In practice, a load which corresponds to cab height at the front and is reasonably streamlined would offer less resistance than the flat load, because it would reduce the vacuum drag behind the cab. Any large discrepancy between these statements and practical results is probably attributable to unsuitable gear ratios.
Critical Speed Over 80 m.p.h.
At 45 m.p.h. rolling resistance would account for 62 h.p. and wind resistance for approximately 43 h.p., the total of 105 h.p. showing that 5 h.p. is lost in the transmission. This vehicle would not attain its critical speed until it exceeded 80 m.p.h., at which an engine output of 286 b.h.p. would be required.
A speed of 55 m.p.h. could be provided by a rated output of 153 b.h.p., comprising a rolling resistance equivalent to 82 b.h.p., a loss to wind resistance of 63 b.h.p. and a sacrifice of 8 h.p. in the transmission. When travelling at the maximum speed of the eight-wheeler (35 m.p.h.) rolling resistance would account for 45 b.h.p. and wind resistance would absorb about 26 h.p., which stand in strong contrast to the performance equivalents of the eight-wheeler and show that very careful iittention must he paid to wind resistance when considering the most appropriate gear ratios.
If the wind resistance of the sixwheeler equalled that of the eightwheeler an engine output of 110 b.h.p. would provide a maximum speed of less than 33 m.p.h., and an output of some 250 b.h.p. would be required for a speed of 55 m.p.h.
Carrying a load which created a wind resistance equal to the average of the resistance values of the • two vehicles, an engine output of 110 b.h.p. would give a top speed of about 37 m.p.h. This would be slightly above the critical speed, and comparisons with the other hypothetical results quoted indicates the extent to which wind resistance can absorb power, sometimes unnecessarily.
Not until facilities become available for wind-tunnel tcsts of complete vehicles will it be possible to analyse the effects of cab and body shape on wind resistance and of the large variations associated with different forms of load in the case of a platform vehicle. .
Eight-wheeler on Hills
Reverting to power required for hill-climbing, reference to Fig. 1 shows that an engine output of 125 b.h.p. will enable the maximum-load eightwheeler to climb a gradient of 1 in 10 at a speed of 7-1m.p.h. It also indicates that if the power available were increased by half to 187.5 b.h.p. the vehicle could climb this gradient at a speed of about Ii m.p.h., representing a gain in speed of some 50 per cent.
These results may be compared with the value of raising the engine output a similar amount in terms of maximum speed on the level. In this case the speed potential of the vehicle is increased from 35 m.p.h. to about 45 m.p.h., a proportionate gain of less than 30 per cent.
It is also important to note that a greater improvement in power output would give a proportionate gain when the vehicle was climbing a 1-in-10 gradient, a speed of 15 m.p.h. being provided by an output of 250 b.h.p. When employed to raise the maximum speed on a level road, the highest attainable rate would be little more than 51 m.p.h., an overall gain of approximately 50 per cent. compared with the 100 per cent. advantage when climbing steep gradients.
" Artie " Performance
An analysis of the performance curves of the six-wheeled articulated outfit (Fig. 2) indicates that the standard power output of 110 b.h.p. provides a road speed of slightly more than 8 m.p.h. on a gradient of 1 in 10 and that increasing the output by half should enable the vehicle to climb at a speed slightly in excess of 12 m.p.h.
This also represents an m.p.h. gain proportionate to the increase in b.h.p.. and a similar proportionate gain applies to doubling the output, which gives a speed of around 16 m.p.h.
On the level the much more favourable wind resistance of the 20-tongross vehicle would enable advantages to be obtained by increases in power output comparable proportionately to the improvements provided in the case of the eight-wheeler. Much smaller gains would normally be expected because of the higher operating speeds; if it were necessary that the eight-wheeler should operate at 68 m.p.h. the power output would have to be increased to around 500 b.h.p.
Charts of the London-Birmingham motorway show that up gradients on the outward run from London of less than 1 in 300 account for about 17 miles and that down gradients total about 13 miles, the remaining 25 miles being substantially flat. An assessment based on a division of the motorway into sections of various lengths, so that each section is of substantially uniform gradient, shows that six miles on the outward run will involve climbing gradients steeper than 1 in 100, the maximum, gradient being 1 in 45 for a distance of 1,200 yd.
Average Gradient 1 in 70 On the return journey a vehicle will climb gradients steeper than I in 100 for nearly nine miles, the steepest section of 1 in 34 covering a distance of about 600 yd. In both cases the average gradient for the distances quoted is about 1 in 70.
• Some indication of the speed potential on gradients in this category of the types of vehicle under review may be assessed by obtaining the theoretical total power required on gradients of 1 in 40 and 1 in 100, as shown by the performance curves. For example, if the eight-wheeler were required to operate at its rated speed of 35 m.p.h. on a gradient of 1 in 100 it would be necessary to increase engine output by 30 per cent. from 125 b.h.p. to 170 b.h.p. The power required for this speed on a gradient of 1 in 40 would involve an increase of 104 per cent. to 255 b.h.p.
The rated power of 125 b.h.p. would provide a speed of about 29 m.p.h. on a gradient of 1 in 100 and about 20 m.p.h. on -a gradient of 1 in 40. In practice, average speeds on the gradients would be higher if the sections were approached at the vehicles' maximum speeds, because the momentum or inertia would add to the effective power available for at least a proportion of the distance.
The 20-ton six-wheeler would require an output of about 155 b.h.p. for climbing a gradient of 1 in 100, compared with 110 b.h.p. for its rated speed of 45 m.p.h. on the level (an increase of about 41 per cent.). If no sacrifice were to be made when climbing a gradient of 1 in 40 it would be necessary to equip the vehicle with an engine producing 250 b.h.p., an increase of approximately 140 per cent.
It will be seen that the increments in power to maintain the speed of the six-wheeler when climbing the gradients mentioned are greater proportionately than those applicable to the eight-wheeler, despite the higher rating of the lighter vehicle. This is explained by the greater wind resistance of the eight-wheeler and the
higher proportionate increase in the power available for hill-climbing when the road speed is reduced.
These background technicalities represent an over-simplification of the many variables involved in assessing the performance of a vehicle. To many they may appear to be an overelaboration. They undoubtedly reveal the urgent need for scientific research for analysing wind-resistance factors and the ways in which greater use can be made of available power output.
The value of full exploitation of power will be the more necessary if engines of much greater output are fitted to vehicles engaged in motorway services, because efficiency is measured in fuel economy as well as power.
Power-curve Change Such research may also involve extensive development work in connection with the form of power curve required by a typical oil engine. The much-vaunted ability of the oil engine to produce a high torque at low speeds may Jose its value when challenged by a performance characteristic giving higher torque values at high speed, which would greatly enhance the vehicle's climbing ability on lesser gradients, without employing a large number of gear ratios.
It is significant that turbocharged -engines are being fitted to vehicles undergoing tests at continuous high speeds.. Turbocharging may enable greater use to be made of the power available as well as, or as an alternative to, raising the maximum power.