324 



NA TURE 



[May 13, 1909 



lo approach the ideal than anything else. The electric 

 system of ignition enables the moment of ignition to be 

 varied exactly as required, giving very complete control 

 over the speed and power of the engine within limits. 

 However, these limits constitute the weak points in what 

 inight otherwise be a perfect system. Hot as the sparic 

 is, it is unable to ignite with readiness mi.xtures which 

 have more than a certain percentage of air, and the 

 ignition, being so extremely local, prevents the flame in 

 a weak mixture being readily transfused throughout the 

 whole of the charge. Improvements in the original system 

 have been in the direction of substituting mechanical for 

 chemical means of production of the current. Owing to 

 the fact that a slight difference in the mixture or in the 

 amount of compression entails an alteration in the time 

 when the charge must be ignited in order to obtain the 

 best result, and owing to the difliculty of maintaining 

 each cylinder of an engine in identical conditions in these 

 respects, it is obvious that an ignition system that does 

 not take such variations into account cannot be perfect. 



The Ackermann system of steering, invented nearly 100 

 years ago for horse-drawn vehicles, is now almost 

 universally employed for all road motors, except traction 

 engines. Each of the steering wheels turns separately on 

 a vertical pivot, which should, theoretically, pass centrally 

 through the vertical plane of the wheel and its contact 

 with the ground. An obstacle met by the wheel would 

 then have no tendency to disturb the steering of the car. 

 The said arrangement is diflScuIt to obtain mechanically, 

 and a compromise is sometimes made by inclining the 

 pivots or by inclining the wheels. It is not easy to see 

 how the best types of the present system can be improved, 

 although it must be admitted that none is perfect. 



Horizontal cylinders may be lubricated by feeding oil 

 through a hole in the cylinder on to the piston, allowing 

 a portion to flow through a hole in the piston to lubricate 

 the gudgeon pin. The oil is drained away at the front 

 end of the cylinder, and is not used again. In vertical 

 engines splash lubrication is generally employed for the 

 lubrication of the piston, gudgeon pin, and in many cases 

 the crank shaft and other engine bearings. The oil in 

 this system is thrown up from the crank chamber by the 

 crank dipping into it. At the best it seems to be a happy- 

 go-lucky method of a most unscientific order; the only 

 thing which can be said in its favour is that in actua'l 

 practice it has been found to work. For crank shafts and 

 similar bearings a forced feed system would be better, 

 provided some perfect system for road motors could be 

 found of freeing entirely the oil from grit before re- 

 feeding it to the bearings. Bath lubrication of gear 

 wheels is efl'ective as regards lubrication, but absorbs 

 power in churning up the oil. 



There is still a good deal that can be effected in design 

 in reducing friction by the substitution of ball or roller 

 bearings for plain ones in suitable places, and by the 

 use of metals having a low coefficient of friction. There 

 are many ways in which power can be lost between the 

 engine and the road wheels. None of the many forms 

 of friction clutch can be depended upon not to slip in 

 the way that a clutch which is positively engaged can. 

 The use of Hooke's universal joints involves loss in trans- 

 mission, as is demonstrated by the rapidity with which 

 they often wear. The total transmission losses are not 

 accurately known under road conditions, but it may be 

 indirectly estimated that such losses may amount to from 

 20 per cent, to 40 per cent., or even more. 



The advantages of pneumatic tyres, owing to their 

 resilience and low resistance, are counterbalanced by their 

 high cost, rapid wear, and vulnerability. Methods of de- 

 creasing the vulnerability are only obtained at the sacrifice 

 of other, and possibly more important, qualities. Owing 

 to the large area of contact with the ground, and con- 

 sequent low pressure per unit of contact area, the coefiicient 

 of friction is so small that skidding occurs if the road 

 is greasy. No remedy has been found which does not 

 impair the action of the tyre as a pneumatic one. Again, 

 owing to the rapidity of recovery of the tyre on passing 

 over an obstacle, oscillatory movement of the vehicle is 

 started, and, given favourable conditions of speed and 

 road, may be maintained, or even increased, to a 

 dangerous extent. \ partial remedy exists in the shock 

 NO. 2063, VOL. 80] 



ab.-orber applied between the sprung and unsprung por- 

 tions of the vehicle. An inherent defect of the pneumatic 

 tyre is its dust-raising properties. The tyre raises the 

 dust, and the eddies produced by the passage of the car 

 scatter it far and wide. This subject is one which is 

 attracting the attention of the authorities representing road- 

 •makers and users. So far, the only effective remedy has 

 consisted in treatment of the surface of the road. 



The study of the composition of the e.xhaust gases is 

 of importance. It ought to be possible to ensure that the 

 exhaust gases contain not more than i per cent, of carbonic 

 oxide. Governing by retarding the ignition is effective, 

 buf is objectionable on account of its liability to increase 

 the percentage of CO in the e.xhaust ; it is also unscien- 

 tific, and very wasteful of fuel. 



To obtain an average speed of twenty miles per hour, 

 experience tells us that the maximum speed will not be 

 less than 50 per cent, greater than the average during 

 some periods of the journey ; assuming a moderate efficiency 

 of transmission of power, the provision of an engine cap- 

 able of giving I brake-horse-power per cwt. of the gross 

 weight of the vehicle and its load of passengers, &c., would 

 not be e.xcessive. 



Deductions made from data known to be approximately 

 correct for the speed, power, and wind area of various 

 cars having ordinary touring bodies lead to the formula 

 P = oooi7AV^, in which P = resistance in lb. per square 

 foot, A = projected area of car in square feet, and 

 V = velocity in feet per second. E.xperiments are needed to 

 provide data as regards the form of car offering the least 

 resistance to the air. 



The gross ton-miles which should be obtained from a 

 gallon of petrol of about 0720 specific gravity at a speed 

 of twenty miles per hour should not fall below thirty under 

 ordinary conditions. There is room for improvement in 

 this. iVIany other items, tyres especially, have to be con- 

 sidered, which swell the bill to such an extent as to render 

 the cost of fuel but a small part of the whole. 



The weight of a pleasure motor-car is high compared 

 to the useful load of passengers. The useful weight in 

 this case would be about one-quarter of the weight of 

 the vehicle. Medium-weight passenger or goods' vehicles 

 may carry a useful load of three-quarters the weight of 

 the unladen vehicle ; heavy vehicles having a slow speed 

 may carry a load equal to the weight of the vehicle. It 

 would appear that some improvement may be reasonably 

 looked for in the reduction of the weight of the car as 

 compared with its useful load. 



Brakes on the steering wheels give immunity from 

 skidding, but are very difficult to arrange for. It is best 

 to apply both brakes required bv law to the driving wheels, 

 rather than to have one of them applied to the secondary 

 transmission shaft. The distance in which a car can be 

 pulled up without damage to the tyres on an ordinary 

 road and under normal conditions may be approximately 

 found from the formula S = oo4V", where V is in miles 

 per hour and S is the distance in yards in which the car 

 should come lo rest. At ten miles per hour it should stop 

 in 4 yards, and at twenty miles per hour in 16 yards. 

 These distances are greater than is desirable, and also 

 greater than most drivers would be prepared to admit, 

 probably owing to time, and not distance, being the factor 

 that a driver judges by when called upon to stop quickly. 

 Improvement is only to be sought for in increasing the 

 surfaces of adhesion, as by braking all four wheels, or 

 by more equal distribution of the braking effect than we 

 have at present. 



Petroleum spirit is practically the only fuel employed ; 

 other fuels which might be used are petroleum, paraffin, 

 benzol, and alcohol. Suction gas producers may be used 

 for the heavier classes of vehicles. 



TH£ " EROMOIL " PROCESS. 



A BOUT five years ago Mr. G. E. H. Rawlins intro- 

 "'"^ duced, as a practical method of making photographs, 

 a process described fifty years previously by Poitevin. 

 Paper coated with gelatin is sensitised by soaking it in 

 a solution of potassium bichromate, dried, and exposed 

 under a negative. Where light has acted the gelatin is 

 rendered less able to absorb water, so that if the print 



