Chapter 17
Horse Power
The term horse-power is misleading, but it was found necessary very shortly after the invention of the steam engine to have some means of expressing the power of an engine.
Before the introduction of steam, horses and other beasts of burden were largely used for driving machinery, for winding minerals, etc. The horses were harnessed to a “gin,” or sort of capstan arrangement, the animals travelling round in a circle. From experiments James Watt made with strong horses, belonging to some London brewers, he fixed 33,000 foot-pounds as one horse-power. This is 1 lb. lifted 33,000ft. high in a minute, or 33,000 lbs lifted 1ft. high in a minute. For instance, if a horse drawing a load on a road covers 330ft. in a minute (a fair walking pace), and the draught or direct pull on the load he is drawing behind him is 100 lbs., he is doing exactly one horse-power. The draught or direct pull on the load can be taken by a dynamometer or spring balance. But a horse can exert much more than a mechanical horse-power for a short period, such as in starting heavy loads and drawing vehicles up steep short inclines. A horse for a short time will give three or four mechanical horse-power.
A horse harnessed to a carriage will draw that carriage easily with five or six persons in it, but if that carriage is to be mechanically propelled four or five horse-power will be necessary. This appears a paradox, but on a little consideration it will be seen that the horse cannot draw this weight all day; he is probably exerting more than one horsepower, using up his energy in three or four hours, whereas a steam or other engine would, and does, give its power off continuously. A London omnibus horse only runs about fourteen miles per day, and taking the speed of the omnibus at six miles an hour, his work is really done in 2h. 20m.; hence relays of horses are necessary.
Another reason is that the power of the engine is given as taken at the fly-wheel or crankshaft. The friction of gearing, chains, belts or spur wheels, and sometimes three or four revolving shafts, consuming considerable power. In some experiments made in America with motor cars it was found that in some cases as much as forty per cent, of the power of the engine was absorbed in this way. Hence the less gearing and fewer revolving shafts in a car there are, the higher efficiency of the machinery.
The ordinary high bicycles that were in use twenty years ago were probably more mechanically efficient than the modern cycle; but the high centre of gravity and the difficulty of mounting has caused them to be no longer used.
Ball and roller bearings, by getting rid of the rubbing surfaces, largely reduce friction and increase the efficiency of the machinery both of cycles and motor cars.
Horse-power may be either Indicated horse-power (generally expressed as I.H.P.) or actual or brake (B.H.P.) The former is the maximum power exerted on the piston as taken by an indicator. The indicator is explained in most elementary books relating to steam or gas engines. It consists of a cylinder fitted with a piston, which presses against a spring. One end of the piston rod is connected to a lever or beam. The long end of the lever carries a pencil, which traces a line on paper on a cylindrical drum, which drum has a reciprocatory motion imparted to it by the piston of the engine. It must he understood that the cylinder of the indicator is connected to the engine cylinder by a pipe fitted with a cock, by which the steam, or pressure from a gas engine explosion, is admitted to the indicator.
The pencil traces on the sheet of paper on the reciproratory drum a curve - see fig. 53, which is an indicator diagram from a Trusty oil engine.
The vertical height represents pressure per square inch. This is ascertained by dividing the diagram into ten vertical areas, and with a scale supplied with the instrument reading the pressure for each tenth part of the stroke; the mean of these represents the actual pressure per square inch on the piston throughout the stroke. To find the total pressure on piston the area must be multiplied by the pressure. To find the indicated horse-power, the total pressure on piston is multiplied by the piston speed, or one-fourth speed in an Otto cycle gas engine, which has only one working stroke in four, the products being then divided by 33,000.
As an example, a gas engine has a cylinder 4 3/8in. diameter and a stroke of 12in., and runs 300 revolutions per minute.
The area of a cylinder 4 3/8in. diameter is 15 square inches, and assuming the mean pressure is 40 lbs. per square inch, this gives a total pressure of 600 lbs. on piston. The piston speed of the engine is 600ft. per minute, but as one stroke in four only is a working stroke it must be taken as 150ft. This multiplied by pressure on piston equals 90,000 foot-pounds, and divided by 33,000 equals 2.7 indicated horsepower, or to put it in another way,
Area in inches x pressure x working speed of piston
33,000
The above example of a gas engine would be written thus:
15 X 40 X 150 = 2.7 indicated horse-power.
33,000
The brake horse-power is the real or actual power delivered by the engine. It is so called by being ascertained by a brake or dynamometer. If an engine were used for hauling a weight out of a deep mine the weight multiplied by the speed it travels would give the actual horse-power, but as it would be inconvenient to have a pit of unlimited depth the weight is allowed to slip off the fly-wheel or rope pulley, and the weight multiplied by the speed at the rim of the wheel gives the foot-pounds developed by the engine.
A convenient arrangement of brake is shown in fig. 54, where A is a fly-wheel or pulley; W is a weight that is lifted by the engine by means of the cord running partly round the fly-wheel or pulley. The tension on the cord is kept by a spring balance C, the reading of which must be deducted from the weight W. Small weights must be added or subtracted from W till the maximum load of the engine is reached. The reading of the spring balance will vary as weights are added, or subtracted from the main weight W. The cord which must be well greased is led through three or more wooden checks D to keep it in its place in the wheel. It is sometimes difficult to arrange the proper tension on the cord; therefore the spring balance can be moved to the right or left, so that more or less of the circumference of the wheel be embraced by the cord, or the cord may completely encircle the wheel, and be attached to a hook in a beam above, as shown by the dotted line E. The power is ascertained by multiplying the speed of the rim of the flywheel by the weight sustained.
For instance, if the fly-wheel A has a peripheral speed of 3,000ft. per minute, and the weight on the cord (less the reading of the spring balance) be 50 lbs., we have 3,000 X 50 divided by 33,000 = 4.85 horse-power.
In arranging a brake care must be taken that the weight is secured by a loose cord or chain to the ground, so that should the cord seize the weight is not thrown over the fly wheel or into the machinery of the engine; such accidents do sometimes happen.
Another form of brake is shown in fig. 55. Here two blocks of wood are cut to fit the circumference of the wheel, and the tension is adjusted by the screws F, one of which should have a spring under the nut. The reading is taken by the spring balance C. The angle formed by the centre of wheel H, the point of the attachment of the spring balance C, and the fixed attachment of balance K, must be a right angle, otherwise the readings will not be correct. In this case the line H G is taken as the radius of an imaginary fly-wheel.
The wood blocks must have checks or flanges on each side to prevent them coming off the wheel, and must be well lubricated with grease, otherwise they would soon fire.
In running friction brakes on small light pulleys there is a danger of cracking the wheel by unequal expansion. The rim soon gets hot, the boss and spokes are cold, and then a spoke will often break.
The tractive effort to drive a car varies with the surface of the road; on a good macadamised road it may be taken as 50 lbs. to 60 lbs. per ton at low speeds. That means that if a horse were to draw a car weighing one ton, and the pull he exerted were read by a dynamometer or spring balance, the tractive effort would he about 50 lbs. or 60 lbs. With rubber tyres it is less, and less again with pneumatic tyres. Dust, mud, and stones increase the power very much.
Assume a car loaded weighs one ton, at ten miles per hour, the tractive force would probably be not less than 70 lbs. on a dead level road. Ten miles an hour equals 880ft. per minute, and if the tractive force 70 lbs. be multiplied by the speed in feet 880 we have 61,600 foot-pounds, or a little under two horse-power; but it has been mentioned above that the gearing may absorb as much as forty per cent of the engine power, so that two and three-quarter brake or actual horse-power will be required to drive this car on dead level road. But on a hill gravity comes into consideration; on a hill of one in ten, that is, what is called a steep hill, one-tenth of the weight of this car, or 224 lbs., must be added to the tractive force, for the car rises 1 ft. vertically for every 10ft. it goes forward. We therefore find that fourteen and three-quarter horse-power would be required to drive the one-ton car up the one in ten rise at ten miles an hour, but there would be also an increase of friction in the extra strain on machinery, bearings, and gearing, and probably sixteen or seventeen horse-power would be nearer the mark. This shows how important it is to have plenty of power for hill-climbing.
Assume, also, an undulating road twelve miles long with the up and down grades the same. Over this road two cars have to run; one with plenty of power, so that it can run uphill and down at the legal limit - twelve miles an hour. It runs over the road in just one hour. The other car can only go uphill at six miles an hour; it therefore runs the six miles on the down grade in thirty minutes, but the six miles uphill occupy one hour, and it takes half an hour more to cover the twelve miles than its more powerful rival.
The French makers when commencing to build cars in 1893-4 allowed about three-quarter or seven-eighths horse power for each passenger. Since that time the power has been increasing rapidly. The modern voiturettes have five horse-power for three or four passengers; the large Panhard and Daimler cars twelve and sixteen horse-power for four or six. Of cars other than bicycles and tricycles, the Benz is probably the lowest power per passenger, but the simplicity of the transmission gear causes less loss of power than in many others.