MOTOR & CYCLING

Taranaki Daily News, 30 August 1930, Page 4 (Supplement)

MOTOR & CYCLING

WHAT IS HORSE-POWER?

A SIMPLE EXPLANATION POWER PI'LL AND PERFORMANCE(By "Autoscribe.") . There J6,no more misunderstood or more abused term in the jargon of motoring than “horse-power,” which means different things to half a dozen motorists, and nothing at all to the seventh. Time and again the writer has had queries as to the exact moanina of horse-power, and though it is difficult to explain the subject without croina into a mass of technicalities, the attempt is now being made. ' As one motorist put it, “how is it that a 32 horse-power ear stuck on a.bad piece of road can be pulled out by a couple oi draught-horses, while alongside Will be a 40 horse-power car, equally helpless in the mud?” It seems good reasoning that if only two horses are needed to extricate the first car, then a little extra horse power .will solve the trouble. But it does not.

Most people know of Janies Wait, as builder of the first really successful steam engine, but they do not know' him as the first to define what horsepower is, nor why he did it. When Watt first invented his engine the principal use before steam engines lay in the pumping of water from the coal mines of England, a job that had always been done by horses working on rotary pumps. As the mines got down to greater depths it became more and more difficult for the horses to cope with the pumping, and at one stage it looked as if many of the mines would have had to be closed down. Then Watt invented Tiis steam engine, .but it was necessary to show the owners that this would do the work of horses better, and more cheaply than the horses themselves. To do this Watt 'had Io estimate horse-power carefully, and after many tests, into- which it is unnecessary to go, he found that one horse-power equalled 33,000 foot-pounds of work per minute. "Watt found that a mine-horse would exert a continuous pull of 1501 b while walking at 21 miles per hour, so defined horse-power as 150 pounds times 220 feet per minute. Thus, in an extremely simple way he was able to demonstrate the capabilities of his steam engine in direct comparison with horst's. The radial arm of the pump at which Watt’s horses worked was 20 feet, and he assessed their work in terms of footpounds, which is exactly the same’ way as motor-car power is measured. Although horse-power is always used to designate the power of a car, torque would actually be more correct, for this is the rotating effect produced by .tha engine on the crankshaft, and can be measured in terms of foot-pounds. When a car engine is being tested for horse-power it is removed to a bench, and a pully with a radius of one foot is attached to the fly-wheel. A suit- , able brake band is applied to the pulley, and the band is kept from turning by attaching a spring balance to it.°

The radius of one foot was mentioned above. The circumference of a circle with such a radius is G.2S feet, and once you know the torque, and the revolutions of the engine per minute, it is possible to work out the horse power. Take an instance of an engine making 500 revolutions per minute with a torque of 1051 b.. This gives 1036 footpounds of work produced per revolution (165 x 6.28) and multiplying that by 500 (the revolutions per minute) we get 518,000 foot-pounds per minute. Dividing this by Watt’s figures of 33,000, we find that the engine horse-power is 15.7.

The method of horse-power assessment given above is the one used for fixing the brake horse-power of a ear, which in reality is the actual net horsepower developed by the motor when actually under test. Brake horse-power reaches its maximum at a certain number of revolutions per minute, according to the design of the motor, and after reaching the peak output it will show a decrease, even although the revolutions per minute are increased. The rated horse-power bears no relation to the brake horse power, as it is determined by a formula, and takes no account of variations in engine design, which may increase efficiency. Certain standards have now been fixed, and these arc the same in both America and England. There are two formulas, each of which takes into account . various engine measurements. The first is the R.A.C. (Royal Automobile Club of Great Britain) and the. second the N.A.C.C, (National Automobile Chamber of Commerce, U.S.A.). The ease of a sixcylinder popular car on the New Zealand market will illustrate the difference between brake' horse-power and rated horse-power, the former being 46 while the latter is only 20. The above details regarding hors’- i power skim very lightly over the subject, avoiding many technical sideissues, which are important but which confuse the issue to the ordinary motor- j ist. A knowledge of how horse-power ‘ is assessed does not necessarily help the motorist at all, but it provides him with an explanation as to why certain cars have maximum speeds, and why hill climbing ability taries. Weight of the car, and the build of the body, which affects wind resistance, are responsible for great variations in the actual performances of cars of the same horse-power, while the gearing will also affect hill climbing ability. Moreover - the hill climbing ability is really a question of the reserve force available at the rear wheels in proportion to the weight of the ear. That Is one of the reasons why American cars, with their more powerful engines, will climb hills on “top” while English and Continental cars will have to be taken down to “second’’ to do the same grade. From the foregoing the novice will have learnt enough to know that horsepower, really does not count for a great deal, and that performance over hills, acceleration in traffic, and general smoothness of running are more important. Buying a car solely on a horse-power basis would be misleading, and might often be the cause of disappointment later.

DIESEL PROSPECTS.

PROGRESS IN AMERICA. American engineering circles are still divided in their opinions of the Diesel motor, according to two statements recently made by leaders for both sides of the question. One American authority on engine design has declared that his company does not believe the Diesel principle is applicable to the motor-car, despite the fact that this company has gone far in the development of-that engine for aircraft purposes. The authority for this statement is none other than Mr. Alvan Macauley, president of the Packard Company, who is well-known for his company’s development of the Diesel aero engine. On the other hand, Mr. C. L. Cummins, of the Cummins Engine Company, says, in a paper read before the Society of Automotive Engineers, that “the application of a Diesel engine to an automobile has developed many interesting and unusual problems, as -well as some very astonishing results in the transportation of weight. Mr. Cummins called attention to the fact that he has experimented with the Diesel in two types of ears; a sevenpassenger limousine, W’hich weighed approximately 6000 pounds, and a roadster w’hich weighed 4500 pounds. The limousine was driven, in onp test, approximately 6000 miles, during which time the fuel consumption varied from 25 to 35 m.p.g., according to speed and the number of traffic delays. This car was geared 2J to 1, and aside from being a little sluggish on the pick-up from zero to 20 miles per hour, the performance was practically the same as with the standard equipment originally used in the car. The motor of this car was of the fourcylinder type, with a 41 by Gin. bore and stroke, designed to operate normally at a maximum of 1000 r.p.m. Revolutions were increased to about 1200, which gave the car a top speed of 52 to 55 miles per hour. The ordinary transmission was used and no other changes from standard were made in the motor, which was primarily designed for marine service, and was heavier than the one removed from the car, the w’eight difference being approximately 400 pounds. In a drive from Indianapolis to New York, undertaken with this car in cold weather, almost every conceivable traffic condition was encountered, but the engine proved sufficiently flexible.