Real Horsepower

It is of interest to compare the efficiency of horses, in converting into useful work the nutrients they consume, with the efficiency of farm tractors or other engines. In Missouri experiments when horses worked at the usual rates, they converted into actual work about 20 to 25% of the energy they expended during the time they were working. The rest of the energy is converted into heat and is lost, so far as useful work is concerned.

However, to gain a true idea of the efficiency of the horse as a source of power, we must compute his over-all efficiency for the entire day of 24 hours. this is the percentage of the total or gross energy in his daily feed, which he is able to convert into useful work. Based on the amount of feed required per day, a 1500 lb horse working 8 hours a day at a rate of 1 horse power will have an over all efficiency of about 8.9%.

However, farm horses do not work every day, though they must eat every day. There fore the over all efficiency for the entire year is much lower than for a 24 hour period in which the horse does a good day’s work. On the average, the yearly over all efficiency of farm horses, which usually work only about 800 to 1000 hours a year, is about 2 to 3%.

In tests of farm tractors by the University of Nebraska, gasoline tractors converted 13%, on the average, of th4e energy of the gasoline into the work of draft. This is probably higher than the efficiency of the average tractor, for these tractors were handled by experts and were in ideal mechanical condition for these tests. (Diesel tractors have a considerably higher mechanical efficiency than gasoline tractors.)

In comparing this efficiency of 13% for tractors with the efficiency of 8.9% for horses in converting the energy of their feed into draft when working a full day, we must consider the following: The horse is supplied with feed in crude form, and a considerable part of it is indigestible and therefore of no value in the production of work. The gasoline or kerosene tractor, however, is not supplied with crude fuel, but with highly refined fuel from which the waste portions have been removed. Moreover, the horse repairs his body continuously, while the tractor cannot replace the daily wear of its parts.

Feeds and Feeding (1954) by Frank B. Morrison