=Testing the Plant.=--When the engine has been selected the consulting engineer supervises its installation, and, after this is completed, carries out tests in order to determine whether or not the guaranteed power and consumption are attained. The methods employed in testing a gas-engine are both complex and delicate. The quality of the gas, the proportions of the elements forming the mixture, the time and the method of ignition, the temperature of the cylinder-walls, the temperature and the pressure of the gas drawn into the cylinder, all these are factors which have a decided bearing upon the results of a test. If these factors be not carefully considered the conclusions to be drawn from the test may be absolutely wrong.
Indicators of any type should not be indiscriminately employed; only those specially designed for gas-engine purposes should be used. Indicator cards are in themselves inadequate, and should be supplemented by the records of explosion-recorders.
The calorific value of the gas should be measured either by the Witz apparatus or by means of any other calorimeter.
In interpreting the diagrams and records some difficulty will be encountered. Sometimes it happens that a particular form of curve is attributed to a cause entirely different from the real one. It happens not infrequently that engineers, whose experience is confined to engines of one make and who have not had the opportunity to make sufficient comparisons, draw such erroneous conclusions from cards.
To recapitulate what has already been said, the testing of gas-engines requires considerable experience and cannot be lightly undertaken. Special instruments of precision are necessary. The author has very often been called upon to contradict the results obtained by experts whose tests have consisted simply in ascertaining the engine power either by means of a Prony brake, or by means of a brake-strap on the fly-wheel. The brake gives but crude results at best; it is a means of control, and not an instrument of scientific investigation.
Something more than the mere power produced by an engine should be ascertained. The tests made should throw some light upon the reasons why that power cannot be exceeded, and show that the necessary changes can be made to cause the engine to operate more economically and to yield energy of an amount which its owner has a right to expect. The indicator and the recorder are testing instruments which clearly indicate discrepancies in operation and the means by which they may be corrected. The tests made should determine whether the power developed is not obtained largely by means of controlling devices which cause premature wearing away of the engine parts.