This has been even more than the others a matter of patience and detailed attention to small and apparently unimportant points. The ignition system which was at one time under electrical control and with only one igniter per cylinder head, has now been made entirely mechanical with two or more independent igniters per cylinder end, so that one will ignite the charge if the other fails. Pilot lights on each of these igniters show whether they are working or not and enable difficulties to be cured before shut downs from the failure of both can occur.

The type of design which has the strength to resist the heavy internal pressures due to the gas explosion, and at the same time is so free from inequalities as to avoid heating strains, and the cracking and failing of cylinder heads and pistons which come from that cause, has gradually been evolved. The system for supplying water for cooling the pistons and piston rods has been developed so that the formation of air and steam pockets is prevented, and the overheating and consequent cracking and premature ignition which arose from those causes are thereby avoided.

In this country the center crank construction has generally been discarded* in favor of the overhung single crank because the latter requires only one main bearing and definite knowledge of its adjustment and condition and of the stresses upon it are obtainable, whereas when the center crank is used, two main bearings are required, the racking stresses which may be developed by the maladjustment of one of them are enormous and entirely impossible to determine. Nevertheless some of the most recent gas blowing engines built by the Mesta Machine Co. have returned to the center crank construction on the ground that a smaller and lighter crank gives the same strength and has less overhang.

The valves of the engines have been put in convenient and accessible locations so that they can be readily removed for examination or replacement, and at the same time so that dirt will not lodge upon their seats, with the consequent destruction of the latter.

All of these details and many more have been subjected to the most rigid, patient, and penetrating investigation, so that the causes which led to these difficulties have either been avoided altogether or their results have been overcome to the best advantage.

Expense Of Operating And Maintenance

The thorough cleaning of the gas eliminated at one stroke a great number of the operating difficulties of the gas engine, while hand in hand with this went the knowledge obtained by both designers and operators from the patient investigation and improvement of details described under the fourth heading, so that great plants of gas engines may now be seen running apparently with but little more attention and no more repairs than would be required by an equal number of steam engines of similar power.

The Reduction Of The Gas Engine To Standard Practice

The gradual elimination of the experimental work required in the early days of gas engine development, and the fact that a number of reliable and experienced engine builders have undertaken its manufacture under competitive conditions, have all tended to reduce the first cost of these engines, while the standardization of designs and better adaptation of the means to the end sought have lowered their rate of obsolescence so that the amortization period is longer and this in conjunction with the lower first cost has greatly reduced the overhead charges.