Fig. 2.—Plate Showing Heavy, Slow Speed Internal Combustion Engines Used Only for Stationary Power in Large Installations Giving Weight to Horse-Power Ratio.

Four Cylinder Diesel Engine for Marine Use
250 Pounds per Horsepower
Two Cycle Marine Engine
50-100 Pounds per Horsepower
600-800 R.P.M.
Single Cylinder Vertical Farm Engine
150 Pounds per Horsepower—Speed 400 R.P.M.
Two Cylinder Four Cycle Tractor Engine
75 Pounds per Horsepower
800 to 1000 R.P.M.
Four Cylinder Four Cycle Automobile Power Plant
Weighs about 25 Pounds per Horsepower
1200 to 2000 R.P.M.

Fig. 3.—Various Forms of Internal Combustion Engines Showing Decrease in Weight to Horse-Power Ratio with Augmenting Speed of Rotation.

Eight Cylinder “Vee” Automobile Engine
15 to 18 Pounds per Horsepower
Speeds 1500 to 2000 R.P.M		Two Cylinder Air Cooled Motorcycle
Engine weights 8-10 Pounds Horsepower
Speed 3000 R.P.M.
Six, Eight or Twelve Cylinder Water Cooled Aviation Engine, Tandem or V Form
4 to 6 Pounds per Horsepower
Speed 1500 R.P.M. Direct Coupled—2000 R.P.M. Geared Drive
Seven or Nine Cylinder Revolving
Air Cooled
Speed 1200 R.P.M. 2.8 Pounds per Horsepower		Fourteen or Eighteen Cylinder
Revolving Air Cooled Aviation Engine
Speed 1200 R.P.M.
2 Pounds per Horsepower

Fig. 4.—Internal Combustion Engine Types of Extremely Fine Construction and Refined Design, Showing Great Power Outputs for Very Small Weight, a Feature Very Much Desired in Airplane Power Plants.

Of all the types enumerated above engines having less than eight cylinders are the most popular in everything but aircraft work. The four-cylinder vertical is without doubt the most widely used of all types owing to the large number employed as automobile power plants. Stationary engines in small and medium powers are invariably of the single or double form. Three-cylinder engines are seldom used at the present time, except in marine work and in some stationary forms. Eight- and twelve-cylinder motors have received but limited application and practically always in automobiles, racing motor boats or in aircraft. The only example of a fourteen-cylinder motor to be used to any extent is incorporated in aeroplane construction. This is also true of the sixteen- and eighteen-cylinder forms and of twenty-four-cylinder engines now in process of development.

The duty an engine is designed for determines the weight per horse-power. High powered engines intended for steady service are always of the slow speed type and consequently are of very massive construction. Various forms of heavy duty type stationary engines are shown at [Fig. 2]. Some of these engines may weigh as much as 600 pounds per horse-power. A further study is possible by consulting data given on [Figs. 3] and [4]. As the crank-shaft speed increases and cylinders are multiplied the engines become lighter. While the big stationary power plants may run for years without attention, airplane engines require rebuilding after about 60 to 80 hours air service for the fixed cylinder types and 40 hours or less for the rotary cylinder air-cooled forms. There is evidently a decrease in durability and reliability as the weight is lessened. These illustrations also permit of obtaining a good idea of the variety of forms internal combustion engines are made in.