The Benz Motor
In the Kaiser prize contest for aviation motors a four-cylinder Benz motor of 130 by 180 mm. won first prize, developing 103 B. H. P. at 1290 R. P. M. The fuel consumption was 210 grams per horse-power hour. Total weight of the motor was 153 kilograms. The oil consumption was.02 of a kilogram per horse-power hour. This motor was afterward expanded into a six-cylinder design and three different sizes were built.
The accompanying table gives some of the details of weight, horse-power, etc.
| Motor type | B | FD | FF | |||
| Rated horse-power | 85 | 100 | 150 | |||
| Horse-power at 1250 r.p.m | 88 | 108 | 150 | |||
| Horse-power at 1350 r.p.m | 95 | 115 | 160 | |||
| Bore in millimeters | 106 | 116 | 130 | |||
| Stroke in millimeters | 150 | 160 | 180 | |||
| Offset of the cylinders in millimeters | 18 | 20 | 20 | |||
| Rate of gasoline consumption in grams | 240 | 230 | 225 | |||
| Oil consumption in grams per b.h.p. hour | 10 | 10 | 10 | |||
| Oil capacity in kilograms | 36 | 4 | 4 | 1⁄2 | ||
| Water capacity in litres | 5 | 1⁄2 | 7 | 1⁄2 | 9 | 1⁄2 |
| The weight with water and oil but with two magnetos, fuel feeder and air pump in kilograms | 170 | 200 | 245 | |||
| The weight of motors, including the water pump, two magnetos, double ignition, etc. | 160 | 190 | 230 | |||
| The weight of the exhaust pipe, complete in kilograms | 4 | 4 | .8 | 5 | 1⁄2 | |
| The weight of the propeller hub in kilograms. | 3 | 1⁄2 | 4 | 4 | ||
The Benz cylinder is a simple, straightforward design and a very reliable construction and not particularly difficult to manufacture. The cylinder is cast of iron without a water jacket but including 45 degrees angle elbows to the valve ports. The cylinders are machined wherever possible and at other points have been hand filed and scraped, after which a jacket, which is pressed in two halves, is gas welded by means of short pipes welded on to the jacket. The bottom and the top of the cylinders become water galleries, and by this means separate water pipes with their attendant weight and complication are eliminated. Rubber rings held in aluminum clamps serve to connect the cylinders together. The whole construction turns out very neat and light. The cylinder walls are 4 mm. or 3⁄16′′ thick and the combustion chamber is of cylindrical pancake form and is 140 mm. or 5.60 inch in diameter. The valve seats are 68 mm. in diameter and the valve port is 62 mm. in diameter.
The passage joining the port is 57 mm. in diameter. In order to insert the valves into the cylinder the valve stem is made with two diameters and the valve has to be cocked to insert it in the guide, which has a bronze bushing at its upper end to compensate for the smaller valve stem diameter. The valve stem is 14 mm. or 9⁄16′′ in diameter and is reduced at its upper portion to 91⁄2 mm. The valves are operated through a push rod and rocker arm construction, which is 7⁄16′′ and exceedingly light. Rocker arm supports are steel studs with enlarged heads to take a double row ball bearing. A roller is mounted at one end of the rocker arm to impinge on the end of the valve stem, and the rocker arm has an adjustable globe stud at the other end. The push rods are light steel tubes with a wall thickness of 0.75 mm. and have a hardened steel cup at their upper end to engage the rocker arm globe stud and a hardened steel globe at their lower end to socket in the roller plunger.
The Benz cam-shaft has a diameter of 26 mm. and is bored straight through 18 mm. and there is a spiral gear made integrally with the shaft in about the center of its length for driving the oil pump gear. The cam faces are 10 mm. wide. There is also, in addition to the intake and exhaust cams, a set of half compression cams. The shaft is moved longitudinally in its bearings by means of an eccentric to put these cams into action. At the fore end of the shaft is a driving gear flange which is very small in diameter and very thin. The flange is 68 mm. in diameter and 4 mm. thick and is tapped to take 6 mm. bolts. The total length of cam-shaft is 1038 mm., and it becomes a regular gun boring job to drill a hole of this length.
The cam-shaft gear is 140 mm. or 51⁄2 inches outside diameter. It has fifty-four teeth and the gear face is 15 mm. or 19⁄32′′. The flange and web have an average thickness of 4 mm. or 5⁄32′′ and the web is drilled full of holes interposed between the spur gear mounted on the cam-shaft and the cam-shaft gear. There is a gear which serves to drive the magnetos and tachometer, also the air pump. The shaft is made integrally with this gear and has an eccentric portion against which the air pump roll plunger impinges.
The seven-bearing crank-shaft is finished all over in a beautiful manner, and the shaft out of the particular motor we have shows no signs of wear whatever. The crank-pins are 55 mm. in diameter and 69 mm. long. Through both the crank-pin and main bearings there is drilled a 28 mm. hole, and the crank cheeks are plugged with solder. The crank cheeks are also built to convey the lubricant to the crank-pins. At the fore end of the crank cheek there is pressed on a spur driving gear. There is screwed on to the front end of the shaft a piece which forms a bevel water pump driving gear and the starting dog. At the rear end of the shaft very close to the propeller hub mounting there is a double thrust bearing to take the propeller thrust.
Long, shouldered studs are screwed into the top half of the crank-case portion of the case and pass clean through the bottom half of the case. The case is very stiff and well ribbed. The three center bearing diaphragms have double walls. The center one serves as a duct through which water pipe passes, and those on either side of the center form the carburetor intake air passages and are enlarged in section at one side to take the carburetor barrel throttle.
The pistons are of cast iron and carry three concentric rings 1⁄4 inch wide on their upper end, which are pinned at the joint. The top of the piston forms the frustum of the cone and the pistons are 110 mm. in length. The lower portion of the skirt is machined inside and has a wall thickness of 1 mm. Riveted to the piston head is a conical diaphragm which contacts with the piston pin when in place and serves to carry the heat off the center of the piston.
The oil pump assembly comprises a pair of plunger pumps which draw oil from a separate outside pump, and constructed integrally with it is a gear pump which delivers the oil under about 60 pound pressure through a set of copper pipes in the base to the main bearings. The plunger oil pump shows great refinement of detail. A worm wheel and two eccentrics are machined up out of one piece and serve to operate the plungers.
Some interesting details of the 160 horse-power Benz motor, which is shown at Fig. 246, are reproduced from the “Aerial Age Weekly,” and show how carefully the design has been considered.
Maximum horse-power, 167.5 B. H. P.
Speed at maximum horse-power, 1,500 R. P. M.
Piston speed at maximum horse-power, 1,770 ft. per minute.
Normal horse-power, 160 B. H. P.
Speed at normal horse-power, 1,400 R. P. M.
Piston speed at normal horse-power, 1,656 ft. per minute.
Brake mean pressure at maximum horse-power, 101.2 pound per square inch.
Brake mean pressure at normal horse-power, 103.4 pound per square inch.
Specific power cubic inch swept volume per B. H. P., 5.46 cubic inch; 160 B. H. P.
Weight of piston, complete with gudgeon pin, rings, etc., 5.0 pound.
Weight of connecting rod, complete with bearings, 4.99 pound; 1.8 pound reciprocating.
Weight of reciprocating parts per cylinder, 6.8 pound.
Weight of reciprocating parts per square inch of piston area, 0.33 pound.
Outside diameter of inlet valve, 68 mm.; 2.68 inches.
Diameter of inlet valve port (d), 61.5 mm.; 2.42 inches.
Maximum lift of inlet valve (h), 11 mm.; 0.443 inch.
Area of inlet valve opening (π d h), 21.25 square cm.; 3.29 square inches.
Inlet valve opens, degrees on crank, top dead center.
Inlet valve closes, degrees on crank, 60° late; 35 mm. late.
Outside diameter of exhaust valve, 68 mm.; 2.68 inches.
Diameter of exhaust valve port (d), 61.5 mm.; 2.42 inches.
Maximum lift of exhaust valve (h) 11 mm.; 0.433 inch.
Area of exhaust valve opening (π d h), 21.25 square cm.; 3.29 square inches.
Exhaust valve opens, degrees on crank, 60° early; 35 mm. early.
Exhaust valve closes, degrees on crank, 161⁄2° late; 5 mm. late.
Length of connecting rod between centers, 314 mm.; 12.36 inches.
Ratio connecting rod to crank throw, 3.49:1.
Diameter of crank-shaft, 55 mm. outside, 2.165 inches; 28 mm. inside, 1.102 inches.
Diameter of crank-pin, 55 mm. outside, 2.165 inches; 28 mm. inside, 1.102 inches.
Diameter of gudgeon pin, 30 mm. outside, 1.181 inches; 19 mm. inside, 0.708 inch.
Diameter of cam-shaft, 26 mm. outside, 1.023 inches; 18 mm. inside, 0.708 inch.
Number of crank-shaft bearings, 7.
Projected area of crank-pin bearings, 36.85 square cm.; 5.72 square inches.
Projected area of gudgeon pin bearings, 22.20 square cm.; 3.44 square inches.
Firing sequence, 1, 5, 3, 6, 2, 4.
Type of magnetos, ZH6 Bosch.
Direction of rotation of magneto from driving end, one clock, one anti-clock.
Magneto timing, full advance, 30° early (16 mm. early).
Type of carburetors (2) Benz design.
Fuel consumption per hour, normal horse-power, 0.57 pint.
Normal speed of propeller, engine speed, 1,400 R. P. M.