Fairbanks-Morse 31A18

Naturally, as things progress – engines got bigger.  For Fairbanks-Morse, the 31A18/31AD18 was the largest production engine they made…at least until the 1960’s…but we will get to that later.    Fairbanks-Morse introduced their line of 31A engines around 1945 or so, calling them the new “En-bloc” engines, meaning “as a whole” in the dictionary.   While previous FM engines were made of various castings, bases, liners, air box, exhaust belt, etc., the new 31A line used a one-piece cast block, in which the cylinder liners went into (not on top of like earlier models). The cam and main bearing pockets were cast right into the block, ensuring perfect alignment every time (or so they touted), as well as featuring an integral oscillating scavenging blower.   The 31A series was offered in a 6 ¼” and an 8 ½” bore for marine and stationary service, as well as a giant 18” bore engine for stationary power generating service, and thus the 31A18 was born.

Fairbanks Morse 10 Cylinder 31A18

The 31A18 used an 18” bore, with a 27” stroke, rated at 277 RPM.   FM also offered the 31A18 in a dual fuel model, the 31AD18 – which used diesel as a pilot fuel for natural gas operation.   The engines were offered in the following configurations:

– 6 Cylinder, 2100HP, 140,000lb/143,000lb (31AD18)
– 8 Cylinder, 2800HP, 193,000lb/197,000lb (31AD18)
– 10 Cylinder, 3500HP, 247,000lb/251,000lb (31AD18)

End cutaway view of the 31AD18

One of the few options offered was in way of the scavenging blower – The engine could be equipped with an oscillating vane blower, or none at all, with the scavenging air being supplied by a separate motor driven blower in the plant.   It appears most applications went with the separate motor driven unit.     Lubricating oil was supplied by a duplex system with an engine driven gear pump handling piston cooling, main bearings, crankpins, camshaft, injection pumps and blower (if equipped).  A separate pair of Madison-Kipp lubricators driven off the main camshaft supplied oil to each cylinder liner wall by means of 6 lines going to each. The camshaft also chain drives the fuel oil service pump.  Jacket water cooling is handled off a main header (remember, there is no internal water passages in the cast block) on the lower potion of the block, up through the liners and heads, and out through an upper header.   The engine has no water pumps of its own (common in large stationary engines), with separate motor driven pumps for the closed loop soft water in the engine, and a raw water system for the heat exchangers.  

Main casting of the 10 Cylinder model. Note the casting line between the first 4 Cylinders and the last 6. The blocks were cast as 4 Cylinder blower/non blower sections and 6 Cylinder sections.
The 31A18 used a rather unique piston. While it was not a floating piston, it did in essence use a carrier, however it was bolted in place through the skirt. The crown was a separate piece with studs, which held it all together.

The 31A/31AD18 were essentially the same engine except for the dual fuel equipment.   In addition to the standard equipment, the 31AD18 used an additional chain driven upper camshaft to control the gas admission valves, supplied by a separate header on the top end of the engine. 

The rather unorthodox method of driving the upper camshaft and fuel pump.

Around 1955, Fairbanks Morse added to the 31A18 line, by introducing a 12-cylinder option, putting out 4200HP.   Production of the 31A18 lasted until the late 1960’s – well past the production of all of the other FM engines that were not model opposed Piston models.  While FM does not support the 31A18 line anymore, there are still numerous examples of these engines still in service in various municipal power plants.   While the 31A18 family was the largest production Fairbanks Morse engine, it was not the largest. That honor would go to the 38A20 Opposed Piston engine.

A pair of 38AD18 engines in an unknown power plant. For the life of me I can not find the source of this photo. If anyone recognizes it please let me know so I can properly credit it.

Midway Island, part of the Midway Atoll in the Pacific is home to a small power plant with a single FM 38AD18. Check out this link for photos of the Midway power plant. Note that this engine has the attached engine driven blower.

Midway Island Power Plant Photos

FM 31AD18 running at the Rochelle Illinois power plant. Note the large grey pipe coming up from the floor in the front of the engine, this is the air intake, as this engine does not have the attached blower. This plant is home to many old engines including FM’s, Worthingtons and Nordbergs. Video from youtube user dzlrod

As always, I welcome and and all comments, additions, corrections and anything else.