The Fairbanks Morse 38D8⅛
Two stroke oppose piston diesel engine
Authored by: Martin Leduc, August 2000
Brought to you by www.dieselduck.net, comments to email@example.com
While working for the Canadian Coast Guard, I had the opportunity to work on the CCGS Tanu. Tanu is 170' fishing patrol vessel. It was built in Victoria, BC, in 1968 by the Yarrows Shipyard. The design was based on a corvette hull, which was a popular warship design back in the second world war.
The main engines of the Tanu are Fairbanks Morse engines built in Dorval, Quebec. I had heard quite a deal about them, because they are of an interesting design for a diesel engine. The Tanu, along with it's sister ship on the east coast, have two such engines.
I understand that these engines are somewhat common in industry. It's welded design make this engine easy to build and repair. At the time of it's introduction, the size to power ratio was quite impressive. With the slim, compact design it is easy to see why they are found in the railroad industry and early diesel submarines. The power generation industry is also a common user of the opposed piston engine.
The Fairbanks Morse 38D8 1/8 on the Tanu is an inline eight cylinder engine although they are available in 4, 5, 6, 8, 9 10, 12 cylinder inline configuration. The bore measures 8 and 1/8 inches and the stroke is 10 inches. The total displacement of 1037 cubic inches per cylinder produces 170 bhp at 720 rpm. It is a two stroke, opposed piston engine. This means that there are two crankshafts in the engine, one at the bottom end, and one at the top end. The power developed in the cylinder is transferred to two pistons. One is connected to the top crankshaft and the other to the bottom crankshaft. The top and bottom crankshafts are connected through what is called a "vertical drive" situated at the driving end of the engine.
Valve covers, aka cylinder heads, do not exist with this engine. As with all two stroke engines, intake ports are cast into the cylinder liner. Additionally the cylinder liner also has exhaust ports. The exhaust and intakes ports are open and closed by the action of the two pistons. The intake ports are at the top while the exhaust ports are located at the bottom of the liner.
As with all two stroke engines, these Fairbanks Morse cannot function without the help of a supercharger. On these engines the positive charge of air is provided by a single roots type blower. The blower is mounted on the driving end and driven by the top crankshaft. The air is distributed to the cylinders by way of an air intake manifold situated along the intake ports near the top end of the engine. The exhaust is collected by an exhaust manifold near the bottom end, with the uptake taken from the front of the engine.
Once the exhaust ports are exposed, the gas expansion, from combustion, ceases. The intake ports then become exposed allowing the positive charge of air to enter the cylinder. This chases, in a "uniflow scavenging" fashion, the exhaust gases out and prepares a fresh charge of air ready for compression. The upper pistons reaches "bottom dead center" 12 degrees after the lower piston, as they head on a collision course, compressing the fresh air in the cylinder.
At 9 degrees before top dead center, the diesel fuel oil is introduce through two conical type hydraulic injectors. The fuel is pressurize to 2000 - 2100 psi by a jerk type fuel pump. It is mounted upside down and driven by a camshaft on the port side of the engine. A Woodward governor controls the rack settings.
The engine has a capacity of about 725 liters of lubricating oil. The oil is a standard "cd" or better, two stroke motor oil. It is filtered thought a housing containing 12 individual filters. The oil is then passed through a oil cooler before being sent back to the engine. The oil pressure for this engine is around 23 psi at full rpm (720). Additionally the oil is treated by an Alfa-Laval oil purifier.
The engine is cooled by a closed system with a capacity of about 300 liters. The heat from that system is transferred to an open sea water system through a heat exchanger. Like most diesel engines the temperature is about about 78 - 82 Celsius in the engine.
The engine is started by introducing compressed air at 1400 kPa (about 200 psi) into each cylinder. The compressed air is introduced to the cylinder by way of a pilot valve controlled by a distributor driven by the lower crankshaft on the forward starboard side of the engine.
The power of the two engines is transferred to a common gear box through Vulcan Sinclair fluid couplers. These are of a British design, used extensively during the second world war. It behaves much like the torque converter in your car's automatic transmission. The gear is roughly a 3:1 ratio, which turns the controllable pitch propeller at about 250 rpm.
Coltec Industries in the United States have taken over the Fairbanks Morse engine line, as well as the Alco line. They can be found on line through Martin's Link Page.
I found theses pictures since I first wrote this piece, and now also have enough room on my web servers to put them up. Enjoy. Martin May 2007.
This picture illustrate the difference in size between the 8 cyl Fairbanks and the engine block of a 6 cylinder Caterpillar 3306, a similar size engine as found on large highway tractor trucks.
This diagram gives us a cross section (for-aft) of the Fairbanks. The heart of the operation is the cast iron liner, the rest of the engine is welded. Also note that there is two injection nozzle per cylinder.
This diagrams shows piston position during the one cycle. Notice the crank lead between the top and lower crankshaft.
These diagrams of the roots type blowers found on the aft end of the engine, driven by the top crankshaft. Air inlet is at the top.
Rocky Adams sends us his comments and input on this article in 2007
" These engines were originally designed so that you could remove the upper pistons out through the top just as you did with the lowers through the bottom. However, the US Navy, while negotiating a large contract with F-M told the manufacturer that the engine was four inches too tall to be serviceable in submarines. Fairbanks then redesigned the engine taking four inches out of the block height above the intake manifolds and shortening the upper connecting rods by four inches.
The difference between the upper and lower crankshaft placements are obvious in the cutaway drawing you provided. This allowed the engines to be installed in USN's submarines. It also made removal of the upper piston much more difficult because now the crankshaft was too close to the top of the cylinders to give enough space. I'm sure you remember that to remove an upper piston it must be separated from the 'dummy piston' inside it. the upper con rod and the 'dummy' are then removed through the top of the engine but the piston itself must be lowered down through the cylinder, after the lower piston and rod assembly are removed, and taken out the bottom."
John sends us his comments and pictures, in 2010
" When I was in the Navy back in the 60‘s our Destroyer Escort had four of these engines and they were the 10 cylinder models.
When they were rebuilding one of the engines out at sea I noticed they were throwing some of the old pistons overboard. I talked them into giving me one. After cleaning it for days with emery cloth, I had it chromed and the ships name and all the guys in my division engraved on it.
Thought you might get a kick out of a few pictures of it. It is eleven and a half inches high, eight inches in diameter and weighs thirty five pounds. "
Tracey in San Francisco, in May 2011, sent us this link to US submarine diesel engine manual. From it, are the neat diagrams of Fairbansk Morse engines, below.