- New Life for a 1960-era Turbocharged Diesel -
More Power AND Reduced Emissions
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Although it seems unusual, there exists in 2020 a large number of railroad locomotives, tugboats, stationary powerplants and other powerplant applications which use a turbocharged 2-stroke diesel engine, (the GM Electromotive Division EMD-645), the design of which is well over 50 years old.
The 645 was first introduced on a testbed in 1964. That year, Electro-Motive manufactured a demonstrator in what became its series of turbocharged, 645 series railroad switching locomotives. This locomotive used the 16-cylinder 645 engine which produced hefty 3,000 horsepower.
The 645 engine series was in production from 1965 through the late 1990s, but has since been discontinued.
Figure 1 below shows a 12-cylinder version installed in a locomotive.
Figure 1: A 12-Cylinder 645 in a Locomotive
The EMD-645 is a 45-degree V-block engine that has a cylinder bore of 9-1/16 inches and a stroke of 10 inches. Two-stroke diesels need forced induction to operate, and the 645 was built in 8, 12, and 16 cylinder roots-blower versions, and in 8, 12, 16, and 20 cylinder turbocharged versions. The turbocharged engines, in stock form, produce 1525, 2305, 3070 and 3600 HP respectively, at 900 RPM.
Think about that:-- 3600 HP at 900 RPM requires 21008 lb-ft of torque!
This engine is a "uniflow" design two-stroke engine, meaning that the intake air is fed into the cylinders under pressure from piston-ported passages in the bottom of the cylinder, and burned exhaust gas flows out through the poppet valves in the cylinder head.
At roughly 120 degrees past TDC, the four exhaust valves in each cylinder head open and allow most of the burned exhaust to leave the cylinder. Near the bottom of the stroke, the piston uncovers the intake ports and air rushes into the cylinders from the pressurized plenum in the block and continues to purge burned exhaust gas out the top. As the piston starts upward, it closes off the intake ports and the big compression event begins.
The EPA has issued increasingly-restrictive emissions requirements for all such industrial diesels. Many of the newer engines use sophisticated computer controls which provided a huge advavtage in terms of emissions compliance.
The GM-645 is completely mechanical - it uses a cam-driven fuel injector for each cylinder.
Figure 2 below shows the valve gear for a single cylinder, with two valve-operating rocker arms (one for each pair of valves, with a bridge) and the center rocker arm operating the fuel injection piston.
Figure 2: Valve and Injection Components for One Cylinder
Although the engine is no longer in production, it is still widely used, and aftermarket replacement parts are readily available. Hatch & Kirk, one of the major producers of replacement parts for this engine, contracted with EPI, Inc. to develop a new cylinder head and other components to help bring the GM-645 into emissions compliance.
First, we developed a new cylinder head which exactly replaces the original design, but which flows 22% more air at the same depression and valve lifts as the original. H&K generated a new casting for our design and put it into production.
Figure 3 below shows the development head on the EPI flowbench, with the new porting system completely formed in clay.
Figure 3: New Cylinder Head in Development
In addition, EPI made modifications to the inlet ports in the cylinder which increased the flow approximately 11% and added a significant amount of swirl to the incoming air charge.
Figure 4 below shows the development cylinder on the EPI flowbench. This cylinder, together with the conical flow adapter at the bottom, was over 28 inches tall, and just barely fit onto the flowbench.
Figure 4: Cylinder Liner in Development
The third improvement that EPI made for this engine was to develop an injection-cam lobe profile which takes advantage of the well-known pilot-injection technology. This lobe injects a small amount of fuel at the appropriate time (the "pilot injection") which rapidly initiates combustion in the cylinder by eliminating the long delay required for a large amount of fuel to absorb enough heat from the compressed air to ignite. Approximately 5 - 10 milliseonds later, the main injection begins, and because the pilot injection has already initiated combustion, and because of the large swirl vortex introduced during intake, the incoming fuel burns much more raipdly and more completely.
Figure 5 below shows a rocker arm, exhaust valve, and a camshaft for one cylinder compared to a smallblock Chevy camshaft and valve.
Figure 5: Valve Gear Size Comparison
In addition to the EPI improvements, the chief engineer at H&K devised a specially-shaped projection ("the tower") that is located in the center of the piston bowl. After the engine is running under load, the "tower" becomes red-hot. As incoming fuel impinges on the hot "tower", the fuel absorbs heat very rapidly so it is ready to combust as soon as it encounters free oxygen molecules. The "tower" also accomplishes a wider dispersion of the incoming fuel jet, so the incoming fuel molecules are more likely to encounter unbonded oxygen molecules.
The combined results of these changes were spectacular. The target emissions requirements were substantially exceeded, AND the engine power output increased over 25%.
