Two-stroke diesel engine
A two-stroke diesel engine is a diesel engine that works in two strokes. A diesel engine is an internal combustion engine that operates using the Diesel cycle. Invented in 1892 by German engineer Rudolf Diesel, it was based on the hot-bulb engine design and patented on February 23, 1893. During the period of 1900 to 1930, four-stroke diesel engines enjoyed a relative dominance in practical diesel applications. Charles F. Kettering and colleagues, working at the various incarnations of Electro-Motive and at the General Motors Research Corporation during the 1930s, advanced the art and science of two-stroke diesel technology to yield engines with much higher power-to-weight ratios than the two-stroke diesels of old. This work was instrumental in bringing about the dieselisation of railroads in the 1940s and 1950s.
All diesel engines use compression ignition, a process by which fuel is injected after the air is compressed in the combustion chamber, thereby causing the fuel to self-ignite. By contrast, gasoline engines utilize the Otto cycle, or, more recently, the Atkinson cycle, in which fuel and air are mixed before entering the combustion chamber and then ignited by a spark plug.
Two-stroke internal combustion engines are simpler mechanically than four-stroke engines, but more complex in thermodynamic and aerodynamic processes, according to SAE definitions. In a two-stroke engine, the four "cycles" of internal combustion engine theory (intake, compression, ignition, exhaust) occur in one revolution, 360 mechanical degrees, whereas in a four-stroke engine these occur in two complete revolutions, 720 mechanical degrees. In a two-stroke engine, more than one function occurs at any given time during the engine's operation.
- Intake begins when the piston is near the bottom dead center. Air is admitted to the cylinder through ports in the cylinder wall (there are no intake valves). All two-stroke Diesel engines require artificial aspiration to operate, and will either use a mechanically driven blower or a turbo-compressor to charge the cylinder with air. In the early phase of intake, the air charge is also used to force out any remaining combustion gases from the preceding power stroke, a process referred to as scavenging.
- As the piston rises, the intake charge of air is compressed. Near top dead center, fuel is injected, resulting in combustion due to the charge's extremely high pressure and heat created by compression, which drives the piston downward. As the piston moves downward in the cylinder, it will reach a point where the exhaust port is opened to expel the high-pressure combustion gasses. However, most current two-stroke diesel engines use top-mounted poppet valves and uniflow scavenging. Continued downward movement of the piston will expose the air intake ports in the cylinder wall, and the cycle will start again.
In most EMD and GM (i.e. Detroit Diesel) two-stroke engines, very few parameters are adjustable and all the remaining ones are fixed by the mechanical design of the engines. The scavenging ports are open from 45 degrees before BDC, to 45 degrees after BDC (this parameter is necessarily symmetrical about BDC in piston-ported engines). The remaining, adjustable, parameters have to do with exhaust valve and injection timing (these two parameters are not necessarily symmetrical about TDC or, for that matter, BDC), they are established to maximize combustion gas exhaust and to maximize charge air intake. A single camshaft operates the poppet-type exhaust valves and the Unit injector, using three lobes: two lobes for exhaust valves (either two valves on the smallest engines or four valves on the largest, and a third lobe for the unit injector).
- The power stroke begins at TDC ([0°]; injection of fuel leads TDC by 4° [356°], such that injection of fuel will be completed by TDC or very shortly thereafter; the fuel ignites as fast as it is injected), after the power stroke the exhaust valves are opened, thereby greatly reducing combustion gas pressure and temperature, and preparing the cylinder for scavenging, for a power stroke duration of 103°.
- Scavenging begins 32° later, at BDC–45° [135°], and ends at BDC+45° [225°], for a scavenging duration of 90 degrees; the 32° delay in opening the scavenging ports (constraining the length of the power stroke), and the 16° delay after the scavenging ports are closed (thereby initiating the compression stroke), maximizes scavenging effectiveness, thereby maximizing engine power output, while minimizing engine fuel consumption.
- Towards the end of scavenging, all products of combustion have been forced out of the cylinder, and only "charge air" remains (scavenging may be accomplished by Roots blowers, for charge air induction at slightly above ambient, or EMD's proprietary turbo-compressor, which acts as a blower during start-up and as a turbocharger under normal operational conditions, and for charge air induction at significantly above ambient, and which turbocharging provides a 50-percent maximum rated power increase over Roots-blown engines of the same displacement).
- The compression stroke begins 16° later, at BDC+61° [241°], for a compression stroke duration of 119°.
- In EFI-equipped engines, the electronically-controlled unit injector is still actuated mechanically; the amount of fuel fed into the plunger-type injector pump is under the control of the engine control unit (in locomotives, locomotive control unit), rather than the traditional Woodward PGE governor, or equivalent engine governor, as with conventional unit injectors.
Specific to GM two-stroke (6-71) and related on-road/off-road/marine two-stroke engines:
- The same basic considerations are employed (the GM/EMD 567 and the GM/Detroit Diesel 6-71 engines were designed and developed at the same time, and by the same team of engineers and engineering managers).
- Whereas some EMD and Detroit Diesel engines employ turbocharging, only such EMD engines employ a turbo-compressor system; such Detroit Diesel engines employ a conventional turbocharger, in some cases with intercooling, followed by the usual Roots blower, as a turbo-compressor system would be too costly for these very cost-sensitive and highly competitive applications.
- Burmeister & Wain (part of MAN Diesel since 1980), double-acting diesels for marine propulsion from 1930 onwards, also made by shipbuilders under licence
- Detroit Diesel, uniflow engines for on- and off-road trucks, on-road buses and stationary applications
- Doxford, opposed piston slow speed marine diesel engines.
- Electro-Motive Diesel, uniflow diesel engines for marine, railway and stationary applications
- Fairbanks-Morse, opposed-piston diesel engines for marine and stationary applications.
- Foden, FD series of diesel engines for commercial vehicle, marine and industrial power.
- Gray Marine, uniflow diesel engines for marine applications
- MAN Diesel & Turbo, crosshead diesel engines for marine propulsion
- Mitsubishi Heavy Industries, crosshead diesel engines for marine propulsion
- Napier & Son, Napier Deltic and Napier Culverin opposed-piston valveless, supercharged uniflow scavenged, two-stroke Diesel engines.
- Rootes Group, the Commer TS3 engine for trucks
- Wärtsilä, crosshead diesel engines for marine propulsion
- Sloan, Alfred P. (1964), McDonald, John, ed., My Years with General Motors, Garden City, NY, USA: Doubleday, LCCN 64011306, OCLC 802024. Republished in 1990 with a new introduction by Peter Drucker (ISBN 978-0385042352).
- Walshaw, T.D. (1953), Diesel engine design (2nd ed.), London, England: George Newnes Ltd, LCCN 54029678.
- Horsepower for naturally aspirated engines (including Roots-blown two-stroke engines) is usually derated 2.5% per 1,000 feet (300 m) above mean sea level, a tremendous penalty at the 10,000 feet (3,000 m) or greater elevations, which several Western U.S. and Canada railroads operate, and this can amount to a 25% power loss. Turbocharging effectively eliminates this derating