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, Tue, 12 Jul 2005 05:47:11
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The two stroke makes every stroke a power stroke. On EMD two stroke diesel engines, there are four valves in the head, all exhaust, with the injector located in the center. As the piston goes down to the bottom of the stroke, inlet ports in the cylinder wall are exposed. Air is forced into those ports and exhaust gas is flushed out from bottom to top out the exhaust ports. Note that this requires that the intake 'boost' pressure be higher than a turbo charger exhaust turbine back pressure. These engines almost always have a positive displacement blower to move the air. With a turbo charger only, the engine probably could not be started up. These 'GM' engines are also found in trucks, buses, marine, generators etc. These engines with the positive displacement blowers can be 'enclosed' in a turbo charger cycle as well. This is what was done for the LAV's that GM built for the USMC. A twist on this with locomotive engines was to have the turbo charger gear driven to allow for startup. Then as the engine got on power, the over-run clutch allowed the turbine to then spin up faster than the gear train. This worked well. EMD engines were evolving when I left and they were introducing a 4 stroke engine. It had a typical turbocharger configuration, except that the turbocharger was about the size of a VW beatle.
With a two stoke, every stroke is a power stroke, and the engines can be fueled/tune at a lower rate than a 4 stroke and then it works well at lower stress levels than a 4 stoke that needs higher CR's to make the same power. One seeks efficiencies, but operating life is also a very important issue.
With these heavy diesel engines, the fuel is very thick and needs to be heated to be fluid enough for pumping and injectors. The slow PRM's allow for the fuel to be injected over a relatively long period of time. The thick oil still does not spray as well as a modern automotive diesel at 30,000 PSI. By emulsifying a very small amount of water into the fuel, the entrained water in the spray droplets explodes during combustion, just like water in a hot oily frier. The explosions break up the oil and promote better combusion and specfic fuel consumption is reduced. Of course, the fuel injectors need to be designed to cope with this. Automotive injectors would be quickly damaged.
The engine is designed by a company in Finland with plants in different parts of the world. It makes sense to build these engines in near shipyards.
, Wed, 13 Jul 2005 04:20:36, Wed, 13 Jul 2005 13:15:07
, Tue, 12 Jul 2005 10:02:18, Tue, 12 Jul 2005 05:47:11
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