| Ulrich wrote:|
Thank you for the answers...and what would the advantages of a 4 stroke be?
Would some of the environmental concerns about two stroke outboards and lawnmower engines also apply to locomotive engines?
erikem posted the advantages of a 4-stroke over a two stroke, but there's also a turbo lag situation that doesn't occur with the two stroke engines by design - that's why GE locomotives take so long to load. If you could make a GE load as fast as an EMD, it will belch thick, black smoke due to turbo lag (where the injectors increase fuel delivery before the air can keep up).
The environmental concerns of two stroke gasoline engines do not apply to two stroke diesel engines. On a gasoline two stroke engine, cylinder scavenging is accomplished by pressurizing the crankcase with the downstroke of the piston, forcing an air/fuel mixture (drawn into the crankcase by the vacuum of the piston on its upstroke) through ports in the bottom of the cylinder. Due to the role of the crankcase in this design, the engine cannot be oiled conventionally and oil must be added to the fuel.
During the scavenging phase, this air/fuel mixture pushes the spent exhaust through an opposing set of ports in the cylinder sidewall. Some of this unspent air/fuel mix will scavenge out of the exhaust port along with the spent gases. Also, the oil gets burned in the cylinder during normal operation and both of these factors contribute to the increased hydrocarbon content of the exhaust on a two stroke gasoline engine.
On a two stroke diesel engine, the fuel is direct injected into the cylinder, so during scavenging only air will mix with the exhaust - no fuel available at this point in the cycle. Also, since the crankcase on a two stroke diesel does not play a role in engine scavenging, the engine can be lubricated in a conventional, pressure type delivery system (with oil in the crankcase as well as in a separate tank).
Since the crankcase does not play a role in scavenging, a two stroke diesel must be fed air in a slight pressure. This means employing either a Roots type blower (the 6-71 Roots blower so lovingly embraced in automotive drag racing owes its existence to a two stroke diesel), or a Roots blower with a turbocharger (as in some Detroit Diesel designs) or a hybrid turbocharger driven by the engine through an overrun clutch (as used on turbo EMD engines). Without this positive displacement of air, the engine cannot scavenge and will not run.
Typically, a two stroke diesel cylinder employs intake ports on the low end of the cylinder (above BDC), usually surrounding the entire cylinder. Exhaust is removed through camshaft-operated poppet valves in the cylinder head (like a 4-stroke engine).
The two stroke engines also, for a given power rating, tend to have less total displacement and lower maximum RPM than the equivalent 4-stroke engine - i.e. the EMD 16-710G3 (two stroke) runs at 900 RPM (Notch 8) vs. a GE GEVO (4-stroke) which runs @ 1050 in Notch 8.