The process of reducing constraints to steam flow is usually referred to as “Internal Streamlining”. Internal streamlining produces two benefits – it raises the Front-End Limit resulting in increased steam flow through the cylinders which in turn increases the upper limit of power output for the engine. It also has the effect of reducing flow resistance and thus energy losses at any given steam flow, thereby increasing efficiency, reducing steam and fuel consumption etc.
Streamlining may involve any or all of the following:
- Increasing steam pipe diameter to reduce steam velocity and consequent pressure drops;
- Removing or reduce sharp bends to lower pressure drops;
- Increase steamchest volume to reduce steamchest pressure drop at high steam flow rates. Ideally the steamchest volume should equal the volume of the cylinder that it connects to;
- Enlarge port openings as much as possible without excessive increase in clearance volume;
- Rounding the edges of bridge bars and port edges and tapering the edges of valve lands (see diagrams below), can drawmatically reduce resistance to steam flow into and out of cylinders and minimize “wire-drawing” near the point of valve closure;
- Increase valve travel increases the velocity at which the valve travels over the ports at any given cut-off, thereby reducing the period during which the valve throttles the steam flow as it closes over (or opens over) the port, thereby “sharpening” the valve events.
- Enlarging the exhaust passages including blastpipe area frees up the flow of expanded (i.e. large volume) exhaust steam out of the cylinder, thereby reducing back-pressure and the quantity of steam retained in the cylinder at exhaust port closure, and thereby allowing more steam to flow through the cylinder.
Streamlining reduces pressure drops wherever it is applied, and in consequence it reduces energy losses. Streamlining therefore has the effect of increasing engine (and locomotive) efficiency, thus reducing fuel consumption for a given energy output, and increasing drawbar work produced by a given energy input.
It should be noted that “streamlining” the steam passages of a steam locomotive involves a lot more than “polishing the ports” of a car engine!
Porta illustrated his ideas for streamlining of valve land and port edges in a proposal that he put forward for improving the design of the A1 Tornado as below. He also calculated the reduction in pressure drop that could be achieved by such simple modifications:
In the case of the upper diagram, Porta calculated that the pressure drop through the improved design would be just 17% of the original, which would have the same effect as increasing the valve diameter by 2.4 times – i.e. 24” instead of 10”.
For the larger valve/port opening in the second diagram, Porta calculated the pressure drop through the improved design would be 31% as compared to the original, which would have the equivalent effect of enlarging the valve to 21.5″ diameter in place of 10″.
Edge Lands and Drifting Steam
It may be noted in the above diagrams that Porta illustrates a very thin edge land on the admission end of the valve head, and a rather thicker one on the exhaust end. The reason for this is that the land has to be thick enough to be able to resist the inertial and frictional forces that derive from the motion of the valve and which may be transmitted to the land by the ring that it retains.
At the admission end, the steam pressure inside the steam chest should be high enough to prevent the ring from contacting the land and imparting its own inertial and friction-derived forces onto the land. Hence the land should carry very little load. However steam pressure is very much lower at the exhaust end of the piston head with the result that the edge land has to be thick enough to withstand the ring loads that are applied to it.
It may be deduced therefore that valves with narrow admission edge lands rely on the presence of sufficient steam-chest pressure to keep the edge ring from coming into contact with the land. Most importantly, where such valves are used, it is essential that sufficient steam pressure is maintained inside the steamchest when the locomotive is drifting. This minimum required pressure is not likely to be high: for instance, in the case of the 5AT when drifing at 200 km/h, the steamchest pressure required to keep the admission rings from contacting the edge lands is estimated to be only 274 kPa. Nevertheless, failure to maintain steam pressure in the steamcest during drifting, may result in damage to, or even failure of, the land at the admission ends of the valve heads.