There have been many and varied practices adopted by drivers and/or imposed by railway authorities, for "drifting" of locomotives - i.e. running at speed without power such as when descending a bank. On page 101 of his book "The Red Devil and Other Tales from the Age of Steam", Wardale emphasise the complexity of the problem by listing no less than 13 criteria to be satisfied by any applied method of drifting - viz:
- Minimum (ideally zero) cylinder steam consumption;
- Minimum (ideally zero) cylinder power;
- No net negative cylinder work;
- Cylinder pressure to remain above smokebox pressure to avoid sucking smokebox gases into the cylinders;
- Adequate cushioning of the reciprocating masses to be provided by compression at the end of the stroke;
- Require no snifting or by-pass valves of any type, since the former result in cooling of the cylinders and spoilt lubrication through oxidation of the oil, and both were prone to leakage;
- Require no separate drifing valve;
- Give smooth riding of the locomotive without damage to any of its parts and without longitudinal vibrations that could be transmitted to the trailing load;
- Require no special attention on the part of the driver, i.e. be such that the technique could be expected to be correctly implemented in normal service;
- Where piston valves with thin admission edge lands were fitted, maintain adequate steamchest pressure (approximately 400 kPa) at high speeds to prevent the admission edge valve rings from touching these lands;
- Must not cool the cylinders;
- Give no excessive high cylinder temperature, thus maintaining good lubrication conditions;
- Give minimum oil consumption in the case of locomotives fitted with variable stroke drive for the mechanical lubricator.
In order to determine the optimum drifting method for his Class 19D conversion, Wardale took sample indicator diagrams when running using three methods of drifting:
- valves in mid-gear; throttle shut;
- valves in full-gear; throttle cracked open.
- valves in mid-gear; throttle cracked open.
In case 1, he detected a momentary vacuum in the cylinder; in case 2, the steam flow became far too high; case 3 was found to be satisfactory, and was therefore recommended as the operating regime to be adopted for both his 19D and Class 26 locomotives.
In his reply to a question about the high piston speed of the 5AT, Wardale included a supplementary observation that "Drifting has to be with a limited amount of drifting steam which carries away friction-generated heat which would otherwise build up during high-speed drifting".
See also "Snifting, Drifting and By-Pass Valves".