FRUSTRATIONS OF FUEL EFFICIENCY PART ONE: PRELUDE AND FEED WATER HEATERS
The fuel efficiency of a steam engine is the ratio of what you get out in terms of mechanical work – as manifested by tractive-effort and sustained draw-bar pull – over the potential heat energy of the fuel loaded into the firebox. Throughout its long history, the fuel efficiency of the conventional Stephensonian steam locomotive was lamentable, only achieving figures of over 10% in its final years, and this fact played a major role in its demise at the hands of diesel and electric alternatives. The root causes of this inefficiency are multiple and varied, subjects that have been written about in learned papers, books and numerous articles, often involving complex graphs and equations. In this present series I hope to simplify the problems, and then discuss how locomotive engineers around the world have tried to tackle them – with varying degrees of success and many expensive failures.
Referring to the bar chart in Fig. 1, the total length [K] is the heat available from a given amount of fuel, which can be calculated from its calorific value. Hard coals that were once readily available in the UK had typical values of around 12,500 BTU per lb (3,300 kcals per lb), with Welsh steam coal providing the highest. Coke and dry wood had somewhat similar values but occupied much greater space in the tender. Anthracite, abundant in the USA, has a high calorific value but is difficult to light and keep burning.1 Soft brown coals or lignite, commonly found on the continent of Europe, had much lower values, with peat at the bottom of the scale. Fuel oils have the highest value per unit volume of all.
A. The heat needed to raise the temperature of the water in the boiler to its boiling point. It will be larger if that of the feed-water in the tender/tanks is low.
B. The heat needed to convert the water to steam at the required pressure – the ‘latent’ heat. This is a thermodynamic
