Gas producing combustion system

The GPCS is a single-stage gas producer which achieves an improved combustion and steaming rate, reduces the emission of black smoke and overcomes the problem of clinkering. The most serious waste of fuel in a conventional steam locomotive is the loss of unburned coal particles from the fuel bed through the exhaust because of the rapid flow of air through the grate.^[9][10]

The GPCS relies on the gasification of coal on a low temperature firebed so that the gases are then fully burnt above the firebed. To achieve this, the amount of air being drawn up through the firebed is minimised, while the main sources of air required for combustion is through secondary air inlets located in the firebox sides and through the vertically sliding firedoor. The air inlets contain swirl inducers to spread the incoming air inside the firebox.^[1][2][8][9]

With the GPCS, the coal is heated to drive off the volatile components which are then burned in the secondary air admitted above the grate. The result is improved combustion, thereby minimising black smoke, which is evidence of incomplete combustion and unburnt coal particles being ejected through the exhaust. Note the clear exhausts in the picture below of numbers 141 and 155 double-heading on the climb from Bongwana to Nqabeni.^[1][9]

Lempor exhaust

One visible difference between the original and the rebuilt locomotives is the megaphone-shaped Lempor chimney, which contains a cylindrical mixing chamber and a wide angled diffuser. The object of the latter is to convert the kinetic energy of the combustion gas and exhaust steam mixture to a pressure as close to atmospheric as possible.^[1]

Ejector pump efficiency depends on the length-to-diameter ratio of the exhaust chimney. Although less obvious, both Wardale GPCS locomotives, Class 19D no. 2644 and Class 26 no. 3450, have exactly this shape of chimney, but because of height constraints in large locomotives, their chimneys had to be doubled or even tripled to achieve the correct proportions, hence the double exhausts of the two Wardale locomotives. Narrow gauge locomotives like the Class NG G16, on the other hand, have plenty of height available for the exhaust, so a single long chimney was achievable.^[1]

Other visible differences are larger mechanical lubricators and external drive from the valve spindle, a vacuum ejector exhaust muffler just in front of the safety valves, pipes conveying exhaust and ejector steam to the ashpan for mixing with the primary air, the secondary air inlets in the firebox sides with spark arresting plates outside them, and the snifting and bypass valves which were removed and blanked off since the modified locomotive drifted in mid-gear with steam supplied from the drifting valve.^[1]

Performance

In comparative testing, no. 141 achieved a fuel saving of 25% compared to a standard Class NG G16 Garratt, a performance which was easily maintained in regular service. Power outputs of over 700 indicated horsepower was achieved and the locomotive was probably capable of achieving a maximum of 800 indicated horsepower, some 25% more than the Class 91-000 narrow gauge diesel-electric locomotive. The improvement in performance and economy of the locomotive was brought about by the higher grate limit by means of the GPCS and better flow past the improved valves, valve motion and exhaust. Other basic elements of the locomotive remained more or less the same.^[1]

The cost of the work paid off financially within twelve months and led to a proposal to develop a Class NG G17 Garratt. That proposal, however, never became reality since the line's farming produce traffic was gradually lost to road transport on the improving road network, a decline which eventually led to the ACR's demise.^[8][11][12]