In the case of coal-fired locomotives, there exists what is called the “grate limit” which is defined as the point where any increase in the rate of fuel supply will produce no increase in power output. This occurs when all coal that is added to the firebox gets drawn straight through the boiler tubes by the combustion airstream and blown out through the chimney. In a well designed locomotive (and in most, if not all, oil-fired locomotives) the point at which the cylinders are unable to handle any additional steam that the boiler might be capable of supplying (the "front-end limit”) will be reached long before the grate limit is reached. Whereas all coal-fired locomotives will emit some amount of unburned (i.e. wasted) fuel at medium to high power outputs, a well designed oil-burner should not.
In his Fundamental Design Calculation FDC 1.3, Wardale presents a step-by-step calculation of fuel consumption of 1400 kg of gas-oil (diesel) per hour for the 5AT when operating at maximum drawbar power. Under “average service conditions” he conservatively estimates that the fuel consumption will be around 60% of this value.
Wardale also estimates the 5AT’s fuel consumption when burning coal. Basing his calculation on burning coal with a calorific value of 30 MJ/kg (7165 kcal/kg), he derives a coal consumption rate of 2.27 tonne/h after allowing for carry-over losses based on the use of a Gas Producer Combustion System (GPCS) firebox with 30% of combustion air entering through the grate.
Whilst not estimating the locomotive’s performance with coal firing, he notes that there would be some loss in power output because a proportion of the exhaust steam must be diverted through the firegrate (as part of the GPCS) reducing the smokebox vacuum, and because of the need to incorporate a spark arrestor which will inhibit combustion airflow.