Diesel engines slowly eclipsed those powered by steam as the manufacturing and operational efficiencies of the former made them cheaper to own and operate. While initial costs of diesel engines were high, steam locomotives were custom made for specific railway routes and lines, and as such economies of scale were difficult to achieve.
[11] Though more complex to produce with exacting manufacturing tolerances (1/10,000th of an inch (0.0025 mm) vs. 1/100th of an inch (0.25 mm) for steam), diesel locomotive parts were more conducive to mass production. As such, while the steam engine manufacturer
Baldwin offered almost five hundred steam models in its heyday,
EMD offered fewer than ten diesel varieties.
[12]
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews), and the operating environment is much more attractive, being much quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel engines can be started and stopped almost instantly, meaning that a diesel locomotive has the potential to incur no costs when not being used. Steam locomotives require intensive maintenance, lubrication and cleaning before, during and after use. Preparing a steam locomotive for use can take many hours, especially if the locomotive is being fired from cold. However it is still the practice of large North American railroads to use straight water as a coolant in diesel engines instead of coolants that incorporate anti-freezing properties. This results in diesel locomotives being left idling when parked in cold climates instead of being completely shut down. Still, a diesel engine can be left idling unattended for hours or even days, especially since practically every diesel engine used in locomotives has systems that automatically shut the engine down if a problem such a loss of oil pressure or coolant loss occur. A steam locomotive, by comparison, may be kept in readiness between uses with a small fire to maintain a slight heat in the
boiler, but requires regular and frequent attention to maintain the fire and the level of water in the boiler.
Moreover, maintenance and operational costs of steam locomotives were much higher than diesel counterparts even though it would take diesel locomotives almost 50 years to reach the same horsepower output that steam locomotives could achieve at their technological height.
[13] Annual maintenance costs for steam locomotives accounted for 25% of the initial purchase price. Spare parts were machined from wooden masters for specific locomotives. The sheer amount of unique steam locomotives meant that there was no feasible way for spare part inventories to be maintained.
[14] Steam engines also required large quantities of coal and water, which were expensive variable operating costs.
[15] Further, the
thermal efficiency of steam was considerably less than that of Diesel engines. DieselÂ’s theoretical studies demonstrated potential thermal efficiencies for a compression ignition engine of 73% (compared with 6-10% for steam), and an 1897 one-cylinder prototype operated at a remarkable 26% efficiency.
[16] By the middle of the twentieth century, Diesel locomotives had effectively replaced
steam engines.
[15]
