A Presto log (originally Pres-to-Log) was an artificial fuel for wood-burning stoves developed in the 1930s as a means of recycling the sawdust from sawmills. They were made of clean, dry sawdust, wood shavings and green waste that was formed into logs by machines under great pressure without any binders or glues. Other brands of artificial logs used paraffin or natural binders.
Doc Brown developed his own presto logs sometime in 1885 from pressed wood and pulverized anthracite (a hard and dense variety of coal), which he treated with chemicals known only to himself. The logs were color-coded green, yellow and red in increasing order of the amount of heat each produced. Doc used them secretly in the furnace of his blacksmith's workshop to avoid having to continually feed wood to the fire.
These logs were used to power the steam locomotive that pushed the DeLorean time machine up to 88 mph. Each colored log would react differently, getting the fire hotter, thus raising the steam pressure in the boiler, making the train run faster than it ever could on conventional fuels. With each colored log, the train would emit smoke of that color that mingled colorfully with the steam expelled from the smokestack.
The green log ignited at 500 degrees Fahrenheit and pushed the train up to 35 mph and a firebox temperature of around 1400 degrees. At this point the yellow log fired and increased the speed all the way to 70 mph, as well as causing the temperature to soar to 2000 degrees. At this point the red log explosively ignited, causing superficial damage to the locomotive's smokestack. As the temperature escalated to dangerous levels, the extreme heat blew off the firebox door and produced such a surplus of steam that the boiler began to lose its structural integrity, popping off rivets and threatening to explode. The speed increased to 88 mph just before the train reached the end of the track, causing the DeLorean to enter temporal displacement. This left the locomotive in a wreck of a condition, and it plunged off the end of the incomplete bridge over Shonash Ravine. Just before impact with the ground, the boiler seemed to finally self-destruct. This is quite possible, as the locomotive's fall would have caused all the water in the boiler to surge forward, exposing its firebox crown, which without the cooling effect of the water would have rapidly exceeded its tolerances and imploded, triggering a catastrophic boiler explosion. This suggests that the temperature did indeed briefly reach 2500 degrees Fahrenheit, the point at which the steel/iron firebox of the locomotive would have melted.