According to this Bosch page, their glow plug was developed in the early 1920's.
Yet the diesel engine was around for at least 20 years prior to that; the French used it in a submarine in 1904, for instance.
Given that diesel technology relies on auto-ignition, how did they get the engine up to temperature without a glow plug?
Glow plug is merely an aid for starting the engine. There are many designs still produced today without any glow plugs (eg military diesels run without any electricity), and even many modern diesel engines can be started if glow plugs fail (unless onboard computer prevents that).
Diesel's auto-ignition comes from heat generated by adiabatic compression of air just before injection. Glow plugs just add a little heat so you need less cranking before compression heats the engine up into operating temperature.
When I was little, my dad had a yacht which had been a northern European coastguard patrol vessel. He bought it as surplus military and converted it into a yacht. The engine, a Stork Werkspoor diesel, well that was quite a story. It was massive, almost 6' tall, with a gigantic flywheel. Each cylinder had a shelf in the casting, on which you placed a blow torch. They had to be lit about half an hour before you wanted to start the engine. When the individual cylinder heads were red hot my dad stood on a handle and pushed himself down from the roof, taking real good care the flywheel was positioned in just the right place, or it would kick back (I tried, it almost launched me through the roof). That engine drove a massive 5 blade prop with variable pitch, so no gearbox, you adjusted speed just like an aircraft, by changing the pitch of the prop.
I've not seen it done personally but I've been told that some old (WWII) military engines would be warmed up prior to starting in any number of ways which anecdotal included the use of lit marine flares.
Apparently some Russian tanks with Diesel engines would be started by cranking them with Petrol engines or even warming them over a fire.
To expand on the answer above, many old vintage diesels ran a 'hot bulb' design. There would be a section of the head where an open flame would be used to heat it, so the engine could ignite the lower grade fuel oils/diesels without the need for extremely high compression.
One method that is still used - but only on vehicles with metal air filter canisters is to remove the filter and put burning paper there - the hot air makes all the difference. Once running put the filter back in...
Another method is to heat the air inlet pipe with a blowtorch again : metal not plastic...
And yes I have seen both : heavy goods, agricultural and forestry are likely candidates...
I don't know what was used before glow plugs. But you can use a hair dryer to blow warm air into the air inlet of a diesel motor and it will make it start easily in cold weather. I've done this many times and it works like a charm.
If the compression ratio is high enough (around 22-1), there will be enough heat from the compression. Most diesel engines are turbo charged to increase power. The compression ratio is lower because the air is partly compressed by the turbine, but not while starting, so the glow plugs provide the extra heat needed.
A long, long time ago. I heard that a wick was lit and inserted into the combustion chamber. There must have been an access plug. The glowing wick provided a hot ember to ignite the air/fuel mixture for so many cycles before it was consumed and blown out the exhaust. An old Greek named Donias whose grandfather was fisherman told me this story as a memory from his youth.
I have a Volvo Penta md7a in my sailboat that starts right up with no need for glow plugs or preheating. I can even hand crank it should the batteries die by using a decompression lever that hold the valves open so you can get the substantial flywheel up to starting speed. Air compression and a squirt of fuel is all that’s needed.
