My writing time has taken some hits in the last few weeks, but the weather has hugely improved. It got up to 72 here today, so with joyous enthusiasm I took a long walk. As usual, something occurred to me, this time when a badly adjusted dump truck went past and bathed me in fumes. Ahh! Dieselpunk!
The insight followed soon after: If the world went to hell for some reason and I had to build a vehicle, it wouldn't be steam. It would be Diesel. With some study and care, you could render farm animals or even roadkill and make Diesel fuel. Diesel engine technology requires machining and some skill, but not exotic materials nor computer models. (The same is true of gasoline engines, but gasoline is harder to make than Diesel.) You can do it in a garage. Clean rooms not required.
I had some experience in thinking about recovering technology after a societal crash while creating the Drumlins world. The glitch there is that all the inadvertent colonists' knowledge was in computers, but they didn't have the critical mass of technology to make more computers, nor even fix the ones that broke. (Quick! We need ten pounds of indium! Jimmy, Sam! Go dig around and see what you can find!) So when the computers died, their technology died too, and they were back to a medieval style of life that might have stayed medieval except for the Thingmakers that shared the planet with them. Advanced technologies build on simpler technologies, which in turn depend on simpler technologies still. It made me wonder if there were a sort of minimum technology level, one that, with common sense, an oral tradition, and few old books, might be constructed more or less from scratch.
Speculation: Steampunk might be a consequence of ignorance (i.e., we don't know enough yet) whereas Dieselpunk might be a consequence of a sort of poverty of connectedness (i.e., our societal matrix is neither large enough nor rich enough to build what we might find in old books or otherwise imagine, even if we knew how.)
It occurred to me that there was an interesting plateau of sorts between about 1920 and 1940. Most of the stuff that existed in 1940 existed in a slightly cruder form in 1920. During those two decades, we got better at doing the stuff we did before, but we didn't invent a great deal of truly new stuff. WWII changed everything, of course, and nuclear energy and transistors and many exotic materials showed up by 1950.
The era 1920-1950 was the Golden Age of Back Room Science and Technology. You could do lots of interesting things with an engine lathe, a microscope, a slide rule, a gas stove, a source of electricity, and raw materials you could buy at the local drugstore, hardware store, and feed store. The science was straightforward, the technology simple. Most important of all, it was still possible to be a generalist. A hundred books (Ok, maybe two hundred) could teach you most of what we knew in the hard sciences. You could usefully master physics, chemistry, and math in less than a lifetime. Specialization has always existed, of course, but I think it became mandatory after 1950. After that, you could no longer hear street traffic near a university for Asimov's Sound of Panting.
If the population of Earth were reduced by three quarters (especially by something limited to human beings, like a very nasty flu virus) high-tech civilization might no longer have the critical mass of human skill it would take to maintain itself. The computers would work for awhile, but after they died, all the support infrastructure (chip foundries etc.) would die with them, and what would be left after a few decades would be less Mad Max than Dieselpunk.
All that's debatable, of course, and I could be completely wrong. I bring it up only as an insight obtained by getting out in the sunlight for the first time this cold season and making the blood pump a little. I'm taking notes on a fictional setting involving a new Ice Age, and now I'm sure it'll be a Dieselpunk culture, with no computers but a great deal of steel, vacuum tubes, carbon black soot, and internal combustion. Neanderthals, too: Brute muscle mattered before WWII in a way it may never matter again. And airships--hey, they were huge in the '30s! (Why let the steampunkers have all the fun?)
Now for the time and energy to finish what I'm working on now so I can get on to The Gathering Ice.