Short version: Dieselpunk is allowed to have internal combustion. That’s really all it is.
But from there, you extrapolate what effect that tech level has on your society. At that point, war is mechanized, you’re looking at WWI and WWII level tech, with planes, tanks and machine guns. And machines guns were one of the reasons the first great war was so frelling bloody. People have more ways of killing larger numbers of people faster than ever had before, and this tends to mean that Dieselpunk is less often about discovery and invention like Steampunk, and more often about social struggle.
Also, the tech in Steampunk is more often a new thing, or if not new, something wonderous that few people have access to, while in Dieselpunk, the tech is frequently ever-present, and the average person is likely to have ridden on some form of mass transit at some point in their lives. The technology is no longer new, and mass production increases accessibility. You have trams and subways, trains, and other public transportation. You can have cars and motorcycles, and snowmobiles, and jetskis (“Waterworld”) if you want. The horse has been, or is in the process of being replaced in all but the most, shall we say, traditional communities.
That last one’s pretty huge. The horse has been around as transportation for a long time – thousands of years longer than mechanical vehicles, and that shift is a major turning point in history.
Post Apocalyptic stuff is often Dieselpunk, and I can tell you why it usually ends up Dieselpunk and not Steampunk. It’s because why would we get thrown back to steam level tech, if we had internal combustion? If there’s a loss of tech, it’s likely to go back to the last level where the average person had access to the technology and could do routine maintenance on it, and find someone who can fix it if it breaks down. Computers, it’s completely plausible that computers wouldn’t make it far into a Post Apocalyptic world. That processor – if it burns out – and they burn out eventually, and not infrequently either – the average person can’t fix it, and can’t produce a replacement without sophisticated equipment.
Here, I’m defining sophisticated as anything that the average person has no understanding of, and no picture of what it looks like or how it works. I have a mental picture of a monkey wrench, and an idea of how it’s used. I’ve seen a car jacked up to replace the brake pads – that’s not that complex, and looking at the actual brake mechanism, it makes sense. I do technical support for computers, so I have a general idea of what most computer parts look like – I’ve replaced parts in my computers and my husband’s, and done it myself, without having to take it in to a computer surgeon. But I could not have created the replacement, or, in the case of a broken part as opposed to an upgrade, I could not have repaired the part, or found anyone locally who could.
And I think that’s part of the fascination with Dieselpunk. The technology level is assumed to be around that last level where you can take your vehicle in to a local shop to have it fixed, and not need to have parts shipped in from someplace else in the world where they make new parts. Granted, sometimes these days we do still have to have parts shipped in for our cars, but that’s because we’re moving away from that local based ability to maintain our technology. I had the opportunity to talk to S. M. Sterling at Keycon, a few years ago, and he said the best places to find information on how to build things, is the encyclopedias from those years, because, unlike the encyclopedias these days, they had full instructions on how to build anything you wanted.
But on top of the tech level, there’s also the vibe of the era it represents. Part of the appeal of Steampunk is the romanticism of the Victorian era. Dieselpunk, though, we’re looking at an era of social upheaval. The world was faced with two world wars, and entire countries had to choose sides and fight. Young men coming home from war came home to their jobs having been filled by women, and their women demanding the vote. People of colour, too, demanding to be treated the same as everyone else. Huge shifts in social responsibility and equality. It was a time when people weren’t yet jaded by the lies told by politicians, and and it was worth fighting for a cause you believed in. People still believed that they could make a difference in the world.
That’s why I think Dieselpunk fiction and movies tend to be ones that delve into those sorts of themes. It’s what draws me to it, and why I write it, myself. I hope it will be what draws people to my writing.