What's the Difference Between Regular and Diesel Engines?

I've never really owned a car.  I guess I technically own half of my wife's temperamental 2002 Nissan Sentra now, since I'm pretty sure that's what marriage means, but I've never actually gone out of my way to own and operate a motor vehicle.  As a result, I have an almost childishly simplistic understanding of what the hell makes cars go; I know there's an "engine" inside, which burns "fuel" to turn the "wheels," but saying I'm hazy on the specifics is probably being unfair to haze.  Unsurprisingly then, the question of what makes a diesel engine different from a regular old engine managed to not occur to me for over 32 years, for the same reason that you've probably never wondered what makes a mountain gorilla different from a lowland gorilla (apologies to gorilla owners and/or gorillas among my readership, obv.).  It wasn't until a week ago while I was absently staring at a gas station sign (note to self future possible blog entry: why the 9/10 of a cent thing?) that it occurred to me that some cars (and, apparently, all big trucks) need a special kind of fuel called "diesel," and that why that is exactly was almost certainly a Thing I Should Probably Know.  To the wikipedias!

What secrets lurk in his comically oversized head?

A happy side effect of figuring out why diesel engines are different from regular engines was that I had to finally figure out how regular engines work in the process.  A happy side effect of that side effect was that I learned that the basic spark-ignition internal combustion engine found in most cars is called an "Otto-cycle engine," which made me laugh because I am basically a seven year old with an advanced degree. Anyway, Otto-cycle engines work pretty much how I'd always guessed they do.  You fill a combustion chamber above a piston with some mixture of fuel and air, then apply a high voltage across a spark plug to cause a small electrical arc, which makes the fuel/air mix explode and pushes the piston down.  If you have a bunch of pistons in a row and you time the explosions right, you can make them do useful work, like spin a drive shaft.    I built a potato gun in college that worked on basically the same principle, although the "useful work" in question was "firing potatoes at the grad student dorms until they called the cops on us."

The Diesel engine cycle (coincidentally invented by Rudolf Diesel in 1892) is actually quite a bit more clever, at least from an engineer's standpoint.  It's the same basic process as a normal engine cycle (ignite some fuel, push a piston with the explosion, move some shit), but instead of relying on a spark to do the igniting you just compress the air in the combustion chamber and use the resulting heat to light the fuel.

That's a crap explanation, so here's a more detailed one.  Basically, what we're doing here is exploiting the ideal gas law, which you probably encountered in high school chemistry at some point (PV=nRT, remember?).  The ideal gas law says that, all other things being equal, if you increase the pressure of a gas the temperature is going to increase accordingly.  So with the piston fully extended from the combustion chamber, you're going to fill the chamber with air.  When the piston moves back up into the chamber (via the motion of the engine cycle itself), it's going to effectively make the chamber much smaller, compressing the air inside.  The increased pressure causes increased temperature, which will eventually (at pressures of about 600 psi) exceed the auto-ignition temperature of the vaporized fuel.  Once you've reached that threshold (which if you've designed the engine properly is also the top of the piston's cycle), injecting some vaporized fuel into the chamber will cause it to spontaneously ignite, pushing the piston out of the chamber with no external ignition system needed.  You've replaced spark plugs with the laws of physics essentially; like I said, it's clever.

I'm lazy today so I'm just cold swiping images (this is from automobilehitech.com).  It's a pretty good cross-sectional illustration of the combustion chamber and piston at each point in the Diesel cycle. 

Obviously there's a bit of a chicken/egg problem here: if the ignition cycle relies on the engine already running with enough power to compress the air to its critical pressure, how do we actually start the engine?  If you know anything about cars right now you're probably saying "duh," but as I discussed at length earlier this is all fairly new to me.  Anyway, you start the engine the same way you start a regular engine: by using an electric starter motor to spin the engine until it's moving fast enough for the ignition cycle to take over.  A disadvantage of using heated air vs. spark plugs as your fuel ignition system is that cold weather, which results in both a lower starting air temperature and a cold engine block around the combustion chamber, can make it much harder to get the engine started.  This is usually worked around by using electric engine block heaters, although you can get cute and do stuff like use a fuel with a lower auto-ignition temperature than diesel gasoline (ether is popular if you can get it) to get the engine running until it's up to operating temperature.

So in the Diesel engine, we've got a clever but functionally identical analog to the Otto-cycle design, which begs the question: why use one over the other?  The big advantage of Diesel engines is efficiency; because you have to compress the combustion chamber quite a bit more in order to get the fuel-air mixture to spontaneously ignite, Diesel engines have much higher compression ratios than standard engines.  Higher compression ratio means more piston travel distance which means more work done per combustion cycle; the end result is that Diesel engines can be up to 50% more fuel-efficient than their Otto-cycle counterparts.  The downside is that all that extra piston travel makes the engine block much bigger and heavier, which can entirely negate the extra engine efficiency in smaller vehicles where the engine is a large fraction of the total weight.  As a result, the places you'll usually see Diesel engines are in applications where engine weight isn't a huge deal-- big trucks, ships, tanks, and stationary stuff like generators.  They do show up in cars (particularly in Europe), but maintaining an efficient engine weight in smaller vehicles generally means sacrificing some power, although the fact that you don't have an explosive fuel-air mix in the chamber until just before combustion time means you can play all kinds of games with turbo-charging the compression cycle via increased pressure without worrying about the engine blowing up on you.

The other huge advantage of Diesel engines is the fuel itself.  It's based on petroleum, like gasoline, but unlike gasoline it's pretty much just distilled petroleum; you don't have to do a bunch of stuff to it afterward to turn it into a usable fuel, which means it's much cheaper and cleaner to make.  Petroleum distillate also has a way lower vapor pressure than gasoline, meaning that if you spill some you don't have that whole rapid-outgassing-of-explosive-fumes issue you do with regular gasoline.  Unless you're committing arson (note to self: if you ever need to burn down a building, don't buy diesel fuel even if it's cheaper) that's generally a good thing.  It's also relatively easy to make a diesel fuel substitute ("biodiesel") from all kinds of organic material, and unlike gasoline substitutes like ethanol you can pretty much just run pure biodiesel in a stock diesel engine without issues.  As people come up with increasingly clever and efficient ways of synthesizing biodiesel on useful scales, that one's going to become a big deal.

All of my gearhead/engineer friends are always gushing about Diesel engines, and now I understand why.  Being able to yank out the entire spark ignition system and replace it with a clever application of the laws of physics is the kind of elegant solution to a problem that engineers spend their whole careers trying (and often failing) to come up with; the fact that it actually results in a more efficient engine with a more flexible fuel system is just gravy, honestly.  And since said gearhead friends are probably reading this, I'm well aware that I glossed over a lot of subtleties of engine design and optimization here, so feel free to point them out in the comments.