Tuesday, January 17, 2012

A Girl's Guide to Diesel Engines: Overview

It looks so simple...
I've used an engine almost every day since I was 16, ferrying myself to work, school, the grocery store, and home to Arkansas. I've driven, when you really think about it, an obscene amount, blithely adding to global warming (yes, I know it doesn't exist!) the entire way but without a clue as to how the engine was propelling me down the road. Embarrassing? Pretty much. But perhaps Rudolph Diesel, inventor of the diesel engine in 1892, will be kind enough to forgive me.

Diesel invented an engine that works by drawing air into a cylinder and compressing it until it is very hot (more than 1,000 degrees F). Just before the air is fully compressed, fuel is injected, creating a mini-explosion inside the cylinders. The expanding gases from the explosion push the pistons down, which are attached to the crankshaft by a connecting rod. The crankshaft, via the transmission, turns the propeller, and off you go. All it needs - besides water, impellers, filters, oil, belts, gaskets, etc. - is air and fuel.

Which leads me to the second part of this discussion - what's the difference between a gasoline engine and a diesel? Well, they are both internal combustion engines, which is kind of like saying they are small bombs encased in steel that we sit behind while driving. But they are small bombs in slightly different ways. A diesel engine compresses air and then adds fuel, which ignites in the incredibly heat. However, a gasoline engine compresses fuel and air and adds a spark for ignition. So while diesel engines are a delicate dance between air and fuel, gasoline engines have a third element - spark.

So why not use a gasoline engine on a boat? There are a number of reasons why diesel is preferable, the first one being that diesel fuel doesn't explode. That's a pretty nice feature when you're in the middle of the ocean and a giant fireball is the literally the last thing you want to see coming out of your fuel tank. Also, diesel engines are far more efficient than gasoline engines, and they last a heck of a lot longer, hence the reason you see so many diesel engines from the '70s still in use on boats today. Finally, diesels are a more simplified engine. They don't have as many electrical parts as their gasoline cousins, and electrical parts are highly susceptible to corrosion, a big consideration in a salt water environment. 


  1. Diesel's original conception was for an engine that ran on vegetable oil that common workers could use to power their tractors and trucks.

  2. Okay, my brain is engaged. So this explains why the transmission is named thus? It 'transmits' energy to the propeller? I will google that to find out. Also, I cannot remember the law of thermodynamics that talks about compression causing heat. I suppose that's how diamonds are made. I will be on the google this morning, thanks!

  3. Joshua, I didn't know that, though I've always liked the concept of being able to motor around on used cooking oil. Might be a bit stinky, but hey it's cheap!

    And Melissa, I asked my resident science geek about your questions, and yes, you are exactly right about the transmission! As for why the air gets hot when compressed, I'll try to explain it. Vlad was tossing around the word "entropy" a lot when I asked him, which he usually explains as the universe striving for disorder (he calls it freedom and has used it as an excuse to not clean up the garage). Anyway, I'm not totally sure how entropy plays in, but the gist of it is this. Air is a gas, and a gas is a more energetic state than a liquid or solid. When you compress the air, you're pushing it into a more solid state, and as it moves toward this solid state the air releases energy as heat. Additionally, all those molecules moving around closer and closer to each other causes friction, which also causes heat. A diamond, on the other hand, is caused by heat and pressure. Whew, that's a lot of science early in the morning!

  4. I think it is marvelous that you are taking the time to do this series of posts!

    1. Thanks! I hope it's useful, and it has been refreshing to break out of my non-mechanical comfort zone.

  5. Essentially, the reason gas heats up is that the molecules are freely moving in a gas, and so reducing the volume of the gas causes the molecules to strike each other and the walls of the compression chamber more often, which is released as heat (because of entropy). You could compress gas at a speed using various methods where the heat it was gaining was equal to the heat bleeding off, so that the gas would not seem to heat up, but energy is still lost (because of entropy). However, that would be too slow to power an engine, so you need the quick and dirty compression, causing rapid temperature increase, which ignites the fuel. In a gas engine, you compress a fuel/oxy mixture less to increase the heat and more because compressed gas blows up bigger (the spark causes the molecules to spontaneously combust, instead of the heat of the gas). So, the two types of engines are two applications of the same phenomenon. Insofar as I understand it.

    Diamonds are made by heat and pressure, yes, but you wouldn't feel the heat in a natural diamond, because it happens over such a long period of time. It doesn't require a volcano or anything, just a lot of pressure on a collection of carbon. Synthetic diamonds require a lot of heat, because you are compressing a million year procedure to into a few hours.