WHY DIESEL ENGINE COMPRESSION IS SO IMPORTANT!

Hi everyone. So I've been wanting to make this video for a while. For my beginners and my people that aren't, are still learning about diesels, this is going to be about compression ignition. A lot of people understand that diesels do not have spark plugs, but there's not a wide understanding outside of the diesel industry on why that is. So I wanted to take an opportunity while my students have their engines apart to show you what we're talking about. So as you can see they've got the cylinder heads off their engines. They've got the cylinder heads on the table here. So we're going to talk about why compression is important in a diesel engine and how that compression takes the place of spark plugs for the ignition. Okay so to start with I want you to look at the features of this cylinder head. So this is a cylinder head off of a mid-range Cummins and you can see that looking at it that the surface is very smooth and the valves are just briefly kind of recessed into the cylinder head, but there are really no features on this head. It's flat. So unlike an automotive cylinder head where you would determine the shape of the combustion chamber by the shape of the cylinder head and where the valves were, our diesel engine combustion chambers are going to be defined by the shape of these pistons. So you can see that these pistons are dished down in deep here. We call these a toroidal type piston, toroidal, because it has the crown in the center, it has the valley around it. On these direct injection four-stroke engines the injector sprays fuel directly into the combustion chamber and it spreads out into this area. So here we see a piston that's at top dead center and over here we can see one that's near bottom dead center, right? So you got to imagine that if the cylinder head's sitting flat on top of this guy that when you take all the air, especially pressurized air coming from a turbo, and you fill this area up with pressurized area and then that piston comes up and all the air that was in here is now squeezed into this space right here, right? So the basic law of physics is when you take air and you compress it into a small space, you're going to make it extremely hot. So the reason that diesels can get away with not using spark plugs to ignite a fuel-air mixture, number one, the diesels don't have a fuel-air mixture. During the intake stroke the only thing entering the cylinder is going to be air, pressurized air from a turbo usually. So that air then gets compressed by the piston and when it comes up to top dead center like this, we're going to, not at this point, but right before top dead center, we're going to begin injecting fuel and this air has been heated up so much from the compression that it ignites the fuel by itself without the need of a spark assist. So that would be the difference between what diesels we call them compression ignition engines because they use hot compressed air to ignite the fuel and versus a spark ignition engine which needs a spark to help ignite an air-fuel mixture. So that also means that we have much higher pressures in these cylinders and those higher pressures, if you remember the formula for force right, pressure times area equals force. The force that engines create is called torque and diesel engines are capable of creating a lot more torque because they have a lot more pressure in the cylinders because of the compression ratio. One of the things that we have to do in order to be able to pull enough air in and then squeeze it, compress it until it's hot enough to ignite fuel, is we typically have to have a higher compression ratio than a gasoline engine. So what is a compression ratio? A compression ratio is a measurement of volume from when a piston is at bottom dead center, how much volume is available in a cylinder compared to top dead center. So if we considered this one part, this would be the 13 parts or 14 parts. So a typical automotive application you might have a 9 to 1, 8 to 1, 10 to 1 ratio, get up into performance sports car slightly higher than that right. But diesels can start out at where those performance autos go and start out at 12 to 1, 13 to 1 and get up as high as 25 to 1 in some cases. So we're bringing in a lot more volume of air and squeezing it into a much smaller area. So that pressure combined with the compression ignition is what gives us the pressure against a small area here that gives us the torque that makes engines the preferable engine for towing, diesels the comparable engine for towing because of that increased torque. So I hope this little brief tutorial helped you understand how diesels work, what the difference between spark ignition and compression ignition is. If you have any questions or want me to expand on this any further, please leave a question in the comments and I really appreciate it. Also please go check out our online stores at Shopify and Etsy. Every purchase you make on Shopify and Etsy helps us buy materials to increase our training and to do projects on classic cars and they help to support the furry friends of Charlie's Garage which anybody with pets knows gets expensive. So I appreciate your support, thank you for your time, thank you very much for watching this video. 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