Basic Auto Tech - By Dr. Tom

[05Roush]

As many of you know, Dr. Tom has written a series of auto tech articles for our newsletter. We thought it could be helpful to have them on the website as well!

Enjoy!
__________________________________________________ ____________

Every journey starts with a first step. All of us share an interest in cars: domestic or imported, car, van, truck or bike. All are propelled along by an internal combustion engine consuming gasoline, diesel oil, or a mixture of gas and oil. Some municipalities use "Liquified Natural Gas" or even propane as a fuel in fleet vehicles. All of the difficult work in understanding an engine has been mastered by people that have made our lives easier. They invented, designed and refined them; we only have to understand them. All engines have a few basic design features in common, and when the basics are considered, then the various methods for getting more power out of them make more sense. I'm not an authority on this topic but I know that I can share what I know with those that may not, and we can all have a better understanding of what goes on under the hood. This will be starting out very basic, so that when modifications for more power are covered all of us including anyone that hasn't turned a wrench will see why they work. And off we go. It has been said that an engine is just a big air pump, but there is more to it than this. Lets consider a single piston engine, because then a four, five, six, eight, ten or twelve cylinder engine will be doing the same thing, mostly. (mostly to be reviewed later). If you light a mixture of gasoline and air it will burn violently. When this explosion takes place in a tube with one end sealed off, pressure is driven away from the closed end. If a moveable piston is placed in the tube, then the expanding gasses will push the piston down the cylinder until it falls out of the open end, much like a bullet traveling down the barrel of a gun. (How the fuel got there, how it was lit and where it goes next is coming up, but for now just consider the piston as a oversized bullet in a cylinder.) If we air out the tube, and push the piston up against a fresh mix of gas and air, we can light it with another spark and do this again, and again. This up and down movement (or back and forth depending on how you look at it, would be fine for the "First Flight Pogo Stick Club" but not for us. We need to convert the up/down motion to rotational movement (and just in case anyone is wondering about rotary engines such as those in some Mazdas, we'll get to them later). If we connect the piston to a crank, then the linear motion would be converted to rotation. The piston would have to be connected to a rod that would push against the crank and allow the crank to spin freely. This would be our "connecting rod". The joint that the piston has with the connecting rod would have to flex back and forth, like our hand does with our forearm, to allow the other end to rock back and forth as the crank turns. This pin that joins the piston and connecting rod is aptly named the "wrist pin". The piston is sealed tightly to the cylinder so that gasses can't seep around the piston. The components that create the seal and lets the piston slide along is a set of rings; usually three, sometimes four, and each very specalized. These encircle the piston and fit in precise grooves so that while the piston has clearance to the walls of the cylinder, the rings stick out slightly and are in intimate contact with the cylinder walls. If we wanted to measure a precise amount of fluid we could use a long glass tube with gradations down the length and a valve on the bottom, called a buret. We need something to accurately measure fuel so we can mix it with the right amount of air for our car...Hey, lets call it a CAR-BURET-OR!! (Remember, someone else already did the hard work for us...). We'll get into how carburetors work and the different types of fuel injection and other variations later. We'll need a way for fresh flamable mixture to get into our cylinder, and spent fumes out, at specific times to have our contraption work. A passage delivers fuel and air from our carburetor to an "intake valve" that opens into the cylinder, above the piston, at the correct moment. The space created by the receeding piston that is connected to the spinning crankshaft, is filled with combustable mixture; the piston reaches the bottom of it's travel, and the intake valve closes. When the piston reaches the bottom of the cylinder it changes direction and now compresses the mixture into a quickly decreasing space. (I made reference to a gun before for a good reason, the space where the ignition and explosion will take place over the piston is called the chamber, or more accurately the "combustion chamber". When the mix is really packed into the small space left in the chamber, a spark is delivered to ignite it and force the piston down, pushing the crankshaft around as it does. It travels back up the cylinder and a second valve opens allowing the "exhausted" gasses a way out, pushed along by the rising piston. The exhaust valve closes and our cylinder is ready to repeat the series of events. (1) Intake, (2) Compression, (3) Combustion, (4) Exhaust, a "four cycle engine". We can see that these events need to be timed in a synchronous fashion for all these steps to occur. The valves, and also the spark, are usually (but not always) linked to the rotating crankshaft by two gears connected by a chain or a toothed belt with the gears rotating in the same direction. This is the "timing assembly" that lets it all run over and over again. Next time, how carburetors work and why did the valves open when they did, and where did the spark come from, and maybe what's up with fuel injection, and why does my old car smoke and why did my mechanic change my timing belt, and more. Thanks for taking this walk with me. -Dr.Tom