Week 3 in the Basic Auto Mechanics class was all about the mechanical setup and functions of the engine. We went over the components that make up the engine and some simple troubleshooting tips to follow when there is a problem. This was followed by shop time where we were let loose and told to disassemble to top end of the motor.
Classroom time lasted quite a while this week as we went over how an engine actually works. Starting with what is required to get an engine to run. The three requirements that an engine needs are compression (air), fuel, and spark. Without one of these items, the engine will not run. It was then explained to us that the intake valves control the fuel air mixture in the engine and that the exhaust valves dispose of the used gasses.
At this point we went over the four stroke cycle of a gas engine. The intake stroke comes first, it is when the intake valve is open and the piston is moving down in the bore. This fills the bore with the air/fuel mixture that is necessary for combustion. The compression stroke is next, when the piston is moving up in the bore and squeezing all of the air/fuel to the top of the chamber. As it reaches the top, the spark plug will ignite the mixture and create the next stroke. This is all controlled with ignition timing, we were not educated on the exact way this occurs but the teacher did mention BDC (Before Dead Center) as the controlling mechanism that tells the spark plug when to ignite. FYI: I know that this is controlled with the distributor in an older car or electronically in newer vehicles. The next stroke is the power stroke where the burn of the air/fuel mixture pushes the piston back down into its bore. The last stroke is the exhaust stroke where the burnt gasses from the power stroke are pushed up the cylinder and out of the chamber through the exhaust valve. At this point the cycle starts over again.
We went over the parts of the engine next. We were shown the piston, connecting rod, wristpin, and crankshaft. These were small little display items that the teacher had made for explanation but they were very accurate in their setup. He was able to demonstrate the strokes that were explained in the previous section and how those actions turned the crankshaft to produce power. When examining the pistons, it was explained to us that there are 3 rings that seal the piston in the cylinder. The bottom thicker ring is the oil ring which keeps the oil from entering the combustion chamber. The two smaller rings on top are the compression rings which keep the gasses from mixing with the oil. Not on display in the class was an engine block (those are outside in the shop) but it was stated that on the outside of the piston bores are the water jackets which keep the cylinders cool while the engine is running.
Now there was a small troubleshooting section in which we went over the types of smoke that you may see and what it means when you see it.
- Blue Smoke – This is typically seen on an old engine where there are problems with the piston rings or oil seals.
- White Smoke – This is anti-freeze burning, so somehow there is coolant getting into the combustion chamber.
- Black Smoke – This occurs when you are missing one of the 3 necessities to get an engine to run. The black is usually the partially burnt fuel that was burned in the exhaust stream because it did not get burned in the combustion chamber.
The valve train was next. This consists of the lifter, pushrod, rocker arm, and the actual valve. I always thought that the camshaft was part of the valve train but I guess not…. However, the camshaft was covered in this section and we saw how the camshaft was supported by bearings and how the lobes move around in a set timing to open and close the valves. The high point of the valve is how far open the valve will be when it is at its peak. The duration is how long the valve is open at peak. We briefly touched on performance in this section and he explained that you could get more performance by allowing more air/fuel into the combustion chamber. This can be achieved with a higher lift camshaft or with a longer duration camshaft.
The last part of the classroom session, and the end of my notes, was about oil. It was strongly stated that oil is cheaper than metal and to use the correct oil for the application. We learned that the w is a measure of viscosity and that the next number is the weight of the oil. We also were told that there are detergent oils and non-detergent oils. Detergent oils suspend particles within it and when the oil goes through the oil filter the particles are stripped from the oil. In non-detergent oil, the particles will sink to the bottom and come out when the drain plug is removed. You would not want to use detergent oil in an application that does not have a filter.
Now it was shop time! We pulled over two motors that were on engine stands and were told to take apart the top end, all the way down to the block. This consisted of removing the exhaust manifold, the intake manifold, the valve cover, and the head. The engines were missing bolts and a lot of the bolts were not too tight so it went very quickly. It’s a lot easier to work on an engine on a stand than in your car! When the motor was apart, we were shown the valves, the valve springs, and the retainers. This was all very basic, this is how it goes together, kind of stuff but was interesting to have it explained to me by a real mechanic with lots of experience.
We were also told about the difference between an interference engine and a non-interference engine while we were in the shop. In an interference engine, the valves will hit the pistons if the timing belt breaks. In a non-interference engine they will not. At this point he stressed the importance of properly maintaining the timing belt and performing the maintenance at the required intervals.
So far the experience of going to these classes has been quite enjoyable and I have left every class with a little more knowledge than I had going in. I mainly took the class to make sure that I had a clue as to what I was doing before really getting heavy into the repairing of automobiles. I find myself wishing the class was more advanced but I do realize it is a basic auto mechanics class and is geared to a beginner with very little knowledge. It’s nice to see that I have a clue!