There are three basic problems that can occur: a bad fuel mixture, insufficient compression, or insufficient spark. Beyond that, there are thousands of smaller issues that can cause problems, but these are the “big three.” Based on the simple engine we discussed, here’s a quick rundown of how these issues can affect your engine:
A bad fuel mixture can happen in a number of ways:
The gasoline is gone, so the engine has air but no fuel. The intake may be blocked, so there is fuel but not enough air. The fuel system may be supplying too much or too little fuel to the mixture, meaning combustion can’t work properly. There may be impurities in the fuel (such as water in the tank), preventing the fuel from burning. Insufficient compression: If the charge of air and fuel can’t be compressed correctly, the combustion process won’t work properly. Insufficient compression can happen for the following reasons:
Your piston rings are worn (causing the air/fuel mixture to leak out of the piston during compression). The intake or exhaust valves aren’t sealing properly, causing another leak during compression.
There’s a hole in the cylinder. The most common “hole” in a cylinder occurs where the top of the cylinder (which holds the valves and spark plugs, also known as the cylinder head) joins the cylinder itself. Typically, the cylinder and cylinder head are bolted together with a thin gasket pressed between them to ensure a good seal. If the gasket breaks, small holes will appear between the cylinder and the cylinder head, and these small holes will cause leaks.
Lack of Spark: The spark may be absent or weak for the following reasons:
If the spark plug or the wire leading to the spark plug is worn, the spark will be weak. If the wire is cut or missing, or the system that sends the spark along the wire is not working properly, there will be no spark.
If the spark occurs too early or too late in the cycle (that is, the ignition timing is off), the fuel will not ignite at the correct time. There are many other things that can go wrong. For example: If the battery is dead, you can’t start the engine. If the bearings that allow the crankshaft to turn freely are worn, the crankshaft can’t turn, and the engine won’t run. If the valves don’t open or close at the right time or at all, air can’t get in, exhaust can’t get out, and the engine won’t run.
If there’s no oil, the piston can’t move up and down freely in the cylinder, and the engine will seize. In a properly functioning engine, all of these things are working well. An engine doesn’t need to be perfect to run, but you’ll probably notice when it’s not. As you can see, an engine has many systems that help it do its job of turning fuel into power. We’ll look at the different subsystems used in an engine in the next few sections.
Most modern engines use what’s called an overhead camshaft. This means that the camshaft is located above the valves as shown here. The cams on the shaft actuate the valves directly or through a very short connecting rod. Older engines use camshafts located in the oil pan near the crankshaft.
A timing belt or timing chain connects the crankshaft to the camshafts, which synchronize the valves with the pistons. The camshafts rotate at half the speed of the crankshaft. Many high-performance engines have four valves per cylinder (two for intake and two for exhaust), an arrangement that requires two camshafts per bank of cylinders, hence the term “double overhead cam”.
The ignition system generates a high-voltage charge and transmits it to the spark plugs via ignition wires. The charge first flows to the distributor, which you can easily find under the hood of most cars. The distributor has a wire extending from the center with four, six, or eight wires (depending on the number of cylinders) extending from it. These ignition wires carry the charge to each spark plug. The engine is timed so that only one cylinder receives a spark from the distributor at a time. This method provides maximum smoothness.
Engine Valve Train and Ignition System
Most engine subsystems can be implemented using different technologies, and better technology can improve the performance of an engine. Let’s take a look at all the different subsystems used in a modern engine, starting with the valve train.
The valve train consists of the valves and the mechanism that opens and closes the valves. The opening and closing system is called a camshaft. There are lobes on the camshaft that move the valves up and down.
