The main parts of the gasoline engine are quite similar to those of the diesel engine. The two engines differ principally in that the gasoline engine has a carburetor and an electrical ignition system.

The induction system of a gasoline engine draws gasoline from the fuel tank and air from the atmosphere, mixes them, and delivers the mixture to the cylinders. The induction system consists of the fuel tank, the fuel pump, the carburetor, and the necessary fuel lines and air passages. Flexible tubing carries the fuel from the tank to the carburetor, while the intake manifold carries the fuel-air mixture from there to the individual cylinders. The fuel-air mixture is ignited by an electric spark.

The carburetor is a device used to send a fine spray of fuel into a moving stream of air as it moves to the intake valves of the cylinders. The spray is swept along, vaporized, and mixed with the moving air. The carburetor is designed to maintain the same mixture ratio over a wide range of engine speeds. The mixture ratio is the number of pounds of air mixed with each pound of gasoline vapor. A rich mixture is one in which the percentage of gasoline vapor is high, while a lean mixture contains a low percentage of gasoline vapor.

The electrical ignition system is designed to deliver a spark in the combustion chamber of each cylinder at a specific point in that cylinder's cycle of operation. A typical ignition system includes a storage battery, an ignition coil, breaker points, a condenser, a distributor, a spark plug in each cylinder, a switch, and the necessary wiring.

There are two distinct circuits in the ignition system-the primary and the secondary. The primary circuit carries a low-voltage current. The secondary circuit is high voltage. The battery, the ignition switch, the ignition coil, and the breaker points are connected in the primary circuit. The secondary circuit, also connected to the ignition coil, includes the distributor and the spark plugs.

The storage battery is usually 6, 12, or 24 volts. One terminal is grounded to the engine frame, while the other is connected to the ignition system.

The ignition coil, in many respects, is similar to an electromagnet. It consists of an iron core surrounded by primary and secondary coils. The primary coil is made up of a few turns of heavy wire, while the secondary coil has a great many turns of fine wire. In both coils, the wire is insulated and the coils are entirely separate from each other.

The breaker points form a mechanical switch connected to the primary circuit. They are opened by a cam that is timed to break the circuit at the exact instant that each cylinder is due to fire. A condenser is connected across the breaker points to prevent arcing and to provide a better highvoltage spark.

The distributor, connected to the secondary or high-voltage circuit, serves as a selector switch that channels electric current to the individual cylinders. Although the breaker points are connected in the primary circuit, they are often located in the distributor case. The same drive shaft operates both the breaker points and the distributor.

The spark plugs, which extend into the combustion chambers of the cylinders, are connected by heavily insulated wires to the distributor. A spark plug consists essentially of a metal shell that screws into the spark plug hole in the cylinder, a center electrode embedded in porcelain, and a side electrode connected to the shell. The side electrode is adjusted so that there is a small space (gap) between it and the center electrode. This gap varies depending on the engine. When the plug fires, an electric spark jumps across the gap between the electrodes.

When the engine is running, the electric current in the primary circuit flows from the battery through the switch, the primary winding in the ignition coil, the breaker points, and then back to the battery. The high voltage is produced in the secondary winding in the ignition coil, then flows through the distributor to the individual spark plugs and back to the ignition coil through the engine frame. It is interesting to note that the high voltage that jumps the gap in the spark plugs does not come from the battery but is produced in the ignition coil.

The ignition coil and the condenser are the only parts of the ignition system that require an explanation. The soft iron core and the primary windings function as an electromagnet. The current flowing through the primary windings magnetizes the core. The same core and the secondary windings function as a transformer. Variations in the primary current change the magnetism of the core, which in turn produces high voltage in the secondary windings.

With the engine running and the breaker points closed, low-voltage current flows through the primary circuit. When the breaker points open, this current is interrupted and produces high voltage in the secondary circuit. The electricity, which would otherwise arc across the breaker points as they are separating, now flows into the condenser.

The principal purpose of the condenser is to protect the breaker points from being burned. The condenser also aids in obtaining a hotter spark.

The contact-point ignition system is an older type. The electronic ignition system is of the newer type. The basic difference between the contactpoint and the electronic ignition systems is in the primary circuit. The primary circuit in the contactpoint system is opened and closed by contact points. In the electronic system, the primary circuit is opened and closed by the electronic control unit.

The secondary circuits are practically the same for the two systems. The difference is that the distributor, ignition coil, and wiring are altered to handle the higher voltage that the electronic ignition system produces.

One advantage of this higher voltage of approximately 47,000 volts is that spark plugs with wider gaps can be used. This results in a longer spark, which can ignite leaner fuel-air mixtures. As a result, engines can run on leaner mixtures for better fuel economy and lower emissions.

Another difference is that some electronic ignition systems have no mechanical advance mechanisms-centrifugal or vacuum. Instead, the spark timing is adjusted electronically.

The starting system of the gasoline engine is basically the same as that of the diesel engine. The generator keeps the battery charged and provides the current to operate the lights and other electrical equipment. The starter motor draws current from the battery and rotates the flywheel and crankshaft for starting.

SUMMARY

This chapter was designed to give you a brief understanding of diesel and gasoline internalcombustion engines. You will find these engines on all ships in the Navy. It will be of great value to you to learn more about them by reading the referenced material given throughout this chapter.