Small Engines

Small engines are machines converting energy from oil, gasoline or electricity into motion or Mechanical Work. The rotary motion of a shaft from the engine produces the mechanical workings for driving pumps, generating power, refrigeration compressors and a million other electrical components. Engines are usually classified according to the form of energy they utilize, as steam, compressed air, and gasoline; the type of motion of their principal parts, as reciprocating and rotary; the place where the exchange from chemical to heat energy takes place, as internal combustion and external combustion; the method by which the engine is cooled, as air-cooled or water-cooled; the position of the cylinders of the engine, as V, in-line, and radial; the number of strokes of the piston for a complete cycle, as two-stroke and four-stroke; the type of cycle, as Otto (in ordinary gasoline engines) and diesel; and the use for which the engine is intended, as automobile, airplane and marine engines. Engines are often called motors, although the term motor is sometimes restricted to engines that transform electrical energy into mechanical energy.

Diesel Engine

A type of internal combustion engine in which heat caused by air compression ignites the fuel. At the instant fuel is injected into a diesel engine’s combustion chambers, the air inside is hot enough to ignite the fuel on contact. Diesel engines, therefore, do not need spark plugs, which are required to ignite the air-fuel mixture in gasoline engines. Diesel engines burn a petroleum product similar to kerosene, jet fuel, and home heating oil.

Diesel engines are more efficient and less expensive to operate than gasoline-powered engines, partly because diesel fuel costs less. Diesels consume less fuel and emit fewer waste products. A disadvantage of the diesel engine is the production of sooty, smelly smoke, but modern diesels generally run cleaner and with less odor than older models. German engineer Rudolf Diesel invented the diesel engine in the 1890s. The engines initially used powdered coal for fuel. By 1897 Diesel had built a compression-ignition engine that ran on kerosene.

Diesel engines were more efficient than the steam engines of the 1800s and became popular. They were also big and heavy, suitable mostly for the shipping and railroad industries. They are still the engines of choice for big and small, high and low speed yachts.

Components

Diesel engines are similar in appearance to, and have many of the same components as, spark-ignition engines. Diesels have one or more cylinders (usually four, six, or eight). Pistons inside the cylinders are connected by rods to a crankshaft. As the pistons move up and down in their cylinders, they cause the crankshaft to rotate. The crankshaft’s rotational force is carried through a transmission to a drive shaft, which turns a propeller causing the vessel to move in the direction selected.

Because a diesel engine compresses air inside the cylinders with greater force, the engine block, pistons, connecting rods, crankshaft, and other components must be stronger than those of a gasoline engine with the same power output. As a result, diesel engines tend to be heavier and more expensive to manufacture.

A diesel engine also needs a more powerful Starter Motor, which turns the crankshaft to initiate ignition. It often has an extra battery to provide power to the starter motor.

How It Works

Most diesel engines use a four-stroke cycle: the piston’s first, downward stroke draws in air; the second, upward stroke compresses it; the third, downward stroke, following combustion, delivers power; and the fourth, upward stroke expels waste gases.

At the end of a piston’s compression stroke, a fuel injector sprAY fuel into the combustion chamber. Air temperature inside the chamber at that point is about 540° C (about 1000° F). The fuel ignites, causing a rapid expansion of hot air that forcefully pushes the piston downward. That downward power stroke turns the crankshaft.

Compression Ratio

The amount of compression in an engine cylinder is a ratio of the original volume and the final volume. A compression ratio of 2:1 means the air has been compressed to half its original volume. A ratio of 3:1 indicates compression to one-third of the original volume. The compression ratio of a gasoline engine varies from about 6:1 to 10:1. The compression ratio for a diesel engine varies from about 12:1 to 24:1. The diesel’s higher compression ratios are necessary to create air temperatures hot enough to ignite fuel on contact.

Cold Starts

Diesel fuel is not as flammable as gasoline, so diesel engines are more difficult to start when cold, especially in winter. To solve this problem, each combustion chamber has a glow plug containing a small coil of wire that heats up when an electric current passes through it. Briefly activating the glow plugs before starting a diesel engine preheats the combustion chamber and makes starting easier.

Diesel Fuel

Modern diesel engines burn a petroleum product similar to kerosene, jet fuel, and home heating oil. Diesel fuel is less expensive to produce than gasoline and safer to handle. Due to lower volatility (tendency to vaporize) and a higher flash point (the temperature at which diesel fuel ignites), it is less likely to catch fire during an accident.

There are three grades of diesel fuel. They are classified according to the ease with which they ignite, the lowest temperature at which the fuel will flow, and viscosity (resistance to flow). Individual grades of fuel are better suited for certain engines and operating conditions.