Basic Engine TypesFitness Equipment
Although alike in major mechanical aspects, oil and gas engines differ in fuels and fuel handling, when and where fuel and air are brought together, how much the charge is compressed, and how it is ignited. Many variations of these elements exist but the four examples shown below will serve to illustrate the basic considerations involved and to provide a firm basis for understanding today's engine types.
As in the familiar automobile engine, the low-compression gas engine mixes fuel and air outside the cylinder, before compression. If natural gas is burned, a mixing valve may replace the carburetor used with liquid fuel.
The gas fuel is proportioned to the air to produce a nearly perfect mixture; that is, just enough air to burn the gas completely. This mixture flows into the cylinder and is compressed. Near the end of compression, an electric spark ignites the inflammable mixture, which burns rapidly. Cylinder pressure rises rapidly and acts on the piston to move it down on its power stroke.
Since compressing a gas raises its temperature, the mixture may get hot enough to self-ignite before the end of compression. Such pre-ignition causes loss of power, and compression pressure must be limited in this type of engine. To do this, compression ratio becomes a major factor in establishing what the cylinder compression pressure will be.
Cylinder volume at the beginning of a stroke, divided by its volume at the end, is the compression ratio. Pressure at the end of compression is roughly proportional to the ratio; in general, the higher the compression ratio, the higher will be the maximum pressure reached during combustion. Also, the higher the expansion ratio, which in most engines corresponds to the compression ratio, the more efficient the engine.
Although it is desirable to have a high compression ratio, the nature of the fuel imposes limits in engines where a nearly perfect mixture is compressed. With natural gas, for example, the compression ratio might run about 5:1, for a compression pressure of about 120 psig. Also, pre-ignition might become a limiting factor.
In contrast to engines in which fuel and air mix before compression are those in which only air is compressed and fuel enters near the end of compression. In the diesel, prime example of this class, heat of compression is used to ignite the fuel.
In a typical diesel engine, air is compressed to about 450 psig, which brings its temperature up to about 1000F. When finely atomized oil is sprayed into this heated air, it ignites and burns. In the diesel, therefore, the high compression ratio necessary for reliable ignition means inherently high efficiency.
Because, in practice, compression ratio above those needed for ignition do not improve overall efficiency much, it is customary to go no higher. Pressure and temperature resulting from a given compression ratio depend on engine speed, cylinder size, and other design factors. Typical compression pressures in diesels range from 450 to 600 psig or more, with small, high-speed engines generally having higher compression pressures than larger units.
Since natural gas has about the same heating value per pound as does fuel oil, if a diesel were operated at the same output on natural gas instead of on oil, the cylinder would be charged with a weight of gas about equal to that of the oil. Thus, a lean mixture would result. In fact, the mixture would be so lean that it would not self-ignite at the temperature reached by the end of compression.
In dual-fuel engines, therefore, a small quantity of oil (called pilot oil) is injected near the end of compression. It is ignited by the compressed mixture and burns like the oil fuel injected into a standard diesel. Its burning supplies enough heat to ignite the gas/air mixture. For precise control of the pilot oil, a separate set of fuel pumps and nozzle is added.
Because the quantity of air admitted to cylinders remains the same at all loads, the fuel/air mixture tends to become extremely lean at partial loads - so lean as to lead to incomplete combustion and poor fuel economy. To maintain a desirable mixture ratio, most dual-fuel designs include some means of reducing the air quantity at partial loads.
High-Compression Gas Engines
When operation is solely on gas, it is desirable to gain the good fuel economy of high compression without the cost of pilot oil. By using higher intensity ignition systems and slightly richer mixtures with lower compression ratios than in duel-fuel engines, today's gas engines can yield performance close to that of duel-fuel units, without pilot fuel.