Aircraft technical Basics: TM 1-405, Aircraft Aircraft Engines, 1941: II - Classification and Description of Engine Types

SECTION II - CLASSIFICATION AND DESCRIPTION OF ENGINE TYPES

9. General. - a. Aircraft engines may be divided into two general classes: liquid-cooled and air-cooled. Although the air-cooled engine has been more generally used in the past, the modern liquid-cooled engine is increasing in popularity due to its excellent streamlining characteristics and the ease with which its heat may be dissipated.

b. Either of the above classes may be further divided into types, based on the arrangement of the cylinders with respect to the crank-case. These types are illustrated in figure 5.

(1) The most common types of air-cooled engines are radial (single or double row), in-line, and opposed.

(2) The most common types of liquid-cooled engines are V, in-line, opposed, X, and double V.

10. In-line type. The cylinders of the in-line type engines are arranged in a single row on the crankcase, either in an upright or inverted position. Although this type is not suitable for use where high horsepower output is required, it is an entirely satisfactory design for low-performance airplanes. Practically all of the modern in-line engines are of the inverted type. This method of construction is particularly advantageous from the standpoint of stream-lining and visibility. The high position of the propeller shaft in the inverted engine also allows for low landing gear design due to the increased propeller clearance. The maximum number of cylinders in in-line engines is usually limited to six, to facilitate cooling and avoid prohibitive weight per horsepower.

11. V-type. The cylinders of V-type engines are arranged on the crankcase in two rows or banks forming a V. The two banks of cylinders are directly opposite each other, which permits two sets of connecting rods to operate on the same crankpin. This arrangement greatly reduces the weight per horsepower as compared to the in-line type with very little sacrifice in the head resistance of the aircraft. A typical engine of this type is illustrated in figures 6 and 7.

12. Double V- or fan type. - In an attempt to raise the limit of available horsepower per unit of weight, a double V-type, liquid-cooled engine has been developed, which consists of four banks of cylinders arranged in a fan shape. The chief advantage in such an engine is that it gives practically the same horsepower as two single V-type engines without materially increasing the frontal area required for a single V-type engine. The two crankshafts are employed to drive the propeller.

13. X-type. In the X-type engine two of the four banks of cylinders are arranged on one side of the center line of the engine while the other two are equally spaced on the opposite side in the form of the letter X. This arrangement permits the use of only one crankshaft in contrast to the two crankshafts in the double V-type.

14. Opposed or flat type.   In the opposed type engine the cylinder banks are arranged horizontally, in two rows, on opposite sides of the crankcase. A single crankshaft- of conventional design is employed and the engine may be constructed with any even number of cylinders. Due to the flat shape of the engine. it is particularly adaptable for streamlining and makes possible wing engine installations.

Figure: 5. Typical aircraft engine cylinder arrangement.

15. Radial type.  - a. In single-row radial engines the cylinders extend radially from the center line of the crankshaft in any odd number, usually between 3 and 9. The cylinders are evenly arranged in the same circular plane. A single-throw, 360° crankshaft is used, to witch all of the connecting rods are attached resulting in a minimum number of working parts as well as a considerable saving in weight.

Figure 6. - Right  rear view of typical V-type engine.

b. The double-row radial engine resembles two single-row engines combined on a single crankshaft. The conventional number of cylinders used is either 14 or 18. which in effect would be either two 7-cylinder engines or two 9-cylinder engines. A two-throw, 180° crankshaft is used to permit the cylinders of each row to be alternately staggered on the common crankcase. This alternate staggering allows the cylinders in both the front and rear rows to receive sufficient air from the slip stream for proper cooling. A typical double-row radial type engine is illustrated in figures 8 and 9.

16. Cylinder numbers. - The numbering of engine cylinders is by no means standard in procedure. Some manufacturers number their engines from the drive end and some from the opposite end. In-line and V-type engine cylinders are usually numbered from the end opposite the propeller shaft, as shown in figures 10. 11. and 12. Radial-type engines usually have their cylinders numbered counter-clockwise as viewed from the propeller shaft end, number one being the top or vertical cylinder. The usual cylinder numbering of both single- and double-row radial engines is illustrated in figures 13 and 14.

Figure 7. -  Left front view of typical V-type engine.

17. Firing orders.      a. The order in which firing impulses take place in any internal-combustion engine depends upon the type and design. The following factors are the most important features which govern the firing order:

FIGURE 8. - Rear view of typical double-row radial engine

FIGURE 9. - Side view of typical double-row radial engine

(1) Primary factors.

(a) Number of cylinders to be fired.

(b) Arrangement of cylinders on the crankcase.

(c) Direction of rotation of the crankshaft.

(d) Crankshaft type.

(2) Secondary factors.

(a) Type and arrangement of the cam mechanism.

(b) Design of the induction system.

(c) Desirability of minimum stresses obtainable through a logical order of firing impulses.

b. Figures 10 to 14 inclusive illustrate the most logical firing orders of conventional engines.

Figure: 10. - Cylinder numbering and firing order of 4-cylinder in-line engine.

Figure: 11.--Cylinder numbering and firing order of 6-cylinder in-line engine.

FIGURE 12. - Cylinder numbering and firing order of 12-cylinder V-type engine.

FIGURE 13. -Cylinder numbering : I firing order of 9-cylinder single-row radial engine.

Figure. 14.   - Cylinder numbering and firing order of 14-cylinder double-row radial engine.