Reciprocating Engines Study Guide Ch. 1 PDF

STUDY UNIT ONE INTRODUCTION TO RECIPROCATING ENGINES 1.1 The Four Stroke Cycle 1.2 Valves 1.3 Pistons and Cylinders This study unit contains outlines of major concepts tested, a portion of the FAA test questions and answers regarding r...

STUDY UNIT ONE INTRODUCTION TO RECIPROCATING ENGINES 1.1 The Four Stroke Cycle 1.2 Valves 1.3 Pistons and Cylinders This study unit contains outlines of major concepts tested, a portion of the FAA test questions and answers regarding reciprocating engines, and an explanation of each answer. The remaining topics on reciprocating engines are covered in Study Unit 2. CAUTION: Recall that the sole purpose of this course is to expedite your passing the FAA knowledge test. Accordingly, all extraneous material (i.e., topics or regulations not directly tested on the knowledge test) is omitted, even though much more information and knowledge are necessary to perform a mechanic’s duties. Click here to access the FAA Powerplant Handbook. 1.1 THE FOUR STROKE CYCLE 1. The sequence of events in a four stroke cycle reciprocating engine occurs as follows: a. Intake of the fuel-air charge as the piston moves downward with the intake valve open. b. Compression of that charge as the piston moves upward with intake and exhaust valves both closed. c. Before the piston reaches top dead center on the compression stroke, the ignition event occurs. 1. Usually, ignition is timed to occur 20° to 30° before top dead center. d. The power stroke happens as the piston is driven downward by the pressure of hot gases. 1. Combustion should be complete just after top center on the power stroke. e. Finally, during the exhaust stroke, the upward motion of the piston drives the exhaust gases out of the cylinder. 2. As the piston nears the end of the exhaust stroke, the intake valve begins to open. This maximizes the fuel-air charge drawn into the cylinder on the intake stroke. a. However, if the intake valve opens too early, hot combustion gases may cause a backfire through the induction system. b. The intake valve closes just after bottom dead center on the compression stroke. 3. The exhaust valve opens before bottom dead center on the power stroke. a. The exhaust valve remains open briefly as the piston starts down on the intake stroke, allowing exhaust gases to exit under their own inertia. https://www.gleim.com/aviation/avmech/amp/main.php?showAction=show&action=Continue 11/15/24, 3 34 PM Page 1 of 4 :  4. Valve overlap is defined as the number of degrees of crankshaft rotation during which both valves are off their seats. a. Valve overlap occurs at the end of the exhaust stroke and beginning of the intake stroke. 1.2 VALVES 1. Proper valve timing and valve overlap will increase volumetric efficiency and lower cylinder operating temperatures. a. This also promotes better scavenging of the exhaust gases. 2. The operating temperature valve clearance is greater than the cold valve clearance due to the thermal expansion of the cylinder head. a. Therefore, setting hot clearance values on a cold engine will result in excessive valve clearance once the engine is hot. 1. The valve will open late and close early. a. The total duration of open valve time will be shortened. 2. Excessive valve clearance reduces the valve overlap period, reducing volumetric efficiency and power. 3. If valve clearance is inadequate, the valves will not seat positively during start and engine warm-up. a. If the exhaust valve clearance is inadequate, the exhaust valve will not be on its seat long enough to transfer heat properly, resulting in a burned exhaust valve. 4. On an engine equipped with a floating cam ring, the cam bearing clearance must be eliminated when making valve adjustments. 5. Hydraulic valve lifters are designed to operate with zero clearances. They eliminate the need for the regular adjustment of valve clearances that is required with solid lifters. a. Hydraulic lifters use oil pressure to achieve zero valve clearance. When they are flat and empty, a specified amount of valve clearance should be measurable. b. If the clearance on hydraulic valve lifters is out of tolerance, change out the push rod for one of correct length. 6. Metallic-sodium filled exhaust valves reduce operating temperatures: the sodium inside melts at operating temperature, carrying heat away from the head of the valve. 7. Some valve stems have a safety circlet installed to prevent the head from falling into the combustion chamber in the event that the valve stem breaks. 8. Using two or more valve springs together, with different coil diameters and pitches, eliminates valve spring surge, allowing proper spring action throughout the RPM range of the engine. 9. Valves that are ground to a feather edge may cause preignition. a. Also, the feather edge will burn, requiring frequent repair or replacement. 10. A contour or radius gauge is used to check valves for stretch. 11. Worn valve guides allow oil in the rocker arm boxes to pass between the valve stems and the valve guides, https://www.gleim.com/aviation/avmech/amp/main.php?showAction=show&action=Continue 11/15/24, 3 34 PM Page 2 of 4 :  leading to high oil consumption. 1.3 PISTONS AND CYLINDERS 1. The piston displacement of a single cylinder is simply the cross-sectional area of the cylinder multiplied by the stroke length. a. Multiply this by the number of cylinders to get to the total piston displacement for the entire engine. b. The three or four digit number included in an engine designation usually indicates the total displacement of the engine in cubic inches. 1. For example, “R985-22” is an engine with 985 cubic inches of total piston displacement. 2. A cam ground piston provides a better fit at operating temperatures. Its shape is such that, when it expands at operating temperature, its mass expands around the wrist pin boss to make the piston nearly round. 3. Full floating pistons pins can rotate freely in position at both the piston and the connecting rod. 4. Piston rings provide the seal between the piston and the cylinder. a. Cast iron rings may be used in chrome plated cylinders, and chrome plated rings may be used in plain steel cylinders. However, chrome rings cannot be used in chrome cylinders. b. End-gap clearance is measured by placing the rings in the cylinder and measuring the end gap with a feeler gauge. 5. Piston velocity is always at a maximum 90° after top dead center and after bottom dead center. a. Piston velocity is at a minimum at top dead center and bottom dead center, where the piston is changing direction. b. The above applies regardless of stroke or RPM. 6. The compression ratio compares the volume of the cylinder with the piston at bottom dead center with its volume of the space at top dead center. a. Generally, higher compression ratios mean increased power. b. The maximum compression ratio is impacted by the detonation characteristics of the fuel used, the design limitations of the engine, and the degree of supercharging. 7. Volumetric efficiency refers to the ratio of the fuel-air mass actually taken into the cylinder to the mass that would fill the cylinder volume under the existing atmospheric conditions. a. The closer this ratio is to 1, the higher the efficiency. b. Improper valve timing, sharp bends in the induction system, and high carburetor air temperatures will all decrease the mass of fuel-air entering the cylinder and therefore decrease the volumetric efficiency of the engine. c. Full throttle operation and low cylinder head temperature both increase volumetric efficiency. 8. On a cylinder wall, the greatest amount of wear occurs near the top of the cylinder where the heat is the greatest. a. Nitriding is utilized to harden cylinders and increase their resistance to wear in their extreme environment. 1. Because of this and their relatively thin walls, aircraft cylinders have smaller oversizes than automobile cylinders. https://www.gleim.com/aviation/avmech/amp/main.php?showAction=show&action=Continue 11/15/24, 3 34 PM Page 3 of 4 :  9. Choked or taper-ground cylinders are slightly smaller near the top where they screw into the cylinder head than at the middle of the cylinder. When operating temperatures are achieved, this shape yields a more nearly straight bore and better fit for the pistons. 10. When removing a cylinder, first rotate the crankshaft so that the piston is at top dead center. 11. One of the best indicators of combustion chamber problems is the spark plug condition. Copyright © 1995-2024 Gleim Publications, Inc. and/or Gleim Internet, Inc. Terms and Conditions of Service | Privacy Policy Phone: (800) 874-5346 | Fax: (352) 375-6940 https://www.gleim.com/aviation/avmech/amp/main.php?showAction=show&action=Continue 11/15/24, 3 34 PM Page 4 of 4 :