Reciprocating Engines - Powerplant Written
If the crankshaft runout readings on the dial indicator are plus .002 inch and minus .003 inch, the runout is
.005 inch. ] - Crankshaft run-out is measured by clamping a dial indicator to a solid part of the engine and placing the arm of the indicator against the part of the crankshaft where the run-out reading is to be measured. Place the indicator at Zero with the arm against the crankshaft. Rotate the crankshaft for a complete revolution. The total run-out is the difference between the negative and the positive readings. If the positive reading is +0.002 and the negative reading is -0.003, the total run-out is five thousandths of an inch (0.005 inch).
Engine crankshaft runout is usually checked 1. during engine overhaul. 2. during annual inspection. 3. after a 'prop strike' or sudden engine stoppage. 4. during 100-hour inspection.
1 and 3. ] - Crankshaft run-out is checked to determine whether the crankshaft of a reciprocating engine is bent. Crankshaft run-out is checked during each engine overhaul and after each sudden stoppage of the engine.
Which of these conditions will cause an engine to have an increased tendency to detonate? 1. High manifold pressure. 2. High intake air temperature. 3. Engine overheated. 4. Late ignition timing.
1, 2, 3.] - Detonation occurs when the fuel-air mixture burning in a cylinder reaches its critical pressure and temperature. Detonation may be caused by the high pressure and temperature resulting from high manifold pressure, high intake air temperature, or an overheated engine. Late ignition timing reduces engine power but it does not cause detonation.
A nine-cylinder engine with a bore of 5.5 inches and a stroke of 6 inches will have a total piston displacement of
1,283 cubic inches. ] - The piston displacement of a reciprocating engine is the total volume swept by the pistons in one revolution of the crankshaft. Find the piston displacement of one cylinder by multiplying the area of the piston in square inches by the stroke, which is measured in inches. The total piston displacement is the volume of one cylinder, measured in cubic inches, multiplied by the number of cylinders. Area = 0.7854 x bore^2 = 0.7854 x 30.25 = 23.75 square inches Volume = piston area x stroke = 23.75 x 6 = 142.55 cubic inches Piston displacement = volume x number of cylinders = 142.55 x 9 = 1,282.95 cubic inches
Using the following information, determine how many degrees the crankshaft will rotate with both the intake and exhaust valves seated. Intake opens 15°: BTDC. Exhaust opens 70°: BBDC. Intake closes 45°: ABDC. Exhaust closes 10°: ATDC.
245°. ] - The intake valve closes 45° of crankshaft rotation after the piston passes bottom dead center, moving upward on the compression stroke. Both valves are closed at this point, and they both remain closed until the piston passes over top center and comes down to 70° before bottom dead center on the power stroke. At this time the exhaust valve opens. Both valves are on their seats for 45° + 180° + 20°, or 245°.
Which of the following will decrease volumetric efficiency in a reciprocating engine? 1. Full throttle operation. 2. Low cylinder head temperatures. 3. Improper valve timing. 4. Sharp bends in the induction system. 5. High carburetor air temperatures.
3, 4, and 5.] - The volumetric efficiency of a reciprocating engine is the ratio of the weight of the fuel-air charge taken into the cylinder, to the weight of a charge that would completely fill the entire volume of the cylinder at the same pressure. Anything that decreases the weight of the air entering the cylinder decreases the volumetric efficiency. Improper valve timing, sharp bends in the induction system, and high carburetor air temperature will all decrease the volumetric efficiency.
Ignition occurs at 28° BTDC on a certain four-stroke cycle engine, and the intake valve opens at 15° BTDC. How many degrees of crankshaft travel after ignition does the intake valve open? (Consider one cylinder only.)
373°. ] - The crankshaft rotates 28° on the compression stroke after the ignition occurs. The crankshaft rotates 180° on the power stroke. The crankshaft rotates 165° on the exhaust stroke before the intake valve opens. The total crankshaft rotation between the time ignition occurs and the time the intake valve opens is: 28° + 180° + 165° = 373°.
At what speed must a crankshaft turn if each cylinder of a four-stroke cycle engine is to be fired 200 times a minute?
400 RPM.] - Each cylinder in a four-stroke-cycle engine fires every other revolution of the crankshaft. If a cylinder is to fire 200 times in one minute, the engine will have to be turning at 400 RPM.
The volume of a cylinder equals 70 cubic inches when the piston is at bottom center. When the piston is at the top of the cylinder, the volume equals 10 cubic inches. What is the compression ratio?
7:1 - The compression ratio of a reciprocating engine is the ratio of the volume of a cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke. If the cylinder has a volume of 70 cubic inches with the piston at the bottom of its stroke and 10 cubic inches with the piston at the top of its stroke, the compression ratio is 7:1.
Which fuel/air mixture will result in the highest engine temperature (all other factors remaining constant)?
A mixture leaner than a manual lean mixture of .060. ] - Lean mixtures burn more slowly than rich mixtures and a mixture leaner than the manual-lean mixture of 0.060, or approximately 17:1, will possibly be burning as the gases are forced out past the exhaust valve. This will cause serious overheating of the exhaust valve.
By use of a differential pressure compression tester, it is determined that the No. 3 cylinder of a nine-cylinder radial engine will not hold pressure after the crankshaft has been rotated 260° from top dead center compression stroke No. 1 cylinder. How can this indication usually be interpreted?
A normal indication. ] - In a nine-cylinder radial engine, each cylinder fires 80° of crankshaft rotation after the cylinder before it, in firing order. When the crankshaft is rotated 260° after the piston in cylinder number one is at top dead center on its compression stroke, the piston in cylinders 7 and 8 are near the top of their strokes. The piston in cylinder 3 is near the bottom of its power stroke and its exhaust valve is open. It is normal for a cylinder not to hold air pressure when its piston is near the bottom of its power stroke and its exhaust valve is open.
What will be the likely result if the piston ring gaps happen to be aligned when performing a differential-pressure compression check on a cylinder?
A worn or defective ring(s) indication. ] - The joints of the piston rings must be staggered around the circumference of the piston in which they are installed to reduce blowby. If the gaps are not staggered, a differential compression check will give the indication of worn or defective rings.
When does valve overlap occur in the operation of an aircraft reciprocating engine?
At the end of the exhaust stroke and the beginning of the intake stroke] - Both the intake and exhaust valve are open at the same time, only during the period of valve overlap. Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke. The intake valve opens a few degrees of crankshaft rotation before the piston reaches the top of the exhaust stroke. The exhaust valve remains open until the piston has moved down a few degrees of crankshaft rotation on the intake stroke.
What does valve overlap promote?
Better scavenging and cooling characteristics. - Valve overlap is the portion of crankshaft rotation during which both the intake and the exhaust valves are off of their seats at the same time. Adequate valve overlap increases the volumetric efficiency of the engine. It aids in the scavenging of the burned exhaust gases, and it gives the engine better cooling characteristics. This is done by ensuring that the fuel-air charge in the cylinder is rich enough for proper operation and not diluted with exhaust gases.
To what altitude will a turbo charged engine maintain sea level pressure?
Critical altitude. - The critical altitude of a turbocharged aircraft engine is the altitude above which the turbocharger can no longer produce sea level manifold pressure and the engine cannot maintain its rated horsepower.
What is required by 14 CFR Part 43 Appendix D when performing an annual/100-hour inspection on a reciprocating engine aircraft?
Cylinder compression check.] - The only alternative listed here that must be included in a 100-hour inspection as specified in 14 CFR Part 43, Appendix D, is the cylinder compression check. A compression check, and specifically a differential compression check, tells much about the internal condition of the cylinders. It gives an indication of the seal provided by the valves and the condition of the piston rings.
Increased water vapor (higher relative humidity) in the incoming air to a reciprocating engine will normally result in which of the following?
Decreased engine power at a constant RPM and manifold pressure.] - The amount of energy released by a burning fuel-air mixture is determined by the weight of both the fuel and the air in the mixture. Water vapor weighs only about 5/8 as much as dry air, and when an engine takes in air with a high relative humidity, it produces less power at the same RPM and manifold pressure than it would produce if it were taking in dry air.
Which of the following is a characteristic of a thrust bearing used in most radial engines?
Deep-groove ball. ] - Deep-groove ball bearings are used as the thrust bearing in most radial engines. This type of bearing is the best of those listed for reducing friction while carrying both thrust and radial loads.
An aircraft reciprocating engine using hydraulic valve lifters is observed to have no clearance in its valve-operating mechanism after the minimum inlet oil and cylinder head temperatures for takeoff have been reached. When can this condition be expected?
During normal operation. ] - There is no clearance in the valve-operating mechanism when an engine equipped with hydraulic valve lifters is operating normally and the minimum oil and cylinder-head temperatures for takeoff have been reached. Hydraulic valve lifters are used because they remove all of the clearance between the rocker arm and the tip of the valve stem. By keeping all of this clearance removed, the valves operate with less noise and less wear.
Which of the following engine servicing operations generally requires engine pre-oiling prior to starting the engine?
Engine installation.] - When a new or freshly overhauled engine is installed in an aircraft, it must be pre-oiled. The oil tank is filled, and oil is pumped through all the passages until pressure registers on the oil pressure gauge in the cockpit. Pre-oiling ensures that all the bearings will be adequately lubricated before the oil pump begins to pump oil through the system normally.
What is the best indication of worn valve guides?
High oil consumption. ] - High oil consumption is the only alternative that would indicate worn valve guides. When the valve guide wears, oil from the rocker box flows down the valve stem and is burned.
Which of the following would most likely cause a reciprocating engine to backfire through the induction system at low RPM operation?
Lean mixture. ] - A lean fuel-air mixture burns slower than either a rich or a chemically-correct mixture. There is a possibility that a lean mixture will still be burning as it is pushed out through the exhaust valve. During the time of valve overlap, when both the intake and the exhaust valves are open, the burning exhaust gases can ignite the fresh fuel-air charge being taken into the cylinder through the intake valve. This can cause a backfire through the induction system.
Which condition would be the least likely to be caused by failed or failing engine bearings?
Low oil temperatures.] - All of the alternatives except low oil temperature would likely be caused by failed or failing engine bearings in a reciprocating engine. Low oil temperature would be the least likely of these alternatives.
Which bearing is least likely to be a roller or ball bearing?
Master rod bearing (radial engine) ] - The master rod bearing in a radial engine is always a plain bearing. Rocker arm bearings may be either ball, roller, or plain type and the crankshaft main bearings for radial engines are usually ball bearings.
Which statement is correct regarding engine crankshafts?
Moveable counterweights serve to reduce the torsional vibrations in an aircraft reciprocating engine.] - Torsional vibration caused by firing impulses of the engine are minimized by the installation of moveable counterweights suspended from certain crank cheeks. These moveable counterweights, called dynamic dampers, rock back and forth and act as pendulums, changing the resonant frequency of the rotating elements, thus reducing the torsional vibration.
On which part of the cylinder walls of a normally operating engine will the greatest amount of wear occur?
Near the top of the cylinder.] - In normal operation, an aircraft engine cylinder wears more at the top than in the center or at the bottom. This greater wear is caused by the heat of combustion decreasing the efficiency of the lubrication at the top of the cylinder.
After spark plugs from an opposed engine have been serviced, in what position should they be reinstalled?
Next in firing order to the one from which they were removed and swapped bottom to top. ] - When spark plugs have been cleaned, gapped and tested, they should be installed in the cylinder next in firing order to the one from which they were removed, and they should be swapped from bottom to top.
What is likely to occur if a reciprocating engine is operated at high power settings before it is properly warmed up?
Oil starvation of bearings and other parts. ] - No aircraft engine should be operated at high power settings before it is properly warmed up and the oil is warm enough to flow freely through all the passages. High power operation with cold oil can cause oil starvation to the bearings.
How may it be determined that a reciprocating engine with a dry sump is pre-oiled sufficiently?
Oil will flow from the engine return line or indicator port. ] - When pre-oiling a dry-sump reciprocating engine, you know there is oil in all the passages when oil flows from the engine return line or from the port to which the oil pressure gauge is connected]
If air is heard coming from the crankcase breather or oil filler during a differential compression check, what is this an indication of?
Piston ring leakage. ] - A hissing sound heard at the crankcase breather during a differential compression check is caused by air leaking past the piston rings. Exhaust valve leakage is heard at the exhaust stack or muffler and intake valve leakage is heard at the carburetor inlet.
Which type of bearings are generally used for connecting rods and cam shafts?
Plain. ] - Master rods used in radial engines have plain bearings in both their big end that fits around the throw of the crankshaft and the small end that fits around the wrist pin in the piston.
What could cause excessive pressure buildup in the crankcase of a reciprocating engine?
Plugged crankcase breather. ] - The crankcase of a reciprocating engine is vented to the outside air through a breather pipe. If the breather should ever become plugged, the crankcase pressure can build up to a point at which the oil consumption increases drastically. It is possible that the oil can then be forced into engine accessories damaging them.
What is the basic operational sequence for reducing the power output of an engine equipped with a constant-speed propeller?
Reduce the manifold pressure, then the RPM. ] - When reducing the power of an engine equipped with a constant-speed propeller, it is important that the manifold pressure be reduced by retarding the throttle before the RPM is reduced with the propeller pitch control. If the wrong sequence is used, the high manifold pressure and the low RPM can produce cylinder pressures high enough to seriously damage the engine.
What is an advantage of using metallic-sodium filled exhaust valves in aircraft reciprocating engines?
Reduced valve operating temperatures. ] - Some aircraft engine exhaust valves are hollow and are partially filled with metallic sodium. When the engine is operating, the sodium melts and as the valve opens and closes, the molten sodium sloshes back and forth in the valve. When it is in the head, it absorbs heat. When it is in the stem, it transfers this heat to the valve guides. Sodium-filled valves reduce the valve operating temperature.
Which of the following will be caused by excessive valve clearance of a cylinder on a reciprocating aircraft engine?
Reduced valve overlap period. - If both the intake and exhaust valves in a cylinder have excessive clearance, the valve overlap period will be reduced. Valve overlap is the time between the end of the exhaust stroke and the beginning of the intake stroke when both valves are off of their seats. If the intake valve clearance is too great, the intake valve will open late. If the exhaust valve clearance is too great, the exhaust valve will close early. Late opening of the intake valve and early closing of the exhaust valve, shorten the period of valve overlap.
When is the fuel/air mixture ignited in a conventional reciprocating engine?
Shortly before the piston reaches the top of the compression stroke. ] - Ignition occurs in a reciprocating engine somewhere around 30° of crankshaft rotation before the piston reaches top center on the compression stroke. By timing the ignition to occur when the piston is in this position, the maximum pressure inside the cylinder is reached just after the piston passes over top center and starts down on the power stroke.
Which statement is correct regarding a four-stroke cycle aircraft engine?
The intake valve closes on the compression stroke. ] - The intake valve in a four-stroke-cycle aircraft engine closes somewhere around 60° after bottom center on the compression stroke. The exhaust valve opens about 70° before bottom center on the power stroke. The intake valve opens about 20° before top center on the exhaust stroke. The exhaust valve closes about 15° after top center on the intake stroke.
Which statement pertaining to fuel/air ratios is true?
The mixture ratio which gives the best power is richer than the mixture ratio which gives maximum economy. ] - The fuel-air mixture used for the engine to produce its best power is richer (there is more fuel for the air) than a mixture that gives the maximum economy. The best power mixture is about a 12:1 mixture (12 parts of air to one part of fuel), and the maximum economy mixture is about 16:1.
As the pressure is applied during a reciprocating engine compression check using a differential pressure tester, what would a movement of the propeller in the direction of engine rotation indicate?
The piston was positioned past top dead center. - When performing a differential compression check on a reciprocating engine, the piston of the cylinder being tested is placed on top center of the compression stroke and air is put into the cylinder. If the air causes the propeller to turn in the direction of normal rotation, the piston is not on top dead center, but is slightly past top center.
Why does the smoothness of operation of an engine increase with a greater number of cylinders?
The power impulses are spaced closer together. ] - One of the main factors that affect the smoothness of operation of a reciprocating engine is the closeness with which the power impulses are spaced. The greater the number of cylinders, the closer the power impulses are together and the smoother the engine will operate.
Which of the following is most likely to occur if an overhead valve engine is operated with inadequate valve clearances?
The valves will not seat positively during start and engine warmup.] - Overhead valves in an air-cooled engine have their smallest clearance when the engine is cold. This clearance opens up to several times the cold clearance when the engine is at its operating temperature. If the valve clearance is too small, the valves will likely not seat positively when the engine is cold during start and engine warm-up.
If the hot clearance is used to set the valves when the engine is cold, what will occur during operation of the engine?
The valves will open late and close early. ] - The cylinder head of an air-cooled engine expands much more than the pushrod. Because of this, air-cooled engines equipped with solid valve lifters (this applies primarily to radial engines) have a much larger valve clearance when the engine is hot than when it is cold. If the valves are adjusted to the hot (running) clearance when the cylinder is cold, the clearance in the valve train will be too great when the engine is at its normal operating temperature. The valves will open late and close early. The cam will have to turn farther to open the valve and the valve will close before the cam has turned to the normal valve-closing position.
How many of the following are factors in establishing the maximum compression ratio limitations of an aircraft engine? 1. Detonation characteristics of the fuel used. 2. Design limitations of the engine. 3. Degree of supercharging. 4. Spark plug reach.
Three. ] - The maximum compression ratio of an engine is limited by the ability of the engine to withstand detonation in its cylinders. Of the alternatives given with this question, three of them are factors affecting the engine's ability to withstand detonation. The detonation characteristics of the fuel used is a limiting factor. Fuels having a low critical pressure and temperature must not be used with high compression engines. The design limitations of the engine are important, because engines that are not designed strong enough to withstand high cylinder pressures, must not have a high compression ratio. The degree of supercharging is extremely important, because the cylinder pressures are a function of both the initial pressure in the cylinder (the pressure caused by the supercharger) and the compression ratio. The only alternative that does not limit the compression ratio is the spark plug reach.
What tool is generally used to measure the crankshaft rotation in degrees?
Timing disk. ] - A top dead center indicator is used to show when the piston in cylinder number one is on top dead center. A timing disk is clamped to the propeller shaft and positioned so the pointer, which is held straight up by a weight on one end, points to zero degrees. As the crankshaft is rotated, the pointer indicates on the scale of the timing disk the number of degrees the crankshaft has rotated.
What is the purpose of a power check on a reciprocating engine?
To determine satisfactory performance. ] - A power check of a reciprocating engine is a check to determine that the engine is developing the correct static RPM and manifold pressure. The purpose of this check is to determine that the engine is performing satisfactorily.
What is the purpose of installing two or more springs on each valve in an aircraft engine?
To eliminate valve spring vibration or surging. ] - Every mechanical device has a resonant frequency. If the valve is operating at the resonant frequency of the valve spring, the spring will lose its effectiveness and will surge, allowing the valve to float. By using two or more valve springs wound with a different pitch and a different size wire, the resonant frequency of the springs will be different and there will be no engine RPM at which point, the valves will float.
What is the principal advantage of using propeller reduction gears?
To enable the engine RPM to be increased with an accompanying increase in power and allow the propeller to remain at a lower, more efficient RPM. ] - The horsepower produced by a reciprocating engine is determined by its RPM. The higher the RPM, the greater the power. But the efficiency of a propeller decreases as the blade tip speed approaches the speed of sound. In order to get the best of both conditions, many of the more powerful aircraft engines drive the propeller through a set of reduction gears. Reduction gears allow the engine to turn fast enough to develop the required power. At the same time, the propeller tip speed is kept low enough that the tips do not approach the speed of sound.
What is the purpose of the safety circlet installed on some valve stems?
To prevent valves from falling into the combustion chamber. ] - Some aircraft-engine poppet valves have a groove cut in their stem that is fitted with a safety circlet, a small snap ring that grips the valve stem in this groove. If the tip of the valve stem should ever break off in operation, this safety circlet will contact the top of the valve guide and prevent the valve from dropping into the cylinder.
If an engine cylinder is to be removed, at what position in the cylinder should the piston be?
Top dead center.] - When removing a cylinder from an aircraft engine, rotate the crankshaft until the piston is at top center on the compression stroke. In this position, the pushrods can be most easily removed. The piston will be all the way out of the crankcase so the wrist pin can be slipped out to remove the piston with the cylinder.
Which of the following conditions would most likely lead to detonation?
[ Use of fuel with too low an octane rating.] - Detonation is an uncontrolled burning of the fuel inside the engine cylinders. The fuel-air mixture actually explodes, rather than burning evenly as it should. The octane rating of a fuel is a measure of its detonation resistance. If a fuel with too low an octane rating is used, the fuel-air mixture is likely to detonate when the engine is developing full power.
One cause of afterfiring in an aircraft engine is
[An excessively rich mixture.] - After-firing, or torching, is the burning of the fuel-air mixture in the exhaust manifold after the mixture has passed through the exhaust valve. After-firing is usually caused by operation with an excessively rich mixture, such as would be caused by overpriming, improper use of the mixture control when starting, or by poor ignition.
How is proper end-gap clearance on new piston rings assured during the overhaul of an engine?
[By placing the rings in the cylinder and measuring the end-gap with a feeler gauge.] - The end gap in piston rings is measured by placing the piston ring inside the cylinder and pushing it up with the top of the piston so that it is square in the cylinder bore and in line with the cylinder flange. With the ring in this position, measure the distance between the two ends of the ring with a feeler gauge.
On which strokes are both valves on a four-stroke cycle reciprocating aircraft engine open?
[Exhaust and intake.] - Both the intake and exhaust valve are open at the same time only during the period of valve overlap. Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke. The intake valve opens a few degrees of crankshaft rotation before the piston reaches the top of the exhaust stroke. The exhaust valve remains open until the piston has moved down a few degrees of crankshaft rotation on the intake stroke.
Standard aircraft cylinder oversize's usually range from 0.010 inch to 0.030 inch. Oversize on automobile engine cylinders may range up to 0.100 inch. This is because aircraft engine cylinders
[Have relatively thin walls and may be nitrided.] - Thin-walled aircraft-engine cylinders may be ground oversize, but the amount they may be ground is much less than is allowed for the much thicker-walled cylinders normally used in automobile engines. Some aircraft-engine cylinders may not be re-bored at all. The engine manufacturer's recommendations must be followed in detail regarding any re-boring operation.
Which of the following would indicate a general weak-engine condition when operated with a fixed-pitch propeller or test club?
[Lower than normal static RPM, full throttle operation.] - The condition of an engine is shown by a full power check made with a fixed-pitch propeller or test club which furnishes a constant and a known load on the engine. If the engine is not producing its full power, it will not produce the correct static RPM at full throttle. The static RPM will be too low, and low static RPM with a fixed propeller load indicates a 'weak' engine.
Compression ratio is the ratio between the
[cylinder volume with piston at bottom dead center and at top dead center.] -The compression ratio of a reciprocating engine is the ratio of the volume of the cylinder with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke.
Excessive valve clearances will cause the duration of valve opening to
[decrease for both intake and exhaust valves.] - Excessive valve clearance will cause the valves to remain open for a shorter period of time than they would have with a normal clearance. The cam must turn farther to open the valve and the valve will close before the cam has turned to the correct valve-closing position.
Some cylinder barrels are hardened by
[nitriding.] - The walls of an aircraft-engine cylinder are subjected to a great deal of wear as the iron piston rings rub against them. The walls of some cylinders are treated to increase their hardness and resistance to wear. There are two methods of hardening these surfaces: hard-chrome-plating and nitriding. Nitriding is a process in which the surface of the steel cylinder wall is changed into a hard nitride by an infusion of nitrogen from the ammonia gas used in the nitriding heat treatment process.
During ground check an engine is found to be rough-running, the magneto drop is normal, and the manifold pressure is higher than normal for any given RPM. The trouble may be caused by
a dead cylinder.] - A dead cylinder will cause an engine to run rough. Because the throttle will have to be opened farther to get the same RPM, the manifold pressure will be higher than it would on an engine with all the cylinders firing. A dead cylinder will not show up on a magneto check.
The valve clearance of an engine using hydraulic lifters, when the lifters are completely flat, or empty, should not exceed
a specified amount above zero. - Hydraulic valve lifters are used to keep all of the clearance out of the valve system when the engine is operating and the lifters are pumped up. When the lifters are completely flat, there will be clearance in the system of a specified amount above zero.
Backfiring through the carburetor generally results from the use of
an excessively lean mixture. ] - Backfiring through the carburetor is often caused by the use of an extremely lean mixture. A lean mixture burns slowly. If it is still burning when the intake valve opens, the burning mixture will ignite the fresh fuel-air charge and cause a backfire in the induction system.
If an engine with a stroke of 6 inches is operated at 2,000 RPM, the piston movement within the cylinder will be
at maximum velocity 90° after TDC.]- The piston in a reciprocating engine is not moving when it is at the top and bottom of its stroke. As it leaves top dead center, it accelerates from zero velocity to a maximum velocity, which is reached when it is 90° beyond top dead center. It then decelerates to zero velocity at bottom dead center.
If the intake valve is opened too early in the cycle of operation of a four-stroke cycle engine, it may result in
backfiring into the induction system. ] - The intake valve opens when the piston is moving upward at the end of the exhaust stroke. Opening at this point allows the low pressure caused by the inertia of the exiting exhaust gases to assist in starting the fuel-air mixture flowing into the cylinder. If the intake valve opens too early, some of the burning exhaust gases could flow into the intake manifold and ignite the mixture. This would cause a backfire in the induction system.
During the inspection of an engine control system in which push-pull control rods are used, the threaded rod ends should
be checked for the amount of thread engagement by means of the inspection holes.] - When installing push-pull control rods in an aircraft, you can determine that the rod end is properly screwed onto the rod by trying to pass a piece of safety wire through the inspection hole in the rod. If the rod end is screwed into the rod far enough to cover the hole, there are enough threads engaged to give the connection the full strength required.
Direct mechanical push-pull carburetor heat control linkages should normally be adjusted so that the stop located on the diverter valve will be contacted
before the stop at the control lever is reached in both HOT and COLD positions. ] - When rigging any engine control in an aircraft, the stop on the component being actuated must be contacted before the stop in the cockpit. The control linkage has enough spring-back in both directions that after the stop on the diverter valve is contacted, the control can be moved in both directions until it contacts the stop in the cockpit. When the control is released, it will spring back a few degrees.
(1) Cast iron piston rings may be used in chrome-plated cylinders. (2) Chrome-plated rings may be used in plain steel cylinders. Regarding the above statements,
both No. 1 and No. 2 are true. ] - Statement (1) is true. Only cast iron piston rings can be used in nitrided or chrome-plated cylinders. Statement (2) is also true. Chrome plated rings can be used in plain steel cylinders.
Full-floating piston pins are those which allow motion between the pin and
both the piston and the small end of the connecting rod. ] - A full-floating piston pin is free to rotate in both the piston and the small end of the connecting rod. Full-floating piston pins are usually a push fit in the piston. They are kept from damaging the cylinder walls as they move up and down by soft aluminum or brass plugs in the ends of the pin.
The actual power delivered to the propeller of an aircraft engine is called
brake horsepower. ] - The actual horsepower delivered to the propeller of an aircraft engine is called brake horsepower. This name is used because brake horsepower was originally measured with a prony brake loading the engine with mechanical friction. Modern measurements of brake horsepower are made with a dynamometer which loads the engine with electrical or fluid-flow opposition.
An engine misses in both the right and left positions of the magneto switch. The quickest method for locating the trouble is to
check for one or more cold cylinders.] - If an engine misses on both magnetos, the quickest way to find the cylinder that is not firing is by running the engine at the RPM at which it misses the most consistently and by feeling the exhaust stack at the cylinder head. The exhaust stack of the cylinder that is not firing will be much cooler than those of the cylinders that are firing normally.
If the exhaust valve of a four-stroke cycle engine is closed and the intake valve is just closed, the piston is on the
compression stroke. ] - The intake valve closes when the piston is moving upward on the compression stroke. At this time, the exhaust valve is already closed.
The floating control thermostat, used on some reciprocating engine installations, helps regulate oil temperature by
controlling air flow through the oil cooler. ] - The floating-control thermostat controls the oil cooler air-exit door. It maintains the oil temperature within the desired limits by controlling the air flow through the oil cooler.
Excessive valve clearance in a piston engine
decreases valve overlap.] - Excessive valve clearance in a reciprocating engine will decrease the valve overlap. The intake valve will open late and the exhaust valve will close early.
If the ignition switch is moved from BOTH to either LEFT or RIGHT during an engine ground check, normal operation is usually indicated by a slight
drop in torque meter pressure indication. ] - All certificated aircraft reciprocating engines have dual ignition. During normal operation, both ignition systems are operating. During a magneto and engine ground check, the engine is operated at the speed specified by the engine manufacturer and the ignition system is switched from BOTH magnetos to each magneto separately. When the engine operates on a single magneto, the fuel-air mixture in the cylinder is ignited at one point only causing a slight drop in engine power. The RPM will drop slightly. When the magneto check is performed, a drop in torquemeter pressure indication is a good supplement to the variation in RPM.
Valve overlap is defined as the number of degrees of crankshaft travel
during which both valves are off their seats. ] - Valve overlap is the number of degrees of crankshaft rotation that both the intake and exhaust valves are off their seat at the end of the exhaust stroke and the beginning of the intake stroke. Valve overlap allows a greater charge of fuel-air mixture to be inducted into the cylinder.
If the oil pressure of a cold engine is higher than at normal operating temperatures, the
engine's lubrication system is probably operating normally. ] - Many large aircraft reciprocating engines have a compensating oil pressure relief valve that allows the oil pressure for cold oil to be considerably higher than it allows for warm oil. This higher pressure allows the thicker, higher viscosity oil to be forced through the engine bearings. The plunger of the oil-pressure relief valve is held down by two springs when the oil is cold. However, when the oil warms up, a thermostatic valve opens and allows oil pressure to remove the force of one of the springs. For normal operation, only one spring holds the pressure relief valve on its seat.
Some aircraft-engine poppet valves have a groove cut in their stem that is fitted with a safety circlet, a small snap ring that grips the valve stem in this groove. If the tip of the valve stem should ever break off in operation, this safety circlet will contact the top of the valve guide and prevent the valve from dropping into the cylinder.
excessively enriching the fuel/air mixture.] - An aircraft engine produces power by converting the chemical energy in the fuel into heat energy as the fuel-air mixture is burned inside the engine cylinders. The efficiency of this energy interchange is determined by the ratio between weight of the air and the weight of the fuel in the mixture. The air at high altitude is less dense (weighs less) than the air at sea level, and the fuel metered into the same volume of air will cause the fuel-air mixture at high altitude to become excessively rich. There will be too many pounds of fuel per pound of air for the most efficient production of power.
A hissing sound from the exhaust stacks when the propeller is being pulled through manually indicates
exhaust valve blow-by. ] - A hissing sound heard at the exhaust stacks when an aircraft engine is pulled through by hand is an indication that an exhaust valve is leaking. There is exhaust valve blow-by.
A condition that can occur in radial engines but is unlikely to occur in horizontally opposed engines is
hydraulic lock. ] - Radial and inverted engines have some cylinders below the crankcase, and when the engine is idle, oil will leak from the crankcase, past the piston rings, and fill the combustion chamber. This condition is called a hydraulic lock. If this oil is not removed before the engine is started, the piston will move against the noncompressible oil and cause serious damage.
An increase in manifold pressure with a constant RPM will cause the bearing load in an engine to
increase. ] - The cylinder pressure applied to the crankshaft through the connecting rod bearings is determined by the compression ratio of the engine and the manifold pressure. If the manifold pressure for a given RPM is increased, the bearing load imposed on the crankshaft will increase.
The horsepower developed in the cylinders of a reciprocating engine is known as the
indicated horsepower. - Indicated horsepower (IHP) is the horsepower developed in the cylinders of a reciprocating engine without reference to friction losses. Shaft horsepower and brake horsepower are the actual usable powers that do include friction losses.
The five events of a four-stroke cycle engine in the order of their occurrence are
intake, compression, ignition, power, and exhaust. ] - The five events that take place in a reciprocating engine during each cycle of its operation are: Intake -- The fuel-air mixture is taken into the cylinder. Compression -- The fuel-air mixture is compressed as the piston moves upward (outward) in the cylinder. Ignition -- As the piston nears the top of its stroke, an electrical spark ignites the mixture so it burns and releases its energy. Power -- As the fuel-air mixture burns, it forces the piston downward. This movement of the piston rotates the crankshaft and performs useful work. Exhaust -- After the piston has reached the bottom of its stroke and done the most of its useful work, the piston pushes upward, forcing the burned gases out of the cylinder.
If fuel/air ratio is proper and ignition timing is correct, the combustion process should be completed
just after top center at the beginning of the power stroke. ] - The ignition of the fuel-air mixture in the cylinder of a reciprocating engine is timed so it occurs when the piston is about 20 to 30 degrees of crankshaft rotation before reaching top center on the compression stroke. If the mixture ratio and ignition timing are both correct, the fuel-air mixture will be all burned shortly after the piston passes over top center. The expanding gases caused by absorbing heat from the burning mixture will exert the maximum amount of push on the descending piston during the power stroke.
Excessive valve clearance results in the valves opening
late and closing early. ] - Excessive clearance in the valve train will cause the valves to open late (the cam will have to turn farther before the valve is opened) and close early (the valve will close before the cam rotates to the normal closing position).
If an engine operates with a low oil pressure and a high oil temperature, the problem may be caused by a
leaking oil dilution valve. ] - Some aircraft engines are equipped with an oil dilution system in which gasoline is put into the lubricating oil before the engine is shut down. Oil dilution is used in cold weather to make a cold engine easier to start. If an oil dilution valve should leak and allow gasoline to flow into the oil supply during normal operation, it will cause the oil to be too thin (to have too low a viscosity) for normal operation. The oil pressure will drop and the oil temperature will go up.
An unsupercharged aircraft reciprocating engine, operated at full throttle from sea level, to 10,000 feet, provided the RPM is unchanged, will
lose power due to the reduced density of the air drawn into the cylinders.] - The power produced by an aircraft reciprocating engine is determined by the weight of the air that is mixed with the fuel and burned. At altitude, the density of the air (its weight per unit volume) is less than it is at sea level. Therefore, for the same RPM, an unsupercharged engine will take in less weight of air to combine with the fuel and the engine will lose power.
If the oil pressure gauge fluctuates over a wide range from zero to normal operating pressure, the most likely cause is
low oil supply. ] - Oil pressure fluctuation ranging from zero to the normal operating pressure is most likely caused by a low oil supply. When the pump picks up oil, the pressure is normal, but when it draws air, the pressure drops to zero.
To reduce the power output of an engine equipped with a constant-speed propeller and operating near maximum BMEP, the
manifold pressure is reduced with the throttle control before the RPM is reduced with the propeller control. ] - When changing the power setting of an engine equipped with a constant-speed propeller, it is important that the manifold pressure be reduced by retarding the throttle before the RPM is reduced with the propeller pitch control. If the wrong sequence is used, the high manifold pressure and the low RPM can produce cylinder pressures high enough to seriously damage the engine.
(1) Preignition is caused by improper ignition timing. (2) Detonation occurs when an area of the combustion chamber becomes incandescent and ignites the fuel/air mixture in advance of normal timed ignition. Regarding the above statements,
neither No. 1 nor No. 2 is true. ] - Statement (1) is not true. Preignition is the ignition of the fuel-air mixture before normal ignition is timed to occur. It is caused by incandescent objects in the cylinder. Detonation produces enough heat in a cylinder that carbon particles can become incandescent and ignite the mixture early. Statement (2) is not true. Detonation is the spontaneous combustion of the unburned charge ahead of the flame front after ignition has occurred. Preignition can cause the fuel-air mixture to burn in the cylinder long enough to heat the unburned mixture to its critical temperature. At this point, it explodes rather than burns. This instantaneous release of energy can overheat the cylinder producing enough pressure to damage the piston and connecting rod.
The primary purpose in setting proper valve timing and overlap is to
obtain the best volumetric efficiency and lower cylinder operating temperatures. ] - Valve overlap is the angular travel of the crankshaft during the time both the intake and exhaust valves are off their seats, and is used to increase the volumetric efficiency of the engine. The exhaust valve remains open until after the piston has started down on the intake stroke to allow the maximum amount of burned exhaust gases to leave the cylinder. The intake valve opens shortly before the piston reaches the top of its travel on the exhaust stroke. The inertia of the exhaust gases leaving the cylinder when the intake valve opens, helps start the fresh fuel-air charge to flow into the cylinder. By timing the valves and ignition to occur at the proper time, the mixture will not be burning as the piston is moving downward, and the cylinder walls will not become overheated.
Grinding the valves of a reciprocating engine to a feather edge is likely to result in
preignition and burned valves. ] - If a valve is ground with a feather edge (a thin edge) the heat in the cylinder will cause the thin area to glow red hot and this will ignite the fuel-air mixture before the correct time for ignition. This will result in preignition and burned valves.
Some aircraft engine manufacturers equip their product with choked or taper-ground cylinders in order to
provide a straight cylinder bore at operating temperatures.] - Some aircraft engine cylinders are ground with the diameter at the top of the barrel, where it screws into the head, slightly smaller than the diameter in the center of the barrel. This is called choke grinding. The large mass of the cylinder head expands more when heated than the smaller mass of the cylinder barrel, so the diameter of a choke-ground cylinder becomes uniform when the engine is at its operating temperature.
The primary concern in establishing the firing order for an opposed engine is to
provide for balance and eliminate vibration to the greatest extent possible. ] - The firing order of an opposed engine is designed to provide for balance and to eliminate vibration as much as possible.
Valve clearance changes on opposed-type engines using hydraulic lifters are accomplished by
push rod replacement. ] -When assembling an opposed engine equipped with hydraulic valve lifters, if the valve clearance is not within the allowable limits, install a pushrod of a slightly different length.
Engine operating flexibility is the ability of the engine to
run smoothly and give the desired performance at all speeds. ] - One of the required characteristics of an aircraft engine is operating flexibility. Operating flexibility is defined by the FAA as the ability of an engine to run smoothly and to give the desired performance at all speeds.
A characteristic of dyna-focal engine mounts as applied to aircraft reciprocating engines is that the
shock mounts point toward the engine's center of gravity. ] - Aircraft reciprocating engines are often mounted in a type of suspension called dynamic suspension, or dyna-focal engine mounts. Dyna-focal mounts absorb the vibrations of the engine about the center of gravity of the engine-propeller combination and isolates these vibrations from the aircraft structure. The shock mounts all point toward the engine-propeller center of gravity
One of the best indicators of reciprocating engine combustion chamber problems is
spark plug condition. ] - The condition of the spark plugs taken from the cylinders of a reciprocating engine is a good indicator of the condition of the combustion chamber of the engine. Spark plugs can show when detonation has been occurring, and they can show up an excessively worn valve guide and induction system filter leaks.
If metallic particles are found in the oil filter during an inspection,
the cause should be identified and corrected before the aircraft is released for flight. ] - Anytime metallic particles are found on the oil screen of an aircraft engine, their source and the cause for their being in the oil system must be determined and corrected before the aircraft is released for flight.
Before attempting to start a radial engine that has been shut down for more than 30 minutes,
turn the propeller by hand three to four revolutions in the normal direction of rotation to check for liquid lock. ] - There are some cylinders below the center line of a radial engine, and it is possible for oil to drain down, past the piston rings, into these lower cylinders while the engine is not operating. When a radial engine has been shut down for a half hour or so, it should be checked for a liquid lock (oil in the lower cylinders) by pulling the propeller through in the direction of normal rotation by hand for at least two complete revolutions of the crankshaft. If oil has collected in any of the lower cylinders, the spark plugs must be removed from these cylinders and all of the oil drained out.
During overhaul, the disassembled parts of an engine are usually degreased with some form of mineral spirits solvent rather than water-mixed degreasers primarily because
water-mixed degreaser residues may cause engine oil contamination in the overhauled engine. ] - Extreme care must be used if any water-mixed degreasing solutions containing caustic compounds of soap are used for cleaning engine parts. Such compounds, in addition to being potentially corrosive to aluminum and magnesium, may become impregnated in the pores of the metal and cause oil foaming when the engine is returned to service.
During overhaul, reciprocating engine exhaust valves are checked for stretch
with a contour or radius gauge.] - One recommended way of checking exhaust valves for stretch is by measuring the diameter of the valve stem with a vernier outside micrometer caliper at a point specified by the engine manufacturer. If the valve has stretched, the stem diameter will be smaller than it should be. Another way of determining if a valve has been stretched is by using a valve radius gauge to see if the radius between the valve stem and head is the same radius the valve had when it was manufactured.
