Reciprocating Engines

Small general aviation aircraft mostly use horizontally opposed piston engines; radials still fly, but their use is very limited (FAA-H-8083-32B, p. 1-1). The ACS lists ten knowledge areas for this subject (FAA-S-ACS-1, AM.III.A). The handbook values below are for illustration only; for real work, use the manufacturer's instructions (FAA-H-8083-32B, p. 10-25).

Types of reciprocating engines

Engines are classed by cylinder arrangement or by cooling. All piston engines finally reject their heat to the air: air-cooled cylinders carry thin fins, while liquid-cooled engines use a coolant and radiator at the cost of extra weight (FAA-H-8083-32B, p. 1-3 to 1-4).

• Inline: usually an even number of cylinders and one crankshaft; with the cylinders below the crankshaft it is called inverted.
• V-type: two inline banks, generally 60° apart, named by a V and the displacement in cubic inches, such as V-1710.
• Radial: rows of three, five, seven or nine cylinders around a central crankcase; still found in older cargo aircraft, warbirds and crop sprayers.
• Opposed: two banks directly opposite each other with the crankshaft between them, giving a low weight-to-horsepower ratio, a narrow silhouette and low vibration.

Engines are also classed by operating cycle: four-stroke, two-stroke, rotary and diesel. A two-stroke completes its cycle in one crankshaft revolution, with intake and exhaust during the same stroke, and is used in ultralight, light-sport and experimental aircraft. A rotary engine turns a three-sided rotor inside an elliptical housing. A diesel needs no spark: the piston compresses air to a high pressure and temperature, fuel is sprayed in at maximum pressure, and that heat ignites it. Engines that run on Jet A use a version of this cycle, and diesels are built in two-stroke and four-stroke forms (FAA-H-8083-32B, p. 1-25 to 1-27). Smaller two-strokes generally run on gasoline premixed with 2 percent oil (1:50); others have an oil injection pump (FAA-H-8083-32B, p. 11-4).

The four-stroke cycle and combustion

Most certificated aircraft piston engines use the four-stroke (Otto) cycle. Its five events are intake, compression, ignition, power and exhaust, and the four strokes take two crankshaft revolutions (720°), so each cylinder fires once every two turns. The bore is the cylinder's inside diameter; the stroke is the piston's travel between top dead center (TDC) and bottom dead center (BDC) (FAA-H-8083-32B, p. 1-25). Timing is given in crankshaft degrees and marks the start of valve opening. Typically the intake valve opens 8° to 55° before TDC and closes 50° to 75° past BDC, using the momentum of the incoming charge; ignition comes 20° to 35° before TDC; and the exhaust valve opens well before BDC, while pressure remains to push the gases out (FAA-H-8083-32B, p. 1-26 to 1-27).

Because the intake opens early (valve lead) and the exhaust closes late (valve lag), both valves are open around TDC. This valve overlap improves volumetric efficiency, helps scavenging and lowers cylinder temperature (FAA-H-8083-32B, p. 1-18 and 1-26). Too much valve clearance shortens the overlap and richens the idle mixture; too little lengthens it and leans the idle mixture (FAA-H-8083-32B, p. 10-34).

Detonation and pre-ignition

Detonation is the explosive burning of the unburned charge ahead of the flame front after the spark. Light detonation may give no flight deck warning, so it is often found only at overhaul, as dished piston heads, collapsed valve heads, broken ring lands or eroded parts. Rich mixtures resist detonation, which is why mixtures are enriched at high power (FAA-H-8083-32B, p. 10-29 to 10-30). Pre-ignition starts before the spark, from a hot spot such as a carbon deposit or a cracked valve, and shows as roughness, backfiring and a sudden rise in cylinder head temperature (FAA-H-8083-32B, p. 10-30).

Opposed engine construction

• Crankcase: the foundation of the engine, usually cast or forged aluminum alloy and split into two halves (FAA-H-8083-32B, p. 1-5 to 1-8).
• Crankshaft: forged from a strong alloy such as chromium-nickel-molybdenum steel. Each throw has a journal that turns in a main bearing, a crankpin (usually hollow) for the connecting rod, and crank cheeks. Dynamic dampers, small pendulums on the cheeks, reduce vibration (FAA-H-8083-32B, p. 1-8 to 1-9).
• Connecting rods: plain rods with a two-piece bearing; always reinstall a rod in the same cylinder and position (FAA-H-8083-32B, p. 1-11).
• Pistons and rings: aluminum alloy forgings, usually cam ground so they become round when hot. Compression rings sit at the top, oil control rings below them and an oil scraper ring at the skirt. Rings are usually cast iron; chrome-plated rings are for steel cylinder walls and must never be used in chrome-plated cylinders (FAA-H-8083-32B, p. 1-12 to 1-13).
• Cylinders: an aluminum alloy head is heated and screwed onto a chilled steel barrel. The bore is usually nitrided; worn barrels can be restored by chrome plating, and some are choked (smaller at the top) (FAA-H-8083-32B, p. 1-14 to 1-15).
• Valves: faces are ground to 30° or 45°, often with stellite. Sodium-filled stems carry heat away; the sodium melts at about 208 °F, and such a valve must never be cut open (FAA-H-8083-32B, p. 1-17 to 1-18).
• Valve train: the camshaft turns at one-half crankshaft speed and drives tappets, push rods and rocker arms. Hydraulic lifters hold valve clearance at zero (FAA-H-8083-32B, p. 1-20 to 1-21).
• Bearings: plain bearings take mainly radial loads, ball bearings take radial and thrust loads, and straight roller bearings take radial loads only (FAA-H-8083-32B, p. 1-22).

The propeller end is always the front, and left and right are named as seen from the accessory end. Opposed-engine cylinder numbering is not standard, so check the engine manual; one six-cylinder opposed firing order is 1-4-5-2-3-6 (FAA-H-8083-32B, p. 1-15 to 1-16).

Radial engine construction

In each row of a radial engine, one piston connects to the crankpin through a master rod, and the others connect to the master rod through articulated rods and knuckle pins, so an 18-cylinder, two-row engine has 2 master rods and 16 articulated rods (FAA-H-8083-32B, p. 1-9). The knuckle pins travel in elliptical paths, which upsets timing; unevenly spaced knuckle pin holes or a compensated magneto make up for it (FAA-H-8083-32B, p. 1-10). A single-row radial uses a single-throw (360°) crankshaft, and a cam ring turning at reduced speed works the valves (FAA-H-8083-32B, p. 1-8 to 1-19).

Radial cylinders are numbered clockwise from the rear, with No. 1 at the top; in a double-row engine the odd numbers are in the rear row. A single-row radial fires the odd cylinders in order and then the even ones, so a nine-cylinder engine fires 1-3-5-7-9-2-4-6-8. For a 14-cylinder double row, add 9 or subtract 5 to find the next cylinder (FAA-H-8083-32B, p. 1-16).

Power and efficiency

One horsepower is 33,000 ft-lb per minute, or 550 ft-lb per second (FAA-H-8083-32B, p. 1-28). The key formulas:

In PLANK, P is indicated mean effective pressure in psi, L the stroke in feet, A the piston area in square inches, N the power strokes per minute (rpm ÷ 2 for a four-stroke engine) and K the number of cylinders (FAA-H-8083-32B, p. 1-30). Brake horsepower is the power delivered to the propeller shaft, found from torque measured on a dynamometer or Prony brake. Friction can take as much as 10 to 15 percent of indicated horsepower (FAA-H-8083-32B, p. 1-31 to 1-33), and the mechanical efficiency of the average aircraft engine approaches 90 percent (FAA-H-8083-32B, p. 1-35).

Manifold pressure times compression ratio gives the pressure just before ignition: about 210 in. Hg at 30 in. Hg and 7:1 (FAA-H-8083-32B, p. 1-29). Higher compression turns more heat into work but raises cylinder head temperature and the risk of detonation. Much of the heat is wasted: 40 to 45 percent leaves with the exhaust (FAA-H-8083-32B, p. 1-34). Volumetric efficiency compares the charge drawn in, corrected to standard conditions, with the piston displacement. Part throttle, long narrow intake pipes, sharp bends, high carburetor air or cylinder head temperature, incomplete scavenging and improper valve timing lower it; true supercharging can push it past 100 percent (FAA-H-8083-32B, p. 1-35 to 1-36).

Ground operation

Turning a propeller can start the engine if a magneto switch is on (FAA-H-8083-32B, p. 8-6). Before moving one by hand, disconnect the spark plug leads, confirm the magneto switches are off with the P-leads grounded, close the throttle, set the mixture to idle cutoff, set the brakes, chock the wheels and stay out of the propeller arc (FAA-H-8083-32B, p. 8-20). A fireguard with a CO2 extinguisher stands by for every start. If a fire starts, keep cranking to start the engine; if it will not start and the fire burns on, stop and let the fireguard put it out, aiming CO2 into the air intake (FAA-H-8083-30B, p. 1-14).

• Pre-oiling: after overhaul or long idleness, force oil through the engine before the first start, then run it within 4 hours (FAA-H-8083-32B, p. 8-12 to 8-13).
• Hydraulic lock: oil or fuel can drain into the lower cylinders of a radial. Remove a spark plug from the lower cylinders and pull the propeller through in the direction of rotation, never backward (FAA-H-8083-32B, p. 10-25).
• Warm-up: oil pressure should show within 30 seconds; if it is not normal within 1 minute, shut down. Warm up at the speed where the engine runs most smoothly, and never lean the mixture to hurry it (FAA-H-8083-32B, p. 10-25 to 10-26).
• Magneto check: a fast rpm drop points to spark plugs or the harness, a slow drop to ignition timing or valve adjustment, and no drop to a faulty ground, so the engine could fire if the propeller is turned (FAA-H-8083-32B, p. 10-25 to 10-27).
• Power check: with the propeller on its low-pitch stop, a higher than normal manifold pressure for a given rpm usually means a dead cylinder or late timing; a low one suggests early timing.
• Shutdown: cowl flaps open, carburetor heat cold, idle at 800 to 1,000 rpm, mixture to idle cutoff, and ignition off only after the propeller stops (FAA-H-8083-32B, p. 10-28 to 10-29).

Maintenance and inspection

The differential compression test measures leakage past the valves and rings. Test soon after shutdown, with the piston at TDC on the compression stroke and 80 psi regulated pressure (FAA-H-8083-32B, p. 10-39 to 10-40). Keep clear of the propeller, because air pressure can turn it if the piston is off TDC. A loss of more than 25 percent of the input pressure is cause to suspect the cylinder, but recheck after running the engine at least 3 minutes (AC 43.13-1B, par. 8-14). Air at the exhaust means the exhaust valve, at the intake or carburetor the intake valve, and at the crankcase breather the piston rings (FAA-H-8083-32B, p. 10-40 to 10-41).

Cylinders are replaced for low compression, high oil consumption, excessive valve guide clearance, loose intake pipe flanges, loose spark plug inserts or cracks, always as a complete assembly (FAA-H-8083-32B, p. 10-41). If one hold-down cap screw is loose or broken, discard all of that cylinder's cap screws, and reinstall push rods in their original places without turning them end for end (FAA-H-8083-32B, p. 10-42 to 10-43). The cold cylinder check finds dead or intermittent cylinders by comparing cylinder head temperatures right after a run on one magneto (FAA-H-8083-32B, p. 10-43 to 10-44).

The manufacturer sets the time between overhauls (TBO). A top overhaul reconditions cylinders, pistons, rings and valves without splitting the crankcase and is really a repair; a major overhaul completely reconditions the engine in ten steps, from receiving inspection to testing and break-in (FAA-H-8083-32B, p. 10-1). Parts are inspected visually, structurally and dimensionally against the manufacturer's table of limits, and a bent crankshaft must not be straightened (FAA-H-8083-32B, p. 10-2 to 10-9). When the manufacturer gives no procedure, AC 43.13-1B lists acceptable methods (AC 43.13-1B, par. 8-1).

Storage and preservation

Combustion leaves moisture and corrosive by-products, so an idle engine corrodes. Active storage means at least one continuous hour of running with oil at 165 °F to 200 °F, for up to 30 days; temporary storage is 30 to 90 days and indefinite storage over 90 days (FAA-H-8083-32B, p. 8-18). For longer storage the engine is run on a preservative oil mixture, each cylinder is sprayed through the spark plug holes, and the crankshaft must not be turned afterward. Dehydrator plugs hold silica gel treated with cobalt chloride: bright blue means low humidity, lavender about 30 percent, and pink fading to white about 60 percent. Below 30 percent relative humidity, corrosion does not normally take place (FAA-H-8083-32B, p. 8-19). Humidity indicators are checked every 30 days on engines in wooden shipping cases and every 180 days in metal containers (FAA-H-8083-32B, p. 8-21).

Worked example

A six-cylinder four-stroke engine has a 5.00-inch bore and a 4.50-inch stroke. On a test stand at 2,400 rpm, its indicated mean effective pressure is 160 psi, the dynamometer shows 230 bhp, and fuel flow is 103.5 lb/hr.

1. Displacement. Area = 0.7854 × 5.00 × 5.00 = 19.635 in². Displacement = 19.635 × 4.50 × 6 = 530 cubic inches.
2. Indicated horsepower. Convert the stroke: L = 4.50 ÷ 12 = 0.375 ft. N = 2,400 ÷ 2 = 1,200. ihp = 160 × 0.375 × 19.635 × 1,200 × 6 ÷ 33,000 = 257 hp.
3. Friction and efficiency. Friction hp = 257 − 230 = 27 hp. Mechanical efficiency = 230 ÷ 257 = 0.895, or about 89.5 percent.
4. Economy. BSFC = 103.5 ÷ 230 = 0.45 lb per bhp per hour.

Common mistakes

• Using rpm for N in PLANK. A four-stroke cylinder fires once every two revolutions, so N = rpm ÷ 2.
• Leaving the stroke in inches in PLANK. L must be in feet.
• Putting the diameter instead of the radius into πr², which makes the area four times too big.
• Working out compression ratio from displacement alone instead of total volume at BDC.
• Turning a radial backward to clear a hydraulic lock, which forces liquid into the intake.
• Fitting chrome-plated rings to a chrome-plated cylinder.
• Treating a magneto check with no rpm drop as good news; it can mean an ungrounded magneto.