Reciprocating Engine: Construction and Operating Principles — Study Notes

Crankcase

• The foundation of an engine is the crankcase. It contains the bearings and bearing supports in which the crankshaft revolves.
• It must provide a tight enclosure for the lubricating oil and support various external and internal mechanisms of the engine.
• It also provides support for attachment of the cylinder assemblies and the powerplant to the aircraft.
• It must be sufficiently rigid and strong to prevent misalignment of the crankshaft and its bearings.
• Cast or forged aluminum alloy is generally used for crankcase construction because it is light and strong.

Cylinder Pads or Cylinder Mounting Pads

• Machined surfaces on which the cylinders are mounted.
• Provided with a suitable means of retaining or fastening the cylinders to the crankcase.
• The general practice to secure the cylinder flange to the pad is to mount studs in threaded holes in the crankcase.
• The inner portion of the cylinder pads is sometimes chamfered or tapered to permit installation of a large rubber O-ring around the cylinder skirt, which effectively seals the joint between the cylinder and the crankcase pads against oil leakage.

Mounting Lugs

• Spaced about the periphery of the rear of the crankcase or the diffuser section of a radial engine.
• Used to attach the engine assembly to the engine mount or framework provided for attaching the power plant to the fuselage of single‑engine aircraft or to the wing nacelle structure of multi‑engine aircraft.
• May be integral with the crankcase or diffuser section or detachable, as in flexible or dynamic engine mounts.
• The mounting arrangement supports the entire power plant including the propeller, and is designed to provide ample strength for rapid maneuvers or other loadings.

TYPES OF ENGINE CRANKCASE

• A typical horizontally opposed engine crankcase consists of two halves of cast aluminum alloy that are manufactured either with sand castings or by using permanent molds.
• A. Opposed Engine Crankcase
• B. Radial Engine Crankcase
  • Nose Section: mounted at the front of a radial engine crankcase and bolts directly to the power section. The nose section usually houses and supports a propeller governor drive shaft, the propeller shaft, a cam ring, and a propeller reduction gear assembly if required.
  • Main or Power Section: represents the section of the crankcase where the reciprocating motion of the pistons is converted to the rotary motion of the crankshaft. It supports crankshaft bearings, where cylinders are mounted.
  • Diffuser or Supercharger Section: generally made of cast aluminum alloy or magnesium. This section houses the supercharger and its related components. It is also called the fuel induction and distribution section. Provides housing for attachments of induction pipes and manifold pressure lines.
  • Accessory Section: typical accessory section houses gear trains containing both spur- and bevel-type gears that drive various engine components and accessories. It has mounting pads for fuel pump, oil pump, tachometer generators, etc.

Crankshaft

• The crankshaft is the , and it may be of construction. Its main purpose is to the of the piston and connecting rod for rotation of the propeller or helicopter transmission. A shaft located at specified points along its length. Since crankshafts must be very strong, they generally are forged from a very strong alloy, such as . backbone of the reciprocating engine, single-piece or multi-piece, transform the reciprocating motion into rotary motion, composed of one or more cranks, chromium-nickel-molybdenum steel.
• The four-throw construction may be used either on four‑cylinder horizontal opposed or four‑cylinder inline engines. The six-throw shaft is used on six‑cylinder inline engines, 12‑cylinder V‑type engines, and six‑cylinder opposed engines. For radial engines, it may be the single‑throw, two‑throw, or four‑throw type.
• Three Main Parts:
  • Journal: is supported by, and rotates in, a main bearing. It serves as the center of rotation of the crankshaft.
  • Counterweights and Dampers: helps balance the crankshaft. Its function is to relieve the whip and vibration caused by the rotation of the crankshaft.
  • Crankpin: the section to which the connecting rod is attached. It is off-center from the main journals and is often called the throw.
  • Two crank cheeks and a crankpin make a throw.
  • Crank Cheek or Crank Arm: connects the crankpin to the main journal. In some designs, the cheek extends beyond the journal and carries a counterweight to balance the crankshaft.
• Crankshaft Balance:
  • Crankshafts are balanced for static balance and dynamic balance.
  • A crankshaft is statically balanced when the weight of the entire assembly of crankpins, crank cheeks, and counterweights is balanced around the axis of rotation.
  • A crankshaft is dynamically balanced when all the forces created by crankshaft rotation and power impulses are balanced within themselves so that little or no vibration is produced when the engine is operating.
  • A dynamic damper is merely a pendulum that is fastened to the crankshaft so that it is free to move in a small arc. It is incorporated in the counterweight assembly.

Cylinders

• The portion of the engine in which the burning and expansion of gases take place; provides the combustion chamber where power is developed.
• Four major design considerations for the cylinder assembly:
  • Must be able to withstand internal pressures.
  • Be lightweight to keep engine weight down.
  • Have good cooling characteristics for efficient cooling.
  • Be comparatively durable and easy to manufacture, inspect, and maintain.
• The air-cooled engine cylinder commonly used is the overhead valve type.
• Each cylinder is an assembly of two major parts: cylinder head and cylinder barrel.
• At assembly, the cylinder head is heated to expand and then screwed down on the cylinder barrel, which has been chilled, producing a gas-tight joint. Majority of cylinders use an aluminum head and a steel barrel.
• Cylinder Head contents: intake and exhaust valve ports, spark plugs, and valve actuating mechanisms. The greatest improvement in air‑cooling resulted from increasing cooling fin area in certain sections.
• Cylinder Barrel: usually made of a steel alloy forging; inner surface hardened to resist wear of the piston and piston rings that bear against it. Hardening is typically done by exposing the steel to ammonia or cyanide gas during chroming.
• Chromium-plated cylinders should use a suitable coating; ceramic chrome (Cermicrome) and nickel and carbide coatings have minimized cylinder wear.

Valves

• The fuel/air mixture enters the cylinders through the intake ports and through the exhaust ports. The valves used in aircraft engines are the conventional poppet type. They are shaped as either mushroom or tulip heads.

Valve Construction

• The valve head has a ground face that forms a seal against the ground valve seat in the cylinder head when the valve is closed (often ground to 30° or 45°; in some engines: intake-valve face is 30°, and the exhaust-valve face is 45°).
• valve faces may be hardened with stellite for durability against high-temperature corrosion and wear.
• The valve stem acts as a pilot for the valve head and rides in the valve guide installed in the cylinder head. The valve stem is surface hardened to resist wear.
• Some intake and exhaust valve stems are hollow and partially filled with metallic sodium.

Valve Operating Mechanism

• At a particular instant, both valves are open at the same time (end of the exhaust stroke and beginning of the intake stroke). This overlap permits better volumetric efficiency and lowers the cylinder operating temperature.
• Valve timing is controlled by the valve operating mechanism and is referred to as valve timing.
• Valve lift (distance that the valve is lifted off its seat) and valve duration (length of time the valve is held open) are determined by the shape of the cam lobes.
• The portion of the lobe that gently starts the valve operating mechanism moving is called the ramp or step.
• Valve overlap, valve duration, and timing are critical for engine breathing and performance.

Valve Operating Mechanism - Radial

• The valve mechanism of a radial engine is operated by one or two cam rings, depending upon the number of rows of cylinders.
• The cam ring is a circular piece of steel with a series of cams on its outer surface. The surface of these lobes and the space between them (on which the cam rollers ride) is known as the cam track.
• Cam Ring Location:
  • Single-row radial: cam ring placed between propeller reduction gearing and front power section.
  • Twin-row radial: second cam added between rear power section and supercharger.

Valve Operating Mechanism - Tappet / Valve Lifters

• The tappet or valve lifter assembly consists of:
  • A cylindrical tappet, which slides in and out in a tappet guide installed in one of the crankcase sections around the cam ring.
  • A tappet roller or face, which follows the contour of the cam ring and lobes.
  • A push rod or socket, which transmits motion toward the rocker arm.
  • A rocker arm assembly, which transmits the lifting force to the valve tip.
• The function of the tappet assembly is to convert the rotational movement of the cam into reciprocating motion and to transmit this motion to the push rod, rocker arm, and then to the valve tip, opening the valve at the proper time.
• A lubrication hole is drilled through the tappet to allow engine oil to flow to the hollow push rods to lubricate the rocker assemblies.

Valve Operating Mechanism - Solid Lifters / Hydraulic Tappets

• Solid lifters (cam followers) generally require manual valve clearance adjustment to ensure valve closure.
• Hydraulic tappets (lifters) eliminate the need for any valve clearance adjustment mechanism; valve clearance is automatically kept at zero when the engine is running.

Rocker System

• The push rod transmits lifting force to the rocker arm via a hardened steel ball on each end of a tubular push rod; one ball end fits into the socket of the rocker arm.
• The rocker arms transmit the lifting force from the cams to the valves.
• Rocker arms are supported by a plain, roller, or ball bearing, or a combination of these, which serves as a pivot.
• Each valve is closed by two or three helical springs. Multiple springs reduce vibration and failure risk due to heat and metal fatigue.

Camshaft

• The valve mechanism of an opposed engine is operated by a camshaft.
• The camshaft is driven by a gear that meshes with another gear attached to the crankshaft.
• The camshaft always rotates at one-half the crankshaft speed $\frac{ ext{RPM}<em>{cam}}{ ext{RPM}</em>{crank}} = \frac{1}{2}.$

Piston

• The piston of a reciprocating engine is a cylindrical member that moves back and forth inside a steel cylinder.
• It acts as a moving wall and the boundary of the combustion chamber.
• Two primary materials:
  • Forged pistons: typically aluminum alloy (e.g., AMS 4140)
  • Cast pistons: Alcoa 132 Alloy
• Pistons perform two primary functions:
  • Draws fuel and air into the cylinder, separates the fresh charge from burned gases, and transmits the force of combustion to the crankshaft via the connecting rod.
  • Purges the cylinder of burned exhaust gases after combustion.

Parts of a Piston

• Piston Head: top surface exposed to heat of combustion.
• Ring Grooves: grooves machined around the piston to house piston rings (up to six rings are possible).
• Ring Land: the portion of the piston between the ring grooves.
• Piston Skirt: extends to assist in aligning the piston in the cylinder.
• Piston Pin Boss: enlarged area inside the piston that forms the piston skirt and provides bearing area for the piston pin to connect the piston to the connecting rod.

Piston Head Designs

• Some older designs are not used in modern high-powered engines due to insufficient strength or wear resistance.
• Recesses may be machined in the piston head to prevent interference with the valves.
• Slipper-type pistons were used in the past.
• Cam-Ground Pistons: modern pistons ground to produce an elliptical shape at room temperature; when the engine warms to operating temperature, dimensions change so the piston becomes round in the bore.

Piston Pin

• The piston pin (also called the wristpin) connects the piston to the connecting rod and allows relative motion.
• Modern aircraft engines commonly use a full-floating piston pin, allowing rotation in both the piston and the connecting rod.
• Piston pins are categorized as:
  • Stationary: restrained in place by a setscrew
  • Semi-floating: restrained in the connecting rod by a clamp or slot in the pin
  • Full-floating: free to rotate in both piston and connecting rod; requires a piston-pin bearing
• Wristpin / gudgeon pin / full-floating pin terminology reflects this function and motion.

Piston Ring End Gaps

• End gaps allow for expansion and contraction and accommodate cylinder wall irregularities.
• Types of end gaps: Butt, Step, Angle.
• Piston ring end gaps are staggered to prevent compression loss.

Types of Piston Rings

• Compression Rings: seal the combustion gases from entering the crankcase. They are placed in ring grooves immediately below the crown of the piston.
• Common practice in aircraft engines: two or three compression rings per piston.
• Cross-section shapes include Rectangular, Wedge Shaped, and Tapered.
• Function: minimize blow-by and maintain compression.

Oil Rings / Oil Control Rings / Scraper Rings

• Oil Rings control the amount of oil that reaches the cylinder walls.
• Oil Control Rings: located below the compression rings; one or more per piston; form a film on the cylinder wall.
• Oil Scraper Rings: typically have a beveled face and are located in the groove at the bottom of the piston skirt, scraping excess oil away from the piston and reducing oil entering the combustion chamber.
• The combination of compression rings and oil rings regulates the oil film thickness on the cylinder wall and helps prevent oil burning.

Connecting Rods

• The connecting rod must be strong enough to remain rigid under load and light enough to reduce inertia forces.
• Common materials: steel alloy; aluminum can be used for low-horsepower engines.
• End types:
  • Crankpin End: connects to the crankshaft.
  • Piston End: connects to the piston.
• The connecting rod is the link that transmits forces between the piston and the crankshaft.

Types of Connecting Rods

• Fork-and-Blade Rod Assembly: used primarily in V-type engines; the forked rod is split at the crankpin end to fit between the prongs of the blade rod.
• Plain-Type Connecting Rods: used in inline and opposed engines; the end attached to the crankpin is fitted with a cap and a two-piece bearing.
• Master-and-Articulated Rod Assembly: common in radial engines; one piston in each row connects to the crankshaft via a master rod, and the other pistons in the row connect to the master rod via articulated rods. A split-type assembly is a special form.
• Knuckle Pins: solid construction except for oil passages drilled in the pins to lubricate knuckle pin bushings.

Propeller Reduction Gearing

• Most propellers must operate at a specific speed for maximum efficiency. Therefore, the engine can run at higher speeds while the propeller runs at a slower, more efficient speed.
• A propeller reduction gear system permits the propeller to turn slower than the engine to achieve this relationship.

Types of Propeller Reduction Gearing

• Spur planetary reduction gearing: consists of a large driving gear (sun gear) splined to the crankshaft, a large stationary gear (bell gear), and a set of small spur planetary pinion gears mounted on a carrier ring.
• Bevel planetary reduction gearing: driving gear with beveled external teeth attached to the crankshaft; a set of mating bevel pinion gears is mounted in a cage attached to the end of the propeller shaft. More compact and suitable for smaller gear steps.
• Several types of lower powered engines can use spur and pinion reduction gear arrangements.

Propeller Shafts

• Tapered Propeller Shafts: used on most early, low-powered engines. The shaft tapers toward the propeller end; the threaded end receives a propeller retaining nut.
• Splined Propeller Shaft: provides increased strength for higher power. A spline is a rectangular groove machined into the propeller shaft.
• Flanged Propeller Shafts (most modern horizontally opposed engines): a flat flange is forged onto the end of a crankshaft and a propeller is bolted to the flange; additional support is provided by a short shaft forward of the flange and studs around the flange circumference.

Reciprocating Engine Operating Principle

• An internal combustion engine is a device for converting heat energy into mechanical energy.
• Reciprocating engines operate on the basic principle of converting chemical energy (fuel) into mechanical energy through combustion inside cylinders.
• There are two primary reciprocating engine designs: Spark Ignition (SI) and Compression Ignition (CI).
• The main difference between SI and CI is the process by which the fuel is ignited.

Spark Ignition (SI) Engine

• The SI engine has served as the powerplant of choice for many years.
• Spark-ignition four-stroke engines remain the most common design in general aviation aircraft today.
• The main parts of a spark-ignition reciprocating engine include the cylinders, crankcase, and accessory housing.

Compression Ignition (CI) Engine

• To reduce operating costs and improve reliability, several manufacturers are turning to compression ignition as a viable option.
• Often referred to as diesel or jet-fuel engines, CI engines have the advantage of utilizing readily available and lower-cost diesel or jet fuel.
• A CI engine first compresses the air in the cylinder, raising its temperature to trigger automatic ignition when fuel is injected into the cylinder.

Cylinder Arrangements

• Depending on the engine manufacturer, cylinder arrangements can be designed to utilize spark or compression ignition and operate on either a two-stroke or four-stroke cycle.

Two-Stroke Cycle

• In a two-stroke engine, the conversion of chemical energy into mechanical energy occurs over a two-stroke operating cycle.
• The intake, compression, power, and exhaust processes occur in only two strokes of the piston rather than the four strokes of a four-stroke engine.
• Because a two-stroke engine has a power stroke upon each revolution of the crankshaft, it typically has a higher power-to-weight ratio than a comparable four-stroke engine.
• Due to inherent inefficiency and higher emissions of early two-stroke designs, their aviation use has been limited. Recent advances have mitigated many of these negatives.
• The typical sequence in a two-stroke cycle involves: inlet opens to admit air-fuel mixture, piston compresses, ignition and power, exhaust scavenging, and restart of the cycle.
• Illustrative depiction (textual): Inlet port opens; air-fuel mixture enters; piston moves upward to compress; a spark ignites the mixture; combustion expands the gas pushing the piston down (power stroke); waste heat is exhausted.
• See: Inlet stroke, compression stroke, power stroke, exhaust stroke as the four functional events that occur over two piston movements.

Four-Stroke Cycle

• The four-stroke cycle involves the intake, compression, power (expansion), and exhaust processes occurring in four separate piston strokes.
• The order is: a) Intake Stroke; b) Compression Stroke; c) Power Stroke; d) Exhaust Stroke.
• This cycle yields smoother operation and lower emissions compared to two-stroke designs.
• Four-stroke cycle visuals (textual):
  • Intake: air/fuel enters through an open intake valve as the piston descends.
  • Compression: both intake and exhaust valves are closed while the piston ascends, compressing the mixture.
  • Power/Expansion: fuel-air mixture is ignited (via spark in SI); gas expands and pushes piston down.
  • Exhaust: exhaust valve opens and spent gases are expelled as the piston rises.
• Four-Stroke Cycle Repeats: subsequent cycles continue as long as fuel is supplied and the engine runs.

End of Presentation