Reciprocating Engine Day 1-4

Heat Engine

Converts chemical energy (fuel) into heat energy; heat energy is then converted into mechanical energy

Types of Heat Engines

Internal Combustion Engine and External Combustion Engine

Internal Combustion Engine (comparison features)

Combustion location inside the engine; working fluid is gas or fuel-air mixture (e.g., air, gasoline); heat transfer directly inside the engine; examples include car engine, piston engine; more efficient and compact; internal cooling (e.g., radiator); high power, mobile applications

External Combustion Engine (comparison features)

Combustion location outside the engine; working fluid is liquid (e.g., water) or gas; heat transferred externally; examples include steam engines, Stirling engines; less efficient and larger; external cooling; lower power, stationary applications

Ericsson Cycle

Composed of 2 Isothermal and 2 Isobaric processes; a governing cycle of external combustion engine

Sterling Cycle

Composed of 2 Isothermal and 2 Isochoric processes; a governing cycle of external combustion engine

Reciprocating Engine

A type of heat engine that derives its name from the back-and-forth movement of its pistons; operates by inducing a mixture of air and fuel into a cylinder, compressed by a piston, ignited, and the rapid rise in temperature forces the piston down the cylinder, converting heat energy into mechanical energy

Reciprocating Engine linear-to-rotary conversion

Linear movement of the piston is converted into rotary motion by a connecting rod and crankshaft

Classification of Reciprocating Engine by Cylinder Arrangement

Radial Engine, In-line Engine, V-Type Engine, Opposed Type Engine

Classification of Reciprocating Engine by Method of Cooling

Air Cooling and Liquid Cooling

Two basic types of Radial Engines

Rotary-type (large torque) and Static-type

Single-row Radial Engine

5-9 Cylinders (odd numbers)

Multiple-row Radial Engine

14-18 Cylinders

Characteristics of a Radial Engine

Greatest drag of all types; high power-to-weight ratio; problems in cooling; helped revolutionize aviation with high power and dependability

In-Line Engine

Generally has an even number of cylinders that are aligned in a single row parallel with the crankshaft; can be liquid-cooled or air-cooled

Advantages of an In-Line Engine

Small frontal area allowing better streamlining; when inverted, allows greater propeller ground clearance for shorter landing gear; commonly used in tailwheel aircraft

Disadvantages of an In-Line Engine

Low power-to-weight ratio; rearward cylinders of an air-cooled engine receive very little cooling air; limited to only 4-6 cylinders (prone to torsional vibration); most designs confined to low- and medium horsepower engines used in light aircraft

V-Type Engine

Arranged in two in-line banks that are 45, 60 to 90 degrees apart; has 8-12 cylinders, liquid-cooled or air-cooled; capable of producing more horsepower than an in-line engine; compact packaging and less vibration compared to in-line engine; complex construction and maintenance

Opposed Type Engine

Most popular engine used in light aircraft; always even number of cylinders; can produce 36-400 horsepower

Characteristics of an Opposed Engine

High power-to-weight ratio; small frontal area which minimizes aerodynamic drag; vibrate less than other engines (cancel impulses); most efficient, dependable and economical type for light aircraft

Crankcase

Foundation of a reciprocating engine; houses the main bearings for the crankshaft, supports the cylinders, and provides mounting faces and attachment points of the other main engine casings; most are made of Cast Aluminum Alloy

Opposed Engine Crankcase manufacturing

Manufactured with sand casting or permanent mold (thinner walls means less tendency to crack)

Cylinder Pads

Surface where a cylinder is mounted to a crankcase

Transverse Webs

Support main bearings and a set of camshaft bosses; adds strength to the case

Opposed Engine Crankcase oil function

Forms a sealed chamber for the lubricating oil; passages are drilled into the case halves to deliver oil to the moving parts within the crankcase; a vent to the atmosphere is normally provided to avoid gas pressure build-up

Crankcase Breather/Positive Crankcase Ventilation

Removes unwanted gases (called "blow-by") from the crankcase to prevent build-up; blow-by consists of combustion byproducts (unburned fuel, water vapor, acids) that slip past the piston rings during the power stroke, normally caused by worn/broken piston rings

Effects of a plugged crankcase breather

Crankcase pressure can build up to a point at which oil consumption increases drastically, sludge formation occurs, and efficiency is reduced; oil can be forced into engine accessories, damaging them

Radial Engine Crankcase - Nose Section

At the front; domed shape; supports propeller governor drive shaft, cam ring, reduction gear, etc

Radial Engine Crankcase - Power Section

Where reciprocating movement occurs

Radial Engine Crankcase - Supercharger Section

Houses the supercharger; where air is compressed and distributed to the cylinders

Radial Engine Crankcase - Accessory Section

Houses magnetos, carburetor

Engine Mounting Points (Low-HP)

Mounting lugs are casted on crankcase

Engine Mounting Points (High-HP)

Mounting lugs are bolted on crankcase

Engine Mounting Points (Radial engine)

Mounting points are around the supercharger section

Four Basic Power Delivering Parts

Cylinder, Piston, Connecting Rod, Crankshaft

Crankshaft

Backbone of a reciprocating engine; converts the reciprocating or linear motion of the pistons into rotary motion, transmits torque to the propeller, and provides the drive for accessories; generally forged from strong alloys such as Chromium-Nickel Molybdenum Steel

Parts of a Crankshaft

Main Bearing (Journal/Main Journal), Crankpin, Crank cheek or crank arm, Counterweights and Dampers

Crank Throw

The offset of the crankshaft, also called piston stroke

Main Bearing Journals

Support the crankshaft; at least two; transmit stress from crankshaft to crankcase; nitrided to resist wear

Crankpins

The pistons are attached by the connecting rods to the crankpin; nitrided; offset from the crankshaft axis; usually hollow to reduce weight (oil passage); has a sludge chamber; usually 180 degrees apart

Crankcheek/Crankarm

Connects the crankpin to the crankshaft; extended to form counterweights for balancing; has drilled passageways to permit oil to flow from the main journal to the crankpin

Counterweights and Dampers

Unbalanced shaft causes extensive vibrations

Crankshaft Balance principle

The crankshaft must be accurately balanced to minimize vibration; when a shaft has to transmit a torque or twisting moment it must flex to some extent and spring back again when released

Statically Balance

Weight of an entire engine assembly is balanced around its axis of rotation; out of balance is when, placed at edge knife blocks, it rotates to one position

Dynamically Balance

A crankshaft is dynamically balanced if it can rotate at operating speed without creating vibration; using counterweights, the centrifugal forces and power pulses are offset

Dynamic Damper/Moveable Counterweight

A weight fastened to a crankshaft's crank cheek assembly in such a way that it is free to move back and forth in a small arc; serves to reduce torsional vibrations in an aircraft reciprocating engine

Torsional effect minimization

Shafts should be as short as possible and adequately supported and counter-weighted to minimize torsional effects

Tachometer

An instrument that measures and displays the rotational speed of the engine's crankshaft in revolutions per minute (RPM)

Timing disk (degree wheel)

A circular disk marked with degrees (0°–360°) used to measure the exact angular position of the crankshaft during engine maintenance and timing adjustments

Single-Throw or 360 Degree Crankshaft

Used on single-row radial engines; consists of a single crankpin with two main journals that support the crankshaft in the crankcase

Two-Throw Crankshaft

Set 180 degrees from each other; sometimes used on two cylinder opposed engines; may consist of either one or three pieces

Four-Throw Crankshaft

Two throws are arranged 180 degrees apart from the other two throws; used on 4 cylinder opposed and in-line engines

Six-Throw Crankshaft

Consists of 4 main bearings and six throws which are 60 degrees apart; used by 6 cylinder opposed and in-line engines or 12 cylinder V-type engines

Bearing

Any surface that supports and reduces friction between two moving parts; typically used in main journals, crankpins, connecting rod ends, accessory drive shaft; designed to withstand axial and radial loads while allowing rotation with minimum friction and wear

Radial load

Acts perpendicular to the shaft's centerline

Axial load

Acts parallel to the shaft's centerline

Plain Bearings

Typically subject to radial loads only; generally used for crankshaft main bearings, cam ring and camshaft bearings, connecting rod end bearings, and master rod bearing; flange-type plain bearing is often used as axial thrust bearings in opposed-type reciprocating engines

Bushing

Smaller plain bearing used to support various accessory drive shafts; its purpose is to reduce friction between two surfaces sliding against each other

Oilite Bushing

The heat produced by friction draws the impregnated oil to the bearing surface to provide lubrication during engine operation

Ball Bearings

Have the least amount of rolling friction; well suited to withstand axial/thrust loads; the balls are held in place and kept evenly spaced by the bearing retainer, while the inner and outer bearing races provide a smooth surface for the balls to roll over

Deep-Groove Ball Bearings

Subtype of ball bearing that feature deep, rounded raceway grooves that closely conform to the balls, allowing support of both radial and axial loads; commonly used in radial engine as crankshaft main bearing

Roller Bearings

Provide greater contact area and corresponding increase in rolling friction; two types are straight roller bearings and tapered roller bearings

Straight roller bearings

Subjected to radial loads only; used in high-power engine as crankshaft main bearings

Tapered roller bearings

Cone-shaped inner and outer races which allow the bearing to withstand both radial and thrust loads

Bearing failure signs

If a bearing fails or is in the process of failing, metal to metal contact is occurring; the friction which accompanies this generates a great deal of heat and can cause high oil temperatures

Connecting Rods

Transmit the forces of combustion to the crankshaft; convert the linear movement of the pistons into rotary movement of the crankshaft; made of durable steel alloy, though aluminum can be used with low horsepower engines; has a crankpin end and a piston end

Plain Type Connecting Rod

Used in opposed and in-line engines; the piston end is fitted with a bronze bushing to accommodate the piston pin; the crankpin end is usually fitted with a two-piece bearing (plain bearing)

Plain Type Connecting Rod design purpose

Connecting rods are designated to a single cylinder and are often matched with pistons for balance and crankpins for fit

Master and Articulated Rod

Connecting rod assembly used in radial engines

Master Rod

The piston end of a master rod contains the piston pin bearing; crankpin bearing (master rod bearing) must be able to withstand the radial loads placed on the rod assembly

Articulate Rod

Each articulated rod is hinged to the master rod by a knuckle pin

Knuckle Pins

Pressed into the master rod so they do not rotate in the flange holes

Full Floating Knuckle Pins

Have a loose fit that allows them to rotate in both the flange holes and articulated rods

Knuckle pin retention

In either type of installation, a lock plate on each side retains the knuckle pins and prevents lateral movement

Master Rod movement

Full Circle Movement; master rod attached to crankshaft crankpin, that's why it forms a perfect circle

Articulated Rods movement

Elliptical Movement; articulated rod attached to master rod big end, that's why elliptical

Fork-and-Blade rod assembly

Used in V-type engines; forked rod is split at the crankpin end to allow space for the blade rod to fit between the prongs

Piston primary functions

Draw fuel and air into a cylinder, compress the gases, and purge burned exhaust gases from the cylinder; transmit the force produced by combustion to the crankshaft

Piston materials

Usually made of Aluminum Alloy AMS 4140 for forged pistons and Alcoa 132 for cast pistons

Piston temperature and pressure

Piston temperature may exceed 4000°F and pressures as high as 500 psi or up

Piston Head

Directly exposed to the combustion chamber

Ring Grooves

Cut into a piston's outside surface to hold a set of piston rings

Ring Land

Portion between the piston ring grooves

Piston Pin Boss (Wrist Pin Boss)

Provides bearing area for a piston pin to attach the piston to the connecting rod (enlarged area)

Piston Skirt

Prevents tilting (rocking), and helps transfer side thrust to the cylinder wall; provides greater heat transfer to the engine oil

Flat Head (piston)

Basic design (most common)

Flat Head with Valve Recessed

Provides clearance for the intake and exhaust valves when they open near top dead center

Domed Head

Increased compression ratio

Cupped Head

Lowering the compression ratio (helps prevent detonation/knock when running on lower-octane aviation gasoline)

Cam Ground Piston

Piston machined with a slightly oval shape to compensate for thermal expansion; the piston's diameter perpendicular to the piston pin is slightly larger than the diameter parallel to the piston pin; provides a better fit within the cylinder at operating temperatures

Piston Rings functions

Prevent pressure leakage from the combustion chamber, control oil seepage into the combustion chamber, and transfer heat from the piston to the cylinder walls

Piston Ring seal formation

Piston rings are spring-loaded and press against the cylinder walls; when properly lubricated, they form an effective seal

Piston Ring materials

Usually made of high-grade gray cast iron or chrome-plated, mild steel

Piston Ring Gap

Point where a piston ring is split; measured using a feeler gauge; upon reaching operating temperature this gap closes due to thermal expansion

Piston ring gap staggering

Must be staggered/offset to prevent combustion gases from leaking past the rings into the crankcase

Blow-by

Combustion gases leaking past the rings to the crankcase; results in loss of power and increases oil consumption

Worn or stuck piston rings symptom

Cause excessive blue smoke (burning oil) to be ejected from the exhaust pipe

Seated piston ring

New piston rings require some wear-in during engine operation so that the ring contour matches the cylinder wall; a ring that matches its cylinder is considered to be seated

Types of Piston Rings

Compression Rings and Oil Rings

Top Compression Ring

Seals combustion gases under high pressure and temperature; also transfers a lot of heat; design is rectangular or slightly barrel-faced