Module 16: Piston Engine – Comprehensive Study Material

1. Module Overview

Module 16 of the EASA Part-66 syllabus (Appendix I) covers the theory, construction, operation, and maintenance of piston engines used in aircraft. This module is essential for B1.2 licence holders (aeroplanes with piston engines) and provides the foundational knowledge required for safe maintenance, troubleshooting, and certification of piston engine installations.

The module encompasses the following key areas:

16.1 Fundamentals: Engine configurations, operating principles, and mechanical components
16.2 Engine Performance: Power output, efficiency, and performance parameters
16.3 Cylinder and Valve Assemblies: Construction, wear mechanisms, and maintenance
16.4 Ignition Systems: Magneto operation, timing, and fault diagnosis
16.5 Lubrication Systems: Oil circuits, pumps, filters, and cooling
16.6 Engine Performance and Propeller Interaction: Load matching and system integration
16.7 Carburettors and Fuel Systems: Mixture control and fuel delivery
16.8 Reduction Gearing: Gear types, wear patterns, and inspection
16.9 Propeller Systems: Constant-speed propellers and governors
16.10 Maintenance Practices: Inspection, testing, and return-to-service criteria

2. Key Concepts Explained in Detail

2.1 Engine Fundamentals and Construction

2.1.1 Engine Configurations

Piston engines used in aviation are classified by cylinder arrangement:

Horizontally Opposed Engines

Most common configuration in modern general aviation
Cylinders arranged in two banks of 180° apart
Advantages: low frontal area, good cooling, reduced vibration (primary and secondary forces balanced)
Examples: Lycoming O-320, Continental IO-360

Radial Engines

Cylinders arranged radially around a central crankcase
Used in older and some specialised aircraft
Odd number of cylinders (5, 7, 9) for even firing intervals
Require special attention to lower cylinder oil drainage during shutdown

Inline and V-Engines

Less common in modern aviation
Inline: cylinders in a single row
V-type: two banks at an angle (typically 60° or 90°)

2.1.2 Crankshaft and Bearing Surfaces

The crankshaft converts reciprocating motion of pistons into rotary motion. Key considerations:

Main Bearing Journals

Support the crankshaft in the crankcase
Subject to various wear patterns that indicate specific faults:
Scoring and discolouration: Typically indicates oil starvation or insufficient oil pressure, leading to metal-to-metal contact
Blue discolouration: Indicates overheating, usually from oil starvation or excessive clearance
Hammered or brinelled appearance: Characteristic of high-impact loading from detonation or pre-ignition, producing rapid pressure spikes exceeding bearing material fatigue limits
Wear on lower halves: In horizontally opposed engines, excessive propeller thrust load causes wear on lower halves of main bearing journals

Crankshaft Runout

Measured at centre main bearing journal
Must not exceed manufacturer's specified limits (typically 0.03–0.05 mm)
Exceeding limits requires replacement or overhaul before return to service

2.1.3 Compression Ratio and Volumes

The compression ratio is a fundamental design parameter:

Formula: CR = (Swept Volume + Clearance Volume) / Clearance Volume

Where:

Swept Volume (Vs) = volume displaced by piston from BDC to TDC
Clearance Volume (Vc) = volume remaining in cylinder at TDC

Example Calculation:

Given: CR = 8.5:1, Swept Volume = 0.5 litres

8.5 = (0.5 + Vc) / Vc

8.5 Vc = 0.5 + Vc

7.5 Vc = 0.5

Vc = 0.0667 litres (66.7 cm³)

Typical aviation piston engine compression ratios range from 6.5:1 to 9.5:1, depending on fuel grade and supercharging.

2.2 Engine Performance

2.2.1 Volumetric Efficiency

Volumetric efficiency (ηv) is the ratio of actual air inducted into the cylinders to the theoretical maximum at ambient conditions:

ηv = (Actual air mass inducted) / (Theoretical maximum air mass)

Factors affecting volumetric efficiency:

Throttle position: Opening the throttle reduces restriction in the induction system, allowing greater air mass to enter cylinders at the same RPM, thus increasing volumetric efficiency
Manifold pressure: Directly related to throttle position and air density
Valve timing: Affects the time available for air intake
Induction system design: Manifold length, diameter, and smoothness
Air temperature: Colder air is denser, improving volumetric efficiency

Relationship with Manifold Pressure:

Manifold pressure (MAP) indicates the absolute pressure in the intake manifold
Higher MAP (closer to atmospheric) means less restriction and higher volumetric efficiency
At full throttle, MAP approaches ambient atmospheric pressure (minus filter and duct losses)

2.2.2 Abnormal Combustion

Detonation

Spontaneous, explosive combustion of the end-gas ahead of the flame front
Causes rapid pressure spikes that can damage bearings (hammered appearance), pistons, and cylinder heads
Causes: low fuel octane, high manifold pressure, high cylinder head temperature, advanced ignition timing

Pre-ignition

Ignition of the fuel-air mixture before the spark occurs
Caused by hot spots in the combustion chamber (glowing carbon deposits, sharp edges, overheated spark plugs)
Leads to extreme pressure and temperature, potentially causing severe engine damage

Dieseling (Run-on)

Engine continues to run after ignition is switched off
Caused by glowing carbon deposits in the combustion chamber igniting the fuel-air mixture
More common in engines with heavy carbon buildup
Not caused by faulty ignition switch (which would prevent spark, not cause running without spark)

Backfiring

Combustion occurs in the induction system (through the carburettor)
Often caused by a lean mixture that burns slowly and may ignite the incoming charge
Accelerator pump failure or incorrect adjustment can cause lean mixture during acceleration

2.3 Cylinder and Valve Assemblies

2.3.1 Cylinder Construction and Wear

Cylinder Barrel

Typically made of nitrided or chrome-plated steel
Bluish discolouration around cooling fins indicates overheating
Causes: blocked cooling fins, malfunctioning cowl flaps, incorrect fuel/air mixture

Cylinder Bore Wear

Measured for out-of-round and taper
Out-of-round exceeding manufacturer's limits requires replacement or reconditioning
Excessive wear leads to high oil consumption and poor sealing

Cylinder Head

Aluminium alloy with cast-in valve seats and guides
Cracks are serious defects compromising structural integrity
Standard practice: replace cylinder assembly (repairs typically not approved)

2.3.2 Valve Assemblies

Valve Clearance

Critical adjustment affecting engine operation
Too small: Valve may not fully close when engine heats up and components expand. Valve held slightly off its seat allows hot combustion gases to escape, overheating and burning the valve
Too large: Causes noisy operation, reduced valve lift, and potential valve damage

Valve Seat Recession

Occurs when lead-free fuel is used in engines designed for leaded fuel
Lead provides a lubricating cushion preventing micro-welding and wear
Without lead, valve seats wear rapidly, especially exhaust valves

Valve Damage Modes

Burning: from poor seating or excessive heat
Spalling and pitting on camshaft lobes: typically caused by abrasive contamination in oil, removing protective oil film and causing metal-to-metal contact and fatigue

2.3.3 Compression Testing

Differential Compression Test

Applies regulated air pressure (typically 80 psi) to the cylinder at TDC on compression stroke
Measures leakage percentage
Acceptable limit for normally aspirated engines: typically 25% maximum leakage
Example: 80 psi applied, stabilises at 70 psi = 12.5% leakage (within limits)

Interpreting Leakage Paths:

Air from oil filler cap/crankcase breather: Leakage past piston rings into crankcase
Air from intake: Leaking intake valve
Air from exhaust: Leaking exhaust valve
Air from cooling fins/exterior: Cracked head or cylinder

Wet vs Dry Compression Test

Add oil to cylinder and re-test
If pressure rises significantly: worn piston rings (oil temporarily seals them)
If pressure remains low: valve or head leakage

Propeller Rotation During Tests

Must rotate in normal direction of rotation
Reverse rotation can cause exhaust valve to be pushed open by piston during overlap, leading to false low readings and potential valve damage
Rotation also expels oil or fuel that may have accumulated, preventing hydraulic lock

2.4 Ignition Systems

2.4.1 Magneto Systems

Basic Operation

Self-contained ignition generating unit (no external power required)
Permanent magnet rotating past coil windings generates high voltage
Produces spark at the correct time in the compression stroke

Magneto Timing

Critical after installation or maintenance
Position crankshaft at TDC on compression stroke (both valves closed)
Install magneto so drive gear aligns with timing mark
Ensures spark occurs at correct point before TDC

E-gap (Internal Timing)

Set during overhaul
Point of maximum magnetic flux change in the coil
Not typically adjusted in the field

2.4.2 Magneto Drop Testing

Procedure

Run engine at idle (typically 800–1000 RPM)
Switch to one magneto and note RPM drop
Return to both, then test other magneto

Acceptable Limits

Typical maximum drop: 120 RPM or 10% of engine RPM (whichever is greater)
Some manufacturers allow up to 175 RPM
Difference between magnetos should not exceed 60–90 RPM

Interpreting Results

Excessive drop on one magneto (e.g., 150 vs 50 RPM): Fault in that ignition circuit (fouled plug, faulty lead, magneto internal issue)
Equal drops on both: Normal operation or engine-related issue (affects both systems equally)
200 RPM drop: Exceeds typical limits, requires investigation before return to service

Common Faults Causing Excessive Drop

Fouled or worn spark plugs
Damaged ignition leads
Magneto internal wear or timing issues
Incorrect spark plug gap

2.5 Lubrication Systems

2.5.1 System Components and Function

Oil Circuit

Oil pump (typically gear type) draws oil from sump
Oil passes through filter and cooler
Distributed to bearings, cylinder walls, and valve train
Returns to sump by gravity

Oil Pressure vs Temperature

Normal oil pressure with high temperature: indicates partially blocked oil cooler (oil still flowing but not dissipating heat)
Low oil pressure: faulty relief valve, low oil level, worn pump, or excessive bearing clearance

2.5.2 Oil Types and Specifications

Oil Grades

Only correct grade and specification per Aircraft Maintenance Manual (AMM) must be used
Using incorrect oil can damage the engine
Common grades: SAE 15W-50, SAE 20W-50 (multigrade); SAE 50, SAE 60 (monograde)

Oil Dilution Systems

Found on some radial and inline engines
Injects fuel into oil before shutdown
Thins oil for easier cold starts in extremely low temperatures
Fuel evaporates from oil once engine warms up

2.5.3 Pre-oiling Procedure

Purpose

Distributes oil to all bearing surfaces, cylinder walls, and valve train components before engine rotation or start
Prevents dry-start damage after storage or overhaul
Not primarily for checking oil pressure or flushing (secondary benefits only)

When Required

After engine storage without preservation (typically > 30 days)
After overhaul or major repair
After extended periods of inactivity

2.5.4 Oil Contamination and Wear

Abrasive Contamination

Causes spalling and pitting on camshaft lobes
Removes protective oil film, causing metal-to-metal contact and fatigue
Sources: dirty oil, failed filters, wear particles from other components

Oil Starvation

Causes scoring and discolouration of bearing journals
Results in metal-to-metal contact
Sources: low oil level, blocked oil passages, failed pump, excessive bearing clearance

2.6 Carburettors and Fuel Systems

2.6.1 Carburettor Operation

Float Chamber

Maintains constant fuel level
Float-operated needle valve controls fuel inlet
Saturated float: Loses buoyancy, sinks, needle valve remains open, fuel level rises, resulting in overly rich mixture

Accelerator Pump

Provides extra fuel during rapid throttle opening
Prevents lean mixture during acceleration
Failure or incorrect adjustment: Causes lean mixture, leading to backfiring through carburettor during acceleration

Idle System

Provides fuel at low throttle settings
Partially blocked idle jet affects both magnetos equally (fuel issue, not ignition)

2.6.2 Mixture Control

Lean Mixture Symptoms

Backfiring through carburettor during acceleration
Rough running at idle
High cylinder head temperatures
Reduced power output

Rich Mixture Symptoms

Rough running
Carbon fouling of spark plugs
High fuel consumption
Low cylinder head temperatures

2.7 Reduction Gearing

2.7.1 Purpose and Types

Reduction gearing allows the engine to operate at higher RPM while driving the propeller at lower, more efficient speeds.

Types:

Spur gear (parallel shafts)
Planetary gear (compact, coaxial)
Bevel gear (right-angle drives)

2.7.2 Gear Wear Patterns

Pitting

Small cavities on contact surfaces
Caused by high contact stress due to misalignment, concentrating load on small area
Insufficient lubrication causes scuffing or scoring (different pattern)
Excessive backlash causes noise and impact loading (not typically pitting)

Scuffing/Scoring

Caused by insufficient lubrication
Metal-to-metal contact and welding

2.8 Propeller Systems

2.8.1 Constant-Speed Propeller Operation

Governor Function

Maintains selected RPM by adjusting propeller blade angle
Flyweights: Centrifugal devices sensing engine/propeller RPM
When RPM deviates from selected value, flyweights move a pilot valve, directing oil to change propeller pitch

Response to RPM Changes:

RPM decreases: Governor reduces blade angle (fine pitch) to decrease aerodynamic load, allowing engine to speed up and return to set RPM
RPM increases: Governor increases blade angle (coarse pitch) to increase load, reducing RPM

Hydraulic Systems:

Oil pressure from governor moves blades towards fine pitch
If governor fails to supply pressure, propeller remains in coarse pitch
Counterweights or springs move blades towards coarse pitch (fail-safe)

2.8.2 Propeller-Engine Matching

Oversized Propeller

Increases load on engine
Reduces ability to reach rated RPM
Increases heat generation, leading to higher cylinder head temperatures

Undersized Propeller

Allows engine to over-speed
Reduces efficiency and performance

2.8.3 Governor Faults

Hunting or Surging

RPM fluctuates erratically at fixed throttle setting
Caused by incorrectly adjusted propeller governor
Governor constantly changes propeller pitch, causing RPM fluctuations
Not caused by air leaks (steady rough running), sticking valves (regular misfire), or accelerator pump (transient response only)

2.9 Engine Maintenance and Inspection

2.9.1 Propeller Strike

Definition

Sudden stoppage due to propeller striking an object

Required Action

Tear-down inspection or overhaul per manufacturer's instructions
Internal damage may not be visible externally
Check crankshaft, connecting rods, and bearings for damage
Critical safety requirement under Part-66 and manufacturer's instructions

2.9.2 Cylinder Inspection

Cracks

Serious defect compromising structural integrity
Standard practice: replace cylinder assembly
Repairs typically not approved for cracks in engine components

Discolouration

Bluish tint on cylinder barrel: overheating
Causes: blocked cooling fins, malfunctioning cowl flaps, incorrect fuel/air mixture

2.9.3 Dynamic Balancing

Purpose

Ensures combined effects of rotating and reciprocating masses are balanced
Reduces vibration for engine smoothness and longevity
Not simply about weight measurement or counterweight position (though related)

3. Important Formulas and Regulations

3.1 Key Formulas

Compression Ratio:

CR = (Vs + Vc) / Vc

Where:

Vs = Swept Volume (m³ or litres)
Vc = Clearance Volume (m³ or litres)

Volumetric Efficiency:

ηv = (Actual air mass inducted) / (Theoretical maximum air mass)

Magneto Drop:

Maximum acceptable = 120 RPM or 10% of engine RPM (whichever is greater)

Differential Compression Leakage:

Leakage % = [(Applied Pressure - Stabilised Pressure) / Applied Pressure] × 100

3.2 Regulatory References

EASA Part-66 (Regulation (EU) No 1321/2014, Annex III)

Module 16 syllabus defines knowledge requirements for B1.2 licence
Knowledge levels: 1 (overview), 2 (general knowledge), 3 (detailed theory)

Key Module 16 Sub-topics:

16.1 Piston Engine Fundamentals (Level 2/3)
16.2 Engine Performance (Level 2)
16.3 Cylinder and Valve Assemblies (Level 2/3)
16.4 Ignition Systems (Level 2/3)
16.5 Lubrication Systems (Level 2/3)
16.6 Engine Performance and Propeller Interaction (Level 2)
16.7 Carburettors and Fuel Systems (Level 2)
16.8 Reduction Gearing (Level 2)
16.9 Propeller Systems (Level 2)
16.10 Maintenance Practices (Level 2/3)

Manufacturer's Documentation

Aircraft Maintenance Manual (AMM)
Engine Maintenance Manual (EMM)
Overhaul Manual (OHM)
Service Bulletins and Airworthiness Directives

4. Common Relationships Between Concepts

4.1 Throttle Position and Volumetric Efficiency

Opening the throttle → Reduced induction restriction → Greater air mass → Higher volumetric efficiency → More power output

4.2 Valve Clearance and Engine Temperature

Insufficient valve clearance → Valve held off seat when hot → Combustion gas leakage → Valve overheating → Valve burning

4.3 Oil Contamination and Component Wear

Abrasive oil contamination → Protective oil film removed → Metal-to-metal contact → Spalling and pitting on camshaft lobes

4.4 Abnormal Combustion and Bearing Damage

Detonation/pre-ignition → Rapid pressure spikes → Bearing fatigue → Hammered/brinelled appearance on journals

4.5 Compression Test Leakage Paths

Leakage Location	Indicates
Oil filler cap / crankcase breather	Piston ring leakage
Intake system	Intake valve leakage
Exhaust system	Exhaust valve leakage
Cooling fins / exterior	Cracked head or cylinder

4.6 Magneto Drop and Ignition Faults

Excessive drop on one magneto → Fault in that ignition circuit (plugs, leads, magneto)

Equal drops on both → Engine-related issue or normal operation

4.7 Propeller Governor and RPM Control

RPM decrease → Flyweights move → Pilot valve directs oil → Blade angle decreases (fine pitch) → Load decreases → RPM increases

5. Typical Exam Focus Points

5.1 Diagnostic Reasoning

Candidates must be able to interpret symptoms and identify root causes:

Bearing Damage Patterns:

Scoring/discolouration → Oil starvation
Blue discolouration → Overheating
Hammered appearance → Detonation/pre-ignition
Lower half wear → Propeller thrust load

Compression Test Results:

Air from crankcase → Ring wear
Air from intake/exhaust → Valve leakage
Pressure improves with oil → Ring wear
Slow steady drop → Valve leak (vs rapid drop for ring/bore issues)

Magneto Drop Interpretation:

Excessive drop on one magneto → Fault in that circuit
Equal drops → Normal or engine-related
Drop > 120 RPM → Investigate before return to service

5.2 Maintenance Actions

Return to Service Criteria:

Any component exceeding service limits must be replaced or overhauled
Cracks in cylinder heads require replacement
Propeller strike requires tear-down inspection

Correct Procedures:

Magneto timing: TDC compression stroke, align drive gear with timing mark
Compression test: Rotate propeller in normal direction, position piston at TDC
Pre-oiling: Before start after storage or overhaul
Oil addition: Only correct grade/specification per AMM

5.3 System Interactions

Propeller-Engine Matching:

Oversized propeller → Reduced RPM capability, increased CHT
Governor faults → Hunting/surging

Fuel System Effects:

Saturated float → Rich mixture
Accelerator pump failure → Lean mixture, backfiring on acceleration

5.4 Calculation Skills

Compression Ratio Calculations:

Given CR and swept volume, find clearance volume
Given CR and clearance volume, find swept volume

Differential Compression:

Calculate leakage percentage from applied and stabilised pressures
Determine if within acceptable limits (25% typical)

5.5 Safety Considerations

Hydraulic Lock Prevention:

Rotate propeller in normal direction before compression test
Expels accumulated oil/fuel

Propeller Strike:

Mandatory tear-down inspection
Hidden internal damage possible

Cracked Components:

Replace, do not repair (unless approved)
Structural integrity compromised

Summary

Module 16 provides the comprehensive knowledge required for safe maintenance of aircraft piston engines. Key themes include:

Understanding normal operation to recognise abnormal conditions
Diagnostic reasoning to identify root causes from symptoms
Correct maintenance procedures per manufacturer's instructions
Regulatory compliance with Part-66 requirements
Safety prioritisation in all maintenance decisions

Successful candidates demonstrate integration of theory with practical application, understanding how different systems interact and how component failures manifest in observable symptoms.