Module 16: Piston Engine – Comprehensive Study Material

1. Module Overview

This module provides the foundational knowledge required for certifying staff working on piston-engine-powered aeroplanes under EASA Part-66. It covers the design, construction, operation, and maintenance of reciprocating (piston) engines, including their associated systems. The syllabus is structured into sub-modules (16.1 to 16.8) that progress from fundamental principles to complex system interactions.

The module is divided into the following key areas:

16.1 Fundamentals: Engine construction, materials, and operating principles
16.2 Engine Performance: Power, efficiency, and performance parameters
16.3 Engine Fuel and Fuel Metering Systems: Carburettors and fuel injection
16.4 Ignition Systems: Magnetos, spark plugs, and timing
16.5 Lubrication Systems: Oil types, pumps, filters, and cooling
16.6 Exhaust Systems: Manifolds, mufflers, and turbocharging
16.7 Propeller Systems: Constant-speed propellers and governors
16.8 Engine Controls: Throttle, mixture, and propeller controls

Knowledge levels range from Level 1 (overview) for basic principles to Level 3 (detailed theory) for critical maintenance procedures and troubleshooting.

2. Key Concepts Explained in Detail

2.1 Engine Fundamentals and Construction (16.1)

Crankshaft and Counterweights

The crankshaft converts reciprocating motion of the pistons into rotational motion. It is a critical structural component that must maintain precise dynamic balance. Crankshaft counterweights are individually weighed and marked with their part number and weight in grammes. This marking is essential because:

Each counterweight is manufactured to a specific weight tolerance
They must be reinstalled in their original position to preserve the dynamic balance of the rotating assembly
Incorrect installation can cause severe vibration and premature bearing failure

Crankshaft Journal Inspection

Crankshaft journals (the bearing surfaces) can develop scratches or nicks during operation. The correct action depends on the severity:

Minor scratches within limits: Polish to remove stress risers using approved methods
Scratches exceeding limits: Replace or repair according to approved data (AMM or engine manufacturer's manual)

Cylinder Construction and Inspection

Cylinders in radial and horizontally opposed engines are critical components that require regular inspection. Cylinder heads are particularly susceptible to cracking in high-stress areas, especially:

Around spark plug holes
Around valve seats
Between the valve seats and spark plug holes

Inspection methods include:

Thorough visual inspection (primary method)
Non-destructive testing (NDT) such as dye penetrant or eddy current if cracks are suspected

Cylinder Installation

When replacing a cylinder, the hold-down nuts must be tightened evenly in a crisscross pattern to avoid distortion of the cylinder base and crankcase. After a warm-up cycle, the gasket may compress, requiring re-torque to the specified value. This procedure is mandatory per the AMM.

2.2 Engine Performance and Valve Systems (16.2)

Valve Clearance

Valve clearance (tappet clearance) is the gap between the valve stem and the rocker arm or tappet when the valve is closed. This clearance is critical because:

Too small: The valve cannot fully close, allowing hot combustion gases to escape past the valve. This causes:
Overheating of the valve
Burning of the valve face and seat
Loss of compression
Reduced engine performance
Too large: The valve opens late and closes early, reducing valve lift and duration, which affects engine breathing and performance.

Typical clearances are specified as a range (e.g., 0.008–0.012 inches for Lycoming O-360). Any value within this range is acceptable; adjusting to the midpoint is not required.

Hot Magneto Drop

A "hot" magneto drop (a drop that increases when the engine is hot) is often caused by excessive exhaust valve clearance. As the engine heats up, the valve train expands, but if clearance is excessive, the valve may not fully close, leading to poor sealing and a rough drop. This is a classic symptom that distinguishes valve issues from ignition component faults.

Compression Testing

Differential compression testing measures the cylinder's ability to hold pressure. The procedure involves:

Position the piston at TDC on the compression stroke (both valves closed)
Apply a standard test pressure (typically 80 psi)
Measure the leakage (e.g., 25/80 means the cylinder holds 55 psi)

Critical safety point: The piston must be at TDC on the compression stroke to prevent the propeller from being forced to rotate when air pressure is applied. At TDC, the piston has minimal leverage on the crankshaft.

Typical limits:

Maximum allowable leakage: 25% (20/80) or 30% (24/80) depending on engine
A reading of 25/80 (31%) exceeds the common 25% limit and is unserviceable

During the test:

Throttle must be fully open to allow maximum air into the cylinder
Mixture must be in idle cut-off to prevent fuel from being drawn in (fire hazard)
Ignition must be off

2.3 Fuel and Fuel Metering Systems (16.3)

Float-Type Carburettors

The float-type carburettor operates on the principle of atmospheric pressure and venturi vacuum. Key components include:

Float chamber: Maintains a constant fuel level
Venturi: Creates a low-pressure area to draw fuel
Main metering jet: Controls fuel flow at high power
Idle circuit: Provides fuel at low throttle settings
Accelerator pump: Compensates for rapid throttle opening
Mixture control: Adjusts fuel/air ratio

Idle Circuit

The idle circuit provides fuel at low throttle settings when the venturi vacuum is insufficient to draw fuel through the main jet. An incorrect idle mixture adjustment (too lean or too rich) results in rough running or misfiring at idle, while the main metering jet only affects higher power settings.

Accelerator Pump

When the throttle is opened rapidly, the sudden increase in airflow temporarily leans the mixture. The accelerator pump injects a shot of fuel into the airstream to compensate, preventing hesitation. This is a small piston or diaphragm pump activated by throttle movement.

Idle Cut-Off

Moving the mixture control to idle cut-off closes the mixture valve, stopping fuel flow to the discharge nozzle. The engine stops because it is starved of fuel. Residual fuel in the float bowl does not sustain running because it is not drawn into the discharge nozzle without the mixture valve open.

Troubleshooting Rich Idle

A rich mixture at idle can be caused by:

Clogged air filter (insufficient air)
Idle mixture screw set too rich
Float level too high

Note: Low fuel pressure or low float level would cause a lean condition, not rich.

Continuous-Flow Fuel Injection

Continuous-flow fuel injection systems deliver fuel directly to each cylinder intake port. The primary advantage over a carburettor is:

More balanced mixture distribution between cylinders
Reduced risk of carburettor icing
Improved performance and fuel economy

Water in Fuel

Water in fuel is a serious hazard as it can cause engine failure. The correct action when water is found in a fuel sample:

Drain the fuel until the sample is clear of water
Continue the pre-flight
If water persists, further investigation is required

2.4 Ignition Systems (16.4)

Magneto Timing

Magneto timing is set by:

Positioning the crankshaft at the specified BTDC on the compression stroke (per AMM)
Adjusting the magneto so that the breaker points just begin to open
This corresponds to the firing point for that cylinder

The procedure uses a timing disc (degree wheel) on the propeller flange and a continuity light (buzzer) to detect when the points open. Some engines use a strobe light for dynamic timing.

Ignition Timing Tolerance

Ignition timing must be set to the specified value with tight tolerances (typically ±1°). A deviation of 3° is not acceptable. Advanced timing can cause detonation, which can severely damage the engine.

Magneto Drop Check

The magneto drop check is performed during ground run-up:

Run the engine at a specified RPM (typically 1700–2000 RPM)
Switch from "BOTH" to "LEFT" and note the RPM drop
Switch back to "BOTH" and then to "RIGHT" and note the RPM drop
Maximum allowable drop is typically 120 RPM (varies by engine)
Maximum difference between magnetos is typically 60 RPM

A drop exceeding the limit indicates a fault in that magneto, such as:

Faulty spark plug
Faulty lead
Internal magneto component failure

Isolating Faulty Cylinders

To isolate a faulty spark plug or cylinder:

Run the engine on one magneto at a low RPM (typically 1000 RPM)
Short (ground) each cylinder in turn using the ignition switch or a momentary grounding lead
If shorting a cylinder causes no additional RPM drop, that cylinder is not contributing power, indicating a fault in that cylinder's ignition

Wet Spark Plugs

Wet spark plugs indicate that fuel is reaching the cylinders but not igniting. This is often due to:

Over-priming
Rich mixture

The correct action is to clear the flood by cranking with the throttle open and mixture at idle cut-off as per the pilot's operating handbook.

2.5 Lubrication Systems (16.5)

Oil Dilution System

Some radial and inline engines are equipped with an oil dilution system. This system:

Injects a small amount of fuel into the oil before shutdown in cold weather
Thins the oil, reducing drag on the starter
Allows the engine to crank faster for easier starting
Once the engine warms up, the fuel evaporates from the oil

Pre-Oiling Procedure

Pre-oiling is performed after an engine has been inactive for a period to ensure that oil reaches all critical components before the engine is started. This prevents damage from lack of lubrication during the initial cranking.

Oil Level Check

The most accurate time to check oil level is:

After a stabilization period (typically 10–15 minutes after shutdown)
This allows oil to return from the galleries and components to the sump
Checking immediately after shutdown gives a false low reading

Oil Analysis

Metal particles in oil indicate abnormal wear or damage inside the engine:

Normal wear produces very fine particles
Visible metal chips are a red flag requiring further investigation
Oil analysis (spectrometric) can identify specific metals and their sources

High Oil Consumption

High oil consumption without external leaks is typically caused by oil passing through the combustion chamber via:

Worn piston rings
Worn valve guides
Defective valve stem seals

This oil is burned, leading to high consumption and possibly blue exhaust smoke.

Oil Temperature Issues

High oil temperature can be caused by:

Blocked oil cooler (reduces heat dissipation)
Faulty oil pump (reduces flow)
Oil cooler bypass valve stuck open (oil bypasses the cooler)

A stuck-open pressure relief valve would cause low oil pressure, not necessarily high temperature.

Breather System

The engine breather (crankcase ventilation) system vents internal pressure and oil mist to the atmosphere. Small oil deposits near the breather outlet are normal. A partially blocked breather line can cause pressure build-up and oil leakage.

Cold Weather Operations

At low temperatures, oil viscosity increases, leading to inadequate lubrication during start. Most manufacturers recommend pre-heating to at least 0°C (some to 10°C) for piston engines.

2.6 Exhaust Systems and Turbocharging (16.6)

Exhaust System Inspection

The primary reason for checking for cracks and leaks in the exhaust manifold and muffler is to prevent carbon monoxide (CO) from entering the cockpit/cabin. CO is a serious safety hazard as it is odourless and can cause incapacitation.

Exhaust Smoke Diagnosis

Black smoke: Rich mixture
Bluish-white smoke: Oil burning (worn valve guides or rings)
White smoke with sweet smell: Coolant leak

Turbocharger Waste Gate

The waste gate is a variable valve in the exhaust system that diverts exhaust gas flow around the turbine. By adjusting the waste gate position:

Turbocharger speed is regulated
Boost pressure (manifold pressure) is controlled
Overboosting is prevented

2.7 Propeller Systems (16.7)

Constant-Speed Propeller Operation

Constant-speed propellers use engine oil pressure to adjust the propeller blade pitch. The governor:

Senses engine RPM
Adjusts oil pressure to the propeller hub
Changes blade angle to maintain the selected RPM

Governor Functional Check

The proper functional check of a constant-speed propeller governor during a ground run is to:

Cycle the propeller through its full range of RPM
Monitor engine RPM and oil pressure
Verify that the governor and propeller pitch change mechanism are working correctly

Governor Failure Modes

Engine accelerates beyond governed RPM: The propeller cannot move to a higher pitch (stuck low-pitch mechanism). The governor cannot increase the blade angle to absorb power.
Propeller does not respond to governor adjustments: The governor is not receiving oil pressure, so it cannot adjust pitch.
RPM fluctuations during flight: Low oil pressure prevents the governor from making necessary pitch changes.

2.8 Engine Controls (16.8)

Control Cable Inspection

A frayed control cable is a safety hazard and must be replaced, not lubricated or adjusted. The carburettor heat control is a critical engine control; any defect must be rectified before further flight.

Control Cable Lubrication

The correct lubricant for control cables is specified in the AMM or maintenance manual. Using the wrong lubricant can attract dirt or damage the cable.

3. Important Formulas, Regulations, and Procedures

3.1 Key Formulas

Manifold Pressure (Naturally Aspirated)

At sea level, a naturally aspirated engine should develop close to ambient atmospheric pressure (approximately 29.92 inHg) at full throttle. A reading of 20 inHg indicates a significant restriction.

Propeller Track Tolerance

Typical limit: 0.125 inches (3.175 mm) for most piston engines. This ensures balanced thrust and reduces vibration.

Compression Test Limits

Maximum allowable leakage: 25% (20/80) or 30% (24/80) depending on engine
Minimum acceptable reading: 70/80 psi (varies by engine)

Magneto Drop Limits

Maximum allowable drop: 120 RPM (varies by engine)
Maximum difference between magnetos: 60 RPM (varies by engine)

3.2 Regulatory References

Part-145.A.50 (Certification for Return to Service)

A certifying staff must not certify an aircraft for return to service if it is known to be unairworthy. A crankshaft flange run-out out of limits is a structural/safety issue; the aircraft must be grounded and maintenance performed per the AMM.

Part-66 Module 16 Syllabus

The module is aligned with Appendix I of Regulation (EU) No 1321/2014, Annex III. Knowledge levels:

Level 1: Overview (familiarisation with basic concepts)
Level 2: General knowledge (understanding of principles and procedures)
Level 3: Detailed theory (in-depth knowledge for troubleshooting and certification)

3.3 Critical Procedures

Cylinder Hold-Down Nut Torquing

Tighten nuts evenly in a crisscross pattern
Follow specified torque sequence per AMM
After warm-up cycle, re-torque to compensate for gasket compression

Magneto Timing Procedure

Position crankshaft at specified BTDC on compression stroke
Install timing disc on propeller flange
Connect continuity light to magneto points
Adjust magneto until points just begin to open
Lock magneto in position
Verify timing per AMM

Compression Test Procedure

Remove spark plugs from cylinder being tested
Position piston at TDC on compression stroke
Open throttle fully
Set mixture to idle cut-off
Ensure ignition is off
Apply test air pressure (80 psi)
Measure leakage
Compare to AMM limits

Engine Pre-Heat Requirements

Below 0°C: Pre-heat required for most engines
Some manufacturers specify pre-heat below 10°C
Pre-heat ensures adequate oil flow and lubrication during start

4. Common Relationships Between Concepts

4.1 Oil System and Propeller Governor

The constant-speed propeller governor uses engine oil pressure to adjust blade pitch. Therefore:

Low oil pressure → Governor cannot adjust pitch → RPM fluctuations
High oil temperature → Reduced oil viscosity → Reduced governor effectiveness
Oil dilution → Reduced viscosity for cold starts → Normalises as engine warms

4.2 Valve Clearance and Engine Performance

Excessive clearance → Hot magneto drop, valve noise, reduced performance
Insufficient clearance → Valve burning, loss of compression, overheating
Correct clearance → Optimal performance, proper valve cooling

4.3 Fuel System and Engine Operation

Rich mixture → Black smoke, rough idle, fouled spark plugs
Lean mixture → Backfiring, overheating, loss of power
Water in fuel → Engine failure, hesitation, rough running
Clogged air filter → Rich condition, reduced manifold pressure

4.4 Ignition Timing and Engine Protection

Advanced timing → Detonation risk, engine damage
Retarded timing → Loss of power, overheating, hard starting
Correct timing → Optimal performance, safe operation

4.5 Compression and Engine Condition

Low compression → Loss of power, oil consumption, rough running
Uneven compression → Engine vibration, misfiring
Compression test results → Indicate ring, valve, or head condition

5. Typical Exam Focus Points

5.1 Critical Safety Items

Carbon monoxide from exhaust leaks is the primary reason for exhaust inspection
Water in fuel requires immediate draining and investigation
Frayed control cables must be replaced, not deferred
Crankshaft flange run-out out of limits requires grounding
Engine mount cracks require replacement or approved repair

5.2 Diagnostic Skills

Smoke colour diagnosis: Black = rich, bluish-white = oil, white sweet = coolant
Magneto drop interpretation: Exceeding limits = fault, hot drop = valve clearance
Compression test interpretation: Below limits = unserviceable
Oil analysis: Metal particles = internal damage
Manifold pressure readings: Low at full throttle = restriction

5.3 Procedure Knowledge

Cylinder installation: Crisscross torquing, re-torque after warm-up
Compression test: TDC on compression stroke, throttle open, mixture idle cut-off
Magneto timing: Points just opening at specified BTDC
Oil level check: 10–15 minutes after shutdown
Pre-oiling: After long inactivity

5.4 Component Identification

Accelerator pump: Compensates for rapid throttle opening
Waste gate: Regulates turbocharger boost
Oil dilution: Thins oil for cold starts
Counterweight markings: Preserve dynamic balance

5.5 Tolerance and Limit Knowledge

Valve clearance: Range acceptable, not midpoint
Ignition timing: ±1° tolerance
Magneto drop: 120 RPM maximum (typical)
Propeller track: 0.125 inches (3.175 mm)
Compression: 25% maximum leakage (typical)

5.6 Troubleshooting Scenarios

Engine overspeeds beyond governed RPM → Stuck low-pitch mechanism
Propeller unresponsive to governor → No oil pressure to governor
High oil temperature → Blocked cooler or bypass valve stuck open
Rough idle, smooth at power → Idle circuit problem
Wet spark plugs → Over-priming or rich mixture

Summary

Module 16 requires a thorough understanding of piston engine systems and their interrelationships. The key to success is understanding not just what to do, but why. Every procedure has a safety or performance rationale, and every symptom has a logical cause. Certifying staff must be able to:

Identify components and their functions
Diagnose faults from symptoms
Perform maintenance procedures correctly
Interpret test results against limits
Apply regulatory requirements for return to service

The material in this module forms the foundation for safe and effective maintenance of piston-engine-powered aeroplanes. Mastery of these concepts is essential for the Part-66 examination and for professional practice as certifying staff.