Chapter 8: Induction Systems

Fuel-Air Mixtures and Induction Systems

  • Key goal: burn fuel with the correct amount of oxygen (air) to achieve efficient combustion and prevent engine damage.
  • Ideal fuel–air ratio by weight: 1:12 (1 part fuel to 12 parts air).
  • Chemically correct mixture: 1:15, but a 1:12 mixture provides better power output in practice.
  • Combustion range (air–fuel ratio where combustion can occur): between 1:9 (rich) and 1:18 (lean).
    • Rich mixture: excess fuel remains after combustion, leading to unburned fuel.
    • Lean mixture: shortage of fuel, leaving some oxygen unburned.
  • Consequences: too little or too much fuel reduces efficiency and can damage the engine.
  • Induction systems in piston engines: carburetor system and fuel injection system.
  • Starting point for both systems: collect air from the external environment via an air intake (usually at the front of the aircraft).
  • Air preparation: air goes through an air filter to remove dust and debris.
  • Alternate air source: in case the air filter blocks, an alternate air source (inside the engine cowling) can be used, either manually or automatically.

Carburetor (Float-Type Carburetor Overview)

  • The float-type carburetor is common in older piston engines. It uses a float in the float chamber to regulate fuel delivery.
  • Air path: air from the intake passes through a venturi (a narrow section that accelerates air).
    • Venturi effect: acceleration lowers static pressure (Bernoulli principle: increased velocity → decreased pressure).
  • Air–fuel mixing controls: a butterfly valve (throttle valve) upstream of the venturi regulates air flow.
  • Fuel delivery in the venturi: a fuel jet connected to the float chamber draws fuel from high-pressure area to low-pressure venturi region, creating a fuel–air mixture.
  • Optional components for better mixing: an atomizer and diffuser in the fuel jet to help vaporize fuel and improve mixing.
  • Fuel flow through the jet is controlled by the butterfly valve position:
    • High power (throttle wide open): more air flows through venturi, larger pressure drop, more fuel drawn through the jet.
    • Descent or lower power: butterfly valve partially closed, less air flow, less fuel drawn.
  • Float chamber and fueling: the float keeps a constant fuel level and operates a needle valve to admit or stop fuel from the main fuel system.
    • High fuel demand (e.g., throttle fully open in climb): fuel in chamber drops, float drops, needle valve opens, more fuel enters.
    • Low fuel use: float rises, needle closes, preventing more fuel entry.
  • Accelerator pump: a small manual pump activated by throttle linkage provides a extra squirt of fuel when the throttle is opened quickly, reducing power lag.
  • Idling system: delivers fuel slightly upstream of the butterfly valve to keep the engine running at idle when air flow is low; bypasses the venturi to supply fuel to the other side of the butterfly valve.
  • Sea-level calibration: carburetors are calibrated for sea level air density; at altitude, air density is lower, causing a richer mixture if the mixture control is not adjusted.
  • Mixture control (lean/rich adjustment): a needle in the float chamber restricts fuel to the fuel jet to lean the mixture.
  • Leaning procedure:
    • Start with full rich at lower altitudes/take-off and descent below 2,000 ft.
    • As altitude increases, lean to improve fuel efficiency (cruising above 5,000 ft).
    • Monitor rpm: as you lean, rpm should rise; reach a peak and then begin to fall if too lean.
    • If rpm falls or engine runs rough, move mixture slightly toward the richer side.
    • Some aircraft use Exhaust Gas Temperature (EGT) gauges to find the optimum mixture at peak EGT.
    • Repeat leaning whenever cruising altitude or power setting changes.
  • Mixture control uses to shut down engine (idle cut-off): closing the mixture stops fuel entry into the float chamber, causing engine to stop due to fuel starvation.
  • Practical operation notes:
    • Full rich is recommended below 2,000 ft during take-off, landing, and descent.
    • Full rich also recommended during high-power cruise (more than ~75% power) for cooling.
    • Leaning is beneficial during cruise above 5,000 ft for fuel efficiency.
  • Two main problems with float-type carburetors:
    1) Not suitable for abrupt maneuvers (aerobatics): fuel flow relies on gravity and can be interrupted.
    2) Carburetor ice (fuel or throttle ice) can form around the venturi region.
  • Carburetor icing characteristics:
    • Ice can form even when outside air temperature is above freezing (0°C / 32°F) because venturi cooling can drop air temperature significantly (up to ~35°C).
    • Icing is more likely around the venturi and near the butterfly valve during low-power settings due to enhanced cooling.
    • Prevention: use carburetor heat (warm air) when extended operation at low power is required; do not overuse carb heat, as it reduces power due to less dense air.
    • Ground icing is also possible during taxiing when the engine is cold; a proper engine run-up helps prevent/correct icing.
    • First signs: rough engine operation and degraded power; if untreated, engine may stop due to fuel starvation.

Fuel Injection Systems (Modern Piston Engines)

  • Fuel delivery improvements over carburetors:
    • Fuel is delivered under pressure to each cylinder by fuel pumps. Systems typically include two pump types:
    • Engine-driven pump: operates when the engine is running (powered by the engine).
    • Electric boost/auxiliary pump: used during starting and as a backup if the engine-driven pump fails.
    • Fuel-air mixing happens after the air passes the throttle, but fuel is injected into the intake manifold just before each cylinder (some systems inject directly into the cylinder).
  • Continuous-flow fuel injection (common in many aircraft):
    • Initial components arrive similar to carburetor air path, but fuel does not mix with air in the venturi.
    • Fuel is sent to a Fuel Control Unit (FCU) linked to throttle and mixture controls.
    • FCU regulates how much fuel is delivered to each cylinder based on throttle/mixture settings.
    • Fuel travels from FCU to a Fuel Manifold Unit (FMU), then to separate fuel discharge nozzles for each cylinder.
    • Nozzles spray fuel into the inlet just before the intake valve; fuel mixes with incoming air as the intake valve opens.
    • This arrangement is known as a continuous-flow system because fuel is continuously injected and drawn into cylinders on intake stroke.
  • Direct fuel injection (for larger engines):
    • Fuel is injected directly into each cylinder via high-speed spray nozzles.
    • Benefits include rapid vaporization and better mixture control per cylinder; reduces carburetor ice risk, since fuel is injected after the venturi region.
  • Components and flow path (typical minimum): air intake → throttle butterfly valve → FCU → FMU → fuel discharge nozzles → intake valves/cylinders; air and fuel mix just before entry to the cylinder.
  • Advantages of fuel injection over carburetion:
    • Eliminates carburetor ice (fuel injected after venturi region).
    • Maintains correct mixture per cylinder, improving efficiency and power delivery, especially during abrupt maneuvers.
    • Generally better fuel efficiency and reliability.
  • Common issues with fuel injection:
    • Hot starting (vapor lock): after shutdown, residual fuel remains in warm lines inside the cowling, vaporizes and blocks fuel flow; solved by using the electric boost pump to pressurize lines and clear vapor.
    • Fuel contamination risks: dirt or water can clog fine fuel lines or injector nozzles.
  • Intake icing (relevant to injection systems, though not caused by carburetor):
    • Intake icing can still occur at the air intake or along the intake path where air turns, especially at certain angles or humidity.
    • Icing more likely when flying through moisture and at certain temperatures; ice blockage can severely degrade power or cause engine failure.
  • Broader performance considerations:
    • Even with correct fuel–air mixture, engine performance can still drop at high altitude due to thinner air.
    • Some aircraft employ supercharging to maintain power at altitude.
  • Intake system environment and hazards:
    • Inlet ice (impact ice) can form on the air intake or inside the inlet manifold, reducing air flow and power.
    • Figure references: intake ice can visually occur around the intake (Figure 8.5).

Altitude Effects and General Considerations

  • Air density decreases with altitude, causing a richer mixture if the pilot does not re-lean after altitude change.
  • For carburetor-equipped aircraft, leaning is often necessary during cruise at higher altitudes to maintain efficiency and prevent rough running.
  • For injection systems, altitude effects are less about mixture drift and more about overall air density and engine efficiency; some engines use supercharging to compensate for altitude losses.
  • Operational guidance summary:
    • Carburetor-equipped: lean into cruise after reaching desired altitude; use full rich for takeoff/landing/descents below 2,000 ft or during high-power cruise (>75% power);
    • Injection-equipped: rely on FCU/FMUs to maintain proper fuel delivery; monitor for hot starting and contamination; manage intake icing risk with proper inlet protection and anti-icing as needed.

Practical Implications and Safety Considerations

  • Correct fuel–air mixture is critical for smooth running, power, and engine longevity; both systems require proper handling of mixture control and power settings.
  • Leaning must be adjusted when changing altitude or power settings to avoid rough running or overheating.
  • Icing management is essential in carburetor-equipped aircraft; carb heat is a critical tool but reduces power, so use judiciously.
  • Modern fuel-injected systems reduce many carburetor-related issues but introduce others (hot starts, contamination, and intake icing) that require awareness and proper procedures.
  • The induction system is a key determinant of engine performance, efficiency, and safety, with implications for fuel economy, emissions, and risk of engine failure due to fuel starvation or icing.

Quick Reference: Key Ratios, Percentages, and Thresholds

  • Ideal fuel–air ratio (by weight): 1:12
  • Chemically correct mixture: 1:15
  • Combustion viable range: 1:9 ext{ to } 1:18
  • Cruise power reference: greater than 75 mph{ ext{ }}
  • Typical cruise power range: 55 ext{ to }65 ext{%}
  • Leaning altitude guideline: lean for cruise above 5{,}000 ext{ ft}
  • Carburetor icing temperature window: commonly occurs around 5^ ext{C} ext{ to } 25^ ext{C} (41°F to 77°F), but can occur in warmer, moist conditions as well.
  • Temperature drop in venturi due to Bernoulli effect can be significant (up to around 35^ ext{C}$$) which promotes icing.

Common Figures (Referenced in Text)

  • Figure 8.1: Cross-section of a basic float-type carburetor and venturi components.
  • Figure 8.2: Idling system and mixture control in a simple float-type carburetor.
  • Figure 8.3: Carburetor ice formation around venturi and butterfly valve.
  • Figure 8.4: Basic components of a simple fuel injection system (FCU, FMU).
  • Figure 8.5: Intake ice (impact ice) around air intake or inlet manifold.

Final Takeaways

  • Induction systems are designed to mix air and fuel efficiently to achieve the desired combustion across a range of operating conditions.
  • Carburetors are simple and robust for older or smaller aircraft but are susceptible to icing and fuel-delivery lag; require leaning and carb heat management.
  • Fuel injection systems provide precise fuel metering, better mixture consistency across cylinders, improved efficiency, and no carb ice, but come with hot-start and contamination concerns.
  • Understanding and managing mixture, power, and icing are essential for safe and efficient flight operations, especially at varying altitudes and operating conditions.