Chapter 4: Piston Engine Variations

Piston Engine Variations

• Piston engine limitations: produces limited thrust, affecting how large an aircraft can be. Early solutions included adding more piston engines (e.g., large four-piston engine airliners in the 1940s) or increasing the number of cylinders (e.g., 12-cylinder engines in WWII fighters).
• Gas turbine (jet) engines now power most large aircraft. This chapter covers important piston-engine improvements to extract maximum performance.

Compression Ignition (Diesel) Engine

• Traditional piston engines discussed earlier used AVGAS (aviation gasoline) and spark plugs for ignition (spark-ignition).

• Compression ignition (diesel) engines run on diesel or jet fuel (jet fuel is the same fuel used in gas turbine engines) and have the same basic components as spark-ignition engines: cylinders, pistons, valves, connecting rods, and a crankshaft, typically operating on a four-stroke cycle.

• Key difference: ignition occurs via compression and fuel injection, not by a spark plug.

• Four-stroke cycle stages:

  • Intake Stroke: The piston moves downward and only air enters the cylinder (unlike spark-ignition engines where a fuel-air mixture enters at this stage).
  • Compression Stroke: The piston moves upward, compressing the air to a very high pressure and temperature. The compression ratio is typically much higher than in AVGAS engines, usually around ext{CR} \approx frac{20}{1} or 20:1.
  • Power Stroke: When the piston reaches the top, fuel is injected into the hot, compressed air. The hot air causes the fuel to spontaneously ignite, forcing the piston downward (no spark plugs required).
  • Exhaust Stroke: Exhaust gases are expelled through the exhaust valve.

• Diesel engines have long been used in cars, but aviation uptake has been slow. This is changing as jet fuel is often more accessible and cheaper, and diesel engines are typically more efficient.

• Figure reference: Figure 4.1 shows how a compression-ignition engine ignites fuel by injecting it into very hot, compressed air in the cylinder.

Two-Stroke Engine

• The four-stroke engine remains the most common design in small aircraft; however, modern two-stroke piston engines are appearing, mainly in very light aircraft such as ultralights.

• A two-stroke cycle completes the entire process in two strokes (one revolution of the crankshaft):

  • Stroke 1 (Intake, Compression, and Exhaust): The fuel-air mixture is forced into the cylinder via an inlet port while exhaust gases exit through an exhaust port located near the top of the cylinder. As the piston moves upward, it also compresses the incoming mixture.
  • Stroke 2 (Power Stroke): The piston closes off both the exhaust and fuel-air mixture ports. The mixture is then ignited, pushing the piston downward.

• Figure reference: Figure 4.2 illustrates a basic two-stroke cycle.

• Notes: Historically, two-stroke engines were not suitable for aircraft, but newer designs show promise due to compactness and fewer moving parts. They have also been developed using compression ignition.

Supercharging

• A piston engine can be naturally aspirated (the density of received air depends solely on ambient conditions). Air density is highest at sea level and decreases rapidly with altitude due to falling atmospheric pressure (e.g., around 18,000 feet, pressure is about half).

• In naturally aspirated engines, lower air density reduces the density of the fuel-air mixture and thus the engine’s power output at altitude.

• Supercharging compensates for reduced air density, enabling higher-performance operation at altitude. Supercharging is achieved with either a supercharger or a turbocharger.

• General principle: both systems compress incoming air before it enters each cylinder, offsetting the effects of lower ambient air density at higher altitudes, so the engine can maintain more sea-level-like power.

• Limitation: at some altitude, the supercharger/turbocharger can no longer compensate, and performance degrades relative to sea level.

• One major drawback of supercharging is that compression raises air temperature, which can lead to detonation if temperatures get too high. This limits how much air can be compressed.

• Very high compression can also damage the inlet manifold (air-delivery pipes).

• The core component of a supercharging system is the compressor, typically a centrifugal compressor with an impeller and diffuser:

  • Air enters the center of a rotating impeller and is accelerated outward by centrifugal force.
  • The diffuser surrounds the impeller and converts the accelerated air into high-pressure air.
  • After the diffuser, the compressed air is directed to each cylinder.

• The main difference between a supercharger and a turbocharger is how the compressor is driven:

  • Supercharger: Compressor is driven directly by the engine (geared to the crankshaft), usually via belts or a chain. They are not normally used in modern piston engines because they consume a portion of the engine’s power to drive the compressor, reducing overall efficiency.
  • Turbocharger: Compressor is driven by the engine’s exhaust gases, via a turbine connected to a shaft that drives the compressor.

• Figure reference: Figure 4.3 depicts a supercharger’s compressor being driven by the engine.

Turbocharger and Wastegate

• In a turbocharger, exhaust gases drive a turbine that spins a compressor, increasing the pressure of the air entering the cylinders.

• A critical component is the wastegate, which regulates compressor speed by diverting exhaust gases away from the turbine. This prevents over-compression, which could cause detonation or other issues.

  • When the wastegate is open, exhaust gases bypass the turbine, resulting in no turbocharging.
  • When the wastegate is closed, exhaust gases pass through the turbine, maximizing turbocharging.

• The wastegate can be either fixed or variable:

  • Fixed wastegate: Position is set before flight; the pilot cannot adjust it in flight.
  • Variable wastegate: Position can change in flight, either pilot-controlled or automatic.
  • Automatic wastegates are more complex but reduce pilot workload; different automatic settings exist, including altitude turbocharging (also called normalizing), which adjusts the wastegate to keep engine operation like sea level as high as possible.

• Figure reference: Figure 4.4 shows the turbocharger compressor driven by exhaust gases and controlled by a wastegate.

Reduction Gearbox

• In many small aircraft, the propeller is directly connected to the engine (direct-drive), meaning the propeller spins at the same speed as the engine. Most engines are designed to keep propeller speed below approximately ext{rpm} \lesssim 2700.

• Some piston engines run at very high speeds, e.g., up to around 5000 rpm, but if the propeller were directly driven at such speeds, the tips could exceed the speed of sound, causing excessive noise and a substantial loss of thrust.

• To maintain efficiency and manage propeller tip speed, aircraft with high-speed engines use a reduction gearbox between the engine and the propeller. This allows the engine to run fast while the propeller turns more slowly, improving overall efficiency.

• The reduction gearbox effectively lets the engine operate at higher speeds while the propeller remains at an efficient, lower speed.

• Figure reference: Figure 4.5 shows a reduction gearbox between the engine and the propeller.

• Summary: Modern piston engines gain benefits from higher-speed operation (power and efficiency) but require support systems (supercharging, turbocharging, and reduction gearing) to maintain performance, efficiency, and propulsive effectiveness at various flight conditions. The chapter notes that these enhancements come with their own trade-offs and maintenance considerations, and that all piston engines rely on a range of supporting systems to run smoothly.

• Connections to broader context and real-world relevance:

  • Early aviation solved thrust limitations by distributing power across multiple engines or using larger piston engines, but jet/turbine propulsion has largely superseded this approach for large airframes.
  • Diesel (compression-ignition) aviation engines offer potential efficiency gains and better fuel availability with jet fuel, influencing modern aircraft design and fuel strategy.
  • Two-stroke designs offer compactness and simplicity, finding niche applications in ultralights and light aircraft.
  • Supercharging and turbocharging extend high-altitude performance, enabling high-performance small aircraft to operate closer to sea-level power envelopes at altitude.
  • Reduction gearboxes enable high engine speeds to be used without sacrificing propeller efficiency and noise considerations, broadening the design space for piston-powered aircraft.

• Formulas and key numbers:

  • Compression ratio in diesel engines: ext{CR} \approx 20:1
  • Typical modern piston engine propeller speed limit: ext{rpm}_{ ext{propeller}} \lesssim 2700
  • High engine speeds in some designs: ext{rpm}_{ ext{engine}} \approx 5000\; ext{rpm}

• Figures referenced:

  • Figure 4.1: Compression ignition operation (fuel injection into hot, compressed air).
  • Figure 4.2: Basic two-stroke cycle.
  • Figure 4.3: Supercharger compressor driven by the engine.
  • Figure 4.4: Turbocharger driven by exhaust gases and wastegate operation.
  • Figure 4.5: Reduction gearbox between engine and propeller.

• Note on terminology:

  • “Supercharging” generally refers to forced induction by device driven by the engine (supercharger).
  • “Turbocharging” refers to forced induction driven by exhaust gases (turbocharger), with wastegate controlling boost.
  • “Normalizing” is another term for altitude compensation in automatic wastegate settings to maintain sea-level performance at higher altitudes.