Chapter 4: Piston Engine Variations

Question-and-answer flashcards covering piston engine variations, including diesel (compression ignition), two-stroke designs, supercharging, turbocharging, wastegates, and reduction gearboxes.

What is a main disadvantage of a basic piston engine, and how was this addressed in the 1940s?

It produces limited thrust; addressed by adding more piston engines or more cylinders (e.g., large four-piston airliners and 12-cylinder engines on fighters).

What fuels do compression ignition (diesel) piston engines use, and how do they ignite the fuel?

They run on diesel or jet fuel; fuel is injected into hot compressed air and auto-ignites, so no spark plugs are required.

In a compression ignition engine, what enters the cylinder during the intake stroke?

Air only enters the cylinder (no fuel-air mixture).

What is the typical compression ratio in a diesel engine?

About 20:1.

How is ignition achieved during the power stroke of a diesel piston engine?

Fuel is injected into the hot, compressed air and spontaneously ignites (auto-ignition).

Why has uptake of compression ignition piston engines in aviation been slow, and what is changing?

Diesel uptake was slow, but is increasing because jet fuel is more accessible/cheaper and diesel engines are often more efficient.

What is a two-stroke engine, and where is it mostly used in aviation?

A cycle completed in two strokes (not four); now seen mainly in very light aircraft like ultralights.

In a two-stroke engine, what happens during Stroke 1 (intake/compression/exhaust)?

Fuel-air mixture is forced into the cylinder via an inlet port; exhaust gases exit through the exhaust port; as the piston moves up, it compresses the incoming mixture.

What happens in Stroke 2 (power stroke) of a two-stroke engine?

The piston closes off both the exhaust and fuel-air ports; the mixture is ignited, pushing the piston down.

What is a naturally aspirated engine, and how does air density affect its power?

An engine that relies on ambient air density; lower air density at altitude reduces the density of the fuel-air mixture and decreases power.

What is supercharging, and what devices achieve it?

A method to compensate for reduced air density at altitude by compressing intake air using a supercharger or a turbocharger.

How does a centrifugal compressor in a supercharger operate?

Air enters the center of a rotating impeller and is accelerated outward by centrifugal force; the diffuser converts the accelerated air into high-pressure air.

What is the main difference between a supercharger and a turbocharger in terms of driving force?

A supercharger is driven by the engine (geared to the crankshaft); a turbocharger is driven by exhaust gases via a turbine.

What is a wastegate, and why is it important?

A valve that controls the turbine’s speed by regulating the amount of exhaust gases passing through the turbine to prevent over-boost (detonation).

What is a fixed wastegate versus a variable wastegate?

A fixed wastegate has a pre-set position; a variable wastegate can change position in flight (pilot-controlled or automatic).

What is altitude turbocharging (normalizing)?

An automatic setting that adjusts the wastegate to keep the engine running like sea level as high as possible.

What is a reduction gearbox and why is it used in some piston engines?

A gearbox between the engine and propeller that slows the propeller relative to the engine to prevent tip speeds approaching or exceeding the speed of sound, improving efficiency.

Why can’t the propeller spin at the engine’s high speed in many small aircraft, and what is the typical propeller rpm?

High engine speeds can push propeller tips toward or beyond the speed of sound, causing noise and thrust loss; most small engines keep propellers below about 2,700 rpm.

How does a reduction gearbox contribute to engine efficiency and performance?

It allows the engine to run at high speed where it’s efficient while keeping the propeller at a slower, efficient speed.