EnginesandSystems

The engine in almost every training aircraft is a four-stroke piston engine, and the cycle it runs is the same one a car engine runs, arranged differently.

Intake: the piston moves down with the intake valve open, drawing a mixture of fuel and air into the cylinder. Compression: both valves close and the piston moves up, squeezing the mixture into a fraction of its original volume — which raises its temperature and makes it burn far more energetically. Power: the spark plugs fire near the top of travel, the mixture burns, and the expanding gas drives the piston down. This is the only stroke that produces work. Exhaust: the exhaust valve opens and the piston moves up, pushing the burnt gas out.

Two revolutions of the crankshaft for one power stroke per cylinder. A four-cylinder engine therefore produces two power strokes per revolution, which is what makes the running reasonably smooth.

Aircraft engines are usually horizontally opposed — cylinders lying flat in two banks either side of the crankshaft — and air-cooled, with fins on the cylinders and baffling that directs airflow over them. Both choices are about the same two things: frontal area and weight.

An aircraft piston engine has two spark plugs in every cylinder, fired by two entirely separate magnetos. This is unusual — cars have one plug per cylinder and take their ignition from the battery — and it is done for two reasons.

The first is redundancy. A magneto is a self-contained generator: it makes its own electricity from the engine’s rotation and needs nothing from the aircraft’s electrical system. Lose the alternator and the battery both, and the engine keeps running. Lose one magneto and the engine keeps running on the other, with a small power loss.

The second is combustion quality. Two flame fronts starting from opposite sides of the combustion chamber burn the charge faster and more completely than one, which produces more power and less residual unburnt fuel.

This is why the run-up before every flight includes a magneto check: the pilot switches to one magneto at a time and watches for the expected small drop in RPM. No drop at all means one magneto is not actually being switched off, which means a live ignition system on an engine everybody believes is dead — a hazard to anybody who moves the propeller by hand.

A gas turbine does exactly what a piston engine does — draw air in, compress it, burn fuel in it, extract work from the expansion — with one structural difference that changes everything. In a piston engine the four events happen in sequence in the same place. In a turbine they happen simultaneously in different places, and the gas flows through continuously.

Five sections, in order. The inlet slows and straightens the incoming air. The compressor raises its pressure, in a series of stages, each stage a rotating disc of blades followed by a stationary ring of vanes; a modern engine may have a dozen or more stages and a pressure ratio above 40:1. The combustor introduces fuel and burns it continuously, at constant pressure. The turbine sits directly behind the combustor and extracts energy from the hot gas — and its first job is to drive the compressor in front of it, through a shaft that runs down the middle of the engine. Whatever energy is left goes out of the exhaust.

That shaft is the key idea. A turbine engine is a machine that spends most of its output driving its own compressor. In a typical turbojet, the great majority of the turbine’s work goes into compression, and the useful output is what remains.

The consequence is that there is no separate power stroke and no reciprocating mass. A turbine has far fewer moving parts than a piston engine of equivalent output, all of them rotating in one direction, which is why turbines are both lighter for their power and dramatically more reliable.

A pure turbojet takes all its air through the core and produces thrust by throwing a modest amount of air backward very fast. It works, and at high supersonic speeds it is the right answer, but at airliner speeds it is inefficient and extremely loud.

The insight behind the turbofan is that thrust is mass flow multiplied by velocity change, and that the same thrust can be produced by moving a lot of air a little faster or a little air a lot faster. Moving a lot of air slowly is far more efficient, because the wasted energy left behind in the jet goes up with the square of the velocity.

So a turbofan puts a large fan at the front, driven by the turbine, and routes most of the air around the core rather than through it. On a modern high-bypass engine the ratio is somewhere around 9:1 to 12:1, and the great majority of the thrust — commonly quoted at around 80 per cent — comes from the bypass air. The core has become, in effect, a gas generator whose job is to drive the fan.

This also explains the shape. The enormous diameter of a modern airliner engine is not the core getting bigger; the core is comparatively small and buried. It is the fan.

The fuel system feeds it. Tanks in the wings, a selector valve the pilot controls, an engine-driven pump, and usually an electric boost pump for starting and as a backup. Fuel quantity gauges in light aircraft are notoriously imprecise, which is why the fuel check before flight is a visual check of the tanks and a calculation, not a look at a gauge.

The electrical system runs an alternator driven by the engine, a battery, and a bus that distributes power to everything through circuit breakers. Its most important property in a piston aircraft is what it is not responsible for: the engine’s ignition. A complete electrical failure in a light piston aircraft is a serious problem for the radios, the lights and some instruments, and the engine does not notice.

The vacuum system, in older aircraft, drives the gyroscopic instruments using an engine-driven pump. It is a single point of failure for the attitude and heading indicators, and its failure is difficult to detect quickly because the gyros slow down gradually rather than stopping — which is precisely the failure mode most likely to be missed in cloud.

On larger aircraft, hydraulics operate the landing gear, flaps, brakes and flight controls, and pressurisation and anti-icing run on bleed air tapped from the compressor. Cabin pressurisation works by pumping in more air than is allowed out through a controlled outflow valve; the cabin altitude is set by how much is let out, not by how much is pumped in.