Petrol against diesel
Both engine families run the same four strokes with the same moving parts; the differences are in how the fire starts and how power is set:
Means of ignition — petrol ignites its pre-mixed charge with a timed spark; diesel injects fuel into air so hot from compression that it ignites by itself, needing no ignition system at all (the glow plugs some diesels carry are cold-starting aids, not ignition).
Maximum compression ratio — petrol is limited to roughly 7:1 to 9:1, because squeezing a ready-to-burn mixture harder makes it hotter until it explodes on its own (the detonation section takes this up). Diesel compresses only air, so it can and must go far higher, typically 15:1 to 25:1: high enough that the compression heat itself lights the injected fuel.
Regulating the supply to the cylinder — the petrol engine's throttle valve chokes the amount of mixture admitted, so power is set by how much charge each cylinder receives. The diesel engine admits a full, unthrottled charge of air on every induction stroke and sets power purely by how much fuel is injected into it. The classic shorthand: the petrol engine meters quantity (of mixture), the diesel meters quality (the fuel content of an unchanging air charge).
Pollution from the exhaust — petrol engines burn a mixture that is often richer than chemically ideal, leaving carbon monoxide and unburned fuel in the exhaust, and AVGAS adds lead. The diesel burns lean in excess air, producing less carbon monoxide from lead-free fuel, at the cost of more soot and oxides of nitrogen.
The fuels and their colours
The fuel-system module introduced the piston fuels; what matters here is matching fuel to engine at the filler neck. Petrol engines burn AVGAS (aviation gasoline), and the grades are dyed so a glance at a fuel sample identifies them:
AVGAS 100 — dyed green: 100-octane gasoline with the full lead content.
AVGAS 100LL — dyed blue: the same 100-octane rating with less lead (LL, low lead), and the grade most of the training fleet burns.
Aviation diesel engines normally burn JET-A1, the same kerosene the turbine fleet uses: available at practically every airport in the world, which is a large part of the aviation diesel's reason to exist. The densities to memorise repeat the fuel module's figures: about 0.72 kg per litre for AVGAS and about 0.8 kg per litre for diesel fuel, so a litre of diesel fuel is roughly a tenth heavier, and mass and volume must never be confused when fuel is ordered in one and logged in the other.
The octane rating is the number in the grade names: a fuel's resistance to detonation, measured against a reference scale on which a detonation-resistant reference fuel (iso-octane) scores 100 and a detonation-prone one (normal heptane) scores 0. The higher the octane rating, the harder the fuel is to explode by pressure and heat alone, and the more compression and power its engine can safely use. The rating says nothing about the energy in the fuel: it grades manners, not strength.
Detonation
Detonation is combustion gone explosive. Normally the spark lights the charge at one point and a flame front burns smoothly across the chamber in a few milliseconds, pushing the piston progressively. If the last unburned pocket of charge, squeezed and heated by the advancing flame, reaches its self-ignition temperature before the flame arrives, it explodes all at once: pressure spikes instead of swelling, and the shock hammers the piston crown and the cylinder walls.
The causes in a petrol engine are anything that makes the end charge hotter or more willing to self-ignite: fuel of a lower octane rating than the engine demands, high cylinder pressures from high power settings, hot induction air, an overheated engine, and a mixture leaned toward the setting that burns hottest. The conditions that promote it in service follow directly: high manifold pressure combined with low rpm, take-off and climb power held with the mixture too lean, a hot day, a long slow climb with little cooling airflow, and refuelling with too low a grade. In a diesel engine the equivalent disease has a different trigger: ignition there is self-ignition by design, but if injected fuel accumulates before it lights (a long ignition delay), because the engine is cold or the fuel ignites too reluctantly, the accumulated charge then burns violently in one lump, giving the same hammering pressure rise (diesel knock).
The effects are mechanical and thermal at once: hammering pressure waves, rapidly rising cylinder head temperature, scorched and eroded piston crowns, broken rings, loss of power, and, if it continues, destruction of the cylinder. Recognising it is harder in the air than in a car: behind the propeller's noise the knock is usually inaudible, so the pilot's evidence is rough running, unexplained loss of power, and a cylinder head temperature climbing for no visible reason. The response, treated fully in the mixture lesson, amounts to removing the causes: richer mixture, less power, more cooling.
Pre-ignition
Pre-ignition is the other combustion disease, and the distinction between the two is a favourite exam mark: detonation is the charge finishing its burn explosively after normal ignition; pre-ignition is the burn starting too early, before the spark, lit by a glowing hot spot in the chamber: an incandescent carbon deposit, an overheated spark plug electrode, or an exhaust valve edge running too hot.
The effects follow from the timing: the charge burns while the piston is still travelling up its compression stroke, so the engine fights its own combustion, losing power drastically and running rough, while the affected cylinder overheats savagely; sustained pre-ignition can melt a hole clean through a piston crown. The causes are whatever creates the hot spot, and the two diseases feed each other: detonation's overheating creates the hot spots that trigger pre-ignition, and pre-ignition's early burn raises pressures that provoke detonation. A diesel engine has no classic pre-ignition: during its compression stroke the cylinder holds only air, and air cannot ignite early because there is nothing in it to burn until injection.
Water, volatility and vapour lock
Checking the fuel for water is done with a fuel tester: a clear cup filled from each tank's sump drain and from the system's lowest filter point, held up to the light. Water is denser than fuel and does not mix with it, so it shows as a distinct layer or as globules at the bottom of the sample, and draining is repeated until the sample comes up clean. The occasions: before the first flight of the day, after every refuelling, and whenever water had a chance to get in or settle out, such as after rain or a large temperature drop overnight (which condenses moisture from the air space in a part-empty tank).
Three handling properties complete the fuel picture:
Volatility — a fuel's readiness to vaporise. Petrol is highly volatile, which makes cold starting easy but its vapour dangerous; diesel fuel and kerosene are far less volatile, safer to handle and harder to cold-start.
Viscosity — resistance to flow. Diesel fuel is more viscous than petrol, thickens noticeably in cold weather, and its pumps and filters must be designed for it.
Vapour locking — volatility's direct consequence: if fuel in a warm line or pump vaporises, the bubble of vapour blocks the flow of liquid, because a pump designed for liquid moves vapour poorly, and the engine is starved.
Heat, low pressure and high fuel volatility promote vapour locking, which is why it strikes petrol systems on hot days, at altitude, and during restarts of a heat-soaked engine, and it is one of the jobs the fuel-system module's electric boost pump exists to prevent: pressurising the line keeps the fuel liquid.