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Most people drive a machine they were never shown.

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Move the mixture. Watch what it costs.

This is not a video of a simulation. Flame speed, flame temperature and exhaust temperature are computed from the mixture you set, not read from a table tuned to look responsive. Drag it lean and watch where the heat actually goes.

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The whole subject, and how it connects

Twenty roots. One machine.

This is the real dependency graph, not a picture of one. Braking needs energy conversion. Emissions needs air and fuel. Open any of them — nothing is locked, and there is no front door.

  1. Energy & ConversionEverything a vehicle does is energy changing form. Start here and the rest follows.
  2. CombustionA flame front crossing a cylinder in about two thousandths of a second.
  3. Air & FuelGetting the right amount of both to the right place at the right moment.
  4. IgnitionTiming is not a setting. It is the difference between power and a broken piston.
  5. Rotating AssemblyTurning a straight push into a turning force, thousands of times a minute.
  6. Valvetrain & BreathingAn engine is an air pump. How well it breathes is how much it makes.
  7. Lubrication & FrictionMetal never touches metal. A film of oil a few microns thick is doing all the work.
  8. Cooling & Heat RejectionEngines want to be hot. The cooling system stops them being too hot, not cold.
  9. Electrical FundamentalsMost "electrical faults" are not electrical. They are connections.
  10. Electrochemistry & BatteriesStoring energy in a chemical bond and getting it back out on demand.
  11. Electric Machines & Power ElectronicsWhy an electric motor makes full torque before it has turned once.
  12. Transmission & Torque MultiplicationTrading speed for force, because an engine only makes power in a narrow band.
  13. Driveline & DifferentialsThe outside wheel travels further through a corner. Something has to allow that.
  14. Suspension & Weight TransferKeeping the tyres on the road, and deciding which ones carry the load.
  15. Steering & GeometryWhat an alignment actually corrects, and why the car self-centres.
  16. Braking & Energy DissipationTurning motion into heat fast enough that the tyres get a chance.
  17. Tyres & Contact PatchFour patches the size of your hand. Everything else is negotiation.
  18. Materials & FastenersA bolt is a spring you tension on purpose. Torque is only a proxy for that.
  19. Sensing, Control & DiagnosticsWhat a fault code tells you, and — more usefully — what it does not.
  20. Emissions & AftertreatmentCleaning up what combustion leaves behind, and why it needs the mixture exact.

Orange is where combustion lives. Cyan is where electrons do the same job. Grey belongs to both, which is most of it.

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How deep it actually goes

From a number everyone quotes to a code everyone misreads.

One path, four roots, no leaps. This is an excerpt — the lesson itself carries the derivations and the measurements.

  1. 01Air & Fuel14.7:1

    The ratio

    Fourteen point seven kilos of air to one of fuel. Not because it makes the most power — it does not — but because it is the ratio at which every molecule of fuel finds exactly enough oxygen to burn, with none of either left over.

  2. 02Sensing & Controlλ = 1.00

    How the engine knows

    The engine cannot measure what it injected. It measures what came out. A lambda sensor in the exhaust reports whether there was oxygen left over, and the ECU corrects on the next breath. That loop is the whole of closed-loop fuelling.

  3. 03Sensing & ControlSTFT + LTFT

    The correction, written down

    Those corrections are the fuel trims. Short-term is the ECU arguing with the sensor right now; long-term is what it has learned to expect. A trim near zero means the engine's model of itself is right.

  4. 04Air & Fuel+20% idle → +5% @ 2500

    The differential

    Now the useful part. A trim of +20% at idle that falls to +5% at 2500 rpm is a vacuum leak, and the reason is arithmetic: a fixed leak is a large fraction of a small airflow and a small fraction of a large one.

  5. 05Air & Fuelthe other way round

    What it is not

    A trim that is fine at idle and climbs under load is not a leak. That is fuel delivery falling behind — a tiring pump, a restricted filter, an injector that cannot keep up. Same symptom, opposite slope, different job.

  6. 06DiagnosticsP0171

    The code

    System too lean, bank one. It sets when the trim needed to hold lambda runs out of room. The code tells you the engine is adding fuel it should not need. It does not tell you why — the slope of the trim does.

By the time you reach the code, it is not a lookup. It is a conclusion.

Read the full lesson

Five ways in

The tree does not have a front door.

Nobody is made to sit through a beginner module to reach the thing they came for. Pick whichever of these sounds like you — or ignore all five and open the tree.

Who made this

Everything above is free, and stays free. It is made by a workshop in Ravenhall that would rather you understood your car than trusted us blindly. When you want hands on it, we're here — and you'll know enough to check our work.

MC Mechanical Service Centre — Ravenhall, VIC 3023

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