Last reviewed 1 Oct 2026 · 10 min read
1. Fuel system
Petrol engine (carburettor system)
Fuel tank → fuel filter → fuel pump (mechanical or electric) → carburettor → intake manifold → cylinder.
The carburettor atomises petrol and mixes it with air in the right proportion. Principle: air flowing through a venturi creates a low pressure (Bernoulli), which draws fuel from the float chamber through the main jet.
Parts: float chamber (keeps a constant level), float and needle valve, venturi (choke tube), main jet, idling jet, throttle valve, choke valve, accelerator pump (extra fuel on rapid acceleration).
Air–fuel ratio: stoichiometric about 14.7 : 1 (by mass); rich (about 12 : 1) for starting and full power; lean (about 16–18 : 1) for economy. Petrol injection (MPFI) has replaced carburettors in vehicles for better control.
Diesel engine fuel system
Fuel tank → lift (feed) pump → primary (sedimentor) filter → secondary filter → injection pump (high pressure) → high-pressure pipes → injectors → combustion chamber, with a return line carrying excess fuel back to the tank.
| Component | Function |
|---|---|
| Fuel tank | Stores fuel; has a filler cap with a vent, a drain plug and a strainer |
| Lift (feed) pump | A diaphragm or plunger pump driven by the camshaft; supplies fuel at low pressure (about 0.5–1.5 bar) to the injection pump; has a hand priming lever for bleeding air |
| Fuel filters | Primary (water separator, coarse) and secondary (fine paper element, 2–10 µm) — dirt and water quickly damage the pump and injectors |
| Injection pump | Delivers precisely metered fuel at very high pressure (150–2000 bar) at the right time to each cylinder. Types: in-line (multi-plunger, Bosch type), rotary distributor (DPA, VE), common rail (modern) |
| Governor | Controls the engine speed (limits the maximum speed and keeps idling steady) by regulating the fuel delivered; mechanical (centrifugal flyweight) or electronic |
| Injectors | Spray atomised fuel into the combustion chamber; nozzle with holes (multi-hole for direct injection, pintle/throttle for indirect); a spring-loaded needle opens at a set opening (popping) pressure; need testing and cleaning |
| High-pressure pipes | Equal length for all cylinders (to equalise timing) |
| Fuel priming and bleeding | Removing air from the system (vent screws) after servicing |
Injection pump operation (in-line): a cam on the pump camshaft drives a plunger in a barrel; the plunger delivers fuel to the injector when it covers the port, and the delivery ends when a helical groove on the plunger uncovers the spill port; rotating the plunger (by the control rack, linked to the governor) changes the effective stroke, i.e. the quantity delivered. A delivery valve gives a sharp cut-off and keeps the line pressure.
Combustion chambers (diesel)
| Type | Description | Features |
|---|---|---|
| Direct injection (open chamber) | Fuel injected directly into a bowl in the piston | Best fuel economy, easy starting, high injection pressure, harsh combustion (tractor engines commonly use DI) |
| Indirect injection (pre-combustion, swirl, turbulence chambers) | Fuel injected into a separate pre-chamber connected to the main chamber | Smoother, quieter, lower injection pressure; poorer economy and harder cold starting (needs glow plugs) |
Diesel combustion — knock
Diesel knock is caused by a long ignition delay (too much fuel accumulates before ignition, then burns suddenly). It is reduced by higher cetane fuel, higher compression ratio, higher intake temperature, smaller delay.
2. Air intake and exhaust
- Air cleaner — removes dust from the intake air: dry (paper) element, oil-bath (the air is passed over an oil pool; coarse dust is trapped), or cyclone pre-cleaner; a clogged cleaner raises fuel consumption and emits black smoke; dust causes rapid cylinder wear. Clean or replace on a schedule (more frequently in dusty fields).
- Intake manifold — distributes the air (or mixture).
- Turbocharger — an exhaust-driven turbine turns a compressor that raises the intake air pressure (boost 0.5–1.5 bar), increasing air mass and so power (by about 20–40 %) and economy; often with an intercooler to cool the compressed air.
- Exhaust manifold, silencer (muffler), spark arrester (in some forestry/farm uses) — reduce noise and fire risk; exhaust smoke colours: black (rich/ incomplete combustion, clogged air filter, bad injector), white (unburnt fuel, water, cold engine), blue (lubricating oil burning).