Trade Theory and Fundamentals of the Vocational Trade (PDFM/RMPDFM)
This domain tests how well you understand the systems of power-driven farm machinery - diesel engines, fuel and air delivery, cooling, lubrication, transmission, hydraulics and 12 V electrical circuits - and the principles that explain their behaviour in service.
Core rules
- A four-stroke diesel engine completes suction, compression, power and exhaust in two crankshaft revolutions; ignition is by heat of compression, not a spark plug.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume. Higher CR raises thermal efficiency but increases peak pressure and NOx.
- Injection timing is specified in degrees before top dead centre (BTDC); valve clearance is set cold or hot exactly as the manual states.
- The three-point linkage and draft/position control transfer implement weight and load; PTO speed is engine-speed dependent, so match implement rpm to the rated 540 or 1000 PTO.
- Lubrication is pressure-fed to bearings and splash to cylinder walls; low oil pressure means stop, not top up and continue.
Worked example
A single-cylinder engine has a swept volume of 500 cm³ and a clearance volume of 50 cm³. Compression ratio = (500 + 50) ÷ 50 = 550 ÷ 50 = 11:1. If the clearance volume is increased to 62.5 cm³ by a thicker gasket, CR = 562.5 ÷ 62.5 = 9:1. The reasoning: only the clearance volume changes; swept volume is fixed by bore and stroke. A lower CR reduces efficiency and can make cold starting harder.
Traps
Do not confuse swept volume with total cylinder volume; do not quote injection timing in mm; do not set valve clearance on a hot engine unless specified; do not assume PTO rpm equals engine rpm.