Diesel Engines
Fundamentals
Diesel ignition is by heat of compression alone, typically 16:1 to 18:1, so cranking speed, ring sealing, and valve sealing matter more than spark. Understand injection pressure and timing, and the major fuel system families: mechanical (inline or rotary pump), unit injector, HEUI (hydraulically actuated, oil pressure driven), and common rail. Know the air path - turbocharger, charge-air cooler, intake throttle or EGR valve - and the aftertreatment chain: DOC, DPF, SCR with DEF dosing.
Diagnostics you must know
- Cylinder cutout / contribution test: disabling each injector in turn; the cylinder whose cutoff causes no change in rpm or smoothness is the weak one.
- Compression: diesel cranking compression is high (commonly in the 300–600 psi range; always use the OEM spec).
- Blowby: measured at the oil fill or breather; excessive flow points to rings or piston damage.
- Fuel return and balance rates: high return flow from an injector can cause a low-power or hard-start complaint.
Traps
Never crack a high-pressure common-rail line while the engine runs - pressure can exceed 20,000 psi. Questions also test that a DPF regeneration needs specific conditions (operating temperature, speed, soot load) and that contaminated or diluted DEF causes derate codes rather than a mechanical fault.
Worked example
A truck runs rough at idle. A cutout test shows no rpm change when cylinder 4 is disabled, but a change for all others. Cylinder 4 is not contributing; next step is a compression test and injector return-flow check on that hole before condemning the injector.