Engine and Fuel Systems Flashcards

What is the primary purpose of a diesel engine's unit injector (UI) system compared to a common rail system?

Answer: Each unit injector generates its own high pressure, eliminating a separate high-pressure pump

In a unit injector system, the injector itself generates high pressure via a rocker-arm-driven plunger, unlike common rail which uses a central pump.

A technician finds gray, milky residue on the oil filler cap of a diesel engine. What is the most probable cause?

Answer: Coolant intrusion into the engine oil

A milky or gray emulsion on the oil cap indicates coolant mixing with engine oil, typically from a head gasket failure or cracked cylinder head.

On a turbocharged diesel, what is the purpose of the charge air cooler (CAC or intercooler)?

Answer: Cool the compressed intake air to increase air density and power

The charge air cooler reduces intake air temperature after turbo compression, increasing air density so more oxygen is available for combustion.

Which measurement tool is used to check diesel injection timing by detecting pressure rise in the fuel system?

Answer: Diesel timing light with clamp-on transducer

A diesel timing light uses a clamp-on transducer on the injection line to detect the pressure pulse when injection occurs, indicating timing relative to crankshaft position.

A coach engine develops a 'fuel knock' that is distinct from normal diesel combustion knock. What condition causes fuel knock?

Answer: Over-advanced injection timing causing rapid pressure rise

Over-advanced injection timing causes fuel to ignite too early, creating an abnormally rapid pressure rise rate that produces a characteristic sharp knock.

When replacing a diesel fuel injector, why must the injector hold-down torque be tightened to specification?

Answer: To prevent combustion gas blow-by past the injector seat and seal

Proper torque ensures the injector sealing washer or copper crush washer seats correctly, preventing high-pressure combustion gases from escaping past the injector.

A diesel engine's DPF (diesel particulate filter) requires frequent active regeneration cycles. What underlying condition most likely causes this?

Answer: Excessive oil consumption or fuel contamination causing excess soot production

Excessive soot from burning engine oil or fuel dilution overloads the DPF faster than normal, triggering more frequent regeneration events.