ABYC DIESEL ENGINE EXAM COMPLETE QUESTIONS AND
ALL ANSWERS VERIFIED NEWEST VERSION (100% PASS
GUARANTEE)
1. What is the primary difference between a diesel and gasoline engine?
Diesel engines use compression ignition while gasoline engines use spark
ignition. Diesel fuel ignites from the heat of compressed air rather than from a
spark plug.
2. What is the typical compression ratio of a marine diesel engine? Marine
diesel engines typically have compression ratios between 14:1 and 25:1,
compared to 8:1 to 12:1 for gasoline engines.
3. What are the four strokes in a four-stroke diesel engine cycle? Intake,
compression, power (combustion), and exhaust.
4. How does a two-stroke diesel engine differ from a four-stroke? A two-
stroke completes the cycle in one crankshaft revolution (two strokes) rather than
two revolutions (four strokes), and typically uses a blower for scavenging.
5. What is cetane number and why is it important? Cetane number measures
the ignition quality of diesel fuel. Higher cetane numbers (typically 40-55)
indicate better ignition characteristics and smoother engine operation.
6. What causes diesel fuel to ignite in the combustion chamber? The high
temperature generated by compressing air to approximately 1000°F (540°C)
causes diesel fuel to auto-ignite when injected.
7. What is the purpose of glow plugs in a diesel engine? Glow plugs preheat
the combustion chamber to facilitate cold starting by raising air temperature
before compression.
8. What is diesel knock and what causes it? Diesel knock is a harsh
combustion noise caused by excessive ignition delay, resulting in too much fuel
igniting simultaneously. It's often caused by low cetane fuel or cold engine
temperatures.
9. What is the typical operating temperature range for a marine diesel
engine? Most marine diesels operate between 160°F and 195°F (71°C to 91°C),
monitored at the cooling system.
,10. What is specific fuel consumption? Specific fuel consumption (SFC) is the
amount of fuel consumed per unit of power produced, typically measured in
pounds per horsepower-hour (lb/hp-hr) or grams per kilowatt-hour (g/kWh).
11. What is turbocharger lag? Turbocharger lag is the delay between throttle
application and boost pressure buildup, caused by the time needed for exhaust
gases to spin the turbine up to speed.
12. What is the purpose of intercooling on a turbocharged diesel?
Intercooling cools the compressed air from the turbocharger before it enters the
engine, increasing air density and power output while reducing combustion
temperatures.
13. What is fuel injection timing? Fuel injection timing refers to when fuel is
injected relative to piston position, typically measured in degrees before top
dead center (BTDC).
14. What causes black smoke from a diesel exhaust? Black smoke indicates
incomplete combustion due to insufficient air, excessive fuel, or poor fuel
atomization.
15. What causes white smoke from a diesel exhaust? White smoke typically
indicates unburned fuel due to low compression, cold engine temperature, or
poor fuel quality. It can also indicate coolant entering the combustion chamber.
16. What causes blue smoke from a diesel exhaust? Blue smoke indicates oil
burning in the combustion chamber, typically from worn piston rings, valve
guides, or turbocharger seals.
17. What is the purpose of the injection pump? The injection pump precisely
meters and pressurizes fuel for delivery to the injectors at the correct timing and
quantity.
18. What is a common rail fuel system? A common rail system uses a high-
pressure fuel rail that supplies all injectors, with electronic control over
injection timing and duration for each cylinder independently.
19. What is pilot injection? Pilot injection is a small pre-injection of fuel
before the main injection event to reduce combustion noise and NOx emissions
by creating a smoother pressure rise.
20. What is EGR (Exhaust Gas Recirculation)? EGR recirculates a portion of
exhaust gas back into the intake to reduce peak combustion temperatures and
NOx emissions.
, 21. What is the purpose of a diesel particulate filter (DPF)? A DPF captures
and stores diesel particulate matter (soot) from the exhaust, which is
periodically burned off during regeneration.
22. What is SCR (Selective Catalytic Reduction)? SCR is an emissions
control technology that injects diesel exhaust fluid (DEF/urea) into the exhaust
to convert NOx into nitrogen and water.
23. What is the difference between direct and indirect injection? Direct
injection sprays fuel directly into the main combustion chamber, while indirect
injection uses a pre-combustion chamber where initial combustion occurs
before spreading to the main chamber.
24. What is volumetric efficiency? Volumetric efficiency is the ratio of actual
air intake volume to theoretical maximum displacement volume, indicating how
effectively an engine breathes.
25. What is brake horsepower (BHP)? Brake horsepower is the actual usable
power output measured at the engine's crankshaft or flywheel, after accounting
for internal friction losses.
26. What is the purpose of a governor on a diesel engine? The governor
automatically controls engine speed by regulating fuel delivery, preventing
over-speeding and maintaining steady RPM under varying loads.
27. What is mechanical vs. electronic governing? Mechanical governors use
springs and flyweights, while electronic governors use sensors and computer
control for more precise speed regulation.
28. What is the typical fuel pressure at the injectors in a common rail
system? Modern common rail systems operate at 20,000 to 30,000 PSI (1,400
to 2,000 bar), though some newer systems exceed 35,000 PSI.
29. What is cavitation in a cooling system? Cavitation occurs when coolant
boils locally due to low pressure areas, creating vapor bubbles that collapse and
cause erosion damage to metal surfaces.
30. What is detonation in a diesel engine? Unlike gasoline engines, diesels
don't experience true detonation. However, excessive ignition delay can cause
similar harsh combustion events, often called diesel knock.
Section 2: Fuel Systems (Questions 31-60)
31. What are the main components of a diesel fuel system? Fuel tank, supply
and return lines, fuel filters (primary and secondary), lift pump, injection pump,
injectors, and fuel/water separator.
ALL ANSWERS VERIFIED NEWEST VERSION (100% PASS
GUARANTEE)
1. What is the primary difference between a diesel and gasoline engine?
Diesel engines use compression ignition while gasoline engines use spark
ignition. Diesel fuel ignites from the heat of compressed air rather than from a
spark plug.
2. What is the typical compression ratio of a marine diesel engine? Marine
diesel engines typically have compression ratios between 14:1 and 25:1,
compared to 8:1 to 12:1 for gasoline engines.
3. What are the four strokes in a four-stroke diesel engine cycle? Intake,
compression, power (combustion), and exhaust.
4. How does a two-stroke diesel engine differ from a four-stroke? A two-
stroke completes the cycle in one crankshaft revolution (two strokes) rather than
two revolutions (four strokes), and typically uses a blower for scavenging.
5. What is cetane number and why is it important? Cetane number measures
the ignition quality of diesel fuel. Higher cetane numbers (typically 40-55)
indicate better ignition characteristics and smoother engine operation.
6. What causes diesel fuel to ignite in the combustion chamber? The high
temperature generated by compressing air to approximately 1000°F (540°C)
causes diesel fuel to auto-ignite when injected.
7. What is the purpose of glow plugs in a diesel engine? Glow plugs preheat
the combustion chamber to facilitate cold starting by raising air temperature
before compression.
8. What is diesel knock and what causes it? Diesel knock is a harsh
combustion noise caused by excessive ignition delay, resulting in too much fuel
igniting simultaneously. It's often caused by low cetane fuel or cold engine
temperatures.
9. What is the typical operating temperature range for a marine diesel
engine? Most marine diesels operate between 160°F and 195°F (71°C to 91°C),
monitored at the cooling system.
,10. What is specific fuel consumption? Specific fuel consumption (SFC) is the
amount of fuel consumed per unit of power produced, typically measured in
pounds per horsepower-hour (lb/hp-hr) or grams per kilowatt-hour (g/kWh).
11. What is turbocharger lag? Turbocharger lag is the delay between throttle
application and boost pressure buildup, caused by the time needed for exhaust
gases to spin the turbine up to speed.
12. What is the purpose of intercooling on a turbocharged diesel?
Intercooling cools the compressed air from the turbocharger before it enters the
engine, increasing air density and power output while reducing combustion
temperatures.
13. What is fuel injection timing? Fuel injection timing refers to when fuel is
injected relative to piston position, typically measured in degrees before top
dead center (BTDC).
14. What causes black smoke from a diesel exhaust? Black smoke indicates
incomplete combustion due to insufficient air, excessive fuel, or poor fuel
atomization.
15. What causes white smoke from a diesel exhaust? White smoke typically
indicates unburned fuel due to low compression, cold engine temperature, or
poor fuel quality. It can also indicate coolant entering the combustion chamber.
16. What causes blue smoke from a diesel exhaust? Blue smoke indicates oil
burning in the combustion chamber, typically from worn piston rings, valve
guides, or turbocharger seals.
17. What is the purpose of the injection pump? The injection pump precisely
meters and pressurizes fuel for delivery to the injectors at the correct timing and
quantity.
18. What is a common rail fuel system? A common rail system uses a high-
pressure fuel rail that supplies all injectors, with electronic control over
injection timing and duration for each cylinder independently.
19. What is pilot injection? Pilot injection is a small pre-injection of fuel
before the main injection event to reduce combustion noise and NOx emissions
by creating a smoother pressure rise.
20. What is EGR (Exhaust Gas Recirculation)? EGR recirculates a portion of
exhaust gas back into the intake to reduce peak combustion temperatures and
NOx emissions.
, 21. What is the purpose of a diesel particulate filter (DPF)? A DPF captures
and stores diesel particulate matter (soot) from the exhaust, which is
periodically burned off during regeneration.
22. What is SCR (Selective Catalytic Reduction)? SCR is an emissions
control technology that injects diesel exhaust fluid (DEF/urea) into the exhaust
to convert NOx into nitrogen and water.
23. What is the difference between direct and indirect injection? Direct
injection sprays fuel directly into the main combustion chamber, while indirect
injection uses a pre-combustion chamber where initial combustion occurs
before spreading to the main chamber.
24. What is volumetric efficiency? Volumetric efficiency is the ratio of actual
air intake volume to theoretical maximum displacement volume, indicating how
effectively an engine breathes.
25. What is brake horsepower (BHP)? Brake horsepower is the actual usable
power output measured at the engine's crankshaft or flywheel, after accounting
for internal friction losses.
26. What is the purpose of a governor on a diesel engine? The governor
automatically controls engine speed by regulating fuel delivery, preventing
over-speeding and maintaining steady RPM under varying loads.
27. What is mechanical vs. electronic governing? Mechanical governors use
springs and flyweights, while electronic governors use sensors and computer
control for more precise speed regulation.
28. What is the typical fuel pressure at the injectors in a common rail
system? Modern common rail systems operate at 20,000 to 30,000 PSI (1,400
to 2,000 bar), though some newer systems exceed 35,000 PSI.
29. What is cavitation in a cooling system? Cavitation occurs when coolant
boils locally due to low pressure areas, creating vapor bubbles that collapse and
cause erosion damage to metal surfaces.
30. What is detonation in a diesel engine? Unlike gasoline engines, diesels
don't experience true detonation. However, excessive ignition delay can cause
similar harsh combustion events, often called diesel knock.
Section 2: Fuel Systems (Questions 31-60)
31. What are the main components of a diesel fuel system? Fuel tank, supply
and return lines, fuel filters (primary and secondary), lift pump, injection pump,
injectors, and fuel/water separator.
