Diesel Engine Fundamentals Summary

• The compression ratio is the volume of the cylinder with piston at TDC divided by the volume of the cylinder with piston at BDC.
• Bore is the diameter of the cylinder.
• Stroke is the distance the piston travels from TDC to BDC, and is determined by the eccentricity of the crankshaft.
• The combustion chamber is the volume of space where the fuel air mixture is burned in an engine. This is in the cylinder of the engine.
• The following components were discussed and identified on a drawing.
  
  1. Piston and rod
  2. Cylinder
  3. Blower
  4. Crankshaft
  5. Intake ports or valve(s)
  6. Exhaust ports or valve(s)
  7. Fuel injector

• Ignition occurs in a diesel by injecting fuel into the air charge which has been heated by compression to a temperature greater than the ignition point of the fuel.
• A diesel engine converts the energy stored in the fuel’s chemical bonds into mechanical energy by burning the fuel. The chemical reaction of burning the fuel liberates heat, which causes the gasses to expand, forcing the piston to rotate the crankshaft.
• A four-stroke engine requires two rotations of the crankshaft to complete one cycle. The event occur as follows:
  • Intake – the piston passes TDC, the intake valve(s) open and the fresh air is admitted into the cylinder, the exhaust valve is still open for a few degrees to allow scavenging to occur.
  • Compression – after the piston passes BDC the intake valve closes and the piston travels up to TDC (completion of the first crankshaft rotation).
  • Fuel injection – As the piston nears TDC on the compression stroke, the fuel is injected by the injectors and the fuel starts to burn, further heating the gasses in the cylinder.
  • Power – the piston passes TDC and the expanding gasses force the piston down, rotating the crankshaft.
  • Exhaust – as the piston passes BDC the exhaust valves open and the exhaust gasses start to flow out of the cylinder. This continues as the piston travels up to TDC, pumping the spent gasses out of the cylinder. At TDC the second crankshaft rotation is complete.
• A two-stroke engine requires one rotation of the crankshaft to complete one cycle. The events occur as follows:
  • Intake – the piston is near BDC and exhaust is in progress. The intake valve or ports open and the fresh air is forced in. The exhaust valves or ports are closed and intake continues.
  • Compression – after both the exhaust and intake valves or ports are closed, the piston travels up towards TDC. The fresh air is heated by the compression.
  • Fuel injection – near TDC the fuel is injected by the injectors and the fuel starts to burn, further heating the gasses in the cylinder.
  • Power – the piston passes TDC and the expanding gasses force the piston down, rotating the crankshaft.
  • Exhaust – as the piston approaches BDC the exhaust valves or ports open and the exhaust gasses start to flow out of the cylinder.

• A mechanical-hydraulic governor controls engine speed by balancing engine speed (mechanical flyweights) against hydraulic pressure. As the engine speeds up or slows down, the weights move the hydraulic plunger in or out. This in turn actuates a hydraulic valve which controls the hydraulic pressure to the buffer piston. The buffer piston is connected to the fuel rack. Therefore, any motion of the buffer piston will control fuel to the cylinder by adjusting the position of the fuel rack, which regulates the amount of fuel in the injectors.
• Most mid-sized to large diesel engines have (as a minimum) the following protective alarms and trips.
  • Engine overspeed alarm/trip
  • High water jacket temperature alarm
  • High exhaust temperature alarm
  • Low lube oil pressure (alarm and/or trip)
  • High crankcase pressure alarm

Congratulations, you have completed the knowledge section of the course.

You may now complete the course by successfully passing the course quiz.