Engine Theory and Diagnosis

Chapter 16 Motive Power Theory: Engines

Lesson Objectives

  • Describe heat engines and their operation.

  • Explain the physics of engine operation.

  • Explain force, work, and power.

  • Describe four-stroke engine arrangement, operation, and measurement.

  • Describe the purpose and function of spark ignition engine components.

  • Explain two-stroke and rotary engine operation.

Heat Engines and Their Operation

  • Principles of Thermodynamics:

    • Thermodynamics relates heat to other energy forms (mechanical, electrical, chemical).

    • Heat expansion leads to pressure when contained, which moves mechanical components.

    • Combustion: Heat from burning air and fuel drives the engine.

Types of Heat Engines

  1. External Combustion Steam Engine:

    • Fuel is burned outside, creating steam in a boiler.

    • Steam builds pressure that moves a piston in a cylinder.

    • Drawback: Slow steam generation.

  2. External Combustion Stirling Engine:

    • Fuel burned outside.

    • Uses two cylinders connected by passages.

    • Power cylinder heated, expands air, and moves piston.

    • Air cycles back and forth but output is fixed.

  3. Internal Combustion Engine:

    • Burns fuel inside the cylinder.

    • Most common for transportation:

      • Gasoline (spark ignition)

      • Diesel (compression ignition)

  4. Engine Types:

    • Reciprocating Piston Engine:

      • Pistons move back and forth; crankshaft converts this to rotary motion.

    • Rotary Engine:

      • Utilizes rotating motion and avoids losses from reciprocating parts.

Spark Ignition (SI) and Compression Ignition (CI) Engines

  • SI Engine:

    • Air-fuel mixture compressed and ignited by spark from a spark plug.

  • CI Engine:

    • Air is compressed to heat, igniting diesel fuel, negating spark plugs.

Force, Work, and Power

  • Force: The effort causing a push or pull (needs to generate movement).

  • Work: Done when force moves an object.

  • Power: The rate at which work is performed.

Four-Stroke Engine Arrangement

  1. Otto Cycle:

    • Four strokes: intake, compression, ignition, power, and exhaust.

    • Top Dead Center (TDC): Far from crankshaft.

    • Bottom Dead Center (BDC): Close to crankshaft.

  2. Basic Four-Stroke Operation:

  • Involves defined steps: Intake, Compression, Power, Exhaust.

  1. Displacement-on-Demand Engines:

    • Cylinders can be shut off to reduce emissions during cruising by closing valves or stopping fuel injection.

  2. Compression Ratio (CR):

    • Measures compression size changes affecting performance.

    • Related to piston top/combustion chamber size/valve timing.

  3. Variable Compression Ratio Engines:

    • Adapt compression ratios for operational efficiency.

Atkinson and Miller Cycle Engines

  • Efficiency: Utilize different timing strategies for power and efficiency.

  • Atkinson Cycle: More efficiency within specific RPM range (2000-4500 RPM).

  • Miller Cycle: Incorporates superchargers for efficiency boosts.

Spark Ignition Engine Components

  • Function: High-voltage ignition of air-fuel mixture after compression leads to thermal expansion and power generation, followed by exhaust.

  • Assembly Types:

    • Bottom end includes crankshaft, bearings.

    • Top end holds cylinder heads and combustion chambers.

    • Short Block: Parts below the head gasket to above oil pan.

    • Long Block: Adds cylinder heads and valve train components.

Two-Stroke and Rotary Engine Operation

  • Two-Stroke Engine:

    • Air-fuel mixture cycles through crankcase and cylinder during piston movements.

    • Combustion drives piston down, exhaust occurs upon reaching BDC.

  • Rotary Engine:

    • Uses rotor movement in an epitrochoid path to handle air-fuel mixtures.

    • Efficient but has a complex seal system.

Engine Mechanical Testing (Chapter 17)

Process

  • Strategy-based diagnosis focusing on root cause identification through various tests.

  • Initial steps: start engine, visual inspection, review service history, locate TSBs.

Testing Methods

  1. Cranking and running compression tests:

    • Evaluates cylinder's ability to compress air.

    • Required: engine at temperature, throttle open, spark plugs out.

  2. Cylinder Leakage Test:

    • Identifies leaking areas and assesses cylinder health.

    • Tests air leakage's source: combustion chamber issues or valve integrity.

  3. Power Balance Test:

    • Determines cylinder performance by disabling one at a time and observing RPM drop.

Summary Questions

  • Testing and diagnosing skills are crucial for identifying engine performance issues, focusing on pressure, temperature, and compression aspects.

  • Techniques and tools include scan tools, vacuum gauges, and cylinder leakage testers to ensure efficient engine diagnostics.