14.1 Four-Stroke and Two-Stroke Engine Cycles

Internal combustion engines power our world, and understanding their cycles is key. Four-stroke engines, with their intake, compression, power, and exhaust strokes, are the workhorses of modern vehicles. They offer better efficiency and lower emissions than their two-stroke counterparts.

Two-stroke engines complete the same processes in just two piston movements. While they pack more power in a smaller package, they're less efficient and produce more emissions. Both types have their place, with four-strokes dominating cars and two-strokes excelling in small, portable applications.

Thermodynamics of Internal Combustion Engines

Four-Stroke Internal Combustion Engine Cycle

• A four-stroke internal combustion engine cycle consists of four distinct piston strokes: intake, compression, power, and exhaust
• The intake stroke begins with the piston at top dead center (TDC) and ends with the piston at bottom dead center (BDC)
  • During the intake stroke, the intake valve is open and the fuel-air mixture is drawn into the cylinder
• The compression stroke occurs as the piston moves from BDC to TDC with both valves closed
  • The fuel-air mixture is compressed and its temperature increases during the compression stroke
• Ignition of the compressed fuel-air mixture occurs near the end of the compression stroke
  • The ignition causes a rapid increase in pressure and temperature, driving the piston down during the power stroke
• The exhaust stroke begins as the piston reaches BDC and the exhaust valve opens
  • Combustion products are expelled from the cylinder as the piston moves back to TDC during the exhaust stroke

Thermodynamic Analysis of Four-Stroke Cycle

• The four-stroke cycle can be represented on a pressure-volume (P-V) diagram
  • The enclosed area on the P-V diagram represents the net work output of the cycle
• Thermal efficiency of the four-stroke cycle is influenced by several factors
  • Compression ratio, fuel properties, and operating conditions all impact the thermal efficiency
  • Higher compression ratios generally lead to increased thermal efficiency (up to a certain limit)
  • Fuel properties such as octane rating and energy content affect efficiency and performance
  • Operating conditions like engine speed, load, and air-fuel ratio also influence efficiency

Otto, Diesel, and Dual Cycles

Otto Cycle

• The Otto cycle is a four-stroke cycle that uses a spark plug to ignite a pre-mixed fuel-air charge
  • Typically used in gasoline engines with a carburetor or fuel injection system
• Ignition occurs at a constant volume in the Otto cycle, resulting in a rapid pressure rise
• The Otto cycle typically operates at a lower compression ratio compared to the Diesel cycle
  • Lower compression ratios are used to avoid pre-ignition or knocking (uncontrolled combustion)
  • Common compression ratios for Otto cycle engines range from 8:1 to 12:1

Diesel Cycle

• The Diesel cycle is a four-stroke cycle that uses compression ignition
  • Air is compressed to a high temperature and pressure before the fuel is injected
• Fuel injection and combustion occur at nearly constant pressure in the Diesel cycle
• The Diesel cycle operates at a higher compression ratio than the Otto cycle
  • Higher compression ratios result in higher thermal efficiency
  • Typical compression ratios for Diesel cycle engines range from 14:1 to 24:1
• Diesel engines are often used in heavy-duty applications (trucks, buses, construction equipment) due to their high torque output and efficiency

Dual Cycle

• The Dual cycle, also known as the mixed cycle, combines features of both the Otto and Diesel cycles
• The Dual cycle uses a pre-mixed fuel-air charge like the Otto cycle but injects additional fuel during the combustion process, similar to the Diesel cycle
• The Dual cycle aims to achieve a balance between the advantages of the Otto and Diesel cycles
  • Improved efficiency compared to the Otto cycle
  • Reduced emissions compared to the Diesel cycle
• Dual cycle engines are less common than Otto or Diesel engines but have been used in some automotive applications (Mazda SKYACTIV-X engine)

Two-Stroke Engine Cycle Analysis

Two-Stroke Internal Combustion Engine Cycle

• A two-stroke internal combustion engine completes the four processes (intake, compression, power, and exhaust) in just two piston strokes
• The intake and compression processes occur during the upward stroke of the piston, while the power and exhaust processes occur during the downward stroke
• Intake and exhaust ports are used instead of valves, with the piston itself controlling the opening and closing of these ports
• The two-stroke cycle relies on the positive displacement of the incoming fuel-air mixture to expel the exhaust gases, a process known as scavenging

Scavenging and Thermal Efficiency in Two-Stroke Engines

• Scavenging efficiency is crucial for two-stroke engine performance
  • Incomplete scavenging can lead to reduced power output and increased emissions
  • Scavenging efficiency is affected by factors such as port design, timing, and gas dynamics
• Two-stroke engines typically have higher power-to-weight ratios compared to four-stroke engines
  • The increased frequency of power strokes in two-stroke engines results in higher specific power output
• Thermal efficiency of two-stroke engines is often lower than that of four-stroke engines
  • Incomplete scavenging and increased heat loss contribute to lower thermal efficiency
  • Typical thermal efficiencies for two-stroke engines range from 20% to 30%, while four-stroke engines can achieve 30% to 40% efficiency

Four-Stroke vs Two-Stroke Engines

Advantages of Four-Stroke Engines

• Four-stroke engines generally have higher thermal efficiency and lower fuel consumption compared to two-stroke engines
  • The separation of intake, compression, power, and exhaust strokes allows for better control over the combustion process and more complete scavenging
• Four-stroke engines typically produce lower emissions due to more efficient combustion
  • The ability to incorporate emission control technologies (catalytic converters, EGR systems) further reduces emissions
• Four-stroke engines are more commonly used in automotive and industrial applications
  • The majority of modern passenger vehicles and heavy-duty engines use four-stroke designs

Advantages of Two-Stroke Engines

• Two-stroke engines have higher power-to-weight ratios and simpler mechanical designs compared to four-stroke engines
  • The increased frequency of power strokes results in higher specific power output
  • Simpler designs with fewer moving parts can lead to reduced manufacturing costs and easier maintenance
• Two-stroke engines are often found in smaller, lightweight applications
  • Common applications include motorcycles, chainsaws, outboard motors, and small generators
• Two-stroke engines can be more compact and lighter than equivalent four-stroke engines
  • The reduced size and weight make them suitable for portable and space-constrained applications

Disadvantages of Two-Stroke Engines

• Two-stroke engines often have higher hydrocarbon emissions and require more frequent lubrication compared to four-stroke engines
  • Incomplete scavenging can result in unburned fuel being expelled with the exhaust gases, increasing hydrocarbon emissions
  • Two-stroke engines typically require a mixture of oil and fuel for lubrication, which can lead to increased oil consumption and emissions
• Two-stroke engines may have shorter lifespans than four-stroke engines due to increased wear and tear
  • The higher operating speeds and increased heat generation can accelerate component wear
  • More frequent maintenance and rebuilding may be necessary to maintain performance and reliability