Two-Stroke Engine Scavenging Process

Two-Stroke Engine

• To obtain higher power output, the two strokes used for gas exchange are suppressed and substituted by a scavenging process, which involves displacing burned gas with a fresh charge pressurized in an external compressor or blower.
• The simplest design uses the crankcase to act as a blower, varying in volume in opposition to the cylinder volume.
• The intake and exhaust valves can be replaced by ports in the cylinder liner, controlled by the piston motion, making the design more compact.

Two-Stroke Operation

• The two strokes are:
  • Compression Stroke: the piston compresses the cylinder charge, and toward the end of the stroke, combustion is triggered via SI or fuel injection (CI).
  • Power Stroke: the hot burned gases expand, pushing the piston down, making mechanical work, and expelling burned gases from the cylinder.
• Toward the end of the power stroke, the scavenging ports open, and the pressurized charge displaces the burned gases, starting a new cycle.

Fuel Comparison: Petrol vs Diesel

• Diesel engines have an environmental advantage: they get better mileage and require less refining.
• Refining crude oil into gasoline incurs energy costs, and limited refineries in the US contribute to increased gasoline needs, making diesel fuel a more environmentally friendly option.

Engine Cycle Comparison: Otto vs Diesel

• The main differences between Otto and Diesel engines are:
  • No fuel in the cylinder at the beginning of the compression stroke in Diesel engines, preventing autoignition.
  • Diesel engine uses compression ignition instead of spark ignition.
  • Fuel is injected directly into the combustion chamber, and the first part of the power stroke occurs at constant pressure.
  • Higher compression ratios can be achieved in Diesel engines, resulting in higher temperature and pressure compared to Gas engines.

Cost of Ownership: Gasoline vs Diesel Engines

• Diesel engines are more expensive to purchase and maintain than gas engines.
• Diesel oil changes are more expensive than gas oil changes.
• Fuel costs are higher for Diesel engines, averaging $3.27 per gallon, but Diesel engines are more efficient, making them a better option for those who drive extensively.

Vehicle Comparison: Gasoline vs Diesel Variants

• Diesel engines are suitable for those who drive extensively, carry heavy cargo, or tow big loads.
• Gas engines are better for those who drive less than 12,000 miles per year and prefer a car with fast acceleration.

Exhaust Emissions Treatment: Gasoline vs Diesel

• Diesel engines emit more nitrogen oxides (NOx) due to heated air in the engine, making them greater pollutants overall.
• An aftertreatment system is needed to reduce harmful exhaust emissions from internal combustion engines.

Fuel Conversion Efficiency

• The ratio of the work produced per cycle to the amount of fuel energy supplied per cycle.
• Typical values for fuel conversion efficiency:
  • SI: 270 g/kW-hr (0.47 lbm/hp-hr)
  • CI: 200 g/kW-hr (0.32 lbm/hp-hr)

Air/Fuel & Fuel/Air Ratios

• The air/fuel ratio is the ratio of the mass of air to the mass of fuel supplied to the engine.
• Typical values for normal operating ranges:
  • SI: 12 ≤ A/F ≤ 18 (0.056 ≤ F/A ≤ 0.083)
  • CI: 18 ≤ A/F ≤ 70 (0.014 ≤ F/A ≤ 0.056)

Volumetric Efficiency

• The volume flow rate of air into the intake system divided by the cylinder volume displacement rate.
• Measures the effectiveness of the engine's induction process.
• Typical values: 80 to 90% for Normally Aspirated (NA) engines.

Engine Specific Weight & Specific Volume

• These parameters indicate the effectiveness of engine design and material usage.
• CI engines have higher volumetric efficiencies than SI engines.

Thermodynamic Analysis: Carnot Cycle vs Actual ICE Conditions

• The Carnot cycle offers the maximum possible energy conversion efficiency, but it's not suitable as a reference ideal cycle for ICEs.
• The ideal reference cycles are the Otto cycle for SI engines and the Diesel cycle for CI engines.
• These cycles account for the characteristics of the combustion process in ICEs.

Engine Emissions

• Important engine operating characteristics include levels of NOx, CO, unburned hydrocarbons (HC), and particulates.
• Concentrations of gaseous emissions in the exhaust gases are usually measured in parts per million (ppm) or percent by volume (% vol).
• Two commonly used indicators of engine emissions levels are Specific Emissions and Emission Index.

Specific Emissions vs Emissions Index

• Specific Emissions: mass flow rate of pollutant per unit power (typical units: μg/J, g/kW-hr, and g/hp-hr)
• Emission Index: mass flow rate of pollutant divided by the fuel flow rate (typical units: μg/J, g/kW-hr, and g/hp-hr)

Internal & External Combustion Engines

• Internal combustion engines (ICEs) convert chemical energy of fuel into mechanical energy through a combustion process.
• Examples of ICEs include gas turbines, reciprocating and rotary ICEs.
• External combustion engines, on the other hand, operate according to a thermodynamic closed cycle, where the working fluid undergoes thermodynamic transformations in a closed loop without any need of being replaced. Examples include steam turbine plants and Stirling engines.

Fuel Conversion Efficiency

• Fuel conversion efficiency is the ratio of the work produced per cycle to the amount of fuel energy supplied per cycle.
• Typical heating values for hydrocarbon fuels: 42 - 44 MJ/kg (18,000 - 19,000 Btu/lbm)
• Fuel energy supplied by the fuel may not be fully released as thermal energy due to incomplete combustion.

Air/Fuel & Fuel/Air Ratios

• Air/fuel ratio is the ratio of the mass of air to the mass of fuel supplied to the engine.
• Typical values for normal operating ranges:
  • SI: 12 ≤ A/F ≤ 18 (0.056 ≤ F/A ≤ 0.083)
  • CI: 18 ≤ A/F ≤ 70 (0.014 ≤ F/A ≤ 0.056)

Volumetric Efficiency

• Volumetric efficiency is the volume flow rate of air into the intake system divided by the cylinder volume displacement rate.
• Measures the effectiveness of the engine's induction process.
• Typical values: 80 to 90% for Normally Aspirated (NA) engines. CI engines have higher volumetric efficiencies than SI engines.

Engine Specific Weight & Specific Volume

• Engine specific weight and specific volume indicate the effectiveness with which the engine designer has used the engine materials and packaged the engine components.

Engine Design & Performance Data

• Mean Piston Speed (Sp): measures comparative success in handling loads due to inertia of parts and/or engine friction.
• Brake Mean Effective Pressure (bmep): reflects the product of volumetric efficiency, fuel/air ratio, and fuel conversion efficiency.
• Power per Unit Piston Area: measures effectiveness with which piston area is used, regardless of cylinder size.
• Specific Weight: indicates relative economy with which materials are used.
• Specific Volume: indicates relative effectiveness with which engine space has been utilized.

Specific Fuel Consumption (sfc)

• Specific fuel consumption is the amount of fuel consumed by the engine per unit power output.
• Measures how efficiently the engine is using the fuel supplied to produce work.
• Low values of specific fuel consumption are desirable.

ICE Efficiency: Breakdown of Energy Losses

• Combustion losses: significant losses due to the combustion process requiring a remarkable amount of time to be completed.
• Gas exchange or pumping losses: in 4S naturally aspirated engines, the pressure during the intake stroke is lower than the environmental pressure, while the opposite happens during the exhaust stroke, resulting in a negative pumping work.

Power, Torque, & Mean Effective Pressure

• Power, torque, and mean effective pressure can be used to compare engines with different sizes.
• Engine-specific power can be related to either engine displacement or the piston unit area, which is proportional to the product between bmep and mean piston speed.

Road-Load Power

• Road-load power is the power required to drive a vehicle on a level road at steady speed.
• It is used to overcome both rolling resistance and aerodynamic drag.
• An approximate formula for road-load power is:

P_rl = (CR * Mv * g) / (ρa * CD * Av) * Sv^2

where: CR = coefficient of rolling resistance Mv = mass of vehicle g = acceleration due to gravity ρa = ambient air density CD = drag coefficient Av = frontal area of vehicle Sv = vehicle speed