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What is the purpose of splitting the fuel conversion efficiency into different factors?
What is the purpose of splitting the fuel conversion efficiency into different factors?
What does the mechanical efficiency 𝜂m consider?
What does the mechanical efficiency 𝜂m consider?
What is the ideal thermodynamic cycle for SI engines?
What is the ideal thermodynamic cycle for SI engines?
What is the purpose of modifying the Carnot cycle?
What is the purpose of modifying the Carnot cycle?
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What is the ratio between the brake work Wb and the indicated work Wi?
What is the ratio between the brake work Wb and the indicated work Wi?
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What is the maximum possible energy conversion efficiency for any thermodynamic cycle?
What is the maximum possible energy conversion efficiency for any thermodynamic cycle?
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What does the indicated efficiency 𝜂i comprise?
What does the indicated efficiency 𝜂i comprise?
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What is the ideal reference cycle for CI engines?
What is the ideal reference cycle for CI engines?
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What is the purpose of analyzing the fuel conversion efficiency?
What is the purpose of analyzing the fuel conversion efficiency?
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What does the fuel conversion efficiency 𝜂f equal to?
What does the fuel conversion efficiency 𝜂f equal to?
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Typical air-fuel ratios for CI engines range from 12 to 18.
Typical air-fuel ratios for CI engines range from 12 to 18.
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Volumetric efficiency measures the effectiveness of the engine's combustion process.
Volumetric efficiency measures the effectiveness of the engine's combustion process.
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The engine specific weight is typically higher for CI engines than SI engines.
The engine specific weight is typically higher for CI engines than SI engines.
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The specific emissions are typically measured in units of g/kW-hr.
The specific emissions are typically measured in units of g/kW-hr.
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The emissions index is a measure of the volumetric efficiency of the engine.
The emissions index is a measure of the volumetric efficiency of the engine.
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CI engines typically have lower volumetric efficiencies than SI engines.
CI engines typically have lower volumetric efficiencies than SI engines.
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The specific volume of an engine is a measure of its engine specific weight.
The specific volume of an engine is a measure of its engine specific weight.
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Concentrations of gaseous emissions in the exhaust gases are usually measured in parts per billion (ppb).
Concentrations of gaseous emissions in the exhaust gases are usually measured in parts per billion (ppb).
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The Emissions Index is a measure of the mass flow rate of pollutant per unit power.
The Emissions Index is a measure of the mass flow rate of pollutant per unit power.
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The unit of measurement for Specific Emissions is typically kilograms per second (kg/s).
The unit of measurement for Specific Emissions is typically kilograms per second (kg/s).
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The Emissions Index is a dimensionless quantity.
The Emissions Index is a dimensionless quantity.
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The equations for Specific Emissions and Emissions Index are identical.
The equations for Specific Emissions and Emissions Index are identical.
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Particulates are typically measured in parts per million (ppm) or percent by volume (% vol).
Particulates are typically measured in parts per million (ppm) or percent by volume (% vol).
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The Relationships Between Performance Parameters are used to express engine performance in terms of engine design and operating parameters.
The Relationships Between Performance Parameters are used to express engine performance in terms of engine design and operating parameters.
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Volumetric efficiency is a measure of an engine's ability to induct air and fuel at the same ratio.
Volumetric efficiency is a measure of an engine's ability to induct air and fuel at the same ratio.
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A higher brake mean effective pressure indicates a more efficient use of air and fuel in an engine.
A higher brake mean effective pressure indicates a more efficient use of air and fuel in an engine.
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Engine specific weight is a measure of the engine's power output per unit of engine weight.
Engine specific weight is a measure of the engine's power output per unit of engine weight.
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Brake specific emissions is a measure of the amount of pollutants emitted by an engine per unit of power output.
Brake specific emissions is a measure of the amount of pollutants emitted by an engine per unit of power output.
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A lower specific fuel consumption indicates that an engine is using more fuel to produce a unit of power.
A lower specific fuel consumption indicates that an engine is using more fuel to produce a unit of power.
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Power per unit piston area is a measure of an engine's ability to handle loads due to inertia of parts and engine friction.
Power per unit piston area is a measure of an engine's ability to handle loads due to inertia of parts and engine friction.
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Engine specific volume is a measure of the relative economy with which engine space is utilized.
Engine specific volume is a measure of the relative economy with which engine space is utilized.
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Typical air-fuel ratios for SI engines range from 12 to 18.
Typical air-fuel ratios for SI engines range from 12 to 18.
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Volumetric efficiency measures the effectiveness of the engine's combustion process.
Volumetric efficiency measures the effectiveness of the engine's combustion process.
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The engine specific weight is typically lower for CI engines than SI engines.
The engine specific weight is typically lower for CI engines than SI engines.
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The specific emissions are typically measured in units of kW-hr/g.
The specific emissions are typically measured in units of kW-hr/g.
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The emissions index is a measure of the engine specific weight.
The emissions index is a measure of the engine specific weight.
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CI engines typically have higher volumetric efficiencies than SI engines.
CI engines typically have higher volumetric efficiencies than SI engines.
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The specific volume of an engine is a measure of its engine specific weight.
The specific volume of an engine is a measure of its engine specific weight.
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The road-load power equation takes into account only the rolling resistance.
The road-load power equation takes into account only the rolling resistance.
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The coefficient of rolling resistance (CR) is typically between 0.1 and 0.2.
The coefficient of rolling resistance (CR) is typically between 0.1 and 0.2.
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The drag coefficient (CD) for cars is typically between 0.1 and 0.2.
The drag coefficient (CD) for cars is typically between 0.1 and 0.2.
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In SI engines, the flame speed is relatively insensitive to the air/fuel ratio.
In SI engines, the flame speed is relatively insensitive to the air/fuel ratio.
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Volumetric efficiency is a measure of the engine's combustion process effectiveness.
Volumetric efficiency is a measure of the engine's combustion process effectiveness.
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The engine specific weight is typically lower for CI engines than SI engines.
The engine specific weight is typically lower for CI engines than SI engines.
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The specific emissions are typically measured in units of g/liter.
The specific emissions are typically measured in units of g/liter.
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The emissions index is a measure of the engine's efficiency.
The emissions index is a measure of the engine's efficiency.
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CI engines typically have higher volumetric efficiencies than SI engines.
CI engines typically have higher volumetric efficiencies than SI engines.
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The specific volume of an engine is a measure of its engine efficiency.
The specific volume of an engine is a measure of its engine efficiency.
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Internal combustion engines exploit the conversion of mechanical energy into chemical energy.
Internal combustion engines exploit the conversion of mechanical energy into chemical energy.
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External combustion engines operate according to a open thermodynamic cycle.
External combustion engines operate according to a open thermodynamic cycle.
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Gas turbines are an example of external combustion engines.
Gas turbines are an example of external combustion engines.
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Hydrocarbons are typically used as fuels in internal combustion engines.
Hydrocarbons are typically used as fuels in internal combustion engines.
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The heat generated by oxidation reactions of elements such as carbon or hydrogen produces a pressure decrease in the fluid.
The heat generated by oxidation reactions of elements such as carbon or hydrogen produces a pressure decrease in the fluid.
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Stirling engines are an example of internal combustion engines.
Stirling engines are an example of internal combustion engines.
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Internal combustion engines convert the chemical energy of a fuel into electrical energy.
Internal combustion engines convert the chemical energy of a fuel into electrical energy.
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Study Notes
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
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Description
Learn about the scavenging process in two-stroke engines, where burnt gas is displaced by a fresh charge pressurized in an external compressor or blower. Explore the design and mechanism of this process.
