Engines
What is Engines?
An engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. The expansion of the hot combustion products then directly drives a mechanical motion.
Key formula / rule: Thermal Efficiency
Key points
- Understand the working principles of different engine types.
- Analyze thermodynamic cycles relevant to engines.
- Calculate engine performance parameters.
- Identify factors affecting engine efficiency and emissions.
Common exam trap
Confusing the ideal cycles (e.g., Otto, Diesel) with actual engine cycles.
Definitions
- Term
Internal Combustion Engine (ICE)
- Meaning
An engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. The expansion of the hot combustion products then directly drives a mechanical motion.
- Term
External Combustion Engine (ECE)
- Meaning
An engine where the combustion of a fuel takes place outside the engine, and the heat generated is transferred to a working fluid (like water or air) which then expands to produce work.
- Term
Compression Ratio (r)
- Meaning
The ratio of the volume of the cylinder and combustion chamber when the piston is at the bottom dead center to the volume when the piston is at the top dead center.
- Term
Cutoff Ratio (α)
- Meaning
In a Diesel cycle, the ratio of the volume at the end of the constant pressure heat addition process to the volume at the beginning of this process.
Learning objectives
Understand the working principles of different engine types.
Analyze thermodynamic cycles relevant to engines.
Calculate engine performance parameters.
Identify factors affecting engine efficiency and emissions.
Formulae
- Name
Thermal Efficiency
- Note
Ratio of net work output to total heat input.
- Expression
\ηth = \frac{Wnet}{Qin}
- Name
Specific Fuel Consumption
- Note
Fuel consumption rate per unit net power output. Lower SFC indicates better efficiency.
- Expression
SFC = \frac{\dot{m}_f}{\dot{W}_{net}}
- Name
Otto Cycle Ideal Efficiency
- Note
r = compression ratio, \γ = ratio of specific heats (cp / cv).
- Expression
\ηth, Otto = 1 - \frac{1}{r\γ-1}
- Name
Diesel Cycle Ideal Efficiency
- Note
r = compression ratio, \γ = ratio of specific heats, \α = cutoff ratio (V4/V2).
- Expression
\ηth, Diesel = 1 - \frac{1}{r\γ-1} \left[ \frac{\α\γ - 1}{\γ (\α - 1)} \right]
Prerequisites
Thermodynamics
Heat Transfer
Basic Mechanics
Common mistakes
Confusing the ideal cycles (e.g., Otto, Diesel) with actual engine cycles.
Incorrectly applying thermodynamic laws to engine processes.
Ignoring heat losses and friction in performance calculations.
Misinterpreting efficiency definitions (e.g., thermal vs. mechanical).
Keywords
Engine
Internal Combustion Engine
External Combustion Engine
Otto Cycle
Diesel Cycle
Rankine Cycle
Thermal Efficiency
Specific Fuel Consumption
Thermodynamic Cycle
Work Output
Heat Input
Practice preview
What is the primary function of a carburetor in a spark-ignition engine?…
easy
Which thermodynamic cycle is ideally associated with the working of a spark-ignition internal combustion engine?…
easy
For the same compression ratio and heat rejection, which of the following statements is true regarding the thermal efficiencies of ideal Otto and Diesel cycles?…
medium