Diesel Engine: Principles, Applications, and Comparison
Operating Principle of Diesel Engines
A diesel engine works by the ignition of fuel injected into a combustion chamber containing air at a temperature above the fuel’s autoignition temperature.
Advantages and Disadvantages of Diesel Engines
The main advantage of diesel engines compared to gasoline engines is their low fuel consumption. The primary disadvantages of these engines are their higher initial price and maintenance costs.
Applications of Diesel Engines
Diesel engines are used in various applications, including:
- Agricultural machinery
- Railway propulsion
- Marine propulsion
- Cars and trucks
- Tracked vehicles
- Aircraft propulsion
- Industrial power generation
Characteristics of a Diesel Engine
A major difference between a diesel and a gasoline engine lies in the fuel injection process. A diesel engine injects fuel directly into the cylinder during a portion of the power stroke. This technique improves the efficiency of the diesel engine.
Diesel fuel has a higher energy density than gasoline. On average, a gallon of diesel fuel contains approximately 147 x 106 Joules, while a gallon of gasoline contains 125 x 106 Joules. This explains why diesel engines generally have better mileage than their gasoline equivalents.
Parts of a Diesel Engine
Key components of a diesel engine include:
- Block
- Cylinder Head
- Crankshaft
- Flywheel
- Piston
- Camshafts
- Ducts
- Nozzles
- Valves
- Injection Pump
- Transfer Pump
Importantly, in the diesel cycle, one should never confuse the four-stroke engine with the idealized thermodynamic cycle, which only covers two strokes.
We could also say that the diesel cycle takes into account different variables, including compression. The engine operates with a working fluid that is initially air-only.
The four strokes are: intake, compression, combustion (power), and exhaust.
The Ericsson and Stirling Cycles
It has been shown that the combined effect of regenerative heating and cooling increases the thermal efficiency of a gas turbine power cycle. It is interesting to examine what happens when the number of stages for both cooling and heating becomes infinitely large. In such circumstances, the isentropic processes of compression and expansion become isothermal. The cycle can then be represented by two constant-temperature stages and two constant-pressure processes with regeneration.
Such a process is called the Ericsson cycle.
In this cycle, the fluid expands isothermally from state 1 to state 2 through a turbine, producing work and reversibly absorbing heat from a high-temperature reservoir. After that, the fluid is cooled at constant pressure in a regenerator from state 3 to state 4. The fluid is then compressed isothermally, requiring an input of work and the reversible removal of heat to a low-temperature reservoir. Finally, the fluid is heated at constant pressure to the initial state.
Stirling Cycle
The Stirling cycle is a thermodynamic cycle used in Stirling engines, aiming for optimal performance. It is similar to the Carnot cycle. In this cycle, the fluid undergoes two isothermal transformations and two constant-volume (isochoric) transformations.
Ericsson Cycle
In the Ericsson cycle, the fluid undergoes two isothermal transformations and two isobaric (constant-pressure) transformations.
Efficiency of the Stirling and Ericsson Cycles
The Stirling and Ericsson cycles are completely reversible, like the Carnot cycle. Therefore, according to Carnot’s principle, all three cycles will have the same maximum thermal efficiency when operating between the same temperature range.
Brayton Cycle
Also known as the Joule cycle or Froude cycle, the Brayton cycle is a thermodynamic cycle consisting of an adiabatic compression stage, an isobaric heating stage, and an adiabatic expansion stage of a compressible thermodynamic fluid. Because the gases expand at a higher temperature (qh), the work obtained from the expansion process is greater than that required for compression. The net work of the cycle is the difference between the two.