1. Thermodynamics, Lesson 4-4: The Air Standard Diesel Cycle
MECE-251
Thermodynamics, Lesson 4-4:
The Air Standard Diesel Cycle
Compression-ignition
Air is compressed to a high pressure and temperature.
Combustion occurs spontaneously when fuel is injected.
This type is
preferred for high-power applications and when fuel economy is required.
used in heavy trucks and buses, locomotives and ships, and auxiliary power units.
A diesel engine is one of several air standard power cycles that are in common use for transportation systems throughout the world.
They key attribute of these engines is that combustion of the fuel is initiated by virtue of the high temperature and pressure present at the peak of the compression stroke in the engine cylinders.
A second law of thermodynamics analysis allows us to predict that under these high temperature and pressure conditions, the fuel combustion reaction will proceed spontaneously, without the presence of an initiating spark. Thus, there are no spark plugs in a diesel engine.
Compression ignition engines are commonly used in heavy truck and rail applications today. Many modern rail applications employ a diesel electric system, wherein the combustion ignition engine drives a rotary generator to produce electricity.

2. Introducing Engine Terminology
Displacement volume: volume swept by piston when it moves from top dead center to bottom dead center
Top dead center
Stroke
Bottom dead center
Let’s consider a single cylinder in a compression ignition diesel engine. The purpose of the engine is to move the piston up and down. The piston is connected to the piston rod, which in turn connects to the crank shaft. The cylinder is filled with make-up air from the intake valve. Diesel fuel is injected at the top of the cylinder. The other valve is used to exhaust the combustion products. When one cylinder is on the power stroke, as a result of fuel combustion, the other cylinders are in other positions of their process. The mechanical timing between cylinders allows one cylinder to provide power to the vehicle, and motion for the other cylinders when they are not in the power stroke.
When the piston is at the highest position inside of the cylinder, this is called top dead center.
When the piston is at the lowest position inside of the cylinder, this is called bottom dead center.
The distance of travel between top dead center and bottom dead center is called the stroke.
The displacement volume of each cylinder is equal to the cross sectional area of the piston times it stroke.
The compression ratio is the ratio of the maximum volume over the minimum volume.
Compression ratio, r : volume at bottom dead center divided by volume at top dead center
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

3. Introducing Engine Terminology
Four-stroke cycle
Four strokes of the piston for every two revolutions of the crankshaft
Diesel Intake stroke
With the intake valve open, piston stroke draws a fresh charge of air into the cylinder.
Let’s look at a four stroke cycle for a diesel engine. Each stroke constitutes one thermodynamic process. One stroke is completed when the piston moves from top dead center to bottom dead center, or in the reverse. Two strokes, or processes, are completed with each revolution of the crankshaft. Four strokes, or processes, are completed in two revolutions of the crankshaft.
During the first stroke, the intake valve opens to admit fresh air to the cylinder, and the exhaust valve is closed to trap the air in the cylinder. As the piston moves from top dead center to bottom dead center, fresh air fills the cylinder and the intake valve closes.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

4. Introducing Engine Terminology
Compression stroke
With both valves closed, piston compresses charge, raising the pressure and temperature, and requiring work input from the piston to the cylinder contents.
For diesel compression-ignition engines, combustion is initiated by injecting fuel into the hot compressed air.
During the compression stroke (or process) both the intake and exhaust valve are closed.
This portion of the cycle looks just like a piston compression cycle for a compressor. The air inside of the cylinder is compressed, and its temperature rises.
As the piston approaches top dead center, fuel is injected into the hot gas and combustion is initiated.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

5. Introducing Engine Terminology
Power stroke
The gas mixture expands and work is done on the piston as it returns to bottom dead center.
Exhaust stroke
The burned gases are purged from the cylinder through the open exhaust valve.
The power stroke is caused by the expanding gases that result from combustion of the fuel and air mixture.
The expanding gases do work upon the piston as the piston approaches bottom dead center.
At this point, combustion has ceased, and the exhaust valve opens. The piston once again moves to top dead center, pushing the exhaust gases out of the exhaust valve.
The mechanical cycle is ready to repeat.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

6. Air-Standard Diesel Cycle
The Diesel cycle consists of four processes in series:
Process 1-2: isentropic compression.
Process 2-3: constant-pressure heat addition to the air from an external source.
Process 3-4: isentropic expansion.
Process 4-1: constant-volume heat transfer from the air.
The Diesel cycle has a two-step power stroke: process 2-3 followed by process
We have just seen that the diesel cycle consists of 4 sequential processes, each corresponding to one stroke of the piston.
The first process is isentropic (or adiabatic) compression. This stroke is just like the air compressor that we saw before.
The second process is constant pressure heat addition to the air, from the burning fuel, which begins the power stroke.
The third process consists of isentropic (or adiabatic) expansion, which concludes the power stroke.
The fourth process is constant specific volume exhaust of the gases from the cylinder.
Notice that the power stroke begins during the later part of the compression process and extends all the way through the expansion process.
Let’s take a look at an animation of the diesel cycle.
Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.

7. Air-Standard Diesel Cycle Animation
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Reference: Moran, Shapiro, Boettner, Bailey: Fundamentals of Engineering Thermodynamics, 7th Edition, Dec 2010, © 2011, Wiley.