00663 - Machines

Course Unit Page

SDGs

This teaching activity contributes to the achievement of the Sustainable Development Goals of the UN 2030 Agenda.

Affordable and clean energy Responsible consumption and production Climate Action

Academic Year 2021/2022

Learning outcomes

The course objective is to show the main phenomenaaffectingdesign and performance ofthe following machines:

- centrifugal pumps

- displacement pumps

- hydraulic turbines (Francis, Pelton, Kaplan)

- Internal combustion engines

The course structurefollows the path from the general physical law to its application for the given machine, showing how the physical behavior affects on one side the design, and on the other side the machine performance.

Particular attention is devoted to the machines regulation: this is the basis for a deeperstudy on this topic, that can be attained with other courses.

Course contents

1- Thermodynamics cycles: Otto, Diesel, Sabathè. T-s  and p-V Diagrams. Evaluation of Sabathè cycle thermodynamic efficiency. Thermodynamic model hypothesis (ideal machine, ideal media, closed cycle).

2- T-s diagrams: comparison of Otto-Diesel cycles for a fixed compression ratio, with fixed heat, and with different compression ratios, but same peak pressure.

3- Average exhaust gas temperature.

4- Limit cycle, effects of specific heat, mixture composition variations and CO-CO2 equilibrium.

5- Indicating diagrams; indicated efficiency, mechanical efficiency.

6- Combustion, A/F ratio, lambda, chemical balance (C43H84).

7- Factors influencing engine power. Definition of combustion efficiency, volumetric efficiency. Definition of available thermal power for si and ci engines. Indicated and Brake Mean Effective Pressure. Brake Specific Fuel Consumption.

8- engine speed and load effect on mechanical, combustion, and indicated efficiencies.

9- Performance curves: Power, Torque, BSFC, for si and ci engines. Downsizing and downspeeding effects.

10- Power control: effects on indicating diagram, and efficiency, comparison of the partial load behavior of si and ci engines .

11- The combustion phenomenon: description on the basis of generalized Arrhenius equation. Premixed and diffusive combustion. Auto-ignition of an homogeneous mixture. Induction time.

12- Flame front propagation. Kernel formation, laminar flame speed; T, p and lambda dependencies. Turbulent combustion speed.

13- Diffusive combustion propagation, mixture etherogeneity effects, comparison with flame front propagation, consequencies on power regulation and pollutant emissions.

14- Combustion propagation nd cylinder pressure formation.

15- Combustion model, determination of net rate of heat release based on cylinder pressure. Definition of MFB50, effect of Spark Advance on ROHR.

16- Cycle-to-Cycle variability; MFB50 distribution; bell-shaped curve.

17- The need to vary SA with speed and load.

18- Abnormal combustions in SI engines: pre-ignition, knock. Effects of design, environment and control parameters. Octane number.

19- Diffusive combustion. CI engines combustion. Heat release and cylinder pressure with ci combustion. Limitations related to the peak of heat release rate and diffusive combustion slowness.

20- Cetane number. Combustion Management with common rail systems. Differences between si and ci engines in terms of fuel specifications (Octane/Cetane numbers), consequences on power control.

21- ICE pollutant emissions. Main pollutants and causes of formation. Regulations aimed at pollutants reduction.

22- Pollutant emissions sensitivity to speed, load, lambda (si engines). Three way catalyst with AFR feedback control.

23- Effects of EGR and SA on pollutant emissions. Secondary air, catalyst heating.

24- Pollutants formation in CI engines. Operating conditions and control parameters effect (load, prail, pboost).

25- Pollutants reduction in diesel engines: DOC, EGR, DPF, SCR, LNT.

26- Control systems: torque control in si and ci engines. Torque management (control diagrams).

27- Si engines control: the need for instantaneous air flow estimation. Speed-density, Alfa-N, MAF systems. Injector and coils management.

28- Open-loop determination of fuel mass and injection time.

29- Average piston speed, effects on inertial stresses.

30- Effect of the number of cylinders on output power (with fixed displacement).

31- Energy conservation for closed systems.

32- Energy conservation for opened systems. Fluid equation of motion, Euler equation, kinetic energy equation.

33- Centrifugal pumps: drawing, compensation of axial forces.

34- Speed triangle, theoretical head.

35- Actual head, head with null flow.

36- Pumps in series and parallel. Determination of the operating point.

37- Hydraulic efficiency; mechanical efficiency, volumetric efficiency. Hydraulic similitude, theoretical and actual iso-hydraulic efficiency curves. 

38- Head-flow diagram sensitivity to speed variations. Pump control: bypass, discharge valve, speed control.  

39- Buckingham theorem for the determination of dimensional parameters. Flow coefficient, Head coefficient, power coefficient.

40- Specific diameter, specific speed. Balje diagram.

41- Centrifugal pump design.

42- Pump priming. Cavitation: NPSH definition, sensitivity to flow.

43- Experimental determination of NPSH: layout description.

44- Axial pumps: design, speed triangle.

45- Volumetric pumps: displacement, theoretical and actual Dp-Q diagrams. Vane pump drawing.

46- determination of work and power absorbed.

47- Volumetric pump control: discharge valve, bypass, speed control, fast bypass valve.

48-gear and lobes pumps drawings.

49- reciprocating pumps.

50- Hydraulic turbines: available energy, work, efficiencies.

51- Pelton Turbine: design (drawing), water flow. Distributor efficiency, flow control. Blades design.

52- Speed triangles, maximum efficiency condition. Performance diagrams.

53- Maximum and minimum number of blades.

54- Limitations to head and flow. Multiple nozzles turbines.

55- Francis turbine. Design (drawing), balance of axial forces. Energy recovery with the diffuser. cavitation.

56- Speed triangles, maximum efficiency condition. Parameters affecting specific speed. Performance, design change with specific speed increases. 

57- Kaplan turbines: design (drawing), speed triangles, performance.

Readings/Bibliography

The content of the course will be available in pf documents at www.wirtuale.unibo.it. Some topics may be deepened useing the following textbooks.

V. Dossena, G. Ferrari, P. Gaetani, G. Montenegro, A. Onorati, G. Persico, Macchine a Fluido, Città Studi Edizioni, 2015

G. Cornetti, F. Millo, Macchine a Gas, Il Capitello, Torino, 2015

Teaching methods

Lectures are carried out using the blackboard; to deepen some of the topics powerpoint presentations, data and animations (or images) will be video-projected. 

The topics will be organized as follows:

- machine description

- machine functional modeling

- determination of equations describing the machine behavior

- performance optimization, determination of rules for optimal design

- power control

Attending the class is not mandatory, although highly advisable for a better understanding of the crucial concepts.

Assessment methods

Oral examination: the student will answer three questions concerning the three main topics (Pumps, Turbines, Internal Combustion Engines). Each question is related to one of the 57 topics reported in the course content list. A rank from 0 to 10 is given for each answer: the final score is the sum of the three single grades.

The answer will be considered complete if the description of the machine or its behavior is carried out involving the mathematical analysis (equations, physical models), graphical description (drawings and graphs) and general considerations showed during the lectures (that can be found in the textbooks or on the material given to the students).   

The Course Macchine T (6 CFU) is one module of an integrated Course (Macchine e Sistemi Energetici, 12 CFU): the final grade for the integrated course is the average of the grades obtained by the student in the exams of the two modules. The final grade will be rounded up to the nearest whole number. The vote '30 cum laude' is considered as a 31, so to get a 30 cum laude overall, the student should be in one of the following conditions:

- rating of 30 cum laude in both the modules

- rating of 30 cum laude in one module, and 30 in the other

The exams calendar is available well in advance of the exams dates, and the students can enroll from 10 days to 5 days before the date of the exam. An ID is required to take the exam.

If the number of students attending the exam is higher than 10, the first part of the exam (first question) will be written. The written test will be carried out in the first hour of the exam. If necessary, the exam will be conducted online, using teams or zoom. If the number of participants to the online exam will be higher than 10, the first question will be replaced with a written test carried out using EOL. The test will feature open and closed quick questions, and it will allow accessing the oral part of the exam. 

Teaching tools

Parts of books, presentations and multimedia are used in the lectures and shared to deepen some topics of the course. The shared material covers all the topics of the program, and it is available through the University data exchange platform. The student will be able to access the material only after enrolling in the distribution list of the course for the considered year.

Office hours

See the website of Enrico Corti