AME 436, Prof. Paul Ronney

Midterm Exam Study Guide

March 26, 2008

Format of the exam

The midterm exam will be open book, 90 minutes long. You may use any reference materials you want, but laptops, Pocket PCs, etc. capable of running GASEQ, aircycles4recips.xls, etc. will NOT be permitted. The exam will have three types of questions: (1) numerical problems, for example chemical equilibrium/adiabatic flame temperature, (2) graphical problems, for example P-V and T-S diagrams, and (3) short-answer questions.

Material covered

The exam may cover any material through the end of section on Unsteady Flow engines (i.e. material on Thrust, Compressible Flow, and Airbreathing Propulsion won’t be on the exam). The material covered on the midterm includes:

• Classifications of IC engines; advantages and disadvantages of each type

• Introduction to combustion

• Fuel types

• Chemical thermodynamics

• Stoichiometry

• Heating value

• Adiabatic flame temperature

• Isentropic expansion with frozen and equilibrium products

• Elementary combustion theory

• Chemical reaction rates

• Homogeneous reaction

• Premixed flames (deflagration)

• Effects of turbulence

• Non-premixed flames

• Unsteady flow engines

• Design parameters

• rc, Vd, N

• Performance parameters

• Indicated and Brake torque, power, MEP

• Efficiency - thermal, mechanical, volumetric

• Emissions

• Ideal-gas cycle analysis

• KNOW T-S AND P-V DIAGRAMS BACKWARDS AND FORWARDS!

• Otto and Diesel cycles and variations (e.g. complete expansion)

• Cycle comparisons

• Fuel-air cycles

• Modifications to ideal cycles

• Slow burn

• Friction

• Heat loss

• Combustion in unsteady flow engines

• Knock

• What is it and why is it bad?

• Effect of fuel type and fuel structure

• Effect of operating conditions

• Flammability/misfire limits

• Incomplete combustion / flame quenching

Last year’s midterm exam (should look familiar) (Average score was 72/100)

Instructions:

Open book exam. Use any reference materials you want, other than laptop computers, Palm Pilots, Pocket PCs, etc. capable of running spreadsheets like AIRCYCLES4RECIPS.XLS or GASEQ. 90 minutes allowed. Show all work; partial credit given if you can show you have a valid approach!

Problem #1 (Chemical thermodynamics) (25 points total) A new process has been invented to produce and store ozone (O₃) safely and economically (yeah right…) Ozone decomposes exothermically to form O₂ via the reaction

O₃ ® 1.5 O₂.

Thermodynamic data (not all of this data is needed!):

	O₃	O₂	O
Dh_f^o (J/mole)	1.427 x 10⁵	0	2.492 x 10⁵
Molecular weight (kg/mole)	0.048	0.032	0.016
C_P (J/mole K) (temperature-averaged)	57.18	36.49	20.84
K_i (equilibrium constant) at 2500 Kelvin (dimensionless)	3.01 x 10^-7	1	1.44 x 10^-2

a) (8 points) What is the heating value of ozone “fuel” in J/kg if the combustion product is O₂ only (no O₃, no O)?

b) (8 points) Estimate the constant-pressure adiabatic flame temperature of ozone if the combustion product is O₂ only (no O₃, no O) and the initial temperature is 300K.

c) (9 points) At a temperature of 2500 K, what is the mole fraction of O atoms at equilibrium in the products? For this part assume the products contain O and O₂ but do not contain O₃.

Problem #2 (the dreaded P-V and T-s diagrams) (5 points each diagram; 40 points total)

Consider the “baseline” ideal Diesel cycle shown on the P-V and T-s diagrams. Sketch modified P-V and T-s diagrams if the following changes are made. Unless otherwise noted, assume in each case the initial temperature and pressure, compression ratio, fuel mass fraction, heating value, etc. are unchanged. Where useful for clarity, label plots with phrases like “this area = that area,” “these two temperatures are the same,” etc. In some cases there may be no change to the P-V or T-s diagram.

a) The compression ratio is increased (same maximum volume)

Same max. volume for modified cycle

b) Part way through the constant-pressure burn, knock occurs which causes the remainder of the burn to occur instantaneously at constant volume

c) The intake valve closes late (i.e. after part of the compression stroke has started; the pressure stays at ambient pressure and no compression occurs until after the intake valve closes) in such a way that the pressure after the expansion is ambient (i.e. the cycle has been converted to a complete-expansion cycle).

Same max. and min. volumes for modified cycle

d) A new lubricant is used that decreases rubbing friction

Problem #3 (Cycle analysis) (10 points total).

For each of the cycle modifications in problem 2, will the brake thermal efficiency increase, decrease or remain the same? Explain very briefly. (You should be able to do this even if your diagrams aren’t right.)

(a)

(b)

(c)

(d)

Problem #4 (Engine performance) (25 points total) The following 5 changes to a premixed-charge engine are being considered:

1) Increase the displacement volume by a factor of 2

2) Increase the compression ratio by a factor of 2

3) Increase the intake pressure by a factor of 2

4) Increase the engine rotation rate (N) by a factor of 2

5) Increase the turbulence intensity by a factor of 2 using a different piston shape (N not changed)

Briefly explain:

a) Which of these would increase the thermal efficiency the most? Assume knock is not a factor.

b) Which of these would increase the volumetric efficiency the most? Assume knock is not a factor.

c) Which of these would increase the brake power the most? Assume knock is not a factor.

d) Which of these would decrease the tendency to misfire the most? Assume knock is not a factor.

e) Which of these would decrease the tendency to knock the most?