What is Mechanical Engineering?

What is mechanical engineering?

Paul D. Ronney
Associate Professor
Department of Aerospace and Mechanical Engineering
University of Southern California, Los Angeles, CA 90089-1453

Slides on-line at:
http:/carambola.usc.edu/ENGR101

Definition of mechanical engineering

If it needs engineering but it doesn't involve electrons, chemical reactions, arrangement of molecules, life forms, isn't a structure (building/bridge/dam) and doesn't fly, a mechanical engineer will take care of it

(if it does involve electrons, chemical reactions, arrangement of molecules, life forms, is a structure or does fly, that's OK too)

ME Curriculum

• Basic sciences - math, chemistry, physics

• Breadth, distribution

• Tools

• Computer graphics, computer aided design
• Electronics
• Experimental engineering & instrumentation
• Mechanical design - nuts, bolts, gears, welds
• Computational methods - convert continuous mathematical equations into discrete equations

F = ma Æ F = m = m

• Core "engineering science"

• Mechanics

• Statics: SF = 0
• Dynamics: SF = ma

• Strength of materials:

• Mechanical stress:

• Vibration of beams:

• Fluid mechanics - F = ma applied to a fluid
• Thermodynamics

• 1st Law - energy is conserved - "you can't win"
• 2nd Law - entropy always increases - "you can't break even"

• Heat transfer

• Conduction - q = -kA

• Convection - q = hA(T_f - T_w)

• Radiation - q = seFA(T⁴ - T_w⁴)

• Control systems

• Synthesis

• Senior seminar

• Senior design project - "capstone"

Examples of industries employing ME's

• Automotive

• Combustion
• Engines, transmissions
• Suspensions

• Aerospace (w/ aerospace engineers)

• Control systems
• Heat transfer in turbines
• Fluid mechanics (internal & external)

• Computers (w/ computer engineers)

• Heat transfer
• Packaging of components & systems

• Construction (w/ civil engineers)

• Heating, ventilation, air conditioning (HVAC)
• Stress analysis

• Electrical power generation (w/ electrical engineers)

• Steam power cycles - heat and work
• Mechanical design of turbines, generators, ...

• Petrochemicals (w/ chemical, petroleum engineers)

• Oil drilling - stress, fluid flow, structures
• Design of refineries - piping, pressure vessels

• Robotics (w/ electrical engineers)

• Mechanical design of actuators, sensors
• Stress analysis

Personal experience #1 - automotive engineering

Why internal combustion engines? - alternatives & their limitations

• External combustion - steam engine

• Heat transfer is too slow (100x slower than combustion)
• 10 B-747 engines = large coal-fueled electric power plant

• Electric vehicles

• Batteries are heavy - 1000 lbs/gallon of gasoline equiv.
• Fuel cells better, but still nowhere near gasoline •
• "Zero emissions" myth - exports pollution

• Solar

• Need 30 ft x 30 ft collector for 15 hp
(Arizona, high noon, mid-summer)

• Nuclear

Who are we kidding ???

• Moral - hard to beat gasoline-fueled premixed-charge IC engine for

• Power/weight & power/volume of engine
• Energy/weight & energy/volume of liquid hydrocarbon fuel

Things you need to understand before you invent the clean 100 mpg 1000 hp engine, revolutionize the automotive industry and shop for your retirement home on the French Riviera

• Room for improvement - factor of 2x in efficiency

• Ideal Otto cycle engine with CR = 8: 52%
• Real engine: 30% maximum
• Differences because of

• Heat losses
• Friction losses
• Throttling losses

• Room for improvement - in pollutants

• Pollutants are a non-equilibrium effect

• Burn: Fuel + O2 + N2 Æ H2O + CO2 + CO + UHC + NO
• Expand: CO + UHC + NO "frozen" at high levels

• With slow adiabatic (no heat loss) expansion:

CO + UHC + NO Æ H2O + CO2 + N2

...but we can't slow down the expansion or make it adiabatic

• Room for improvement - very little in power

• IC engines are air processors

• Fuel takes up little space - air flow = power

• Limitation on air flow due to

• Limitation on rotation rate
• Friction
• Mechanical strength
• Slow burn
• "Choked" flow past intake valves

Idea for improvement

• Throttleless Premixed-Charge Engines

• Engines frequently operated at lower than maximum torque output (throttled conditions)

• Throttling adjusts torque output of engines by reducing intake density through decrease in pressure via fluid-dynamic losses ( P = rRT)

• Throttling losses substantial at part load

• Our idea - Throttleless Premixed-Charge Engine

• U. S. Patent No. 5,184,592

• Use intake temperature increment via exhaust heat transfer to reduce r

• Increasing Tintake leads to leaner lean misfire limit - use air/fuel ratio AND Tintake to control torque

• Substantially improved fuel economy (up to 16 %) compared to throttled engine at same power & RPM

• Emissions

• (Untreated) NOx performance > 10 x lower than throttled engine
• Meets federal emission standards (0.4 g/mi)
• CO and UHC comparable to throttled engine

Conclusions - engines

• IC engines are the worst form of vehicle propulsion, except for all the other forms

• Despite over 100 years of evolution, IC engines are far from optimized

• Any new idea must consider many factors, e.g.

• Where significant gains can & cannot be made
• Cost
• Resistance of suppliers & consumers to change

Personal experience #2 - Fire safety in spacecraft

• Motivation

• Gravity influences combustion through buoyant convection

• Hot gases rise - changes heat & fuel transport, flame shape

• Case study - "Flame balls"

• Zeldovich, 1944: stationary spherical flames ("flame balls")

have solutions for unbounded domain only in spherical geometry (no "flame cylinders" or "flame slabs" possible)

• T ~ 1/r unlike propagating flame (T ~ e^-r) - properties dominated by long 1/r tail (with r³ volume effects!)

• Theory (1985): flame balls are unstable

• Ronney (1990): seemingly stable, stationary flame balls accidentally discovered in drop-tower experiments; confirmed in parabolic aircraft flights (Ronney et al., 1993)

• Only seen at µg, light fuel (e.g. H₂), weak mixtures near extinction limits

• New theory (1990): window of stable conditions when radiant heat loss is present

• Practical value of flame ball studies

• Stationary spherical flame - simplest interaction of chemistry & transport - test combustion models
• Spacecraft fire safety - flame balls exist in mixtures outside one-g extinction limits
• May be relevant to near-limit turbulent combustion of H₂ - proposed for Low-Emission Vehicles

• Implementation of space experiment

• Experimental apparatus

• Combustion vessel - cylinder, 32 cm i.d. x 32 cm length
• 15 gas mixtures
• Ignition system - spark
• 2 video cameras
• Temperature - fine-wire thermocouples, 6 locations
• Radiometers (4), chamber pressure, acceleration (3 axes)

• Space Shuttle mission MSL-1 (April 4 - 8, 1997) & MSL-1R (July 1 - 16, 1997), Combustion Module-1 (CM-1) facility

• Trained as backup crew member for missions

• Results - surprises

(1) Much less buoyancy-induced drift than expected - may be hours before drifting into walls (partially understood now)

(2) Flame balls drifted apart - mutual repulsion - consuming each other's fuel (mostly understood now)

(3) Data drastically affected by impulses caused by small thrusters used to control Orbiter attitude (understood now)

(4) 2 missions, 26 burn tests, wide range of pressure, diffusivity, radiation properties, # of balls, etc., yet every single flame ball, without exception, produced between 1.0 and 1.8 Watts of radiant power !!!!! (not understood at all)

• Summary - fires in space

• First premixed gas combustion experiment in space
• Weakest flames ever burned, either on the ground or in space (flame ball: 1 Watt; birthday candle: 50 watts)
• Many surprises despite 13 years of ground-based experiments & modeling

• Space flight training _ "the right stuff" any more

Summary - Mechanical Engineering

• Perhaps broadest engineering discipline

• Everybody needs ME's

• Core material permeates all engineering systems (fluid mechanics, solid mechanics, heat transfer, control systems, ...)