Final Engine Lab Quiz

Alkenes

(also known as olefins) • At least one double chemical bond • Diolefins: feature 2 double bonds

Alkanes

(also known as paraffins) • All single chemical bonds

Cycloparaffins

(or cycloalkanes) • Carbons form a ring structure

2 stroke cycle

1-2 - Isentropic Expansion (power stroke) • 2-3 - Exhaust blowdown (exhaust port open, intake port closed) • 3-4-5 - Intake, and exhaust scavenging (Exhaust port open, intake port open) - Intake air pushes most of remaining exhaust gases • 5-6 - Exhaust scavenging (Exhaust port open, intake port closed) - Piston pushing more exhaust gases out • 6-7 - Isentropic compression (both ports closed)

Diesel Engine (1892)

4 stroke, Compressed ignition

Otto Engine (1876)

4 stroke, compressed charge

4 stroke vs 2 stroke

4 stroke: 4 piston movements (Intake, compression, power, exhaust) 2 stroke: 2 piston movements (Intake/power, compression/exhaust)

BSFC

A fuel efficiency metric Allows different engines to be directly compared

What is an internal combustion engine?

A heat engine that converts chemical energy in fuel into mechanical energy - Heat engine: a device that takes thermal energy and converts it into usable work • Internal: combustion happens in an interior space

Eddy Current Dynamometers

A metallic disk rotating in a magnetic field As disk rotates, it conducts electricity, creating eddy currents in the disk Magnetic field adjusted to increase eddy currents (load) Energy from eddy currents is absorbed as heat Torque on disk measured, converted to power Our eddy current dyno: For John Deere Turbo Diesel

Hydraulic Dynamometer

Absorb engine energy in water, oil or some other fluid Engine powers pump that circulates fluid Adjustable valve controls head loss in system (load of pump) Our hydraulic dyno: For Tecumseh 2-stroke gasoline engine Uses silicone fluid Measures pressure on valve, converted to torque

Dual Cycle

An attempt to combine elements of Otto and Diesel cycles: - 1-2 - Isentropic compression - 2-x - Constant Volume Heat Addition - x-3 - Constant Pressure Heat Addition - 3-4 - Isentropic expansion - 4-1 - Constant Volume Heat Rejection • Used in modern day CI engines - More efficient combustion process - Still allows for higher compression ratios

Alkynes

At least one triple chemical bond

Fuel Input Type

Carbeureted - air and fuel are mixed before entering the cylinder; fuel drawn into chamber by pressure differential Fuel Injected - fuel is injected into air at certain locations -Throttle Body - upstream injection -Direct Injection - injection in cylinder

Air Fuel Ratio

Describing mixture of incoming fuel/air ma/mf

Electric Dynamometers

Engine attached to electric generator, then electricity absorbed by some load Total Resistance adjusted to change load Our Electric Dynos: Troy-Bilt gasoline generator has Air cooled heating elements Yanmar Diesel generator has Water cooled heating elements

Water Brake Dynamometers

Engine energy converted into mechanical energy from circulating water Torque of casing due to moving water measured Our Water Brake Dyno: For Chevrolet V-6 Load of dyno remains constant; speed of engine varied by transmission Torque measured using scale and moment arm

Enthalpy of formation

For each product molecule, the enthalpy of a substance at a specified state due to its chemical composition • Summing these values for each product yields enthalpy of combustion

Lenoir Engine (1860)

Fuel not compressed (2 stroke) Used for early automobiles, but mainly for printing presses, water pumps and machine tools

Octane Number (ON)

Fuel property that describes knock resistance, or how well a fuel will not self-ignite • The higher the number is, the less likely knocking will take place • Two reference fuels: • Iso-octane - ON of 100 • N-heptane - ON of 0

Effect of Inert Gases on Reaction

Gases that do not participate in the reaction • Will affect equilibrium composition but not K P

Isomers

Have the same number of atoms of each element, but different structures

Diesel Cycle

Ideal Cycle for compression ignition engines - 1-2 - Isentropic compression - 2-3 - Constant pressure heat addition - 3-4 - Isentropic expansion - 4-1 - Constant volume heat rejection

Otto Cycle

Ideal Cycle for spark-ignition engines - 1-2 - Isentropic compression (BDC to TDC) - 2-3 - Constant volume heat addition - 3-4 - Isentropic expansion (TDC to BDC) - 4-1 - Constant volume heat rejection

Thermal Efficiency Comparison

If the 3 cycles had the same compression ratio: otto > dual > diesel If the 3 cycles had the same peak pressure: diesel > dual > otto

Equilibrium Constant

K P is a function of temperature only (not pressure) • Values found in tables for common reactions • The K P of the reverse reaction is 1/K P • The larger the K P , the more complete the reaction • If stoichiometric coefficients are doubled, K P is squared.

Avoiding Knock

Limiting compression ratio • If Octane number is lower, compression ratio must be lower • Hydrocarbons chain length • Longer chain length usually yields lower Octane number • Side chains will usually yield higher Octane number • Fuel Additives • Limiting deposits on the combustion chamber walls • Deposits reduce heat transfer through chamber walls and increase temperature inside • Avoiding "hot spots" on combustion chamber • Starting ignition later in compression stroke

Air Input Types

Naturally Aspirated - no pressure boost system (most engines) Supercharged - intake air pressure increased with a compressor driven by crankshaft Turbocharged - intake air pressure increased with a compressor driven by a turbine using exhaust gases

Reciprocating Engines

Piston inside a cylinder moves back and forth as gases expand/contract • Reciprocating energy is typically converted to rotational energy through a crankshaft

Equilibrium in a hydrocarbon combustion reaction with air

Possible Products: • Carbon dioxide • Carbon monoxide • Nitrogen monoxide • Nitrogen dioxide • Various hydrocarbons • Water vapor • Hydrogen gas or ion • Oxygen gas or ion • Nitrogen gas or ion • Hydroxide ion

Reciprocating vs. Rotary

Reciprocating - pistons move back and forth in cylinder, then transfer energy to crankshaft Rotary - engine block built around a large non concentric rotor and crankshaft

SI vs. CI

SI: combustion in engine is initiated by a spark from a spark plug (typically gasoline) CI: fuel self ignites when it is introduced to high temperature air (typically diesel fuel)

Equivalence Ratio

Sometimes referred to as the "fuel equivalence ratio" • Relates the AF ratio of the engine to the stoichiometric ratio: • φ=1: stoichiometric reaction (fuel/oxygen completely consumed) • φ>1: fuel "rich" reaction (fuel leftover after reaction) • φ<1: fuel "lean" reaction (oxygen leftover after reaction)

Classifying Combustion Reactions

Stoichiometric (theoretical) combustion - fuel and oxygen react completely, leaving only products (no reactants) at finish • Complete combustion - all fuel reacts completely (some oxygen can remain) • Incomplete combustion - some fuel remains after reaction is complete

Self-Ignition Temperature (SIT)

Temperature where air/fuel mixture will ignite without ignition source (like spark plug) • Usually happens when air/fuel mixture is compressed to a high degree (and temperature increases) • Intentional in Diesel Engines, not intentional in gasoline engines • Ignition Delay - the time period between the air/fuel mixture reaching SIT and the mixture igniting

BMEP

The average pressure that exists in the combustion chamber during the entire cycle

Volumetric efficiency

The efficiency with which the engine can move the air/fuel mixture (or exhaust) in or out of the combustion chamber • When air is drawn into the combustion chamber, the pressure of the air is lower than ambient, which reduces the density • Lower density implies less air (oxygen), which limits the amount of fuel to be added • 100% implies that the air pressure in the combustion chamber is close to ambient conditions before compression/combustion • <100% implies that the air pressure in the combustion chamber is lower than ambient • >100% implies that the air pressure in the combustion chamber is higher than ambient

Adiabatic Flame Temperature

The temperature of products of combustion if no heat is lost to the surroundings

2 stroke Applications

Typically used in small engine applications (chain saws, leaf blowers, small scooters/motorcycles) • Large amounts of exhaust gases remain in engine throughout cycle - Usually have lower exhaust temperatures because of diluted air/fuel mixture

Standard Reference State

Used to compare energy levels of reactions for different molecules • Standard reference assumed to have zero energy • Traditionally at 25 deg C and 1 atm of pressure • Values have superscript of "°"

How is Octane Number determined?

Using test engine with variable compression ratio • Test fuel is run through engine, and compression ratio is adjusted until knocking is experienced • Then a mixture of iso-octane and n-heptane is run through the engine, varying the percentages of each until the same level of knocking is experienced

Knocking

When gasoline engines reach self-ignition temperature • Combustion process not controlled in this case • Traditional combustion will involve a "flame front" moving from ignition source towards piston • Knocking will have combustion happening at other locations in cylinder creating conflicting pressure waves • Causes ragged pressure changes during compression/power strokes • Usually accompanied by additional noise and vibrations in the engine • Will likely happen if compression ratio is 11 or above

Determining Equilibrium State

Write out balanced stoichiometric equation • Write out actual equation (including remaining reactants) • Write conservation of mass equations for each element in reaction and total moles • Convert pressure to atm • Find expected K P value from tables • Solve for actual equation coefficients (iteration likely)

Aromatics

feature a ring structure and double bonds (like benzene, C 6 H 6 )

Brake Power

the actual work available at the crankshaft (what is typically measured)

Air Standard Assumptions

• Additional simplifications for internal combustion engines: - The working fluid is air, which continuously circulates in a closed loop, and always behaves as ideal - All the processes involved are internally reversible - Combustion is replaced by external heat addition - Exhaust is replaced by heat rejection

Fuel "rich" combustion

• More fuel than necessary for stoichiometric reaction • Fuel will be leftover at the end of the reaction • Air/Fuel Ratio will be lower than stoichiometric • Equivalence Ratio will be greater than 1 • Often seen when engine is being started or when under high load (like acceleration)

Fuel "lean" combustion

• Not enough fuel to react with all oxygen • Oxygen will be leftover at the end of the reaction • Air/fuel ratio will be higher than stoichiometric • Equivalence Ratio will be less than 1 • Often seen when engine is under light load (like cruising speed)