Internal Combustion Engines Midterm Quiz

Otto Cycle

ideal cycle for spark-ignition engines

Throttle body

upstream injection

Air intake process

▪ Naturally aspirated ▪ Supercharged ▪ Turbocharged

Otto Engines

4 stroke, compressed charge (1876)

Diesel Engines

4 stroke, compression ignition (1892)

E85

85% ethanol, 15% gasoline; only a few vehicles manufactured can handle this fuel (sometimes referred to as "flex-fuel" vehicles (gasoline alcohol mixtures)

E10

90% gasoline, 10% ethanol; all vehicles manufactured can handle this fuel (gasoline alcohol mixtures)

Diesel Cycle

Ideal Cycle for compression ignition engines

innovations that led to the rise of the IC engine

Pneumatic rubber tires were invented in 1888, which made automobiles more practical and created a much larger market for engines

Carbureted

air and fuel are mixed before entering the cylinder; fuel drawn into chamber by pressure differential

Stoichiometric ratio

all fuel and oxygen are consumed

CNG

compressed (natural gas/methane)

CI

compression ignition- fuel self ignites when it is introduced to high temperature air (typically diesel fuel)

4-stroke:

four piston movements (intake, compression, power, exhaust)

fuel injected

fuel is injected into air at certain locations

Octane Number

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

Direct injection

injection in cylinder

Turbocharged

intake air pressure increased with a compressor driven by a turbine using exhaust gases

Supercharged

intake air pressure increased with a compressor driven crankshaft or electric motor

LNG

liquefied (natural gas/methane)

two-stroke cycles

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

How is Cetane Number determined?

o Centane number determined in similar fashion to octane number

How to limit knocking?

o Limiting compression ratio o Hydrocarbons chain length o Fuel additives o Limiting deposits on the combustion chamber walls o Avoiding "hot spots" on combustion chamber o Starting ignition later in compression stroke

Fuel rich

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

Fuel lean

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

Enthalpy of formation

o The enthalpy of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states (25 degC, 1 atm pressure).

• Air Standard Assumptions (additional simplifications for internal combustion engines)

o The working fluid is air, which continuously circulates in a closed loop, and always behaves as ideal o All the processes involved are internally reversible o Combustion is replaced by external heat addition o Exhaust is replaced by heat rejection

Standard reference state

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

Adiabatic flame temperature

o the temperature of products of combustion if no heat is lost to the surroundings o Must solve for h-bar values (representing Tad) o More unknown variables than equations

Cetane Number

quantifies ignition delay time, lower number implies longer ignition delay

SI

spark ignition - combustion in engine is initiated by spark from a spark plug (typically gasoline)

Self-Ignition Temperature

temperature where air/fuel mixture will ignite without ignition source (like spark plug)

Brake Power

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

BMEP:

the average pressure that exists in the combustion chamber during the entire cycle (Pa)

2-stroke:

two piston movements (intake/power, compression/exhaust)

Knocking

when gasoline engines reach self-ignition temperature, combustion process not controlled in this case

Natural Gas/Methane- at least one advantage and disadvantage of each:

• Advantage: Fairly abundant, especially in US • Disadvantage: low energy density/ limited vehicle range/ large fuel tank needed

Hydrogen- at least one advantage and disadvantage of each:

• Advantage: potentially widely available (in water) • Disadvantage: difficult to store, transport and refuel (very volatile/ flammable/ explosive)

Biofuels- at least one advantage and disadvantage of each:

• Advantage: sources widely available; low cost; low emissions • Disadvantage: low energy content, high specific fuel consumption; competes with food production; may gel at low temperatures

Alcohols- at least one advantage and disadvantage of each:

• Advantage: wide variety of sources • Disadvantage: low energy content

3 differences between ideal cycle and real engine

• Real engines are not closed systems • Gases inside engine are not always air and not always ideal • There are heat losses during the cycle of a real engine

Diesel engines

• Stoichiometric ratio: 14.5:1 • Can range from 14 to 70

Gasoline engines

• Stoichiometric ratio: 14.6:1 Can range from 6 to 25

Complete combustion

• all fuel reacts completely (some oxygen can remain)

Stoichiometric (theoretical) combustion

• fuel and oxygen react completely, leaving only products (no reactants) at finish

2nd Generation Biofuels

• made from biomass o Woody (inedible) crops, agricultural residues or waste plant material

1st Generation Biofuels

• made from food crops on arable land (Ethanol & Biodiesel)

Biodiesel

• produced from oils or fats through transesterification o Mostly consists of fatty acid methyl esters (FAMEs) o Most common sources are vegetable oils and animal fats

incomplete combustion

• some fuel remains after reaction is complete

Diesel cycle process

▪ 1-2 - Isentropic compression ▪ 2-3 - Constant pressure heat addition ▪ 3-4 - Isentropic expansion ▪ 4-1 - Constant volume heat rejection

5 processes that make up ideal cycle

▪ 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

Otto Cycle Processes

▪ 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

BSFC

▪ A fuel efficiency metric ▪ Allows different engines to be directly compared

Eddy Current Dynamometer

▪ A metallic disk rotating in a magnetic field ▪ As disk rotates, it conducts electricity, creating eddy currents in the disk ▪ Magnetic field adjusted to increased eddy currents (load) ▪ Energy from eddy currents is absorbed as heat ▪ Torque on disk measured, converted to power

Fuel input

▪ Carbureted- 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

Electric Dynamometer

▪ Engine attached to electric generator, then electricity absorbed by resistance ▪ Total resistance adjusted to change load

Water Brake Dynamometer

▪ Engine energy converted into mechanical energy from circulating water ▪ Water flow rate adjusted to change load ▪ Torque of casing due to moving water measured

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