A/C Powerplant: Internal-Combustion Engine Theory and Performance (CH.4)

Heat Engines

• Both reciprocating and gas-turbine

• Use heat energy to produce power

• Law of Conservation of Energy

  • Energy can neither be created nor destroyed

  • Perpetual Motion Machines cannot exist.

• Reciprocating engines use heat to expand gasses, create pressure, push piston, rotate crankshaft, producing power and work

Work

• Product of force and displacement

• Units: Joule (SI), Foot-pound

• Work = Force ⋅ displacement

Power

◦ Energy transfer per unit of time

◦ Units: Watt (SI), Horsepower

◦ Power = Work/Time

◦ Torque * RPM

Torque

◦ Rotational equivalent of linear force

◦ Units: N⋅m (SI), pound-force-feet (pound-foot)

◦ Torque = Force ⋅ Arm

Horsepower

• This is a linear function, work/time, or power

• Engines are also measured in torque, a rotational function

• All to say, foot-pounds and pound-feet are not interchangeable

• Engine torque is measured in pound-feet

Horsepower

• Indicated Horsepower (ihp)

  • Power converted from heat to mechanical energy

• Brake Horsepower (bhp)

  • Horsepower delivered by the engine to a propeller or other device

  • Measured by a Prony Brake (dynamometer) to derive torque, then multiplying by RPM to determine power

• Friction Horsepower (fhp)

  • Power necessary to overcome friction of moving parts

  • bhp = ihp - fhp

Boyle’s Law

• P1V1 = P2V2

• For a constant temperature, as volume
  decreases, pressure increases

Charles’s Law

V1/T1= V2/T2

◦ For a constant pressure, as temperature
increases, so does volume

Gay-Lussac's law

P1/T1 = P2/T2

◦ For a constant volume, as pressure
increases, so does temperature

Combined Gas Law

PiVi/Ti= PfVf/Tf

Engine Operating Fundamentals

• Cycle – complete sequence of events
  returning to the original state

• Engine cycle – sequence of events in an engine

• Four Stroke, Five event Otto cycle

  • Strokes: Intake, Compression, Power, Exhaust

  • Events: Intake, Compression, Ignition,
    Combustion, Exhaust

Stroke

• The distance through which the piston travels

• During each stroke, the crankshaft rotates 180 degrees

• Limits of travel are Top Dead Center (TDC) and Bottom Dead Center (BDC)

• Each revolution of the crankshaft has 2 strokes, one up, one down

Compression Ratio

• Ratio of volume of space when piston is at BDC to volume available with piston at TDC

• The higher the compression ratio, the more the fuel air mixture compresses before combustion

• More Mechanical energy is extracted from a given volume of fuel/air mixture at higher compression ratios

• MORE POWER

Intake Stroke

• Beginning at TDC

• With intake valve open and exhaust closed, piston Moves Down

• Intake valve remains open until the compression stroke is being performed (as much as 60 degrees) to utilize the inertia of intake mixture and increase volumetric efficiency

  • The ratio of the fuel-air charge being burned to the piston displacement

  • Aka: the volume of air entering the cylinder compared to the volume of the cylinder

Compression Stroke

• Piston moves from BDC to TDC

• Intake valve closes

• Ignition by the spark plugs occurs a few degrees before TDC

• If ignition timing is correct, combustion completes just after TDC

• If timing is off, the piston is moving down as combustion finishes, leading to loss of power

Power and Exhaust Strokes

• Expanding gasses push piston down to BDC, rotating crankshaft and propeller

• The Exhaust valve opens before BDC, allowing pressure to equalize and exhaust to escape

• Piston moves from BDC to TDC, forcing out remaining exhaust gasses

• Near TDC, intake valve opens

Valve Overlap

• Angular distance of crankshaft rotation in which both valves are open

• The exhaust valve opens before BDC on the power

• The intake valve opens before TDC on the exhaust stroke

• For a time, both valves are open

• Allows exhaust gasses to exit at the earliest possible moment and intake to begin and take advantage of inertia due to pressure gradients, increasing volumetric efficiency

Valve Lag

The opening or closing of a valve after TDC and BDC

Valve Lead

The opening or closing of a valve before TDC or BDC

Duration

The time the valves are open

Backfiring

• Backfiring occurs when the intake valve opens too early or becomes stuck open

• Exhaust gasses travel out the intake and ignite incoming mixture

Aftering

• Afterfiring occurs when unburned fuel enters the exhaust where sufficient oxygen mixes with it to cause combustion

• This occurs often in airplanes when throttle is rapidly reduced

Two-Stroke Engines

• Intake Event

  • The upwards piston motion creates low pressure in the crankcase, drawing
    mixture through a check valve (previous cycle’s compression event)

  • The piston moves down and intake port opens, allowing mixture in crankcase to enter cylinder (previous cycle’s exhaust event)
    

• Compression Event

  • Piston moves up, compressing mixture as the check valve opens, allowing more mixture to enter crankcase (next cycle’s intake event)

• Ignition and Power Events

  • Spark plugs fire and combustion occurs, pushing piston down

  • Exhaust Port opens as piston approaches BDC and intake port opens right after (next cycle’s intake event)

Two-Stoke Advantages

• May be designed to not use traditional valves

• Fire every revolution rather than every other

• Lighter

Two-Stoke Disadvantages

• Must mix oil with gas

• Some exhaust is mixed with inducted mixture and some inducted mixture mixes with exhaust leading to less fuel efficiency and higher emissions

Diesel Engines

• Note: Jet A is “dry diesel” or diesel without lubricants mixed

• Compresses mixture to the point it combusts without the need for a spark

• Glow plugs or igniters are used in engine starting, but may turn off during operation

• Higher compression ratios than “normal” engines

Displacement

• Piston Displacement is the cross-sectional area of a cylinder bore multiplied by the distance a cylinder travels in on full stroke

• Multiplying piston displacement by the number of cylinders gives engine
  displacement

Manifold Pressure

• The pressure of the mixture prior to the intake valve

  • i.e. the amount of air entering the engine

• Measured in inches of Mercury (inHg)

• In a normally aspirated engine (no turbocharger or supercharger), air density decreases with altitude, lowering manifold pressure

• Critical altitude is the altitude at which an engine will maintain a given horsepower output

Altitude engines

Turbocharged or supercharged; compress air, increasing manifold pressure

Atmospheric density

Affected by atmospheric pressure, temperature, and humidity

Density Altitude

• must be calculated to accurately predict engine performance

• Altitude corrected for nonstandard pressure and temperature

• Altitude in the International Standard Atmosphere where air has the same density as the atmosphere being compared

• High Density Altitude = Bad for performance, air is not dense

Detonation

• Temperature and pressure in cylinder become high enough for instantaneous burning

• Caused by

  • Excessively lean mixture

  • Lower octane fuel

  • Insufficient engine cooling

Preignition

• Early combustion of the fuel-air mixture due to hot-spot

• Hot carbon deposits or spark plug electrodes, often due to excessively rich mixture

• Leads to roughness of engine and reduction of power

Best Power mixture

Mixture which provides maximum power at a specific RPM

Best Economy mixture

Mixture which provides highest brake specific horsepower (bsfc)