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AGK: Piston 1 (4 Piston Eng. Lube & Cooling (Oil: ⬇️friction, cooling,…
AGK: Piston 1
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4 stroke cycle (Otto Cycle) - ICPE
- Induction > Compression > Power > Exhaust
- Top dead centre (TDC), Bottom Dead Centre (BDC)
- Stroke = TDC - BDC (180deg crank rotation)
- Power stroke only one that generates power!
- 2 revs of crank needed for 4 stroke
- Typical Firing seq: 1 -3-4-2
Types
- Rotary: Fixed crankshaft with cylinders rotating around it
- Radial: Fixed cylinders with central rotating crankshaft
- In Line: cylinders in line,⬇️frontal area, cooling problem ∴usually only 4 or 6
- Inverted In Line: Clearance over eng. for good viz. Risk of hydraulicing.
- Horizontally opposed most common - cylin & piston horizontal either side of central crankshaft
Good viz, smaller than in-line (can fit into nacelle = less drag), No hydraulicing, more even cooling
- V: ⬆️cylinders without ⬆️engine length
Components
- Piston: Crown > Compression Ring (cast iron. Carbon in it = self-lube) > Oil control ring (even spread oil in cylin. > Skirt ( joined to con-rod small end by Gudgeon pin)
- Connecting Rod
- Small end joined to piston via Gudgeon pin
- Big End (2 halves) joined to crankshaft big end bearing/journal
- Crankshaft: Steel (case = iron), main journal (oil ways), BE journals. Throw = BE to crankshaft. Throw = 1/2 stroke
- Cylinder head + cylin. Barrel. Head has more cooling fins cos of combustion
- Cam shaft: EXAM: Rotates at 1/2 speed of crankshaft. Driven by crankshaft. Controls poppet valves open/close
- Ports: Inlet bigger than exhaust
- Valves (Poppet): Inlet bigger than exhaust
- Valve Gear: Cam lobe > hydr. tappet > push rod >rocker arm > clearance > valve (via stem)
Springs rtns valve to open. Springs wound different direction to stop valve rotating
- Sodium filled exhaust valves for cooling
Calcs
- Swept volume = how much charge can be displaced
- Clearance volume = vol. between TDC and top of cylinder (where piston can’t go)
- \( Compression \;Ratio = \frac{Swept\;Volume\;+\; Clearance \;Volume}{Clearance\; Volume} \)
Piston Eng. Power
- Work = Force * Dist.
- Power (HP) = Work / Time (rate of Work)
- 1hp = 330lbs coal pulled 100ft in 1 min = 33000 lb ft/min
- Friction (Prony) brake - measure hp (tries stop eng from turning)
- \( Brake\;HP\;(actual\;HP) = \frac{Force\;*\;Moment\;arm\;*\;2pi\;*RPM}{33000} \)
- \( Brake\;HP\;(actual\;HP) = Torque\;*\;RPM \)
- \( Indicated\;HP = \frac{PLANE}{33000} \)
- P = Indicated Mean Effective Pressure (mean press. put on piston in power stroke)
- L = Length of stroke
- A = Area of piston crown
- N = Number of POWER strokes per min
- E = Number of cylinders
.
- P * A = Force
- L = Distance
- N * E = rate
- BHP = IHP (theoretical HP) - FHP (friction HP)
Factors Affecting Power Output - DME
- Density - affected by Press, Temp, humidity
- Mixture Ratio - becomes richer as density ⬇️correct manually
- Exhaust Back Press. - as press. ⬇️ with alt., scavenging ⬆️ cos less work to get exhaust out
Efficiency - TMV
- Thermal - only 30% of fuel = work
\( Thermal\;Eff. = \frac{Work\;Out\;(BHP)}{Work\;(Energy)\;in\;fuel} \) **
- Mechanical - ratio of BHP to IHP
- % of power in the eng. to turn prop
- typically 80-85%
- Volumetric - ratio of vol. of charge drawn c/f swept vol (induction)
- Indication of how well eng. is breathing
- typically 85%
⬆️ Volumetric Efficiency - Practical cycle
- Ineffective crank angle - Close to TDC & BDC v. lil linear movement of piston even though angular rotation speed of crankshaft unchanged
- Practical 4-stroke cycle - counter Ineffective crank angle
- Inlet Lead and Lag
- Ignition Advanced
- Exhaust Lead & Lag
- Valve Overlap
.
- SFC ( Lb (or Kg) / BHP/ Hour )
- The mass of fuel used per unit HP produced per unit time
5 Piston Fuel Sys.
- Carburettion = mixing fuel and air
- 15:1 is optimal
- monitor exhaust gas temp, highest = 15:1
- 9:1 Full rich, best for TO cos unburnt fuel cools cylinders so hi RPM
- 12:1 Best Power (Fast crz)
- 16:1 Best SFC crz
- Detonation= spontaneous combustion post ignition. Triggered by shockwave from ignition. Excess hi press. at TDC, knocking. Power ⬇️ markedly, cylin. temp ⬆️ = damage
- Caused by: Incorrect octane fuel, weak mix ratio, hi eng temp, if temp or press of mix raised b4 it's burnt i.e. low RPM, hi manifold press.
- Preignition: Ignition of fuel b4 sparkplug. Caused by carbon deposits + Hi temps. Same symptoms as detonation
- Pre-ignition worsen with ⬆️ RPM whereas detonation ⬇️ as RPM ⬆️
- Pre-ignition unique characteristic – "Running on" = eng. continues to run for a short period after ignition turned off
- Octane Rating - resistance to detonation
- Normal Heptane - easy detonates = zero
- IsoOctane - hard to detonate = 100
- Octane #s >100 called performance #s
- Additives
- Tetra Ethyl Lead = ⬇️combustion temp & ⬆️ flame rate but Lead Oxide corrosive
- Then added Ethylene Di-bromide convert Lead Ox. to Lead Bromide but lead poisoning,⬇️ mental capacity
- Most popular additiv now aromatic hydrocarbons and alcohol
- AVGAS80 - RED
- AVGAS100 - GREEN
- AVGAS100LL (Low Lead) most common - BLUE
- Internal Combustion = Convert chemical energy to heat energy by burning fuel with air internally
- Boyle's law: P 𝜶 1/V
- Charle's law: V𝜶T
- Combined gas law: PV/T = constant
- Eqn. of continuity (Mass flow) (Alien vs Predator) AV⍴ = Mass flow = constant
- Bernoulli's: Ps + Pdyn = constant
- Mechanism for Heat Transfer: Conduction, Convection (via circulation of a fluid), Radiation (via EM waves)
- Newton's laws
- F (Thrust) = m*a (Newton's 2nd)
- Momentum = m*v
- Work (Joules) = F*d
- Power (Watt) = Rate of doing work = W/t
- Torque = F*dist (and depends on angle applied)
- Efficiency = a measure of output c/f input