Comprehensive Guide to Piston Engines: Otto Cycle, Mechanics, and Performance

The Otto cycle describes the sequence of events in a standard four-stroke piston engine.
The cycle consists of four distinct strokes of the piston, where a stroke is defined as the piston moving from Top Dead Center (TDC) to Bottom Dead Center (BDC) or vice versa.
1. Intake Stroke
- The piston moves from TDC to BDC.
- The intake valve is open.
- A fuel/air mixture enters the cylinder.
2. Compression Stroke
- The piston moves from BDC to TDC.
- Both the intake and exhaust valves are closed.
- The fuel/air mixture is compressed.
- The spark occurs at a specific interval to initiate combustion, typically between $20^{\circ}$ and $35^{\circ}$ before the piston reaches TDC.
3. Power Stroke
- Both the intake and exhaust valves remain closed.
- Combustion occurs, creating pressure that pushes the piston from TDC to BDC.
- This stroke is responsible for producing useful work.
4. Exhaust Stroke
- The exhaust valve is open.
- The piston moves from BDC to TDC.
- Burnt gases are expelled from the cylinder through the exhaust port.

Intake Valve Closing
- The intake valve typically does not close exactly at BDC; it closes between $50^{\circ}$ and $75^{\circ}$ after BDC.
- This delay allows more fuel/air mixture to continue entering the cylinder even as the piston begins to rise, leveraging the inertia of the incoming air.
- This design improves volumetric efficiency.
- It ultimately increases the overall performance of the engine.
Valve Overlap
- Definition: Valve overlap is the specific period in the cycle when both the intake and exhaust valves are open simultaneously.
- Purpose:
  - Improves the scavenging of exhaust gases (using the outgoing flow to help pull in the fresh charge).
  - Helps the cylinder fill more effectively with a fresh charge.
  - Improves engine efficiency.
- Formula: $\text{Valve Overlap} = \text{Valve Lead} + \text{Valve Lag}$
Exhaust Valve Opening during Power Stroke
- The exhaust valve is designed to open before the piston reaches BDC during the power stroke.
- Purposes:
  - To expel burned exhaust gases out of the cylinder through the exhaust port as early as possible.
  - To improve scavenging.
  - To help cool the cylinder.
  - To prevent the dilution of the incoming fresh charge with leftover exhaust.

Piston travel is not uniform relative to crankshaft rotation.
When the piston is at or near the top center, it moves more distance per degree of crankshaft rotation than it does when it is at the bottom center.
Piston Velocity:
- Velocity is at its highest point at the $90^{\circ}$ position of rotation.
- Velocity is at its lowest point at the top and bottom center positions.
Because of these dynamics, valve lead and lag are greater near BDC than at TDC.

Hydraulic Lock
- Condition: A state where liquid fuel or oil accumulates inside the cylinder.
- Consequences: Any attempt to start an engine while in a state of hydraulic lock can cause severe mechanical damage to the piston, connecting rod (con-rod), and valves.
- Precaution: Always check for hydraulic lock before attempting to start a radial engine if it has been shut down for more than $30\text{ minutes}$ .
- Procedure to Eliminate Hydraulic Lock:
  1. Ensure the Magneto is OFF.
  2. Remove the spark plugs.
  3. Rotate the propeller by hand.
  4. Drain the excess fluid from the cylinders.
  5. Reinstall the spark plugs and inspect the engine for any damage.
Overpriming
- Definition: The introduction of excessive fuel into the engine during the priming process.
- Effects:
  - Flooded engine.
  - Difficulty in starting.
  - High risk of an induction fire.
- Corrective Action:
  1. Set the Mixture control to IDLE CUT-OFF.
  2. Set the Throttle to FULL OPEN.
  3. Crank the engine until the excess fuel clears.
  4. Restart the engine using the normal prescribed procedure.

Work
- Definition: Work is performed if a force is applied to an object and that object moves. If there is no movement, no work is done.
- Formula: $W = F \times D$
- Variables:
  - $W = \text{Work}$
  - $F = \text{Force}$
  - $D = \text{Distance}$
- Units:
  - English: Foot-pound ( $\text{ft-lb}$ )
  - Metric System: Joule ( $J$ )
Power
- Definition: Power is the time-rate of doing work.
- Formula: $P = \frac{F \times D}{t}$
- Variables:
  - $P = \text{Power}$
  - $F = \text{Force}$
  - $D = \text{Distance}$
  - $t = \text{Time}$
- Units:
  - English: Foot-pound per second ( $\text{ft-lb/s}$ )
  - Metric System: Joules per second ( $J/s$ )

Indicated Horsepower (IHP)
- This is the total power actually developed within the engine cylinders.
- It does not account for power lost to friction components.
Friction Horsepower (FHP)
- This represents the power lost in overcoming internal friction.
- This power is used to drive engine accessories and internal components.
Brake Horsepower (BHP)
- This is the actual useful power available at the crankshaft.
- It is measured using a dynamometer.
Mathematical Relationship:
- $IHP = BHP + FHP$

Definition: Detonation is an abnormal combustion that occurs after normal ignition has taken place. The fuel/air mixture explodes violently rather than burning smoothly.
Detection Methods:
- Engine roughness.
- Loss of power.
- High cylinder head temperature (CHT).
- General engine overheating.
- Audible knocking or pinging sounds.
Causes:
- Excessive engine temperature.
- High manifold pressure.
- Use of low octane fuel.
- Incorrect ignition timing.
- An excessively lean fuel/air mixture.
Effects and Damage:
- Damage to the piston.
- Damage to the cylinder.
- Damage to the spark plugs.
- Chronic engine overheating.