chapter 3 Flashcards
What is an Air-standard cycle?
A simplified model for engine cycle analysis, assuming gas behaves as an ideal gas with constant specific heats and operates in a closed cycle.
What are the key assumptions in an Air-standard cycle?
Gas is treated as an ideal gas with constant specific heats
Operates as a closed cycle
Combustion is modeled as heat input
Exhaust energy rejection is modeled as heat transfer
Constant pressure intake and exhaust strokes
Isentropic compression and power strokes
combustion occurs at constant volume (SI), constant pressure (CI), or a combination (Dual cycle)
All processes are assumed reversible.
What are some real-world deviations from the ideal Otto cycle?
Valve timing effects, combustion requires time, heat transfer losses reduce efficiency, gas flow friction and piston friction cause deviations, exhaust blowdown starts before BDC and takes finite time.
What is the Otto cycle?
The air-standard model for most four-stroke Spark Ignition (SI) engines.
What are the four major strokes of the Otto cycle?
(1) Intake Stroke: Air-fuel mixture enters at constant pressure (~1 atm). (2) Compression Stroke: Isentropic compression increases pressure and temperature. (3) Power Stroke: Combustion occurs at constant volume, pushing the piston. (4) Exhaust Stroke: Expels gases at nearly constant pressure
What happens in an SI engine cycle at part throttle?
Butterfly valve restricts air-fuel intake, lowering intake pressure, greater negative pump work during intake stroke, reduced net work output, superchargers or turbochargers can increase pressure but may cause knock.
How does the thermal efficiency of a real IC engine compare to the ideal Otto cycle?
Real IC engines have lower efficiency due to heat transfer losses, combustion inefficiencies (~95-98%), exhaust blowdown occurring before BDC, finite valve opening/closing times, and mass losses from piston blowby and crevice flows (~1%).
What is the Exhaust Process in an engine?
(1) Blowdown – Driven by a large pressure difference when the exhaust valve opens. (2) Exhaust Stroke – Piston forces out remaining gases. (3) Exhaust Residuals – Some exhaust gas remains (~3-7%, up to 20% in extreme cases). (4) Isenthalpic Mixing – Residual exhaust gas heats incoming air, reducing density and efficiency.
What is the Diesel cycle?
Similar to the Otto cycle, but with constant pressure combustion instead of constant volume combustion. Uses compression ignition, has a longer combustion process, operates at higher compression ratios, and is generally more efficient than Otto cycles.
What is the Cut-off Ratio (β) in the Diesel cycle?
The volume change during combustion in a Diesel engine.
What is the Dual Cycle?
A hybrid of the Otto and Diesel cycles, featuring combustion occurring in two stages: constant volume combustion followed by constant pressure combustion. It is the basis for modern CI engines.
What is the Pressure Ratio (α) in the Dual cycle?
The pressure rise during combustion in a Dual Cycle engine.
How do Otto, Diesel, and Dual cycles compare?
Diesel Cycle has the highest thermal efficiency for the same input conditions and peak pressure. Diesel Cycle operates at a higher compression ratio than Otto Cycle. The Atkinson Cycle is an Otto or Diesel Cycle with an extended expansion stroke, improving efficiency but requiring complex mechanisms.
What is the Miller Cycle?
A modified Otto cycle that uses valve timing to increase the expansion stroke relative to the compression stroke. More efficient than the Otto cycle, requires variable valve timing, and is often turbocharged or supercharged.
How does the Miller Cycle compare to the Otto Cycle?
Miller Cycle is more efficient due to a longer expansion stroke. Both are limited by temperature constraints to avoid knock. Miller Cycle results in lower exhaust temperatures and higher efficiency.
What is a Two-Stroke Cycle?
A cycle where the power stroke occurs every two strokes instead of four, making it simpler and lighter. Used in small engines (motorcycles, chainsaws, etc.) and very large low-RPM engines. Not practical for automobiles due to high emissions.
How does a Two-Stroke SI Engine Cycle work?
(1) Isentropic power stroke (2) Exhaust blowdown begins (3) Intake and exhaust scavenging (exhaust port opens, intake opens, BDC, intake closes) (4) Compression stroke (fuel enters early, spark ignites near TDC) (5) Constant volume heat input
What is the Sterling Cycle?
External combustion engine with a regenerator that recycles heat between cycles. High efficiency, low emissions, and flexible fuel use. Not commonly used due to high weight, sealing issues, and cost.
What is the Lenoir Cycle?
No compression, only intake, combustion, power stroke, and exhaust. Lower efficiency than Otto or Diesel cycles. First commercially produced IC engine.
