Atkinson-Miller Cycle and Engine Performance
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Atkinson-Miller Cycle and Engine Performance

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Questions and Answers

What characterizes the cycle that rejects more heat?

  • It requires less displacement volume.
  • It has higher thermal efficiency.
  • It operates at a higher pressure than ambient pressure.
  • It has lower thermal efficiency. (correct)

    • In the Atkinson-Miller cycle, what is required for a greater work output?

  • Shorter compression strokes.
  • Less displacement volume.
  • Longer adiabatic expansion. (correct)
  • Higher ambient pressure than cylinder pressure.

    • What does the actual compression ratio in the Atkinson-Miller cycle imply?

  • It is less than the geometrical compression ratio. (correct)
  • It has no impact on the thermal efficiency.
  • It is equal to the displacement volume.
  • It is greater than the geometrical compression ratio.

    • Why is it easier to discharge combustion gases in the real operating cycle?

    <p>The cylinder pressure is higher than ambient pressure.</p> Signup and view all the answers

    What is the relationship between thermal efficiency and the ratio 'r' in the Atkinson-Miller cycle?

    <p>Thermal efficiency increases significantly if 'r' is low.</p> Signup and view all the answers

    How is the real operating cycle experimentally determined?

    <p>By measuring pressure as volume changes.</p> Signup and view all the answers

    What is another name for the pressure diagram in the real operating cycle?

    <p>Indicator cycle.</p> Signup and view all the answers

    What is the main limitation of the Atkinson-Miller cycle related to displacement volume?

    <p>It requires a larger displacement volume than other cycles.</p> Signup and view all the answers

    What factor does NOT decrease volumetric efficiency?

    <p>Increased mean piston speed</p> Signup and view all the answers

    How is the pressure drop ($, , , , , \Delta p $) in the intake system calculated?

    <p>$\Delta p \propto \sum \zeta_i$</p> Signup and view all the answers

    When does the intake valve close in relation to the bottom dead center (BDC)?

    <p>Well after BDC</p> Signup and view all the answers

    What impact does increasing the duct length (L) have on volumetric efficiency?

    <p>It decreases volumetric efficiency</p> Signup and view all the answers

    What is indicated by the term 'overlap period' in valve timing?

    <p>Both valves are open simultaneously</p> Signup and view all the answers

    How does the diameter of the duct (de) affect heat transfer in the intake system?

    <p>Decreased diameter increases surface/volume ratio</p> Signup and view all the answers

    Which of the following statements regarding the intake system friction losses is true?

    <p>Friction losses are proportional to the square of air velocity</p> Signup and view all the answers

    Which of the following statements about the exhaust valve timing is correct?

    <p>The exhaust valve opens before the BDC</p> Signup and view all the answers

    What factor primarily limits mean piston speed in an engine?

    <p>Fluid dynamic losses or mechanical stresses due to inertia forces</p> Signup and view all the answers

    Which performance parameter of an engine is defined as the ability to do work?

    <p>Torque</p> Signup and view all the answers

    What is the formula for calculating power in relation to torque and engine speed?

    <p>P = 2πNτ</p> Signup and view all the answers

    Which power measurement represents the power available at the crankshaft?

    <p>Brake power (Pb)</p> Signup and view all the answers

    What index quantifies the work done by gases inside the cylinder during the indicated cycle?

    <p>Indicated mean effective pressure (imep)</p> Signup and view all the answers

    Mechanical efficiency can be calculated as the ratio between which two quantities?

    <p>Indicated power and brake power</p> Signup and view all the answers

    What does bmep stand for in the context of engine performance?

    <p>Brake mean effective pressure</p> Signup and view all the answers

    How does mechanical efficiency typically change with an increase in speed at constant load?

    <p>It decreases due to increased friction power</p> Signup and view all the answers

    Which of the following factors primarily determines the stability of the torque curve?

    <p>Response to variations in external load</p> Signup and view all the answers

    What does the term 'specific power' refer to in engine performance metrics?

    <p>Power per unit volume</p> Signup and view all the answers

    Which of the following best describes the relationship between indicated power and brake power?

    <p>Indicated power is always greater than brake power</p> Signup and view all the answers

    When the torque curve is described as unstable, what effect does it have on finding a new equilibrium point?

    <p>A new equilibrium point is not found.</p> Signup and view all the answers

    What impact does a decrease in load at constant speed have on mechanical efficiency?

    <p>It decreases mechanical efficiency because friction power remains fairly constant</p> Signup and view all the answers

    How is the shape of performance curves influenced?

    <p>By a combination of thermal, volumetric, and mechanical efficiencies</p> Signup and view all the answers

    In motor applications, how can user device requirements typically be categorized?

    <p>Constant speed with varying torque, variable speed with constant torque, and hybrid</p> Signup and view all the answers

    What indicates a descending torque curve when responding to an external load?

    <p>Torque difference decreases</p> Signup and view all the answers

    Which parameter is essential for measuring engine performance at different rotational speeds?

    <p>Torque output</p> Signup and view all the answers

    How are indicated thermal efficiencies represented in performance calculations?

    <p>As a function of $imep$ and $ au_i$</p> Signup and view all the answers

    What is the main consequence of high gas inertia in engine performance?

    <p>Greater heat losses</p> Signup and view all the answers

    What does the variable $ au_b$ represent in engine performance?

    <p>Brake torque</p> Signup and view all the answers

    What is the main role of poppet valves in a 4-stroke engine?

    <p>To regulate intake and exhaust flow areas.</p> Signup and view all the answers

    What happens to volumetric efficiency when cross-sectional areas of intake and exhaust ports are maximized?

    <p>Volumetric efficiency may decrease at low speeds.</p> Signup and view all the answers

    Which of the following valve timing events facilitates gas exchange effectively at TDC?

    <p>IVO 10°-40° BTDC.</p> Signup and view all the answers

    In terms of pressure drop caused by the inlet valve, what percentage contributes to the overall pressure drop?

    <p>50-70%.</p> Signup and view all the answers

    What is the primary disadvantage of fixed valve timing and lift in traditional engines?

    <p>Reduced performance under varying loads.</p> Signup and view all the answers

    What geometric characteristics of a valve seat help prevent gas leakage under high cylinder pressure?

    <p>Conical shape.</p> Signup and view all the answers

    How does the velocity distribution in a real flow differ from an ideal flow assumption?

    <p>Velocity is not uniform in the cross-sectional area.</p> Signup and view all the answers

    What is one of the main objectives of valve lift design?

    <p>Minimize mechanical friction at surfaces.</p> Signup and view all the answers

    What happens during the overlap period in valve timing?

    <p>Unburnt charge may enter the exhaust port.</p> Signup and view all the answers

    What impact does increasing the number of valves have?

    <p>It can increase the intake flow area.</p> Signup and view all the answers

    Which factor is crucial for achieving high volumetric efficiency at higher engine speeds?

    <p>Maximizing port cross-sectional area.</p> Signup and view all the answers

    What does the inlet valve closing lag influence in an engine's operation?

    <p>Shape of the volumetric efficiency curve.</p> Signup and view all the answers

    Which component of the valve is mainly responsible for driving valve movement?

    <p>Valve stem.</p> Signup and view all the answers

    What is likely to happen if the flow area of the valve is smaller than optimal?

    <p>Increased pumping work and reduced efficiency.</p> Signup and view all the answers

    Study Notes

    Atkinson-Miller Cycle

    • Prolonging adiabatic expansion to pa -> Atkinson-Miller cycle
    • Requires larger displacement volume (v4’>v4=v1)
    • rexpansion>rcompression
    • Can be operated in the same cylinder if valves are left open
    • Actual compression ratio lower than geometrical
    • Efficiency increase only significant with low r
    • Lower work output for same displacement volume than other reference cycles

    Real Operating Cycle

    • Determined by measuring cylinder pressure with varying volume
    • Pressure diagram called “indicated” cycle
    • Mean piston speed correlated with several factors
    • Limited by fluid dynamic losses or mechanical stresses
    • Between 8 and 18m/s
    • Geometrically similar engines with same mean piston speed have same velocity field

    Power and Torque

    • Key engine performance parameters
    • Torque measures ability to do work
    • Power measures rate at which work is done
    • P =  = 2 N
    • Defined as power/unit volume (P/V)
    • Weight/power ratio (engine mass)/P
    • Power per piston area P/Ap

    Brake and Indicated Quantities

    • Internal power not all available at crankshaft
    • Indicated power Pi
    • Friction power Pf
    • Brake power Pb
    • Pi − Pb = Pf
    • Indicated mean effective pressure (imep)
    • Independent of engine size
    • Pi = imep V

    Mechanical Efficiency

    • Ratio of brake and indicated power
    • m = Pb/Pi
    • Factors influencing mechanical efficiency: mechanical friction, pumping work, driving engine accessories
    • Decreases with increasing speed at constant load
    • Decreases with decreasing load at constant speed

    Performance Curves

    • Power and torque obtained at maximum load at each rotational speed
    •  i  imep  b  bmep
    • Determined by indicated thermal, volumetric and mechanical efficiencies
    • i v fluid dynamic losses m

    Stability of Torque Curve

    • Stable or unstable based on engine response to sudden load variation
    • Stable curve will have difference between b and i tend to decrease
    • Unstable curve will have torque difference tending to increase
    • Stability also depends on whether curve is descending or ascending

    Coupling with User Device Requirements

    • Constant speed, variable torque: electric generator
    • Turbomachinery:

    Volumetric Efficiency

    • Decreases with: lower mean piston speed, higher L, lower de, and higher Tw

    Friction Losses

    • Proportional to the square of the air velocity
    • Pressure drop also proportional to square of mean piston speed
    • Friction is distributed and concentrated
    • Can be estimated with p /  =  i vi2 / 2

    Valve Timing

    • Intake valve opens before TDC and closes after BDC to exploit inertia
    • Exhaust valve opens before BDC and closes after TDC to exploit inertia
    • Valves open during overlap period

    Introduction

    • Poppet valves regulate intake and exhaust flow in 4-stroke engines.
    • Valves and ports are crucial flow restrictions causing high pressure drops.
    • The pressure drop can reduce volumetric efficiency and increase pumping work.
    • Inlet valves are responsible for 50-70% of the pressure drop in the intake system.

    Valve and Port Geometry

    • Valve head controls the minimum flow area.
    • Valve stem transmits the camshaft command to drive valve movement.
    • Valve seat ensures a tight seal to prevent gas leakage, with a conical shape.
    • Valves are opened by sliding along the stem axis towards the piston.

    Design Recommendations

    • Maximize cross-sectional areas to improve volumetric efficiency and power at high engine speeds.
    • Large curvature radius and minimal valve guide protrusion minimize friction losses.
    • Intake valves are typically larger than exhaust valves to enhance volumetric efficiency and mitigate heat transfer issues.

    Valve Size and Cylinder Bore

    • Ideal case: The maximum valve diameter is proportional to the cylinder bore.
    • Real case: The intake and exhaust valve diameters are measured as a proportion (0.42-0.50) of the cylinder bore.
    • The valve diameter varies according to valve design, such as flat, wedge, or hemispherical heads.

    Number of Valves

    • Increase intake flow area either by:
      • Increasing the number of valves to effectively use the cylinder section.
      • Inclining valve axes in a domed head surface.

    Valve Timing

    • Valve motion requires time due to laws of dynamics.
    • Exhaust Valve Opening (EVO): A compromise between work loss at the end of the expansion stroke and work during the exhaust stroke.
    • Exhaust Valve Closing (EVC): Optimizes gas inertia during the overlap phase.
    • Intake Valve Opening (IVO): Ensures sufficient valve opening at TDC for gas exchange in the clearance volume.
    • Intake Valve Closing (IVC): Facilitates fresh charge entry due to inertia.

    Inlet Valve Closing Lag

    • Influences volumetric efficiency curve.
    • A compromise between backflow at low speeds and unexploited inertia at high speeds.
    • Overlap period creates two issues:
      • Unburnt charge flowing into the exhaust port causing UHC and CO emissions (long EV lag at low speeds).
      • Residual gas flowing back into the intake port causing slower combustion and UHC/CO emissions (large IV lead at partial throttle).

    Valve Lift

    • Valve motion is determined by the camshaft, revolving once per two crankshaft revolutions.
    • Balance is achieved between:
      • Maximum volumetric efficiency.
      • Prevention of oscillatory motions, vibrations or impacts.
      • Minimizing mechanical friction.
    • Valve lift determines the minimum flow area until the port becomes the restricting factor.

    Variable Valve Timing

    • Conventional engines have fixed valve timing and lift curves.
    • Variable valve timing improves performance and thermal efficiency by:
      • Shifting timing angles with fixed duration and lift.
      • Discrete control of timing and lift.
      • Complete flexibility in timing and lift control.

    Valve Flow

    • Ideal case: Steady, one-dimensional, isentropic, and adiabatic flow of a perfect gas in a converging nozzle emptying into a large volume.
    • Mass flow rate is calculated using a formula considering the cross-sectional area, pressure, and temperature.
    • Choking occurs at a critical pressure ratio, resulting in a choked mass flow rate.

    Real Case

    • Real flows diverge from the ideal case due to non-ideal gas conditions, non-uniform velocity distribution, friction, and heat transfer.
    • A coefficient C is introduced to account for deviations between real and ideal mass flow rates.
    • C is a function of Reynolds number and Mach number for similar geometry.

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    Description

    This quiz covers the principles of the Atkinson-Miller cycle, focusing on its adiabatic expansion and efficiency compared to other cycles. Additionally, it explores real operating cycles determined by cylinder pressure and key engine performance parameters like power and torque. Test your understanding of these concepts in automotive engineering.

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