Atkinson-Miller Cycle and Engine Performance

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.

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.