Variable Compression RatioByMyles BohonRobert DoroskoNana Noel

MAE 442Dr. KlangMay 2006

OutlineTraditional automobile engine compression ratioWhat it isHow to calculateCharacteristicsDisadvantagesStrategies for the reduction of fuel consumptionVariable compression ratioWhat it isAdvantagesDisadvantagesWays to modify CRDesign ExamplesInvestigation of MCE-5 ConfigurationConclusionReferences

Compression Ratio (CR)What is it?

Is the ratio by which the fuel/air mixture is compressed before it is ignited.

Limited in conventional engines due to auto-ignition.

Determines how efficiently the engine can utilize the energy in the fuel.

CR How is it determined?

CR =

-or-

Where b=cylinder bore (diameter) s = piston stroke length Vc = volume of the combustion chamber

sb

CR - CharacteristicsCR of traditional engine is fixed and is a compromise across a wide range of operating conditions.

CR for passenger cars ranges between 8:1 and 10:1, while CR for diesel engines can be much higher.Typical Compression Ratios

Disadvantages of Fixed CRBecause CR cannot vary to fit a wide range of operating conditions:

Power output is reduced

Fuel efficiency is not optimized

Pollution from combustion is not minimized.

A reduction in fuel consumption is achieved by increasing the work output and/or decreasing the frictional and pumping losses.

Fuel Consumption Reduction Strategies

Variable Compression Ratio (VCR)What is it?

The combustion chamber volume, Vc, is altered to meet the power needs to the vehicle.At low power levels, high CR captures fuel efficiency benefits.At high power levels, low CR prevents knock.

VCR - advantagesCR modified to meet power demand (CRs range from 7:1 to 21:1)Increased fuel efficiencyClaims of up to 30% reduction in fuel consumption Adding variable valve actuation and turbo-charging further improves fuel efficiency (7-10% additional reduction in fuel consumption)Reduced combustion emissions

Increased Efficiency and Pollution Reduction with VCR:

VCR - DisadvantagesNew technology results in high research and development and manufacturing costs.Reliability is not proven.Consumer reactions are unknown and unpredictable.Repairs and maintenance initially may be difficult and costly.

Ways to Modify CRMoving the cylinder head.Variation of combustion chamber volume.Variation of piston deck height.Modification of connecting rod geometry.Moving the crankpin within the crankshaft (effectively varying the stroke).Moving the crankshaft axis.

Ways to Modify CR:

A: articulated cylinder headB: hydraulic pistonsC: eccentrics on bearingsD: multilink rod-crank mechanismsE: additional piston in cylinder headF: gear-based mechanisms

VCR Design ExamplesSaab Tilting Monohead DesignArticulated cylinder head2 part engine block allows cylinder head to be lowered closer to the crankshaft to dynamically alter VcCR varied by adjusting the slope of the monohead in relation to the engine blockUpper block is pivoted about the hinge point using a hydraulic actuator

Unfortunately, Saab shelved project due to cost.

VCR - Saab Tilting Monohead DesignLower Engine BlockActuatorUpper Engine BlockPivot PointIncreased Volume

VCR - Saab Tilting Monohead Design123

MCE-5 VCR Engine BlockThe control jack is connected to a control rack. Between the control rack and the piston rack is a gear wheel, which has a fulcrum supported by the connecting rod. Up/down movement of the control jack effectively changes the TDC placement, therefore increasing or decreasing the compression volume.

MCE-5 VCR Piston Arrangement

MCE-5 VCR in ActionControl Jack: up and down movement results in the displacement of the control rack and ultimately a change in the volume of the combustion chamber.Control Rack

MCE-5 VCR Gear and Piston Arrangement

MCE-5 VCR Engine BlockGear WheelPistonsCrankshaftControl jackControl Rack

Piston KinematicsWith the MCE-5 configuration, kinematics never varies, regardless of CR.Allows VCR to be easily implemented on future SI engines.

Rotational Speed (MCE-5 configuration)

Rotational speed limit of crank is comparable to conventional SI engines.Expected key speeds:Max torque speed at 2,000 rpm Max power speed at 5,500 rpmLimit speed at ~ 7,000 rpm (not due to the load applied to gears, but to the tensile force applied to the connecting rod).

MCE-5 CR ControlCRs adjustable from 7:1 to 20:1.Fast response (

Frictional Losses In PistonsConventional engines have losses due to contact with the piston skirt.But the VCR (MCE-5 configuration) piston only moves up and down, greatly reducing the frictional losses by eliminating radial forces and moments.

Reliability and Durability (MCE-5 configuration)Due to the decreased radial forces between the piston and the skirt, stress and ultimately wear are greatly reduced.Therefore, low piston wear and no piston slap lead to a better piston and ring durability.

Overall size and vehicle integration (MCE-5 configuration)

Size is comparable to conventional engine blocks with less power and similar displacement, bore, and stroke.Because MCE-5 requires no unique attachments and requires no new technologies for its integration into existing systems, it connects easily with pipes and peripherals.

ConclusionsVariable Compression Ratio engines have great potential to increase engine power and fuel economy.When coupled with technologies such as turbocharging, variable valve actuation, and direct fuel injection, the effectiveness of the system is further increased.Once established, this technology will likely become commonplace due to increasing energy and environmental concerns and the ease of integration.

ReferencesPower Plants, Dr. Richard Gould http://www.mae.ncsu.edu/courses/mae442/gould/index.htmlAutomotive Engineering Fundamentals, Stone, R. and Ball, J., 2004Websites:http://en.wikipedia.org/wiki/Main_Pagehttp://www.eere.energy.gov/vehiclesandfuelshttp://www.tc.gc.ca/en/menu.htmhttp://www.mce-5.comhttp://www.saabnet.comhttp://www.vehicular.isy.liu.se/Lab/SVC/index.htmlhttp://www.prodrive.com/uploads/Benefits%20and%20challenges%20of%20VCR.pdf