Variable Compression RatioBy

Myles BohonRobert Dorosko

Nana Noel

MAE 442Dr. KlangMay 2006

Outline• Traditional automobile engine

compression ratio– What it is– How to calculate– Characteristics– Disadvantages

• Strategies for the reduction of fuel consumption

• Variable compression ratio– What it is– Advantages– Disadvantages– Ways to modify CR– Design Examples– Investigation of MCE-5 Configuration

• Conclusion• References

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

volcylindervolcylinder

minmax

s

b

CR - Characteristics

• CR 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 CR

• Because 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

Fuel Consumption Reduction Strategy

Sources of effectiveness Engine strategies

1 - Increased effective expansion ratio

Improved indicated efficiency

•High Fixed Compression Ratio (FCR)•Variable Compression Ratio

2 - Downsizing •Pumping losses reduction•Internal heat transfer surface reduction•Flame travel distance reduction•Friction losses reduction

•High supercharging•Increased average BMEP•10:1 Compression Ratio (FCR)•From 8:1 to 16:1 Compression Ratio (VCR)

3 - Downspeeding •Pumping losses reduction•Friction losses reduction

•High supercharging•Increased average BMEP

4 - Unthrottled load control •Pumping losses reduction •Load control by air/fuel ratio•Load control by Late Intake Valve Closing

5 - Engine mechanical efficiency improvement

•Friction losses reduction •Reduction of all friction sources

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 - advantages

• CR modified to meet power demand (CR’s range from 7:1 to 21:1)

• Increased fuel efficiency– Claims 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 - Disadvantages

• New 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 CR

• Moving 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 head

B: hydraulic pistons

C: eccentrics on bearings

D: multilink rod-crank mechanisms

E: additional piston in cylinder head

F: gear-based mechanisms

VCR – Design Examples

• Saab Tilting Monohead Design– Articulated cylinder head– 2 part engine block allows cylinder head to be

lowered closer to the crankshaft to dynamically alter Vc

– CR varied by adjusting the slope of the monohead in relation to the engine block

– Upper block is pivoted about the hinge point using a hydraulic actuator

• Unfortunately, Saab shelved project due to cost.

VCR - Saab Tilting Monohead Design

Lower Engine Block

Actuator

Upper Engine Block

Pivot Point

Increased Volume

VCR - Saab Tilting Monohead Design

1 2 3

MCE-5 VCR Engine Block

• The 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 Action

Control 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 Block

Gear Wheel

Pistons

Crankshaft

Control jack

Control Rack

Piston Kinematics

• With 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 rpm– Limit 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 Control

• CR’s adjustable from 7:1 to 20:1.• Fast response (<100 ms to move from max CR

to min CR) .• Electronically controlled sensors allow the

engine to quickly tailor itself to the ideal operating conditions.

• Control jacks function as hydraulic ratchets, moving piston TDC location (changing the compression volume and therefore the CR).

Frictional Losses In Pistons

• Conventional 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.

• 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.

Reliability and Durability (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.

Overall size and vehicle integration (MCE-5 configuration)

Conclusions

• Variable 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.

References• Power Plants, Dr. Richard Gould

http://www.mae.ncsu.edu/courses/mae442/gould/index.html• Automotive Engineering Fundamentals, Stone, R. and

Ball, J., 2004• Websites:

– http://en.wikipedia.org/wiki/Main_Page– http://www.eere.energy.gov/vehiclesandfuels– http://www.tc.gc.ca/en/menu.htm– http://www.mce-5.com– http://www.saabnet.com– http://www.vehicular.isy.liu.se/Lab/SVC/index.html– http://www.prodrive.com/uploads/Benefits%20and%20challenges%20of

%20VCR.pdf