|Cat’s variable compression ration engine relies on an eccentrically-mounted crankshaft. Click to enlarge.|
As part of its research and development into HCCI (Homogeneous Charge Compression Ignition) regimes for meeting coming heavy-duty engine emissions requirements, Caterpillar has developed—and filed a patent on—a variable compression ratio (VCR) engine.
A variable compression ratio engine offers the potential to increase combustion efficiency and decrease emissions under varying load and speed conditions.
Caterpillar is part of a Department of Energy-funded research project on high-efficiency, clean combustion along with ExxonMobil (fuels and combustion chemistry), Sandia National Laboratories (optical diagnostics, fuel spray and combustion, fuel effects), and IAV (closed loop control, transient controls, vehicle calibration, sensors).
|The VCR mechanism. Click to enlarge.|
Benefits of the variable compression ratio engine with an HCCI regime include high-load operation at low-compression ratios; elimination of the cold-start issue with a low-compression ratio; improvement of light-load combustion stability and the emissions of HC/CO; the optimization of load, emissions and fuel economy; and the enabling of engine braking.
The crankshaft in the 15-liter engine is mounted eccentrically in the cradle which sits in the cylinder block. Rotation of the cradle moves the crankshaft which alters the compression ratio.
|Hydraulic pistons control the cradle. Click to enlarge.|
Hydraulic pistons drive the cradle up and down. As a result, the engine can change its compression ratio from 8 to 15:1.
The VCR engine is by no means a panacea for highly-efficient and clean combustion, but, notes Kevin Duffy from Caterpillar, it is “a valuable development and exploration tool to understand tradeoffs of numerous controls and combustion parameters [with HCCI].”
Caterpillar is not alone in its investigations of variable compression ratio engines.
Many of the earlier initiatives focused on smaller-displacement, light-duty engines. A VCR could enable significant engine downsizing in highly boosted engines, and was seen as a possible solution to bringing gasoline engine efficiency closer to that of diesels.
Among the research and development efforts:
In 2000 Saab introduced a prototype VCR engine at the Geneva Auto Show.
Three European car manufacturers (PSA Peugeot Citroën, Volvo and Renault) two powertrain development companies (FEV Motorentechnik and Le Moteur Moderne) and VKA at Aachen University of Technology worked on VCR engines as part of a 2002 European effort, Project VCR.
A French development company, MCE-5, is working on a variable compression ratio engine block.
An earlier DOE project involving Argonne National Laboratory, AVL Powertrain, Computer Systems Management, Envera and Ricardo developed a light-duty variable compression ratio engine prototype in 2001.