| Brake specific fuel consumption (BSFC) curves for a 5.7L V8, a 3.6L Turbo-DI V6 (both with actual data), and a 2.2L VCR 4-cylinder engine (projected data) from a 2011 report by Envera. All three engine BSFC curves correspond to an engine speed of 2000 rpm.Click to enlarge. |
The National Energy Technology Laboratory (NETL) of the US Department of Energy (DOE) has awarded Envera LLC a $2.8-million contract (DE-EE0005981) to develop and to test a high-efficiency variable compression ratio (VCR) engine with variable valve actuation (VVA) and an advanced high-efficiency supercharger to improve fuel efficiency.
The new engine will achieve high efficiency using the Atkinson Cycle with an 18:1 compression ratio, combined with aggressive engine downsizing. To attain high power levels, new high-efficiency supercharging technology will be used to boost the engine and provide V8-like performance from only four cylinders. Envera estimates that the new engine could deliver up to a 40% improvement in fuel economy over a conventional port-fuel injection engine at a relatively low cost while also delivering performance, torque and flex-fuel capability.
Valve events will also be adjusted with the VVA to trap more air in the cylinders when power is needed. Engine displacement is 2.4L. A low 8.5:1 compression ratio will be used during supercharged conditions to accommodate the higher power levels. The engine can also be combined with a hybrid drivetrain to attain even greater mileage improvements.
The engine will include new technologies developed by Envera and Tier-1 supplier Eaton. Envera is responsible for development of the engine and providing the VCR system. Eaton will provide a new advanced supercharger and variable valve actuation (VVA) technology. Current plans call for testing the engine in a full-size pickup truck or utility van.
Background. Envera specializes in the development of advanced technologies for improving automotive fuel economy. Prior advanced technology engine development and prototype build projects for DOE include a port fuel-injected VCR engine for the Argonne National Laboratory; a common-ratio turbo-diesel VCR engine for the Oak Ridge National Laboratory; and a gasoline direct-injection / port fuel-injection VCR engine for the Oak Ridge National Laboratory.
In a 2010 report (revised in 2011) for NETL, Envera described the technology and the its approach to achieving a high-efficiency VCR engine.
Engine downsizing is a key tool in improving fuel economy. However, to meet torque and power requirements, a smaller engine needs to do more work per stroke, Envera notes. This is typically accomplished by boosting the incoming charge with either a turbo or supercharger. Current production gasoline engines are limited in the degree of engine boosting by detonation (combustion knock) at high boost levels. In addition, the charger needs to be responsive and efficient while providing the needed boost.
VCR technology can eliminate engine knock at high load levels by reducing compression ratio to ~9:1 (or whatever level is appropriate) when high boost pressures are needed. By reducing the compression ratio during high load demand periods, there is increased volume in the cylinder at top dead center (TDC) which allows more charge (or energy) to be present in the cylinder without increasing the peak pressure. Cylinder pressure is thus kept below the level at which the engine would begin to knock. When loads on the engine are low the compression ratio can be raised to as much as 18:1, Envera said.
Using variable valve control (VVC), the Envera VCR engine will run most of the time on the Atkinson cycle. When high torque values are required the valve settings are adjusted to maximize the amount of intake air trapped in the cylinders, as needed for maximizing power and torque.
Variable valve control can be used to adjust the “effective compression ratio” by adjusting the amount of air trapped in the cylinder—trapping more air in the cylinder provides a higher effective compression ratio and more power, while trapping less air in the cylinder provides a lower effective compression ratio and less power.
Unfortunately, the opposite compression ratio values are needed. A low compression ratio is needed at high power to avoid detonation, and a high compression ratio is needed at low power levels to provide higher engine efficiency. VCR is unique in its ability to provide the correct compression ratio when needed. Unlike VVC systems, the Envera VCR mechanism adjusts the physical size of the combustion chamber and is able to provide the ideal compression ratio settings at all power levels.—Envera report to DOE
Combining VCR and VVC delivers much higher efficiencies are attained at low load by increasing the mechanical compression ratio with VCR, reducing pumping losses with VVC, and operating the engine using the high-efficiency Atkinson cycle. High power output levels are attained by reducing compression ratio with the VCR to avoid detonation, boosting the engine, and adjusting the valve timing with the VVC to trap as much intake air as possible in the engine cylinders.
| Envera variable compression ratio mechanism from a 2011 report to DOE. Click to enlarge. |
The Envera VCR mechanism uses an eccentric carrier approach to adjust engine compression ratio. The crankshaft main bearings are mounted in this eccentric carrier and pivoting the eccentric carrier 30 degrees adjusts compression ratio from 9:1 to 18:1. The eccentric carrier is made up of a casting that provides rigid support for the main bearings, and removable upper bearing caps. Oil feed to the main bearings transits through the bearing cap fastener sockets. The eccentric carrier design was chosen for its low cost and rigid support of the main bearings.
A control shaft and connecting links pivot the eccentric carrier. The control shaft mechanism features compression ratio lock-up at minimum and maximum compression ratio settings. Envera selected the control shaft method of pivoting the eccentric carrier due to its lock-up capability. The control shaft can be rotated by a hydraulic actuator or an electric motor.
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