Road testing of a new variable-compression ratio (VCR) combustion system in Europe has been “very successful to date” as OEMs validate the FEV-developed technology, reports Dean Tomazic, Executive Vice President and CTO of FEV North America.
“We’re working with several customers and there are more to follow. We’ve been conducting test-fleet operations for many months,” with 50 engines in use, Tomazic told Automotive Engineering.
He described FEV's 2-step VCR, which offers a 3-5% increase in fuel efficiency, as “a relatively big ‘hammer’ to employ for CO2 reduction. It can significantly extend the operating window, allowing you to operate at higher power levels without incurring (engine) knock.”
With the expanding European test program, the VCR technology is “gaining momentum” toward productionization, with FEV in discussions with potential Tier 1s, Tomazic noted. The modular technology can be adopted as a clean-sheet or retrofit for diesel, gasoline, flex-fuel engines in any cylinder configuration.
FEV has been in the vanguard of VCR design, development and testing since the 1990s. In 2009 the company published an SAE technical paper (http://papers.sae.org/2009-01-1457/) describing the 2-stage VCR system now under test by OEMs. The mechanical centerpiece is a clever adjustable-length connecting rod featuring a rotating eccentric eye within the rod’s “little end.”
“It’s a simple, passive system, requiring just a bit of hydraulics and a 2-way valve to lock the system into positions ‘A’ and ‘B,’” Tomazic explained. The con rod’s unique design incorporates two small hydraulic pistons, each within a dedicated chamber; the hydraulics in the two small cylinders serve only as a locking function to stabilize the mechanism in the ‘A’ position.
“We basically drain one chamber and make the other one accessible to low-pressure oil that comes through the crankshaft and the con rod into that chamber,” Tomazic said. The chamber fills up and the oil flows back through a check valve. The primary (large) piston is moved up and down relative to the rod exclusively by the mass and inertial forces.
Transitioning from compression ratio ‘A’ to ‘B’ is achieved within 0.2 to 0.6 s. According to Tomazic, a typical ratio change in a gasoline engine would be from 11:1 to 15:1. The piston’s maximum vertical lift threshold of 1.5 to 2 mm (.06 to .07 in) is adjusted in real time according to load and available fuel quality, via inputs from knock and fuel-octane sensors.
FEV engineers have evolved the VCR using one of the company’s proprietary analysis toolsets known as CMD (Charge Motion Design), based on optimized CFD. Compared with fixed-ratio and full-variable compression-ratio designs, the 2-stage VCR enables higher potential fuel economy in spark-ignited ICEs, particularly as average peak firing pressures (up to 170 bar/2466 psi) increase.
Some powertrain engineers have commented that even with the presumed cost benefit of mass production, FEV’s sophisticated con rod would be many times more expensive per unit than a current-production conventional steel rod. Tomazic argues that the greater complexity is part of the industry’s investment in advanced ICE technologies to meet the next phase of CO2 regulations.
“It’s getting more expensive to reduce emissions and improve fuel economy,” he noted. “OEs are weighing variable-compression-ratio engines versus electrification. They’re finding 2-step VCR is very cost competitive, whereas five years ago it wasn’t because there was lower-hanging, lower-cost fruit available to meet their targets.”