|Working principle of FEV’s two-stage VCR system. Click to enlarge.|
Use of a variable compression ratio (VCR) could be one technology approach to reducing fuel consumption in highly boosted (e.g., downsized, turbocharged) gasoline engine, as operating an engine with higher compression ratios at low load can increase fuel economy compared to operating an engine with fixed compression ratio.
At SAE World Congress 2013, engineers at FEV reported on their progress on a two-stage variable compression ratio (VCR) system based on a variable length conrod with eccentric piston pin suspension. The small end of the connecting rod in the FEV systemis equipped with an eccentric sleeve housing the wrist pin. By rotating the eccentric sleeve, the effective connecting rod length, and thus the compression ratio, can be varied. (Diagram at right.)
The system, which has been in development for more than four years, could be adapted to existing engines with moderate effort, they suggest. The technology could also enable more fuel efficient engines through improved support for alternative fuels or gasoline controlled autoignition, they also sugest.
The application of downsizing to reduce fuel consumption in context with boosting presents a disadvantage because of the higher knock sensitivity at higher engine loads. At a given boost level and fuel quality this can only be compensated for by either spark retardation or adjustment of the compression ratio.
Despite the fact that the majority of the boosted engines in contrast to the naturally aspirated engines are equipped with direct fuel injection, today’s boosted engines are designed for RON 95 gasoline with a compression ratio (CR) of approximately 9.6. In comparison, naturally aspirated engines have a higher compression ratio in the order of 1 to 1.5 units.
In the United States, there is a strong demand that downsized boosted engines can be operated with regular fuel instead of premium fuel which has a lower RON compared to the common fuel used in Europe, but it contains 10% of ethanol which can be taken into account for the CR discussion.
Depending on the base fuel used for the ethanol blend the added ethanol can increase the octane rating of the fuel or with comparable octane rating to non-ethanol fuel the ethanol content can significantly change the latent heat of vaporization and leads to an additional charge cooling effect which can be used advantageously especially with direct injection engines.
The lower compression ratio at full load results in lower pressures and temperatures at the end of compression, leading to a beneficial location of the center of combustion which improves efficiency. A higher compression ratio at part load without knock limitation is always beneficial in terms of thermal efficiency and thus fuel consumption. The application of a variable compression ratio mechanism lends itself to resolve this conflict.—Kleeberg et al.
Two main types of variable compression ratio systems have been proposed over the years: continuous and two-stage. There have been a number of concepts proposed to enable such VCR systems. Broadly, they can be characterized into three groups: unconventional cranktrains; systems that vary the distance between the crankshaft and the cylinder head; and variable kinematic lengths. The FEV variable length connecting rod is part of the third group.
In the FEV system, the length of the connecting rod can switch between two positions, altering the distance between the piston head and the cylinder head and thus the compression ratio. To vary the conrod length, FEV uses the rotation of an eccentric bearing in the connecting rod small end and a two-cylinder system within the conrod along with a 3/2 way valve. Activation of the 3/2 way valve can be via several options, including via cam discs.
Because all functional elements are concentrated into only one component, the connecting rod, the system can be a cost effective VCR solution as no expensive and power consuming actuators are needed, FEV suggests.
The VCR system can be applied on any reciprocating engine and adjusted to any engine architecture. Range of the variation of the length of the conrod is dependent on the required minimum and maximum compression ratio as well as the stroke.
The transition duration of the system is 0.6 seconds, and is independent from oil temperature and the corresponding operating point. FEV says that the increase in friction of the engine is minimal.
Given a fuel consumption improvement potential of 5-7%—depending upon drive cycle and vehicle/powertrain combination—the cost of the VCR system is competitive, and comparable to other fuel consumption reducing technologies, FEV said. FEV has adapted and tested the two-step VCR system on several engines and one demonstration vehicle.
Looking forward, the system can also be used to enable the use of fuels with different octane rating, such as bio-fuel or flex-fuel concepts, and to compensate for worldwide gasoline octane rating fluctuations.
Kleeberg, H., Tomazic, D., Dohmen, J., Wittek, K. et al., “Increasing Efficiency in Gasoline Powertrains with a Two-Stage Variable Compression Ratio (VCR) System,” SAE Technical Paper 2013-01-0288, 2013, doi: 10.4271/2013-01-0288
D. Tomazic, H. Kleeberg, S. Bowyer, J. Dohmen, K. Wittek, B. Haake (2012) Two-Stage Variable Compression Ratio (VCR) System to Increase Efficiency in Gasoline Powertrains. (DEER Conference 2012)
Wittek, K., Tiemann, C., and Pischinger, S., “Two-Stage Variable Compression Ratio with Eccentric Piston Pin and Exploitation of Crank Train Forces,” SAE Int. J. Engines 2(1):1304-1313, 2009, doi: 10.4271/2009-01-1457
Pischinger, S., Wittek, K., Tiemann, C. (2009) Two-stage Variable Compression Ratio with Eccentric Piston Pin. MTZ worldwide doi: 10.1007/BF03227930