Opposed-piston engines (OPEs) have been around since 1858 and serious variable compression ratio (VCR) devices have been available since the early 1950s. Most applications were primarily research endeavors, with the goal of creating mechanisms that can achieve the benefits of VCR reliably, durably and for reasonably low cost.
Before exploring previous OPEs that have experimented with VCR, it is perhaps appropriate to examine the variable compression ratio benefits that have already been identified and achieved. These include:
- Limiting cylinder peak pressures to 140 bar while allowing higher boost pressures and increasing BMEP to 40 bar
- Reducing SFC at partial load by increasing compression ratio
- Reducing wear rates due to limited peak pressures
- Easier engine starting at low ambient temperatures (-25° C)
- Lower start-up emissions and smoother idling
- Limiting engine structure and bearing loads while increasing power
- Increasing expansion ratio
- Multi-fuel capability
There are three recorded OPEs that have experimented with VCR. All are of the rocking beam type. Two OPEs achieve VCR through eccentric mechanisms within their central pivots (Figures 1 and 2) and the third also achieves VCR through eccentric mechanisms within its end pivots (Figure 3).
Most internal combustion engines require substantial modification to their design to achieve a VCR capability, regardless of the various known concepts that were employed. Inevitably, a virtual redesign of the engine and all that that involved was unavoidable.
Dr. Wilfred (Bill) Mansfield of BICERA faced this dilemma during the early 1950s. His solution: the invention of the VCR piston as a virtual drop-in replacement. Initially, his research focused primarily on a Mirrlees 2 TLB/E3 diesel engine, recording a BMEP of 20 bar, peak cylinder pressure of 90 bar, SFC of 228 g/kWh and brake thermal efficiency (BTE) of 42%—a substantial increase in all-around performance.
Research continued with a patent for a VCR piston Otto cycle gasoline engine (GB762074), developed in 1956. For these experiments, a modified BMW L6 1.5L automotive engine was used, operating a compression ratio between 6.5:1 and16.5:1, and fueled by 70 octane gasoline. In back-to-back tests, with and without VCR pistons, the SFC was reduced by 25% during partial load running.
News of Dr. Mansfield’s work reached Continental Aviation & Engineering Corp. in the U.S. A licensing agreement was drawn up and they embarked on a major power improvement program. With the application of VCR pistons, the AVCR 1100 -V12 diesel increased from 410 kW to 1100 kW. Details of this are written up in the Society of Automotive Engineers (SAE) International technical papers: 762A, 660344 and 760051.
In 1962, Dr. Mansfield patented a VCR for two-cycle operation. Previous VCR pistons for Otto cycle applications had moveable piston crowns, which would not be suitable for two-stroke engines. Properly designed two-cycle engines require a precise relationship between the piston crown position for both inlet and exhaust port timing. A moveable piston crown would disturb such timing. So Dr. Mansfield’s design is based on a fixed piston crown, but with a moveable combustion chamber within the piston. See patent GB902707, 1962.
This two-cycle VCR design is equally applicable to OPE uniflow cylinders and modernized versions of Dr. Mansfield’s invention are depicted in Figures 4 and 5.
For those engine manufacturers seeking to meet future emissions legislation and the goal of 50% BTE, the VCR piston is a technology that can help. Perhaps now is the time to revive Dr. Mansfield’s work.