Pinnacle 4-stroke opposed-piston SI engine shows fuel economy improvements of between 30-60% at light loads, 12-30% at medium loads over conventional
|Comparison of ISFC (@42.9 MJ/kg) (4000 RPM) between 250cc Pinnacle and 200cc poppet valve engine. Source: Pinnacle. Click to enlarge.|
Pinnacle Engines, the developer of a high compression ratio, four-stroke, spark-ignited (SI), opposed-piston, sleeve-valve architecture engine (earlier post) reports that the most basic configuration of its engine has shown improvements in indicated specific fuel consumption (ISFC) of between 30-60% at light load and 12-30% at medium-load compared to a comparably sized conventional poppet-valve technology engine. There is no ISFC improvement at high load.
The results represent a light-load indicated efficiency improvement of 15-30% (up to 40.9%). The company presented these latest test results in a paper at the SAE 2012 World Congress in Detroit.
|Net indicated efficiency of the Pinnacle Opposed-Piston engine, CR = 15:1, 87 Octane. Source: Pinnacle. Click to enlarge.|
The engine architecture has two different configurations: a fixed-geometry lower content version targeted at markets in developing countries (e.g., for scooters and tuk-tuks) and a variable geometry, higher-content version (e.g., variable compression ratio (VCR); variable valve train (VVT); external EGR; turbocharging; direct injection) to support more advanced and multi-cylinder applications for automotive. The testing reported in the paper is on a single-cylinder, fixed-geometry basic configuration.
Pinnacle has signed a licensing and joint development agreement with a major, as yet unnamed, Asian vehicle OEM, with lower power products targeted for two- and three-wheel applications in the developing country markets expected to be available in the first quarter of 2013. The company has recently completed a 110cc production-intent prototype, which it began testing in February.
|Pinnacle Engines’ 4-stroke opposed-piston engine. Click to enlarge.||Pinnacle Engines’ 110cc production-intent prototype packaged for a motorcycle application. Click to enlarge.|
The test engine for the paper was a 250cc single-cylinder unit with dual-camshaft-actuated reciprocating sleeve valves for induction and exhaust. The sleeve valves are linearly actuated and seal on an angled valve seat. Compression ratio was set at 15:1, and the fuel used was 87 octane gasoline. For a comparison engine, Pinnacle used a 200 cc, conventional two-valve, air-cooled, carbureted model.
Opposed-piston geometry has a low surface area-to-volume ratio relative to conventional poppet-valve engines, which offers advantages including reduced heat loss during the expansion stroke. Knock resistance can also be attributed to the sleeves’ ability to operate at significantly lower temperatures than conventional poppet valves.
In general, the low surface area and low heat transfer and low knock combustion chamber enable the combustion strategy to operate in an over-compressed mode.—Tony Willcox, Pinnacle Engines
|Active sleeve cooling. The sleeve valves have a short heat rejection path directly to lube oil. During the valve open event, all but the top few millimeters are drawn past a lip seal into oil. Source: Pinnacle. Click to enlarge.|
At high loads, the Pinnacle opposed-piston engine also utilizes the Cleeves cycle, an operating mode in which spark timing is highly delayed from best power spark advance, to enable knock-free operation. Although delayed spark timing reduces engine efficiency at high loads, the compromise to improve part-load efficiency is beneficial to vehicle mileage, Pinnacle says. (The high compression ratio and rapid late combustion provide sufficient effective expansion to maintain comparable full-load torque, EGTs and efficiency.)
In other words, Pinnacle will trade-off high-load efficiency improvements for significant improvements under low- and medium-load conditions, where most drivers spend most of their time.
Pinacle has focused so far in its testing on steady-state load/speed/lambda points. For a given test point, speed, throttle, and mixture (i.e., air-fuel equivalence ratio, lambda) are fixed, and spark timing is adjusted until either maximum brake torque (MBT) is reached or incipient knock criteria are met.
With the extended lean operation of the sleeve-valve engine, NOx emissions remain low for light to medium loads and rise back to typical levels during the brief enrichment periods required to maintain competitive BMEP levels at higher loads.
Thus the Pinnacle engine offers the twin benefits of low emissions and improved efficiency at a low cost—an especially attractive proposition for their initial target market. Early testing on the 110cc production-intent unit has shown a slight further increase in indicated efficiency above that shown by the 250cc unit, Pinnacle noted.
These improvements in indicated efficiency along with expected friction levels show promise in improving real-world vehicle efficiency in the lowest-cost configuration of the engine. Significant additional improvements are expected in a higher-cost market than can take advantage of a simple VCR system in combination with VVT, EGR, and a three-way catalyst.
With additional combustion stability enabled by high compression ratios at light loads, extended EGR levels can further reduce pumping losses, which should assist in closing the gap to throttle less, spark-ignited IC engine operation. Furthermore, popular four-stroke gasoline fuel efficiency improving techniques such as direct injection and turbocharging for downsizing/down speeding should have an additive, and in some cases, compounding efficiency benefit when combined with the advantages offered by the opposed-piston, sleeve-valve architecture.—Willcox et al.
Willcox, M., Cleeves, J., Jackson, S., Hawkes, M. et al. (2012) Indicated Cycle Efficiency Improvements of a 4-Stroke, High Compression Ratio, S.I., Opposed-Piston, Sleeve-Valve Engine Using Highly Delayed Spark Timing for Knock Mitigation. SAE Paper 2012-01-0378 (http://papers.sae.org/2012-01-0378)