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Pinnacle’s opposed-piston scooter engine can meet NOx target with lean operation and ignition delay; little fuel economy hit

In a paper presented at the recent Small Engine Technology Conference organized by JSAE and SAE International, Michael (Tony) Willcox from Pinnacle Engines reported that the 110cc version of the 4-stroke, spark-ignited (SI), opposed-piston, sleeve-valve architecture engine (earlier post) can use lean operation and ignition delay to meet Euro 4 NOx emission targets without aftertreatment and with only a negligible impact (< 1.5%) on the fuel consumption benefits of the engine (about 25-27% better than a comparable conventional engine in the relevant drivecycle).

Further, Pinnacle noted, recently developed hardware designed for additional lean limit extension is under test. The new hardware is anticipated to leave margin with Euro 5/6 NOx regulations using an oxidation-only catalyst for aftertreatment.

For the target market (the Asian scooter market), the low NOxcapability of the Pinnacle engine provides a significant cost benefit to manufacturers. For such small vehicle applications, manufacturers are seeking a fuel-efficient means of reducing engine-out NOx to avoid the cost of implementing a 3-way catalyst and closed-loop-controlled fuel injection system.

While the Pinnacle 110cc engine is expected to cost about $25 more per unit to build than a current conventional engine, those engines are looking at an increased build expense of $75-$80 to add on the fuel injection system, lambda sensor and fuel pump controls required to meet emissions requirements, plus the cost of exhaust treatment, said Monty Cleeves, Pinnacle founder, President and CTO. When the entire vehicle costs around $700, that’s a significant difference.

It is well understood that in-cylinder NOx production occurs under high combustion temperatures and that extended fresh-air lean operation or ignition delay can reduce it. However, combustion stability often prevents traditional architectures from operating sufficiently lean to enable drive cycle emissions to be met, and, because three-way catalysts and near-stoichiometric closed-loop EFI systems are nearly as expensive as the engine itself in these vehicles, NOx is often controlled with rich operation by reducing the available oxygen for NOx formation and lowering peak temperatures. Fuel efficiency and cost savings are achievable if the engine’s ignition limits are sufficient to maintain stable combustion at a lean enough mixture ratio to meet NOx emissions without compromise to customer drive feel. Pinnacle Engines’ 4-stroke opposed-piston sleeve-valve engine has shown very good combustion stability with highly dilute mixtures and has shown promise in meeting certain NOx emissions regulations without a three-way catalyst.

—Willcox and Cleeves (2013)

The gist is that we are able to run a lot leaner than other people can with stable combustion. What can we use that for? First, lower temperature, meaning less heat loss, more efficient combustions, better fuel economy...and, fortuitously, lower NOx.

—Monty Cleeves

Our engines historically have run from 50-80% dilution tolerance at the 4,000 rpm speed relevant to the drive cycle, so the NOx emissions have been quite attractive. The purpose of this study was to look at the mix ratio and spark timing, and weigh the balance of the combination to give us certain emission criteria and quantify the fuel consumption associated with varying degrees of dilution levels and spark advance delays.

—Tony Willcox

The engineers tested a prototype 100cc engine on a dyno to gather steady-state fuel consumption, emissions, and combustion stability data over a matrix of speed, load, mixture ratio, and combustion phasing. Four engine control modes were evaluated with the goal of reducing NOx and minimizing fuel economy impact.

They evaluated the contributions to total vehicle fuel usage and emissions production using steady-state flow data weighted by time spent at a respective load and speed from a tested world harmonized motorcycle test cycle (WMTC).

Among the findings were:

  • Meeting current and anticipated future regulations depends on HC and catalyst design as the current Automotive Research Association of India (ARAI) spec for WMTC cycle 2-wheeler Class 1 vehicle is HC + NOx (1.08 g/km). Operation with Pinnacle Engine’s lean-burn existing hardware mapped for best efficiency (MBE) contributes ~0.27 g/km of NOx to the total allowable sum.

  • Reducing NOx solely with combustion delay is not as fuel efficient as utilizing leaner operation, or a combination of the phasing delay and leaner operation.

  • The engine configuration tested looks to be capable of Euro 4 NOx emissions without after treatment with approximately 6% COV (Coefficient of variation of net IMEP, a measure of combustion instability). Fuel economy impact is < 1.5% reduction from best efficiency mode.

  • Deviation from MBE (maximum brake efficiency) to target minimum NOx at a 5% COV limit can reduce NOx emissions by nearly 60% for less than a 1.5% reduction in fuel economy. An 80% reduction in NOx from MBE to 0.05 g/km (Euro 5/6) looks possible with 10% COV.

  • With respect to best efficiency operation, engine-out HC emissions increase approximately 10-25% at 5% COV level for operation with either delayed from MBE combustion, more dilute mixture than MBE, or a combination of the two. Lean operation is a stronger HC driver than combustion phasing.

  • Engine-out CO emissions are not anticipated to change significantly from MBE (<2% for a deviation to 5% COV). Small reductions are anticipated for methods that utilize leaner operation, with no notable change expected for combustion phasing variations.

Separate from the combustion stability and NOx testing, Pinnacle ran a 400-hour durability test on its engine and essentially found no change over time in power, torque, oil consumption or mechanical components (e.g., valve and seal), Cleeves said.

That's very beneficial to us; we don't have to debug and figure out the problems [resulting form failures of the engine in the durability testing]. We can focus on the combustion.

—Monty Cleeves


  • Michael A. Willcox, James M. Cleeves (2013) “Drive Cycle Fuel Economy and Engine-Out Emissions Evaluation Using an Opposed-Piston Sleeve-Valve Engine with Lean Operation and Ignition Delay for NOx Control” (JSAE 20139064/ SAE 2013-32-9064) doi: 10.4271/2013-32-9064



What about this engine as a range-extender in a bev. Im interrested to buy.


Any idea how many hp it generates ?


Currently working on PE research,it`s pretty good engine.only available on motor cycle so far but they may make small engine for car in 2015.
very good fuel economy.

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