The two-stroke engine has been favored for small recreational vehicles because of its high power density, low cost and simple, lightweight design, Oh said.

However, it has been blighted by very high unburned hydrocarbon (HC) emissions that, as a result of ever more stringent regulations, have caused it to be almost entirely forced out of the market in favor of four-stroke replacements. This is due to raw, externally premixed fresh charge comprising air, fuel and lube oil being short-circuited into the exhaust during the scavenging process. The lost available energy also results in poor off-design point fuel efficiency. Both problems are addressed in part with gasoline direct injection, but are only decisively eliminated (apart from trace emissions from the lube oil) using hydrogen as the energy carrier.

...The motivations for targeting this segment are manifold. For one, the [California Air Resources Board] CARB-established limit of HC+NO_x emissions in the 7.4 kW rated power class represents a 92% reduction in the current Tier 4 (4 Star) regulations from Tier 1, from 67 to 5 g/kWh. A presently voluntary 5 Star designation would further halve that limit to 2.5 g/kWh. This has forced almost all two-stroke gasoline marine outboards out of the market, while engines producing over regulated limits are still being manufactured and sold under a corporate fleet averaging system by trading credits from engines generating less pollutants than the mandate with those emitting more. Even with the move to four-stroke engines, additional complexity will be necessary—possibly exhaust after-treatment—in order to fully meet regulations. This will incur significant extra costs that are proportionally highest yet least tolerable in the lowest power rating segments. The second motivation is a purely pragmatic one, since a modest output powerplant will also have concomitantly modest needs for hydrogen storage to achieve a targeted autonomy.

—Oh and Plante

Oh’s hydrogen engine matched the rated power of the original gasoline engine, with an achieved best-point gross indicated thermal efficiency is 42.4%. The brake thermal efficiency at rated power is 32.3%.

ICOMIA cycle NOx emissions and hydrogen consumption, efficiency-optimized calibration. Oh and Plante. Click to enlarge.

Weighted over the entire 5-mode ICOMIA (International Council of Marine Industry Associations) duty cycle, the gross indicated thermal efficiency is 36.3%; brake thermal efficiency is 25.5%; and brake specific NO_x is 2.46 g/kWh, the majority of which is in the Mode 5 (maximum rated power) operating point that accounts for only 6% of the duty-cycle time. The NO_x emissions can be reduced by 18% to 2.01 g/kWh with a 3.3% fuel consumption penalty.

The base of the hydrogen engine is a series-production 2009 model year two-stroke carbureted gasoline marine outboard with 9.9 hp (7.4 kW) rating. Initially the team looked at indirect and semi-direct injection strategies. These were hampered by low power and abnormal combustion events such as knocking, pre-ignition and backfiring, which worsened with increasing engine load. As a result, Oh and Plante abandoned these approaches and focused on a cost-effective direct-injection system.

Because the design target included low system cost and production viability, exotic hydrogen storage and one- off fuel delivery solutions were precluded. The duo selected compressed gaseous hydrogen at 350 bar and adapted air-assisted injectors from Synerject (earlier post).

They developed a new cylinder head yielding an increased compression ratio to incorporate the hydrogen direct injection system as well as to maximize thermal efficiency. The chosen geometry is a hemispherical bowl-in-head that is offset away from the exhaust ports with approximately central vertical positioning of the injectors and closely located sparkplugs. Generous squish promotes charge turbulence near TDC for rapid mixing and short combustion duration. The geometric compression ratio is variable from 12 to 14.5:1 using metal head gaskets of varying thicknesses.

Everything else—pistons, porting and crankcase, bearings, exhaust, etc.—were carried over from the production engine.

Lubrication proved to be an issue. Initially, they used an off-the-shelf, synthetic-blend, TCW3-rated two-stroke engine oil. However, operation with hydrogen fuel resulted in severe black sludge on the piston crown and in the combustion chamber; hard varnish and rust on the cylinder liner and piston ring pack; and a foamy, white, mayonnaise-like substance indicative of aqueous emulsion and/or hydrogenation in the crankcase and ports. Switching to an unadditized Group III (severely hydro-processed) oil eliminated those issues.

In their study, Oh and Plante observed that late fuel injection is the key driver in raising the thermal efficiency, but at the expense of increased NO_x emissions. They suggested that the cause of this is increasing charge stratification.

Since the low injection pressure limits the extent of realizable late timings and it is unlikely that the compression ratio can be further raised without increasing the tendency for abnormal combustion and NO_x emissions, additional simultaneous efficiency improvement and NO_x reduction will require substantial redesign with detailed determination of the sources of thermodynamic losses and novel approaches to their minimization. To this end, the effects of charge motion and stratification are to be investigated closer with direct in-cylinder optical observation, experimental measurements and CFD analysis. Wall-heat transfer is identified as the predominant loss factor based on a literature review, combining the small engine dimensions and very high compression ratio—with the consequent disadvantageous exposed surface-to- volume ratio—and the peculiarities of hydrogen combustion with its small quenching distance and high flame speed. Therefore, the systematic mitigation of wall heat transfer by imposing designed regimes of charge stratification and flow motion in the combustion chamber for reduced thermal convection is the subject of further work.

A gross indicated thermal efficiency of 45% and engine-out, cycle-weighted indicated specific NO_x emissions of 1 g/kWh are targeted.

—Oh and Plante

Hydrogen two-strokes. Although Oh and Plante are targeting the recreational market with their development work, a low-cost, efficient and low-emitting small displacement two-stroke engine could have a significant impact in transportation markets in Asia.

In 2004, Asif Faiz and Surhid Gautam, both of the World Bank, published a paper in the International Journal of Vehicle Design exploring different technical and policy options for reducing emissions from 2-stroke engines.

In South Asia, these vehicles account for about 60% of the motor vehicle fleet and contribute significantly to air pollution, resulting in adverse health effects, particularly for urban dwellers. They are a major contributor to particulate matter (PM) and hydrocarbon emissions, besides visible smoke. PM emissions from a typical 2-stroke engine used in South Asia are an order of magnitude higher compared to a 4-stroke engine of equivalent size. Poor vehicle maintenance, misuse of lubricants, and adulteration of gasoline exacerbate emissions from these vehicles.

—Faiz and Gautam (2004)

In their paper, Faiz and Gautam suggested that emissions from existing 2-stroke gasoline engines could be reduced by using the correct type and quantity of lubricant, improving vehicle maintenance, and improving the quality of gasoline. For new vehicles, they suggested redesigning 2-stroke engines to decrease scavenging losses, and installing catalytic converters to reduce tailpipe emissions. Other technical options they identified included replacing the 2-stroke engine with a 4-stroke gasoline engine—or by switching to cleaner alternative fuels such as liquefied petroleum gas, compressed natural gas, or electricity.

Resources

• David Oh and Jean-Sébastien Plante (2012) A Hydrogen-Fueled, Direct-Injected, Two-Stroke, Small- Displacement Engine For Recreational Marine Applications With High Efficiency And Low Emissions. (ASME ICEF2012-92047)

• Faiz, Asif; Gautam, Surhid (2004) Technical and policy options for reducing emissions from 2-stroke engine. International Journal of Vehicle Design, Volume 34, Number 1 doi: 10.1504/IJVD.2004.003891