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API Publication 4592-1994: Evaluating Oxygenated Fuel Effects on Two-Stroke Cycle Outboard Engines
A Technical Review of Performance, Emissions, and Durability Implications for Marine Engine Design and Fuel Regulation
API Publication 4592-1994 (scanned version) is a landmark investigation into the effects of oxygenated fuels on the performance of two-stroke cycle outboard engines. Published by the American Petroleum Institute, this document addresses the challenges and opportunities presented by reformulated gasolines—blended with oxygenates such as ethanol and MTBE—on marine engines. As environmental regulations drove changes in fuel composition in the mid-1990s, API 4592 provided essential empirical data to help engine manufacturers, fuel suppliers, and regulators understand the trade-offs in power, efficiency, emissions, and durability. This technical article explores the scope, test methodology, key findings, and compliance considerations from this publication.
Scope and Objectives
The primary objective of API Publication 4592 was to quantify the effects of oxygenated fuels on the performance, exhaust emissions, and material compatibility of two-stroke cycle outboard engines. The study covered:
- Fuel Types: Unleaded gasoline blended with ethanol (E10, E15) and MTBE (up to 15% oxygen weight), compared with a baseline conventional gasoline.
- Engine Platforms: Multiple popular two-stroke outboard engines ranging from 50 hp to 200 hp, representing typical recreational and commercial applications.
- Metrics: Peak power, fuel consumption, exhaust emissions (HC, CO, NOx), and wear after accelerated durability testing.
The scope excluded four-stroke engines and focused exclusively on spark-ignited two-stroke outboards—the dominant marine engine technology at the time. The study was designed to support industry decision-making during the phase-in of oxygenated fuels under the Clean Air Act Amendments.
Technical Methodology and Test Requirements
Testing followed a rigorous protocol to ensure repeatability and comparability. Engines were first broken in on baseline fuel, then operated on each test fuel under controlled laboratory conditions. A water brake dynamometer measured power and torque at wide-open throttle over a speed sweep from 1500 to 5500 rpm. Emissions were sampled using a raw exhaust gas analyzer system calibrated for marine wet conditions. Fuel consumption was measured gravimetrically.
A key aspect of the methodology was the simulation of real-world operation through a 100-hour cyclic endurance schedule that included idle, cruise, and full-throttle segments. This allowed assessment of deposit formation, spark plug fouling, and component wear. The following table summarizes representative results observed with different oxygenated blends:
| Fuel Blend | Peak Power Change (%) | Fuel Consumption Change (%) | HC Emissions Change (%) | CO Emissions Change (%) | NOx Emissions Change (%) |
|---|---|---|---|---|---|
| Baseline (0% oxygenate) | — | — | — | — | — |
| E10 (10% ethanol) | −1.5 to −2.0 | +3.0 to +5.0 | −10 to −15 | −5 to −10 | +2 to +5 |
| E15 (15% ethanol) | −3.0 to −4.0 | +6.0 to +8.0 | −15 to −20 | −10 to −15 | +5 to +8 |
| MTBE 15% (by oxygen weight) | −0.5 to −1.0 | +1.0 to +2.5 | −5 to −10 | −2 to −5 | +1 to +3 |
The data indicate that oxygenated fuels consistently reduce HC and CO emissions, with ethanol providing larger relative decreases than MTBE, but at the cost of increased fuel consumption and slight power reduction. NOx emissions increased moderately with both oxygenates, a common trade-off when leaning the air-fuel mixture.
Key Findings and Implementation Highlights
API 4592 documented several critical findings that have influenced both engine design and fuel regulation:
- Emissions Benefits: E10 reduced total HC+CO emissions by up to 20%, supporting its use as an ozone reduction strategy in non-attainment areas.
- Power and Efficiency: Power losses of 1–4% were acceptable to most recreational users, but could affect high-performance applications. Fuel consumption increases required adjustment of fuel storage and refueling practices.
- Durability Concerns: High ethanol blends (E15 and above) caused increased cylinder bore wear and elastomer swelling in fuel system components. Standard Buna-N seals showed accelerated degradation, while Viton and PTFE exhibited better compatibility.
- Deposit Formation: Oxygenated fuels reduced combustion chamber deposits in many tests, but some low-volatility MTBE blends led to increased intake system deposits without proper additive packages.
Implementation Tip: When designing two-stroke marine engines for markets using oxygenated fuels, manufacturers should plan for a ~2% power reduction and higher volumetric fuel consumption. Adjusting fuel injection timing and calibration can partially mitigate these effects without compromising emissions benefits.
Key Warning: The study found that sustained operation on E15 resulted in up to 30% higher cylinder wear compared to baseline fuel. Marine engine owners should avoid using fuel blends containing more than 10% ethanol unless the engine manufacturer explicitly approves higher levels.
The publication also emphasized the need for proper fuel formulation—specifically the use of corrosion inhibitors and deposit control additives in oxygenated blends. Fuel suppliers are advised to ensure that marine gasoline meets ASTM D4814 requirements with appropriate oxygenate limits.
Compliance and Regulatory Considerations
API Publication 4592-1994 served as a foundational document in the regulatory dialogue between the U.S. Environmental Protection Agency (EPA), the California Air Resources Board (CARB), and the marine industry. During the early 1990s, the shift toward reformulated gasoline (RFG) under the Clean Air Act Amendments raised concerns about the impact on marine engines, which were not yet subject to stringent emissions controls at the federal level.
Key compliance takeaways include:
- Fuel Labeling: The study supported the requirement for clear labeling of ethanol content at marine fuel Stations to prevent misfueling with high-blend fuels not suited for two-stroke engines.
- Warranty and Durability: Engine manufacturers used the data to establish fuel composition limits in their warranties, typically capping ethanol at 10% and requiring the use of fuel stabilizers in stored fuel.
- Emission Certification: As CARB and later EPA began to certify marine engines, the baseline emissions data from API 4592 became a reference for determining the effectiveness of aftertreatment and fuel-based strategies.
Regulatory Success Story: API 4592 contributed directly to the EPA’s recognition of oxygenated fuels as a viable means of reducing marine hydrocarbon emissions. It demonstrated that up to 20% reduction in HC and CO could be achieved without costly hardware changes, supporting the phased adoption of RFG in coastal and lake regions.
Critical Compliance Risk: Non-compliance with the fuel composition recommendations derived from this study—especially the use of ethanol blends above 10% or fuels lacking corrosion inhibitors—can void engine warranties and result in premature failure of fuel system components, posing safety risks from fuel leakage and fire.
Although API 4592 is now over thirty years old, its technical conclusions remain largely valid for traditional two-stroke outboards still in service. Modern engine designs incorporate improved materials and fuel management systems that extend compatibility, but the fundamental relationships between oxygenate content, power, and emissions persist. Regulators and manufacturers continue to reference this publication when evaluating new fuel formulations such as E20 or high-octane biofuels.
Frequently Asked Questions
Q: What types of fuels were tested in API Publication 4592?
A: The study tested conventional unleaded gasoline as the baseline, and reformulated blends containing ethanol (E10 and E15, meaning 10% and 15% ethanol by volume) and MTBE (blended to provide up to 15% oxygen by weight). All fuels met ASTM D4814 requirements for volatility and octane.
A: The study tested conventional unleaded gasoline as the baseline, and reformulated blends containing ethanol (E10 and E15, meaning 10% and 15% ethanol by volume) and MTBE (blended to provide up to 15% oxygen by weight). All fuels met ASTM D4814 requirements for volatility and octane.
Q: How were emissions measured in the study?
A: Emissions were measured using an integrated raw exhaust sampling system following SAE J1088. Hydrocarbons (HC) were measured by flame ionization detection, carbon monoxide (CO) and carbon dioxide (CO2) by nondispersive infrared, and nitrogen oxides (NOx) by chemiluminescence. Measurements were taken at multiple steady-state speeds and under the transient acceleration schedule.
A: Emissions were measured using an integrated raw exhaust sampling system following SAE J1088. Hydrocarbons (HC) were measured by flame ionization detection, carbon monoxide (CO) and carbon dioxide (CO2) by nondispersive infrared, and nitrogen oxides (NOx) by chemiluminescence. Measurements were taken at multiple steady-state speeds and under the transient acceleration schedule.
Q: Is API Publication 4592 still applicable to modern outboard engines?
A: While many modern outboards are four-stroke or direct-injection two-stroke designs that respond differently to oxygenates, the fundamental findings on fuel consumption, power, and material compatibility remain relevant. The publication is often cited as a historical baseline when evaluating the impacts of new oxygenated fuel blends on legacy and current engines.
A: While many modern outboards are four-stroke or direct-injection two-stroke designs that respond differently to oxygenates, the fundamental findings on fuel consumption, power, and material compatibility remain relevant. The publication is often cited as a historical baseline when evaluating the impacts of new oxygenated fuel blends on legacy and current engines.
— Published 2026. This article provides a technical overview of API Publication 4592-1994 for informational purposes. Always refer to the current version of the applicable standards for compliance requirements.