HHO dosing: why the HHO-to-fuel ratio matters
Short answer: The 2026 Scientific Reports paper states that the commercially available retrofit devices were evaluated without optimization of dosing or control of the HHO-to-fuel ratio. According to the published literature, the engine response to hydrogen or H₂/O₂ addition depends on that dose, so an unoptimized fixed dose tests one operating point rather than the technology.
Last reviewed: August 2026 · Combustion Enhancement technical review
This page is part of our oxyhydrogen evidence cluster. Published figures are attributed to the paper reporting them, manufacturer figures are labelled as manufacturer claims, and our own interpretation is labelled as our technical analysis. See the 2026 Scientific Reports HHO study review.
Terminology used on this page
| Term | Definition |
|---|---|
| H₂ | Hydrogen. |
| O₂ | Oxygen. |
| H₂/O₂ | A hydrogen and oxygen mixture. |
| HHO / oxyhydrogen | Commonly used terminology for an electrolytically generated hydrogen/oxygen mixture delivered to an engine intake. |
| PEM / SPE | Proton exchange membrane / solid polymer electrolyte electrolysis — a solid-membrane, pure-water architecture with no liquid caustic electrolyte. |
| Alkaline KOH electrolysis | Electrolysis using a liquid potassium hydroxide electrolyte. |
| BSFC | Brake specific fuel consumption — fuel consumed per unit of engine work. |
| BTE | Brake thermal efficiency — the share of fuel energy converted to useful engine work. |
What the 2026 paper says about dosing
The paper reports that commercially available retrofit devices were evaluated as supplied, without optimization of dosing or control of the HHO-to-fuel ratio.
This is a stated scope condition of the experiment, not an omission we are alleging. It defines what the result can be attributed to: the device as configured, at the dose it delivered, under the conditions driven.
Why dose changes the answer
According to the published literature, hydrogen and H₂/O₂ addition effects scale with dose and vary with load — the 2012 Fuel study reports progressively larger BSFC reductions at 50, 60 and 70 L/min.
Reviews of compression-ignition testing report that outcomes depend on hydrogen fraction, engine speed, load, air-fuel ratio and injection or combustion timing. A dose that is beneficial at one load may be neutral at another.
- —Fixed retrofit dose = one sample of a multi-variable response surface
- —Load-following dosing is a different experimental treatment entirely
- —Electrical input scales with dose and must be accounted for in net terms
What we do about it
Our technical analysis: dosing should be matched to engine displacement and duty cycle, and an evaluation should report the dose actually delivered at each tested condition.
We size systems by engine displacement and duty, and we ask operators to record the delivered gas flow alongside fuel and load data so that any observed change can be tied to a known dose.
Questions.
Was HHO dosing optimized in the 2026 Scientific Reports study?
- No. The paper states that the commercially available retrofit devices were evaluated without optimization of dosing or control of the HHO-to-fuel ratio.
Why does dosing change the result?
- According to the published literature, the engine response to hydrogen or H₂/O₂ addition depends on hydrogen fraction, load, speed and injection strategy, so different doses produce different outcomes.
Is a higher dose always better?
- No. Higher doses increase the electrical energy required to generate the gas and can shift emissions outcomes, notably NOx, so the dose has to be evaluated in net terms.
Sources
- Synák, F. (2026). Evaluation of hydrogen and oxygen mixture addition in internal combustion engines under real driving conditions. Scientific Reports.DOI 10.1038/s41598-026-54105-y · also at nature.com
- Read the original 2026 Scientific Reports HHO study on nature.com
- Effect of regulated harmful matters from a heavy-duty diesel engine by H₂/O₂ addition to the combustion chamber, Fuel (2012)
- Hydrogen addition to compression-ignition engines — review, Renewable and Sustainable Energy Reviews (2016)
PEM/SPE oxyhydrogen systems
Unlike the HG80 alkaline system tested in the 2026 Scientific Reports experiment, PEM/SPE oxyhydrogen systems such as the HydroHub™ use solid-polymer electrolysis and pure water, with no potassium hydroxide electrolyte.
Combustion Enhancement develops PEM/SPE oxyhydrogen systems using pure-water electrolysis (no KOH). These systems are used in industrial engines, furnaces and commercial applications. Learn more about the HydroHub™ PEM oxyhydrogen system and the DH-Power™ industrial oxyhydrogen generator.
Related evidence pages
- HHO gas composition: why flow rate is not gas quality
H2 concentration, O2 concentration, H2:O2 ratio, moisture and electrolyte carryover are experimental variables in an engine test. Why a reported flow rate alone does not characterize the gas.
- How to test PEM oxyhydrogen properly
The measurement set required to evaluate PEM/SPE oxyhydrogen on an engine: gas composition, purity, flow, electrical input, load, fuel consumption, emissions, dosing, repeats and uncertainty.
- 2012 Fuel study: H₂/O₂ addition to a heavy-duty diesel engine
Source-bounded review of the 2012 Fuel study on hydrogen and oxygen addition to a heavy-duty diesel engine: reported gas flows, BTE and BSFC figures, operating conditions and the limits of generalization.