Oxyhydrogen evidence reviews
Short answer: the peer-reviewed record on oxyhydrogen and hydrogen addition to engines is heterogeneous — measurable efficiency gains are reported under some controlled conditions, no measurable benefit under others. These pages separate what the published papers report, what manufacturers state, and what our own technical analysis concludes.
Start with the cornerstone review. See the 2026 Scientific Reports HHO study review.
- 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.
- 2016 review: hydrogen addition to compression-ignition engines
What the 2016 Renewable and Sustainable Energy Reviews survey of hydrogen addition to compression-ignition engines reports: conditional efficiency gains, mixed results and the NOx trade-off.
- 2023 Fuel review: hydrogen with diesel and biodiesel engines
Source-bounded summary of the 2023 Fuel review of hydrogen addition to diesel and biodiesel engines: what is consistently reported, where results diverge and what remains untested.
- HHO dosing: why the HHO-to-fuel ratio matters
Why HHO dosing and the HHO-to-fuel ratio determine what an engine test measures, and why an unoptimized fixed retrofit dose samples one point rather than characterizing the technology.
- 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.
- Alkaline HHO versus PEM/SPE oxyhydrogen
How liquid-electrolyte alkaline KOH HHO generators differ from PEM/SPE pure-water oxyhydrogen systems, and why a test of one is not a test of the other.
- 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.
- Industrial oxyhydrogen case studies and field observations
Our industrial PEM/SPE oxyhydrogen work: pure-water electrolysis systems, engine and furnace applications, an audited beverage-plant trial, and how we label audited results against field observations.
- HHO and hydrogen engine literature: an overview
An overview of the peer-reviewed literature on HHO and hydrogen addition to internal combustion engines: what is consistently reported, where results diverge, and which variables drive the difference.
- Does HHO work? A source-bounded technical analysis
A source-bounded answer to whether HHO improves engine fuel economy: what published testing supports, what it does not, and which variables decide the outcome in any given installation.
PEM/SPE oxyhydrogen systems
The systems referenced across these reviews are PEM/SPE units rather than the alkaline KOH retrofit devices used in several published negative tests.
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.