2023 Fuel review: hydrogen with diesel and biodiesel engines
Short answer: The 2023 Fuel review of hydrogen addition with diesel and biodiesel fuelled engines reports condition-dependent efficiency and emissions outcomes, with recurring reductions in smoke and particulates and recurring increases in NOx at higher hydrogen fractions.
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 2023 review reports
According to the published literature, hydrogen addition with diesel and biodiesel produces outcomes that depend on hydrogen fraction, load and injection strategy rather than a uniform improvement.
The review covers published testing across diesel and biodiesel operation and reports that combustion characteristics change measurably with hydrogen addition, with the direction and magnitude of the effect depending on operating condition.
As in earlier reviews, reductions in smoke, CO and particulates are commonly reported alongside increases in NOx at higher hydrogen fractions.
How we use this in evaluation design
Our technical analysis: because the reported response is condition-dependent, an evaluation must span multiple engine loads and dosing levels before any conclusion about a technology is drawn.
We apply the same standard to our own equipment. Where a claim cannot be supported by measurement on the engine in question, we do not publish a number for it.
Questions.
What does the 2023 Fuel review conclude?
- It reports condition-dependent outcomes for hydrogen addition with diesel and biodiesel, with recurring smoke and particulate reductions and recurring NOx increases at higher hydrogen fractions.
Does the review test PEM/SPE oxyhydrogen systems?
- The review covers hydrogen addition to engines generally; it is not a comparison of PEM/SPE against alkaline gas-generation hardware.
Sources
- Hydrogen addition with diesel and biodiesel fuelled engines — review, Fuel (2023)
- Hydrogen addition to compression-ignition engines — review, Renewable and Sustainable Energy Reviews (2016)
- Effect of regulated harmful matters from a heavy-duty diesel engine by H₂/O₂ addition to the combustion chamber, Fuel (2012)
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
- 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.
- 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.