Does reduced visible smoke mean the engine will pass an emissions test?

No. Nothing on this page is a compliance, certification or defect-clearance claim, and no such outcome should be inferred. Visible smoke (opacity) and oxides of nitrogen are measured and regulated separately in most frameworks, and the same literature that reports lower smoke also frequently reports NOx increasing. A reduction in visible plume therefore says nothing about whether a given engine will pass a given test regime.

Why is smoke opacity a more consistent finding than fuel economy?

Because it is easier to measure and less sensitive to the variables that dominate fuel-economy testing. Opacity is read directly off an opacimeter at a stated load and speed, whereas a fuel-economy result depends on duty cycle, driver behaviour, load factor, fuel quality and instrumentation over a much longer window. Independent studies of hydrogen and oxyhydrogen enrichment replicate a smoke reduction far more consistently than they replicate any single fuel-economy figure.

What causes visible smoke in the first place?

Visible exhaust smoke is largely made up of the products of incomplete combustion. Where combustion of the primary fuel is more complete, fewer of those products leave the cylinder, and the plume is less visible. That is the whole mechanism as it is described here — established physics, without any claim about specific soot chemistry.

Do the percentages on this page describe your systems?

No. Every percentage on this page is a finding reported by an independent study on that study's own test engine and conditions, attributed to that study by name. They are not claims about the performance of our products, and we do not publish a headline performance percentage for our own systems. Operators who want a number for their own equipment should run a controlled baseline.

Where does this matter operationally?

In enclosed and semi-enclosed working environments — underground and confined mining areas, plant rooms and ship's engine spaces — visible smoke and exhaust plume affect ventilation conditions and the working environment directly, independent of any fuel-economy consideration. In marine operation it affects plume visibility at port and the experience of crew and passengers. On general heavy equipment it is the difference an operator or bystander can actually see.

Evidence · smoke opacity

Visible smoke reduction.

The effect of hydrogen and oxyhydrogen enrichment on visible exhaust smoke — opacity — is one of the most consistently replicated findings in the peer-reviewed literature on this technology. It is replicated more consistently across independent studies than any fuel-economy figure. The same literature also consistently reports oxides of nitrogen increasing alongside it. Both halves of that sentence are on this page.

Short answer

What does the literature actually report about visible smoke?

Independent peer-reviewed studies of hydrogen and oxyhydrogen enrichment on compression-ignition engines report reductions in measured smoke opacity across a wide span of conditions — roughly single-digit percentages at low hydrogen fractions up to above 40% at the highest fractions tested in the most favourable published cases. The same studies commonly report NOx increasing under the same conditions. These are findings of independent studies on their own test engines, not performance claims about our systems.

Smoke opacity is the most consistently replicated finding in this literature
Reported reductions span roughly single digits to above 40%, per study and per condition
The same studies frequently report NOx increasing — stated plainly, not footnoted
Every figure below is attributed to a named study with its test conditions
No compliance, certification or emissions-test outcome is claimed or implied
Independent studies report these results; our own systems carry no published percentage

Measured quantity: Smoke opacity / smoke units
Consistency: High across studies
Known trade-off: NOx often increases
Regulatory status: No compliance claim

Required disclosure · read this first

NOx often increases in the same studies.

The published work that reports lower smoke, carbon monoxide and unburnt hydrocarbons with hydrogen or oxyhydrogen addition very often reports oxides of nitrogen going the other way. The reason is not controversial: more complete, hotter combustion produces fewer incomplete-combustion products and more thermal NOx. Any honest reading of this literature has to carry both results together.

This matters for how the page should be used. Visible smoke and NOx are measured separately and regulated separately in most frameworks. A less visible plume is an operational and working-environment outcome. It is not evidence about a test result, a certification, or a defect notice, and it must not be read that way.

1 · The studies, one at a time

Attributed per source, with test conditions.

The figures below are not blended into a single headline number, because they were not produced on a single engine or under a single condition. Each row is what that study reported, on its own test hardware, at its own hydrogen fraction and load.

Yılmaz, Uludamar & Aydın (2010), Int. J. Hydrogen Energy 35(20)

Reported: Reduction in measured smoke opacity, together with lower CO and unburnt hydrocarbons, when a small hydroxy (HHO) fraction was inducted.

Test conditions: Single-cylinder and multi-cylinder compression-ignition test engines, diesel fuel, bench test at varied speed with an on-board HHO generator; opacity measured by exhaust gas analyser.

NOx in the same study: The authors report NOx increasing under the same conditions.

https://doi.org/10.1016/j.ijhydene.2010.07.040

Saravanan & Nagarajan (2008), Int. J. Hydrogen Energy 33(6)

Reported: Smoke measured in Bosch smoke units fell substantially at higher loads with hydrogen-enriched intake air — a reduction of roughly a quarter to a third of the baseline reading at the reported optimum.

Test conditions: Single-cylinder direct-injection diesel engine, hydrogen enrichment of the intake air at a fixed flow rate, steady-state loading on a dynamometer.

NOx in the same study: NOx rose with hydrogen enrichment as in-cylinder temperature increased.

https://doi.org/10.1016/j.ijhydene.2007.12.065

Karagöz et al. (2016), Int. J. Hydrogen Energy 41(1)

Reported: Large reductions in smoke — the biggest opacity reductions in this set, in the order of tens of percent and above 40% at the highest hydrogen energy fractions tested.

Test conditions: Single-cylinder diesel research engine, bottled hydrogen inducted at several energy-share levels, constant speed, dynamometer loading, smoke measured by opacimeter.

NOx in the same study: NOx increased with rising hydrogen share; the paper presents this as the principal trade-off.

https://doi.org/10.1016/j.ijhydene.2015.09.064

Uludamar et al. (2016), Int. J. Hydrogen Energy 41(26)

Reported: Lower smoke opacity with hydrogen and oxyhydrogen addition across diesel and biodiesel blends, with the size of the reduction depending on the base fuel and engine speed — several test points in the single-digit to low-double-digit range.

Test conditions: Unmodified multi-cylinder compression-ignition engine, low-sulphur diesel and biodiesel blends, hydrogen and oxyhydrogen inducted at fixed flow, vibration/noise/emissions measured together.

NOx in the same study: NOx generally increased relative to the corresponding baseline fuel.

https://doi.org/10.1016/j.ijhydene.2016.03.179

El-Kassaby et al. (2016), Alexandria Eng. J. 55(1)

Reported: Spark-ignition counterpart: CO and unburnt hydrocarbons fell markedly with HHO addition — the same incomplete-combustion products that make diesel exhaust visible.

Test conditions: Four-cylinder gasoline engine, HHO generator producing a small gas fraction inducted upstream of the intake, bench test at varied speed.

NOx in the same study: NOx increased in this study as well.

https://doi.org/10.1016/j.aej.2015.10.016

Independent studies report the results above. We do not represent them as the performance of our systems, and we publish no headline performance percentage of our own. The wider reading of this evidence base, including its limits, is on evidence review.

2 · Mechanism, at the established-physics tier

Fewer leftovers leave the cylinder.

Visible exhaust smoke is dominated by the products of incomplete combustion. Hydrogen's higher flame speed and wider flammability range improve the completeness and phasing of the burn of the primary fuel. Where combustion is more complete, fewer incomplete-combustion products reach the tailpipe, and the plume is less visible. That is the extent of the mechanism as described anywhere on this site — the gas path itself is set out under how it works.

3 · Why this is a use case on its own

Plume is an operating condition, not just an emissions line item.

Mining, underground and enclosed plant

In confined and semi-confined workings, exhaust plume is part of the atmosphere people work in. Less visible smoke bears directly on ventilation conditions and the working environment, independent of any fuel-economy consideration. Unit selection for site fleets is covered under mining.

Marine and harbour operation

Plume visibility at berth and in harbour is noticed by crew, passengers and everyone on the wharf. It is an operator-experience and local-amenity matter in its own right, again independent of economics.

General heavy equipment and fleet

A visibly cleaner stack under load is the one change an operator or a bystander can see without instrumentation. We describe that as an observable difference in operating conditions, not as an economic or regulatory benefit.

Segment detail: mining, marine, generator sets and agriculture.

The same citation pool underpins hydrogen with biodiesel blends, and the load-dependence visible in these results is examined on idle and part-load duty cycles. Where fuel is carted to site, the economics of any reduction change — remote fuel logistics cost.

4 · What is not claimed here

Boundaries, stated once and clearly.

No emissions-compliance, certification or defect-clearance outcome is claimed or implied. Fitting a system is not represented as helping any engine pass any test or inspection regime.

The percentages on this page are the findings of independent studies on their own test engines, attributed individually. They are not statements about the performance of our systems.

NOx behaviour is disclosed above rather than omitted, because the same body of work that supports the smoke result reports it.

Broader emissions behaviour and how to test it is on emissions reduction; first-party operator data and the citation discipline behind it is on field results.

No projected, typical or expected fuel-saving figure is published. Actual results vary materially with engine condition, duty cycle, load profile, fuel quality, installation and operating conditions. A controlled field evaluation on your own equipment, with baseline data captured before installation, is required before any commercial projection.

FAQ

Visible smoke: common questions, answered honestly.

Does reduced visible smoke mean the engine will pass an emissions test?: No. Nothing on this page is a compliance, certification or defect-clearance claim, and no such outcome should be inferred. Visible smoke (opacity) and oxides of nitrogen are measured and regulated separately in most frameworks, and the same literature that reports lower smoke also frequently reports NOx increasing. A reduction in visible plume therefore says nothing about whether a given engine will pass a given test regime.
Why is smoke opacity a more consistent finding than fuel economy?: Because it is easier to measure and less sensitive to the variables that dominate fuel-economy testing. Opacity is read directly off an opacimeter at a stated load and speed, whereas a fuel-economy result depends on duty cycle, driver behaviour, load factor, fuel quality and instrumentation over a much longer window. Independent studies of hydrogen and oxyhydrogen enrichment replicate a smoke reduction far more consistently than they replicate any single fuel-economy figure.
What causes visible smoke in the first place?: Visible exhaust smoke is largely made up of the products of incomplete combustion. Where combustion of the primary fuel is more complete, fewer of those products leave the cylinder, and the plume is less visible. That is the whole mechanism as it is described here — established physics, without any claim about specific soot chemistry.
Do the percentages on this page describe your systems?: No. Every percentage on this page is a finding reported by an independent study on that study's own test engine and conditions, attributed to that study by name. They are not claims about the performance of our products, and we do not publish a headline performance percentage for our own systems. Operators who want a number for their own equipment should run a controlled baseline.
Where does this matter operationally?: In enclosed and semi-enclosed working environments — underground and confined mining areas, plant rooms and ship's engine spaces — visible smoke and exhaust plume affect ventilation conditions and the working environment directly, independent of any fuel-economy consideration. In marine operation it affects plume visibility at port and the experience of crew and passengers. On general heavy equipment it is the difference an operator or bystander can actually see.

Where this applies

Where visible smoke is the operating constraint.

Underground and enclosed work, public-facing transit and harbour operation — the applications where opacity is noticed before fuel cost is.

Visible smoke reduction.

What does the literature actually report about visible smoke?

NOx often increases in the same studies.

Attributed per source, with test conditions.

Yılmaz, Uludamar & Aydın (2010), Int. J. Hydrogen Energy 35(20)

Saravanan & Nagarajan (2008), Int. J. Hydrogen Energy 33(6)

Karagöz et al. (2016), Int. J. Hydrogen Energy 41(1)

Uludamar et al. (2016), Int. J. Hydrogen Energy 41(26)

El-Kassaby et al. (2016), Alexandria Eng. J. 55(1)

Fewer leftovers leave the cylinder.

Plume is an operating condition, not just an emissions line item.

Mining, underground and enclosed plant

Marine and harbour operation

General heavy equipment and fleet

Boundaries, stated once and clearly.

Visible smoke: common questions, answered honestly.

Does reduced visible smoke mean the engine will pass an emissions test?

Why is smoke opacity a more consistent finding than fuel economy?

What causes visible smoke in the first place?

Do the percentages on this page describe your systems?

Where does this matter operationally?

Where visible smoke is the operating constraint.

Marine →

Public transport & bus →

Waste & refuse collection →

Confined space & underground →

Continue reading.

Hydrogen combustion enhancement

Engine performance improvement

Fuel efficiency

Emissions reduction

Remote-site fuel logistics cost

Generator-set classes

Idle & part-load duty cycles

Hydrogen with biodiesel blends

Installation guide

Safety & compliance

Troubleshooting

Case studies & field results

Compare PEM systems

Does it actually work? Evidence review

How it works

PEM vs alkaline

Sizing combined gas

Four hydrogen technologies

Compared with Hydrogen Fuel Systems

Compared with H2i Technology

Compared with dynaCERT HydraGEN

Compared with HYDI

AU installation, warranty & safety

Mining fleets

Marine

Agriculture

FAQ