HHO is the mixture of hydrogen (H2) and oxygen (O) gases when water (H2O) is separated.  It goes by several names including HHO, Brown’s Gas, Oxyhydrogen Gas, and Hydroxy Gas.  Or it can simply be referred to as hydrogen because when it is in this atomic structure, the atoms of hydrogen, hydrogen, and oxygen are not bonded. It is colorless, odorless, and lighter than air.  Extracting the hydrogen from water is considered to be a possible long-term, renewable, recyclable, and non-polluting energy source.  

Is HHO Safe?

The first point to make about HHO is that it is safe, comparatively, in regards to risks of asphyxiation and fire hazards.  This is because of its diffusivity.  Diffusivity is the rate at which a molecule is dispersed into the air.  That means that it bonds with other gases quickly and is no longer in a reactive state.  HHO, which is not a pollutant to the environment, has a diffusivity of .61-.63 cm2/s.  This is eight times faster than gasoline (.08 cm2/s).  

When working with gases, there is a risk of asphyxiation because the gases displace oxygen.  This can make it difficult to breathe if there is a leak because the gas fills the room.  However, when working with hydrogen gas in a confined space, a small leak “poses little” risk.  And when working with hydrogen in unconfined spaces the risk is “almost negligible” (College of the Desert, 2001).  Gasoline, propane, and diesel leaks have additional dangers even in small leaks because of their toxicity.  Carbon monoxide is a poisonous gas that causes severe health hazard.  Hydrogen is not toxic and poses a lower risk of asphyxiation.      

Hydrogen also poses little threat as a fire hazard even if it leaks.  Any “slight ambient wind, vehicle motion, or radiator fan serve to disperse leaked hydrogen even more quickly with a further reduction of any associated fire hazard” (College of the Desert, 2001).  By contrast, gasoline fumes spreads laterally and diffuse slowly.  This creates a lingering fire hazard or risk of explosion.  Hydrogen, though it still needs to be handled with caution, is safer in comparison to other fuels.      

What are HHO’s Properties?

There are several other properties that make HHO very favorable over hydrocarbon fuels.  The first is its flammability.  Flammability describes the range of concentrations at which a given substance will combust when mixed with air.  There is a lower limit and upper limit.  HHO’s flammability is between 4 and 75%.  This is significant when comparing it to gasoline (1.4 and 7.6%) or diesel (.7 and 5%) because those mixtures can be too lean or too rich while hydrogen will still burn.  This also means that hydrogen can be used at a wide range of fuel-air mixtures.  

Table 1. Diesel, Unleaded Gasoline, and HHO Properties

PropertyDieselUnleaded GasolineHHO (Hydrogen)
Chemical FormulaC12H23C8H18HHO, H, or H2
Flame Temperature (K)260024732318
Autoignition Temperature (K)453-593500-753858
Stoichiometric Air Fuel Ratio14.514.734.3
Diffusivity (cm2/s)0.080.63
Net Heating Value (MJ/kg)42.543.9119.93
Flame Velocity (m/s)0.22-0.250.3-0.52.65-3.25

A second property is autoignition temperature.  Autoignition is the temperature at which a mixture of gasses or vapors ignite spontaneously with no external ignition source.  Hydrogen’s autoignition temperature is between 100 and 300 degrees higher than diesel or unleaded gas.  This allows for higher compression ratios in the engine, which therefore leads to higher thermal efficiency (Dhariwal et al., 2018).  But this does limit hydrogen from being the sole fuel in compression ignition engines, because the temperature is so high.  

Ignition temperature is not to be confused with flame temperature.  Hydrogen actually burns at a cooler temperature than hydrocarbons.  Hydrogen burns at 2318 K, whereas gas is 2473 K and diesel is 2600 K.  So, even though HHO has a high autoignition temperature, it actually burns at a lower temperature.  Research is varied on the exhaust temperatures with the addition of HHO.  Two examples show a 4-5% increase in exhaust temperatures (Dhariwal et al., 2018 or Madyira and Harding, 2014).  Another shows minimum to no change in exhaust temperature (Al-Rousan, 2010).  And a fourth showed a consistent drop of a couple percent in exhaust temperature regardless of engine speed (Musmar et al., 2011).  The variation has shown no more than plus or minus 5% in engine temperature.   

A fourth property is flame speed.  Flame speed is the rate of expansion of a flame front in a combustion reaction.  It is typically measured in meters per second.  Hydrogen’s flame speed is 2.65-3.25m/s.  Gasoline and Diesel are roughly ten times slower.  This allows for additional burn time so the engines can more closely approach thermodynamically ideal engine cycles.  This provides a significant contribution to complete combustion in engines.  HHO Carbon Clean Systems amplifies on those benefits in the article “How HHO Contributes to Complete Combustion.”  

Net Heating Value is a very significant calculation in fuels.  It can be calculated as the energy per volume and the energy per mass.  HHO has a very high energy density, but very low when compared with its volume.  This is what makes it great for space travel.  It has a high energy output and is lightweight but the volume necessary to carry it isn’t a concern.  The Net Heating Value by density is 119.93 MJ/kg compared to gasoline (43.9 MJ/kg) or diesel (42.5 MJ/kg).  When HHO is stored as water, it allows for a high density of hydrogen to be stored in a safe manner.          

Another property is the Air-Fuel Ratio.  The correct air-fuel ratio for complete combustion of a gasoline or diesel engine is about 14.7:1.  The stoichiometric air-fuel ratio for hydrogen is 34:1.  That is 2.3 times higher than gasoline.  This means that HHO “covers approximately 30% of the combustion chamber compared, compared to around 1-2% for gasoline” at ideal conditions (Dhariwal et al., 2018).  This enhances engine economics and emissions.

Overall, HHO offers unique benefits as an energy source because of its energy density, flammability, safety, molecular form, and is an environmentally friendly energy source.  Though it may not be reasonable as a sole fuel source for internal combustion engines, it does act as an excellent supplement and agent for petrol-based engines.          


Al-Rousan, A. A., (December 2010) Reduction of fuel consumption in gasoline engines by introducing HHO gas into intake manifold. International Journal of Hydrogen Energy, 35(23), 12930-12935. https://www.researchgate.net/publication/251580451_Reduction_of_fuel_consumption_in_gasoline_engines_by_introducing_HHO_gas_into_intake_manifold

College of the Desert. (December 2001) https://www1.eere.energy.gov/hydrogenandfuelcells/tech_validation/pdfs/fcm01r0.pdf

Dhariwal, A. K., Nayyar, A., Sharma, D.K. (2018). Utilization of HHO gas with Diesel fuel in stationary compression Ignition Engine. Skit Research Journal, 8(1), 52-59. https://submissions.ijskit.org/index.php/ijskit/article/download/88/75/

Madyira, D. M., Harding, W. G. (14-16 January, 2014). Effect of HHO on Four Stroke Petrol Engine Performance. 9th South African Conference on Computational and Applied Mechanics Somerset West. Department of Mechanical Engineering Science, Faculty of Engineering & the Built Environment, University of Johannesburg, Auckland Park 2006, Johannesburg. https://core.ac.uk/download/pdf/54204275.pdf

Musmar, Sa’ed & A.A, Al-Rousan. (2011). Effect of HHO gas on combustion emissions in gasoline engines. Fuel. 90. 3066-3070. 10.1016/j.fuel.2011.05.013.