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A Skeptical Look at HHO Generators

A friend of mine sent me a link to an interesting device, known as an HHO Generator. HHO stands for 'hybrid hydrogen oxygen'. It's basically an electrolysis unit that runs off your car's electrical system, and sends the hydrogen and oxygen into the engine's air intake. The company selling the device claims gas mileage improvements on the order of 30% to 90%!

I'm skeptical. This seems too much like a perpetual motion machine. For example, you can't hook up an electric motor to an electric generator, and then have the generator power the motor. Resistance in the wires and friction in the moving parts will rob energy from the system and dissipate it as heat into the environment. In fact, if you just had a flywheel of the same inertia on low friction bearings, it would spin longer.

I drew up two quick diagrams to illustrate what I'm getting at with this HHO generator. The first diagram below is a normal car - it burns gasoline to power the engine to turn the wheels. Below that is a car with the HHO setup.

Energy Flow in Conventional Car

Energy Flow in Car with HHO Generator

That extra loop resembles a perpetual motion machine too much. You're taking energy from the engine to split water, then trying to use that as fuel to turn the engine, and hoping to get more energy out than you're putting in. But remember, there's friction in the alternator and resistance in the wires running from the alternator to the HHO generator; when you run current through the water, only some of it goes into splitting the molecules while the rest heats up the water; and then there's also friction in the lines from the HHO generator to the intake. You're losing energy to heat in every step of that process.

Still, my friend bought that system and installed it in his car, and he insists that he's getting better gas mileage than before. So, I've tried to think of reasons why this may be the case. Here are my thoughts.

1) The hydrogen makes the combustion process more efficient, so that the engine converts more thermal energy into kinetic energy. This really seems pretty unlikely, though, especially without doing any additional modifications to the engine. First of all, the increase in efficiency would have to more than offset all the lost energy from the HHO generator. And internal combustion engines are already really efficient, especially modern engines with oxygen sensors and fuel injection that can tailor the air fuel ratio. And automotive companies are under pressure from government regulations (not to mention market forces) to make the engines as efficient as possible. Given the number of engineers working on these engines, and the amount of money manufacturers spend on development, I can't imagine that there's much room for improvement in efficiency.

2) He hasn't done enough tests. Fuel mileage is strongly dependent on driving style and other variables. A lead foot burns a whole lot more fuel than driving conservatively. Sitting at stops signs and traffic lights hurts fuel economy (even though the engine's at idle, you're getting zero miles per gallon during those times). A head wind will hurt you, while a tail wind helps. Properly inflated tires have a noticeable effect. I don't think a few tanks of gas driving around town is enough to smooth out all those variables. You either need to do some really controlled testing (an external fuel tank you can weigh on a closed course), or run thousands of miles with and without the HHO generator for comparison.

3) The hydrogen and oxygen are messing up the oxygen sensors. Engines are usually tuned to run at lower power at a stoichiometric air fuel ratio (AFR). This is when the amount of gasoline and oxygen are matched up perfectly, so there's no fuel left unburnt, and no free oxygen left. However, as the engine's power output increases, if it continued to operate at the stoichiometric AFR, it would burn hot enough to damage engine components. That's why the AFR needs to be enrichened - the extra fuel lowers the combustion temperature, keeping the engine from getting damaged. I've got some first hand experience with that - at work, we hired an 'expert' consultant to help us tune an engine, and he let the exhaust gas temperatures (EGTs) get up to 1750ºF (we usually tried to keep them below 1550º), and it literally burnt the ends off of the spark plugs.

The other problem with running too lean is that the engine could start knocking (when the fuel air mixture explodes instead of burning smoothly). That's another reason the AFR gets enrichened. I've got some first hand experience with that, too. We were tuning the engine another time (at a different shop), and we had a laptop hooked up to the engine computer that gave us real time feedback on all the variables the engine computer was monitoring. We kept advancing the timing (another variable that strongly influences knocking) to try to get the engine operating as efficiently as possible. The guy operating the dyno had run plenty of engines, so he had a good ear for it. The laptop was telling us that the engine was sensing knocking, but the guy running the dyno couldn't hear it, so we figured it was a false signal. After a few more dyno runs, we basically destroyed the engine. When we took it apart and inspected it, it had all the signs of knocking. The moral being - your ear isn't sensitive enough to reliably detect knocking at levels that are still high enough to damage your engine.

If the hydrogen and oxygen are messing up the oxygen sensors, it may be tricking the computer into running the engine leaner. This would improve fuel economy, but at the cost of higher EGTs and increased chance of knocking - both of which will reduce the life of your engine. Unfortunately, there's no way of telling on a more or less stock system. Block temperature is not a reliable indicator of exhaust temperature, because there's plenty of capacity in the cooling system to keep the block temperature low enough. And knocking isn't something you can always catch by ear.

My gut feel is that it's probably option two above. I think that with more testing, my friend will find that the HHO generator is actually hurting his gas mileage. If it turns out to be option three, though, it could be causing some serious damage to the engine. I recommended to him that he at least pull the plugs periodically to see what they look like, and that it might not be a bad idea to invest in some EGT probes and a knock sensor, either.

Anyway, after a little bit of research, I did find a few other sites discussing this (the first link below is the best). It looks like my second option above is the most likely.

It looks like maybe there could be something to these HHO generators in an engine specifically designed for them, but nowhere near as much as many of the scam artists are claiming. Plus it's analagous to octane. High octane fuel doesn't explode as easily, so some of those things that cause knocking (advanced timing & leaner mixture which I already discussed, plus higher compression ratios which I didn't mention) can be pushed harder if you have high octane fuel. So, if you have an engine designed to take advantage of high octane fuel, you can get better efficiencies. But, if you simply run high octane fuel in an engine designed for low octanes, you won't see any difference. Some of the stuff I've seen for hydrogen says that it might allow you to run leaner than with pure gasoline, but your engine and sensors would have to be designed accordingly. Simply pumping it into a stock engine wouldn't give you those benefits.


In order to even be effective (in an engine designed for it), an HHO device would need to produce enough hydrogen to replace approximately 2% of the airflow. To replace just 1%

of the airflow into a 1.8 L engine running at 2400 rpm would require the electrolysis of about two teapoons of water every minute, which would require an input power of about 5,200 watts (7 horse power). For a 12 Volt electrical system, this requires about 430 Amps. The wire you would need to carry this current is about as thick as your index finger, not counting the insulation. Also, the heat loss due to inefficiencies in the electrolysis would cause the water in the HHO device to completely boil out within a handful of minutes. At this rate, 1 quart of water would last only 90 minutes, assuming you could prevent it from boiling away.

Sometimes your own belief in your skepticism produces the optimal results you are seeking. Good job.
Be a little more practical. A scientist is not a skeptic but an explorer and a lover of the possibilities of the unknown.
Not wise to let your belief form your desired result when working on discovery. It's a little bit like being a prosecuting attorney isn't it not? So why are prosecuting something that has been demonstrated to work so beautifully when done correctly. Open your mind son.


I disagree that scientists aren't skeptics. They must be skeptics. It's the only way that they can keep from fooling themselves. Just take a look at this list of congitive biases on Wikipedia. Nobody is immune to these biases, which is why it's so important to follow appropriate protocols. That's why blind experiments were invented.

For gas mileage, there are so many factors that could affect it, that it's especially important to perform a well controlled experiment. Simply gasing up and driving around town is not reliable, especially considering that a person might unconsiously change their driving habits when they've installed a device that supposedly improves gas mileage.

I agree that it's not wise to let preconceived notions bias results of an experiment, but there are some things we know about the universe well enough that we can use them to inform us. The laws of thermodynamics are one of those areas. We know that you can't get more energy out of something than what you put into it. In fact, due to friction and other losses we know that you can't even break even. So, when a salesman claims that a device can improve gas mileage by robbing electrical energy from the alternator, we ought to be very skeptical of those claims.

I write blog entries like this because I don't like seeing people get taken advantage of by hucksters and snake oil salesmen, wasting their money on products that don't work.

The HHO gas is supplied to the engine via intake manifold or carb. The gas then helps your gasoline burn more efficiently, while producing its own combustion.

Error On your account

Subject # 3
AFR -- stoichiometric is 14.7:1 which means 14.7 parts air to one part fuel. Its a ratio. While stoichiometric of gasoline is different then that of alcohol being almost 9:1

It is the point of which the fuel being burned at its most effiecient point.

HH0 is extremely volatile while alcohol is a lazy burn. thus the stoichiometric point will be raised. to more parts air to one part fuel.

It does not trick the sensors. It is still fuel being burned. Your computer is programed to run a specific stoichiometric at idle and while your in a preset open loop configuration. Closed loop will monitor and try to maintain preset values in the computer to manage fuel injector pulsing. Which feeds fuel.

Your computer will monitor many values at once and make changes up to a certain percentage. The percentage of the Subaru computer from my experienced tuning by default is 15%.. Which I can venture to say most others will have a value around the same given you are running fuel injection.

Other values take place in this measurement including the mass air for sensor or (MAF) and the Intake Air Temperature (IAT) combined with the Signal. (Voltage) off the primary and secondary 02 Sensors.

What is happening here exactly is your introducing a fuel source in between the (MAF+IAT). Your adding Fuel and very slightly cooling the intake air temp. Not A whole lot but a margin measurable.

Your computer will see the air traveling through the Air flow sensor and it will have a measurement of xx. lets just say 14.7 for example naturally at an idle your computer will try to match that to 1 part fuel for every 14.7 parts of air.

Your 02 Sensor sees ok i have 1 part fuel to 14.7 parts air, but wait a second I didn't add that fuel, so basically back off the Injector pulsing. "SAVING FUEL"

Now to go even deeper into the controversy. You can only back it off by default of 15% therefor only gaining you 15% in fuel economy. This Number can be increased with a tuning device to whatever you'd like.. but enable to go further you have to produce the HHO quick enough.

Flex Fuel vehicles have systems that when running a ethanol based fuel will require a richer mixture then the 14.7 for maximum efficiency. therefor fuel economy is worse then running gasoline. Using HHO for a fuel source given that it burns so quickly. you can use less of it. raising the stoichiometric point thus raising fuel economy given that your computer will back off the duty cycle on the injectors.

Not all cars will provide the same savings in mileage since all cars have different parameters in the tuning. with the right tuning knowledge, and the right amount of HHO generation. its easily a gas saver.

Now im really going to blow your mind. These systems use electrical current. Providing you use Alternator = greater draw on motor = less MPG

or Multiple Battery's = Weight = cost outweighs savings "replacement" = less MPG plus the added space.

Best savings, would be to use a Home HHO gerating device and compress them into cylinders to feed into Intake Track. = Higher initial Price but greater savings long run. = less Weight. Some may worry about well what about if I crash will they explode.. the answer is no. its not really explosive if you shoot at it or crush it. BUT IT WILL EXPLODE IF THERE IS FIRE. Also a lot of people say DONT COMPRESS HHO.. Its been safely done staying under 350psi "currently testing higher". Safety is still a concern with all the idiots out there. Use at your own risk. Test and Test And Re-Test again to ensure your own safety. Don't go dumping a ton of HHO in your motor if you value your Intake Track. You will pop it right off in pieces trying to shove hydroxy gas in there. Try filling a ballon attaching it to a broom handle and set it on top of a candle. OUTDOORS preferably around july 4th. or oxygen and acetalyne mixture. compare the two then step back and think about the power behind Water.

The proper way to evaluate HHO is to run an engine on a dynamometer test stand at a controlled speed and load. Fuel consumption is measured by taking the weight of a fuel tank with a quick disconnect fitting. Weight is taken before and after a timed test run. The difference equals the weight of fuel consumed. A three-way valve is used to switch the feed from a supply tank to the weigh tank and back again. That marks the beginning and end of the test run. This is all standard practice in any engine lab to measure what is known as brake specific fuel consumption (BSFC). The measurement is repeated one or more times at different HHO gas flow rates. This way, precise, accurate measurements are made under highly controlled conditions. This sort of evaluation has been done a number of times. This study reviews 8 such evaluations. Average and maximum yield values are calculated for each study. This yield value is an attempt to rate the perfomance of the HHO injection with a single number. The results of these studies differed widely. The best results were obtained by injecting a very small amount of HHO. This data and other results suggest that HHO acts as a sort of catalyst. Somehow energy may be diverted from waste streams to useful output which would not violate the laws of physics. In fact, that is the whole purpose of optimizing engine efficiency.

Sorry, forgot to insert URL. Click here for report.

OK, click my name for report?

Charles - I fixed the link in your second comment (though I don't know when I'll have time to actually read it).

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