HHO-only Diesel car
 

After what RedDevil said about HHO (see above), it got me thinking: if it's so explosive, even at relatively low compression, wouldn't it be a good fuel for Diesel vehicles (again, only for city use, not for long-distance driving). Like hydrogen, it's a completely clean fuel, so I'd like to use it for this.

At present, it's mostly tested when mixed with a fossil fuel (gasoline or Diesel), but a quick search revealed that some people have indeed used it as a single fuel (not mixed).

Range is going to be far worse than with hydrogen (since that's compressed), and an HHO electrolyser would need to be fed by a battery (and water tank).
So, like battery-electric vehicles, it would be limited to what energy you can store in the batteries, and conversion efficiency is even far worse (less than half of battery-electric vehicles since we'll be using an internal combustion engine). The upside though is that the conversion would not cost much and is easy to do (if you already have a Diesel engine-powered vehicle). Also, this conversion can be done with Diesel-engine vehicles, whereas hydrogen conversions would only be possible with gasoline-powered vehicles.

So, anyone here at ecomodder knows whether this will work, and does anyone also know the correct air/fuel mixture to use for HHO(-only) ?

LOL running an engine off uncompressed HHO as the sole fuel!!!

Range: 0.3 miles.

How did you calculate the 0,3 miles ?

Also, I do mean to use an on-demand system, so the idea isn't to store the HHO in a tank (uncompressed). Rather the energy comes from a battery that generates uncompressed HHO (on-demand) from a water tank -via an HHO electrolyser obviously-.

So, if you use a same battery as you would use in an electric car, you'd get about half the range of the electric car (well that's the theory at least, in practice, the voltages of those electric car battery packs will be higher than what you need for the electrolyser, so you'll need to use a different battery). But still, I do think you'll get acceptable range (at least for city use).

But to be sure, we'd have to calculate it out.
From hydrogen I know the energy in it is 0,003 kWh/l @1 bar (so uncompressed)
From Brown's gas, I have no gasoline gallon equivalent (kWh/l) information, so I can't calculate it.

Also, for the air/fuel ratio, I'm equally lost. Hydrogen has a ratio of 34.3:1

I'm assuming the air/fuel ratio needed for Brown's gas should be even higher, as I'm assuming it's much more energetic (I once heard it was 4x more energetic). This would then mean you could drive further with it (if you inject it in this proper ratio).

How do you calculate the amount of liters of fuel you need btw per liter of engine (so say for a 1 l engine, or hence a 4 cylinder, 0,25l per cylinder), using a known air/fuel ratio (say 14.7:1 for gasoline) ?

According to this site,
hydrogen has 50,000 Btu's per pound whereas Brown's gas has about 66,000 Btus per pound. So, that means it would (presumably) be 32% more energetic
(66 000 - 50 000 = 16 000 / 50 000 = 0,32 x 100 = 32 )

If so, than it would have 0,00396 kWh/l
(0,003 kWh for hydrogen x 1,32 = 0,00396 )

The same site also mentioned that you could generate 340 litres of Brown's gas (13.6 cu. ft.) per kilowatt-hour. If so, than the amount of energy you could generate per 1 kWh battery would be 1,3464 kWh
(340l x 0,00396 kWh/l) so that would be even more energy than the energy in the battery itself.

This doesn't seem right. I guess there are probably some incorrect figures here, so let's calculate it out another way.

At wikipedia we read at the Brown's gas page:
"Theoretically, a ratio of 2:1 hydrogen/oxygen is enough to achieve maximum efficiency; in practice a ratio 4:1 or 5:1 is needed to avoid an oxidizing flame."

So if we consider that oxygen does not add any energy, and only the hydrogen in the HHO adds energy, then HHO could be
- 50% less energetic then hydrogen (2:1 H2-02 ratio)
- 25% or 20% less energetic then hydrogen (4:1 or 5:1 H2-02 ratio)

Depending on this, it could have:
- 0,0015 kWh/l
(0,003 kWh for hydrogen x 0,5 = 0,0015 )
- 0,00225 kWh/l or 0,0024 kWh/l
(0,003 x 0,75 = 00225 ; 0,003 x 0,8 = 0,0024 )

Regarding the amount of energy you could generate with it:
I could not find any data on the efficiency of HHO electrolysers, but there is data of the efficiency of hydrogen electrolysers (which granted, work at a different way). But still, it gives an idea. Anyway, PEM electrolysers are 94% efficient, alkaline electrolysers are 43-67% efficient. So let's say the HHo electrolysers are 50% efficient. Then how much liter of gas could they generate ?

Calculation:
A 1 kWh battery could then generate 0,5 kWh on HHO gas.
The HHO gas has an energy of (and now we use a range of found data above): 0,00396 kWh/l or 0,0015 kWh/l or 0,00225 kWh/l or 0,0024 kWh/l
So:
0,5 kWh /0,00396 kWh/l = 126 liter
0,5 kWh /0,0015 kWh/l = 333 liter
0,5 kWh /0,00225 kWh/l = 222 liter
0,5 kWh /0,0024 kWh/l = 208 liter

https://media.tenor.com/images/b228c...64bc/tenor.gif

Look, this is the last I'm posting in one of these threads because they become worse and worse as they go on. This probably belongs in the unicorn corral for math failures being interpreted as over unity garbage.

A 2L diesel going down the highway at 60 mph is turning 2000 rpms. This imaginary diesel is drawing 2L into itself every other revolution. 16.7 times/sec. So 33.3L/sec. For simplicity I'm going to say 2:1 air to your atmospheric pressure tank. So about 10L/sec from your 250L tank is 25 seconds. Or 0.4ish miles traveled.

Or generating it from an onboard battery? So you're going to take a 1kw battery, use it at 50% efficiency to make this gas, then burn that gas 30% efficiency in an engine. Thereby getting 167wh of motive force from it and adding a 9ft^2 tank "somewhere" in the car.

You could literally get more than twice this improvement in FE by throwing an extra (small) deep cycle battery in your car and disabling the alternator for a while.

None of this even approaches making basic, logical sense enough for it to even be a valid thought experiment.

Don't bash the fellow. He is going through the natural thaught process as we all have.

When the hype first came around we all said "cool, hydrogen power" then did the calculations to discover that it was not even remotely possible.


He is a little bit late in the game an generally anyone who is capable of crunching the simplest numbers now knows how inefficient hydrogen generation is. Combined with the fact that people have been trying to push hydrogen boosters and other nonsense to the market people are generally fed up with topics related to hydrogen.

Since you derived the idea from my quote I feel entitled to comment, with trepidation.

That statement there is bunk; it is wrong at several levels. That whole website is complete nonsense. And here is why:
It does not matter if the oxygen comes from the air or is contained within the gas itself when it comes to calculating BTU's. Oxygen is just the reactant that oxydates (!) the hydrogen. Only the hydrogen counts as fuel.

Hydrogen has an atomic weight of about 1 unit per atom. Hydrogen gas consists of molecules made up with 2 hydrogen atoms, weighing 2 units per molecule.
Oxygen has an atomic weight of about 16. A molecule of oxygen is 16 times as heavy as a hydrogen molecule; as both are perfect gases, per volume oxygen is 16 times as heavy as hydrogen gas.

Browns gas is a mixture of 2 molecules of hydrogen for every molecule of oxygen. Therefore it is 2/3 + 16/3 = 6 times as heavy as hydrogen, and it only has 2/3 of the hydrogen molecules.

So Browns gas has just 1/9th or 11.111% of the BTU pure hydrogen gas has per weight, and just 2/3ds or 66.666% of the BTU per volume.

So while Brown's gas is less powerful as a combustible than you think, the real problem is that, in essence, it is not a combustible at all. It is an explosive, and as an explosive it has no equals when it comes to bang per weight.
When it explodes it will not complete the reaction; the temperature gets so high that some of the hydrogen and oxygen start to lose their bonds again, and the reaction will only complete when it can shed its heat or mix with the surrounding gases.
Hence the tendency of these mixtures to create a whistling sound when exploding; the gas blows out only partly fused to water molecules, creates a vacuum that though extremely hot, still sucks back in the gas, which lost some heat on the way out and back in, reacts again; blows out, new vacuum, etc.

It may seem strange that such an explosive gas contains only so much power. But a simple gasoline engine uses 14 times the weight of the gasoline it uses on air. That is a lot of air for a little bit of gasoline.
Compared to Browns gas, you exchange the non-combustible oxygen atoms with the same amount of carbon atoms, who are very much combustible - and lighter than oxygen to boot.

Now you wanted to use it as the sole fuel in a diesel engine.
Well, diesel engines ignite by pressurizing the air-diesel mixture to beyond the detonation point. But it won't fire all in one go; it burns slowly enough to spread the bang so it won't knock too hard. Because diesel is a mixture of long and short hydrocarbons it will not all react at once.
If you would get the detonation point right with Brown's gas (if that is possible!) then the reaction would complete or reach its plasma phase within a fraction of a millisecond. While diesel engines are sturdy, they may not be able to withstand that; and if they do, it would be very noisy indeed.

You'd need to mix in at least 3 times as much air than Brown's gas or you'd just be wasting the excess because it could not react completely. But the uncontrollable detonation remains problematic. The largest auto manufacturers break their heads trying to get the diesel-style combustion working for gasoline properly. Pure hydrogen, whether mixed with air or pure oxygen, would only be even harder.

So, it is not like your idea is sound with just a few problems to solve.
It is fundamentally wrong in all of its stages.
You should be aware that hydrogen and Brown's gas have already been researched to the bone. You cannot expect surprises there within existing fields of technology.
You certainly cannot expect to get anything useful without understanding the basics - but if you get to grips with that you'd abandon the project.
Better abandon it now, when you've not yet chanced to kill yourself nor wasted too much money.

Trouble is he does not listen, or at least it looks like he does not want to hear. Instead of that he simply makes another thread with similar content.

I understand and I don't want to hurt his feelings unnecessarily, but I felt obliged to fast-track the process.
Because a 250 liter tank of Browns gas under atmospheric pressure is more deadly than a hand grenade or a small stick of dynamite.

My chemistry teacher ignited a layer of Browns gas foam on a dish washing tile.
Water, soap and maybe one liter of gas filled foam... What could possibly go wrong?
Nothing went wrong.
But no less than 10 teachers from the neighboring classes came storming in to check everyone was all right, and we were impressed with the strength of the classroom windows; none of them broke!

I once witnessed a huge lightning strike in a flagpole less than 10 meters away. I'm unsure whether the bang was harder or not.

Agreed, this is where I also was about 20 years ago.

Use Steam instead, it's safer.

https://www.youtube.com/watch?v=Lb7UAnvIT80

(entertainment value) or a Hydrogen Truck

Yes, I was worried about that too.
I dropped the concept and won't work it out, but just for the hell of it, I'll calculate it out further in this thread.

I indeed don't listen to idle comments. I only listen to comments that are backed up by hard data. So, if you reply to me and include calculations to back up your thoughts, I'll listen.

I've been working on related issues since many years. I never did the calculations on hydrogen though since I also always considered it a lost cause. However, given that it is a completely clean fuel (and little other transport technologies are, -and if they are, they're expensive, ie batteries/electric motors-), I'm reconsidering it, and trying to make it at least better then the current way of using hydrogen.

Batteries and electric motors are the improvement over using a fuel burning engine.

Ok, now some more number crunching:
We determined that HHO can have a energetic value between 0,0015 kWh and 0,00225 kWh, whereas hydrogen has an energetic value of 0,003.
RedDevil mentioned that HHO has 2/3 (66%) of the energy in hydrogen, so 0,00198 kWh. This is in line with this data.
That said, we won't need this data now as we'll calculate the range another way.

The range when using HHO in a Diesel engine:
A Diesel engine (running on Diesel) can have a fuel consumption of say 6l/100km.
Diesel has an energetic value of 37,95 kWh/gallon = 10 kWh/l
So it uses 60 kWh/100km or hence 0,6 kWh/km

A 1 kWh battery can generate (with a 50% efficient HHO electrolyser) 0,5 kWh on HHO gas
Using this in Diesel engine, you can hence cover: 0,83 km (0,5/0,6)

So if you would say have a 50 kWh battery, you'd be able to cover 41,66 km.
I don't think that's bad really. Seems practical if you consider just this data.
But obviously, if the energy isn't transferred suitably in the engine and blows up your engine instead, it's useless.

Next for the amount of gas the electrolyser would need to be able to supply to the engine per minute:
We assume using a diesel engine that consumes 6l/100 km and which drives at a speed of 100 km/h.
6l -> in 1 hour (=60 minutes)
6l -> 0,1 l/min (6/60)
Diesel has an energy content of 10 kWh/l, so 0,1 l = 1 kWh
So we need to supply it 0,1l/min
Those HHO electrolysers can generate up to 10-15 lpm, so that's more than plenty.

O.K, guys. This smallscaleH2 nick must be alternative account of Frank Lee, and now he is loughing madly about how he got us. There is no other explanation.

I would just use the 50kwh battery to power electric motors and go 100 to 200 miles on a charge, with the reliability and repeatability of electronic motors.

Let's do a comparison, as it touches one of the cores of the problem.

A Hyundai Ioniq electric has a 28 kWh battery and a range of over 200 km.
So it has 5 times the range from less than 3/5 of the capacity.
The running costs on electricity alone would be just 12% of your setup.
In other words, yours is 8 times as expensive to run.

If you have a battery the best way to transfer the power within to motion is by using an electric motor and motor controller. The efficiency of that is typically way above 50%.

And it would be fun.
I've driven all kinds of vehicles on gas, diesel, hybrid and electric. Of those the EVs were by far the best when it comes to comfort and power delivery (no gear changes, instant delivery, constant torque, very fast acceleration).

The thought you would have a big battery on board and just use that to make a diesel engine tick over goes against logic. Like if in the time of the first real cars you'd build a mechanical horse to pull carts instead.

Hey Man, I don't resemble that remark! Well, OK.... maybe I do.

Notice how so far I've not touched this with a ten foot pole?

I know, but I think you surely would appreciate absurdity of these last few posts.

This whole thread is comedy gold!

I can't believe I just read through it.

Seriously, what did you expect when the title had HHO in it?


That is like opening an email that says "gain 12 inches with these miracle pills" and the saying I cant believe it was not true.

I know a guy who flies a sled through the air, pulled by 8 unicorns, yelling "HHO HHO HHO"!
He claims he uses no fuel at all.

The Nissan Leaf, with 30 kW-h of energy aboard, already covers more than twice the 50m range of the 50 kW-h battery in the HHO concept you're wildly flailing at, and it avoids all the BS of the HHO stuff. Wouldn't that be the better choice? Skip a bunch of snake oil steps, no snake oil squeezing hardware aboard your vehicle, go farther? I think that's the way to go.

That was genius

You're looking at it from an energy-efficiency perspective.
This project idea was never about energy-efficiency.
Instead, I was looking at it considering these design criteria:
* the fuel needs to be usable in a Diesel engine
* the fuel should (when burned) not emit any carbon or toxic gases
* conversion needs to be relatively cheap to do

Regarding the cost, I don't agree. I agree the fuel costs would be higher, but if you look at the whole thing (so including conversion costs), it would be cheaper -electric motor, battery and motor controller tend to cost a lot-.

I did know about the negative perspective most people have on HHO. What I did never read anywhere however were scientifically based arguments (so including energy calculations, ...). That's why I made the post and did the calculations. What I actually found was that it was a much better fuel than I initially thought. Seifrob and ar5boosted mentioned alternatives like compressed air and steam I could use. Well, compared to that I think HHO is a better energy carrier (if it wasn't for the damage it inflicts on the engine, but for say overbuild Wankels -found in construction equipment-, this might not be the case)

Anyway, it doesn't matter any more as despite meeting my initial criteria, it does damage regular Diesel engines. So it's useless.

A more interesting question to ask now however is: is hydrogen usable in Diesel engines (so not gasoline engines, but Diesel engines) ? If not, what's the reason why it doesn't work in them, and are there any workarounds to fix that problem (like using fuel or combustion chamber pre-heating, adding of a spark-plug inside the combustion chamber, ...) ?
If I find that I can just use hydrogen in Diesel engines, then that would even be better than if I were able to use HHO.

in modern diesel engines, the compression does not ignite the diesel that is in the cylinders. Rather the cylinder compresses and heats up the air and diesel that is injected in to the cylinder combusts because it enters a very hot environment.

Diesel engines achieve this by compressing the diesel to 1000-2000 bars. So you would also have to compress your hydrogen to similar pressures.

As far as i know the old tech uses a pre combustion chamber ehere the diesel is ignited and sucked in to the chlinder while still burning. I have a feeling that your hydrogen would very rapidly all combust and not make it inside the cylinder.


I do not think that you can get the timing right if you just let the engine suck in hydrogen and have the compression ignite it.

I am no expert but you may have to use a spark.

HHO seems like it was used to supplement Diesel and extend the range of British Tanks in WWII. So it does work alongside diesel fuel.

However, HHO all by itself when burnt gives you a metal cutting torch.

https://i.ytimg.com/vi/nATpzINdn4g/hqdefault.jpg

When burnt / ignited / oxidised whatever it will melt the Iron in the engine.

How do you propose getting around that ?

But, more seriously, talking about these things in a theoretical sense won't get you anywhere. HHO has been proven to be in the same category of fuels as Hydrogen-Peroxide. Highly unstable and dangerous.

Sure there may be ways of making it work, but you haven't proposed any yet that haven't been already tried by mainstream Science, Corporations or talented hackers.

If you want a Hydrogen car, you can buy one today:

- https://ssl.toyota.com/mirai/fcv.html

Just $57,500 MSRP

Most people would be skeptical if you can make something better than what Toyota have with their multi-billion investments in the field.

You describe HHO as a good fuel - what makes it good, then, if not its fuel efficiency?
Emissions maybe - but you start off with electric power, and from that point on a regular EV has no emissions at all. Diesel engines typically run lean under light loads, and you need to do the same (e.g. mix in air) to have a chance to make it work. But then you'd produce nitrous oxides - unlike the EV.
With efficiency and the environment out of the room, practicality and safety never in view, what remains?
Hey, there is a battery in your setup too. You actually need a big one to get anywhere.
If we just look at the differences:
EV conversion: + motor, motor controller;
HHO-Diesel: + Diesel engine, cooling system, tranny, exhaust system, electrolyser, power regulator...
I'm not sure your setup would be cheaper. (that's an euphemism)
You could buy a nice used Leaf for the cost of a 50 kWh battery alone.
Yes, definitely. But you might find that a very light fuel like hydrogen works better in a gasoline engine than a diesel engine. Lower compression ratio to prevent detonation, spark ignition, throttle valve - all gasoline engine territory.
Still, if it has to be a diesel engine for whatever reason, chances to get it to work at all are higher with hydrogen than when using Brown's gas.

Have you seen this video HaroldinCR found for you? (permalink) This one:
https://www.youtube.com/watch?v=hbvwvZtCeIU

I will not discuss it it is fake or not, let's take it as canon.
- Now, when you know that it can be done (leave aside that we do not see the car in action). From the youtube video we can figure:
- whole boot of that corvette is filled with hydrogen tanks
- he used gasoline engine, not diesel
- the tanks give around 400 miles range (as he claims)
- to fill these tanks it takes industry-grade electrolyser overnight, and the process cannot be sped up significantly
- you need a hydride (Lithium-6 deuteride) to store hydrogen in safely under pressure around 7 bar (pressure in waterline)
- it is neither easy, nor cheap (and surely not "for free")

Now it depends what do you want to do.
- Personally use eco friendly car? Than sell yours and buy (second hand) electric car. You use electricity anyway and conversion rate in electromobiles is way better

- build yourself eco friendly car? Do an EV conversion, there are plenty of examples here on Ecomodder, even Top Gear guys made one ;-), (see here) and here .

- build yourself hydrogen powered car, just for sake of doing it?, OK, but be prepared it takes much higher skills in theory of combustion and motor engineering, and do not try to reinvent wheel. If I would be in your boots, I would firstly search every successful attempt in literature (printed, as there is less chance to let print false positives. See your local university library). As getting hydride is out of question for backyard engineer, CNG conversion kits would be a good start, study them, see how they do it and why (no one here discussed preheating gas after expansion, for instance). But do not expect to come with easy, safe and cheap solution. There is none. As I already told you, cheapest LPG conversion kit costs around € 800 in my country, CNG conversion kits around € 1400 and they will be more similar to your case due pressures used. So that is your base price. And remember that CNG tanks do not suffer hydrogene embrittlement you need to overcome. I could continue, but I do not see a reason why.

You can make it done, but seems to be extremly difficult, close to impossible, and nowadays, with current state of the technology it simply is not practical to do it. Electromobiles seem to be much better alternative.

- was this enough data for you?
here you acused me to make idle comments. Sometimes it takes more complicated calculation that working with fractions, trust me. My time is too valuable to type all equations on my smartphone. Where I do back-of-the-envelope calculation, I announce it. Where i cite other sources, I announce it. We collectively provided evidence of others people work and these also did the actual number crunching. You came for answers, we provided them. But, do you really listen to what we provided? Than, how can you be so stubborn?
- did you just google HHO powered car before you entered this forum ? Isn't it weird that serious links (BBC, CNN, popular mechanics etc.) only list topics that it cannot be done? yeah, there is world-wide conspiracy for sure.

I am not going to visit this thread again.
howgh

There was a guy that blew him self to bits compressing HHO a few years with an air compressor I will assume he was compressing it to no more than 150psi.
A diesel runs adabadic compression of 700psi minimum.
How's that going to work?

I don't get how this is still a discussion.

https://en.wikipedia.org/wiki/Energy_density

One kilogram of diesel has 13 KWh of energy.

One kilogram of hydrogen contains approximately 39 KWh of energy.

Buuuut... even with perfect electrolysis, you are consuming that *same*39 KWh to produce that one kilogram of hydrogen. More practically, at least double that, due to inefficiency.

https://en.wikipedia.org/wiki/Electrolysis_of_water

Remember, oxygen doesn't count. It doesn't provide power. And most of the weight of water is oxygen. Only a little over 1/10th of your water is hydrogen, by weight. Which means you would have to carry four times as much water as diesel for this, and an expensive EV battery pack, as on the Leaf, would not be enough to electrolyze more than a few liters of water.

In other words, just stick to an electric motor. Cheaper, safer, more efficient.

I like how his last calculation equated 1L of diesel fuel to 1L of uncompressed gaseous HHO.

ROFL!

Thread over.

He did say the calculations seemed wrong.

Didn't seem to realize why.

This is the problem with mixing volume units for liquids and gases.

And why we shold buy, sell and measure our liquid fuels in kilograms (or pounds).

Makes things so much easier to figure out.

One kilogram of uncompressed Brown's gas does not fit in the car, even if you'd utilize the entire passenger space. And that's a good thing. Any volume that's over a gram worth is dangerous.

I'm not sure where samwichse got the idea from that I compared diesel fuel to 1L of uncompressed gaseous HHO.
I only said that "I assumed you'd get about half the range of an electric car", given that you have high efficiency losses when generating HHo gas from a battery.
Also, I don't get why some of you seem to think that I need to store this amount of (uncompressed) gas in a single go onboard the vehicle. I wouldn't be using tanks remember, I would be generating it via an on-demand system (electrolyser). So I don't need to store much HHO gas at all.

Yes. I came to the same conclusion.
See https://ipnpr.jpl.nasa.gov/progress_...42-27/27BB.PDF

I think the best option here is just to mix plain hydrogen (not HHO) with a regular fuel in the combustion chamber. That doesn't clean up the exhaust emissions completely, but at least partially. So it would be an improvement, and I don't need to install a spark plug.
See here, here and here

Oh right, one more thing:

With a conversion, I mean you would have an existing (working) Diesel vehicle you'd convert. So you don't need to buy a new Diesel engine, exhaust system, ... then.

You'd still need the battery, that was my point there. And a big one too.
In effect you need a very big battery and still have way less range than the cheapest Leaf you could buy.
And you'd weigh down the car considerably. Ordinary cars would already be beyond their load capacity from that battery alone. There would not be much space left to load anyting either.
You'd have a chance if you replace the heavy engine and adjacent systems with a motor and controller, and of course you'd get a better range.

You choose diesel as a way of cutting cost, but the rest of the setup will annihilate any gain you could get there. With safety and practicality already gone, cost is no longer relevant either.
This goes nowhere.

Agreed. The only benefit I initially saw regarding the battery was the voltage being lower with this (say 12 to 24 volt rather than 600 V for the Leaf electric motor -and mass-produced 12V batteries tend to be cheaper than high-voltage speciality batteries-). But given that I need so much batteries anyway, cost wouldn't be lower since if I wire the batteries in series, I can increase voltage considerably anyway.

Doesn't matter any more though. I shifted the approach to using hydrogen injection instead.

Build your HHO system and have it powered by an external battery.

Inject it with the fuel, see how much of a benefit you get.

Maybe you can slowly train your car to accept only hho.

Yes, HHO injection (and not HHO-only use) could also reduce the air pollution caused by the vehicle, and commercial HHO injection kits exist already. It's a possible alternative to the hydrogen injection I first proposed (and should be a bit cheaper), although it's somewhat less safe.

Do any other methods exist to reduce the air pollution (I'm not talking about reducing emissions, I really mean reducing the air pollution) ? What I've been thinking about is a sprayer that creates a water mist over the tailpipe outlet so that (a fair portion of) the exhaust gases are washed out. This idea is similar to this. The reason I mention to spray it over the outlet (rather than have it injected in the outlet is that this way, the outlet flow isn't reduced that much, so doesn't reduce performance of the engine. It won't work perfectly (there will still be some air pollution, even when hydrogen injection is also done) and it won't be able to reduce emissions either, but it should improve things compared to not doing it at all.

Plain rainwater could be used (rather than tap water). The only thing I'm not sure of is how much water is needed (per 1h of running) to achieve this, and what sprayer I best use ?