Fumigation of Diesel Engines

This refers to the practice of allowing a small amount of combustible gas (LPG, CNG, natural gas, etc.) into the air intake of a diesel engine for the purpose of increasing engine output or otherwise improving performance.  In a sense, it’s like adding a nitro kit to a race car, giving a quick jolt of extra power when it’s needed.

It can be as simple as what some truck drivers have been doing for many decades now: keeping a BBQ gas bottle in the cab with them, with a rubber hose trailing under the hood and into the air intake.  When they need a little extra oomph to get up a steep grade, they crack open the valve just a bit.  There are also conversion kits available in a variety of price ranges that can be purchased on the internet, with all that that implies.

Proponents also claim that in addition to increased engine output, fumigation reduces pollutants in the exhaust.  The questions are these:  Does it work?  Does it harm the engine?  Is it cost-effective? And finally, Why don’t manufacturers already supply equipment designed to use this technique?

Does It Work?

Anecdotally, truckies that use fumigation insist that it works.  I can certainly see no reason why adding extra fuel to the charge in a combustion cylinder would not result in an increase in pressure and temperature.  It makes sense.  Higher cylinder pressure translates directly into greater force on the piston and therefore torque on the driveshaft.

It is also conceivable that higher combustion temperatures might result in the fuel being burned more completely, leading to lower hydrocarbon emissions and less soot particulates in the exhaust.   On the other hand, higher temperatures will also undoubtedly lead to more nitrogen oxides (NOx) pollutants in the exhaust.

Test reports I’ve read aren’t as clear as the theory, and show that it is a mixed bag.  Under some conditions, exhaust quality is improved across the board, while in others, it is unchanged or even worse.  Under ideal and controlled conditions, the hydrocarbon and particulates can be reduced with only a minimal increase in NOx.  But that result was mainly found on engines for which there was a lot of room for improvement to begin with. 

Does It Harm Engines?

Short answer, yes.  Yes, it most definitely does.  Higher torque and higher combustion temperatures means that pressure in the combustion chamber was, in a word, higher.  That places additional strain on the cylinder head, walls, piston connecting rod, bearings, crankshaft, and on all the bolts holding the engine together.  Diesel mechanics have reported to me seeing stretched head bolts, bent con rods, wiped and scored main bearings, snapped crankshafts, holes burned through pistons, and burnt valve seats.  In other words, over-fumigating a diesel engine will wreck it more thoroughly than almost anything else you could do on purpose. 

The tricky part is that it doesn’t always wreck the engine, and not always right away.  A diesel engine is designed to produce a certain amount of torque, and so its components are designed to withstand a certain maximum stress, plus a margin for safety.  Many older engines were over-built, to be on the safe side.  It didn’t matter if they ended up being much heavier as a result.  But newer engines are more optimized.  Besides being cleaner-burning with lower emissions, they are also lighter weight, which improves the fuel consumption and performance.  But it also means that components are exactly as strong as they need to be, and not much more.  Forcing the engine to produce more torque than it was designed for is really asking for trouble.

Is it cost-effective?

Truckies that use fumigation very moderately (so as not to blow up their engines) say that the gas (LPG, CNG etc) replaces some of the diesel fuel that the engine consumes.  And since gas is cheaper per MJ (energy unit) than diesel fuel, they end up saving a few dollars per trip.  I would point out that they are probably running older equipment that leaves a lot of room for improvement in the efficiency department.  In any case, I hope they are putting that money aside in an interest-bearing account for when they need to buy a new engine.   Knowing truckies, I’d say this is unlikely.

Why Don't Manufacturers Do This?

Since modern, advanced technology diesel engines are designed for optimum performance, efficiency and low emissions, they don’t need extra bits.  They are already fairly clean-burning, and are producing about as much torque as they can without blowing up.  To get more torque, an operator should select a larger engine.  After-market fumigation systems, aside from definitely voiding the warranty, will risk ruining a perfectly good engine.

The Crackpot Zone

"Do thou something stupid for Mine
amusement!"

Ignoring all of the above, some operators out there say, “What if you didn’t even need to BUY the gas?  What if you could generate the gas for free as you drive and save fuel, increase torque, etc etc?”  The idea is to use electrical current from the alternator, pass it through a water cell, and produce hydrogen gas that is then sucked into the air intake. 

One hustler actually told me that God told him to create this product.  Really?  God said that?  Well, I know God, and He’s a real kidder.  He was just winding you up, mate, having a go at you for being so ignorant about science!  He’s up there on His cloud laughing his beard off at you right now.

I did the calculations on this a couple of years ago, and followed it up by analysing test data from a prototype hydrogen gas generator. If your alternator produces 10 amps at 13.5 volts, and if your electrolysis cell is far, far more advanced than just a water bottle with a couple of wires stuck in it (most internet offerings are exactly this) with anything close to optimum gas production, then those 10 amps are going to produce something like 10 g/h of H₂ gas. 

THAT'S TEN GRAMS PER HOUR!!!

But most likely the “gas cell” will not be producing anything measureable.  Sure, you will see a few gas bubbles forming on the wires, but they will not represent anything close to the amount of gas needed to make any difference whatsoever.

A diesel engine consuming something like 12 liters of fuel per hour is using around 10 kg or 10,000 grams of fuel per hour.  Ten grams (or less!) of gas is not going to be even measurable in terms of engine output.  Any effect will be purely “placebo effect.”  Drivers will convince themselves that it’s making a difference because they don’t want to admit they’ve been scammed.  And to prove that they are not victims of a con, they will email me all kinds of anecdotal evidence that the idea works. 

Right.