The Chemistry of Fuel: Petrol vs Diesel #3

Diesel ignites in your motor by compression ignition, and petrol motors ignite by a spark.

This obviously means that diesel motors must run at higher compression than petrol motors.  One implication of this is that diesel motors require stronger batteries to start, as they must do more work in compressing the air in the cylinders.

Traditionally, diesel motors are slower revving and produce their power low in the rev range, whereas petrol motors are higher revving and produce their power higher in the rev  range.  This resulted in diesels only being used in vehicles where a slow revving, torquey motor was an asset, like trucks, trains, and large 4WDs.

In recent times however, the Europeans – notably the Germans – have led the way with revolutionising the diesel motor.  Foremost among these is Audi which it is essentially now unbeatable in the Le Mans 24 hour endurance race.

This revolution in design has resulted in two things.

Turbochargers, although not a new concept, are relatively new on diesel motors.  What they do is to provide high pressure air to the motor, thereby providing an abundance of oxygen to allow the motor to run efficiently at higher revs.

Secondly, and more importantly, common rail fuel injection technology has been developed.  Essentially, this is where the diesel is injected under much higher pressure than the older diesels – up to 20 or 30,000 PSI. The result of this is very fine atomisation of the fuel.  So when it ignites the size of the droplet that must burn is very small and generally is able to burn with high efficiency.

The implication of this is very little black smoke from unburnt fuel, and no smell of diesel.

So modern diesels – turbocharged common rail designs – have an efficiency that the older diesel engines could only dream of.

So now these cars produce lots of mid range torque, and they are able to rev well because of the turbocharger, and they produce extraordinary fuel efficiency.

Let’s now will do a comparison of diesel versus petrol in the same car, let’s say a Hyundai i30.




Engine Size 1600cc 2000cc
Max Torque 255Nm @ 1900rpm 186Nm @4600rpm
Max Power 85kW @ 4000rpm 105kW @ 6000rpm
Fuel Consumption 6.0L/100km 7.6L/100km

So here are the important numbers in the table.  The diesel motor is smaller, uses less fuel, and produces its maximum torque (255Nm) almost off idle at 1900 rpm.  The petrol motor doesn’t produce its maximum torque (which isn’t as much anyway) until the motor is screaming at 4000 rpm.

Anyone who has owned or driven one of these vehicles will know what it’s like – from the moment you take off the motor pulls strongly.  Because it pulls so strongly so well so easily, you don’t have to put your foot down much.  And this is the point – the fuel consumption figures quoted above are delivered under controlled driving conditions.

In reality however, the difference between the diesel and petrol fuel consumption is much greater.  The reason simply is the ease with which diesel motors pull in normal driving, and the fact that you don’t have to floor the accelerator to get them moving.  So in reality a petrol car uses a lot more fuel than its diesel counterparts, even on these figures.

But what of hybrids, I hear you ask.

That’s tomorrow’s topic.

The Chemistry of Fuel: Petrol vs Diesel #2

Petrol and diesel both come from the same place – crude oil.  In other words, they are natural.

That’s right – they’re natural.

Crude oil that is taken out of the ground is comprised of hundreds and possibly thousands of chemicals.  They are separated into their various components by boiling point by a process called fractional distillation.

There are several implications of this.  Firstly, as we have seen, petrol is carcinogenic and diesel isn’t.

Secondly, petrol is flammable and diesel isn’t.  If you toss a match into a bucket of diesel nothing would happen but if you tossed a match into a bucket of petrol it’d be a different story.  The flammability of petrol has resulted in several fires at petrol stations, mostly caused by static electricity that can jump between the driver and the body of the car as they are fuelling it.

The flammability of petrol of course means that it ignites with a spark plug inside your engine, but diesel won’t.

If you accidentally put diesel into your petrol car by mistake then you would find that initially a lot of smoke would come out and then it would just stop.

Diesel must be heated to a high temperature before it will ignite and this is how diesel motors work – the air is heated to above the auto ignition temperature by the compression of the air in the engine and the fuel is then vapourised into the combustion chamber by the fuel injectors.  The tiny droplets of diesel are then ignited by the hot air.

With petrol, the combustion process only starts at one point – at the spark plug.  The flame must then travel across the cylinder until all the fuel is ignited.  This is obviously a less efficient process than diesel where all the droplets ignite at the same time giving a more uniform combustion.

This may be the reason why diesel motors last longer than petrol motors – the uniform combustion places less mechanical stresses on the engine than a petrol engine which has uneven combustion.

Because petrol is a more volatile chemical, by the time the petrol gets inside the combustion chambers it is in the vapour phase, regardless of whether it has come through an old-fashioned carburettor or whether it is through a fuel injection system.

With diesels, however, the less volatile nature of the fuel has meant that in the past the diesel that is injected is not fully vaporised, but present as lots of small droplets, each of which must individually burn.  The implications of this are twofold.

Firstly, the time taken for the drops to burn detracts from the efficiencies of the engine, and secondly, and probably more importantly, often all the fuel simply does not burn.  This is why old diesel cars and trucks have that residual oily diesel smell about them – it’s all the unburnt fuel.

But modern common rail turbodiesel motors are far more efficient, as the result of extremely high injection pressures.  The much higher pressures result in much more efficient vaporisation, and much more efficient burning of the fuel. That’s why modern diesel motors do not give your car an oily diesel smell.  They also do not blow great clouds of black smoke (which is unburnt fuel) which was an issue on older type diesels.

Tomorrow, we’ll look at the implications of these combustion processes on the mechanical efficiency, performance, and economies of diesel and petrol motors.

The Chemistry of Fuel: Petrol v Diesel #1

We hop in our car, press the accelerator and the car moves.

The energy required for this process comes from the chemical energy stored in the fuel that you put in your tank, whether it be petrol, diesel, or LPG.

The fuel reacts with the oxygen in the air to produce water, carbon dioxide, and energy.

If we consider octane (nominally petrol) the reaction is given by

2C8H18 + 25O2 = 16CO2 + 18H2O + energy

So we can see that there are two components required for fuel to burn – the fuel and the oxygen.  So the combustion is actually an oxidation process.  Technically, we could say that the combustion process requires the oxidant and the thing that gets oxidised.

Now, of course, in an internal combustion engine we rely upon oxygen from the air – so, if you like, half the components of the reaction that provide the energy for how the car moves are free.  If you have a rocket, however, you cannot rely upon getting oxygen from the air, as there simply isn’t enough of it – so rocket fuels are a two component mixture, including the fuel and the oxidant, but that’s a story for another day.

Back to petrol.  Because we are relying upon oxygen from the air, sometimes there isn’t enough of it, so instead of the reaction above we get

2C8H18 + 21O2 = 8CO2 + 8CO + 18H2O + energy

so instead of getting all carbon dioxide we now also get carbon monoxide (CO).

Bt we have all seen the black stuff that accumulates in our exhaust pipe – carbon.  This tells us that sometimes there is even less oxygen than this and we get this reaction

2C8H18 + 16O2 = 6CO2 + 6CO + 4C + 18H2O + energy

Or, expressing it in words:

Octane plus oxygen gives carbon dioxide plus carbon monoxide plus carbon plus water plus energy

By the way, this is why you sometimes see water trickling out of exhaust pipes first thing in the morning, and why exhaust pipes go rusty.  When the motor is still cold, the hot water vapour that has just been formed in the engine condenses in the cold metal of the exhaust pipe and so dribbles out the end – rather like an old hillbilly still.

But when the engine warms up and the exhaust pipe warms up it stays in the vapour phase and you don’t see the dribbles any more.

Petrol and diesel are made up of different chemicals.  They are both mixtures and both come from the same source – crude oil – but have different components.  Petrol consists of short chain hydrocarbons, some aromatics, and is highly flammable with a flash point of -44° C.

Diesel on the other hand, has long chain hydrocarbons, is not flammable (it has a flash point of 75 to 80° C)

And this is the first significant difference between the two – petrol contains aromatics and is therefore carcinogenic, whereas diesel does not contain aromatics and is therefore not carcinogenic.

Also petrol is much more volatile (evaporates more easily) and it therefore is not as easy to store or transport.

So right away we can see that diesel is both safer and an easier fuel to handle.  Tomorrow will look at how they burn and what the implications are for the performance of an engine.


Firefighters and Cancer

Reader Robyn writes:

Hi Millsy and Dr Chemical,

Firstly, love your show Millsy.  I listen every morning on my way to work.  Kent is a hoot and has me in stitches.

I heard you this morning and thought I would share my story relating to Fire-fighters and cancer.

My late husband was a fire-fighter in the Royal Australian Air Force for 23 years.

After serving all over Australia, and a stint in the Middle East in 2005, he left the RAAF in May 2006 and we returned to WA.

In July 2006 he was diagnosed with an extremely aggressive form of Non-Hodgkin’s Lymphoma, and after 12 months of chemo and a bone marrow transplant, he passed away on my 41st birthday… the 21st of  June 2007.

He was 44.

As soon as he was diagnosed  in 2006 we lodged a claim with Veteran’s Affairs, as we believed his cancer was caused by his line of work.  After extensive research I discovered that in the USA, Canada and the UK, this form of cancer is recognised as being caused by the fumes and toxins to which fire-fighters are exposed.  .
It is twice as prevalent in fire-fighters as in the average person on the street. Hence they are able to lodge a workers’ compensation claim.

But here in Australia, it is not recognised, so his claim was denied.

However, earlier this year Veterans Affairs contacted me and said that some legislation had changed and they now accepted some liability, as his case fell under the “De-seal/Re-seal” legislation as he had worked at RAAF Base Amberley in Brisbane during the required time and been exposed to the contaminated fuel.  However, they did not concede that any other part of his role had contributed to his cancer.

The children and I received a lump sum payment (death benefit) which was greatly appreciated.
However, considering it cost him his life, it was a measly amount (approx $300k).  I was also required to sign a declaration stating that I would not take any further action against them for further damages.

Hearing your story this morning gives me hope that other fire-fighters, both Defence and Civilian, that may contract cancers relating to their work, will now have more of a chance of these cancer’s being recognised and their families will hopefully be looked after.

Keep up the good work, love the show.



Wow! For several reasons!

I must confess that this is something I hadn’t thought of.  My original comments were about the Fiskville firefighters who has contracted cancer from being exposed to the fumes coming from waste flammable solvents that had been deposited on site for firefighting practice.

But, of course, firefighters in real situations are exposed to similar things, if not worse.

And the danger here is not acute toxicity, but cancer.  Cancer, of course, is a modern disease, only being in existence essentially since the industrial revolution.

As far as I am aware, no one knows what causes it, in the sense of what it actually is.  But it is known that cancer is the result of being exposed to certain types of radiation and chemicals.

From the point of view of the chemicals, the reason that it difficult to work out what is actually happening is because all the chemicals that contribute to cancer are organic, just like our body.  This means that the molecules react with the molecules of our body and are subsumed by them.

For example, benzene, which is one of the more common carcinogens around:

Benzene is a basic six membered carbon ring:

with a shorthand structure that looks like this:

The six membered carbon ring is one of the most basic components of millions of different chemicals in our body.  For example, here are the structures of some alkaloids

so any benzene that we take into our body can participate in literally millions of unwanted chemical reactions in our body, becoming incorporated into the structures of big molecules and so becoming completely invisible.

That is, if you had been exposed to benzene, it would be utterly impossible to prove it, because the benzene molecule will have disappeared inside the structures of other molecules.  The reactions that occur can release all sorts of nasty intermediate compounds of unknown structures, some of which, no doubt, cause cancer.

I’m aware that firefighters wear breathing equipment when they fight fires, but no respirator is going to block out everything, and in any case, I suspect that these are a relatively new phenomenon, and perhaps firefighters of the past were not so well equipped.

I personally find it utterly astonishing that this is not recognised in law.  Utterly astonishing.  There are chemicals in things in houses that were never meant to be burned.  Never mind benzene – the high temperature of a fire is capable of producing all sorts of nasty free radicals, dioxins and other aromatics that you do not want to be breathing.

In fact similar things happened to firefighters that were involved in 9/11.  Although there was not a fire when the buildings collapsed their was certainly plenty of dust – dust that had been pulverised into particles so small that they would pass through most air filters.

And apparently this did indeed result in very high cancer rates not only amongst firefighters after 9/11 but people who were in the general vicinity at the time.

And this really is not rocket science.  Chemists who work with organics have higher rates of cancer then chemists who don’t. And this is common knowledge – one of the highest rates of cancer in the industry is fragrant chemists, as they are forever breathing in fumes of large complex organic molecules.

Hopefully this tragic legacy will change soon – I can only assume that firefighters these days are better equipped.

But it is for those like Robin who must endure the tragic legacy of ignorance of chemical toxicity – not unique to firefighters I might add – that I hope the law changes soon – if anyone ever wanted to put me in the witness box, I’d be happy to explain a few things to them…

New Blog Format

I had a talk with my web developer guy last week and he showed me how to categorise my blog postings.  If you look down the right-hand side you will see the categories that have been created. Dale Carter is his name and he is in Nedlands – awesome guy.

When I get a chance I’ll go through all my postings and put them into categories. This is actually going to be better than what I had originally planned, which was to have different sections of the blog with different subjects in each section.

The difficulty with that of course is that some postings fall into several different categories at the same time.  But by doing it this way – I can put as many category tags on each post as I want.

The other good thing about this is that I get a quick overview of which areas I have been covering, and which areas I have been neglecting.

If you have any ideas for any other categories or areas that I could do, drop me a line in my Q&A section and I’ll see what I can do.