Exhaust Emissions: What they’re made of, and factors that increase or reduce them

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Unleaded fuel is marking its tenth anniversary in Kenya.  The use of lead (pb) as an additive to improve octane ratings is now history, and all the problems associated with the change to other chemicals either did not happen or have been resolved.

There were two reasons for replacing lead.  One was that it is a cumulative “heavy metal” poison, which exhaust fumes put into the air we breathe.  The quantities were very small and the effects were negligible (the whole world ran on leaded fuels for 100 years before the unleaded trend began in the 1990s), but with vehicle populations and general pollution increasing it was a good idea to be rid of it.

The second and more pressing reason for removing lead from petrol was the introduction of catalytic converters, which can help purify or neutralize other (more abundant) toxic elements in exhaust fumes.  The problem was that even the smallest quantities of lead quickly destroy the chemistry of catalytic converters, and make them useless.

Removing lead solved that problem, but created two others.  If lead was removed, other chemicals had to be found to enhance the octane rating of petrol.  The alternatives now universally used are no sweeter than lead in terms of what gets up your nose, but they are harmless to cat converters.

The other problem was that lead has beneficial lubricating properties in the combustion process. The alternative octane improvers do not.   Motor manufacturers worldwide were given advance warning, and started “hardening” parts of  engines so the lubricating properties of lead were no longer necessary, before unleaded fuel became the norm and cat converters became compulsory.

Engines manufactured before the change (late 70s and early 80s) either had to be modified or run some risk of more rapid wear-and-tear by using unleaded fuel.  There were still quite a lot of old engines being used in Kenya when unleaded was introduced in 2002, so leaded fuel continued to be supplied alongside unleaded for some years.

There are now very few engines of 1970s vintage still running (perhaps unleaded fuel helped accelerate their demise) even in our relatively ancient national fleet, and the leaded fuel option is no longer necessary (or allowed).

So what has all this done for us?  The answer is not-a-lot.  Certainly there is a little less lead in the air, but there are other noxious chemicals instead.  Certainly, catalytic converters are helping reduce the toxicity of other exhaust fumes, but for other reasons the pollution from exhausts is increasing.

First, because vehicle numbers in Kenya have trebled since “unleaded” joined the dictionary.  But also because cat converters are just an aid – not a silver bullet – in reducing exhaust pollution.  There are many other and greater factors, including the age and condition of engines,  driving styles, and traffic patterns (stop-start motoring generates vastly more toxic emission than steady speed cruising).

The question is whether we have taken two steps forward and one step backwards, or one step forwards and two steps backwards.     

The biggest misconception in the exhaust gas arena remains:  the notion that  a particular vehicle, running on a particular fuel, will produce a particular quantity of noxious fumes.  The truth is much more complex and variable than that.  

An old-fashioned engine, run on leaded fuels, can produce relatively harmless exhaust gases if the vehicle is well maintained and skilfully driven in sensible traffic systems; while a modern lean-burn engine, run on unleaded fuel, can produce the most noxious fumes if the vehicle is not well maintained, or is clumsily driven in crazy traffic systems.

Bottom line - the basic service condition of a vehicle, the basic way it is driven, and the basic traffic system it operates in, are MORE important than the high-falutin' stuff.   Here's a look at the technical basics: 


All motor vehicles are powered by burning a mixture of fuel and air.

The burning occurs in a confined space, known as the combustion chamber, inside the engine.

The burning generates heat which creates pressure which forces pistons downwards.  Through a system of rods, shafts and gears, this downward motion is converted into rotation of the drive wheels (in the same way that pressing down on the pedal of a bicycle causes the wheel to turn).

The gases generated by the chemical reaction of burning fuel in air are emitted through the exhaust.


All motor fuels consist of hydrogen (H) and carbon (C). These two elements combine in different molecular structures to create different fuel types (petrol, kerosene, diesel) but all are principally Hydrocarbons (HC). Fuels also contain various additives  and various impurities (such as sulphur, S).

Pure air consists mostly of Nitrogen (N), Oxygen (0) and Carbon Dioxide (CO2). It also contains various trace gases, and differing amounts of moisture (water, H2O). 

The action of mixing air and fuel and igniting them causes all these chemicals to react with each other, so the various elements - Hydrogen, Oxygen, Carbon, Nitrogen, additives and impurities - combine in different ways to produce different chemicals.

Which new chemicals are produced will depend on the ratio of the fuel mixture and how evenly and completely the mixture burns when it is ignited.

If the proportions of fuel and air (the mixture) are just right, and the burning process (combustion) is absolutely even and complete, the chemicals produced will be mostly Carbon Dioxide (CO2),  Water (H20) and inert Nitrogen Oxide (N2O).  These have no intrinsic ill-effect* when emitted by the exhaust and dispersed in the atmosphere.

  • Carbon Dioxide, CO2: This already exists in abundance in pure air, and is necessary to plant life which absorbs CO2 and converts it into Carbons (for growth) and Oxygen (which is released back into the atmosphere).
  • Water, H2O: Water, pure and simple.
  • Nitrogen Oxide (N2O): There are three main nitogen oxides - NO, NO2 and N2O. This one, N2O, is inert and harmless.

However, any imbalance in the mixture and any irregular or incomplete burning will produce different chemicals. These include:

  • Hydrocarbons, HCs:  Unburned or partially burned hydrocarbons (soot/visible smoke). These are foul-smelling and toxic, carcenogenic, acidic when they combine with moisture in the air, and cause photochemical smog. They are poisonous to humans, they irritate and destroy mucous membranes in the body, and they make moist air more corrosive (eg faster rusting of corrugated iron roofs). 
  • Carbon monoxide, CO: When inhaled, this colourless, odourless gas combines with haemoglobin in the blood and reduces the blood's ability to transport oxygen. In low concentrations CO causes drowsiness, nausea and dizziness; in high concentrations it causes death. CO is inflammable, and when it burns it generates blue smoke.
  • Nitrogen Oxides, NOx:  Unlike the inert N2O produced by perfect combustion, incomplete combustion generates NO and NO2, and these are harmful.  NO is a colourless and odourless gas, and when it comes into contact with the air it turns into NO2, which is a smelly reddish brown gas which can cause    coughing and insomnia.  It is also a component of photochemical smog and it dissolves easily in water, producing powerful nitrous (HNO2) and nitric (HNO3) acids. Poor combustion also makes the quantity and chemical composition of  additives and impurities worse, involving:
  • Sulphur oxides, SOx:  Most crude oil contains sulphur, which remains as an impurity in refined fuels (especially diesel). This becomes an exhaust emission of mainly sulphurous acid gas (SO2), which dissolves easily in water to make foul-smelling, toxic and corrosive substances such as sulphurous acid (H2SO3) or sulphuric acid (H2SO4).
  • Lead, Pb:  This is an additive used to increase the octane value of petrol, to prevent knocking (pre-ignition from spontaneous detonation) and therefore reduce emission of HCs, CO and NOx. Both alkyl and ethyl leads are very toxic, and can cause cumulative lead poisoning. The alternative octane-enhancers used in so-called "unleaded" fuel are not necessarily less poisonous - they are chosen because lead destroys catalytic converters which help to neutralise CO and NOx emissions.


It can be seen that efficient combustion (the right mixture, perfectly burned) greatly reduces exhaust pollution. It also:

  • improves engine performance - complete burning of the fuel releases all the fuel's energy as power; unburned fuel delivers no power at all and is wasted. 
  • extends engine life -  the hydrocarbons and acidic gases in exhaust emissions which foul the atmosphere also clog and corrode engine parts and degrade lubricants, increasing rate of wear and risk of breakdown.
  • reduces fuel consumption - unburnt fuel is wasted fuel. Power and economy are interlinked in overall "performance".

It follows that inefficient combustion has the opposite effect - it reduces power and economy performance and it reduces engine reliability and engine life.  And it increases pollution.


There are many complex and highly technical factors in engine design which aim for best possible (none are perfect) combustion characteristics. But in simple summary, each engine is designed to be run:

  • at very precise settings (mixture, ignition, timing)
  • on a very precise fuel specification to achieve optimum power, economy, durability and exhaust purity performance. 

Wrong settings

Any deviation from the manufacturer's engine setting specifications, or precise fuel characteristics and purity, will degrade one or more or all of the performance elements.

If the mixture is too rich (too much fuel, not enough air), power output will fall, fuel consumption will rise, and emission of sooty hydrocarbons and CO will increase.

If the mixture is too lean (not enough fuel, too much air), the car will start and idle badly, it will run too hot, and there will be a substantial increase in NOx emissions.

Ignition faults or maladjusted timing (advance and retard) have similar lose-lose consequences.  And in all cases, those ill-effects will apply even if the fuel is the correct specification and absolutely pure.

Adulterated fuel

If the fuel itself is adulterated, the ill-effects will be even more severe, and will degrade performance even if all the engine settings are right. For an engine designed to run on petrol, or on diesel, has no correct setting for a cocktail of petrol and kero, or a stew of diesel and kero.

Adulterated fuel causes an horrendous increase in exhaust pollution, a certain increase in fuel consumption, a probable loss of power, and engine damage - possibly immediate, and definitely in the long-term.

So clearly, an engine in good condition, correctly set and running on the correct and pure fuel for that engine, are fundamental in the quest for reduced exhaust pollution.

Driving style + traffic flows

But even when those conditions are achieved, there is still another basic. How the car is driven.

The general principle is that engines run most efficiently, burning fuel most completely and producing the least toxic exhaust gases, when they are running at even and moderate speeds in the correct gear.

Any deviation from that causes an increase in the toxicity of exhaust gas. Accelerating, braking, idling.  The stop-start rhythms of a traffic jam. These can cause massive increase in exhaust pollution.

The high-tech solutions the modern Western world is talking about - lean-burn engines and unleaded fuel - are looking to  reduce exhaust toxicity by several percentage points.

But engine condition, fuel purity, driving manner and traffic flows can affect exhaust pollution by several hundred percentage points.

They are, in our shoddy motoring environment, a hugely more significant factor than the esoteric notions that the eco-babblers are focussing on.

Of course it would be best if we could get the basics and the high-tech elements right. Immediately. But we can't. We have to prioritise. And we should be careful to get our priorities right.

It is also important to note that all motor vehicle exhaust pollution is only a fraction of overall air pollution. 

A single jetliner in a single take-off produces more exhaust gas than 1,000 cars motoring all day.  And while we burn about 8 million kilos of petrol and diesel every day, we also burn about 10 times that much charcoal, firewood, industrial compounds and  rubbish.

To better understand that perspective, consider the city of New York, and the massive energy consumption and exhaust emissions of its millions of gas guzzling motor vehicles.  Well, all those vehicles use less energy and produce less exhaust pollution than the office and apartment buildings…never mind the factories and the airport.

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