The second law of thermodynamics postulates an increase in entropy in the universe. This bring about the thinking that climate change is a as a result of increase in disorder. The first law of thermo dynamic postulate indestructibility of energy and associated biblical prophecy of destruction of the universe by fire when all the energy in the universe will be converted to heat energy. The second law of thermodynamics is defied by living organisms in photosynthesis where by simple atoms are combined to make complex molecules.

Sunlight an electromagnetic wave is usually stored as bond energy during photosynthesis which results in the formation of organic molecules; fossil fuels are formed from dead organic molecules under pressure over millions of years ago. Burning of fossil fuel results in increase in entropy as high ordered molecules are broken down in to smaller molecules. This result in the release of sun energy that had already been packaged into bond energy millions of years ago and when this is added to current solar irradiation, global warming results. Increase in entropy is not a negative outcome per se as it is out of the process of increase of entropy designers can come up with devices that make human life easy. One good example of such a device is the internal combustion engine that revolutionized mobility. Increase of entropy of gasoline, diesel or liquid petroleum gas during combustion brings about expansion of gaseous products that is converted to useful work. Some of the resulting gases are harmful to the environment and some of which are green house gases. Many innovations are currently being brought forward to replace the internal combustion and do away with the emissions especially through adoption of electric vehicle (EV). The challenge has been the realization of a battery with a large energy density (watt hours per liter) or the improvement of the fuel cell. The persistence of this challenge implies that the internal combustion engine is still more economical over emerging technologies and thus there is need to reduce its impact on the environment. Engine management has been developed by vehicle manufacturers well enough to reduce emissions; the challenge we face as a society is adherence to emissions control measures. For example it is very common for a mechanic in Kenya to recommend the removal of a catalytic converter (zeolite material) in positive ignition (PI) or oxidation catalyst and particulate filter in compression ignition (CI). In this article methods and standards for reducing emission control from vehicles fitted with internal combustion engine will be elucidated. This methods and standards can guide policy makers in road transport policy formulation and ensure a healthy national fleet and reduce CO2 burden in our road transport.

Engine emissions

Engine emissions are determined by several factors including fuel type, engine family, method of aspiration, ignition type, engine operating condition (Transient condition or steady state condition), idling, coasting, overrun breaking, high speed heavy load running, cruising speed acceleration and deceleration [1]. Engine emissions are deduced from the basic combustion equations below:-

For carbon;

1kg carbon requires 2.67 kg of oxygen to produce 3.67 kg of carbon dioxide.

For hydrogen

1 kg of hydrogen requires 8kg of oxygen to produce 9 kg of water.

The above equation applies under ideal situation with nitrogen which makes the highest percentage of air emitted without participating in the reaction [1]. The ideal air fuel ratio is about 14.7:1 and is the theoretical amount of air required to burn fuel completely and gives a ‘lambda’ (λ) value of 1 [1]. In practical situation there is a large range of operating conditions that engines experience exhaust gases consists of several other gases and materials such as

  • Carbon monoxide (CO) – due to rich mixture and incomplete combustion
  • Oxide of Nitrogen ( ) – due to very high temperature
  • Hydrocarbons ( ) – due to poor combustion
  • Particulate matter (PM) – soot and organo- metallic materials
  • Sulphur dioxide ( ) – arise from combustion of small amount of sulphur in diesel fuel.

Engine emissions control has been in the mind of all major vehicle and industrial plant manufactures and all designs of commercially viable engines is a compromise between conflicting interests highlighted below:-

  1. Combustion chamber design – hydrocarbon (HC) emissions usually emanate from unburned fuel in contact with combustion chamber wall [2]. Combustion chamber surface area should therefore be kept as small as possible and with least complicated shape with a spherical shape being theoretically ideal of which it is still far from practical [2]. Nissan Motor Corporation are currently leading in research and development of Homogeneous Charge Compression Ignition (HCCI) engines with prototypes already fabricated and are expected to result in engines that can operate on gasoline, diesel and most alternative fuel with emissions akin to PI engines without Zeolite material with negligible NOX levels and efficiencies akin to CI engines without soot formation [3].
  2. Compression Ratio – Higher compression ratio results in higher thermal efficiency of an engine with better performance and better fuel economy [2]. The drawback to higher compression ratio is increased emissions especially NOX due to high temperatures [2]. This challenge has been overcome with chamber design and the introduction of four valves per cylinder and electronic control [1].
  3. Valve timing – Effect of valve timing on exhaust emissions is quite considerable, one major factor being valve overlap which has a significant effect on reaction temperature and hence an effect on NOX emissions [2]. At higher speeds, a longer in let valve open period increases developed power with a down side of causing a longer valve overlap and at idle this can greatly increase emissions of hydrocarbons HC [1]. This has successfully been addressed with the introduction of electronically controlled valve timing especially Variable Valve Timing intelligent (VVT-i) [5].
  4. Manifold design – Complexity of gas flow in inlet and exhaust manifolds is due to transient change in flow emanating from change in engine speeds and the pumping action in the cylinders [3]. Pumping action cause pressure fluctuations in the manifolds and if the manifolds as well as the induction and exhaust systems are designed to reflect the pressure wave back at the right time great improvement in volumetric efficiency can be achieved [2]. This has resulted in many vehicles to be fitted with variable length induction tracts with longer tracts used at lower engine speeds and shorter tracts at higher engine speeds [2].
  5. Exhaust gas recirculation – primarily used to reduce peak combustion temperature and hence impacts on NOX emissions [2]. A proportion of exhaust gas is usually returned to the inlet side of the engine in an attempt to keep combustion temperature below 1800K [1]. This function is best achieved through an electronically controlled valve operated by an ECU as shown in fig. 1 [1].
  6. Catalytic converters – Catalysts assist in chemical reaction without being consumed in the reaction process. Most catalysts used in vehicle emission control are Zeolite material namely Platinum and Rhodium. At temperatures in excess of 573.15K platinum acts as a catalyst that speeds the conversion of CO (carbon monoxide) to CO2 (carbon dioxide), HC to H2O and CO2 [1]. For correct functioning of a catalytic converter the air fuel ratio must be maintained close to the correct stoichiometry+ which is achieved by the use of exhaust gas sensors and electronic control [3]. Rhodium is catalyst NOX to NO2. Fig. 2 show a three way catalytic converter used in gasoline engines emission control [1]. On CI engines a zeolite can be used to reduce HC emissions but will have less effect on NOX due to the fact that CI engines are always run with excess air to ensure better and more efficient burning of fuel [2]. A catalyst like the one used in PI engines will not strip the oxygen in NOX to oxidize the HC because the excess O2 in air will be used and for this reason special converters for NOX are being intruded in CI engines [2].


  1. Diesel particulate filters – particulate matter (soot) arises from CI engines and consists mainly of carbon with some absorbed HC. Filtration of exhaust products is a method used to remove particulate matter from exhaust gases before they are discharged to the atmosphere [1]. Several forms of filtering mediums are used in trapping PM. Cleaning of the filter is regularly required to avoid filter blockage, in diesel particulate filters (DFPs) controlled burning is applied to convert carbon (soot) to carbon dioxide CO2 (a green house gas) a process known as regeneration [1]. There is need for the development of other cleaning processes that does not result to green house gases. Figure 3 show a 1950 diesel Fordson agricultural tractor without a PM filter emitting black smoke (as a result of particulate matter).

Fig. 3 A 1950 Fordson agricultural tractor without particulate matter filter.

Emission standards are varied all over the world with different countries having different air quality index and some countries having none [4]. The variation arises from the difference in test procedure among different vehicle manufactures and different regulations from country to country.

In Kenya there are no communicated emissions standards or air quality index. The defunct police motor vehicle inspection unit had the start of the art emissions measurement tolls that were never utilized until they were handed over to National Transport and Safety Authority (NTSA) hat has also not put them into proper use. Some of this equipment is described below:-

  1. Dynamometer – This is the instrument used in measurement of engine torque and speed giving the brake horsepower. The brake horse power is usually between 70% to 85% of the indicated power (power developed by expansion of gases in the cylinders) due to friction as well as energy consumed in getting the charge in and out of cylinders [5]. Fig. depicts a 75190 hp brake horsepower and a displacement of 18500 liters. This has lead to increased load capacity and range, although a bit cleaner compared to earlier engines it still burns fossil fuels with associated emissions.

Fig. 4. A 75190 hp, 18500ltrs engine used in marine transport with a high thermal efficiency.

  1. Gas analyzers- Various measurement principles are applied depending on the gas to be analyzed:-

CO and CO2 – A non-dispersive Infrared (NDIR) absorption type analyzer is widely applied [4].

HC – A heated flame ionization detector (HFID) analyzer type is used with gas temperature maintained at 463 K ± 10K [4].

NMHC (Non methane hydrocarbons) – Usually analyzed with heated non methane cutter (NMC) in line with a flame ionization detector (FID) [4].

NOX – Usually analyzed with a chemiluminescent detector (CLD) or heater chemiluminescent detector (HCLD) with a NO2/NO converter [4].

Air to fuel measurement – This determines exhaust gas flow. A lambda (λ) sensor of Zirconia type (mounted on the exhaust pipe at a point where temperatures are high enough to avoid water vapour condensation) is utilized [4].


In a Kenya where the bulk of national fleet composes of pre owned vehicles imported from Japan and Europe there is need to monitor and regulate emission level by adoption of best practices like the global technical regulation (GTR) No. 4 [4]. The shaken (車検) system in Japan make vehicle owning and operating cost to increase with vehicle age, this has led to a situation in which vehicles that a no longer sustainable within this system to be exported to developing nations as second hand cars. The Kenyan system is not badly off as it has set a regulation in which vehicle older than eight (8) years from the time of manufacture cannot be registered in the country. There is a correlation between vehicle age and emissions levels and this informs the setting of the economic life of a motor vehicle [1]. The National Transport and Safety Authority (NTSA) in Kenya has proposed for private vehicle above four (4) from the year of manufacture to be undergoing bi-annual inspection just like commercial vehicles annual inspection to determine their road worthiness. Emission levels should be one of the parameters in determining vehicle road worthiness and NTSA should work closely with NEMA (National Environmental Management Authority), the Chief Engineer (Mechanical) in the ministry of transport, infrastructure, housing, urban development and public works, Kenya bureau of Standards (KBS) and other stake holders in setting of National Emission Standards and approved national test procedure in harmony with GTR No. 4 and the 1998 Agreement. The state of affairs is worse with the motor cycle public transport popularly known as boda boda, as most of them are poorly maintained and carry low capacity. It’s very common to encounter a motor cycle burning engine oil and emitting very noxious gases to the environment in most of our rural town. The idea of empower the youth economically though boda boda operation was very noble, though we need to look in to the environmental and human costs associated with this mode of transport. In fact there are very few insurance companies willing to insure this mode of transport. The traffic act needs to be seriously enforced to bring sanity in this sector. The politician needs to influence the society towards sustainable development activities other than sponsoring activities that will lead to environmental degradation. We all live in this planet and in the face of adversity we are all vulnerable. Climate change is one adversity that threatens to end life on the planet earth, so let every human being exploit the earth’s resources responsibly especially the energy resource.



There is need for Kenya to develop national emissions standards and test procedure which should be strictly enforced for this country to realize a significant reduction in CO2 emissions from the transport industry. We should strive to be early adopters of the electric vehicle (EV), especially for short range transport. Electrified mass transport train system is long overdue for the metropolitan area of Nairobi and the government should accelerate its implementation.



  1. Allan Bonnick, Automotive Science and Mathematics, Elsevier Ltd (UK 2008) ISBN 978-0-7506-8522-1.
  2. Tom Denton, Automobile Electrical and Electronic Systems, Elsevier Ltd (UK 2004) ISBN 13: 978-0-7506-6219-2.
  3. Allan Bonnick and Derek Newbold, A practical Approach to Motor Vehicle Engineering and Maintenance, Taylor and Francis (USA 2011) ISBN 987-0-08-096998-5.
  4. Global Technical Regulation No. 4, Test procedure for compression ignition (C.I.) engine and positive ignition (P. I.) fueled with Natural gas (NG) or Liquefied Petroleum Gas (LPG) with regard to emissions and Pollutants, United Nations global registry (Geneva 2007) ECE/TRANS/180/Add.4
  5. Joseph Heitner, Automotive Mechanics Principles and Practice, Litton Educational Publishing Inc. (India 2001) ISBN 81-7671-015-6


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