by Dave Scott

Just Google ‘exhaust smoke’ and be prepared to sift through approximately 22 100 000 results (0,41 seconds) – an exhausting info overload.

The author has been involved with trucks since 1966 and has never seen a driver fined for excessive exhaust smoke, or a truck owner taken to task for any abnormal emissions. And no wonder – the RTA regulations are so vague as to be unenforceable.

It is not only the exhaust emissions we see (mainly due to particulate matter) that are cause for concern, but the ones we do not see – carbon monoxide and dioxide, nitrogen oxides, hydrocarbon particles and sulphur dioxide – which are truly dangerous.

Visible Smoke

Distinction must be made between particulates and visible smoke: particulate matter is defined as anything that is collectable on a filter (particulates may be present in exhaust emissions even though no visible smoke is apparent); the defining character of exhaust smoke is that it is comprised of solid or liquid aerosol particles that absorb or deflect light.

Just because it is smoke free it does not mean a vehicle exhaust is ‘clean’.  Air contaminants that frequently pour out of exhaust systems – especially very old vehicles – are carbon dioxide, nitrogen dioxide, sulphur dioxide, particulates, ozone, benzene, toluene and xylene, 1,3-butadiene, formaldehyde and acetaldehyde, benzo(a)pyrene. For example – Benzene can be harmful if it is swallowed, inhaled, or touched. It is a member of a class of compounds known as hydrocarbons. Human exposure to hydrocarbons is a common problem.

Efficient, well-maintained diesel engines do not emit clouds of smoke. Modern, turbo-intercooled, electronically managed diesel engines are designed to operate with clear gases at any altitude. The reduction in exhaust back-pressure with increasing altitude speeds up turbine and compressor activity to counter the effects of higher altitudes. Apart from obvious environmental problems caused by diesel fuel exhaust fumes the real issue is that fuel is being wasted through incomplete combustion. In addition, exhaust smoke accelerates servicing requirements and engine wear rates. Exhaust smoke is costly.

Here are the main types of diesel exhaust smoke encountered on the road –

Black exhaust smoke – this is very visible, resulting from large soot particles, indicating incomplete combustion due to localised rich mixtures in a combustion chamber. Here are the main causes of black smoke:

  • Incorrect injector pressures – generally too low
  • Poor injector spray pattern
  • Out-of-spec injection timing
  • Over-fuelling – wrong spill rate or governor setting
  • Inadequate air supply – blocked air filters or intercoolers or worn turbochargers.
  • Abnormally high diesel fuel cetane number

White exhaust smoke – typically pale grey in colour, this is caused by visible liquid fuel particles in an exhaust. This indicates that fuel is passing through a combustion chamber without vaporising or igniting. Constant thick white smoke is usually a sign that coolant is being burnt. The main causes of white smoke are –

  • Low diesel fuel cetane number – check for blending with illuminating paraffin.
  • Engine is over-cooling.
  • Misfires
  • Burned valves.
  • Late injection timing
  • Poor injector spray pattern – fuel impingement on cylinder walls
  • Low compression

Blue exhaust smoke – blue smoke is not diesel fuel related. This comes from burning excessive amounts of crankcase lubricant in combustion chambers. Typical causes are:

  • Worn rings.
  • Worn valve guides.
  • Incorrect honing pattern.

And do not expect exhaust smoke to arrive in distinct black, white, or blue clouds – it could be a mixture of all three depending on the technical problems that are present.

Drivers have an important role to play as foundation observers of exhaust smoke.

The evidence of a wide-spread malpractice in diesel fuel adulteration is found in reported sales of illuminating paraffin (kerosene). Press reports on paraffin data in South Africa point to a two-fold increase in kerosene consumption over the past three years (2020 to 2022), from 600,000 kilolitres annually to over 1.2 million kilolitres.

Here are a few practical starting points to address exhaust emissions:

  • Conduct MBWA (Management By Wandering Around) at cold start to spot and record smoke offenders.
  • Train the drivers to observe and report exhaust smoke on defect reports and start-up check lists.
  • Track and reconcile individual vehicle oil and coolant consumption with excessive smoke reports. Modern coolant systems are sealed, and expansion tanks should not require constant topping-up. Coolant loss is evident in white exhaust smoke.
  • Overfilling on engine lube dipsticks is an evil that will promote blue smoke.
  • It is important to know when smoke is generated – usually a tell-tale sign of damaged piston rings occurs when bluey-grey smoke leaves the exhaust while the vehicle is accelerating.

The full article may be downloaded via this link: https://www.wearcheck.co.za/shared/TB84.pdf

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