Sulphur
Sulphur has a significant deactivation effect on the catalytic converter reducing its efficiency. The conversion efficiency of modern 3-Way catalysts may reduce by 40 % with 900 ppm sulphur compared to 100 ppm sulphur in fuel. Most of the loss in 3-Way catalyst efficiency caused by sulphur is recovered once the engine is operated on low sulphur fuel. More advanced catalysts systems like de-NOx catalysts and the closed-coupled catalysts operating at high temperatures suffer more serious and
poermanent loss of efficiency. NSR de-NOx catalysts require practically sulphur-free (< 5 ppm by mass) fuel for their operation.
Sulphur also has an adverse effect on the working of heated exhaust oxygen sensor by deactivation of platinum electrodes.
Oxygenates
Oxygen containing compounds like alcohols and ethers which may be blended in gasoline are commonly termed as oxygenates. The alcohols and ethers that are permitted as blending components in gasoline are given in Table 8.5. The amount of oxygenates or oxygen content of fuel for use in engines designed to operate on the conventional petroleum fuels is limited mainly due to;
(i) Leaning effect of mixture caused by presence of oxygen in the fuel, which may increase NOx emissions,
(ii) Effect on fuel Reid vapour pressure particularly when blending methanol
(iii) Adverse effect on the fuel system materials.
The oxygen in the fuel is limited to 2.7% mass of O2 maximum in the USA and 2.5% O2 maximum in Europe. Now in the USA, use of methanol as a blending component of gasoline is not permitted due to its toxic nature.
| Table 8.5 |
| Permissible Limits of Oxygenates in Gasoline in Europe |
| Oxygenate Type |
% vol. |
| Methanol, suitable stabilizing agents must be added |
3 |
| Ethanol, stabilizing agents may be necessary |
5 |
| Isopropyl alcohol |
7 |
| Tertiary butyl alcohol |
7 |
| Iso-butyl alcohol |
10 |
Ethers containing 5 or more carbon atoms per molecule |
7 |
Other organic oxygenates or their mixtures defined in the regulations |
2.5% m/m oxygen, not exceeding the individual limits fixed above for each component |
Use of oxygenates in gasoline have the following features;
- Addition of oxygenates results in lower CO and HC emissions primarily due to mixture leaning effect more so in the carburettor vehicles.
- When oxygenates are added, oxygen is attached with some of the fuel molecules and hence an enhanced oxidation of fuel may be expected resulting in somewhat higher reduction in CO and HC emissions than the leaning effect alone.
- The magnitude of CO emission reduction for the closed loop feedback controlled vehicles using 3-Way catalysts is low as the mixture leaning effect occurs only during engine warm-up and acceleration phases. Under normal operating conditions, the feedback system controls air-fuel ratio close to stoichiometric.
- Excessive leaning caused by oxygenates can result in poor vehicle drivability and higher unburned hydrocarbons. Leaning effect may increase NOx.
- Use of oxygenates particularly alcohols also increases emissions of formaldehyde, which is a known carcinogen and has high photochemical reactivity.
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