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Carbon deposits are a continuing problem for cars


March 27, 2000

Carbon deposits are a continuing problem for cars

The days might be long gone when low-compression petrol engines needed regularly to have their cylinder heads de-coked and their spark plugs cleaned of whiskery lead deposits.

But that does not mean deposits are not a continuing problem for car makers.

The emphasis these days has shifted away from the combustion space to the intake areas — the backs of valves, on the surface of intake runners, and in carburettors and fuel injectors.

That’s where the carbon-rich heavier components of the modern petrol blend tend to be deposited as the more volatile constituents evaporate when they contact the hot metal.

The PFI (port fuel injection) systems that are now virtually standard on every modern car are particularly susceptible. The deposits tend to form mainly in the 'hot soak' period after an engine is shut off and is still hot. The stationary petrol trapped in an injector is exposed to a higher temperature for a longer time than during its comparatively swift passage through the injector when the engine is running.

Seepage from injectors still under pressure in an engine that has just been shut down is also believed to play a part in the formation of deposits in intake ports and on the backs of valves. Carburettors, with their myriad of small-diameter fuel orifices, are also vulnerable to hot soak deposits.

Changes to the composition of petrol after the phasing out of lead have also contributed to an increase in intake-area deposits. One of tetra-ethyl lead's primary characteristics was its ability to boost the octane rating of petrol; nowadays, petrol blends require a higher level of unsaturated hydrocarbon compounds to achieve the same rating. These less stable products of the oil refining process are more likely to contribute to deposit formation in an engine's intake system.

Several decades of research have clearly demonstrated a link between intake area deposits and reduced power, worsened fuel economy and increased exhaust emissions. Clogged fuel injectors and carburettor metering orifices contribute to rough running and reduced driveability and their collateral penalties of poor fuel economy and increased maintenance costs.

Deposits on intake valves and ports have a similar negative effect: not only do they reduce full-throttle air flow, but also, during a cold start, they can absorb some of the petrol, thereby ‘leaning out’ the air/fuel ratio and leading to difficult starting and rough idling during warm-up.

There is evidence that modern 'fast burn' engines with their distinctive swirl intake patterns are even more adversely affected by valve and intake port deposits which disrupt the smooth flow of the mixture and so inhibit the formation of the swirl.

Detergent additives designed to keep the throttle bodies of carburettors clean were first added to petrol in the 1950s. Though they accomplished their primary task, they were less effective at keeping the smaller carburettor jets and fuel injector nozzles free from clogging unless used in high concentrations.

During the 1960s a new class of additives with a detergent/dispersant action was introduced. They worked well on keeping manifold bodies and intake ports clean, but had little effect on valve deposits and had poor carburettor and injector clean-up performance.

The first of the so-called deposit control additives was introduced in the 1970s and proved effective in cleaning up — and keeping clean — the critical areas in an engine's intake system. The chemistry of this generation of DC additives was based on polybutene amine; they worked well with leaded petrol, but with unleaded they produced unacceptably high combustion chamber deposits (the lead salts had acted as a catalyst to burn off the carbon).

This led to the development of a new generation of DC additives based on the chemistry of polyether amine, which provides excellent cleaning performance without contributing to combustion chamber deposits itself.

Growing concern over the harmful effects of deposits on exhaust emissions forced the Environmental Protection Agency in the USA to require that after 1995 all petrol sold there should contain additives to prevent their formation. Oil companies in New Zealand, in common with most developed countries, have also adopted a certain level of additives in their petrols.

However, at the minimum standards required by the EPA and adopted by many companies they will barely keep clean an otherwise well-maintained engine in a stable state and cannot clean up already-fouled systems.

And there is evidence that the general standard on cars in this country is far from ideal. A field survey on intake valve deposits conducted two years ago on cars from eight Asia-Pacific countries and the USA gave New Zealand a moderate score, with an average deposit rating of 8.3 on a scale of 10 (clean) to 1 (theoretical worst-case).

However, the survey was limited only to the Auckland area, which uses comparatively low-deposit petrol from Marsden Point. Other parts of the country receive their supplies largely from Australia, where the petrol is higher in olefins and sulphur and which had an average rating of 6.3 in the survey. Laboratory research indicates that this level of deposits is typically accompanied by a power loss of around four to five percent.

Caltex Vortex is based on the latest polyether amine chemistry. The petrol is the result of a technical collaboration between the oil company and Lubrizol, a leading player in the field of fuel and oil additives.

Three years ago Caltex invited tender submissions from all the additive makers for the production of a world-best DC additive. Lubrizol's proposal was accepted and further development work was carried out by the two companies' chemists to optimise its performance. Because the formulation was specifically commissioned, Vortex is exclusive to Caltex, and is likely to become the standard by which all other petrols are judged.

Its performance in standard laboratory procedures has been spectacular. It scored a near-perfect rating in a carburettor keep-clean test. In the equivalent PFI test, three of the injectors in the reference four-cylinder engine were still flowing at better than 99 percent of full capacity at the end of 10,000 miles (16,000 km), while the fourth had dropped to 97 percent.

Clean-up performance was equally as impressive: injectors in a PFI engine which had dropped to 90 and 94 percent of their original flow after less than 4000 km on a reference fuel were bought back to fully clean in one tank of Vortex.

Deposit build-up on the valves of a BMW 318i engine in another 10,000 mile standard test averaged below 50mg. This compares with the EPA standard of 100 mg and an average for the reference fuel of 300 mg.

The reference fuel used in these laboratory tests does not contain any additives, so its performance will be worse than petrols normally available in countries like New Zealand. However, as the field survey results showed, we still have a significant problem with the intake-system deposits which Vortex is formulated to handle.

Further standard tests have demonstrated that Caltex Vortex is neutral with respect to lubrication oils used in both four stroke and two-stroke engines, which makes it suitable for use in the full range of spark-ignition petrol power plants.

Nor does it have an observable effect on the combustion chamber deposits of petrol, even at additive concentrations far higher than will be sold.


Issued for Caltex New Zealand Limited by 141 Palace Plus

Tracey Brown Deborah Pead
Manager Brand Marketing Group Managing Director
Caltex New Zealand Limited 141 Palace Plus
Tel: 021 249 0471 021 612 919
0-4-495 6185 0-9-358 0141

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