Much Ado About 0.005%

What a tangled web we weave …

New direct-injection gasoline/petrol engines will be equipped with particle filters to meet evermore stringent emissions limits, imposed quite arbitrarily by bureaucrats let off the leash to run amok.

Let’s look at the scale of the “problem”:

The particles are produced in the first few minutes of direct-injection petrol engines when the finely sprayed fuel droplets fail to evaporate before being burnt. It’s less of a problem with manifold injection because the fuel has more of a chance to evaporate in the manifold and as it is drawn past the (hot) inlet valve. Direct injection is directly into the combustion chamber.

A continued drive by manufacturers to pursue engine technology that makes engines more efficient and cleaner has driven the adoption of direct injection; which somewhat ironically has resulted in the new “problem” of particulate emissions from petrol engines. Requiring innovation to clean it up. Ultimately at greater cost to the customer.

The conditions under which the droplets do not fully evaporate in time are temporary; during a cold start and for a few minutes afterwards. The microscopic soot particles produced during the warm-up phase are trapped in the particle filter at the exhaust of the engine.

The particles are so small, that it would take 20 of them, stacked end-to-end, to measure a millionth of a metre; or one thousandth of a millimetre.

Because of the filter needing to trap such fine particles, it would eventually clog up, making it progressively harder for the engine to work efficiently and indeed, safely. The filter also traps other larger molecules, but those evaporate easily over time, joining other exhaust gases to the end of the tailpipe, typically via catalytic converter and other emissions controls.

To prevent the particle filter from perhaps bursting into flames because of the soot buildup, engine management systems monitor the state of the filter and will light up one of those mysterious symbols in your dashboard when the driver has to do something about it.  As to when that, if ever happens in a particular car, depends on how it is driven.

When the light (or a similar one; consult your car’s owner’s manual) shows up, one should endeavour to go on a regeneration drive as soon as possible. “Regeneration” simply produces conditions under which the accumulated soot can be burn off safely. Until regeneration is successful, the engine management system will try to produce conditions for regeneration during your normal driving; which can make the car feel odd and to consume significantly more fuel than normal. (e.g. start/stop functionality will be curtailed)

A regeneration drive is, when it occurs early enough, the car being driven at between 60 to 130 km/h (obeying applicable speed limits) for a period of up to half an hour. Regeneration will be complete when the warning light extinguishes.

If such trips are part of your routine, then you may never see that light aglow.

Should regeneration drives not be undertaken or remain unsuccessful, then eventually, an array of warning lights such as these will require a workshop visit where workshop regeneration or worse; filter renewal will have to be undertaken. It is much, much cheaper to go for a long drive at least once a month with the same conditions as the regeneration drive; even if no regeneration warning light is showing yet.

Graphics from Volkswagen Self-Study Programme 558; “Close-coupled Petrol Particulate Filter”
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