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Gasoline Particulate Filter (GPF)

Diesel exhaustTwo years on we can now say that the VW Group diesel emissions scandal has had a definite effect on the UK’s car buying habits. In August the diesel market share of new car sales dipped below a psychologically significant 40% recording, at just 39.6% of sales. By contrast, across 2015 49.8% of new cars were diesel and even in 2016 the figure was 47%.

So as a result both alternatively fuelled cars (hybrid, pure electric, and plug-in hybrid) and petrol are gaining significant market share. This should be, and indeed is, good news in the battle to reduce air quality emissions as quickly as possible, but there is a potential fly in the ointment.

One of the reasons that modern petrol engines offer better fuel consumption than older ones is the move to direct injection, which offers significant efficiency gains under light loads i.e. a lot of real world driving situations. It is estimated that as many as 40% of new petrol engines are direct injection, and the share is increasing year on year. The downside with direct injection is particulate emissions, which traditional indirect fuel injection petrol engines don’t produce.

Why is this important? Well for various reasons there has been a massive amount of coverage of NOx emissions which you may recall have been significantly higher from diesel engines than petrol. For the record, NOx is bad for you. However there has been less discussion about particulate emissions (PM) because the UK meets current legal limits for PM emissions and many have assumed that the problem went away when all diesel engines were required to fit a diesel particulate filter (DPF) which are amazingly effective at catching PM, with 99% commonly caught.

Up until now, petrol engines haven’t been required to fit the equivalent Gasoline Particulate Filter (GPF), which means that all of the direct injection petrol cars currently circulating are throwing PM out the back, and we mean a significant amount. You can’t see it because it is very fine, and that is worrying because it is the ultra-fine portion of PM that is associated with the worst health impacts.

The World Health Organisation declared some time ago that diesel PM is carcinogenic, and both petrol and diesel PM are associated with all sorts of health impacts, including death. To underline this point, when assessing what the safe level of PM is, the WHO came to a rather sobering conclusion; namely there is no safe lower limit, in other words, any amount of petrol or diesel PM that you breath in has an adverse health impact. For us, that sounds like something you want to avoid.

At last, this month European legislation has caught up. As part of the efforts to stop another VW-type scandal, all new models have to undergo extra tests and from September 2018 every new car will be subjected to these tests. PM limits now for the first time apply to direct injection petrol engines which is fantastic news. In announcing the new standards the EU said that GPF would be required to hit the new limits, but within weeks we started picking up noises that not all car manufacturers agreed.

So where does this leave us? Well we believe that most manufacturers will fit GPF across their ranges even if they think they can get through the tests without one, because they recognise the very real risk that the whole emissions scandal story could blow up again if they don’t. However industry insiders have confirmed to us that not all are thinking in this way.

But if they can meet the standard without GPF, why should they fit one? The answer is simple; just like DPF, the petrol filters are amazingly good at their job, so the difference between two models both meeting the new EURO 6 tests will be significant, with the GPF equipped car almost eliminating PM, and the other still emitting meaningful amounts, indeed the limit for the number of individual particles is defined as 6.0×10 ^11 per km. If like us you’re trying to remember how scientific notation works, that means a EURO 6 compliant car can emit 6,000,000,000 individual particles every kilometre. Now remember the World Health Organisation’s assessment that there is no safe lower limit and you can understand why we should all be keen for every direct injection petrol engine to have GPF.

We hope that all manufacturers will see sense and fit GPF as standard. Each filter costs around £25, requires no maintenance, doesn’t increase fuel consumption and is simpler than a DPF because it won’t clog up in urban running as the exhaust from a petrol engine is hot enough to allow the filter to clean itself without any extra help.

In the meantime we’re going to be watching very carefully, and if any manufacturers do chance their arm and try to get by without a GPF, we will include this information in our reviews so that you can make an informed choice.

Andrew Leadbetter

Read more about the new WLTP fuel economy test