Particulate problems: it’s not just diesels

Particulate emissions from diesel cars have long been the bête noir of the fuel, and thus been a focus of European legislation since Euro emissions standards first became mandatory in 1992.

Indeed, limits for PM10s (those ultra-fine and particularly harmful particulates) have been reduced in every update of the regulations, from Euro 1’s 0.14g/km limit through 0.08g/km for Euro 2, 0.05g/km in Euro 3, 0.025g/km in Euro 4 – and the biggie, just 0.005g/km on Euro 5.

Virtually negligible, in other words – which is why so many modern diesels come with diesel particulate traps and other soot-limiting technologies.

It’s also why the newest diesels don’t smoke when drivers put their foot down.

But it is not just diesels that are affected by particulate restrictions. In the latest Euro 5 regulations, introduced in 2009, gasoline cars also have the 0.005g/km target – the first time petrol cars have had particulates regulations.

Why is this? Surely petrol cars don’t actually give out any particulates, due to the less-dense fuel’s spark ignition? Well, largely, no – but for one recent development that has changed things…

It’s because of gasoline direct injection

Direct injection petrol cars are the cause of this regulation change – indeed, it’s only GDI cars that are oblighed to reach a maximum target for partilates.

It seems direct injection with petrol cars is more susceptible to producing fine exahust particulates, so legislation has had to be written to make sure they’re capped.

Why does gasoline direct injection cause particulates? 

The problem is cause by the fact fuel is, yes, injected directly into the combustion champer. This can cause fuel droplets to form a film on the combustion surfaces, leading to unburned bits of fuel and, thus, particulates.

Whereas indirect injection gives the fuel chance to fully mix with air, direct injection doesn’t allow this premix stage. Hence, the need for much more careful combustion control – and also the risk of particulates if those big globs of fuel still emerge despite this.

Legislation has thus stepped in. Many will be surprised that petrol cars can be susceptible to those dreaded PM10s in the same way as diesels, but it’s passed neither the EU nor engine designers by.

I do wonder if the 2009 legislation is part of the reason why direct injection petrol engines are not more commonplace, though – and whether the extra cost of any particulate filters could erode any cost advantage petrol cars currently have over diesels, which offsets their reduced fuel consumption.

Particulates to actually do diesel engines a service then, after years of being the thorn in their side? Time will tell…