Tuesday, 14 July 2015

Mazda 3 2.2 DPF in day to day use

This post will be of no interest whatsoever to 99.8% of people but for those few who run a Mazda Diesel engine or plan on buying one in the near future it may be useful. I'm also pretty sure that much of my experience will be transferable to other modern diesel cars because most of them work in exactly the same way.

For the most part as technology advances things, whatever they may be work better - this has been especially true in the automotive world. Modern cars are more reliable, safer, offer vastly improved performance and have a bewildering array of standard features many of which a few short years ago were only available on the highest spec models as expensive optional extras. They're also kinder to the environment, much kinder - but unlike many of the other advances in the case of modern diesel engines this has come at quite a cost.

Diesel in comparison with petrol is a dirty fuel it tends to burn incompletely, you can see this yourself following heavy vehicles. The clouds of black smoke you often see billowing from exhaust pipes is made up from billions of tiny soot particles from fuel that hasn't quite finished burning. These particulates as they are know cause and exasperate serious health problems like Asthma and they are also carcinogenic.  

To meet stringent emission controls coming into force around the world (Euro 6 for example here in Europe) engine manufacturers have had to work extremely hard. Many innovations have benefited the consumer; improved engine emmisons, drivability and power. Direct fuel injection, multistage turbo charging, lighter engine components are all good things, the same can't be said however for the diesel particulate filter - commonly known as the DPF. You can see the effect of the filter in the picture below. The first picture is off my Mazda 3 2.2 Skyactive diesel, the second is from a Alfa Romeo Guilleta petrol. You'll notice they're both very clean, it's not actually obvious which is which. Before the days of DPF's you'd have been able tell the diesel car just by looking at the blackened tail pipe. DPF's do stop particulates but they also add complexity, cars equipped with them often require more looking after.

DPF's are fitted in the vehicle's exhaust, typically near the headers which is the hottest part of the system. They're a honeycomb trap designed to capture particles made in the combustion of diesel. Typically high volumes of particulates are only made for a short length of time (for example under hard acceleration or during cold starts). The idea is particulates are captured during these times and then slowly burnt off during normal fast running e.g motorway, this is known as passive regeneration. It's great in theory but in practice vehicles don't always generate enough heat to adaquerly clear the DPF.

Manufactures are aware of this and they've designed in an active clearing cycle known as a active regeneration (often just referred to as a regen) The active regeneration is done by injecting additional fuel during the exhaust stroke which is immediately ejected from the cylinders into the exhaust where it burns. This drastically increases exhaust temperature and burns off the trapped particulates, after this all that is left is a tiny amount of ash. The active regenerations don't run all the time, only when required. The DPF has a pressure sensor either side of the trap, the fuller the DPF becomes the greater the pressure differential. From this the cars engine management system knows when to trigger the regeneration.

The problems really kick in if cycles are not allowed to complete (say for example you get where you are going or you only do short journeys), for a reason I don't full understand this leads to excess diesel ending up in the engines oil. Diesel isn't a great lubricant so excess engine wear becomes inevitable. The issues don't stop there, it's actually possible for the sump level to rise so far the engine actually starts to run on it's own sump oil and revs itself to oblivion. Again manufacturers are aware of this but they are limited with solutions. Now to be fair you will get a warning on the dash if you interrupt too many regeneration attempts which indicates the car needs to be driven on a longer journey to clear the DPF but this doesn't stop the oil level rising.

My Skyactive Mazda 3 2.2 Diesel demonstrates the problem in the pictures below. You can clearly see that the oil level rose above the maximum in a little over 1000 miles from new! At the rate it was progressing it was not possible to get to the 12,500 mile service interval. You'll notice an X on the dipstick near the kink, this is Mazda's must change now warning point. If the oil level reaches X you risk destroying your engine which is astonishing in a modern car. Mazda state that an owner should check the oil every 500 miles or weekly and you can see why.

I've run three Mazda 3's, two 185bhp diesels and my current Skyactive Mazda3. In my first two DPF car's I never had the oil rise significantly, as you can see that's not the case for the Skyactive car. My suspicion is that the lower compression ratio of the Skyactive diesel (which makes for a smoother, quieter, higher revving engine) has the side effect of a cooler exhaust gas temperatures which makes it harder for the car to passively clear the DPF. My Mazda's are driven primarily on motorways 28,000 miles a year, if my driving won't clear a Skyactive's DPF few people's driving cycles will.

Interestingly the new Skyactive seems to run a DPF cycle every 150 miles or so, you can tell easily when a cycle is running. Firstly the exhaust note changes when you're off the throttle (you get a booming rumble) and secondly the start stop system is disabled and the car will show you that the engine isn't ready on the fuel economy app. The older 185bp car was not equipped with a start stop system so it was less obvious but I remember only hearing the exhaust note change occasionally, certainly not every couple of days.

I really like Mazda and their diesels are great to drive but think before you buy, if you don't drive on fast roads over longer distances buy a petrol. I feel this is a bit of a shame but the technology behind DPFs is flawed, I'm sure this will be resolved in the future (why they don't heat the DPF using an electrical element for example rather than diesel puzzles me, seems like an obvious solution) but for now they add to the maintenance burden and who wants that?

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