Naked Science Forum
Non Life Sciences => Technology => Topic started by: chris on 17/09/2012 19:45:52
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A daft orange light turned itself on on my car the other day; naturally there's no way to diagnose what this lgiht means without going to a Ford dealer who plug it into a computer that reports the fault (and churns out a bill for 50 quid).
I was informed that the pressure sensor pipes on the particulate filter were leaking air and needed replacing. Ignoring the fact that these daft bits of plastic and the labour to fit them costs hundreds of Pounds, they did provoke an interesting conversation with the mechanic; i hadn't realised that the enhaust system periodically self-cleans by blowing in raw fuel, driving up the temperature to burn off solids on the catalyst surface.
I'd be intrigued to know how this works, how much fuel it consumes and how effective this burning is at doing the clean-up it's supposed to.
Anyone know?
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Most new cars have a ODBII interface. You should be able to pick up an ODBII reader for less than $50 (which is less than your dealership charged you for plugging your car in). Some parts stores will even plug your car into a diagnostic machine (ODBII reader) for free as they are always happy to sell you whatever parts it indicates. I'm not quite sure what the output is like as none of my vehicles are that new, but presumably you can look up the codes or information they provide on the internet for additional information.
As far as the burnoff, the more raw hydrocarbons the DPF gets (or a catalytic converter), the hotter it will burn. Just like adding more wood to a camp fire. In fact, you can damage a catalytic converter with a poorly tuned vehicle. My old Renault Encore had a Cat. It failed the emissions testing years ago, so when I pulled the CAT, it was completely empty, like an empty tube, presumably from burning too hot for too long.
I had thought I had seen a couple percent or so decrease in fuel efficiency with a DPF over a straight pipe due to both back pressure and the regen cycles. However, this article (in abstract) (http://www.sciencedirect.com/science/article/pii/S1352231011002330) seems to indicate no difference, although the test was apparently done in commercial trucks and buses which may be different from smaller passenger vehicles. Unfortunately I'm not seeing other notes at this time.
One additional thing to note about the DPF. There are two methods to blow raw fuel into the exhaust system. It can either inject raw fuel into the cylinders at the beginning of the exhaust stroke, thus re-using the already existing injectors, or it can inject the fuel directly into the exhaust pipe/manifold. The problem with the late in-cylinder injection cycle is that it has a tendency to coat the cylinder walls with fuel. Vegetable based fuels (biodiesel) tends to stick to, and lubricate the cylinder walls, but then is able to get past the rings, and cause oil dilution, and slowly rising oil levels (which is bad). Post cylinder injection, of course, doesn't have this problem.