Cat­alytic con­vert­ers & diesel par­tic­u­late fil­ters

How to make sure your emis­sions sys­tems are up to scratch.

Car Mechanics (UK) - - Contents -

Al­though in­ter­nal com­bus­tion en­gines are com­pact, light­weight and re­li­able sources of power, one of their down­sides is the pol­lu­tants that emerge from the ex­haust. With more ve­hi­cles on the road than ever be­fore, most tech­no­log­i­cal ad­vances in recent decades have been ded­i­cated to re­duc­ing the neg­a­tive im­pact on both the en­vi­ron­ment and hu­man health. Mod­ern ve­hi­cles now con­tain a plethora of emis­sion con­trol sys­tems, in­clud­ing cat­alytic con­vert­ers and diesel par­tic­u­late fil­ters (DPFS), and mo­torists have an obli­ga­tion, both legally and morally, to en­sure they are kept in op­ti­mum con­di­tion. Main­tain­ing and re­pair­ing ex­haust pipe-mounted tech­nolo­gies can also save you money in the long run.

Tak­ing con­trol

Strict en­gine reg­u­la­tion is vi­tal to en­sur­ing the ef­fi­ciency and re­li­a­bil­ity of emis­sions con­trol equip­ment. In the case of petrol en­gines, a cat­alytic con­verter’s abil­ity to work ef­fec­tively over a typ­i­cal ve­hi­cle’s life­time de­pends greatly on the fuel sys­tem main­tain­ing an op­ti­mum 14.3:1 air-to-petrol mix. This op­ti­mum ra­tio is re­ferred to as Lambda 1, which ex­plains not only why Lambda/oxy­gen sen­sors are used within ex­haust sys­tems to mea­sure the gases’ oxy­gen con­tent, and pro­mote the en­gine man­age­ment sys­tem to ad­just the fuelling ac­cord­ingly, but also is why Lambda is mea­sured as part of a non-hy­brid petrol MOT emis­sions test. Should ex­cess fuel be in­tro­duced into the catal­yser due to a rich mix­ture – caused by an en­gine fault, poor main­te­nance, or un­suc­cess­ful push-start­ing, for ex­am­ple – the con­verter’s in­ter­nal hon­ey­comb can over­heat and crack, melt or col­lapse. Un­sur­pris­ingly, its pol­lu­tant re­duc­tion func­tion is ren­dered use­less.

Iron­i­cally, with diesel en­gines, dos­ing fuel into the ex­haust sys­tem is de­sir­able to con­trol soot emis­sions un­der cer­tain con­di­tions. This is not to ben­e­fit the diesel cat­alytic con­verter (which is un­harmed by the process) but rather the diesel par­tic­u­late fil­ter (DPF) that is mounted fur­ther down­stream. Un­like a catal­yser, the fil­ter must be emp­tied reg­u­larly and it does this au­to­mat­i­cally by su­per-heat­ing its con­tents. Should nat­u­ral fast-road driv­ing not gen­er­ate suf­fi­ciently high tem­per­a­tures to

re­gen­er­ate the DPF, the en­gine elec­tron­ics will step in to per­form an ‘ac­tive re­gen­er­a­tion’, which in­tro­duces fuel into the ex­haust sys­tem on pur­pose. The suc­cess of this process re­lies on a spe­cific set of vari­ables be­ing sat­is­fied, which de­pends upon the con­di­tion of other sys­tems and even the fuel level. There­fore, be aware that, for mod­ern diesel en­gines that are equipped with DPFS, a blocked par­tic­u­late fil­ter may be a symp­tom of a fault else­where and not the cause.

What goes wrong with cats

Cat­alytic con­vert­ers are very re­li­able. Most is­sues are caused by ei­ther a rich mix­ture, or con­tam­i­na­tion orig­i­nat­ing from a faulty en­gine com­po­nent, such as a fail­ing turbo and the valve stem seals that dis­charge oil into it, or a head gas­ket that causes the con­verter to be ‘poi­soned’ by sil­i­con from the coolant. Im­pact dam­age may be a fac­tor, too, es­pe­cially if your sus­pen­sion has been low­ered, rais­ing the risk of the cat be­ing struck. Ther­mal shock, which might be caused by driv­ing through deep flood­wa­ter, can crack the del­i­cate in­ter­nals, too. In some cases, bro­ken ce­ramic ma­te­rial can be scoope­dup by the flow­ing ex­haust gases and pinned against the out­let; the re­sult­ing block­age causes back-pres­sure high enough to cause a loss of power un­der load.

In most cases, cat fail­ure in­di­cates an­other fault. Noth­ing lasts for­ever and, af­ter very high mileages have elapsed, the pre­cious met­als can be­come de­tached from the hon­ey­comb brick and get blownout. In­ter­est­ingly, these frag­ments re­main so valu­able that one Bri­tish uni­ver­sity has de­vel­oped a sys­tem by which pre­cious met­als from cat­alytic con­vert­ers can be ex­tracted from road sweep­ings. This does lit­tle to ben­e­fit the car owner, who can­not scoop them up and ‘glue’ them back into the ex­haust sys­tem!

As the cat­alytic con­verter’s per­for­mance drops, the first an owner may know about it is when an MOT emis­sions test is failed, or an en­gine man­age­ment fault code is gen­er­ated. Var­i­ous fuel ad­di­tives have been de­vel­oped that claim to ei­ther clean the catal­yser’s in­ter­nals of soot or even to re­store some of the met­als that have been lost. The suc­cess rate de­pends on the rea­sons why the catal­yser has failed and if the claims made by the ad­di­tive maker are true.

A diesel par­tic­u­late fil­ter re­lies on a cat­alytic con­verter to func­tion, but it is a soot trap and not a catal­yser. Fit­ted to some Euro IV emis­sions ve­hi­cles from around 2004, DPFS ful­fil the manda­tory Euro V en­vi­ron­men­tal di­rec­tive by cap­tur­ing soot, which in­cludes par­ti­cles of un­burned hy­dro­car­bons and oil, rather than ex­pelling them into the at­mos­phere.

The fil­ter medium is housed within a can­is­ter and the core con­sists of nu­mer­ous in­ter­nal chan­nels, each of them are blocked at one end. Pass­ing gases are forced through the por­ous ce­ramic walls, which trap the soot de­posits. In fast, open-road driv­ing, the gases flow through the walls eas­ily and the par­ti­cles be­come trapped at the end of the fil­ter. As the soot is com­bustible, it may burn away nat­u­rally, due to the high tem­per­a­tures cre­ated at fast speeds and el­e­vated loads – this is called 'pas­sive re­gen­er­a­tion'. The pres­sure dif­fer­ence is de­tected by ei­ther one, or two, elec­tri­cal sen­sors, which take a mea­sure­ment via one, or a pair, of hol­low tubes at­tached to the DPF. In slow-mov­ing traf­fic, the gases do not pass through the con­duits as read­ily, mean­ing that trapped soot builds layers in the in­ter­nal chan­nels within the cen­tre of the fil­ter, re­sult­ing in a high pres­sure dif­fer­ence be­tween the fil­ter’s en­trance and exit ports.

Once a pre-de­ter­mined pres­sure is reached – typ­i­cally around a 45% load­ing – the car will su­per­heat the DPF by ei­ther in­ject­ing fuel into the ex­haust sys­tem via a sep­a­rate fuel injector or, more com­monly, the en­gine fuel in­jec­tors fire ex­tra pulses of diesel dur­ing the ex­haust stroke, which passes through the man­i­fold tract and into the catal­yser and DPF. This process – 'ac­tive re­gen­er­a­tion' – de­mands that the car is driven at set speeds and throt­tle po­si­tions for around 20 min­utes, which per­mits the catal­yser and DPF to reach around 600°C. Dur­ing this time, it is com­mon for the en­gine to cre­ate load by ac­ti­vat­ing elec­tri­cal ac­ces­sories (in­clud­ing, pos­si­bly, the air-con­di­tion­ing), trig­ger­ing the ra­di­a­tor cooling fans, clos­ing the EGR valve to raise the in­take air tem­per­a­tures and even re­strict­ing the air­flow, prior to ad­vanc­ing the in­jec­tion tim­ing and dos­ing ex­tra fuel into the ex­haust.

Nat­u­rally, all of these stages rely heav­ily on en­gine man­age­ment set­tings, com­plex al­go­rithms and whether, or not, the right con­di­tions are met for a suc­cess­ful re­gen­er­a­tion. Try­ing to thrash the car hard in short bursts of max­i­mum ac­cel­er­a­tion in low gears tends to in­crease the DPF’S soot load­ing. Un­for­tu­nately, dur­ing pas­sive re­gen­er­a­tion, the en­gine oil is con­tam­i­nated with wasted diesel and this is why you should re­set any ser­vice in­di­ca­tor lamp di­ag­nos­ti­cally post­main­te­nance, change the oil reg­u­larly and never over­fill the sump.

Once the DPF has re­gen­er­ated, the va­cated par­tic­u­lates cause the pres­sure dif­fer­en­tial to drop and the cy­cle re­peats it­self. How­ever, the vapourised soot leaves ash be­hind. This can­not be re­moved in sub­se­quent re­gen­er­a­tions and even on-car clean­ings. While the car’s ECU’S al­go­rithms es­ti­mate the ash lev­els, de­pend­ing on mileage, con­di­tions and the num­ber of suc­cess­ful re­gen­er­a­tions, most driv­ers needn’t worry about hav­ing the DPF re­moved to clean-out the ash un­til at least 150,000 miles have elapsed, pro­vided that main­te­nance ad­vice has been fol­lowed.

Cat­alytic con­vert­ers and DPFS are housed within boxes in­cor­po­rated with the ex­haust sys­tem and mounted close to the en­gine.

Al­ways read the of­fi­cial owner hand­book for your ve­hi­cle, so that you are fa­mil­iar with the na­ture of any warn­ing lamps. Be wary that some diesels il­lu­mi­nate a fas­cia lamp not to in­di­cate a fault but to in­struct the mo­torist to drive the car for longer.

Some models (mainly Peu­geots and Citroëns, plus some Volvos, Maz­das and Fords) re­duce DPF soot burn­ing tem­per­a­tures by dos­ing the diesel au­to­mat­i­cally with a fuel-borne cat­a­lyst, Eolys. This tends to re­quire pe­ri­odic top­ping-up. Note that there are...

De­fects will pre­vent the emis­sions con­trol sys­tems from func­tion­ing cor­rectly. This bro­ken swirl flap lever (ar­rowed) will in­hibit a suc­cess­ful DPF ac­tive re­gen­er­a­tion cy­cle, for ex­am­ple.

The pre­cious met­als used within catal­y­sers and DPFS make them es­pe­cially vul­ner­a­ble to be­ing stolen by thieves with hand-held cut­ting gear – 4x4 ve­hi­cles are par­tic­u­larly at risk, as it is eas­ier to crawl be­neath them. For­tu­nately, var­i­ous se­cu­rity...

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