Specialist focus: Plant hire: Diesel filters are the future

Like it or not, diesel particulate filters (DPFs) are becoming a reality some four years ahead of the legislation that will - more or less - require manufacturers to fit them as standard items. Currently DPFs are required only on certain sites as specified by the London Best Practice Guide (BPG), but other cities will follow and soon machinery in most large conurbations across the UK will require a DPF.

Within the London Low Emission Zone (LEZ) for trucks, buses and vans, those wanting to avoid the surcharge can simply buy a new or fairly modern vehicle. But those having to comply with the BPG have to do everything themselves.

Caterpillar on trial

Hewden is tackling the problem head-on and tasked one of its managers, Roy Bage, to sort things out. Bage selected a Cat 303C CR excavator with a 22kW engine as the trial machine. While this is below the proposed 37kW cut-off, below which filters will not be needed by the BPG, Bage chose the 3.5t model because it is highly utilised in many situations.

"The first problem is that most manufacturers make particulate filters for machines above 100kW, very few make them for machines this small," says Bage. Caterpillar recommended he speak to Mann + Hummel, a company best known for making oil and air filters. However, Mann + Hummel also makes exhaust after treatment, retrofit centrifugal oil spinners and closed crankcase ventilation systems, as well as a sintered metal particulate filter, suitable for engines up to 130kW, designated the SMF-AR.

Bage was attracted to the filter for several reasons:

• the system requires no operator input
• the sintered metal filter element will withstand red diesel's high sulphur level
• it will last the life of the engine
• the element can be cleaned with a pressure washer to remove the accumulated ash deposited by repeatedly burning off the captured particles.

The housing is 180mm in diameter and comes in either 525mm or 660mm length depending on the throughput. On the Cat 303C, the filter directly replaces the standard exhaust muffler and so does not obstruct the operator's view or represent a burn hazard.

Ulrich Lampen, Mann + Hummel's manager for retrofitting DPFs, says the SMF wall-flow element is best described as 'pleated' (see illustration). As the sintered element is inert, a dosing pump adds a liquid catalyst to the fuel at the rate of 1:2,500. As the particulates are formed in the combustion chamber, they are coated by the additive, which reduces to around 180°C the temperature at which the particulates captured in the filter start to burn. If the exhaust temperature is not hot enough to trigger this automatically, there is a battery-powered heating coil at the outlet end of the element.

Regeneration activated

Pressure is monitored both before and after the filter and if the differential reaches a pre-determined level, the system is prompted to regenerate by activating the heating coil. Once the particles have ignited, the burn moves from one end of the element to the other and takes about two minutes, during which time the machine can keep working.

While no operator input is required, there is a readout in the cab showing the backpressure, operating temperature and so on. It also houses warning lights/buzzers to indicate any fault, such as excessive backpressure - usually around 200 milibars - or if the liquid catalyst reservoir is empty.

Space also has to be found on the machine to accom-modate the catalytic fluid reservoir, a dosing pump, pressure sensor, an electronic control unit and the readout.

Low maintenance

Beyond refilling the reservoir, followed by pressing the reset button, regular maintenance comprises removing the element every 2,000hrs/annually using the quick-release clamps, and cleaning off the accumulated ash with a pressure washer. Lampen says there is no problem with the ash going to landfill as it is totally inert, made up of 80% ferrous oxide from the fuel-born catalyst, with the rest coming from burnt engine oil.

Having installed the first system, Mann + Hummel can deliver the components for Hewden's staff to fit subsequent machines. However, there are a lot of settings required after the components are installed and the system has to be reset when the reservoir is refilled. The extended set up time increases the installation cost to around £675 on top of the £3,400 billfor the components.

"I can see that the first machine requires a fair degree of sorting out, but after that it should be very straight forward," Bage says. "I need something my people can fit in a couple of hours with just a live and earth to connect to the electronics and a warning light and buzzer in the cab. The manufacturer that comes up with such a system will be the one that wins all the business."

Bage is still evaluating the system but already there are areas he thinks could be improved. "We could do with a transparent catalytic fluid reservoir so you see how full it is." Lampen says this is already under consideration. Bage's other concern is the amount of electronic setting and resetting required to install and run the system.

Since fitting the DPF, Bage says the machine has been out on various jobs using red diesel and some operators have been told about the device, others have not. "It's been very reliable and we've not had any problems," he says, adding that he is very happy with how the trial is progressing. "One comment we have had when the machine was working indoors, is that the site workers could not smell the diesel - and that has to be good," he concludes.

How a dpf works

Diesel particulate filters fall into two camps: those with replaceable elements, such as those supplied by EHC Teknik, and those with permanent elements, such as the one featured here.

With the first system, the filter element must be replaced once it has become loaded with trapped particles to an extent that the backpressure exceeds a pre-determined level. With the permanent element, before the backpressure gets too high to damage the engine, the partially blocked filter regenerates by burning the contaminants and reducing them to ash.

If the exhaust gas temperature is high enough, regeneration will happen automatically, otherwise some form of heating is required, which can involve electrical elements, injecting diesel or removing the filter element and placing it in an oven.