Article about Diesel Emissions

daiking · 17-05-2006, 20:32

I thought this might interest some of the smogmonster drivers out there, gives an idea of Euro emissions levels. Sorry for infringing copyright, if its any consolation I'd recommend a subscription to PE magazine to anyone, its a very interesting read.

Dust busters
The designers of diesel engines are aiming to see the back of clouds of tiny particles billowing from exhausts. But its proving a difficult challenge. Richard Butler explains

Pollution from traffic is a concern for anybody who has to live or work near a busy road. Over the past 14 years, European emissions standards have tried to push carmakers into producing cars that are more pleasant to be around.

With each set of emissions regulations, the European Union raises the bar that bit higher. The first couple of these hurdles could be overcome by tweaking with injection timings and other methods of achieving more efficient combustion. But, with each stage of targets, new technologies are having to be added to help overcome the challenge.

Part of the need for this application of technology has come because of a shifting emphasis as the stages have progressed. At first, the regulations focused on reducing carbon monoxide but as they have gone on, the focus on oxides of nitrogen (NOx), and particulate matter has increased.

This is a particular challenge for makers of diesel engines, which tend to produce more particulate matter than petrol engines. Up until now, the carbon monoxide and unburnt hydrocarbon limits have been reached through optimising engine design. Introducing more oxygen, along with higher temperatures and pressures in the combustion chamber, has increased the likelihood of complete combustion of the fuel. This has minimised the output of unburnt hydrocarbons and carbon monoxide, which are the result of incomplete combustion. But the side effect of this increase in oxygen and temperature is that the engine becomes hot enough to start to combust the nitrogen in the air, leading to the formation of NOx.

Consequently, it isn’t possible to change the combustion parameters of engines to meet the NOx targets without compromising the gains already made in carbon monoxide and hydrocarbon emission levels.

The Euro IV standard, which already affects all new passenger vehicles, now applies to all new designs of light commercial and heavy goods vehicles. From October it will apply to all heavy goods vehicles, with light commercials falling into line at the start of 2007.

Recirculating the gases
Most makers of passenger and light commercial vehicles have met the Euro IV standard by optimising a technique called exhaust gas recirculation (EGR). As the name suggests, this involves taking the exhaust gases from the engine and feeding them back into the combustion chamber, along with fresh air. This allows the combustion temperature and oxygen excess to be controlled. Ford recirculates up to half of its exhaust gas back through the engine but has to carefully control this amount so that it doesn’t misfire.

Part of the problem is that diesel engines work across a very narrow rev range. Exhaust gas cannot be recirculated when the engine is idling, because it increases the risk of misfiring. Ford says: “The precision of the injection is close to medical levels at idle. We’re injecting are about 1mm3 of fuel into 0.5-litre of fresh air.”

At this point, the engine will be running at 800rpm, yet it would be expected to pull away powerfully at 1,000rpm and reach maximum torque at 1,800rpm. When the engine reaches the top of this range, again it is necessary to avoid recirculated gases being added to the engine in overdose so that it is receiving as much oxygen as possible to enable it to achieve full load. This means the amount of exhaust gas being fed back through the engine needs to be precisely controlled, so that it is added only when the engine can cope with it.

Although EGR is sufficient on its own to reach the Euro IV standards, it is unlikely to be enough to meet the Euro V standards that are in the process of being finalised. These are likely to be even stricter on particulate matter, which will almost certainly require the addition of diesel particulate filters. These will require even greater control of engines if they are to work effectively.

Countries such as Germany are already encouraging the uptake of diesel particulate filters that exceed the Euro IV standards. These use a silicon-carbide ceramic filter to collect the soot from the exhaust and trap it. This part of the action is simple enough. The problem comes from regenerating the filter, making sure it doesn’t just fill up and become clogged. This occurs when the filter is heated to 650°C, at which point the soot is completely burnt.

Unfortunately, diesel engines tend to run under 500°C, and only creep up to 750°C under full load. This presents a problem, particularly for small cars and light commercial vehicles that tend to be driven in slow-moving traffic.

To get around this problem, Ford has developed a system of sensors on either side of the filter, to monitor exhaust pressure and determine how clogged up the filter is getting. Part of the engine management system takes into account how the vehicle tends to be driven.

If it predicts that the vehicle will be driven at a high enough speed to increase the exhaust temperature and regenerate the filter, it will do nothing. But if the vehicle doesn’t often get driven in third gear or above, other strategies have to kick in. They include restricting the air flow into the engine and using additional fuel injections to raise the exhaust gas temperature. This can even get to the stage in which additional fuel is injected so late in the engine cycle that unburnt fuel is intentionally released into the catalytic converter, where it will be combusted catalytically, raising the temperature of the filter that follows it.

Ford says: “The regeneration of the filter requires the same amount of calculation and processing as it does to control the engine’s injection.”

Heavy goods vehicle manufacturers, such as MAN and Scania, that are using EGR to reach Euro IV and, they hope, Euro V, have already fitted particulate filters to their trucks.

Although EGR works well for most cars and light commercials, many producers of heavy goods vehicles (HGV) that spend more of their life running near full load, have decided that a catalytic method of reducing NOx is more effective. DaimlerChrysler has even used this method to make sure it more-than exceeds the emission limits. Selective catalytic reduction (SCR) involves adding another catalyst further down the tail pipe, after a conventional catalytic converter. This catalyst reduces NOx to molecular nitrogen and water.

This allows the engines to reach the EuroIV standards while running hotter and so more efficiently. Paccar, the owner of the Leyland, DAF and Foden brands believes the these efficiencies make SCR a better technology in the cost-concious HGV market. Engine maker Cummins predicts savings of 10% over EGR-only engines. But, unlike conventional catalytic converters, SCR requires the addition of additional reagents. In this case, ammonia or urea has to be sprayed on to the catalyst at regular intervals to break down the NOx that has accumulated.

The most popular reductive agent seems to be AdBlue, a solution of urea and water. It has the advantage of being less toxic and easier to handle than ammonia and has received support from most major manufacturers. However, its use will require a distribution network to be established. It also means vehicles must have a separate tank of reducing agent, which has to be topped up for the catalyst to work. This extra tank and the cost of yet more catalytic material will inevitably increase the cost of the vehicle.

Cost issues
Ford says this cost could be prohibitive. It says: “With three pretty complex units, such an exhaust system, if it came off in an accident, the cost would easily be comparable to the cost of replacing an engine.”

There is also the possibility that users could try to run vehicles without adding reductive agent, resulting in levels of emissions that exceed current standards. Proposals include onboard sensors that would restrict vehicle power output if they detected emissions were exceeding specified levels. But this will add yet more complexity and will not be ready for several years.

The US has already set emissions levels that will almost certainly require the use of SCR, which could effectively eliminate diesels from the market there. Ford thinks this is a missed opportunity. “Diesel would allow Americans to drive the kinds of vehicles they like with the kind of power they’re used to but with 30% better fuel economy,” predicts the company. The final limits for Euro V have not yet been determined for all vehicle classes, but there is concern that the next stage may do the same as in the US.

© PE Publishing, 17 May 2006

Kentish · 17-05-2006, 20:48

Petrols produce particulates too, just wait until they start on the petrol engine.

The way oil is going, you might have 20 more years of petrol/diesel before things really start to get serious.

17-05-2006, 20:48	#2
Kentish Briskodian Join Date: Feb 2003 Posts: 5,244 Members Car: 4 wheels, 4 doors. Thanks: 0 Thanked 11 Times in 7 Posts	Re: Article about Diesel Emissions Petrols produce particulates too, just wait until they start on the petrol engine. The way oil is going, you might have 20 more years of petrol/diesel before things really start to get serious. __________________ Andy Owner of a sense of humour.

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