Gearing Up For Euro 4/5 Emission Compliance

The next European emission standard – Euro 4 – comes into force on October 1 2006, for all newly registered commercial vehicles with a permitted GVW of 6 tons and above. It will be followed just three years later by the Euro 5 emission standard, effective October 1, 2009.

The short space of time between implementing the two standards means that it is necessary to start thinking about Euro 5 whilst Euro 4 technology developments are still on-going. In the case of urban buses, (average period of ownership 12 years) manufacturers should not expect their customers, most of whom carry out maintenance and repairs in their own workshops, to change systems yet again when Euro 5 comes in. Such a change would have negative effects regarding the procurement of replacement parts and special tools, employee training and documentation.

There is also a voluntary “EEV” (Enhanced Environmentally-friendly Vehicle) standard for commercial vehicles which, in some cases, prescribes even lower emission levels than Euro 5 in some. One has to be aware that this “standard”, which is not a legal requirement, (nor ever will be) constitutes what could be characterised as a moving target in that it will be adapted in line with the Euro 6 limits when these are defined.

For commercial vehicle engine manufacturers, the situation is aggravated by the fact that there are two further emission standards – each with different introduction dates, limit values and test cycles – namely the EPA standard for the NAFTA countries and the JES standard for Japan. This means that engine manufacturers have three different standards to contend with.

These standards focus on four types of emissions:

CO (carbon monoxide)
NMHC (non-methane hydrocarbons)
NOx (nitrogen oxides)
PM (particulate matter)

This analysis focuses on nitrogen oxide and particulate emissions.

A great deal of progress has been made with respect to the reduction of commercial vehicle emissions: by the time Euro 5 comes into force, the amount of particulate and soot emissions contained in exhaust gases will have been reduced by up to 97 % whilst nitrogen oxide emissions will have been cut by up to 86%, compared to exhaust-emission levels at the beginning of the 1990s in each case.

There are basically two different approaches that can be adopted in order to meet the increasingly stringent emission requirements, although it should be noted that, from a purely physical standpoint, engine developers find themselves in somewhat of a trilemma, as outlined below.

If the developers modify the engine to reduce nitrogen oxide emissions, the end result is increased untreated particulate emissions and, directly linked to this, higher fuel consumption. On the other hand, if they modify the engine to reduce particulate emissions, fuel consumption is generally lower but, inevitably, untreated nitrogen oxide emissions are higher.

Cooled exhaust gas recirculation (EGR) method

Here the aim is to cut NOx emissions by means of in-engine measures. Lowering the combustion temperature reduces NOx emissions. This happens when some of the exhaust gas is cooled and then directed back into the combustion chamber. Components required for this process include an EGR valve for controlling the recirculated exhaust gas and an EGR intercooler.

Generally, the now increased amount of particulate matter in the untreated exhaust gases has to be lowered to, or below, the required limit in the exhaust-gas flow downstream of the engine. This can be achieved by installing a diesel particulate filter (DPF), for example, although DPFs have to be cleaned periodically, they may also need replacing occasionally. This disadvantage can be avoided by installing a particulate (oxidising) catalytic converter in the form of an “open-loop system” that – whilst much less efficient than the closed-loop filter system – nevertheless requires no cleaning.

Exhaust gas recirculation systems do not require a second fuel or additive. Disadvantages of these systems include their technical complexity, with serious repercussions regarding repairs, downtime and, therefore, lifecycle costs. Furthermore, the EGR intercooler and intake manifold have to deal with combustion residues which can also get into the engine and cause sooting. In addition, greater demands are placed on the vehicle’s cooling system, thereby rendering it even more inefficient, the degree of efficiency having already been adversely affected by the extra fuel consumption due to the principle involved. The increased amount of particulate in the untreated emissions has to be reduced by means of exhaust gas aftertreatment; the systems generally used for this process require the use of special, low-ash oils and/or low-sulphur fuels, neither of which are always readily available. In some cases, the greater demands placed on the engine oil also shorten oil change intervals.

Since EGR technology has already been introduced, manufacturers can reckon with lower development expenditure in the case of technologies that are entirely new to the automotive industry. Theoretically it would be possible to comply with the Euro 5 standard using EGR; however, due to the required high rates of exhaust gas recirculation, EGR is inextricably linked to unacceptable effects on fuel consumption and, therefore, economic efficiency.

Selective Catalytic Reduction (SCR)

DaimlerChrysler AG and its subsidiary EvoBus GmbH – responsible for DaimlerChrysler bus and coach business in Europe – have opted for future-compatible SCR (Selective Catalytic Reduction) technology, as have Volvo, Iveco/Irisbus, Cummins and DAF. Together these manufacturers account for some 80 % of the European truck market and more than 66 % of the European bus and coach market. By 2009 at the latest, when the Euro 5 standard comes into force, these market shares will have grown further still, not least because only SCR technology ensures compliance with the further reduced NOx limits stipulated by Euro 5.

This technology relies on engine modifications to optimise particulate emissions and, in addition, ensures lower fuel consumption than in the case of the EGR system. Naturally CO2 emissions are also reduced as a consequence.

The increased quantity of nitrogen oxides (NOx), inevitably present in the exhaust gas, is converted into harmless nitrogen and water vapour in a catalytic converter following the injection of an aqueous reducing agent (AdBlue) into the exhaust-gas stream. The catalytic converter, mounted in place of the currently fitted silencer, is designed to last for the entire vehicle lifetime, as are the other components in the system. Periodic cleaning or replacement, as in the case of a particulate filter, is not necessary. AdBlue is carried in a separate tank in the vehicle.

BlueTec is the name of the enhanced diesel technology with a downstream emission control system developed for Mercedes-Benz and Setra buses and coaches. Although it has been successfully used for controlling emissions in power stations, heavy industry and shipbuilding for a number of years, this technology – which also attains the limits specified by future standards such as Euro 5 – is new to the automotive sector.

The EU Air Quality Directive, enforceable in all EU member states, stipulates stringent limits for NO2 (nitrogen dioxide) from 2010 onwards. If BlueTec is already specified, there is little point in fitting an exhaust gas recirculation system to reduce the amount of NO2 in the NOx emissions because BlueTec vehicles produce hardly any NO2 at all.

The required 32.5-percent aqueous urea solution – trade name “AdBlue” – is specified to DIN 70070. Previously large quantities of urea solution were used as a base material in the fertiliser and cosmetics industries. It is a non-toxic, non-hazardous material. In Germany AdBlue, alongside other harmless substances such as drinking milk, is classified in water contamination class 1. Today it retails at around ? 0.65 at public filling stations, whereas the price can fall to between ?0.45 and e0.55 if a vehicle depot has its own pump or filling station, and of course depending on how much AdBlue the depot buys in bulk. AdBlue consumption is around 2 to 4 percent of diesel consumption.

The size of the AdBlue Tank fitted in Mercedes-Benz and Setra buses and coaches means that it only needs to be refilled half as often as the diesel tank.

The two filler holes are less than a metre apart so that that the diesel and the AdBlue can be pumped in at the same time. What’s more, it is not possible to admix the diesel and the AdBlue, since the AdBlue nozzles at all filling stations have a solenoid valve whilst the vehicle’s filler neck contains a transmitter – the AdBlue only starts to flow once the valve hits the transmitter. Furthermore it is not possible to pump diesel into the AdBlue tank because the diesel nozzle is too large to fit into the AdBlue filler hole. Many manufacturers – including Univar, Yara/Brenntag, Kruse Chemie, OMV and GreenChem – already offer a wide range of different-sized filling station units which depots can buy or rent.

Since AdBlue starts to freeze at temperatures below -11°C, the tank in the vehicle and the lines are heated both by the coolant and by electrical means. The same applies to the filling station units themselves, as long as they are not housed indoors. Here the tank itself as well as the filler hose and the nozzle are heated electrically. Energy consumption is low due to the tank’s additional insulation. And the temperature is only kept at approx. -6°C, which is more than sufficient for ensuring the fluidity of AdBlue.

Advocates of exhaust gas recirculation cite two arguments against SCR technology from an environmental standpoint:

1. In urban traffic, the exhaust gases are not denitrified in many cases, since the required catalytic converter temperatures are not reached due to the considerable amount of time the urban bus engine spends idling or overrunning.
2. The increased injection and ignition pressures in the BlueTec engines lead to an increased incidence of harmful microparticles due to the finer fuel atomisation and reduced particulate matter.

Both of these arguments have been proven to be unfounded because:

1. In BlueTec engines, the emission control system functions reliably whatever the operating conditions

Field tests carried out by EvoBus at a very early stage of development in a large city with a flat topography proved that the SCR catalytic converter functions reliably whatever the driving conditions. Above all, the high temperature storage capacity of the catalytic converter, which is heated up every time the vehicle starts off, ensures that the required temperature of at least 200°C is maintained, even during prolonged periods of idling.

2. BlueTec engines also drastically reduce microparticulate matter

By design, all diesel engines produce particulate during the combustion process. However, more than 90 % of the particulate matter produced is oxidised during expansion in the cylinder. The amount of particulate matter that leaves the cylinder is therefore always many times lower than the amount of particulate initially formed in the air deficiency areas. Here it is important to note that the increased injection pressure leads to smaller fuel droplets but not smaller soot particles; fuel droplets cannot form soot. For this to happen, the fuel must be in vapour form. But as we know, vapour has no memory of whether it was formed from small or large droplets.

The oxidisation of particles requires, above all, high pressure, high temperature, swirl, time and a large particle surface area. Assuming they have the same mass, lots of small particles form a larger surface area than just a few large particles. It follows that the optimal conditions for particle oxidation prevail only during the combustion process itself. Since the oxidation conditions in the combustion chamber can be described as optimal, the required oxidation time is negligible.

The overall amount of particulate matter can be reduced by improving the oxidation conditions. DaimlerChrysler BlueTec engines are designed with high injection and combustion pressures as well as a suitably shaped combustion chamber so as to ensure the particulate produced is reduced during the expansion phase in the combustion chamber – to such an extent that the Euro 4 and Euro 5 particulate limits are reliably attained.

During the oxidation process in the combustion chamber, the smallest particles are oxidised first as they have the largest surface area and largely predominate at this stage of the combustion process. As expansion continues, the general conditions for particle oxidation become less favourable. The pressure, temperature and degree of turbulence all drop whilst the remaining small particles grow in size by a process of coagulation and adhesion. In this context it can be seen that engines which offer optimised oxidation conditions for the “microparticulate” are especially effective. DaimlerChrysler BlueTec engines take into account these physical requirements and are meticulously designed with this causal chain in mind.

Reconciling emission and immission requirements

New EU Air Quality Directives which limit the percentage of pollutants per cubic metre of air have been in force since January 2005.

The Euro 4 and Euro 5 standards limit emissions of NOx (nitrogen oxides). The Air Quality Directives, on the other hand, prescribe maximum values for immissions of NO2 (nitrogen dioxide) which, as explained above, account for a varyingly large proportion of the total quantity of NOx emitted by engines.

The debate around soot and particulate emissions in urban areas especially is placing local public transport more and more under the spotlight. According to EU Directive 1999/30/EC, the limits for PM10 (<10 micrometers) size particles are only allowed to be exceeded for a maximum of 35 days, effective 01.01.2005. From this date onwards, municipalities are obliged to ensure that the limits are not exceeded. Alongside traffic-management and traffic-relieving measures, further options being considered include (partial) bans on the use of older diesel vehicles or vehicles without particulate filters in certain urban areas. There is, therefore, a fear that even urban buses used for local public transport could be affected by driving bans. However, many municipalities have realised that, at certain points where the particulate is measured, it would not be possible to attain the limits even if all the vehicles that passed by had an electric drive system, i.e. were local zero-emission vehicles.

Although the exceeding of particulate limits has less to do with traffic-related emissions and much more to do with meteorological and other influences, traffic (which of course includes diesel buses) finds itself at the centre of the public debate and provokes a strong reaction from both politicians and the media. An emotive debate about particulate emissions (PM10) and the extent to which urban regular-service buses emit particulate matter is currently raging in several European countries, but the actual facts of the matter are all too often ignored in the main lines of argumentation.

• Two dozen buses today are no louder than a single vehicle from 1970. And whereas noise emissions in 1974 were still 91 decibels, nowadays they have been reduced to below 80 decibels. Here it must be remembered that a three-decibel reduction in the noise level is equivalent in effect to a halving of the subjectively perceived noise emissions.
• A 12-metre long standard regular-service bus with a full complement of passengers on-board requires just 0.5 litres of diesel to carry one passenger a distance of 100 km.
• Low-emission buses play an important role when it comes to complying with the soot limits stipulated in the EU Air Quality Directive. It should also be remembered that only a fraction of PM10 emissions are caused by traffic: in Germany, for example, general road traffic is only responsible for around a fifth of all soot emissions. Buses account for no more than 10 percent of this and, therefore, less than 2 percent of all PM10 emissions. And even most of these emissions come from buses that were designed to comply with earlier Euro standards.

Low primary energy consumption alone makes the urban bus an extremely ecological means of transport. As well having a lower load per unit area than the cars it takes to carry the same number of passengers, urban buses produce much less particulate matter, which is caused by brake and tyre wear as well as the swirling-up of road dirt. Despite this, the repeated demands for additional emission-curbing measures to be developed and implemented quickly – in order to not only meet but also exceed the requirements stipulated in the new emission standards – often target urban buses in particular.

Although the ecological leverage is relatively small with respect to the size of the European bus and coach market – currently around 25,000 units per year compared to almost 300,000 trucks per year – bus and coach manufacturers have been meeting the ecological challenge head-on for a number of years. By way of example, regular-service buses produced by EvoBus GmbH have been available with particulate filters for some considerable time, and there is still an attractive range of retrofittable particulate filters to choose from. Similarly, the range of urban buses with natural-gas drive, hybrid drive field tests and, last but not least, the large-scale field test currently involving 36 hydrogen-powered urban buses are all proof of the work being done in the interests of climate protection.

It should also be noted that the average development expenditure per vehicle is much higher for urban buses than it is for passenger cars or trucks – and this despite drastically increased price pressure in the bus market.

With BlueTec, EvoBus is not just introducing a new emission control system; the installation of a further tank, with specific requirements regarding heating and insulation, also has a profound effect on the design of the vehicle structure. This is particularly true in the case of low-floor urban buses, since installation space is severely limited and stringent requirements have to be met in terms of passenger safety, passenger comfort, reliability, fuel consumption and other operating costs.

In order to fulfil these requirements, design and optimisation measures aimed at reducing noise and weight et al must be backed up by extensive field tests to gauge strength and endurance, hence the extensive pre-production testing programme. In contrast to trucks, whose chassis are based on a largely standardised design, the situation for buses and coaches is aggravated by the fact that, as well as extremely limited installation space, there is also a large number of variants to consider (vertical/horizontal engines, V/in-line engines, different lengths and heights for solo buses, different floors, articulated buses, chassis units, rear-end variants). This degree of complexity increases design and testing expenditure several-fold.

In the case of buses and coaches especially, each new technology has to function reliably in every possible situation and under all possible conditions (driving conditions, temperature conditions), since these vehicles carry a human “cargo”. Passengers do not just want buses to be environmentally friendly, they also expect them to offer value for money, comfort and reliability – and this for a period of 15 years or more.

The “Blue Angel” seal of environmental quality

The Citaro natural-gas buses offered by the Mercedes-Benz Bus and Coach unit – part of EvoBus GmbH, responsible for the European bus and coach activities of DaimlerChrysler AG – are sophisticated urban regular-service buses that measure 12 m or 18 m in length and feature the all-new, extremely powerful yet highly economical M 447 hLAG Euro 4 natural-gas engine with an output of either 185 kW or 240 kW.

Powered by natural gas, the Citaro low-floor urban buses are extremely low-emission vehicles, especially in terms of nitrogen oxides and particulate. Developed specifically for this vehicle, the M 447 hLAG natural-gas engine with a turbocharger, charge-air cooler, lambda control and oxidising catalytic converter operates on the extremely fuel-efficient lean-mix principle. As well as boasting an ex-factory configuration that ensures compliance with the stringent Euro 4 emission standard due to come into force imminently, this particular vehicle goes several stages further, since its engine is certified to the EEV (Enhanced Environmentally friendly Vehicles) standard. EEV prescribes even lower emission limits than the extremely stringent Euro 5 standard, which is not due to come in until 2008/2009. Customers can also specify EEV certification as an option.

The EEV-certified drive system, the low noise emissions and the environmentally friendly production of the buses at the DaimlerChrysler plant in Mannheim have earned these vehicles the “Blue Angel” seal of environmental quality awarded by RAL e.V. This accolade delivers resounding proof of the environmental compatibility of the natural-gas buses. By the end of 2005, well in excess of 200 Citaro natural-gas buses will be working on regular-service routes in a host of European cities.

Fuel cell: the future has already begun

Recent months have seen DaimlerChrysler make giant strides forward with the fuel cell drive system, a project the company has been working on for a number of years. It represents the biggest revolution ever seen in the field of drive engineering. DaimlerChrysler unveiled the first vehicle in Europe to be equipped with a fuel cell that could be operated under practically any everyday conditions – the “Necar l” (New Electric Car) – in Ulm back in April 1994. With the ensuing Necar vehicle concepts and the “Nebus” (New Electric Bus) fuel cell bus prototype presented in May 1997, the company proved that it is now possible to build fuel cells which, although not much heavier and larger than an internal combustion engine, deliver the same power output.

From both an environmental standpoint and in terms of longevity, the fuel cell buses effectively add double the value. Firstly they are local zero-emission vehicles. Secondly, over the longer term, it will be possible to produce the fuel largely using renewable energy sources. In contrast to the hydrogen internal combustion engine, the fuel cell relies on “cold” combustion. It is also practically soundless and thus considerably reduces noise and vibrations throughout the entire powertrain.

The Nebus – a prototype based on the O 405 low-floor urban regular-service bus – was the first Mercedes-Benz bus to put fuel cell technology on the road in 1997 and, since then, has shown proof of its technical capabilities at various locations around the world.

The Mercedes-Benz Citaro urban regular-service bus with fuel cell drive is the direct successor to the Nebus: the 12-metre long low-floor solo vehicle has a range of around 200 kilometres and – depending on the customer’s specifications – can carry up to 70 passengers. The fuel cell unit with a power output of more than 200 kW and the pressurised gas cylinders containing hydrogen compressed to 350 bar are accommodated on the roof of the Citaro buses. The top speed has been electronically limited to 70 km/h.

Fuel cell drives have the best possible chance of becoming the drive systems of the future. They combine the mileage range of conventional internal combustion engines with high efficiency, low fuel consumption and minimal or even zero emissions.What’s more, they are extremely quiet and ensure a high degree of ride comfort. Powered by fuels produced using renewable energy sources, they also signal an end to the dependency on crude oil and other fossil fuels.

Fuel cells themselves produce no emissions whatsoever: when hydrogen and oxygen react to form water, the electrical energy generated is used to supply the electric motor which, in turn, drives the vehicle. Fuel cell vehicles do not produce any harmful emissions: the only “exhaust gas” emitted by fuel cell vehicles, which carry their hydrogen in on-board tanks, is pure water vapour. For fleets of buses, taxis or delivery vehicles in particular, filling up with hydrogen is an ideal solution because these vehicles need only to have a limited range and can always return to a central filling station. This means that a cost-intensive, comprehensive filling station infrastructure is not necessary.

Last but not least, buses with fuel cell drive are not only clean, they are also very quiet, all of which contribute to vastly improve the quality of life in our towns and cities in many respects.

The first Mercedes-Benz Citaro with fuel cell drive was handed over to the Mayor of Madrid on May 5, 2003 during that year’s UITP Congress. Today a total of 33 Citaro buses with fuel cell drive are on the road in Europe and Australia. Three further vehicles are due to go into service in China in September 2005. These zero-emission vehicles will be used daily on demanding regular-service routes. The cities taking part are Amsterdam, Barcelona, Hamburg, London, Luxembourg, Madrid, Beijing, Perth, Porto, Reykjavik, Stockholm and Stuttgart, making this the world’s largest fuel cell project. To date the vehicles have clocked up a total in excess of 900,000 km, with an availability of over 85 percent. By the end of the project, the vehicles will presumably have covered more than a million kilometres. Once the project is over, the data will be evaluated and used as a basis for further projects featuring this new technology.