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How A Tier-4 Engine Works

Do you remember your physics and chemistry teachers telling you to listen carefully because “one day you will need to know all this”? Well, for tractor owners and users, today is that day.

Modern diesel engines are real science in action: More powerful, compact, quieter and economical than ever before. The trouble is that they also now need to work up at higher combustion temperatures to improve fuel efficiency. But just as science teaches us that you can’t get something for nothing, the downside of all the extra heat is that harmful nitrogen oxide (NOx) production increases.

Until recently, designers have engineered lower combustion temperatures, using internal or external cooling systems and emission gas recirculation systems. But while cutting back the NOx levels, these systems can also be accompanied by a higher level of particulate matter (PM) as well as an increase in fuel consumption.

Currently, it is still technically feasible to comply with Stage IIIa emission regulations by modifying existing engines. But many feel this won’t be possible when Stage IIIb for larger engines takes effect in 2011 and Stage IV comes on stream in 2014. This is a predicament already facing the on-highway truck and bus industries, which are currently working to even stricter new rules. Virtually all the commercial truck and bus manufacturers, from DAF through to Volvo, currently rely on SCR technology to comply with the relevant Euro 5 engine emissions regulations that start to apply in 2009.

SCR employs aqueous ammonia or urea mixed in deionised water — the resulting mix is commonly known by its trade name AdBlue — to treat the exhaust gases. It’s important to note that the AdBlue used in the process is not a fuel additive but a “reductant” that is consumed during the chemical reaction.

AdBlue is sprayed in metered amounts into the exhaust gases. With the heat from the exhaust gases, the urea transforms into ammonia (NH3). Inside the catalytic converter, which is made from a chemically inactive noble metal, the ammonia then reacts with the nitrogen oxides to produce nitrogen gas (N2) and water vapour (H2O) — all fairly innocuous and harmless.

The resulting gases emitted through the exhaust pipe contain up to around 60 per cent less nitrogen oxide, at least 80 per cent lower particulate matter and up to about seven per cent less carbon dioxide.

Another benefit of SCR technology is that it allows engine designers to concentrate on optimising combustion and efficiency without worrying about the gas emissions, as all of the exhaust products are cleaned after they leave the engine. This results in the SCR engine’s claim to use three to eight per cent less fuel than a Stage IIIa engine, while delivering the same output.

The drawback, however, is that an SCR-equipped tractor needs an extra tank to hold the AdBlue mix. It takes three litres of AdBlue to treat the exhaust gas from 100 litres of diesel. This means operators of larger tractors have to fill an extra 50-litre capacity tank every other diesel fill. Also, current SCR technology is available on only new machines and it cannot be retrofitted.

While aqueous urea is fairly innocuous, it is quite sensitive. It is a colourless liquid that comprises about 34 per cent urea, which takes on a blue tinge when exposed to sunlight — hence its name. The European market leader is Brenntag GmbH, which supplies AdBlue under

the trade name “Air1.” The company’s Ulf Brockmann says: “It is not difficult to produce AdBlue, but it needs storing carefully.”

AdBlue freezes at -11C and begins to break down at about 30C. Freezing does not pose too much of a problem; all that happens is the emission levels increase. And as soon as the engine is back up to normal running temperature, the SCR system continues to work normally. Also, AdBlue should not be stored for more than a year, because after that time the solution can start to separate into ammonium hydroxide and carbon dioxide.

AdBlue availability is also not an issue, according to its suppliers. They say that the product can be found on forecourts in every corner of Europe, thanks to SCR-equipped and -reliant trucks travelling across the entire continent’s road/motorway network —from Ireland to Russia and from Norway to Portugal. Currently 1,500 petrol stations in Europe sell AdBlue and 10 to 15 new outlets are being added to AdBlue’s network every week.

The cost: AdBlue currently costs about 46p/litre (about $1 per litre) and is consumed on tractors at a rate of three per cent per litre of diesel. That’s three litres for every 100 litres of fuel. But SCR is said to save five per cent in fuel consumption so with the price of diesel, the SCR system could actually lower fuel costs.


After all the SCR theory, it’s now time to focus on how the technology is used in practice. Massey Ferguson introduced its new 370-hp 8690 at the Innov-Agri show in France in early September 2008. The tractor is powered by a Sisu Power Citius “e3” engine.

Not surprisingly, the little “e3” moniker refers to the use of AdBlue and heralds the world’s first tractor to come with SCR installed as standard. The only difference the casual observer spots is the addition, next to the fuel tank, of a small filler cap into which operators fill the AdBlue. This tank holds 52 litres and needs topping up at every other diesel fill.

The AdBlue vaporiser, which is designed by Bosch, is fitted to the right-hand side of the engine, in behind the turbo. This doses the correct quantity of liquid into the exhaust as it leaves the engine but before it enters the catalytic converter, which converts the exhaust into nitrogen and water vapour.

MF says it guarantees that AdBlue will be available locally to supply buyers of SCR tractors. If there is no existing local outlet, it will be supplied by the local MF dealer. AdBlue comes in a full range of container capacities from 1,000-litre IBCs down to practical fivelitre and one-litre sizes. Remember that five litres of AdBlue treats 160 litres of fuel.

But the crucial question remains: What happens if the tractor runs out of AdBlue? If the operator fails to top up the tank, the system sets off a low level warning. If this warning is ignored and the AdBlue runs out, the electronic system automatically cuts the engine power to half its output. Running the engine and SCR without AdBlue doesn’t do any damage but, as Gabriel Menier, MF’s chief engine designer, says: “Work stops being a pleasure.” Note that there’s no way to override the electronic cut-out.

Currently the MF 8690 does not meet the Stage IV emission limits, but it is ‘Stage IV ready’. MF claims nitrogen oxide is cut by 75 per cent and the emissions are cleaner, containing harmless nitrogen and water and much lower PM levels. Moreover, the tractor provides a claimed five per cent fuel saving compared with any alternative emission reduction systems. Why? Well, in simple terms, because the engine is set to work most efficiently without having to make any compromises to meet the emission regulations. That’s the theory. We await the full tractor test with interest.

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