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TECHNOLOGY UPDATE

4th December 1997
Page 52
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Page 52, 4th December 1997 — TECHNOLOGY UPDATE
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Which of the following most accurately describes the problem?

Truck manufacturers and operators have been through the Euro-1 and Euro-2 engine barriers with less pain than was once thought. Now Euro3 is looming the threat is increased fuel consumption in order to comply with the tightening emission standards. Exhaust gas recirculation, already applied successfully in small diesels, could save the day. Gibb Grace explains.

FsF. . St the bad news. The proposed Euro-3 emission standards are so stringent that most industry insiders are convinced that fuel consumption will worsen as a consequence. However, development work carried out by Ricardo Consulting Engineers and the Dutch Ministry of the Environment (VROM)* suggests that Exhaust Gas Recirculation may be one way of reducing the fuel consumption penalty.

Production of particulate matter (PM), oxides of nitrogen (NOx) and fuel consumption are naturally mutually dependent—if any one of the three is reduced, the other two are automatic fly increased. Retarding the fuel injection f ming, for example, reduces NOx and PM, k it worsens fuel consumption.

This d lemma has been at the centre of engine de .,ign since emissions were first controlled ur ler ECE 49 in 1985. However, thanks to develi pments such as improved engine breathin , the use of higher pressure fuel injection an I accurate electronic control, the designe s managed to deliver decreases in NOx at Euro-1, PM and NOx at Euro-2, and yet still ichieve small fuel savings.

Deg te those past successes Euro-3 is looking like a bridge too far, and if the engine makers are :0 meet the emissions limits, it looks as thougl they will have no choice but to accept the en issions/fuel consumption trade-off and pay d e fuel consumption penalty RVI, Scania and ll LAN all predict a worsening in fuel consumr don by as much as 5 to 8% if the propose I levels of PM (0.1g,/kWh) and NOx (5.0g 'kWh) are to be reached. If the fuel consum tion is to be held at the Euro-2 level, a new way of reducing PM or NOx must be found.

NOx is the real bete noir of the diesel, and redwing it to 5g/kWh is difficult Reducing it by using non-selective catalytic reduction of the exhaust by converters looks unlikely at thi 3 stage. The OxyCat type used in the US reiles on the use of almost sulphur-free fuel w. iich is not readily available Over here, and the more effective DeNthe type uses signffiant amounts of diesel in its operation. Selective Catalytic Reduction (SCR) is potenC.ally more effective, but apart from the high cost, is unlikely to be viable in time to help Nith Euro-3. The only other route is Exhaust Cies Recirculation (EGR), used successfully in car/van engines for more than a decade.

THE EGR SOLIMON

NOx is formed as a result of the high temperatures in the combustion chamber, and EGR works by taking some of the spent exhaust gas, which is low in oxygen, and mixing it with fresh air entering the engine.

Diluting the air leads to lower combustion temperatures and reduces the amount of NOx, avoiding the need to retard the fuel injection timing, thus preserving fuel consumption.

To be useful in a truck engine, EGR would

have to work at full load —it is used only at part load in smaller engines. Also in large engines the boost pressure is generally greater than the exhaust pressure, which makes it more difficult to get the exhaust gas back into the inlet manifold.

To test the EGR solution on a truck diesel, Ricardo modified an existing 24-valve Euro-2 engine by adding an EGR circuit The diagram below shows how this was done. The feed was taken from upstream of the turbocharger through a special EGR cooler and via an EGR valve into the inlet manifold.

When run on the E49 emission test cycle the engine achieved the proposed Euro-3 emission Levels with a fuel consumption penalty of 2 to 3% over Euro-2. Considering the combustion had not been optimised for Euro-3 this has to be an especially encouraging result. EGR HARDWARE EGR involves three major new components, a variable geometry turbocharger (VGT), an EGR valve, and an EGR cooler In production, these parts would be attached to the engine. However, in the Ricardo/VROM test their size and layout were far from ideal.

EGR OPERATION The amount of recirculated air the engine can cope with is expressed as a percentage, and varies depending on its load and speed. In the test engine, typical EGR rates were 5 to 8% at full load, 11 to 14% at half load and 22 to 28% at 10% load. EGR was not used at all below the maximum torque speed.

EGR SOFIWARE Getting the design of the hardware right is important in terms of its durability and reliability, but equally important is the design of the electronic control.

In order to avoid poor engine response, or smoke puffs on acceleration, the software has to co-ordinate the airflow, the fuel and the recirculated air. The team designed a mathematical model of the induction system using the mass of air entering the cylinders as the starting point. This was used to detect whether the engine had reached the lower limit of its air/fuel ratio, below which smoke would become unacceptable. Fuelling was then restricted to contain the air/fuel ratio, and this point used to trigger the EGR valve and the VGT.

The necessary fine control of the EGR rate was achieved by altering the VGT vanes using Li pneumatic dctuator.

OPERATIONAL CONSIDERATIONS The Ricardo/VROM tests not only showed that EGR is feasible, but demonstrated in a 1,000-hour test that the engine oil may not be as vulnerable as expected. Not only was the base engine unaffected, hut subsequent analysis of the engine oil showed the all important soot content and Total Base Number remained normal, inferring no likely reduction in current oil drain intervals With the bench tests completed, the EGR system was fitted to a new engine in a standard production truck and tested for driveability. The EGR system was not used for cold starts or during warm-up, and operated only once the engine coolant reached 60°C. Subjective testing of the laden truck showed the engine response was not impaired, and in some cases the engine responded slightly more quickly than the standard engine, presumably bemuse of the VGT. The only changes made were to the software, to ensure the EGR valve closed at minimum fuelling to suit the use of the engine exhaust brake.

EGR puts additional demands on the turbocharger, and for high power engines, compressor ratios of up to 3.5 to 1 will be needed, and the development of either an air-flow sensor, or an air-fuel ratio sensor, suited to heavy duty applications, could further improve EGR control. But, if the manufacturers are to reduce the fuel consumption penalty that seems to come as standard with Euro-3, they will have to invest in some form of additional technology, and from the Ricardo/ VROM work, EGR looks a good candidate.

by Gibb Grace * Demonstrations of a Euro 3 heavy-duty diesel engine using EGR, by Dr C Havenith, Netherlands Ministry of the Environment (VROM), CH Such, BC Porter and AJ Nicol, Ricardo Consulting Engineers.

Variable geometry turbocharger

In the engine speed/load range where EGR is used, the exhaust manifold pressure has to be maintained slightly above that of the intake manifold and this is best achieved with a variable geometry turbocharger (VGT). Normally, the vanes which direct the flow of air on to the rotor are fixed, but in the VGT they are moveable and can thus supply much more air than normal, especially at lower engine speeds. The test set-up had provision for an exhaust back pressure valve, but because the VGT was successful in maintaining she required pressure, it proved unnecessary.

EGR cooler At full power the exhaust gas temperature was typically 570°C, much too hot to reroute back into the inlet manifold, and so it was reduced by a water-cooled heat exchanger. The simple plate and tube cooler used was plumbed into the cylinder block and though a prototype, worked very satisfactorily throughout the test programme. It reduced the full power exhaust temperature from 570 to 200`C, and the half load temperature from 425 to 175C. EGR valve

The EGR valve is an air-operated, poppet type valve which can be opened or shut in response to signals from an electronic processor. During the test the valve was initially sited ahead of the EGR cooler, but it was subsequently shown to function better downstream from the cooler. In the test engine two small valves were used in parallel, but in production an appropriate size single valve would be used. The VGT is continuously variable over its full range, with the actual setting being controlled as a function of engine speed and load.


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