Clean stan­dard

Pump drive from St. Georgen helps reduce nitrogen oxides in diesel emis­sions.

The EURO 6 stan­dard reduces harmful emis­sions from diesel engines signif­i­cantly – those of carbon partic­u­late matter and those of nitrogen oxide (NOx). The strin­gent limits that take effect 1st September 2014 cannot be met just by improving combus­tion in the engine. There­fore, many manu­fac­turers of commer­cial vehi­cles and passenger cars use what is known as a Selec­tive Catalytic reduc­tion System (SCR). In this form of exhaust gas treat­ment, nitrogen oxide emis­sions are reduced by adding an ultra­pure urea solu­tion. A high-perfor­mance, reli­able pump with an ebm-papst drive injects the urea addi­tive. “This will become stan­dard in commer­cial vehi­cles,” states Peter Metzger, Head of Sales Auto­mo­tive at ebm-papst in St. Georgen, confi­dently. For diesel engines of lorries weighing over 3.5 tonnes, the legis­la­tion dictates that the exhaust gases must be treated such that they are largely free of nitrogen oxides. Compared to the EURO 5 stan­dard, the NOx limits are being decreased by 80 per cent, to 500 mg/kWh. “For passenger cars, this is largely depen­dent on the type of engine and future legis­la­tion. The system is not absolutely neces­sary for diesel engines with small volumes, as the emis­sions are already below the currently valid limits,” Metzger explains. “However, we expect the limits to be made even more strin­gent.”

To render nitrogen oxide harm­less, power plants have long added ammonia to the flue gas. However, ammonia is a highly irri­tating, poiso­nous gas. It is a hazardous mate­rial that can be trans­ported only in pres­surised liquid form. There­fore, ammonia is unsuit­able for mobile use in vehi­cles. It takes a chem­ical trick to enable the cleaning process in lorries and passenger cars: The vehicle-compat­ible SCR uses a solu­tion of urea in water as an addi­tive, which breaks down into its initial compo­nents only when strongly affected by heat. This solu­tion is now tried and tested. The exhaust gas, which is prac­ti­cally free of carbon partic­u­late matter, exits the engine into the exhaust gas system. Here, the addi­tive – urea dissolved in distilled water – is metered. The high temper­a­tures break down the urea imme­di­ately into carbon dioxide and ammonia. At the down­stream catalytic converter, this reacts with the nitrogen oxides to create water and nitrogen – thus cleaning the exhaust gas.

On average, this process consumes an amount of satu­rated urea solu­tion equal to about six per cent that of the fuel. Thus a 60-litre tank of addi­tive lasts about 4,000 kilo­me­tres if the diesel fuel consump­tion of a lorry is 25 litres per 100 kilo­me­tres. However, the fuel consump­tion also becomes lower at the same time, as the diesel engine runs more effi­ciently. For this method of decreasing nitrogen oxide emis­sions, Bosch devel­oped a pumping module and a dosing unit. The reduc­tion of nitrogen oxides must work reli­ably over a wide flow rate range. Because urea solu­tion becomes a solid at below-zero temper­a­tures, an internal heater in the dispensing device must thaw the lines quickly after the engine starts. The quan­tity of addi­tive that the pump injects directly into the exhaust gas system must be adapted exactly to the amount of nitrogen oxide in the exhaust gas. During this process, a closed-loop control system with the actual value set by an NOx sensor regu­lates the timing of the metering of the addi­tive for each nozzle at a cycle of 1 Hertz. At a system pres­sure of 9 bar, the dosing unit guar­an­tees the required mass flow rate: from 36 grams per hour when idling up to 7.2 kilo­grams at full load. If the pres­sure drops, the deni­tri­fi­ca­tion is incom­plete – a demanding task for the pump drive. There­fore, Bosch incor­po­rated the drive xperts of ebm-papst St. Georgen into the devel­op­ment. “Bosch was looking for a partner with whom they could develop the motor and elec­tronics,” Metzger adds.

ebm-papst pump drives for a cleaner envi­ron­ment

This resulted in a modular drive concept with EC motor. The modular design makes it possible to keep following the current state of changing legal require­ments. The pump, hydraulic compo­nents and elec­tronics can be replaced indi­vid­u­ally. The drive experts from Germany’s Black Forest region devised an elegant solu­tion to this chal­lenge: The external rotor motors they produce are known for very good synchro­nous running and high torque, even at start-up. The drives are also highly compact. The devel­opers made use of these basic prop­er­ties and further improved the drive for auto­mo­tive appli­ca­tions. They opti­mised the elec­tro­mag­netic compat­i­bility for mobile use and designed the bearing system and all other compo­nents for a long service life in the required temper­a­ture range. The load profiles are typical for auto­mo­tive and place a load on the engine not only when it is in oper­a­tion, but also when it is not moving. In contin­uous oper­a­tion at a constant load, this is over four and a half years, as Metzger stresses: “In a lorry, this corre­sponds to a service life of over one million kilo­me­tres.” The multi-pole motor sits together with the elec­tronic commu­ta­tion on a board and has vari­able lengths. This way the overall height of eight millime­tres provides suffi­cient output for oper­ating a car. The 13-millimetre version corre­sponds to the larger mass flow rate for indus­trial vehi­cles. Both are equipped with a wide voltage input and there­fore enable use in a 12/24-volt vehicle elec­trical system. The high require­ments demanded by the auto­mo­tive industry for supplier compo­nents effort­lessly fulfil the 100 per cent quality control of fully auto­mated manu­fac­turing. Peter Metzger under­scores that exhaust gas treat­ment with SCR will become an impor­tant compet­i­tive factor for the auto­mo­tive industry in the coming years. “The envi­ron­mental require­ments will also continue to rise substan­tially inter­na­tion­ally. For that reason, markets in which exhaust gas treat­ment has hardly played a role to date will quickly incor­po­rate it.”

Clean stan­dard
The dilemma of consump­tion and carbon partic­u­late matter vs. nitrogen oxide