C OVER STORY ON - AND OFF- HIGHWAY ENGINES

NEW GENERATION OF MTU SERIES 2000 ENGINES MTU has developed its Series 2000 engines to meet the emissions requirement of US EPA Tier 4 for off-highway diesel engines in the power range above 560 kW. The result is a family of twelve- and 16-cylinder engines which comply with the emissions guidelines using only exhaust gas recirculation (EGR) and without any exhaust gas treatment measures. A test of a dump truck demonstrated that they also use around 25 % less fuel than the original engine.

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AUTHORS

DIPL.-ING. WERNER KASPER is Senior Manager of Thermodynamics at the MTU Friedrichshafen GmbH in Friedrichshafen (Germany).

DIPL.-ING. (FH) AND MSC ALEXANDER RICHTER is Product Manager Series 2000 Application Construction and Industrial/Mining at the MTU Friedrichs hafen GmbH in Friedrichshafen (Germany).

DIPL.-ING. (FH) HEIKO WINGART is Senior Manager of Design Series 2000 Construction and Industrial at the MTU Friedrichshafen GmbH in Friedrichshafen (Germany).

EMISSIONS THRESHOLDS LEAD TO FURTHER DEVELOPMENT

MTU Series 2000 engines have been proving their mettle around the world in the off-highway applications Construction and Industrial (C&I) and Oil and Gas (O&G) since 1997 [1, 2]. They can be found everywhere from mobile applications such as haul trucks, excavators, large drilling rigs and dockside cranes to stationary applications like pump drive units in the O&G industry. As a result, they have to cope with the extreme conditions to be found in every climate zone on Earth. The driving force behind the fundamental further development of Series 2000 engines was the tightening of emissions limits demanded by US EPA Tier 4 interim (Tier 4i) and by Tier 4 final (Tier 4f), which is due to come into force in 2015 for off-highway diesel engines producing over 560 kW. With a cylinder displacement of 2.23 dm³, the V12 and V16 engines cover a power range from 567 to 1163 kW. A V12 unit from the new series has been undergoing field trials in a 100 t truck in a South African platinum mine since January 2012. This vehicle was repowered by MTU at the end user’s site and after logging up a runtime of 5000 h, positive results are now available. This sixth-generation Series 2000 unit (200006) from MTU Friedrichshafen offers a

series-production concept for this performance range which meets Tier 4i targets using exclusively in-engine technology. DEVELOPMENT TARGETS

The overall design concept is based on a set of concept specifications covering critical purchasing criteria such as : low fuel consumption and life-cycle costs (LCC) : extended maintenance intervals and low maintenance requirement : long service life with up to 20,000 h of operation before the first major overhaul : high power density (weight-related) : good transient characteristics : low amount of heat to be dissipated : compliance with emission limits without the need for additional consumables. The ability to meet emissions targets is a crucial factor for market acceptance and economic elements such as LCC and reliability likewise play vital roles here. The wide application range means that a modular concept is needed to ensure that component options are available to cover all the application variables with just one model type. Within the ISO C1 cycle, ❶, EPA Tier 4i restricts nitrogen oxides (NOx) to 3.5 g/kWh, hydrocarbons to 0.4 g/kWh and particulate matter to 0.1 g/kWh. The so-called not-to-exceed (NTE) zone is calculated from a range of perfor-

❶ C1-cycle and NTE-area exemplarily for power ratings >560 kW in off-highway applications 12I2013

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C OVER STORY ON - AND OFF- HIGHWAY ENGINES

mance data and engine speeds together with the location of the operating point with the lowest fuel consumption. Emissions limits permit ambient temperatures up to 38 °C (100 °F) and working altitudes up to 1680 m (5500 ft). Emissions patterns during dynamic operation are regulated by the so-called Transient Smoke Cycle (TSC), ❷, which stipulates that average opacity must not exceed 20 % during acceleration or 15 % during lugging, with a maximum peak of 50 % which must never be exceeded. These limits must be observed over an EPAdefined useful life of 8000 h. For operators, LCCs are especially relevant. At 87 %, ❸, these are primarily made up by the cost of fuel. Component development and the harmonisation of performance maps were optimised on this basis. The actual engine purchase price represents only 4 % of the total costs. Exclusively commercial use of the engines coupled with high annual runtimes – partly under extreme conditions at heights beyond 2500 m (8202 ft) and temperatures up to 55 °C (131 °F) – demand concepts which can deliver high levels of robustness and reliability. Engines intended for universal use across a range of applications need a wide performance map with maximum torque and rated power available over a wide speed range. And to utilise the engines in the most important markets without emissions regulations, they

❷ TSC for off-highway applications with power ratings >560 kW

need to be able to operate with sulfur levels up to 500 ppm because supplies of low-sulfur fuel cannot be guaranteed worldwide, ❹. COMBUSTION CONCEPT, KEY TECHNOLOGIES

MTU chose cooled exhaust gas recirculation (EGR) to achieve NOx limits. This decision was in keeping with the company´s aim of dispensing completely with SCR technology or other exhaust aftertreatment techniques, even for Tier 4f. Combustion development work on a single-cylinder engine showed that compliance with emissions regulations re-

quires an EGR rate of up to 30 %. MTU´s donor cylinder concept, ❺, makes it possible to achieve this without the disadvantages of conventional EGR technology. To maintain a constant combustion air ratio with the high EGR rate required and at any given power, the density of the cylinder charge has to be increased correspondingly. To achieve this across a wide engine performance map and at the stipulated NTE working altitude, MTU decided on two-stage controlled turbocharging technology. The negative effects of EGR on particulate emissions and fuel consumption are compensated by the positive influence of the common rail (CR) system in conjunction with an optimised combustion chamber. Here, a large-series production 220 MPa CR system was selected. COOLED EXHAUST GAS RECIRCULATION, DONOR CYLINDER CONCEPT

❸ Life-cycle costs for haul trucks

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Modern turbochargers achieve high levels of efficiency. Consequently, the desired charge pressure can be generated from a relatively low exhaust pressure upstream of the turbine. This produces a positive scavenging pressure difference across wide areas of the engine performance map. The low pressure in the exhaust manifold results in low levels of pumping losses in the cylinders and low fuel consumption. The pressure gradient between the exhaust manifold and the charge-air manifold which is needed for exhaust gas recirculation is realised by means of the donor cylinders [3, 4]. The

basic concept involves designating cylinders that will contribute up to 100 % of their exhaust gas volume to EGR and others which feed all of their exhaust gas exclusively to the exhaust turbine. The location of the donor cylinder flap (1 in ⑤) determines the number of donor cylinders. The twelve-cylinder version has four donor cylinders and the 16-cylinder model has five, facilitating stable achievement of the 30 % (approximately) EGR rate. EGR operation is deactivated via regulating valve (2 in ⑤) in order to avoid sooting of the EGR heat exchanger during low load operation or to relieve the engine and cooling system in extreme conditions (very high ambient temperatures, high-altitude operation). The EGR rates required for compliance with emissions limits lie between 10 and 30 % within the NTE zone. From the exhaust gas extraction point downstream of the donor cylinders to EGR infeed into the charge-air line, the recirculated exhaust gas undergoes a pressure drop of up to 900 hPa. In normal high-pressure EGR systems, back-pressure in the exhaust line for all cylinders would have to be built up to a correspondingly high level. With the donor cylinder concept, however, there is a positive scavenging gradient across the relevant area of the engine performance map and this means that charge-air pressure is higher than the pressure in the exhaust line. In EGR mode, the scavenging gradient is only negative for the donor cylinders. The mean pumping losses (and thus fuel consumption) are therefore less than in high-pressure EGR systems with throttling on all cylinders, ❻. To achieve good engine emissions characteristics, combustion conditions must be the same for all cylinders and this means that EGR distribution must be as uniform as possible. Extensive flow simulations show that the best way to achieve EGR homogenisation is to make the mixing tract for charge air and recirculated exhaust gas as long as possible. Effective EGR distribution has been confirmed in tests. TWO-STAGE CONTROLLED EXHAUST TURBOCHARGING

MTU has a long tradition of exhaust turbocharger (ETC) development and manufacture [5]. For reasons of space, the ETC concept chosen here places the 12I2013

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❹ Sulfur content in diesel fuel

turbocharger with the turbine supported directly on the exhaust line. This means that the turbochargers must not weigh more than approximately 50 kg each. For the air flow rate of the V16 engine, MTU‘s ZR 125 ETC series was extended by the addition of a low-pressure (LP) ETC with high flow capacity. With a comparable flow rate, ❼, the new turbocharger is significantly smaller and lighter than its competitor product. The compressor´s enhanced flow capacity was achieved using new blade geometry with splitter blades which are set back. With two-stage turbocharging, the lower pressure ratio at each stage made it possible to lower the compressor pressure rating as compared with compressors for single-stage turbocharging systems, and the maximum circumferential speed was limited to 560 m/s.

Field trials and tests involving performance map harmonisation showed that the design of the two-stage turbocharging system with three turbochargers of roughly equal size (two LP stages and one HP stage) was also ideal with regard to transient characteristics. The air intake direction permitted by specific application conditions and the positions of the two charge-air coolers were decisive in determining the location of the turbocharger configuration. The LP ETCs can be turned through 180° to enable air infeed from the driving end as well as the free end. To secure the ETCs, a carrier housing was developed which accommodates exhaust routing to and from the ETC and also houses the bypass valve for charge-pressure control. The two-stage controlled turbocharging system offers extremely high charge-

❺ Air and exhaust system diagram for 12V 2000-06 engine with two-stage controlled turbocharging and EGR donor cylinder (HP CAC = high-pressure charge-air cooler, LP CAC = low-pressure charge-air cooler, EGR HE = exhaust gas recirculation heat exchanger, EGR flap = exhaust gas recirculation heat exchanger flap)

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C OVER STORY ON - AND OFF- HIGHWAY ENGINES

pressure potential. This permits engine operation in line with emissions limits within the NTE zone but with no power reduction even at high altitudes. To protect the engine, at heights above 2000 m, the EGR rate is reduced correspondingly and it is shut down completely above 3000 m. In thermodynamic terms, engine operation is possible up to 4000 m without power reduction. Above that, ETC speed limits mean that the injection quantity must be restricted. A controlled by-pass around the HP turbine facilitates a reduction in the effective turbine cross-section so that it does not have to be dimensioned for complete exhaust mass flow. The by-pass is closed during acceleration and the small HP turbine reliably generates a rapid build-up of charge-air pressure despite the small amount of exhaust gas available. The turbine cross-section for both stages has been optimised for an exhaust mass flow which is reduced by the EGR rates in steady-state operation. During transient acceleration, the EGR rate is reduced and increased exhaust enthalpy is thus available to the turbines. The relatively small turbine crosssection ensures a rapid build-up of charge pressure. Despite the increased design complexity involved, it was decided to integrate intercooling as this reduces the compression work to be performed at the HP compressor. The result is improved overall ETC efficiency and thus improved fuel consumption. Intercooling also reduces the final compression temperature at the HP stage so that the HP compressor wheel can be made of aluminium alloy instead of titanium. This has a beneficial effect on the mass moment of inertia, ensures good acceleration characteristics for the HP turbocharger and reduces manufacturing costs. Intercooling also ensures that surface temperatures on the charge-air lines remain below the critical safety level of 200 °C (392 °F) which is required in O&G applications.

functioning efficiently under varying operating conditions. The combustion process utilises injection pressures up to a maximum of 220 MPa, mean integral swirl (according to FEV) of approximately 1.15 and a stepped-bowl piston. These features mean that emissions targets can be achieved without post-injection and without diesel particulate filters (DPF). Proven supplier components produced in large-volume series were selected for the injection system. Because of the large number of cylinders and the high engine power, each cylinder bank has its own system consisting of a gear-driven HP fuel pump with an integrated deliv-

ery pump, rail and leak-free injectors. As is standard with stepped piston bowls, the injection jet spray is split in two at the edge of the main combustion chamber in order to improve air/fuel mixture. Particular attention was paid to the geometrical shape of the piston bowl. DESIGN CONCEPT

Major engine dimensions such as stroke, bore and cylinder spacing were taken over from the existing Series 2000 CR, ❽. The new models also inherited charge-air coolers located in the engine Vee, the valve gear system and the cylin-

❻ Consumption map and scavenging pressure difference on the 12V 2000 engine

COMBUSTION

The introduction of EGR places increased demands on the combustion process with regard to particulate emissions. Comprehensive tests were conducted on single-cylinder units in order to achieve a robust combustion process capable of

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❼ ZR 125 low-pressure compressor with high flow capacity for V16 engine

ENGINES FOR HAUL TRUCKS AND FIELD TRIALS UNIT

❽ Series 2000-06 engine profile

der liners. As the compression ratio was fi xed at 16.5 to ensure good cold-starting performance, the high charge-air pressures resulted in a maximum combustion pressure of 22 MPa. In light of this, bearing diameters were increased and bearings with greater fatigue strength were introduced. The robust character of the cylinder units is achieved by utilising steel pistons, reinforced conrods and structurally reinforced cylinder heads with optimised cylinder head bolts. The structural mechanics of the crankcase were optimised without increasing its weight. With cooled EGR systems, sulfurous acid forms if exhaust temperature drops below the dew point. To protect against corrosion, the exhaust system components in question are made of stainless steel and the aluminium chargeair manifolds are coated. The cooling system consists of a hightemperature (HT) circuit for the engine with lube oil and EGR coolers, and a low-temperature (LT) circuit for chargeair cooling. In order to keep the customer-side cooling system to a minimum, the EGR cooler was integrated in the HT circuit and the permissible outlet temperature was raised to 105 °C (221 °F). The LP and HP charge-air coolers are air/water heat exchangers and are located one above the other in the crankcase Vee. This cooling concept benefits transient characteristics due to 12I2013

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the low-volume charge-air column. In addition, the clean air side no longer forms part of the application scope for the customer and, in cold climates, the air can be pre-heated via the coolant. To achieve low charge-air temperatures, both heat exchangers are connected in parallel on the coolant side. Coolant-carrying components are aluminium-free in order to maximise resistance to corrosive stress in cases where coolants have not been correctly prepared.

In quarries and open-cast mines, haul trucks very often have to negotiate long inclines under full load. At maximum speeds of around 50 km/h, air resistance is not a major factor. As a standard, the ratio between total vehicle weight and power is taken to be around 190 to 200 kg/kW. In order to maximise payloads in light of the limited load-bearing capacity of the tyres, more powerful and heavier engines are not an option. Compared with passenger vehicles (midrange vehicle: 20 to 25 kg/kW) and onhighway trucks (approximately 80 to 100 kg/kW), haul trucks have no reserve power so that the rated power must be available at the wheels without any gaps in tractive force. Rated power for passenger vehicles is calculated to generate speed on the flat with air resistance as the dominant driving resistance. Progressively graduated gears are used to ensure that gaps in traction power occur at speeds where air resistance is low and where large reserves of power are usually available even on inclines. The gearboxes on on-highway trucks have a large number of geometrically graduated gears which ensure that no (or only very small) gaps in traction power occur. For haul trucks in the power range up to 970 kW, automatic gearboxes are available from a single supplier only. These have six gears and correspondingly wide gear

❾ Rimpull chart of dump truck with 12V 2000 783 kW engine, gross vehicle weight 160 t

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❿ Acceleration of laden dump truck with 12V 2000 783 kW compared to original engine

ranges. To avoid having to shift into a lower gear due to gaps in traction power, conventional engines require to switch to converter operation. The engine operating point then approaches the rated speed. As a result, neither engine nor converter operate anywhere near their optimum points and this results in poor fuel consumption. This can be avoided by realising an ideal tractive force hyperbola. With the two-stage turbocharging system providing the required potential (taking account of the permitted gearbox input torque) a constant power range was implemented from 1600 to 2100 rpm, ⑥ and ❾. The converter lock-up clutch needs to be released only momentarily for damping of shocks during gear shifting, ❿. MTU repowered a Terex Type TR100 haul truck for use as a field trials unit. In the course of wide-ranging test runs, the transmission control system was optimised in order to fully exploit the engine´s outstanding performance potential. In the course of one and a half years of endurance trials in the rugged world of a South African chrome and platinum mine, almost 5000 h of operation were logged up. Extremely good levels of reliability and excellent fuel efficiency were achieved. In a comparison with the rest of the fleet of identical haul trucks powered by the mechanically governed, nonemissions-certificated original engine

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producing the same rated power and speed with 38 dm³ displacement, the operator identified a 25 % (approximately) saving in fuel consumption over the total period of operation. This was confirmed in a test involving direct comparison. The trials were run using fuel containing 500 ppm sulfur. Those components exposed to the effects of sulfurous acid, such as the EGR cooler, chargeair pipes, lambda sensor and NOx sensor exhibited no irregularities. SUMMARY

Consistent further development based on the proven Series 200 CR has resulted in the creation of a new off-highway engine for C&I and O&G applications. A cooled, high-pressure EGR system incorporating the donor cylinder concept was selected as a key technology designed to achieve compliance with EPA Tier 4i emissions limits. Exhaust aftertreatment is dispensed with completely. The utilisation of a two-stage controlled turbocharging system with intermediate cooling, increased maximum combustion pressure and an optimised combustion process facilitated extremely good fuel consumption across the entire engine performance map as well as outstanding transient characteristics. The results were confirmed in endurance tri-

als over 5000 h of operation in a 100 t haul truck. The development of the new low-pressure ZR 125 turbocharger facilitates optimum use of installation space for the turbocharger group and allows compact construction. As one of the stages in the process leading to Tier 4f, the Tier 4i engine which has been developed already incorporates all the applicationbased interfaces required. Optimisation of the applied technologies for Tier 4f aims to improve fuel consumption while keeping the amount of heat to be dissipated at the same level. REFERENCES [1] Baumann, H.; Haug, F.: Die neuen Motor baureihen 2000 und 4000 von MTU und DDC. In: MTZ Special Edition, 1997, pp. 14-23 [2] Baumann, H.; Baumgarten, C.; Koliwer, M.; Gauss, W.; Schmitz, J.; Wingart, H.: Baureihe 2000 – Kompakte Motoren bis 2000 kW. In: MTZ Special Edition „Sonderausgabe 100 Jahre MTU“, 2009, pp. 44-49 [3] Mattes, P.; Remmels, W.; Sudmanns, H.: Untersuchungen zur Abgasrückführung am Hochleistungsdieselmotor. In: MTZ 60 (1999), No. 4, pp. 234-243 [4] Sudmanns, H.; Bächle, B.; Schmidt, R.-M.: Vorrichtung zur Schadstoffminderung beim Betrieb mehrzylinderiger Brennkraftmaschinen. German Patent DE 43 31 509 [5] Kasper, W.; Wingart, H.: MTU-Baureihe 200006 – Die nächste Generation von Dieselmotoren für Off-Highway-Anwendungen in der Emissionsstufe EPA-Tier 4i. 32 th International Vienna Engine Symposium, 2011

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