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– Optimization focus: Thermodynamics and engine mechanics –
Evaluating key technologies for the downsizing of automotive engines
Stuttgart/Germany, September 2009—The whole is more than the sum of its parts. This applies equally to interactions between components and subfunctions of a modern engine. To investigate the effect of specific technologies designed to optimize fuel efficiency on the system as a whole, MAHLE implements products from all its business units in a compact, high-performance prototype gasoline engine. This demonstrator is designed according to an aggressive downsizing concept, in which a naturally aspirated engine with a 2.4-l displacement can be replaced by a supercharged three-cylinder engine with a mere 1.2-l displacement. The performance and torque of this engine is sufficient to power a midsize passenger car weighing up to around 1.6 metric tons.
In the coming years, downsizing will play a central role for further reducing CO2 emissions in combustion engines. The gasoline engine, in particular, benefits from this modification because smaller engines often operate at higher load ranges—where combustion is more efficient in gasoline engines than at lower loads. The downsizing engine therefore achieves greater fuel efficiency under the same basic conditions. The MAHLE prototype is proof of this: At optimum performance, the two-stage turbo version of the engine achieves a specific fuel consumption of only 234 g/kWh. At
2,000 rpm and a brake mean effective pressure of 4 bar, fuel consumption is 295 g/kWh. Based on these values, a simulation of the New European Driving Cycle (NEDC) in a vehicle weighing
1.6 metric tons indicates a potential improvement in fuel efficiency of greater than 30 percent compared to the 2.4-l reference engine.
To offer sufficient power output and an easily drivable torque characteristic at this unusually high degree of downsizing—standard production vehicles commonly feature values of up to some 30 percent at best—the two-stage turbocharged version of the engine provides a torque of 153 Nm at values as low as 1,000 rpm and 16 bar brake mean effective pressure. In the 2,500-to-3,000-rpm range and at a brake mean effective pressure of 30 bar, the maximum torque of 287 Nm is applied. The demonstrator engine delivers its maximum power output of 144 kW at 6,000 rpm. At the same time, the engine is designed for compliance with EURO 5 emissions standards.
The core technologies for this engine—which derives its performance and torque from only three 400-ccm cylinders—include turbocharging, direct fuel injection, and variable valve train technology. In addition, the engine was specifically designed for minimal weight and optimal frictional loss in the engine. The torque rating, high performance, and low fuel consumption of the engine are achieved based on production-ready MAHLE system and component solutions and special manufacturing processes.
Base motor, power cell, and fuel injection
The demonstrator's closed-deck cylinder crankcase and four-valve cylinder head are made of an aluminum alloy (A356). Both of these components were sand-cast using the MAHLE COSCAST® process. With this process, it is possible to produce narrow wall thicknesses (such as the thin walls separating cylinder bores) and complex geometries. The cylinder head and cylinder crankcase have separate cooling circuits for flexible, demand-based thermal management. Both circuits are fed by a common electric pump. The current cooling requirement is determined by sensors in the multilayer steel cylinder head gasket and the correlation with the coolant temperature.
To improve the tribological properties and optimize heat transfer, the cylinder surfaces are NIKASIL®-coated. A continuous tension bolt connection between the cylinder head, the cylinder block, and the bed plate minimizes bore warpage. Through a sound combination of lightweight design and stringent safety factors—such as in the design of the main bearing pedestals without steel struts—the entire serviceable engine weighs 145 kg.
The piston/connecting rod/crankshaft system—the power cell—is designed for a peak firing pressure of up to 140 bar. To sustain the high specific loads in an efficient compact engine, the connecting rod bearings are treated with a flame-sprayed coating (PVD coating). The piston pin in the small end of the forged connecting rod has an extremely tough diamond-like carbon (DLC) coating. The forged aluminum pistons are fitted with a three-piece ring set and are designed for the use of extremely low-friction rings. The engine is equipped with a piston jet-spray cooling system.
Centrally located piezo injectors are used to achieve precise metering of the fuel, enabling spray-guided combustion. The use of solenoid valve injectors is also an option. The spark plug is positioned slightly off-center with respect to the injector. This constellation lays the proper groundwork for a conceivable stratified-charge operation in the future. At ε = 9.75, the compression ratio is relatively high for a highly pressure-charged engine and can be attributed to the cumulative effect of modifications intended to reduce susceptibility to knocking (such as exhaust gas recirculation). These modifications contribute to the high fuel efficiency of the engine.
Exhaust gas recirculation
The MAHLE EGR technology implemented in the engine ensures a high degree of control accuracy in the metering of recirculated exhaust gas, even for transient gas flows. To control the EGR, a roller valve is used, which enables cylinder-specific adjustment of the EGR rate. In addition, a fast-switching MAHLE air impulse valve utilizes gas dynamics to enable higher EGR rates, if the exhaust gas mass flow rate would otherwise be insufficient due to the prevailing pressure ratios between the exhaust gas pressure and the intake pressure.
The demonstrator engine uses a cooled exhaust gas recirculation system for EGR rates of up to 15 percent. This eliminates the need for mixture enrichment at full load to protect the turbocharger components and reduces susceptibility to knocking by lowering the temperature in the combustion chamber. And because of the lower peak combustion temperature, the engine also emits lower NOx levels. This engine is designed for compliance with EURO 5 emissions standards.
Variable timing in the DOHC valve train
The demonstrator is a four-valve engine with dual overhead camshafts designed for a maximum engine speed of 7,000 rpm. Valve timing can be adjusted independently by means of camshaft phasers on the intake side and exhaust side. Additional variability can be provided at a later point in the form of a controllable valve lift design—for example, through a mechanism enabling a switchover from a low-lift to a high-lift cam profile or through fully variable control of lift and opening times. Low-friction, low-wear roller-type cam followers contribute to the efficiency of the valve train, as do the low overall weight and low moving masses of MAHLE lightweight valves cooled with a sodium filler.
Exhaust gas turbocharging
Today, the demonstrator engine is available in two versions: For the 100-kW-to-120-kW performance class, the design utilizes single-stage exhaust gas turbocharging. For further optimization of the transient engine performance, in addition to the wastegate turbocharger, an exhaust gas turbocharger (EGT) with variable turbine geometry (VTG turbocharger) was reengineered and tested based on a standard diesel specification. The exhaust manifold is water-cooled to limit the thermal requirements of the components. Exhaust gas from an exhaust port is streamed directly to the turbine, resulting in an additional pulse-loading effect.
It is with the two-stage turbocharger design that the demonstrator engine achieves its peak performance. In this design, two exhaust gas turbochargers are configured sequentially. The turbine housing of the high-pressure-stage EGT is integrated in the manifold. Here as well, exhaust gas is drawn into the turbine directly from an exhaust port. The high-pressure stage is activated at low rpms and continues to compress the exhaust gases up to an engine speed of 2,500 rpm. Once this limit has been reached, all of the exhaust flow is redirected to the low-pressure EGT by means of a bypass valve located between the exhaust manifold and the low-pressure EGT. At this point, the second turbocharger takes over the turbocharging function for the entire remaining rpm range up to an engine speed of 7,000 rpm. The compressor casing of the high-pressure EGT (on the air side) features a pressure regulating valve.
The second EGT on the low-pressure stage is flange-mounted on the exhaust side and the air side. This EGT features a wastegate control mechanism. Following compression, air that has been compressed to up to 2.6 bar passes through a charge air cooler before flowing into the air distributor in the air intake system.
Fast electrical wastegate actuator ready for serial production
An electrical wastegate actuator recently introduced by MAHLE—for the first time in a series production application—makes it possible to implement a fast control strategy for the turbocharger. This does away with "turbo lag," therefore resulting in enhanced responsiveness. Moreover, because the exhaust gas back pressure can always be kept to a minimum, fuel consumption is lowered.
With the pneumatic actuators used previously, the control speed was too slow for this, and the wastegate could only be controlled when sufficient charge air pressure was available. The new actuator passes through the entire range of adjustment in just 80 milliseconds.
Owing to its unique kinematics, the newly developed actuator has a significantly higher torque and thus ensures that the wastegate is not pushed open even by exhaust gas pressure pulsations, as was known to occur with the pneumatic actuators used previously. This, too, results in a faster charge pressure buildup and therefore enhanced responsiveness in the engine.
Performing the air management of the engine is the job of the integrated air intake system with a complete intake air path. In addition to fast-actuating throttle flaps acting as control elements, the light, compact intake system is also endowed with air mass meters in the air filter module, as well as plastic air distributors with EGR and blowby induction at the cylinder head flange.
The lubrication system is designed to ensure low friction, low weight, and small dimensions. The MAHLE COSCAST® casting method once again proves valuable, for forming the thin walls of the oil pump body in this case. For jet-spray cooling of the pistons, oil is taken from the return line (on the dirty oil side), reducing the amount of oil that needs to be fed from the lubricant side.
With its demonstrator engine, MAHLE has taken a whole-system approach to prove that innovative downsizing is possible at an advanced level using technology that is production-ready today or has already been implemented in series production. In addition, the engine provides answers to questions about the benefits of the modifications used and, not least, helps assess the cost-benefit ratio.
The MAHLE Group is one of the top 30 automotive suppliers and the globally leading manufacturer of components and systems for the internal combustion engine and its peripherals. Around 45,000 employees work at over 100 production plants and eight research and development centers. In 2008, MAHLE generated sales in excess of EUR 5 billion (USD 7.3 billion).
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