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The new Blackwing 4.2 liter twin turbo V8 engine from General Motors

Blackwing 4.2 liter twin turbo DOHC engine

29. November 2018 | Engineering Service

Blackwing 4.2 liter twin turbo DOHC engine

General Motors has a storied history with V8 architecture engines that are renowned for their performance, durability and refinement. Furthering GMs expertise in V8s, this article describes an all-new 4.2 liter twin turbo, direct injected DOHC engine which is exclusive for use in the Cadillac CT6 V. The engine produces high power at top-end speed but was designed to provide an effortless driving feel by generating best-in-class torque at low and middle engine speeds. The result is an engine capable of meeting the needs of a high power performance sedan while simultaneously providing the crisp throttle response of a luxury automobile. This article provides details of the new engine as well as highlights of individual technologies that support the balanced performance of this engine.

Modern vehicles have experienced a rapid expansion in feature content as the price of computing power drops and consumer expectations increase. This added feature content requires even more packaging space throughout the vehicle. The vehicle under hood area is not immune from these advancements. Added packaging space is required for advanced braking systems, all-wheel-drive transaxles, noise abatement insulation and resonance volumes, and advanced passenger comfort systems taking away space traditionally available for the powertrain. As such, new and innovative designs are needed to create a more compact engine to fit in the space available.

One concept which has become more common in the luxury vehicle market is a turbocharged V8 engine featuring valley mounted turbochargers. This concept provides the benefits of a low volume exhaust system that reduces turbo lag as well as compact packaging to fit in the ever smaller under hood space. With the design freedom of a completely new engine architecture available to the team, it was decided early in the program to adopt the valley mounted turbocharger concept. This enabled the development team to simultaneously meet the transient response, peak torque and peak power targets. In addition, every aspect of the engine, from block height to the connecting rod bearing diameter, was scrutinized for maximum density without sacrificing the engine’s performance or durability and robustness.

Engine performance achievements
Fig. 2: Engine performance achievements

Engine concept
The engine is an entirely new 90° V8 intended for the Cadillac CT6 V, GM’s top-of-range sedan. The main focus of the engine is to develop high torque at low-speeds in order to provide an effortless driving feel. Key performance targets for the engine are shown in Figure 2.
The CT6 V vehicle that the engine is custom designed for was developed to offer an inline 4-cylinder and a 60° V6 as the primary propulsion systems. This made packaging of a DOHC turbocharged V8 a challenge in both width and height. The engine is only available in an all-wheel drive drivetrain further limiting the under hood space for the engine. As a result, the only 90° V8 twin-turbo engine concept that would fit in the vehicle was one where the exhaust and turbochargers were located in the center of the valley with the intake ports on the outside of the cylinder head.

Key specifications of the 4.2 liter twin turbo engine
Fig. 3: Key specifications of the 4.2 liter twin turbo engine

Technical specification
Cylinder block: The cylinder block is a precision sand cast, deep skirt construction from A319 aluminum with a T7 heat treat. The cylinders feature pressed in iron liners while the 4-bolt nodular iron main bearing caps have an additional cross bolting to the skirt to make a stable structure capable of handling the 25.4 bar BMEP.
Given the space available for the engine and AWD differential, every subsystem within the engine was scrutinized for packaging space. For a cylinder block using iron liners, a relatively narrow 96 mm bore spacing and an optimized deck height of 215 mm was achieved. To make the reduced deck height of the cylinder block possible, the outer fasteners for the main bearing cap were angled. This enabled the proper thread engagement for the fastener without encroaching into the piston and hone over travel clearance reliefs as shown in Figure 4.

Transparent view of angled outboard main bearing cap fasteners
Fig 4: Transparent view of angled outboard main bearing cap fasteners

With uneven cylinder filling as an inherent trait of cross-plane crankshaft V8’s, it was critical that the cooling of the combustion chamber be optimized in order to minimize the knock tendencies of the engine and ensure engine durability. With respect to cooling the top of the cylinder bores, a stepped drill was used to flow coolant between the cylinders as shown in Figure 5. This drill directed the coolant from the cooling jacket in the cylinder block to the cylinder head cooling jacket. A metering hole in the head gasket is used to regulate the coolant flow enabling the coolant flow in each cylinder to be metered individually with a unique orifice size based on the temperatures of each cylinder.

CFD results of the flowrate through the inter-bore cooling passage
Fig. 5: CFD results of the flowrate through the inter-bore cooling passage


Cranktrain and powercell: The space available for the cranktrain and powercell was limited by the location of the AWD transfer housing, available packaging space for the starter, and low deck height of the cylinder block. As such, the cranktrain had to be optimized to fit within the given package. To achieve these packaging constraints, the rod bearing diameter was reduced as much as possible to make a small rod path while the main bearing diameter was increased to recapture the required overlap for crankshaft stiffness and strength. The sizes of the bearings plotted on the FEV scatterband in Figure 6 shows how the crankshaft was tailored to fit the packaging constraints of the application.

Main and rod bearing diameter (FEV database)
Fig. 6: Main and rod bearing diameter (FEV database)

The deck height of the engine is also minimized to reduce the overall width of the engine and meet manufacturing clearance requirements when the engine is assembled in the vehicle. To achieve this aggressive target with a relatively long stroke and DOHC architecture, the powercell was scrutinized for minimal length at top dead center. Optimizing the powercell overall length resulted in a short connecting rod that gave an L/R ratio of 3.89. To illustrate how compact this package is, the rod length vs engine stroke for the new V8 engine is plotted on FEV’s scatterband in Figure 7.

Connecting rod length vs. engine stroke (FEV database)
Fig. 7: Connecting rod length vs. engine stroke (FEV database)

The connecting rod is guided by the piston instead of the crankshaft. By reducing the size and relative speed of the bearing area guiding the connecting rod, a reduction in overall friction has been achieved.

Cylinder head: The cylinder heads are Rotocast® from A356 aluminum with a T6 heat treat. This process was selected by the team as it enables superior mechanical properties which are beneficial on a high BMEP engine such as this.

The combustion system was carried over, in part, from the previously released 3.0 L twin turbo V6 LGW engine. The combustion system required several modifications for incorporation into this engine with its valley mounted exhaust. The most prominent was the entrance angle of the intake port which had to be curved upward to facilitate the outboard mounted intake manifold. This upward turned entry drove intake flow to the floor of the port. Significant CFD development was used to ensure the proper amount of tumble was generated with the new design. A comparison between the 4.2L TT and LGW porting is shown in Figure 8.

Comparison between LGW and 4.2L twin turbo porting
Fig. 8: Comparison between LGW and 4.2L twin turbo porting

Extensive work was done to facilitate the cooling of the cylinder head with a single piece cooling jacket. While single piece cooling jackets are relatively conventional, the inboard mounted exhaust ports required a unique coolant flow circuit. In order to achieve a high coolant flow rate through the exhaust valve bridge area, the majority of the coolant is directed from the block into the cylinder head under the exhaust ports, as is the case for most conventional engine arrangements. However, in order for the cooling jacket to degas during coolant fill and in operation, the coolant also exits the cylinder head on the exhaust side of the engine as that is the highest point in the cooling circuit. Thus, the coolant path is designed to enter under the exhaust ports, flow across the combustion chamber, then turn 180° to flow over the exhaust ports where it is collected and returned to the radiator at the front of the engine. Significant CFD development was invested during the development process to ensure that all areas of the head received adequate cooling with a conventional volume coolant flowrate.

Valvetrain and timing drive: The valvetrain was conceptually carried over from the LGW V6 engine with key changes to fit the packaging requirements of the 4.2L TT engine and implement an 8 to 4 cylinder deactivation strategy. The collapsing roller finger followers are actuated via hydraulic pressure that is controlled by cam cover mounted solenoid valves. The development team implemented a cam carrier system to simultaneously reduce friction and provide packaging space for the switching roller finger followers required for cylinder deactivation. This die-cast carrier is complex and houses the camshafts, high pressure fuel pump, and provides worm trail lubrication passages for the cylinder deactivation switching mechanisms.

Intake system and charge air cooling: In order to achieve an extremely crisp throttle response and fast time to torque, the intake volume was minimized by locating the water to air charge air coolers over the cam covers. The intake manifolds are fed directly out of the charge air coolers and cascade over the intake cam side of the cylinder heads into the intake ports. This compact packaging of the intake system, in combination with the low volume exhaust system achieved with a valley mounted turbochargers, was critical to achieving the throttle response targets of the engine without sacrificing the high power targets which requires relatively large turbochargers.
Particular attention was paid to the design of the intake tract such that the charge air coolers were well utilized without the need for restrictive features such as turning vanes. Due to the bank offset of the engine and unique packaging constraints on each side, the charge air coolers were not located in the same position. This required the development of two unique intake tracts with both paths optimized for cooler utilization while maintaining similar volumes. The resulting charge air cooler utilization was above 90 percent for both banks ensuring a balanced charge air temperature for each manifold.

With the charge air coolers being prominently located on the top of the engine, styling cues were included in the housings of each cooler including a “hand crafted with pride” plaque customized and installed by the engine builder on the right charge air cooler. The coolers, accompanied by the aluminum top cover, gives a striking engine appearance when the hood is raised.

Cooling system flow path
Fig. 9: Cooling system flow path

Turbocharging and exhaust system: The engine features two twin-scroll turbochargers that are controlled by electric wastegate actuators. The twin scroll turbochargers feature integral exhaust manifolds for minimum exhaust volume. The turbochargers are mounted to each bank in the valley such that, when combined with the compact intake system previously discussed, an extremely low volume airflow path was realized. Even though the cross-plane V8 does not supply the turbochargers with evenly spaced combustion pulses, the twin scroll turbochargers combined with a low volume exhaust system provides significant transient response and full load benefits without sacrificing classic V8 NVH characteristics.
The valley mounted exhaust manifolds, turbochargers, and catalysts lead to significant thermal challenges under hood. As thermal management was a top concern during the development in order to achieve superior robustness, several heat shielding and valley cooling strategies were tested with multiple strategies implemented. These include direct thermal protection of sensitive components, targeted airflow and heat flow paths, as well as various heat shields. The exhaust manifolds and integrated turbine housings are insulated with contact heat shields. It was found that the contact heat shields with fiber insulation and metallic outer shell provided the best thermal management and required the least packaging space. In addition, because the turbo and manifold heat shields are pre-attached to the parts, assembly sequencing of the hardware in the valley was simplified.

Fuel system: A 350 bar direct injection fuel system was selected to minimize particulate emissions and enable fast combustion at high injection quantities. This system features two intake cam driven fuel pumps with side mounted injectors outside of the valley. Each intake cam features 3 lobes to drive the pumps. A significant amount of multi-body analysis of the camshaft and drive system as well as fuel pressure simulation of the fuel system were conducted to ensure durable operation in combination with optimal engine performance. To meet program targets, the three lobe camshafts were found to better time the fuel pump pulses between the valve events to reduce the peak loading on the timing drive while still providing a consistent fuel delivery.

Lubrication and crankcase ventilation System: At the heart of the lubrication system is a continuously variable vane oil pump. The pump is driven by the nose of the crankshaft and features a PWM controlled solenoid valve that is used to control the eccentricity of the oil pump. The crankcase ventilation system features a valley mounted two-stage oil separator that has dedicated drains at the rear of the engine for the coarse separator and at front of the engine for the fine separator where the separated oil is returned to the oil pan. These drains end below the oil level in the oil pan to prevent crankcase gasses from short circuiting the separator.

The fine separator oil drain is located in a relatively shallow portion at the front of the oil pan. In order to prevent the drain from being uncovered during hard acceleration or cornering, a unique, patent pending design was developed. This design incorporates a reservoir locally around the exit of the drain. The design of this feature was implemented in a cost efficient manner by using one drill for the oil drain and one drill creating the reservoir as can be seen in Figure 10.

Front ventilation separator oil drain reservoir
Fig. 10: Front ventilation separator oil drain reservoir

Engine performance: The engine exhibits class leading performance as can be seen by the power and torque charts shown below. The peak torque of 850 Nm (627 ft-lbs) is available from 2,800 rpm through 4,200 rpm where power peaks at 410 kW (550 hp). The high low-end torque combined with the wide peak torque range offers the Cadillac owner the effortless driving feel that the program targeted from the start of the development.

The torque density of the engine is particularly impressive and matching the best engines on the market even with the breathing dynamics inherent in a cross-plane crankshaft V8 engine. When the scatterband is adjusted to show only V8 engines, the performance is even more impressive.
The results illustrate that this engine is capable of meeting the needs of a high power performance sedan while simultaneously providing the crisp throttle response of a luxury automobile. The engine will fill a critical role for General Motors as it will see exclusive use in the Cadillac CT6 V, GM’s top-of-range sedan.

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RDE-conform gasoline engines

Gasoline Particulate Filters

21. November 2018 | Engineering Service

Gasoline Particulate Filters

The introduction of gasoline particulate filters (GPF) leads to new challenges in terms of calibration. FEV addresses these challenges with new development methods. The methods presented are simulation-supported RDE testing, use of specific worst-case fuels and accelerated ash loading.

The introduction of stricter particulate number (PN) limits in Europe and China is complemented by the measurement of Real Driving Emissions (RDE) on public roads at arbitrary boundary conditions. This creates a challenge to meet the PN limits and a clear trend for all OEMs to apply GPF systems in order to comply with current and future RDE legislations.
Up until now, automotive manufacturers have been able to meet the EU6d-TEMP PN limits for standardized driving cycles, such as the NEDC with the Worldwide Harmonized Light-Duty Vehicles Test Procedure (WLTP) without having to depend on a GPF. The left side of Figure 1 supports this statement, showing PN emissions of approximately 3 x 1011 1/km without a GPF for a J-class vehicle with a turbo DI gasoline engine in a WLTC test. The implementation of a GPF with an efficiency of around 75 percent leads to a further significant PN reduction.

The right hand side of Figure 1 shows influences that are valid under RDE conditions, which can lead to substantially higher particulate emissions compared to nominal WLTP conditions. The first factor shown, is the decrease in the share of the ethanol content in the fuel, e.g. from 10 vol-percent (EU6 fuel respectively “nom. Cond.”) to 0 vol-percent (EU6 worst-case fuel), which is caused by the reduction of the fuel´s oxygen share. Secondly, and also a fuel-related topic, a strong knock resistance of high aromatic content comes with a high particulate formation potential. A payload increase, caused by luggage and/or passengers, leads to engine operation with higher speeds and loads, which further increases the PN output (dark blue bar). Moreover, a sporty or aggressive driving style causes increased particulate numbers, especially when combined with the other illustrated influences. [1]

Abb. 1: PN-Emissionen mit und ohne OPF

>> THE FEV WORST-CASE RDE SIMULATION ENABLES AN EVALUATION OF THE FULL RANGE OF BOUNDARY CONDITIONS

Considering these multiple aspects and all their possible combinations, the extensive integration of GPFs will become mandatory. Consequently, a need to develop suitable tools and methods becomes important, which enables the consideration of all GPF impacts and interactions in an early development phase. The following article discusses the simulation-supported RDE testing and highlights the current GPF hardware trends and calibration subtasks for ultra-low temperatures. It concludes with results of an accelerated GPF ash loading procedure.

Simulation-Supported RDE Testing

Often, the investigation of all influencing boundary conditions for the determination of RDE values requires consideration of several cycles and driving scenarios. If a large number of different cycles need to be tested, this leads to cost and time issues. Figure 2 shows the main RDE boundary conditions and a comparison of the WLTC chassis dyno cycle with an on-road RDE test (conducted on the FEV Aachen RDE track). In addition, a simulation-supported, tailor-made worst-case RDE driving scenario, conducted on a chassis dyno is depicted. The on-road Aachen RDE test is compliant with the law and covers sections in an average mountain range. The best-case condition of each criterion is marked by the center of the diagram, whereas the worst-case condition is marked by the outer border.
As seen in Figure 2, the WLTP covers quite severe speed conditions, and medium vapos and vehicle weight values. The vapos number describes the positive acceleration multiplied by the vehicle speed and is a first characteristic number to evaluate RDE driving. However, the influences of full load acceleration, cold start and positive altitude gain or temperatures are not covered completely. The RDE-compliant FEV Aachen RDE track covers significantly more of the diagram. But, only the FEV worst-case RDE simulation enables an evaluation of the full range of boundary conditions. It is clear that only a holistic simulation approach is able to cover all of these boundary conditions in one cycle.

Fig. 2: Testing at RDE boundary conditions

>> A HIGH NUMBER OF PARAMETERS REQUIRES SIMULATION-BASED TESTS

GPF Hardware and Calibration Overview

Another fact supporting the trend of simulation-supported testing is the wide variety of GPF applications on the market. Figure 3 displays current technology and market trends for GPF applications based on FEV’s in-house analysis. Despite the fact that 75 percent of actual GPFs are coated, FEV expects a long-term trend of uncoated GPFs due to their backpressure advantages [3]. Besides, most GPFs are located in close-coupled position in order to utilize the high exhaust gas temperatures for soot regeneration.

Fig. 3: GPF technology and market trends

The integration of GPFs into a gasoline powertrain brings a number of additional calibration tasks. The major tasks are soot model calibration, monitoring of soot loading (simulation and DP sensor) and safety function calibration. All calibration tasks aim at minimizing the customer impact of the GPF implementation. Typical ECU calibration tasks are listed in Figure 4.

Fig. 4: Calibration tasks

The most important input for GPF calibration is an engine-out soot map. For that, the base engine calibration must be in a mature state, including optimized particulate emissions. The soot map is usually based on worst-case fuel, as these conditions serve as a main driver for the initiation of active regenerations. Tests for GPF loading and (active) regeneration take place at the engine test bench. It is necessary to determine the critical specific soot mass for the filter to prevent thermal shock during regeneration. Furthermore, the corresponding tests generate input data for soot loading and oxidation models, as well as for the backpressure model. With the definition of the critical specific soot mass, the calibration of GPF monitoring and safety functions are possible. Both are of high importance for the impact on drivers. [4]

GPF and Cold-Start Conditions

As the worst-case fuel plays a central role within GPF calibration, FEV applied their fundamental fuel research experience regarding the impacts of ethanol or aromatics content in the fuel to a complying fuel. Since there is no fuel quality sensor in current series applications, the ECU must always consider worst-case fuel.

Fig. 5: GPF soot loading after each 10 cold starts at different ambient temperatures

Figure 5 displays the GPF soot load after ten repeated cold starts at different temperatures. For EU6 certification, fuel with 10 vol-percent ethanol and the chosen SUV with a turbo DI engine, ten cold starts at -20 °C result in a soot load of approximately 0.9 g/l. For the same conditions, the FEV worst-case fuel produces about 50 percent more soot. A reference fuel shows the same behavior.

Ten repeated cold starts at -30 °C led to a specific soot load of 2 g/l, which is the threshold to trigger an active regeneration for the specific application. Considering two cold starts per day, a temperature of -30 °C may become critical regarding the GPF soot loading after only five days of client operation. Thus, it is mandatory to implement an ECU function for active filter regeneration.

Accelerated Ash Loading

In order to consider the aging effects during calibration, accelerated GPF ashing is carried out on the burner test bench. Figure 6 illustrates two oil-based fuel doping experiments on the burner test bench, However, since this aging method is known to produce ash with very high backpressure, the burner hardware has been modified to precisely control the ash properties. The modified hardware can optimize the ash formation and significantly reduce the backpressure, which leads to ash properties comparable to vehicle ash. This improvement was achieved without limiting the shortening factor and full utilization of the potential is still in development.

Fig. 6: Ash loading results

Thus, the burner ash generation is a cost- and time-effective tool for end-of-lifetime investigations with respect to the wide range in which field ash varies for different customer applications. Especially in early development stages, where no durability runs are finished, the burner aging enables GPF aging-effect calibration.

Summary

Due to the future measurements of real driving emissions, an entire new range of influencing factors of vehicle calibration must be considered in order to be certification compliant. This makes a GPF application mandatory. Currently, test cycles do not include RDE worst-case conditions. FEV developed a simulation tool that generates worst-case cycles in order to develop calibrations that guarantee RDE compliance under all boundary conditions.
An FEV market analysis confirms the increasing trend to GPF applications, but also shows that there are a variety of technologies and installations for different vehicle applications available. As the GPF affects the engine operation, new calibration tasks arise. In order to minimize the related calibration effort, the simulation-supported testing is combined with worst-case fuels and cold start conditions. As a result, the soot modelling and regeneration calibration tends to be on the safe side. Accelerated ash loading on the burner test bench addresses the evaluation of aging impacts and allows end-of-life GPFs at a very early development stage.

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