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.
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.
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.
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.
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
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.
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,
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.
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
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
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.
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.