The Opel Signum² concept car has just been debuted at the Frankfurt International Motor Show (IAA). Beneath the hood of the Signum² concept car is the prototype of an all-new, lightweight V8 engine with petrol direct injection, developed in the USA by General Motors Powertrain. Thanks to two camshafts integrated centrally into the engine block and a cylinder-bank angle of only 75 degrees, the 'Global XV8' is so compact it can be installed transversely, as in the Signum², or longitudinally, making it ideally suited for a future career as a global GM engine.
With three valves per cylinder and 4.3-litres displacement, the refined, all-aluminium power unit generates 220kW and accelerates the front-wheel drive concept car to up to 250 km/h (electronically limited). Despite this high power output, it needs only 9.8 litres of super-grade fuel per 100 kilometres (MVEG). This is thanks to the air-assisted injection of fuel directly into the combustion chamber and deactivation of up to four cylinders under partly open throttle. The engine also features variable camshaft timing. The length of the variable intake manifold alters according to engine speed, so that as much as possible of the 400Nm maximum torque is available at low as well as high rpm. A balance shaft beneath the crankshaft gives the Global XV8 the typical smooth-running cultivation of a 90-degree V8 engine.
The Design
The all-new, technology-intensive XV8 concept engine provides the power and smoothness of a premium V8 and the fuel efficiency of a V6. Yet it fits into an engine bay sized for the width of a V6 and the length of a four-cylinder.
"The XV8 is one of the most technologically advanced V8s in the world," said Fritz Indra, GM Powertrain executive director of advanced engineering. "It was our goal to create a compact, fuel efficient V8 that would showcase many advanced technologies all working together. We are learning from this innovative approach, in the same way that many of the early GM engine pioneers experimented with technologies that have made GM a leader in engines for the last century. Although we have no current plans for production in its entirety, many of the technologies are under development for future engine programs."
Packaging, power, fuel efficiency and emissions, along with a combination of advanced technologies, drove the development of the all-new XV8 starting from a "clean sheet" approach.
"There is no V8 in existence that combines air-assisted direct injection petrol, variable inlet systems, cam phasing and Displacement on Demand technology, especially in such a compact package," said Indra. "Normally, engineers try to apply new technology to existing engines, but this often results in compromises because you cannot bring the injector or spark plug in the best position or because you must rely on the existing valve angle, for example. An existing engine could not have worked for all this new technology because the whole combustion process is different between multipoint fuel injection and direct injection. We could only get the best results by designing the combustion chamber, and thus the whole engine, around the best position for the injector and the spark plug."
High Technology Features
Goals for XV8 development, in addition to the compact packaging and outstanding fuel efficiency, included 224kW (300 hp) capability in a performance car-type application. The XV8 runs a very high compression ratio - 10.75:1 - with regular grade petrol. To achieve that type of world-class, direct-injection performance while meeting fuel efficiency demands in the smallest package size possible, GM engineers integrated many different advanced technologies.
Some of the technologies and capabilities of XV8 include:
- Air-assisted direct injection petrol
- Displacement on Demand
- Two cams in-block with variable inlet valve timing and cam phasing
- Narrow 75-degree bank angle
- Active inlet manifold
- Twin oil pumps
- An integrated air compressor
Air-Assisted Direct Injection Petrol
The XV8 was designed to accommodate a combustion chamber that utilizes a direct fuel injection system. It features the air-assisted direct injection petrol system from Orbital Engine Corporation of Australia. The system is integrated into three-valve cylinder heads and dual cams in the block.
"Using three valves-two inlets and one exhaust-gives you more room in the combustion chamber for optimal positioning of the injector and the spark plug, vertical and nearly central in the chamber," Indra said.
Having two cams in the block rather than dual overhead cams provides considerable packaging benefits and combined with the direct injection fuel system, contributes to the XV8's outstanding performance numbers.
"Having this very good combustion chamber process in the small XV8 also lets us re-evaluate the catalytic converters," Indra said. "We made the combustion itself so effective that we can reduce the after-treatment system. That was another prerequisite for the engine and another reason for the clean, complete burn."
Displacement on Demand
GM's Displacement on Demand technology was designed into the XV8 from the start. Displacement on Demand allows the V8 to shut down half of its cylinders seamlessly at predetermined times to significantly reduce fuel consumption without hampering performance.
The XV8's unique twin oil pump design allows the engine to run Displacement on Demand at idle, since the system and cam phasing system have their own dedicated oil pump, which provides enough pressure to deactivate the cylinders at idle and reactivate them immediately upon throttle engagement.
In May 2001, GM announced that it will implement Displacement on Demand in its trucks and SUVs beginning in 2004.
Two Cams In-Block with Variable Inlet Valve Timing and Cam Phasing
The use of a camshaft "phaser" separates the timing functions of the intake and exhaust valves. This is accomplished in the XV8 engine by having two in-block camshafts, one for inlet operation and one for exhaust. The camshafts are located in a vertical plane above the crankshaft and parallel to its centre of rotation. The intake camshaft is the lower camshaft and is approximately in the centre of the block. The exhaust cam is positioned above the intake. Because the intake camshaft rather than the exhaust is "phased," the XV8's camshaft drive provides the ability to better modify and enhance full-load engine torque characteristics. In the stratified combustion mode of operation, it can be used to increase the charge dilution by advancing the intake cam timing. The set-up reduces friction and fuel consumption, particularly at idle and part-load, and also contributes to the engine's outstanding low-end torque.
"With the cams in the block," Indra said, "the valve timing precision is better than with a DOHC configuration. The different heat levels with long belts and chains in a DOHC set-up always changes the valve timing."
Having two camshafts in the engine block with the ability to "phase" one of the cams is unique to GM.
Narrow 75-degree Bank Angle
The XV8's narrow 75-degree bank angle and very small cylinder heads result in an overall engine compactness that is revolutionary. "The 75-degree V8 is a packaging solution that's typical of race cars," said Hayman.
Active Inlet Manifold
The air-assist direct injection system requires port geometries that generate a minimum of "in-cylinder" motion when the system is operating in stratified mode. During homogeneous operating conditions, in-cylinder motion is required in similar fashion to port fuel injected engines. The inlet manifold design supports these design objectives to achieve maximum fuel economy. The resulting design also allows the engine to deliver a broad torque band suited to spirited driving styles, supports the peak power objectives, and fully accommodates the Displacement on Demand system.
Twin Oil Pumps/Balance Shaft Assembly
The XV8 is unique not only in that it has two oil pumps, but also in that the engine's balance shaft doubles as the oil pump drive shaft. The former allows for such functions as cam phasing and Displacement on Demand at idle and the latter contributes to the engine's compact packaging.
Because the XV8 requires extensive hydraulic function, two oil pumps were used in a serial fashion. If the lubrication system was designed with the typical single oil pump, its displacement would have to be substantially increased to provide minimum pressure to the entire engine. The primary pump supplies low pressure filtered oil to the bearings, valve lifters and secondary pump inlet. The secondary pump acts to intensify the pressure for supply to the cam phaser and Displacement on Demand systems. In doing this, parasitic power consumption to the oil pump is minimized.
Because of packaging constraints, the oil pump drive was combined with the balance shaft assembly. To get the necessary 1:1 counter-rotation of the balance shaft, it is driven by a helical gear pressed on the rear of the crankshaft.
"The drive for the pumps is the balance shaft, which has to go opposite engine rotation at engine speed because of our narrow bank angle," Hayman said. "So we get the balance shaft basically for free and this is all packaged in the sump that bolts to the bottom of the block. That is unique. Also, placing the oil pumps at each end of the balance shaft helps to damp vibrations."
Integrated Air Compressor
The XV8's air compressor (used in the direct injection) is integrated into the engine assembly.
"That's another unique aspect of the engine," Hayman said. "The air compressor is part of the engine assembly itself, not just a component bolted onto the accessory drive somewhere as a standalone pump. It's integrated to the back of the cylinder head and all of the fluids are transferred through this interface. This avoids the requirement for the myriad of hoses that would have traditionally been required including the avoidance of having to run a separate air-assist rail."
Conclusion
"Starting with a clean sheet of paper allows engineers to see the whole picture for the first time - not just power or high revolutions or any one specific goal," said Indra. "They really see the requirements of the future and designed the engine around those requirements. It gets excellent fuel economy and has clean emissions, but above all, it satisfies the main requirement anyone has of an engine: It will perform."
|