The amazing General Motors XV8 concept V-8 engine
Shawn pointed to the new XV8 concept engine, shown in the Opel Signum concept car. (Opel is GM's European division, and includes Vauxhall). The all-new engine provides the power of a full-size, high-end V8, but has greater fuel efficiency, the width of a V-6, and the length of a four-cylinder.
With an aluminum block and head, the 4.3 liter XV8 has three valves per cylinder with an air-assisted direct fuel injection system and two camshafts in the block. Power ratings are 300 horsepower (224 kW) and 295 lb-ft (400 Nm) of torque.
Other features include variable inlet systems (currently the main feature of Chrysler's Magnum engines), cam phasing, and displacement on demand (first seen on the ill-fated Cadillac 4-6-8 engines), variable inlet valve timing (common to Toyota and Honda engines), a narrow 75-degree bank angle, twin oil pumps, and an integrated air compressor. A GM spokesman said this combination was possible, in its best form, because of the engine's clean-sheet design: there was no need to compromise new features to co-exist with existing designs. That was especially important for direct injection.
The XV8's compression ratio of 10.75:1 is achieved with regular gasoline.
The all-aluminum 4.3-liter XV8 utilizes a unique three-valves-per-cylinder combustion chamber configuration, supporting the optimization of an air-assisted direct fuel injection system. The configuration features an industry first: two camshafts in the block. The XV8 produces 224 kW (300 horsepower) and 400 Nm (295 lb-ft) of torque.
The air-assisted direct injection gasoline system was developed by Orbital Engine Corp. of Australia, and is integrated into three-valve cylinder heads and dual cams in the block. The three valve system (two inlet valves, one exhaust) provides more room in the combustion chamber for optimal positioning of the injector and the spark plug, vertical and nearly central in the chamber - positioned as they would be in a Hemi engine.
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. The clean burning also means that after-combustion pollution control can be milder.
GM's Displacement on Demand technology allows the V8 to shut down half of its cylinders seamlessly at predetermined times to significantly reduce fuel consumption without hampering performance.
The 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.
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 center of rotation. The intake camshaft is the lower camshaft and is approximately in the center 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. Having two camshafts in the engine block with the ability to "phase" one of the cams is unique to GM.
"With the cams in the block," GM's Fritz 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."
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.
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," GM's Alan 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."
The XV8's air compressor 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 stand alone 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."