Infiniti unveils I4 Variable Compression Turbo engine; targeting 27% improvement in fuel efficiency over V6 engines of similar output
At the Paris Motor Show, Infiniti the new VC-Turbo (Variable Compression Turbo)—the first production-ready variable compression ratio engine. (Earlier post.) VC-Turbo technology combines the power of a high-performance 2.0-liter turbo gasoline engine with the torque and efficiency of an advanced diesel powertrain without the equivalent emissions.
Transforming on demand, Infiniti’s VC-Turbo technology uses an advanced multi-link system to raise or to lower the reach of the pistons, detecting the car’s driving condition and driver inputs, and instantly selecting the most suitable compression ratio. The engine is able to offer any compression ratio between 8:1 (for high performance) and 14:1 (for high efficiency).
With a development target power output of approximately 200 kW (268 hp / 272 ps) and 390 N·m (288 lb-ft) torque, the 2.0-liter VC-Turbo in-line 4 engine will be comparable to certain six-cylinder gasoline powertrains for performance, while significantly outperforming them in efficiency. Infiniti engineers are targeting 27% greater fuel efficiency over similarly-powered V6 gasoline engines, depending upon vehicle application.
|The red line and green line show the difference in piston height. Click to enlarge.|
The four-cylinder VC-Turbo engine achieves lower levels of noise and vibration than conventional internal combustion engines, particularly diesel, and is lighter and more compact than V6 units with comparable power outputs.
Infiniti tested and developed more than 100 engine prototypes in perfecting the technology, covering more than three million kilometers of equivalent road testing and spending over 30,000 hours on the test bed (equal to five million kilometers of on-road testing). The engine is now in its final stages of development on real roads.
Technology. Infiniti has patented more than 300 new technologies specifically designed for application in the new four-cylinder VC-Turbo engine. Among these is the world’s first multi-link system and an electric motor actuator with Harmonic Drive reduction gear, which together enable the engine to transform its compression ratio continuously.
Infiniti connects an electric motor to the Harmonic Drive reduction gear with a connecting control arm. The Harmonic Drive rotates according to the compression ratio required, which then rotates the control shaft at the base of the engine and, in turn, moves the multi-link mechanism.
Changing the multi-link angle adjusts the height of the top-dead-center of the pistons, varying the compression ratio. An eccentric control shaft varies the piston stroke position for all four cylinders at the same time.
The layout of the multi-link mechanism makes the VC-Turbo engine smoother than conventional in-line engines, boasting low noise and vibrations levels similar to those of a V6 engine. The multi-link system means the piston connecting rods are almost vertical during the combustion cycle, rather than moving wider laterally as in a traditional crankshaft rotation.
In addition, the multi-link design operates the ideal reciprocating motion when compared with the traditional connecting rod and crankshaft system. The smooth-running VC-Turbo therefore benefits from uncommonly low levels of vibration and does not require any balance shafts. Conventional four-cylinder engines require a pair of balance shafts to reduce second-order vibrations.
Infiniti benchmarked the 2.0-liter VC-Turbo’s vibration characteristics against other four-cylinder turbocharged engines. In the VC-Turbo, engine vibration noise was reduced from a benchmark average of approximately 30 dB to just 10 dB. Such a low level of vibration makes the VC-Turbo four-cylinder engine almost as smooth and refined as a V6.
Infiniti’s benchmark V6 engine for the latter stages of VC-Turbo development—the manufacturer’s 3.5-liter ‘VQ’ engine—produces 3 dB of vibration noise, only slightly lower than the new engine.
With a variable compression ratio, the VC-Turbo engine is able to switch between both Atkinson and regular combustion cycles, without interruption. Each cycle enables greater combustion efficiency and optimal engine performance as the combustion ratio transforms.
Under the Atkinson combustion cycle, air intake overlaps with the compression cycle in the cylinder, allowing combustion gas to expand to a larger volume for greater efficiency. The VC-Turbo engine employs the Atkinson cycle under higher compression ratios, where the stroke of the pistons is longer. The modern Atkinson cycle—employed in many advanced hybrid engines—allows the intake valves to be open for a short time as the compression stroke starts.
As the compression ratio lowers, to enable greater engine performance, the engine is able to operate a regular combustion cycle: intake, compression, combustion, and exhaust, each stage taking place in distinct, separate phases, resulting in greater performance.
The gasoline VC-Turbo engine employs a combination of both multi-point injection (MPI) and direct injection (DIG), further improving the engine’s ability to balance efficiency and power in all driving conditions.
The DIG system improves combustion efficiency and performance due to the heat-absorbing effect of fuel vaporization—which aids cylinder cooling—and allows the engine to avoid ‘knocking’ at higher compression ratios.
MPI allows for earlier mixing of air and fuel to increase engine efficiency at low loads. Like the compression ratio, the engine can switch instantly between GDI or MPI at regular engine speeds, while both sets of injectors work in conjunction under a combination of high engine speeds and load.
While the VC-Turbo engine’s multi-link system limits the lateral movement of the connecting rods within the engine—a cause of piston friction—Infiniti’s mirror bore coating technology further reduces cylinder friction by 44%. Mirror bore coating technology is a process in which the cylinder walls are sprayed by a special plasma jet. The coating is then hardened and honed, resulting in an ultra-smooth cylinder wall to allow the engine to rotate more smoothly. Mirror bore coating technology has recently been used in Infiniti’s new VR30DDT 3.0-liter V6 twin-turbo engine.
As the engine shifts to allow greater performance or efficiency, a wide-range, single-scroll turbocharger works in conjunction with the VC-Turbo system to maximize acceleration response at any engine speed or compression ratio.
For example, if the driver is cruising at a steady speed but needs to accelerate to overtake, the turbocharger reacts instantly to boost acceleration as the engine shifts seamlessly to a lower compression ratio. The turbocharger enables the four-cylinder VC-Turbo engine to benefit from greater efficiency than a larger, naturally-aspirated six-cylinder engine, yet ensures equivalent performance. The single-scroll turbocharger improves the loss of thermal energy and exhaust pressure, and keeps weight and cost low.
An integrated exhaust manifold is built into the engine’s aluminum cylinder head to further improve packaging, performance and efficiency. The compact size and neater packaging of the cylinder head enables engineers to position the catalytic converter next to the turbo. This reduces the length of the flow path for hot exhaust gases, allowing the catalytic converter to heat up quickly and start the emissions control process sooner. The close proximity to the turbo facilitates the VC-Turbo engine’s immediate accelerative responses.
An electronically-controlled wastegate actuator maintains boost pressure by closely controlling the flow of exhaust gases through the turbocharger. This ensures fuel efficiency and performance under all conditions, even as the compression ratio and engine speeds change.
As the turbocharger recirculates hot exhaust gases back into the engine, the VC-Turbo’s high capacity intercooler lowers the temperature—and therefore the density—of the air, further enhancing the efficiency of the forced induction system.
The four-cylinder VC-Turbo’s high specific power results in greater lubrication and cooling requirements than conventional four-cylinder turbo engines. Development teams addressed this with a two-stage variable displacement oil pump, which enables low-pressure oil distribution at engine speeds below 3,000 rpm. The engine automatically increases oil flow pressure as engine speeds or loads rise (and the compression ratio lowers), ensuring optimum running temperatures at all times.
A multi-way flow control valve (MCV) provides further thermal management. The MCV optimizes the distribution of coolant to the radiator, cabin heater, and oil cooler depending on current conditions and compression ratio. The MCV’s zero-flow coolant flow mode cuts coolant distribution during a cold start to reduce frictional loses while the engine warms up.
Packaging. The new engine’s block and cylinder are made from lightweight aluminum alloys, with the multi-link components manufactured in a high-carbon steel alloy. Compared to the ‘VQ’ V6 engine, the VC-Turbo engine weighs 25 kg less, and occupies a smaller engine bay footprint.
In addition to its downsized nature compared to the larger, naturally-aspirated V6, the layout and design contributes to improved packaging and weight savings throughout the powertrain.
The cylinder head with integrated exhaust manifold is compact in size and enables more intelligent integration with the turbocharger and catalytic converter. Cast in aluminum, the cylinder head and manifold union contributes to the engine’s overall reduction in weight. The mirror bore cylinder coating technology also negates the need for cast iron cylinder liners.
With the multi-link system’s minimal lateral movement, development teams were able to do away with the pair of balance shafts that conventional four-cylinder engines require in order to minimize vibrations. With efficient packaging of its new multi-link components, the VC-Turbo engine will therefore occupy a similar space under the hood in relation to other four-cylinder in-line engines.