INFINITI unveils new QX50 with variable compression ratio engine; 35% improvement in fuel economy over prior FWD V6
At the Los Angeles Auto Show, INFINITI revealed the all-new QX50—a premium mid-size SUV featuring the first production application of the VC-Turbo variable compression ratio engine, transforming on demand. (Earlier post.)
An innovative combustion engine design, the 2.0-liter VC-Turbo engine adjusts its compression ratio to optimize power and efficiency. It combines the power of a 2.0-liter turbocharged gasoline engine with the torque and efficiency of a four-cylinder diesel engine.
The QX50 is the most important vehicle we have ever launched, and it embodies everything the brand stands for: beautiful design, advanced technology and empowering performance. As a premium, mid-size SUV, it’s the right vehicle at the right time in one of the world’s fastest growing segments.—Christian Meunier, INFINITI Global Division Vice President
The VC-Turbo changes its compression ratio seamlessly with an advanced multi-link system, continuously raising or lowering the pistons’ reach to transform compression ratio—offering both power and efficiency, on demand.
The multi-link system uses an electric motor with a unique Harmonic Drive reduction gear to transform its compression ratio. The electric motor is connected to the Harmonic Drive with a control arm; as the Harmonic Drive rotates, the control shaft at the base of the engine rotates, moving the multi-link system within the engine.
As the angle of the multi-link arms changes, it adjusts the top-dead-center position of the pistons—and the compression ratio with them. An eccentric control shaft changes the compression ratio of all the cylinders at the same time. As a result, the engine capacity varies between 1,997 cc (for a low 8:1 ratio) and 1,970 cc (high 14:1 ratio).
A high compression ratio gives greater efficiency, but in certain applications poses the risk of premature combustion (knocking). A low compression ratio allows for greater power and torque, and avoids knocking. In operation, the QX50’s VC-Turbo engine offers any compression ratio between 8:1 (for high performance) and 14:1 (for high efficiency).
The engine delivers 268 hp (200 kW) @ 5,600 rpm and 280 lb-ft (380 N·m) @ 4,400 rpm. The VC-Turbo’s specific power output is higher than many competing turbocharged gasoline engines, and comes close to the performance of some V6 gasoline engines. The unit’s single-scroll turbo ensures immediate accelerator responses, on demand.
Equipped with the VC-Turbo engine, the QX50 delivers gasoline fuel economy of 27 mpg (US combined, front-wheel drive; 26 mpg all-wheel drive). In front-wheel drive specification, this offers a 35% improvement in fuel efficiency over the V6 gasoline engine in the previous QX50, while the new all-wheel drive model’s 26 mpg represents a 30% improvement.
The engine block and cylinder head are cast in lightweight aluminum alloy, while the multi-link components are manufactured in high-carbon steel alloy. Compared to INFINITI’s 3.5-liter VQ V6 engine, the 2.0-liter VC-Turbo weighs 18kg less and requires less space in the engine bay.
The VC-Turbo can switch between both Atkinson and regular combustion cycles without interruption, enabling greater efficiency and performance as it transforms.
Under the Atkinson cycle, air and fuel intakes overlap, allowing the fuel in the combustion chamber to expand to larger volumes for greater efficiency. The INFINITI engine operates the Atkinson cycle under higher compression ratios, with longer piston strokes allowing the intake valves to open for a short time as the compression stroke starts. The Atkinson cycle is commonly used in hybrid engines to maximize efficiency.
As the compression ratio drops, the engine reverts to a regular combustion cycle—intake, compression, combustion, exhaust—in separate phases to enable greater performance.
The VC-Turbo engine combines a number of existing INFINITI technologies to realize its variable nature. The engine employs both MPI (multi-point injection) and GDI (gasoline direct injection) to balance efficiency and power in all conditions:
GDI improves combustion efficiency and performance, and enables the engine to avoid knocking at higher compression ratios
MPI mixes fuel and air earlier, enabling complete combustion in the chamber for greater efficiency at low engine loads
The engine switches between both at regular engine speeds, with both sets of injectors able to work in conjunction under higher loads.
A single-scroll turbocharger maximizes performance and efficiency, enabling immediate throttle responses at any speed or compression ratio. The turbocharger ensures equivalent performance to a naturally aspirated six-cylinder unit. Small enough to aid the compact overall dimensions, a single-scroll system also reduces the loss of thermal energy and exhaust pressure.
An integrated exhaust manifold is built into the aluminum cylinder head to further enhance packaging and efficiency. This enables INFINITI engineers to position the catalytic converter next to the turbo, creating a shorter flow path for hot exhaust gases. This means the emissions control process can start sooner, as the catalytic converter heats up more quickly.
Closely controlling the flow of exhaust gases through the turbocharger, an electronically-controlled wastegate actuator maintains turbocharger boost pressure. This ensures high fuel efficiency and performance under all conditions while minimizing emissions.
The uncommonly smooth VC-Turbo does away with the two balance shafts required in conventional four-cylinder engines, due to the layout of its multi-link system.
The VC-Turbo is smoother than conventional in-line engines, and has the low noise and vibration levels expected of a traditional V6. This is, in part, a result of its multi-link design, where the piston connecting rods are almost vertical during the combustion cycle (rather than moving wider laterally, as they would in a traditional crankshaft rotation). This represents the ideal reciprocating motion, and entirely negates the need for balance shafts. Despite the addition of a multi-link layout, the engine is therefore as compact as a conventional 2.0-liter four-cylinder engine.
The result is uncommonly low levels of vibration. During internal tests, INFINITI benchmarked the engine against four-cylinder engines from rival manufacturers. The VC-Turbo produces reduced engine noise—almost as refined as a V6.
INFINITI’s low-friction ‘mirror bore coating’ contributes to a 44% reduction in cylinder friction, allowing the engine to rotate more smoothly. The coating is applied to the cylinder walls by a plasma jet, then hardened and honed to create ultra-smooth cylinder walls.
The QX50’s 2.0-liter VC-Turbo engine employs an active engine mount vibration damping system, called an Active Torque Rod (ATR), to reduce engine noise even further. The QX50 is the only car in its class to offer this kind of technology. Integrated into the upper engine mount, where the most high-torque noise and engine vibration is generated, the ATR has a G-sensor that detects vibrations. It then creates opposite, reciprocating vibrations, enabling the four-cylinder engine to be as smooth and quiet as a V6, reducing engine noise by 9dB (compared to the current QX50). This helps to make VC-Turbo one of the quietest and smoothest engines in the premium SUV segment.
XTRONIC CVT. The VC-Turbo engine is paired with INFINITI’s new XTRONIC continuously-variable transmission (CVT), a ‘shift-by-wire’ system which eliminates the need for traditional gears. The XTRONIC CVT reacts instantly to driver inputs, with an infinite number of gears matched to the transformation of the engine through different compression ratios.
The transmission adjusts immediately under hard acceleration as the engine transforms to a lower compression ratio. A higher gearing ratio for cruising speeds matches the higher compression ratio of the engine, enhancing fuel efficiency. A high final gear ratio (5.846) creates an instantaneous and decisive acceleration character.
The final drive ratio works seamlessly with the wide-range single-scroll turbocharger, allowing the engine to operate at lower compression ratios, and deliver the immediate acceleration to match the transforming, variable performance delivered by the VC-Turbo engine. Pre-programmed gears allow drivers to change engine speeds on demand, in line with their driving style.
The new XTRONIC CVT adopts the latest transmission technologies to enhance efficiency and acceleration. A torque converter ensures a direct connection between the accelerator pedal inputs and the engine response. An electric oil pump system works in-tune with the Idle Stop system, distributing oil throughout the transmission to enable immediate restarts and enhance fuel efficiency.
This ‘shift-by-wire’ transmission enhances the QX50’s powertrain packaging, with the fully-electronic system replacing the mechanical transmission linkage. This means the XTRONIC CVT takes up less space, eliminating the need for a large transmission case or tunnel mount. This enhances cabin space in the process, and results in a larger central armrest and center console area for maximum elbow room.
The shift-by-wire configuration also empowers the driver with quicker gear shifts, with more immediate and instantaneous shifts in manual mode. Driver inputs are transmitted directly to the CVT by wire, while a short stroke for the shifter itself delivers a high quality, silent gear change feel.