Nissan’s Variable Compression Turbo (VC-Turbo) is a unique engine capable of adjusting variable compression. Unlike traditional engines, which have a fixed compression ratio, the VC-Turbo engine adjusts its compression to the ideal rate depending on the driving style at any given time. As such, this innovative engine design is optimized for both horsepower and efficiency, two performance characteristics that typically clash.
What is a compression ratio?
The compression ratio is the ratio between the maximum cylinder volume of an engine and its minimum cylinder volume. This ratio determines the efficiency and power characteristics of an engine. The idea behind the VC-Turbo is to match the power of a naturally aspirated 6-cylinder engine while achieving the efficiency and torque characteristics of a smaller diesel engine.
What is variable compression?
In a traditional gasoline engine, a piston moves up and down from a central point called the crankshaft, limiting compression to a single gear. For this reason, engineers cannot optimize this type of engine for all driving conditions. High compression ratios result in fuel economy at low revs, but are insufficient for high-end power output. A low compression ratio does the opposite, reducing fuel economy but allowing more power at higher revs.
The VC-Turbo engine is the better of the two solutions to this problem. It uses a multi-link system with an adjustable crankshaft and an actuating arm. This geometry changes the length of the piston stroke and adjusts the engine compression ratio on the fly. By monitoring driver inputs multiple times per second, the engine works with an on-board computer to adjust the compression ratio from 8: 1 to 14: 1 as needed, optimizing power and efficiency seamlessly.
The fully variable engine will choose the ideal compression ratio based on the driver’s throttle input. When the driver calls for power, the internal actuator shortens the piston stroke, allowing compression to drop. At the same time, it harnesses the turbocharger for maximum power. Conversely, during highway cruising and other low horsepower needs, the engine automatically adjusts to a higher compression ratio, providing a much higher level of efficiency.
How does the VC-Turbo engine work?
The VC-Turbo engine runs on the same basic four strokes as any other engine: intake, compression, power and exhaust. However, this engine can completely change the compression ratio to target increased horsepower or increased fuel economy. There are four basic steps to changing the compression ratio in this engine:
- A harmonic drive, essentially an electric motor, rotates an actuator arm.
- The actuator arm rotates a drive shaft, which forces a lower linkage to move up or down.
- The lower link changes the angle of the multiple link, which is connected by an upper link to the piston.
- The upper link moves from the rotation of the multi-link, which causes the piston to move up or down, changing the compression ratio.
So, for example, if the motor was running in its power mode but wanted to switch to efficiency mode, the harmonic motor would run. This rotation would then rotate the drive shaft, which would pull the lower link down and force the upper link up. As a result, this movement would decrease the space between the piston and the cylinder head and increase the compression ratio.
The VC-Turbo engine uses direct injection and orifice injection to achieve the most efficient combustion at low load. Only direct injection is active at higher loads, meeting power demands and allowing precise fuel injection timing. In addition, the distance the exhaust has to travel to reach the turbo is extremely short in this engine, allowing the turbo to be turned very quickly for fast power delivery.
Reliability and durability
Nissan’s VC-Turbo engine is more complex than traditional power plants, and the complexity can lead to reliability issues. Because of this, the automaker has subjected this engine to a much higher standard of testing than usual, including nearly two million miles of real-world driving. Nissan has subjected a fleet of 600 development cars to various climatic and weather conditions to achieve reliability and durability goals. By comparison, a traditional engine undergoes testing with around 50 development vehicles.
Fuel economy and performance are among the most critical driving characteristics, but usually aren’t offered together in a single powertrain. With fully variable compression technology, Nissan’s VC-Turbo delivers both elements – efficient highway cruising and powerful highway passing – all in one system.