Nissan's VC-Turbo Engine - A variable compression inside!

Globally, many automakers adopt unified technologies in the engine to boost their mileage and power as well. Many advancements are there like Supercharging, Turbocharging, Variable Valve Timing (VVT), Honda's i-VTEC, etc.,

Likewise, Nissan bought out an unthinkable technology named a VC-Turbo Engine (Variable Compression Ratio Engine), which can extract more power plus more mileage by altering their compression ratio depends on the loading conditions. It sounds more captivating.

Ok. What is a Compression Ratio?

The ratio of the total volume of the cylinder and the clearance volume where the combustion takes place is known as the compression ratio. In simple words, the ratio of volume before compression and after the compression. For example, assume a compression ratio of 8:1, here the air-fuel mixture inside the total volume has compressed to one eighth its total volume by the movement of the piston from BDC Bottom Dead Center to TDC (Top Dead Center).

In a modern gasoline-powered engine, the compression ratio is about 10:1 to 13.5:1. In the diesel-powered engine, the compression ratio is about 14:1 to 25:1.

Higher compression ratios always give better thermal efficiency with lower fuel consumption and fewer exhaust emissions. Lower compression ratios give more power, especially in high load conditions, but fuel efficiency is lower. This compression ratio is permanent (not variable) in all IC engine-powered vehicles.

How Nissan achieves variable compression?

Nissan designed an intelligent multi-link system, which is a mechanism to alter both the stroke length and engine displacement (CC) by varying the compression ratio. This system has positioned amidst piston and crankshaft instead of connecting rod, included with control shaft and a harmonic drive electronic actuator. A parallelogram or elliptically shaped structure (multi-link) has embodied with a crankshaft. From there, Upper-link has paired with the piston, and Lower-link has paired with the control shaft. An actuator is paired with a control shaft using an actuator arm.

With this mechanism, the compression ratio can be varied about 8:1 (for power) and 14:1 (for efficiency).

It is a complex mechanism, but the work is admirable. Suppose if we press down the accelerator pedal to increase power, the actuator turns the control shaft to some extent using the actuator arm. So the turned control shaft made some angle deviation in the elliptic structured crankshaft (in multi-link). It adjusts the upper-link by reducing the stroke length (traveling distance of the piston has decreased), so the compression ratio gets reduced to 8:1 and produces more power which we required. This mechanism happens inversely while running at slower speeds, which increases the compression ratio to 14:1. To boost air intake, Nissan combines turbocharging technique with variable valve timing (based on Atkinson cycle) and split fuel injection.

Due to complex mechanisms, you may think there is a power loss with an increase in weight... Right!!

Nissan claims that the crankshaft axis and the bore axis are not the same. The crankshaft axis gets offset to some extent from the bore axis, so the Upper-link moves almost linear (up and down). Thus the side force (friction) created by the piston in the cylinder has reduced when compared to conventional types. Therefore weight increased, but the power loss is not there.

Many manufacturers attempted variable compression with more innovative techniques, but commercially Nissan is the first to produce and adopt this VC-Turbo engine in their luxury-branded vehicle Infiniti QX50. While increasing the speed, the sound makes dazzling.