The evolution of the internal combustion engine is still ongoing despite vast quantities of funding moving towards electric motors. The focus for most manufacturers is to get as much power out of each sip of fuel.
Over the years, engines have had extra kit added on to the simple base unit of cylinders that sucked in a fuel/air mix and ignited it. In the early days it was simply about power, regardless of fuel used and to do that a supercharger was added. Then engineers looked at where the valves were placed, the number of them and even how to change how they opened: overhead valve engines became overhead cams to improve the speed of the motor. Variable Valve Timing was developed to change the cam lobe such that the camshaft had an economy cam and a power cam.
Engineers also looked at using waste products to increase the amount of the mix going into the cylinders in the form of turbochargers. They also created valves in the air flow to increase/decrease the amount of air mixed. Fuel injection delivered a precise amount of fuel to the cylinders to try and ensure consistent flow. Another idea was to use the Atkinson Cycle instead of the more traditional Otto Cycle. The Atkinson Cycle has all four strokes in one revolution of the crankshaft whereas the Otto has one stroke per revolution.
Nissan has taken another old idea and modernised it – variable compression in the cylinder. This means that it can have a high compression ratio for economy and a lower ratio for power. They call the engine “VC-T”. What is interesting is that the higher the compression ratio, the higher the efficiency of the motor and so you would expect the compression ratios to be reversed in this engine. However, to boost turbo pressure, you need a lower compression ratio – the compression is done prior to entering the cylinder.
How does it work?
In efficiency mode, the engine has a compression ratio of 14:1. The con-rod is still connected to the crankshaft, however it is connected via a multi-link plate that surrounds the crankshaft. This enables the crankshaft to move sideways which in turn changes the angle of the con-rod inside the cylinder. The crankshaft is connected to a harmonic drive via a control rod and an actuator arm.
When more power is required, the harmonic gear pulls the actuator arm to change the angle of the crankshaft which in turn affects the length of travel of the piston head. Reducing how far up the cylinder the piston can travel changes the compression ratio. In power mode that ratio is only 8:1. As you can imagine, to do this on the fly means that the ECU has to deliver a message to change the harmonic drive so it can be adjusted based on throttle input.
The engine is a combination of lots of the technology described above, including being a 2-litre four cylinder turbocharged Atkinson Cycle motor with variable valve timing and fuel injection. As the initial costs are higher than a normal motor, the engine has been released in an Infiniti first – Infiniti being the luxury sports arm of the group. The 2019 QX50 SUV has been chosen to be the first model to receive this type of motor.