While electrification is all the rage, many car manufacturers are still working hard to make gasoline and diesel engines more efficient. One such company is Nissan, with its variable compression engine technology.
This engine is capable of a number of different power-producing tricks, and it has gained plenty of attention since its unveiling. Let’s take a closer look at what makes this innovative engine so special.
Variable Compression Ratio
Nissan has developed a system that enables its new VC-T engine to vary the compression ratio of its combustion chamber, switching between 14:1 power mode and 8:1 efficiency mode depending on load. This makes it the first gasoline engine with this capability.
The system relies on a crankshaft design with eccentric big-end rod bearings. Hydraulic jacks mounted in the engine block move these gears to raise or lower the piston top dead center position, adjusting the clearance volume (Evans 2009).
A secondary advantage is that reducing the compression ratio also reduces the side force on the piston during its power stroke. That reduces vibration and eliminates the need for a balancer shaft.
Nissan’s development process was highly iterative and included extensive tests of different systems, Raposo says. Its final engine uses multipoint injection for low compression operation and direct fuel injection for high-compression power. This phasing helps contain particulate emissions and ensures that all of the combustion is happening at the proper conditions for maximum thermal efficiency.
Variable Valve Timing
VVT (or variable valve timing) is designed to allow the intake and exhaust valves to open for longer periods of time at higher engine speeds, increasing the amount of air entering the cylinders. This results in a richer burn of the fuel/air mixture, which produces more power.
Although Honda evangelists may have you believe that their VTEC is the first and last word in variable valve timing, Nissan has been experimenting with dynamically altering intake and exhaust valve events for quite some time. The current iteration is called VVEL, and it improves performance in a much more holistic way than Honda’s system.
Like the earlier cam-phasing VVT systems, this one uses 3 rocker arms that are connected together by hydraulic pressure. At low speed, those rocker arms are driven independently by the slow-timing/low-lift right and left cams. At medium speed, those rocker arms are connected together such that they’re both driven by the middle, fast-timing cam.
Variable Valve Lift
Variable valve lift allows the engine to better control airflow in and out of the cylinder. During lower load ranges the system reduces camshaft friction and advances fuel efficiency by easing intake-valve opening duration. This also improves response by allowing more dense air to enter the cylinders.
VVT and VVL are two of the most common dynamic valvetrain features that you can find in today’s mainstream commuter cars and some performance models. They help to reduce emissions and offer better fuel economy, responsiveness, and top end power without making any concessions when it comes to engine output.
The Nissan system is called VVEL and works by using two different cam lobes on the same camshaft; one is shorter with less lift and another is taller with more lift. At lower loads the system uses the short profile and at higher speeds it moves to the taller profile. This enables the system to deliver more power at all engine speeds while helping to reduce emissions and fuel consumption.
Direct Fuel Injection
Direct fuel injection sprays gasoline directly into the combustion chamber, rather than mixing with incoming air as in old-fashioned carburetors. It enables higher compression ratios for better efficiency and power.
A pump delivers low-pressure fuel from the tank to a high-pressure fuel injector rail, where a computer-controlled control module (ECU) proportions a constant stream of metered fuel to each injection nozzle in the intake manifold. The ECU uses signals from sensors to determine engine conditions, such as throttle position and load.
As each cylinder’s intake valve opens, the fuel injectors fire in sequence to deliver a precise amount of gasoline into the cylinders at precisely the right time. This eliminates the need to wait for combustion to occur, which robs engines of torque and horsepower. It also lowers cylinder temperatures, allowing for quicker ignition and better fuel economy.