Sunday, February 11, 2018


For me it's always exciting to chart new technologies, especially when they result in a cost-effective manufacturing process, and a practical application.

I was around in the 60s, when Perth inventor Ralph Sarich came up with the Orbital Engine. Sadly, a great concept which never came to production because of cost and complexity.

We've seen Mazda persist with its version of Felix Wankel's Rotary engine concept - an engine that's even powered Le Mans racers.

Now, from Nissan-Infiniti the first practical and commercialised variable compression engine, and it's brilliant. It makes it's debut in Infiniti's QX-50 crossover.

Here's how it works:
First a cinemagraph shows the principle:

I will use the description from the Infiniti press kit, rather than rewrite it:

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.
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 blend of performance and efficiency represents a compelling alternative to diesel, challenging the notion that only hybrid and diesel powertrains are capable of delivering high torque and efficiency.
The engine delivers 268 hp (200 kW) @ 5,600 rpm and 280 lb ft (380 Nm) @ 1,600 – 4,800 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 is competively efficient, delivering 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.

I think this fabulous, lateral thinking and I commend the team, led by Nissan's Chief Powertrain Engineer, Shinichi Kiga.

When asked why it has taken so long, Kiga replied that the solutions were down to two things - first, it took that long to make the mathematics work; and second, the vastly improved computing power available today to accurately crunch the numbers.

The metal rhombus links the conventional piston connecting rod to the crankshaft to a lower rod attached to an actuator arm. To vary the compression ratio, a harmonic drive rotates the arm, which in turn changes the angle of the multilink, which shortens or lengthens the distance the piston travels.
The multilink reduces the angle of the connecting rod, which reduces side-to-side motion of the pistons. Because the piston motion is closer to vertical, the design does not need counterbalance shafts.
Engineer Kiga says the concept only works with inline engines, as it's too complex to transfer to V engines.

Nissan has issued over 300 patents on its system to protect its intellectual property, but I'm sure it would licence its use to other companies.

Well done Nissan-Infiniti!

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