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Hyundai’s Breakthrough Engine that Answers a 133-year Challenge


Hyundai Motor Group developed the world's first Continuously Variable Valve Duration (CVVD) technology with improved performance, economy, and reduced emissions.

Conventional auto engines generally work on a four-step process of induction, compression, explosion, and exhaust, repeating inside the cylinder. The process converts thermal energy into kinetic energy, generating power to move. The valve is a major determining factor in the efficiency of the heat energy transferred into kinetic energy. A cylinder that opens and closes at the optimal times will maximize engine efficiency.

Engine valves and why they’re important

The intake and exhaust valves need to be accurately calibrated for the piston movements to generate rotational force.

Engine valves are either for intake or exhaust. The intake valve opens during the intake stroke to draw the fuel-oxygen mixture into the combustion chamber. The Exhaust valve opens during the exhaust stroke to discharge the expanded and oxidated gas out of the combustion chamber. Engine valves open and close in less than 0.02 seconds and undergoes about 100 million cycles in the engine’s lifetime.

Although the open-close cycles occur in a couple of hundredths of a second, the cycles are important determining factors of an engine’s power. This is because combustion requires a certain ratio of oxygen from the air; air which can enter and exit only through the valves. The turbocharging technology that has become commonplace in engines is based on forcing in or charging the combustion chamber with an extra gust of air.

Not surprisingly, automakers put forth great investment in R&D for valve technologies. Considering that combustion engines have been around for at least 133 years, it is also interesting to note that the current generation of valve technologies are based on breakthrough technology from only 30 years ago.

Continuous variable valve technology: timing and lift

The first truly innovative valve technology was presented by Porsche, more than a century after the first combustion engine by Carl Benz in 1886. The automobile variable valve timing technology by Porsche was called VarioCam. Continuous Variable Valve Timing (CVVT) is a valve-timing technology that controls the timing of a valve lift event and is often used to improve performance, fuel economy or emissions.

CVVT allows control over the timing of a valve lift

CVVT technology gives control over residual exhaust gas within the combustion chamber. When operating at high speeds, the technology opens the intake valve during the middle and end of the compression stroke (there is a moment of overlap where the intake and exhaust valves are both open, called valve overlap), allowing the maximal expulsion of exhaust gas from the combustion chamber (minimizing residual gas). This gives a great power boost when operating at high speeds.

For operating at lower speeds, the intake valve can be closed later. early intake valve closing during the intake stroke increases the volume of air-fuel mixture, engine output increases. On the other hand, late intake valve closing during the intake stroke decreases the volume of mixture in the chamber, and if fuel injection is decreased to match the decreased volume, the engine output decreases.

In low-load operation conditions, this can improve fuel economy by reducing pumping loss(power needed to compress air inside the piston) and the volume of fuel injected.

Pressing the plunger on a syringe is met with strong resistance if the end is plugged, but far easier if opened

Pumping loss is the power required to perform intake and exhaust pumping functions. Take a syringe for example, where pressing the plunger in while blocking the injection tip is met with more resistance than blocking the tip mid-plunge. CVVT is standard in virtually all automobile engines.

The second innovation in variable valve technology came from BMW in 2001, under the name Valvetronic. BMW’s technology was a continuously variable valve lift system, or CVVL, which varies the height a valve opens.

The technology is exclusive to a limited number of automakers with rigths to the CVVL patnet, such as Hyundai Motor Group, BMW, and Toyota. The Hyundai Motor Group independently developed a 2.0L gasoline CVVL engine in 2012 which debuted with the 7th generation Sonata’s enhancement model.

Overcoming the performance-economy compromise

As we’ve seen so far, variable valve technologies control the timing and lift of the vales to improve performance and fuel economy, but only to a limited extent. It could prioritize either the engine’s performance or fuel economy, but not both; at best, a balanced compromise would have to be reached between the two. An infinitely variable valve intake and lift control that adjusted to provide ideal strokes would achieve performance, fuel economy, and even eco-friendliness, but CVVT and CVVL technologies could not deliver on all fronts. Existing technologies gave control over when and to what height the valves open, but not over how long the valves open.

Not for the lack of attempts, but no other manufacturer successfully took the variable duration technology to market. The two greatest challenges have been difficulty implementing an effective valve drive technology and securing operational reliability.

More specifically, it was the challenge of designing for the appropriate source of drive-power. Electrically-sourced drive-power was proposed but soon scrapped, as electricity would be drawing power from the engine, impacting fuel economy significantly, essentially defeating the purpose.

CVVT controls valve timing, CVVL controls valve height

The second specific challenge was operational reliability. A valve within an engine undergoes about 100 million revolutions over the lifetime of the engine, and even a single valve-event malfunction can prove catastrophic. For example, if the intake valve opens when the piston is compressed and contacts the piston, serious damage may occur. In order to secure operational reliability and prevent such a malfunction, quality-control must be practiced at about 30 times the level of 6-sigma advocated by Jack Welch. It is an extraordinarily demanding level of precision and quality control. Several manufacturers saw limited success with electrical valve-controls, but all failed to commercialize due to long-term reliability. This fact alone speaks volumes about the degree of mechanical precision and quality control achieved by Hyundai Motor. 

The world-first CVVD technology developed by the Hyundai Motor Group allows for control of infinitely variable valve durations. Furthermore, the technology achieved it with a relatively simple mechanical contraption to achieve reliability, while minimizing cost increase; truly an innovation.

The world’s first continuously variable valve duration (CVVD) technology

Open and close valves as desired. The key idea behind CVVD technology

Open and close valves as desired. Hyundai Motor Group technology CVVD is short for Continuous Variable Valve Duration. Here, duration specifically means the duration of the valve event, optimized to the operation of the engine. The Hyundai Motor Group successfully created the most simply structured, mechanically-implemented CVVD mechanism, through countless iterations.

The CVVD component is composed of a drive motor and a variable control unit.

The CVVD system consists of a variable control unit and a drive motor on the camshaft. While ECU turns the CVVD drive motor up to 6000rpm, the variable rotating adjuster moves up and down in 0.5 seconds, and shifts the contact point of the cam lobe, determining how long a valve is open. 

At one end of the adjuster, the valve opens earlier and closes later, extending overlap time. At the other end, the valve opens later and closes earlier, diminishing overlap time.

The rotating motor of the variable control unit changes the cam center axis
The blue areas in the link show how different adjuster settings affect cam revolution speeds. 

CVVD cams share similarities with existing engine cams, but the adjuster link shifts the axis and adjusts cam revolution speeds. Depending on how long the intake and exhaust valves stay open or closed, there are up to 1400 settings that the CVVD system can select from.

CVVLs also work with duration changes, but in terms of peer duration, the CVVL lift is less than half of CVVD. In the case of CVVL, changes in valve duration can hinder lift and result limit necessary air intake and exhaust. CVVD rectifies this limitation, allowing for valve lift with a much wider valve duration window.

The Hyundai Motor Group registered more than a hundred CVVD-related patents per regions around the world, including in Japan, China, and the European Union. More than 120 patents are registered in the U.S. alone.

CVVD is economic, fun to drive, and green

Existing variable valve technologies had to compromise between performance and economy. driving performance required a short valve overlap to maximize airflow, and fuel-economy required longer valve overlap to mitigate downstroke pump loss. Preexisting valve technologies could not achieve both and had to seek a middle-ground or compromise between the two.

CVVD is a breakthrough technology because it can optimize valve overlap duration for high-acceleration and high-economy driving needs, boosting performance and economy up to 4% and 5% respectively. The 5% boost in fuel economy based entirely on a valve system improvement is a giant breakthrough; 5% is the aggregate total of efficiency improvement achieved by all previous valve-timing control in the entirety of the combustion engine’s 133-year history.

CVVD technology can maximize performance and efficiency

Furthermore, combustion efficiency is also improved, decreasing gas emissions by up to 12%, essentially a great eco-friendly technology. Another technology is under development, capable of decreasing emissions by up to 50%. Conventional gasoline engines use a three-way catalyst to convert NOx, HCx, and CO into inert or less harmful gases. However, this conversion rate is lower when the engine is cold or just starting to run, and harmful, unconverted gases are emitted. CVVD’s optimal valve settings not only activate TWC earlier but reduce engine emissions even before TWC is activated.

CVVD is a technology that ensures perfect capture of what was essentially two rabbits in the bush.  

Typically, fuel-efficiency-seeking cars such as hybrids operate on the fuel-economic Atkinson’s Cycle, while high-performance-oriented cars such as those with turbo engines operate on the Miller Cycle. The Otto Cycle works as a compromise between economy and performance. Whichever the cycle, the valve duration is determined and fixed.

CVVD removes the need to pre-determine and fix the cycle; the valve duration can be varied to utilize the benefits of all three cycles.  This means that compromise is no longer necessary, and the engine can deliver both fuel economy and performance. Furthermore, the effective compression ratio of the cylinder may be adjusted anywhere between 4:1 to 10.5:1 essentially in variable-compression ratios.

CVVD tech holds immense applicability

Smartstream G1.6 T-GDi is a powertrain utilizing the world’s first CVVD technology and also features Low-Pressure Exhaust Gas Recirculation (LP EGR) to further optimize fuel efficiency. Additionally, the new powertrain has an Integrated Thermal Management System that rapidly heals or cools the engine to its desired temperature, and a stronger direct injection system that increases fuel spray pressure from 250 bar to 350 bar that works together to boost performance and fuel economy.

The upcoming Sonata Turbo will be powered by the G1.6 T-GDi Engine featuring CVVD technology

Soon set to launch, the upcoming 8th generation Sonata Turbo will be the first to get the new Smartstream G1.6 T-GDi 8-speed automatic engine. There are notable changes from the 7th generation LF Sonata Turbo model’s gasoline 1.6 T-GDi 7-speed DCT. The new G1.6 T-GDi engine with CVVD technology will have significantly improved performance and fuel economy over the earlier turbo engines.

The new Smartstream’s maximum output is 180hp, same as the earlier 1.6 T-GDi model, but the new engine shows improved performance in the daily-driving range, with improved overall acceleration performance. A separate high-performance engine is also under development, featuring CVVD technology. 

The CVVD Smartstream engine will be applied first to mid-size Kia cars, then to Hyundai and Kia mid-to-large SUVs. CVVD technology will also find application in the smaller displacement engines, as well as hybrid drivetrains. In fact, a CVVD-engine-based hybrid model is in development, and the company is reviewing development plans for a 48V mild-hybrid system matched with a CVVD engine.

HCEVs and EVs are changing the way we understand drivetrains and what may be possible. However, 98% of the world’s cars are powered by internal combustion engines. In the next 30 years, that percentage will fall to 30-50%, depending on the study consulted. Nonetheless, 30-50% is a significant number remaining three decades from now. Hyundai Motor Group is not only leading the mobility industry at the forefront of hydrogen fuel cell technology but also from the rear, innovating on preexisting combustion engines, with the aim of taking technological leadership.