Theoretically you could get to a modern Atkinson cycle through the means of variable valve timing, but it depends on the accuracy of the system involved. It also depends on whether you want the engine to fluctuate back and forth between a regular Otto cycle and the Atkinson cycle.
Within the Engineering Explained video, Jason Fenske talks about one way to achieve the Atkinson cycle, that being bleeding off part of the intake charge during the compression stroke. By allowing the intake valve to remain open at the beginning of the compression cycle, you would be allowing the bleed off to occur through reversion back into the intake tract. The engine would only utilize a portion of the intake of air/fuel. This can be handled via variable valve timing (VVT). How well the VVT system is designed and implemented will determine the degree of how well this is accomplished.
According to Review and analysis of variable valve timing strategies—eight ways to approach published in the Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 218(10), pg. 1179-1200, the valve events of a typical spark ignition (SI) engine are:
Intake valve opening (IVO) The inlet valve opens and the air-fuel charge is sucked into the cylinder as the piston moves downward from top dead centre (TDC). It continues until the piston reaches its bottom dead centre (BDC). Generally, opening of the intake valve takes place at around 10 degrees before TDC during the exhaust stroke. Opening of the inlet valve represents the start of the intake stroke as well as the start of intake and exhaust valve overlap.
Exhaust valve closing (EVC) The exhaust valve closes when most of the burned gases have been expelled to the exhaust manifold. This is the end of the exhaust stoke as well as the end of valve overlap. Closing of the exhaust valve takes place at around 10 degrees after TDC during the intake stroke.
Intake valve closing (IVC) Closing of the inlet valve represents the end of the intake stroke and the start of the compression stroke. The inlet valve closes at around 50 degrees after BDC during the compression stroke.
Exhaust valve opening (EVO) Opening of the exhaust valve represents the end of the expansion stroke and the start of the exhaust stroke. The exhaust valve opening takes place at around 60 degrees before BDC.
There are eight different strategies which can be applied to VVT:
- Late intake valve closing (LIVC)
- Early intake valve closing (EIVC)
- Late intake valve opening (LIVO)
- Early intake valve opening (EIVO)
- Late exhaust valve closing (LEVC)
- Early exhaust valve closing (EEVC)
- Late exhaust valve opening (LEVO)
- Early exhaust valve opening (EEVO)
Only some of these are particular to effecting an Atkinson cycle. There are pros and cons to each type variation. The main one which Jason is talking about is LIVC. Some of the pros/cons to LIVC are:
- Improves volumetric efficiency (VE) at higher engine speeds due to mixture high-flow momentum continues to charge the cylinder even though the piston is travelling upwards.
- Decreased pumping losses during part-load conditions and lower NOx emissions with only a slight loss in torque.
- Reduces VE at lower engine speeds due to the intake manifold and cylinder pressures being equal at BDC.
- Propensity for knock at lower engine speeds due to richer mixture and the air-fuel density being lower. This decreases the flame speed and thus enhances knocking.
- Usually requires increased mechanical complexity to implement.
Some of the ways LIVC may be implemented are:
- Additional camshafts or camshaft lobes which get actuated at different engine speeds.
- Camshaft phasing which changes the valve timing at different engine speeds.
According to the paper, LIVC engines demand more spark advance as compared to conventional engines:
... especially at part-loads, because the mixture is permitted a sufficient amount of time to auto-ignite. By advancing the spark it is possible to avoid auto-ignition. The maximum pressure inside the cylinder of LIVC engines was found to be lower than that in conventional engines. This is because the amount of effective mixture left for combustion after the intake stroke is less in LIVC engines.
As far as the EGR valve goes, it can easily be eliminated with enough valve overlap. This is one of the methods used with newer performance engines. It is easily designed into the cam shaft. When the exhaust is held open longer when the intake opens and the piston starts moving down for the intake stroke, reversion ensues and exhaust is sucked back into the cylinder. The end result is the same as what happens when the EGR opens and allows exhaust to be sucked back into the intake. Better yet, with the taking the LIVC approach to gaining an Atkinson cycle, NOx is reduced naturally because of the temperature drop inside the cylinder.
A better way to implement this type of a system may be by utilizing the Koenigsegg FreeValve technology. This technology uses a cam free approach by actuating the valves using a pneumatic/hydraulic solution. Everything is computer driven, which means the valves can opened and closed at will. The open/close events can also be changed on the fly as the engine is running for greater efficiencies. With this the engine isn’t stuck with one or possibly two different cam profiles. It can have as many profiles and event timing changes as needed to provide a more efficient engine. Getting an engine which can benefit from a regular Otto cycle during the slower engine speeds and from the Atkinson cycle during higher engine speeds could easily be done. Better yet, allowing the computer to learn what would be needed to garner the best of everything may just be the ticket for an all-around awesome engine combination.