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Abstract: In diesel engines, VVA can be used to enable Miller type valve timing, low compression ratio, engine braking, internal EGR, swirl control or to improve torque characteristics. In gasoline SI engines, VVA is used to optimize torque characteristics, to reduce cold start HC emissions and to enable throttless operation.
VVA technologies can be applied to both gasoline and diesel engines. In gasoline engines, some of the primary interests in using VVA technologies are improvement of engine torque characteristics across the entire engine speed range, reducing cold start hydrocarbon emissions, cylinder deactivation and reducing engine pumping losses across the intake throttle. In diesel engines, interests include Miller valve timing implementation, secondary valve events for engine braking and internal EGR, improving engine warm-up behavior, control of swirl and tumble flow in the cylinder and improving engine torque characteristics and transient response.
Some important considerations in designing VVA systems include :
- avoiding catastrophic failure (the cam profile can prevent valve-piston contact);
- improving reliability (valve seating velocity and valve-piston clearance requirements meet without sophisticated controls);
- providing repeatability (less subject to trigger-valve variability);
- improving controllability (can provide secondary events without separate triggering);
- providing limp home (limited lost motion);
- adding capability; and reducing parasitic losses.
Gasoline SI VVA
VVA technologies have achieved considerable market penetration is gasoline SI engines. For these applications, various VVA technologies are commonly used to achieve the objectives listed above:
Optimizing engine torque characteristics can be achieved by controlling intake valve closing timing across the engine speed range. At lower engine speeds, earlier IVC is beneficial for volumetric efficiency while at higher engine speeds, a later IVC is better. Cam phasers and discrete VVL systems such as Honda’s VTEC are commonly used for this purpose.
Reducing cold start HC emissions is an important application of VVA in modern SI engines that has no doubt contributed to a widespread uptake of cam phasers in SI engines.
An important application of VVA system in SI engines is their potential to allow the engine to operate throttless or to reduce throttling losses. Throttling creates a significant fuel consumption penalty in SI engines and one of the primary reasons SI engines are less efficient than diesels. Pressures to reduce fuel consumption and GHG emissions will lead to further uptake of this technology in SI engines. Discrete and continuos VVL systems can be used for this application.
Cylinder deactivation can be achieved with various discrete VVL systems as already discussed.
In-cylinder charge motion can also be controlled with various VVA systems. As discussed below for diesel applications, switching the phasing of the intake valves in engines with multiple intake valves/cylinder is one option. Using a discrete VVL system that uses two different maximum lifts for multiple intake valves/cylinder engines as was done by Audi (see above) is another.
In downsized direct injection gasoline engines, ensuring high levels of cylinder scavenging is critical to ensure that low engine speed torque can be maximized and engine fuel consumption minimized. VVA systems are important tools to maximize this scavenging.
An over-expanded engine cycle (sometimes referred to as Atkinson or Miller cycles)—achieved through the use of late intake valve closing combined with a very high geometric compression ratio—is an effective means to reduce part-load pumping losses and improve engine thermodynamic efficiency. This can be achieved by using cam phasers on the intake and exhaust cams.