MCE-5 VCRi: Pushing back the fuel consumption reduction limits

We can replace VCR with VVA

It’s a common mistake to believe that VVA can replace VCR: VVA can under no circumstances replace VCR.

VVA (photos: FIAT Multiair), serves a wide variety of
strategies to reduce fuel consumption and emissions

VVA (Variable Valve Actuation) encompasses a wide variety of technologies known under different commercial names, such as VTech®, Multiair®, Valvetronic® or Variocam+®. We can distinguish two sub-families: VVT (Variable Valve Timing) systems that equip practically all gasoline engines and VVL (Variable Valve Lift) systems that are being used in a more widespread way under different technological configurations. These two systems can be combined.

The aims of VVA are essentially engine filling optimization throughout the entire speed range, load scavenging during supercharging, and dethrottling. Dethrottling reduces pumping losses through the use of the internal EGR and/or by LIVC (Late Intake Valve Closing). This last strategy plans for the expansion-compression of intake gases in an almost adiabatic manner, which can advantageously replace the butterfly valve. VVA is also used in a wide variety of other strategies such as heating the catalyst during cold starts, the control of the exhaust back-flow during supercharging and, possibly, adjusting the EGR rate during compression ignition (CAI-HCCI).

Given this wide assortment of possibilities, we understand why carmakers are so motivated to master VVA.

VCR has two main objectives, neither of which can be met by VVA: hard downsizing and increasing the mean expansion ratio.

Using VCR to serve hard downsizing consists in reducing the compression ratio to highly supercharge the engine without risking hard knocking, wild ping or rumble. To reach the target performances for specific torque and power (MCE‑5 VCRi GDI: 320 Nm/L – 130kW/L), a minimum compression ratio of 6:1 is necessary.

The first error in reasoning comes from the effective compression ratio. It’s perfectly true to say that the engine’s effective compression ratio can be reduced by late intake valve closing (LIVC) using VVA. Having said that, reducing the effective compression ratio via late intake valve closing not only reduces the compression ratio, but also the engine’s effective cubic capacity due to the resulting intake back-flow. In this case, one must compensate for the drop in cubic capacity with an increase in intake pressure (higher supercharging), which increases engine knock. In the end, there is no gain in specific torque or power as opposed to VCR that geometrically reduces the compression ratio, without reducing the engine’s cubic capacity. Hence, VVA is no substitute for VCR.

With regard to the second objective that aims at increasing the mean expansion ratio, though VVA can reduce the effective compression ratio, it cannot increase it. For that, one needs to reduce the dead volume in the combustion chamber, which can only be done geometrically by a VCR engine.

We can also note that VCR will be essential to control compression ignition (CAI-HCCI), and in particular cylinder-by-cylinder VCR control (cylinder selective). Nevertheless, EGR rate control on this type of engine will be managed by a VVA system.

While VVA cannot replace VCR, the opposite is also true, even if some objectives are similar. The best approach is to combine VVA and VCR for the best possible result.

Ultimate CO2: on 65 kW vehicles, VCR + integrated compressor + VVL compared with fixed compression
ratio + GDI + turbo + VVA reduces fuel consumption by 5.5% on NEDC, using MPFI only

Ultimate CO2: on 90 kW vehicles, VCR + integrated compressor + VVL compared with fixed compression
ratio + GDI + turbo + VVA reduces fuel consumption by 6.87% on NEDC, using MPFI only

On 120 kW vehicles, VCR + integrated compressor + VVA compared with fixed compression ratio
+ GDI + turbo + VVA reduces fuel consumption by 7.6% on NEDC in MPFI, and by 16% in CAI GDI

On 160 kW vehicles, VCR + integrated compressor + VVA compared with fixed compression ratio
+ GDI + turbo + VVA reduces fuel consumption by 14.4% on NEDC in MPFI, and by 22% in CAI GDI