Variable Valve Lift - EngineKnowHow

In an internal combustion engine which controls the load with a throttle, such as in a petrol engine the greatest pumping losses in the part load region occur across the throttle (as the cylinder must work to draw the air through the restricted area).  Engines capable of altering the valve lift, profiles and durations can “dethrottle” the throttle and use the valves to control the load (the fresh air supply to the cylinder), reducing pumping losses and therefore fuel consumption.  Typically engines with variable valve lift will run with the throttle 100% open as soon as possible as engine load increases.  Variable valve lift systems can also greatly affect the fluid motion into the cylinder offering additional performance, emissions and fuel consumption gains for both petrol and diesel engines.

 

Variable valve lift systems are currently available in two forms:

 

Discrete – Discrete systems utilise camshaft profile switching technology that offer alternative camshaft profiles, therefore offering discrete valve lifts.  Due to the alternative camshaft profiles, alternative opening and closing speeds, durations and opening / closing times can also be utilised.  Camshaft switching systems are generally oil controlled with examples including Porsche’s VarioCam Plus and Mitsubishi’s MIVEC systems.

 

Continuous – Fully variable valve lift systems offer the ability to operate with the valve lift anywhere between no valve lift till maximum valve lift, phasing the valve opening and / or closing in addition to reopening the valve after closing.  Continuous systems can also be paired with a camshaft phasing systems for further variability.  Continuously variable valve lift systems are currently in production as electric-mechanical and electric-hydraulic systems with each having their limitations.  Examples include BMW’s Valvetronic and Fiat’s Multiair systems.

 

Due to the continuous nature of the fully variable systems, several valve opening and closing strategies can be employed as illustrated below for the intake valve:

Full opening of the intake valve achieves the maximum power and torque at higher engine speeds through maximising the mass of air delivered and trapped in the cylinders.
  Late intake valve opening results in reducing valve overlap and therefore internal EGR and controls the load by throttling the cylinder’s air intake at the intake valve.  This strategy is employed under low engine speed and load conditions such as idle.
  Partial valve opening is utilised as the speed and load increases to control the load at the intake valve whilst employing the increasing internal EGR to further reduce pumping losses by increasing valve opening.
  Early intake valve closing is utilised under low speed, high load conditions to use the faster valve closing time to increase the mass of air trapped in the cylinder prior to the piston acting against the intake charge.
  Reopening of the intake valve allows further control of the fluid motion into the cylinder for increased combustion efficiency in addition to using the intake valve to control the load.

 

This strategy is typically utilised to optimise engine speed and load sites where the vehicle is predominantly driven such as urban city driving.

   No opening of the intake valve is utilised for cylinder deactivation.  Complete closing of the valves results in minimal pumping losses as no air will move through the cylinder.

 

Part Load Example with Variable Valve Lift – 2000rpm, 2bar BMEP

The results below are taken from a 2L, naturally aspirated, spark ignition engine at 2000rpm, 2bar BMEP which was capable of varying the intake valve lift and the duration that the intake valve was open whilst employing a Partial Intake Valve Opening strategy.

Source: University of Kaiserslautern - "Variable Valve Train on the Outlet Side - Potentials and Layout Criteria", Aachener Motoren- und Fahrzeugkolloquium, (R. Flierl, F. Lauer, D. Hosse, S. Schmitt)

Observing the results:

  • The engine without variable valve lift had a fixed valve lift and duration.
  • As the duration that the intake valve was open was decreased, due to the Partial Intake Valve Opening strategy the valve lift would also have been decreasing.
  • The flow of the fresh charge into the cylinder became to be controlled, or “throttled” by the intake valve rather than at the throttle.
  • As the dethrottling of the throttle increased with the decreasing duration, pumping losses were decreasing and therefore less fuel had to be combusted to compensate to attain the target 2bar BMEP.
  • Through the variable valve lift system a fuel consumption saving of 9% was achieved.
  • As the time in which the fresh charge could be inducted was decreasing (due to the duration), the intake charge velocity would have been increasing and therefore the turbulence within the cylinder.