This final section gives a brief summary of the expected potential performance benefits from using early intake valve closing and reduced valve lift as part of a throttleless load control strategy.
Summary From Published Data
The published data is broadly in agreement about the benefits that can be achieved via an early IVC strategy.
The results indicate a significant discrepancy between the theoretical efficiency gains that should result from the reduction in pumping losses and the efficiency improvement that can be demonstrated.
Light load fuel economy improvements of around 12% and mid-load improvements in the order of 5-7% are typical, regardless of the actual type of valve train that is used.
Light Load Combustion Stability
There is a clear tendency for early IVC to reduce the combustion stability under light load operation, and the very early intake valve closing timing that results from mechanical/electromagnetic/hydraulic valve trains tends to increase this effect.
Mechanical valve train systems tend to have a lower valve lift with a longer opening duration such that IVC is closer to BDC, and in some cases combustion problems have not been experienced.
The assessment of high lift, short opening duration valve motions has indicated that the theoretical benefits of reduced pumping losses are obscured by degradation of the combustion process.
The combustion instability and the discrepancy between the theoretical and demonstrated benefits have been attributed to a variety of different factors.
In some cases there are conflicting test results which suggest that the engine behaviour with early IVC may vary significantly between different applications.
It is therefore important that the valve train system is not developed in isolation, but in conjunction with the air intake, fuel delivery, and combustion systems if the benefits of an early IVC strategy are to be maximised.
Potential For Improved Torque and Power
One aspect of variable valve lift and duration systems that has not formed a significant part of the published test data is the opportunity to realise improved full load torque output.
This is because a standard engine configuration is commonly used as the testing baseline, and early intake valve closing fuel efficiency is compared to that of the standard engine with a conventional throttle.
The ability of VVA systems to significantly increase the valve opening duration at higher valve lifts compared to the duration at low lift allows us to optimise the intake valve closing point across the whole speed range.
A large intake and exhaust valve overlap around TDC can be specified at the maximum valve lift without the normal low speed emissions and idle instability penalty because the overlap will not be present at lower valve lifts.
The variation of intake valve lift and closing timing also allows the volumetric efficiency to be maximised over the speed range, improving the full load torque output of the engine.
This final section completes our review of throttleless operation in internal combustion engines.
If you found this discussion informative, please take the time to read the other pages relating to VVA that are contained within our reference section.
Go back to the VVA reference
You may also be interested in taking a look at our variable lift system, which can be used as part of a throttleless load control strategy.
Go to our FlexValve page