Modified Atkinson Cycle Engine

Alternative variable valve timing strategy increases low
speed torque obtainable from Atkinson Cycle Engine.

Virtually all engines utilizing Atkinson cycle technology are used in conjunction with superchargers or electric motors (hybrids). This is due to detonation limited low speed maximum torque deficiencies associated with late intake valve closings utilized in conventional Atkinson Cycle Engines.

The increased light load thermal efficiencies associated with Atkinson engines using late intake valve closings are indisputable.

SAE papers 850074 (Nissan) and 930820 (Honda) document phenomena that could facilitate percent increases in maximum low speed power availability approximately equal to percent decreases in low power availability associated with conventional late close Atkinson Cycle engines.

I came across this early close Atkinson scenario totally by accident while developing variable valve event technology in conjunction with high compression ratios in the early 1980's. Unfortunately the huge increases in low speed torque were accompanied by forfeiting most of the thermal efficiency gains associated with late close Atkinson concept. Eventually I figured out how to have increased maximum low torque and maintain the increased light load thermal efficiency associated with the late close Atkinson Cycle engine. (Patent 4,961,406)

The basic concept could best be described as a Honda i-VTEC engine with a smaller low speed intake valve event and more intake valve phasing in conjunction with an Atkinson cycle expansion ratio. The reduced size intake valve event is run in the advanced position when maximum power is required and retarded when maximum thermal efficiency is desired at light load.

SAE paper 850074 and SAE paper 930820 both acknowledge that earlier than conventional intake valve closings will decrease mixture burn rates. Both papers acknowledge that mixture motion and charge temperature decrease as intake valve closings are advanced. The Honda paper acknowledges that Wide Open Throttle Volumetric Efficiently actually increases, compared to baseline, at low speeds with earlier than normal intake valve closings.

It is amazing that the auto industry did not recognize the aforementioned scenario 20 years ago as a means to capitalize on their research and let
Toyota obtain the competitive edge when Toyota patented the knock control concept 20 years later (patent 6,848,422 whose validity could be questioned in light of 4,961,406 which was not even cited in Toyota's patent).


Nearly everyone agrees that there is no cost benefit associated with hybrid technology at current fuel prices. The fuel efficient aspects of the late close Atkinson cycle could be retained in conjunction with maintaining acceptable low speed torque for a fraction of the cost of hybrid technology if currently available i-VTEC technology were refined to incorporate the attributes of patent 4,961,406.