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Forum » General Tuning Discussion » Otto and Atkinson Cycles

Otto and Atkinson Cycles

General Tuning Discussion

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I was watching the following review of the Lexus RC F (http://youtu.be/ep1OpsEsLWE) with a good explanation of Otto and Atkinson cycles (3:45 mins). It got me wondering about how this is done. Is it simply using a variable valve timing that is heavily retarded on the intake cam to create the Atkinson cycle delayed closing and then advanced back to a standard Otto? I know most tuners aren't going to be too interested in it as we're usually trying to create maximum power but thought it was another interesting topic.

Implementation of Atkinson cycle is becoming more common in modern engines to improve efficiency. It is achieved through the variable valve timing by using a high degree of retard on the intake camshaft. The result of this is that the intake valves remain open through a reasonable portion of the compression stroke. The open intake valves allow the intake charge to be pushed back past the intake valves and into the intake manifold during the early part of the compression cycle which effectively reduces the compression ratio. The aim of the Atkinson cycle engine is to have a higher expansion ratio than compression ratio to allow the combustion pressure at the end of the power stroke to be equal to atmospheric pressure. This means that all of the available energy has been extracted from the combustion process.

The downside though is that the specific power output is reduced due to the fact that the combustion charge is smaller than a comparable Otto cycle. Modern VVT systems allow the engine to run as an Atkinson cycle or Otto cycle depending on the load/rpm.