Roadholder 370 - October 2018

25 fuel induction stroke and the power stroke. Due to this fundamental feature, it can be appreciated that the amount of fuel drawn into the engine is appreciably less than with a 'standard' Otto-cycle motor but the engine still retains the longer 'power' stroke and thereby achieves better thermal-efficiency than conventional designs as the lower power burning charge is retained in the decending power stroke to use up more energy-providing heat (the resultant exhaust gases thus tend to be at a lower temperature to 'normal' engines). This reduced fuel consumption however it also reduced the motor's overall power but, under certain conditions (low load, constant revolutions etc.), this lower power density is outweighed by economy of running. The mechanical methods needed to achieve this asymmetric operation were somewhat complex and altering the characteristics of the engine to meet differing conditions was difficult and rather complex. Although a quite well-known principle, the design was not taken up to any great degree until the features of modern variable-valve technology (VVT) could be deployed to achieve similar characteristics to the 'mechanical' Atkinson cycle. When the Atkinson 'mode' is required, the cylinder inlet valve is held open longer than usual such that it is still partially open during the first part of the compression stroke thus the intake volume of the cylinder is effectively reduced as is the effective compression ratio. The rest of the four stroke cycle is essentially standard thus the expansion (power) stroke remains the same as an 'Otto' engine. The ultimate goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure. When this occurs, all available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio converts more energy from heat to useful mechanical energy - giving a more efficient engine. In a simplistic explanation, when the Atkinson cycle is 'deployed' the engine 'acts' as a motor of smaller capacity e.g. a 3,000cc motor 'becomes' a 2,500cc version.