Air motor for improved engine brake efficiency Design and preliminary experiments

The brake efficiency of a throttled internal combustion engine is reduced at low load operation because of the engine work required to drop the intake manifold pressure. These throttling losses are experienced by all throttled engines operating at less than wide open throttle (WOT). By replacing the throttle plate with a suitable air motor, work can be recovered in an expansion process that reduces the induced air pressure to the same intake manifold pressure as the throttled engine. To maximize the benefits from coupling the air motor to the engine cycle, the air should be returned to a thermal state identical to that of the throttled case at some point prior to combustion. This might be achieved either: (i) prior to cylinder compression via regenerative heat transfer to the inducted air; or (ii) through cylinder compression at an increased compression ratio. The work generated by the Induction Air Motor (IAM) can be directly applied to the engine output thereby increasing the brake efficiency for the same indicated work. This paper reports on the performance of an IAM designed to reduce intake pressure of an engine for low load operation. Increased brake efficiency will be achieved. The IAM design specifications are explored using a numerical model including isentropic efficiency, friction and service life considerations. A prototype has been constructed and was bench tested at flows and pressures comparable to a throttled engine. These tests indicated that the modelled friction was lower than the friction measured during the experiments. From the experiments performed with the prototype, the net performance of an IAM will give efficiency improvements in excess of 5% for an equivalent throttled engine operating at loads in the range up to 10 % of its WOT power.