Development and testing of a HD diesel engine with two-state turbocharging

A research project was carried out in which a DAF engine was equipped with a two-stage turbocharging system. Objectives were to investigate an engine concept with very high BMEP (25-30 bars), maintaining acceptable emissions and fuel consumption, while concentrating on the control system and transient performance.
Two-stage turbocharging has several advantages compared with single stage turbocharging. Two-stage turbocharging enables high specific power levels across a broad speed range. In addition transient behaviour of a two-stage turbocharged engine is improved because of the combination of a small (and fast) High Pressure turbocharger and a larger Low Pressure turbocharger. Especially at high intake manifold pressures (pressure ratio > 3.5) the overall efficiency of two-stage turbocharging is better than with single state turbocharging.
The two-stage system on the engine that was tested consists of two parallel Low Pressure turbochargers, a High Pressure turbocharger with controlled by-pass valve, and a cooler behind the High Pressure compressor. With this configuration BSFC was optimised, while maintaining EURO III emission levels. Also transient tests were carried out, whereby the transient behaviour was compared with that of the current DAF engine.
In the development of the control algorithms for the by-pass valve, simulatons were used with a complete engine model built in Matrixx. These simulations were also used to estimate transient performance of the engine concept, prior to the test program. In addition the engine model was used to analyse the measured data, and to understand several transient effects that play a role.
The results show that two-stage turbocharging is an effective way to increase specific power output while maintaining acceptable fuel consumption and transient performance.