F2010A037
Turbocharging to extend HCCI Operating Range in a Multi Cylinder Engine- Benefits and Limitations
Turbocharging to extend HCCI Operating Range in a Multi Cylinder Engine- Benefits and Limitations
In the last decade there has been interest in Homogenous Charge Compression Ignition- HCCI as a way to increase the efficiency of the gasoline engine. The attractive properties are the increased efficiency due to reduced throttling losses, increased expansion ratio and higher thermodynamic efficiency as a result from the low combustion temperature. The nitrogen emissions are also reduced substantially and the engine can therefore be operated in lean condition without NOx after treatment. The main drawback of HCCI mode is the absence of direct combustion timing control. Therefore all the right conditions for auto ignition has to be set in advance. This lack of direct ignition control sets a high demand on the engine measurement and control system. The operable load range in HCCI is restricted to relative low load due to the rapid combustion with its high pressure rates and therefore high combustion induced noise.
In the last couple of years different car manufactures has showed concept engines operating in HCCI. General Motors has showcased a naturally aspirated HCCI engine operating in the low load regime. To evaluate if the capable load range in HCCI can be extended with supercharging our objective was to turbocharge an engine in HCCI mode. By turbocharging, the in-cylinder dilution is increased and the relative pressure rise rate is decreased leading to less combustion noise for a given load. To be able to cover a normal engine operating range there is currently a need for mode-switch from HCCI to SI but this can be minimized with an increased load range in HCCI mode.
The turbocharged test engine is an in-line four cylinder gasoline engine with a total displacement of 2.2 l. The engine is direct injected of spray-guided design. To achieve HCCI combustion the engine is operated with negative valve overlap by low lift and short duration valve timings where the variable valve timing is used for combustion control.
In this paper the results from the capable load range in turbocharged HCCI mode is divided in small fractions. It is shown which limitations there are and how these influence the operating range. Since the application of HCCI has to meet consumer demand, emission legislation and control capability the real operating range is always a balance between these.
The load range in turbocharged HCCI has been increased substantially compared to a naturally aspirated HCCI engine. The evaluation of different turbochargers and cam profiles has contributed to increased efficiency and load range capability from 1 bar IMEPnet all the way up to more than 6 bar IMEPnet between 1000 to 3000 rpm. In the result there is a comparison between an SI engine and a turbocharged HCCI engine, it is shown that the HCCI engine has up to 35% higher fuel efficiency.
To be able to operate in the whole load range the control strategy is vital and with the added dimensions from turbocharging i.e. control of intake pressure and temperature increases the complexity but at the same time the flexibility is increased to operate the engine in most beneficial way. The effect of intake temperature, boost levels, combustion timing and residual fractions is demonstrated at one test point, 2250 rpm and 400 kPa IMEPnet. In the result it can be seen how different settings affect combustion stability, combustion noise and efficiency.
This abstract is supplemented by a PDF, which can be viewed here.
Session: IC Engines, Goals and Development