Abstract
In this paper, a novel cost-effective pneumatic regenerative stop-start hybrid system, Regenerative Engine Braking Device (RegenEBD), for buses and commercial vehicles is presented. RegenEBD is capable of converting kinetic energy into pneumatic energy in the compressed air saved in an air tank using standard engine braking device and other production type automotive components and a proprietary intake system design. The compressed air is then used to drive an air starter to achieve regenerative stop-start operations. The proposed hybrid system can work with the existing vehicle transmission system and can be implemented with the retro-fitted valve actuation device and a sandwich block mounted between the cylinder head and the production intake manifold.
Therefore, the vehicle operation consists of 3 basic modes, i.e. compression mode (CM), cranking mode and normal firing mode. During the braking operation, the engine switches from the firing mode to CM after fuel injection is switched off. CM is achieved by keeping the intake valves from fully closed throughout the four-strokes by installing the variable valve exhaust brake (VVEB) device on the intake cams. As a result, the induced air could be compressed through the opening gap of intake valves into the air tank through the intake system of proprietary design. The compressed air can then be used to crank the engine directly through the air expander operation or indirectly through the action of an external air starter in production.
A six-cylinder YUCHAI YC6A 7.25 litre diesel engine widely used in the inner city buses in China has been modelled as an air hybrid engine by using Ricardo’s WAVE engine simulation software. The results show that for a 150 litre air tank, it takes 240th engine revolutions for charging the air tank pressure from 1 bar to 6 bar (9.6 seconds at 1500 rpm engine speed). The engine can be cranked directly by the compressed air through fast acting valves or preferably by means of a standard production air starter.
Keywords: Pneumatic hybrid system, air hybrid system, regenerative, stop-start, VVEB