Blowdown Interference on a V8 Twin-Turbocharged Engine 2011-01-0337

The exhaust blowdown pulse from each cylinder of a multi-cylinder engine propagates through the exhaust manifold and can affect the in-cylinder pressure of other cylinders which have open exhaust valves. Depending on the firing interval between cylinders connected to the same exhaust manifold, this blowdown interference can affect the exhaust stroke pumping work and the exhaust pressure during overlap, which in turn affects the residual fraction in those cylinders. These blowdown interference effects are much greater for a turbocharged engine than for one which is naturally aspirated because the volume of the exhaust manifolds is minimized to improve turbocharger transient response and because the turbines restrict the flow out of the manifolds.

The uneven firing order (intervals of 90°-180°-270°-180°) on each bank of a 90° V8 engine causes the blowdown interference effects to vary dramatically between cylinders. These effects are illustrated in this paper for a twin-turbocharged engine with single scroll turbochargers and log style exhaust manifolds. An AVL tool called Gas exchange and Combustion Analysis (GCA) is used to estimate residual fraction for each cylinder based on measured intake port, cylinder pressure, and exhaust port pressure profiles. The uneven firing interval causes imbalance between cylinders in residual fraction (increasing knock), fresh air (increasing CO due to uneven air-fuel ratio), and pumping work. These effects also preclude running high overlap for scavenging at low rpm, and diminish the potential benefits of dual cam phasing at part load.

The advantages and disadvantages of various methods to diminish the magnitude of these effects are investigated through 1D performance simulation and engine dynamometer testing, including the use of twin scroll turbochargers, exhaust camshafts with different exhaust valve opening timings on pairs of cylinders, and a balance tube between exhaust manifolds.