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Blowdown Interference on a V8 Twin-Turbocharged Engine
Abstract:
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.