Functions of valve timing

Answer 1

1 Introduction

Recently, variable valve timing engines have attracted a

lot of attention because of their ability to control valve

events independent of crank shaft rotation, allowing for

reduced pumping loss (work required to draw air into

the cylinder under part-load operation), and increased

torque performance over a wider range than conventional

spark-ignition engine. Variable valve timing also

allows control of internal exhaust gas recirculation (by

control of the valve overlap), allowing for control of

NOx emissions produced during combustion.

Several detailed studies have been performed to show

the benefits of variable valve timing engines. [1] describes

a mechanical variable valve timing system, and

points to the possibility of controlling the air flow into

the engine via the valve openings, thus eliminating the

throttle and reducing pumping losses. In [2] and [3],

varying the intake valve timing was shown to reduce

pumping mean effective pressure while improving fuel

economy and NOx emissions. [4] describes an electrohydraulic

valvetrain based engine and provides a good

summary of the benefits of camless engines. [5] showed

that by optimizing valve events at part-load conditions,

volumetric efficiency, fuel economy, and NOx emissions

0-7803-5519-9100 $10.00 © 2000 AACC

could be improved. More recently, in [6] it was shown

that using the flexibility of an electromechanical valvetrain

to run in 2, 3, and 4 valve modes, as well as

by using cylinder deactivation, fuel economy consumption

could be improved over a wide range of operating

conditions.

More recent attention has focused on the controloriented

modeling and the control of variable valve

timing engines. [7] modeled the cylinder-by-cylinder

breathing dynamics of a careless engine system, including

higher frequency runner dynamics using the forced

oscillator model. The model was then used to obtain a

cycle-averaged model between valve duration and lift

and mean-valued cylinder charge. In the companion

paper, [7], the cycle-averaged model was used with the

cylinder-by-cylinder model to develop and demonstrate

a control approach to estimate cylinder charge online

and use that information to update a feedforward map

between charge demand and valve lift and duration.

However, the model used in [7] and [8] did not include

modeling of the exhaust process and the effects of combustion

on cylinder pressure, and therefore the effects

of combustion on the breathing process and torque produced.

This paper attempts to to demonstrate a more

complete cylinder-by-cylinder model that includes the

exhaust valves, manifold, and breathing process, as

well as the in-cylinder combustion.

2 In-Cylinder Dyanmics

The in-cylinder dynamics consist of the 4 states of

cylinder pressure, temperature, mass, and burned gas

residual fraction, as described in [9]. The cylinder pressure

is obtained from the perfect gas law

Pc~Vcyt =rncytRTcyl (1)

Where Pcyt is the cylinder pressure, Vcul is the cylinder

volume, mcut is the cylinder mass, R is universal gas

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