Page 40 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c01
26 MECHATRONICS Printer: Yet to Come October 9, 2014 7:39 254mm×178mm
Governor reduces
fuel rate to limit
Torque
Lug curve: max speed.
constant fuel rate at max. Mechanical governor
T peak
can operate along a line
with finite slope.
Torque Lower fuel rates Electronic governor can
rise operate along a vertical
line
0 ~ 7%
w max

Low Peak Rated High Speed
idle torque speed idle
speed speed speed
(~600 rpm) (~1200 rpm) (~2100 rpm)
Power

Speed
Fuel
consumption

Speed
FIGURE 1.22: Steady-state engine performance: torque (lug), power and fuel efficiency as
function of engine speed.

of these three curves that defines the capabilities of an engine is the torque-speed curve.
This curve is also called the “lug curve” due to its shape. As the speed is reduced down
from the rated speed, the mean torque generated by the engine increases under constant fuel
rate conditions. Hence, if the load increases to slow down the engine, the engine inherently
increases torque to overcome the load. In order to define the lug curve model for an engine,
we need a table of torque versus the engine speed for maximum fuel rate. A linear inter-
polation between intermediate points is satisfactory for initial analysis. The table should
have, at minimum, the low idle, high idle, peak torque, and rated speed points (four data
points). The points under that curve are achieved by lower fuel rates. As the fuel injection