Environmental Engineering Reference
In-Depth Information
800
5hp
700
10 hp
600
500
15 hp
400
300
500
1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000
Engine speed (rpm)
Figure 1.35 BSFC sensitivity to engine speed
The correlation in Figure 1.35 to FE is one reason why hybrid propulsion
design attempts to lug the engine as much as possible. Lower engine speeds for the
same output power result in lower fuel consumption. Hybrid powertrains that
achieve early upshifts essentially spend more time at lower engine speed, hence
lower BSFC, on average than other strategies, provided the vehicle acceleration
performance is maintained.
1.7.3 ICE basics: Fuel consumption mapping
In Figure 1.36 the BSFC in g/kWh is mapped into the engine torque-speed plane.
As with electric machines, ICEs have regions of bounded efficiency, the contours
of which form fuel islands. Each fuel island has a contour of given efficiency, or
BSFC. The engine peak torque, power and BSFC contours are shown as heavy
dotted, solid and fine solid lines, respectively. The heavy dotted trace is the power
along fuel island maximum gradients. This maximum gradient trajectory represents
the locus of maximum efficiency operating points. Lines of constant power are
shown as hyperbolas in fine dotted traces.
The optimal output power trajectory can be mapped into the engine fuel island
plots by taking the maximum gradient. The power trajectory on maximum gradient
is shown in Figure 1.36. Also, hyperbolas of constant power are shown crossing
over the maximum power line and extending out in speed. It is the job of the
vehicle's transmission to match the road load to the engine power output along a
constant power contour that shifts the engine speed to its most fuel efficient point.
ICEs are most efficient in the 1,500-2,500 rpm range at torque levels approxi-
mately 70% of peak. Outside of this, range specific fuel consumption increases
because of internal
losses (pumping, friction,
less efficient combustion). The
