Page 172 - Understanding Automotive Electronics

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THE BASICS OF ELECTRONIC ENGINE CONTROL 5

BSFC

BSFC is a measurement Fuel economy can be measured while the engine delivers power to the
of an engine’s fuel econ- dynamometer. The engine is typically operated at a fixed RPM and a fixed
omy. It is the ratio of brake power (fixed dynamometer load), and the fuel flow rate (in kg/hr) is
fuel flow to the brake measured. The fuel consumption is then given as the ratio of the fuel flow rate
power output of the (r ) to the brake power output (P ). This fuel consumption is known as the
b
f
engine. brake-specific fuel consumption, or BSFC.
r
BSFC = ----- f
P b
By improving the BSFC of the engine, the fuel economy of the vehicle in
which it is installed is also improved. Electronic controls help to improve
BSFC.

Torque
Torque is the twisting Engine torque is the twisting action produced on the crankshaft by the
force of an engine’s cylinder pressure pushing on the piston during the power stroke. Torque is
crankshaft. produced whenever a force is applied to a lever. The length of the lever (the
lever arm) in the engine is determined by the throw of the crankshaft (the
offset from the crankshaft centerline of the point where the force is applied).
The torque is expressed as the product of this force and the length of the lever.
The units of torque are Nm⋅ (newton meters) in the metric system or ft lb
(foot-pounds) in the U.S. system. (One ft lb is the torque produced by one
pound acting on a lever arm one foot long.) The torque of a typical engine
varies with RPM.
Volumetric Efficiency

Other measurements of The variation in torque with RPM is strongly influenced by the
engine performance volumetric efficiency, or “breathing efficiency.” Volumetric efficiency actually
include volumetric, or describes how well the engine functions as an air pump, drawing air and fuel
“breathing,” efficiency into the various cylinders. It depends on various engine design parameters such
and thermal efficiency. as piston size, piston stroke, and number of cylinders.
Thermal Efficiency
Thermal efficiency expresses the mechanical energy that is delivered to
the vehicle relative to the energy content of the fuel. In the typical SI engine,
35% of the energy that is available in the fuel is lost as heat to the coolant
and lubricating oil, 40% is lost as heat and unburned fuel in exhaust gases,
and another 5% is lost in engine and drivetrain friction. This means that
only about 20% is available to drive the vehicle and accessories. These
percentages vary somewhat with operating conditions but are valid on the
average.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 159