Environmental Engineering Reference
In-Depth Information
ride through a single fault is vital in aerospace, and also a feature that with redundant
systems the repair process can be delayed based on dependability levels of the
backup system.
In automotive systems there are few examples of redundancy. The most
notable examples of redundancy are the safety critical systems of steering, throttle
and braking. Steering, for example, may be either hydraulically or electrically
powered, but a mechanical link remains between the driver and wheels. Federal law
requires that throttle control systems have a primary and secondary means to return
the induction air throttle plate to complete closure in the event of failure. This is
necessary because contrary to common perception, the accelerator pedal controls
airflow into the engine, not fuel delivery. The engine controller, based on throttle
plate position and mass airflow sensing, calculates the necessary amount of fuel to
be injected to meet the driver demand and hold air-fuel mixture at its design level.
Brakes are another example of a redundant system. All vehicles in production have
hydraulic service brakes and a backup mechanical brake or parking brake that has a
cable linkage to the driver. As x-by-wire systems take over safety critical systems,
it will be necessary that dependability levels attain 10
9
or one FIT, that is failure
in time.
Automotive reliability is measured in terms of
R
/1,000, for repairs per thou-
sand vehicles. Then there are the somewhat confusing metrics of
R
/1,000 at 12/12,
or 3/36, and nowadays, 10/150. These somewhat cryptic reliability measures are
carry over from past usage and connote failures per thousand vehicles after
12 months in service, or 12,000 mi whichever comes first. Similarly for 3/36, or
3 years in service or 36,000 mi whichever occurs first. The most recent reliability
metric has been the notion that systems, and especially hybrid systems, must deli-
ver maintenance free service for 10 years or 150,000 mi whichever occurs first.
There are now efforts to extend durability limits to 15 years.
To more fully grasp the implications of these automotive expressions for
reliability, it is necessary to illustrate the commonality of the various metrics. First,
the definition of the terms:
FIT is defined as the number of failures in 10
9
hours of operation.
●
MTBF, or mean time before failure, is defined as 10
9
h/FIT.
●
For exponential probability of failure is assumed.
P
ð
F
Þ¼
1
e
l
t
ð
6
:
27
Þ
P
ð
F
Þ
e
l
t
where
l
is failure rate and
t
time in operation.
From the definitions of reliability, it can be seen that probability of failure, or
time in operation divided by MTBF is approximated as
l
t
. Or, more concisely:
t
MTBF
¼ l
t
P
ð
F
Þ¼
ð
6
:
28
Þ
