Page 138 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 5 .1 Transmissions and driveline

mesh and can be considered in the form of a per- viscosity. Many authors consider the bearing losses to be
centage efficiency or loss figure, i.e. a loss of 3% an order of magnitude smaller than the corresponding
would be an efficiency of 97%, whichever is the most gear losses.
convenient. This split of load related, parasitic and slip losses is
The parasitic losses can be considered to be indepen- important for other related areas of work. The load
dent of the applied torque, these losses can be con- related, parasitic, and slip losses need to be treated
sidered to be ‘drag torques’ and take the form of separately in performance prediction and simulation
a resistance within the transmission. work. Also an overall indication of the losses can
The slip losses that may occur in transmission ele- often be derived for the transmission from simple par-
ments which do not involve a fixed gear ratio. asitic loss testing at zero output torque (i.e. with the
Where the drive is transmitted by gear pairs, the output disconnected so no absorbing dynamometer is
input/output speed ratio is obviously fixed by the required).
tooth numbers on the gears. Where the drive is The efficiency of a transmission unit is particularly
transmitted by another means, the output speed is important during two operating conditions of the
not necessarily a fixed ratio to the input. vehicle:
Cold start, ‘gentle’ drive cycles, urban driving, test
The overall efficiency of a transmission unit needs to take
all three aspects into account to arrive at an overall effi- cycles, etc. The parasitic losses have an impact on
ciency figure. fuel economy, as they are significant compared to the
Although a number of authors in the literature con- drive torque required by the vehicle.
sider all the losses in a transmission together, others have Arduous use, high speed, towing, etc. The friction
considered either the load related or parasitic losses in (load related) losses are roughly proportional to
isolation. In common with this approach we will consider the torque transmitted and can cause very high
the losses separately. Merritt (1971) uses the definitions heat output from the unit. This, in turn, can lead to
of oil churning losses and ‘tooth friction’ when discussing high oil temperatures and even oil breakdown or
gear losses. He and other authors have chosen to elabo- component failure due to insufficient operating
rate on the friction losses at the gear mesh and assume oil film.
the parasitic losses are small by comparison. Test work In summary, these loss mechanisms can be described in
completed by the authors has demonstrated that this is terms of the three categories discussed above:
not the case with the dip lubricated transmission units Load-related losses:
typical of those used in the automotive industry. The
parasitic losses can be significant and need to be consid- Friction losses at the gear tooth mesh point
ered in a wide variety of transmission units. This is par- Load-related bearing friction losses
ticularly the case when the gearbox is operated from Parasitic losses:
a cold start where the lubricant is at or near the ambient
air temperature. In winter, this could obviously be well Oil churning where gears and shafts dip in the oil

below 0 C. bath or foamed oil
In automatic units, the losses associated with the oil Oil displacement at the point where the gear teeth
pump are often the largest cause of parasitic loss. When enter the mesh point
the gearbox requires a high oil pressure the torque re- Windage losses where gears operate in air or oil mist
quired to drive the pump can be significant proportion of Oil seal drag
the torque being transmitted. An example of this is a belt Oil pump drag
CVToperating at its low ratio at low vehicle speeds. The Parasitic losses in bearings due to oil displacement
belt system requires high pressure but the transmitted (and windage) within the bearings
torque is low due to the low road load so the pump load
can be very significant. Drag in clutch packs in autos and CVTs (those not
Most transmission units use rolling element bearings engaged)
rather than plain bearings. As with gears, bearings will Slip losses:
have load related friction losses and parasitic losses due
to the oil movement and windage of the rollers and cage. Slip in the contact zone where drive is transmitted by
For prediction and comparison purposes, the bearing loss friction (i.e. belt–pulley contact in a CVT)
can be treated as part of the gear system losses. It can be Slip that occurs in a fluid drive such as a torque
assumed these are small, and either constant, or behave converter.
in a similar way as the oil churning or gear mesh losses In most non-pumped automotive transmissions the large
relative to the ‘control’ variables of speed, torque and proportion of the load related and parasitic losses come

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