instance, a 2008 Ford Mondeo Hatchback can have between 70 and 137 bhp/tonne,
depending on which engine, body style and trim level is chosen. Hook a caravan on
the back and the power to weight ratio is often nearly halved. One rule of thumb to
aim for here is 40 bhp/tonne of the train weight, ie everything in the outfit that the
engine has to pull - laden car and laden caravan. This is a minimum target figure for
allowing reasonable performance, not delaying other traffic and having sufficient
power for overtaking. Note: this rule does not necessarily apply to older, diesel-
engined cars, where good, low-end torque and relatively large engine capacity will
often make up for any loss of bhp. However, the latest generation of diesels are
tending to have both power output and power delivery characteristics which are more
like those of petrol engines.
Torque is a measure of engine performance which is difficult to visualise or explain.
It is the turning power generated by an engine. Good torque at low engine speeds will
allow easy pulling away, and towing with a minimum of gear changing.
Manufacturers generally only quote maximum torque and the speed at which it is
produced. However many engines nowadays produce 90%+ of maximum torque
from 2500 to 4000 rpm, so a quoted maximum at 4500-5000 rpm need not necessarily
be bad for towing. Even a high engine speed maximum torque can be tolerable if first
gear is low enough to pull away, the gearchange is light and you do not object to more
frequent changes. Alternatively, you can choose an automatic gearbox whose torque
converter usually solves the problem. Ask if the car salesman can show you the
torque/rpm graph for the car you are considering. Manufacturers used to regularly
include graphs of power and torque characteristics in their promotional literature, but
this seems much less common for recent models. If you do find such information,
however, it can be useful for predicting the towing performance. Some examples of
torque curves:
a ‘flat’ torque curve, curve with ‘twin peaks’ - so the max
max torque 135 NM @ 3,500 revs figure of 141 NM @ 5,200 is deceptive
(good for towing) (may cause problems at some speeds)
0
20
40
60
80
100
120
140
1000 2000 3000 4000 5000 6000
Curve with sharp peak at high revs -
less suitable for towing. Max torque
140 NM @ 5,200 revs
(20 Seiten)
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