On 09/04/2016 18:02, Johnny B Good wrote:
On Sat, 09 Apr 2016 17:38:45 +0100, Roger Mills wrote:

On 09/04/2016 11:42, Dave Plowman (News) wrote:
In ,
Roger wrote:
What any car's 0-60 time may be is totally irrelevant to where the
maximum acceleration occurs anyway. Unless comparing apples to
oranges.


The fact remains that a car's power to weight ratio (*not* engine
torque to weight ratio) is a pretty good indicator of accelerative
performance.

It's an oft quoted figure for the masses. Same as BHP is all important
to bar room mechanics. But it doesn't tell the full story. Only that
car makers tend to produce roughly similar engines.

A colleague of mine at Rover studied this is some detail, and plotted a
graph of 0 - 30 times (admittedly not 0 - 60) against power to weight
ratio for a wide range of vehicles. He found a very strong correlation.

But I'll ask you a question. Take a high revving bike engine with a
very high specific BHP per litre and put it up against a lightly
stressed but torquey V8 etc in vehicles with the same power to weight
ratio. Which one will accelerate better?


Why compare a bike engine with V8 car engine?

If you take the same car and equip it with a high revving petrol engine,
and then swap the engine for diesel which produces the same power but at
a lower speed (with a suitable matching gearbox in each case) there will
be very little difference in the accelerative performance - even though
the level of drama may be different.

It depends on the amounts of inertia in the engines relative to the
whole vehicle's inertia. The typical diesel equivalent to its petrol
counterpart will have more inertia, reducing the overall acceleration
performance despite providing the same top speed performance. This
assumes the overall mass of the vehicles remain identical. In practice,
the diesel variant tends to weigh a few pounds more than its petrol
engined counterpart.


I mentioned in an earlier post that you needed to consider the effective
mass of the engine in each gear and add it to the vehicle mass when
calculating acceleration. The diesel engine will have a higher moment of
inertia in absolute terms, but the diesel car will be higher geared - so
the 'effective' mass of the engine probably won't be any greater than
that of the petrol. [And no, I don't have any specific figures in order
to verify that, but that is my hunch.]
--
Cheers,
Roger
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