Cold start
Question: Although this sounds counter-intuitive, I find that my car (a Ford
Mondeo, if that is relevant) always seems to perform better on a damp, cold day
rather than on a dry, warm one. Why should this be?
Answer: There are several ways of improving the power produced by an internal
combustion engine and one of these is responsible for the improved performance
seen by your correspondent.
Cold weather will increase the density of the atmosphere slightly, allowing
more oxygen to be inducted into the cylinder. However on a damp day, the air
that is sucked into the engine actually contains a significant quantity of water
vapour. The water vapour will totally negate this factor so another, more
important, factor comes into play.
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A petrol engine running at low revolutions is quite efficient at burning
fuel. However, at high revs the amount of time available to burn fuel is reduced
and eventually this is the limiting factor. Therefore some of the fuel remains
unburned. The higher temperature from the combustion at higher revolutions
expands the volume of this unburned fuel and consequently of the fuel-air
mixture that remains. At higher revs this produces a significant amount of
additional power.
When water vapour is heated in the combustion chamber it expands in the same
manner and produces extra power because the expansion coefficient of the water
vapour is much greater than for petrol vapour. This explains why the power
output rises despite the reduced amount of fuel-air mixture that can burn due to
the presence of the water vapour.
David Loudfoot
Pleshey, Essex
Answer: On a cold damp day, the air is denser and therefore contains more
oxygen per unit volume. It also contains more water 鈥渄ampness鈥� than normal. The
water drawn into the cylinder is converted into steam, which extends the working
pressure on the working stroke of the piston. Because the pressure is extended
and a higher torque is also produced, this enhances the engine performance.
This effect has been used in some diesel engines where water is emulsified
with the fuel before injection, increasing the power and efficiency of the
engine.
F. A. Warner
Southend-on-Sea, Essex
No borders
Question: Why is it that when I run a bath, the condensation doesn鈥檛 form on
the outer half-centimetre of my mirror? The mirror has a thick wooden frame and
is quite large. I asked my mum, who鈥檚 a scientist, but she couldn鈥檛 think of an
answer.
Answer: If the rim of clear mirror is all around the frame, I suspect that
the cause is whatever you wipe the mirror or polish the frame with. Otherwise,
perhaps parts of the mirror are cooler, maybe touching the tiled wall behind.
When this happens, a mirror actually shows a tile pattern in the mistiness.
More commonly, the clear part of the glass results from a better flow of air.
Subtle air currents can make a big difference. To illustrate this, tape a
horizontal shelf of a few centimetres of plastic or card to the face of the
mirror. If there are convection currents, you will see clear patches at each end
of the shelf where the interrupted air current flows fastest. By experimenting
near the existing clear area, you may see which cause is the most important for
your mirror.
Competition for vapour may cause clear patches in still air. For example,
coarser droplets have a lower vapour pressure because of surface tension. You
will usually find an area around them clear of finer droplets. In a fridge, a
plastic food container with ventilation holes will usually fog up inside, except
for zones round the holes. This results from competition for vapour and from the
inward movement of air as the container cools.
Jon Richfield
Dennesig, South Africa
Fatal attraction
Question: A recent coroner鈥檚 report implicated underwired bras in the tragic
death by lightning of two women in Hyde Park, London, last summer. Does the mere
presence of metallic objects on the body, however well concealed, really
increase the chances of being struck?
Answer: Metal objects do not attract lightning. However, metal does
concentrate the flow of current during a lightning strike. This can be explained
simply by using Ohm鈥檚 law in which, for a given voltage, current flow will be
greater when resistance is lower.
Because your body has a comparatively high resistance, especially when
compared to metal, the metal enjoys a higher current flow, heating up rapidly
and burning the victim. This is why you should remove metal objects from your
body during a thunderstorm. You can put these in a bag or bagpack, as long as
they are not in close contact with your body (nearer than 5 centimetres).
Similar current concentrations occur through the lower-resistance paths
formed by rivulets of sweat and water, leading to fern-like burn patterns on the
body.
In the example mentioned it is likely that the current stopped the victims鈥�
hearts and breathing. It is unusual for a lightning strike victim to die as a
result of burns. Generally, the heart is stopped but recovers fairly quickly and
begins to beat again. However, the victim will often suffocate because the
strike interferes with breathing, which does not restart voluntarily.
If a first-aider can breathe for the victim, recovery is usually possible.
One member of my mountain club team performed artificial breathing for 45
minutes on a young girl who had been struck, and she survived without lasting
damage. Another man I know, who was struck and badly burned, also survived
because his friends were able to breathe for him until his normal breathing
functions returned.
He wrote later: 鈥淚 was struck by lightning four years ago, and revived with 1
minute鈥檚 cardiopulmonary resuscitation. I fought for breath with every ounce of
fight and relaxation I had, and breathed after 3 minutes. Relaxing was the key,
and concentrating. I just devoted myself to the task鈥擨 can breathe, I know
how to breathe, I am going to breathe, breathe . . . OK, I鈥檓 going to pass out,
at which point I pounded on my chest to tell my rescuers I couldn鈥檛 breathe, and
then I moved my diaphragm just a millimetre, then a centimetre, and I didn鈥檛
pass out! I thank all the people who have learned CPR, and I am now a CPR
颈苍蝉迟谤耻肠迟辞谤.鈥�
Jay Hyde
Johannesburg, South Africa
This week鈥檚 question
Skippy: I have observed that while small boys just run, small girls also skip
as they run. Can anyone tell me why this is so?
David Payne
Penarth, Glamorgan