Not 100% relevant, but one spark plug will not succeed in igniting all of the
fuel. I know this from aeroplane engines, which are fitted with two magnetos,
and two spark plugs per cylinder. Part of the engine checks before each
flight involves switching off first one magneto, then the other. You would
expect the RPM to drop by between 100 and 150 (from a starting RPM of between
1800 and 2200 depending on the engine - so that's around a 5-8% drop). Any
more than that means you've probably got a fouled spark plug. And if there's
no drop at all, that means the p-lead which turns the magneto off is probably
broken, and you're still running on all 8 plugs. Thus, the fuel is definitely
burned more efficiently with 2 sparks than it is with 1.
This doesn't necessarily mean that having 5 sparks from 1 spark plug will
improve things. Firstly, each spark in the aeroplane engine is full-strength,
switching off a magneto reduces the total spark strength by half as well as
reducing the number of sparks, and I don't know what proportion of the
RPM/power drop is due to reducing the spark strength and what proportion is
due to reducing the number of sparks. Also, the sparks are located at
opposite ends of the cylinder - one in the top, one in the bottom (this is in
a horizontally-opposed engine), whereas the 5 sparks in the single spark plug
would all be at approximately the same place.
I have flown planes with magneto switches in very inconvenient locations - and
have accidentally turned off one of the magnetos in flight with my left elbow
while operating the throttle on a couple of occasions. The difference in
power was barely noticeable - there was no immediate difference in
performance, but each time, I noticed the plane had lost a little bit of
altitude, and discovered the switched off magneto when investigating.
Unfortunately, I don't know how long the switch had been in this position
before I noticed it, so I can't really put a number on the power loss caused
during flight. And I'm certainly not going to deliberately fly with magnetos
turned off to investigate! But judging by the fact that I there's no large or
immediate difference in cruise performance or RPM, I don't think that an extra
spark makes much difference during cruise.
So how does this relate to car engines.
Well, the cruise performance (i.e. no noticeable difference) relates roughly
to motorway driving - moderate levels of power for extended periods. So I'd
guess the multi-spark plugs would make no difference on the motorway.
The engine checks relate to putting your foot down (aeroplane engines, unlike
car engines, require more power to "rev up" to a certain RPM when stationary
than they do to cruise at the same RPM - there's a windmill effect on the
propeller which reduces the amount of work the engine has to do once you're
moving - so 2200RPM when stationary requires a fairly high power setting). As
I said above, there's a 5-8% decrease in RPM at this power setting when one
magneto is switched off. (This is at about the same RPM as the cruise by the
way - normal cruising RPM is about 2200-2300RPM.)
So it seems that, in a car, there might be a noticeable difference in
acceleration with multiple sparks - notwithstanding the differences between
cars and planes in spark power and placement which I mentioned above. But I'd
be very surprised if there was any improvement in motorway fuel economy. This
seems to be backed up by reports from people who have fitted the plugs and
noticed improved performance. No one has yet said anything about fuel
economy, but someone was going to measure this in a car with a fuel-flow gauge
- any feedback on this yet?
Would be interesting, though, if someone felt like fitting an aircraft engine
to a car. Then, put regular spark plugs on one magneto (well, you'd probably
want to replace the magnetos with alternators for a car - magnetos don't
respond well to sudden changes in RPM) and multi-spark plugs on the other.
Drive down the road, under various conditions, and switch between one the
first and the second alternator, and see if there's any noticeable difference
in performance....... (Actually, this wouldn't be very practical - most
aeroplane engines red-line at 2700RPM! Are there any car engines out there
which have 2 spark plugs per cylinder, where the driver can switch between
them while driving???)
Complete (well, almost complete) change of subject - someone mentioned the
tool for tilting the car on it's side that was demonstrated at the show. I
thought that was a superb idea! Not sure if it would actually take
sufficiently long to bolt the C-bars to the car to mean it's not really
worthwhile for a quick job, but a good idea anyway. But I did feel sorry for
the poor guys who had to stand there demonstrating it - two whole days of
pressing a button on a power-drill, then reversing the drill and pressing the
button again, over and over!
Dean
------------------
> Date: Mon, 11 Mar 2002 12:23:46 -0000
> From: Richard Gosling <richard.gosling@exprogroup.com>
> Subject: RE: Ally Pally Show Review - and too good to be true gadgets x-ma
c-creator="4D4F5353"
>
> Secondly,
> how do the 4 sparks after the first one help? If the first one succeeded
in
> igniting the fuel, the other will not affect anything
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