A few comments...
First, in a top-post, conventional ICE's use reciprocating parts in a
number of places, starting with the valve lifters. Beyond a certain RPM
these parts start "floating", which is one very big reason the RPMs max
out where they do. This effect has a lot to do with the higher redlines
on 4 cylinder engines versus 8 cylinder engines; in conventional designs
the "steepness" of the mechanical rise/fall curves is smaller with more
cylinders which tends to make them more subject to floating. In the bad
old days of breaker points, point-float was a big issue, and dwell time
was a bigger issue. Dwell approached zero as RPMs went up, and the
shallow angles on the distributer cam for a V-8 exacerbated the problem.
I don't know what makes the lightning bolt in most modern cars --
there is more than one option these days -- but if it's CD, then
charging time on the cap must become an issue at higher RPMs. If it's a
flyback effect, as in the old breaker-point scheme, then dwell time
still matters.
All of these problems can be avoided, of course, but that always comes
at a cost. Conventionally one used multiple camshafts to aid the
geometry, multiple (smaller) valves to reduce valve mass, dual-point
systems to increase dwell time, and multiple distributors and coils to
increase dwell time even more. Initial cost goes up, problems occur
more frequently, and tuneups start to get nightmarish with such
arrangements, however. The engineers didn't just "overlook" advantages
to operating at higher RPMs; they decided the tradeoff wasn't worth it
at that time.
Jones Beene wrote:
Subject: Neglected Power Law
The power output from an electrical generator (or ICE) can vary
significantly - with smaller change in rotational speed - RPM. This is a
generally overlooked criterion in present day ICE design: the cubic
power-to-RPM (rule-of- thumb) except in race cars.
Prior to the current emphasis on using a smaller ICE to recharge
batteries, rather than operating solely through a mechanical
transmission, there was little reason to optimize the electrical output
of such an engine. And high RPM can be higher wear and tear with a
piston engine - because of all the friction.
As I pointed out above, just making a reciprocating engine turn faster
without "auto-governing" itself can be a challenge, never mind lifetime
wear issues.
This is not a problem with
a turbine.
IOW - a rotational speed increase can in theory give nearly a *cubic
power law* change in motor/generator output current (at the same
potential for instance). It is actually 'roughly' a cube law since there
are other factors involved, but for the sake of argument - let's call it
a cube-law. This does not imply overunity, as the power required to spin
the device in question also increases in sync with output - but it does
imply increased efficiency and *much smaller size and weight for the
same output.*
Adding 20% more RPM to an alternator will produce roughly (1.2 x 1.2 x
1.2 = 1.73) 73% more current for instance (in a perfect world). So what
happens when you raise the RPM twenty-fold ?
Well, needless to say - both the electrical power output and the
required power input scale somewhat as a cubic power law 20 x 20 x 20 =
8000... Meaning among other thing that the device can be made much
smaller, lighter and so forth for the same power.
File that one away - but keep in mind Fred Sparber's previous posting on
the simple scroll compressor (although other types of compressors can be
used in this proposal). And - also keeping in mind that today's auto
turbocharger spins at twenty or more times the normal engine speed.
Turbochargers can spin at 80-100,000 RPM but are arguably "misused" in
ALL present-day ICE design, because they only supply an air-boost which
can be done in a simpler way. There are said to be efficient only
because they use waste heat. But, and this cannot be denied, they also
use that waste heat very inefficiently ! but since it is waste to begin
with, nobody seems to care muc.
... so what happens when - instead of an air boost - the 'optimum use'
for all of those available high RPMs is implemented : i.e. the former
turbocharger becomes no longer anxilliary but the prime mover itself ?
By redesign the boosted turbine part is spinning a magnet [inside a
coil] at very high speed, instead of a compressor. It can potentially
work out to an extraordinary gain, since maximizing the temperature of
hot exhaust can be trivial, in engineering terms.
Also in that perfect world of auto engine re-design - keep in mind that
ALL (as in 100%) of combustion ICE engines have a torque curve and a
differing RPM curve but will operate most efficiently if and when these
two can be closely aligned. And most of all - if and when the RPM can be
HELD CONSTANT, then overall engine efficiency improves significantly. A
diesel which is maxed at 38% theoretical efficiency at 2,200 RPM might
well be only 32% efficient at either 1,800 or 2,400 RPM, and even less
if the RPM varies up and down instead of staying constant - big difference.
One of the reasons a diesel is efficient is that the peaks of these two
curves - torque and power - are relatively close together anyway,
compared with other engine designs.
You _can_ change the torque curve relative to the RPM curve.
You can _not_ change the power curve relative to the torque curve
(plotted against the RPMs), though, since power is torque times RPM.
So, it's kind of meaningless to talk about the relationship of the
torque curve to the power curve in a particular sort of engine -- that
relationship is fixed for all engine types. If the peaks of the torque
and power curves are "closer together" for diesels, that just means they
can't spin very fast; their torque curves start to fall off quickly with
increasing RPM which keeps the peaks of torque and power close together.
All you're really talking about is the shape of the torque curve: it
peaks quickly and falls off early. That isn't necessarily an advantage.
As you pointed out, the diesel develops quite a bit of torque at low
RPMs, which is useful in a conventional engine/transmission arrangement.
It's irrelevant, however, if you're using a
motor/generator/wheel-motor arrangement. And if you want maximum power
per pound from your motor, you want the torque curve to go up with the
RPMs as long as you possibly can get it, which is quite the opposite of
what the diesel gives you. Again, power is torque times RPM, so, to take
it to an extreme, high torque at zero RPM provides zero power, even
though it's nice to have when you're drag racing. And as you pointed
out above, there are some serious reasons for wanting torque at high RPM
rather than low RPM.
In any case, if you really want torque at low RPM you should switch to
external combustion, of course, and use a steam engine.
And one of the reasons the Prius
hybrid gets better gas mileage is that the setup permits the gasoline
engine to operate longer at the BEST RPM (in terms of the two curves
above).
But the biggest reason is surely regenerative braking.
If they _really_ wanted to keep the gas engine at its efficiency peak
they'd use a motor/generator/wheel-motor rig rather than the fancy
dual-drive transmission that's actually in the cars.
A Prius diesel would be even more efficient. BTW, this variable
of "matching curves" is correspondingly one of the reasons why the
Wankel design is relatively less efficient - i.e. its power curve maxes
out at around triple the speed of its torque curve. Not good for auto power.
But very good for electricity generation and for high power/weight ratios.
What you really said there is that it has a very wide, flat peak in the
torque curve -- that's good, not bad! It means the engine operates well
over a wide range of RPMs. Very peaky torque curves lead to a need for
very fancy transmissions.
Now revive all three of these previously unconnected variables in ICE
redesign - into one ultra-high efficiency scheme [and overlooking the
potential drawbacks for a moment]. What will it look like ?
Well very cool and small! You would be able to easily lift such an
engine for instance. And it is absolutely stunning to me [under the
subject of "overlooked" potential improvements to the auto engine] that
Detroit has not seen this before now. So obvious (to the armchair pundit
at least).
The best possible design, IMHO, based on these variables, for ultimate
fuel efficiency in any ICE powering any vehicle, is going to be
something like this:
1) A very small [single speed] diesel engine of maximum simplicity. The
engine operates either on or off - no variation in RPM is possible - not
even an 'idle'. This drastically simplifies the fuel injector. There is
only a single speed which is exactly where the torque and power curves
are best fitted.
This would convert (most likely) into a small uncooled 2-cycle opposed
piston (Junkers style valveless) 2cylinder constant speed diesel, of
approximately "motorcycle size" (500 cc or less displacement) with a
simple mechanical supercharger (as opposed to a turbocharger). A large
scroll compressor or Roots-type will work. This particular engine with
cermet sleeves can operate uncooled - with all that excess heat going
into the exhaust.
Are you sure you can operate a supercharged diesel without a cooling jacket?
The issue isn't the exhaust heat, it's melting the pistons and cylinder
walls.
2) A total "decoupling" of engine power from vehicle drive power.
3) A very high speed dedicated exhaust driven turbine - driving an
electrical generator or alternator (in excess of 100,000 RPM). The
exhaust is boosted and reheated (as follows below).
IOW this is an integrated hybrid auto design which is 100% driven at the
wheels by a separate electric motor(s), while the power for this motor
comes from a combination of batteries (as in the Prius, or Batt-caps) -
and from a high speed turbine - tiny in size - which turbine drives only
the electrical generator (or alternator). The boosted-exhaust from the
small constant speed diesel - which is not connected to the drive train
at all - should make it all very efficient. Clear as mud? Yes, I realize
that a drawing would be nice at this point, but anyway...
What's the diesel doing for you? It cools and pressure-reduces the
exhaust a lot in the course of spinning itself. What's the point?
Why not just inject the fuel right into the turbine?
The engine is tiny but the turbine is very adequate for 40-50 kWhr and
is using excess air bled from the supercharger and a little extra fuel
added to the normal exhaust to get to optimum turbine speed. Therefore
the exhaust is capable of providing a significant percentage of drive
power to the wheels. The single-speed diesel (in effect) powers the
supercharger, and possibly its own starter - which is also reversible as
a secondary generator- while most of the net electrical power output
comes from a magnetic 'spinner' which by virtue of its high RPM is about
8000 times smaller and lighter than it otherwise would need to be, for
the same power. That is, if it were driven by the ICE engine instead
(as in the Prius, etc).
Is this redesign a match made in heaven or what ?- perhaps the continued
ravings of a single minded perfectionist who knows just-enough to
overlook larger drawbacks?
As with most new concepts in automotive, it is easy to emphasize the
wrong variable -and only a working model will suffice, when all is said
and done.
This working prototype has now been added my "to-do" list... (under the
"first win the lottery" entry)
Jones
