

Duke Engines' compact, lightweight valveless axial engine - anovio
http://www.gizmag.com/duke-engines-axial/33631/

======
jdietrich
This isn't a new engine, it isn't a new idea, and it isn't a particularly good
idea.

Duke Engines have been around since 1993, and built their first prototype in
1996[1]. Axial engines themselves date back to 1911; Their practical use is
limited to torpedoes, where the cylindrical form-factor is an advantage.

Axial engines have inherently high reciprocating mass compared to conventional
piston engines, which is a catastrophic flaw in a performance engine design.
Higher reciprocating mass increases inertia (reducing throttle response) and
increases the forces at the end of the stroke (reducing maximum RPM). They
offer no meaningful advantages in terms of fuel efficiency, and are likely to
be less efficient in many applications due to the difficulty of implementing
existing efficiency technologies (VVT&L, valve deactivation etc)

Both the current Duke engine and their hypothesised next-generation engine
offers poorer specific power than current naturally-aspirated designs. The
cylindrical form-factor is more difficult to package than a traditional piston
engine; Camshafts offer enormous flexibility in terms of layout, allowing the
engine to be squeezed into a multitude of shapes and sizes. Axial engines are
inherently balanced, but balance is practically a non-issue in modern engines,
even for layouts with very poor inherent balance.

[1][http://www.dukeengines.com/technology/overview/](http://www.dukeengines.com/technology/overview/)

~~~
JoeAltmaier
Do the pistons rotate, or does the disk? The disk would have less mass I
suppose.

~~~
desdiv
The pistons rotate. Not just the pistons, but the cylinders and the connecting
rods as well. It's basically the entire engine block rotating against a
stationary cylinder head.

The large rotating mass problem is basically what killed off the pre-WWII-
style rotary engines, and the Duke engine suffer from the exact same problem.

~~~
JoeAltmaier
So why not connect the disk to the shaft, and have it rotate instead? Not a
mechanical engineer, but that would have way less mass, way less clever
mechanical linkages for valving, spark.

~~~
bradleyland
The disc contains systems that are not easily reciprocated: the intake and
exhaust ports, as well as the ignition system. You can be sure that if it were
possible to simply swap the rotation, Duke would have done it. The issue of
reciprocating mass is well known by engineers who work with axial engines.

~~~
JoeAltmaier
I guess I still don't understand. Lots of engines don't put the valves and
ignition on a disk - so don't. The idea of valve-less cylinders via rotating
ports can be done either way - rotating cylinders or rotating shaft - so
reverse them.

I know, that's not a Duke engine. Just wondering who got it wrong the first
day and went down this path. Like the old 'drum memory' systems that rotated
the heads and left the magnetic memory stationary. Didn't take but 2 years to
turn that around and invent disk drives.

~~~
bradleyland
So, you believe that sitting in your armchair, you've identified some
fundamental flaw in the Duke engine design that they overlooked from day 1?

Yes, ported intake/exhaust solves the problem of rotating the disk instead of
the cylinders, but porting comes with its own set of drawbacks. Ask any
engineer who has worked on Wankel Rotary engine design and they'll tell you
all about it. Ported engine designs include the Wankel Rotary design, as well
as 2-stroke, reciprocating, piston-in-sleve (traditional 2-stroke ICE) engine
designs. Both have issues meeting emissions requirements because of inherent
limitations of ported engine designs.

Cam operated valves have some very specific advantages that play a large role
in the ICE's ability to reach current specific output levels. With a ported
engine, you cannot vary the intake/exhaust profiles; with a cam, you can.
Variable overlap in intake/exhaust, as well as variable intake/exhaust opening
area are key aspects of state-of-the-art ICE design. You give up both of these
with ported engine designs.

Wondering about these kinds of things is great, but be conservative with your
assumptions, and generous in your interpretation. It's condescending and
narcissistic to assume that you can take a cursory look at the Duke engine,
wave your hand, and solve a massive design flaw.

~~~
JoeAltmaier
Oh get off your high horse! I'm speculating. You condemned the design in an
earlier post, right? There are all sorts of possible geometries, and many have
been tried; some are found wanting. Elsewhere I speculated about using such an
engine for constant-speed operation e.g. a generator. It might be a win there,
where the ICE responsiveness is not a priority.

Its great to hear from an expert, though one that's clearly invested in the
current popular technology to the exclusion of admitting any benefit to this
one. I thought you'd be right on board with speculating about where this went
wrong. Sorry to have misjudged.

~~~
bradleyland
I'm a novice, not an expert. I have a great interest in cars and engines in
general, and I've pursued them to some degree, but I'm not an engineer. I have
no where close to the level of expertise that the Duke engineers do.

> I thought you'd be right on board with speculating about where this went
> wrong. Sorry to have misjudged.

You've misread me. There's no thing wrong with asking questions. For example,
take the question:

"Why can't we rotate the disc instead of the cylinders?"

Versus your statement

"The idea of valve-less cylinders via rotating ports can be done either way -
rotating cylinders or rotating shaft - so reverse them."

You state this as if it's obvious, and it is obvious. It's also obvious (to
someone with domain knowledge) that it's not a simple matter. I took us part
way down the rabbit hole, and I'm always happy to do that, but it really gets
under my skin when questions are states as presumptuous declarations such as,
"so reverse them." Not to mention this one:

"Just wondering who got it wrong the first day and went down this path."

This presumes that the Duke solution is the wrong one, and your solution is
the right one, but you aren't even familiar enough with ported engine designs
to know the basic drawbacks. Again, I'm not an expert, but I know enough to
know that I can't make these kinds of presumptions. You don't, and my
suggestion is that you should recognize and start with questions rather than
insulting someone else's work.

I know this comes across as a scolding, but I genuinely don't mean to be
harsh. I just felt like I should say something because of the way your writing
came across.

~~~
JoeAltmaier
Cool, I apologize if I offended. But if we're talking radical engine design,
then nothing is off the table. Including solving the existing problems with
the Duke design. They haven't had 2 centuries of tweaking, so criticisms about
where soot collects are getting ahead of things.

And also including turning the problem on its head. If shaft momentum is such
a well-known and pernicious problem, then its not out of line to question why
somebody went down that road at all. Obvious really. Its called returning to
fundamentals, and anybody can do it.

So, what's the problem with using rotation to port air and exhaust, if the
disk is turning instead of the now-stationary cylinder block? A mechanical
linkage between the disk and a ported sleeve should do the trick. If its still
desirable at all - a stationary cylinder block lets you do it the valve way
too if desired.

Anyway, I'm actually astonished that anybody ever thought it was a good idea
to rotate essentially the entire engine. Centripetal forces, heavier bearings,
linkage issues - it invents all sorts of problems. Whereas the idea of pushing
against a tilted disk doesn't require that at all. I can't get over that
fundamental notion, and I'm sure its fair to ask "where did they go wrong"
without being accused of backseat driving. If there's some obvious need to NOT
rotate the disk, I'm all ears. But I didn't read that anywhere.

------
chiph
No mention of meeting fuel economy and emissions requirements. Both of those
are big topics that engine builders have to consider, with the 35mpg fleet-
wide average that cars sold in the US must meet in about a decade. Having
grown up in an age filled with car exhaust from carburated engines, I don't
miss it a bit -- I _like_ having clean air.

One of the comments to the article mentioned the difficulty in keeping high-
pressure seals working. It was in conjunction with Wankel rotary engines,
which have always had problems with the rotor tip seals leaking (ask any Mazda
RX owner...) This engine has the same problem on the intake/exhaust end, as
the piston carrier rotates past the openings.

~~~
BrandonMarc
Thank you. I'd wondered how this would compare to a rotary engine. It's
interesting how enthusiastic the rotary-engine crowd is, since my takeaway was
always "well if it's so much better why isn't it more common".

~~~
MattBearman
As an RX8 owner (and enthusiast), I can tell you Wankel Rotaries aren't
categorically better, or worse. They have strengths and weaknesses:

\- The vast majority of rotaries will need a full rebuild before 100k - worse

\- They rev to 10,000 RPM with an unbelievably smooth power delivery - better

\- I'm lucky to see 22 mpg - worse

\- The engine is so small and light that the RX8 has perfect 50/50 weight
distribution - better

\- If you turn the engine off before it's warm it can flood - worse

\- 231 bhp from a 1.3 litre engine? - better

Oh, and the noise - BETTER!

I do believe with more time and money invested the Wankel Rotary could be
better than a standard Otto Cycle IC engine in every way. But electric is the
future now, so that won't realistically happen.

~~~
IanDrake
Learned stick on my second gen RX-7...

-Almost no torque at low rpm - worse

-Easy to flood, then once flooded major work - worse

-Feed it a quart of oil every 1,000 miles - worse

~~~
MattBearman
No torque at low RPM is true, but I've not found it to be an issue. I use the
engine in the same way as I do on my motorcycle - pretty much always above
4000.

The flooding issue I mentioned. It may have been different with the RX7, but
the RX8 has a de-flood procedure that works 9 times out of 10. And failing
that a bump start usually does it. Although I've never flooded mine, I'm
always careful not to shut her down cold.

Oil usage in the real world is not much worse than many other performance
cars, in fact it's better than most Honda S2000s. But for people who are not
used to having to regularly top up oil I could see it being a ball ache.

~~~
IanDrake
> I've never flooded mine

Neither had I...just never let a valet, mechanic, or relative try to start
your old RX-7 ;)

------
Animats
Axial engines go back a long way, to at least 1917.

[https://en.wikipedia.org/wiki/Axial_engine](https://en.wikipedia.org/wiki/Axial_engine)

That format is widely used in hydraulic systems, and is the basis of
continuously-variable hydraulic transmissions. Classically, it has problems at
high RPMs, but is well behaved at low ones.

It's an idea that might be worth looking at again. With better materials and
controls, it might work. The geometry is more flexible than with Wankel
engines. The elegant Wankel geometry means there aren't many parameters that
can be adjusted to improve combustion. In a piston engine, you can design
piston face geometry, cylinder head geometry and fuel and air injection points
for better combustion. With a Wankel, you're kind of stuck with the geometry.
We'll have to see how this new approach works on pollution control.

------
IanDrake
The only ICE advancement that really seems interesting to me is adding an
extra cycle to inject distilled water into the cylinder for an extra power
stroke.

The idea being that the heat from the prior explosive power stroke is used to
turn the water into steam. Now you're using wasted heat energy and removing
the need for cooling components.

~~~
mikepurvis
I saw an article about that idea a few years ago, and wondered if it might be
possible to combine it with a rotary engine. I ended up creating this rather
odd-looking model:

[http://sandbox.mikepurvis.com/design/engine.svg](http://sandbox.mikepurvis.com/design/engine.svg)

Completely impractical, but an interesting experiment. There's some more
general info on Wikipedia: [http://en.wikipedia.org/wiki/Six-
stroke_engine](http://en.wikipedia.org/wiki/Six-stroke_engine)

------
erik_landerholm
"Duke Engines' 3-liter, five cylinder test mule is already making a healthy
215 horsepower and 250 lb-ft of torque at 4,500rpm – slightly outperforming
two conventional 3 liter reference engines that weigh nearly 20 percent more
and are nearly three times as big for shipping purposes."

Seems convenient to use a pretty lame reference engine to make yourself look
good. 3 liter engines have made near 1000HP for years now.

~~~
mikeytown2
The best naturally aspirated piston engine is 166 hp per liter. The best
naturally aspirated Pistonless rotary engine is 188 hp per liter [1]. The Duke
is at 71 hp per liter; it has a long ways to go but it's already better than a
naturally aspirated Diesel engine (44 hp per liter).

[1]
[http://en.wikipedia.org/wiki/List_of_automotive_superlatives...](http://en.wikipedia.org/wiki/List_of_automotive_superlatives#Highest_specific_engine_output_.28power.2Funit_displacement.29)

~~~
aunty_helen
I always find it interesting that when hp/l stats are quoted they often
neglect to include motorcycle engines... which have been in the 180-210 hp/l
realm for about 5 years now.

And a high hp/l isn't necessarily the best piston engine as it's often at the
cost of massive valve train losses resulting in poor fuel efficiency.

~~~
bradleyland
It's difficult to compare the specific output of engines of dramatically
different displacement. As a general rule, displacement and specific output
are inversely correlated. That's not to say that motorcycle engines aren't an
absolute marvel of engineering and technology, but it's not possible to
compare them directly to engines with twice the displacement because you can't
simply scale up the motorcycle engine and get the same results at the larger
displacement.

------
vlucas
The money quote, for the TL;DR crowd:

"Duke Engines' 3-liter, five cylinder test mule is already making a healthy
215 horsepower and 250 lb-ft of torque at 4,500rpm – slightly outperforming
two conventional 3 liter reference engines that weigh nearly 20 percent more
and are nearly three times as big for shipping purposes. With an innovative
valveless ported design, the Duke engine appears to be on track to deliver
superior performance, higher compression and increased efficiency in an
extremely compact and lightweight package with far fewer moving parts than
conventional engines."

~~~
noir_lord
215hp out of a 3 litre is shockingly low except for a naturally aspirated
carburator engine from the 80's.

For example the 2 litre 2015 Mondeo Turbodiesel gets 210bhp.

The article is interesting but reads like a marketing release, the weight
saving is the interesting take away I think if reliability matches a modern
diesel.

~~~
iaw
I do agree with you but a turbo diesel isn't a valid comparison because it is:

1) turbo charged 2) diesel fueled

An inter-cooled turbocharger setup on this engine would likely net an
additional 100 HP without much issue (presuming the sealing issues aren't
horrific). Generally 100 HP per litre of displacement is the gold standard for
naturally aspirated engines (see Ferrari).

~~~
noir_lord
I used diesel as historically cc for cc they have lagged behind petrol engines
(though that has largely ended).

The turbo is irrelevant though, the IC has a turbo, they work and are reliable
the rotating engine doesn't but that doesn't rule out a basis for comparison.

It comes down to fuel efficiency, emissions, maintenance and cost and I can't
see this engine winning (for cars).

~~~
iaw
The turbo is not irrelevant because it has a direct impact on the ratio of
power delivery / fuel consumption due to the recaptured energy from exhaust
gasses. The volumetric efficiency (ratio of cylinder pressure pre-compression
to atmospheric pressure) of engines with turbo-chargers is almost always >1
while the volumetric efficiency of naturally aspirated engines is always
(excluding rare high-performance vehicles in specific ranges of operation) <1.

I don't have time to do more thorough research right now, but the Ford Duratec
30 series of engines [1] is roughly comparable in both displacement and
cylinder count and produces between 200 and 240 HP depending on configuration
which puts this engine slightly ahead of normal engines from a performance
perspective (smaller form factor and lighter).

Is there any apples to apples comparison that you can find that actually makes
this development look substantially inferior? I get that you used a diesel
because historically that gave the ICE an advantage but with modern diesel
technology and forced induction they aren't the same animal.

[1]
[http://en.wikipedia.org/wiki/Ford_Duratec_V6_engine#3.0_L](http://en.wikipedia.org/wiki/Ford_Duratec_V6_engine#3.0_L)

Edit: changed 'pre-combustion' to 'pre-compression' for clarity on volumetric
efficiency definition.

~~~
noir_lord
In this context it is irrelevant since my comparison was purely one of power
output against displacement.

The Turbo allows a given displacement to output more power for a given
displacement, indeed volvo have 2 litre putting out 450hp (uses staged turbo's
one of which is electrically driven), I _wasnt_ comparing the nearest ICE
equivalent to the rotary.

------
jff
It seems to share some things in common with the rotary, like moving the
ignition chamber past the fuel/exhaust ports, rather than opening/closing
valves. So it will need some way to seal the cylinder during
compression/ignition... will those seals tend to break up at 100,000 miles
like a rotary's apex seals?

------
BrandonMarc
I assume the application most people have in mind is cars. As others have
said, electric _seems_ to be the future there, so while this may be a
revolutionary engine, it's possible the timing won't help it.

What about other applications? Would this work well for marine, aviation,
factory, or other purposes?

------
Roboprog
Looks interesting, but I'm guessing there is a lot of stress on the
recipricator mechanism that the pushrods connect to, pushing against an
incline to cause the output shaft to rotate.

~~~
ew6082
It can be as beefy as it needs to be. There is no constraint there to limit
the design.

It is actually an extremely flexible concept from what I can tell. Need a
longer stroke? Add more angle to wobbler. Need more Torque? Expand the wobbler
diameter.

~~~
MadManE
There is a limit on the available angle that can be "wobbled". Once you exceed
~20 degrees IIRC, the displacements perpendicular to the stroke start causing
problems.

I think the more interesting interaction is between the number of pistons and
the size of the pistons.

------
gills
Wow. That looks inspired by a helicoptor's swashplate driven from the rotor
side. Always fun to see an alternative engine design.

