Once a month SZP opens the airport for visitors. Those of us with
unusual air planes open our hangar doors and people stroll by and ask
questions. Many are questions I have heard over and over again.
The rotary powered RV4 with the cowl off is prominent and draws a lot
of people,
I am scheduled to give a talk about rotaries at AusFly in Narrowmine
NSW Aus next week. To speed the process I am getting a large poster
printed with some of those questions.
Here is a preliminary version. Comments are welcome.
Paul Lamar
Paul,
You know far more about rotary engine durability than I do, but you
continue to bury your head in the sand when it comes to the fuel
efficiency of rotary engines--at least when it comes to cars. The fact
of the matter is that rotaries use 20-30 percent more fuel than
comparable, modern piston engines. That's why even Mazda has given up on
them. The proof of this is incontrovertible and as a rational engineer,
you should accept this fact. That shortcoming doesn't disqualify the
rotary as an aircraft engine, where its other virtues--light weight,
compact size, smoothness, and durability--might be more important.
Csaba Csere
The BSFC is now within 5% of a piston engine
The car problem has more to do with the gear box chosen by Mazda than
the BSFC of the rotary.
My RX8 is doing 3000 rpm at 80 while my 250 HP Honda Odysseys is doing
1800 RPM. This the same reason the 400 HP Corvette gets 25 MPG on the
free way. My 1994 Mercury Cougar was doing 1900 at 80 MPH.
At a steady 70 MPH the RX8 is getting 25 MPG. The same as my old RX2
and the two RX7s I have owned. All had the wrong gearing.
All you need do is look at the OBD data attached.
Another factor is when the Mazda computer leans the engine and when
it does not. Leaning improves the BSFC. Honda circumvents the NOX
emissions laws by not leaning the engine during the smog test. The rest
of the time they lean it frequently. Nox is a big problem for diesels as
nox increases when running lean. Mazda never leans the RX8 engine no
matter what.
I have developed and patented a high speed MSD ignition system that I
hope will knock a few points off of the BSFC.
The exhaust valve, in a 4 stroke, waste 20% of the of the recoverable
energy in turbo compound use. The exhaust normally waste 50% of a gallon
of gas. Harvesting in F1 speak. A turbo compound rotary would therefore
become the most efficient IC in the world maybe short of some diesels.
The F1 hybrid scheme is not the most efficient for free way use,
http://www.rotaryeng.net/Oxford-TC-talk.mp4
If Mazda does not build a turbo compound rotary they are not informed.
Mercedes, Renault , Ferrari and Honda will.
We now have a RB 650 HP 2 rotor that weighs less than 200 pounds in a
time to climb project air plane. It has a controlled leak in the exhaust
manifold because there is too much energy in the exhaust. In other words
it would over boost the engine. This is in addition to the waste gate.
Mazda ought to hire me as a consultant. I would straighten them out
damn quick :-)
Paul Lamar
Paul,
The rotary has relatively short gearing because it has little low-rpm
torque and if the gearing were any taller, it would have no acceleration
in top gear--at least with a manual. Most of the other cars you quote
are non -porting and have automatics, so they kick down when you floor
the accelerator.
Also, there are other cars with similar gearing to the Mazda, such as
a BMW 335i. But that car gets better mileage--over 30 mpg at 70
mph--while delivering far more performance--in a much heavier car.
As to the Honda leaning out during off-cycle driving, I don't know
what your car does, but such a calibration would be highly illegal and I
doubt that Honda, or anyone else, uses it.
Your turbo-compound rotary idea has merit, but it says nothing about
the efficiency of real-world rotaries.
Csaba
Of course. It does need an automatic manual. (Computer controlled hyd
shiifting}.
30 MPG verses 25 MPG is 20% not 30%.
My neighbour has an M3 and I will ask about the 80 MPH RPM.
The piston engine has millions of more hours of R & D.
Due to new apex seal materials the leaning angle can be increased
allowing a higher compression ratio or a better combustion chamber shape.
I had the OBD II on the Honda and I saw a lot of situations where it
went way lean. The gov only cares about the arbitrary driving cycle and
they don't peer over the shoulders of the company EFI programmers.
From a software point of view it is easy to detect the silly driving cycle.
The nox v BSFC situation was discussed in Bruce Briggs book War
against the Automobile.
Paul Lamar
What is not real world about turbo compound. CW made thousands of
R3350 TCs. A Super Connie could stay aloft for 24 hours.
Actually the exhaust valve was the Achilles heel. The head would come
off and take out the turbine. Rotary no problem no exhaust valves.
Paul Lamar
Paul,
Here are the relevant numbers for a 2007 Mazda RX-8 versus a 2007 BMW
335i coupe, both with 6-speed manuals (Car and Driver tests):
RX-8 335i
Weight3060 3557 lbs
rpm@80 mph4020 2974rpm
claimed power 232 300 hp
claimed torque 159 300 lb-ft
0-60 mph6.5 4.9 sec
0-100 mph18.0 12.1 sec
1/4-mile15.0/93 13.6 sec @ 105 mph
50-70 mph top gear 10.9 6.0 sec
EPA city mpg18 19 mpg
EPA highway mpg 24 28 mpg
Observed mpg at 70 25 32 mpg
In what world is the efficiency of the RX-8 engine the same as the BMW
engine, which is moving a substantially heavier car, and delivering much
greater performance, while using less fuel?
You can say that the Mazda gearing is too short for optimal fuel
economy, but if you made it any taller, it would exacerbate the car's
already poor flexibility. Yes, an automated manual with 7 or 8 gears
would help address the fuel economy/performance trade-off, but such a
gearbox would help the BMW's performance and efficiency as well. And
keep in mind that modern piston engines are more efficient than this
8-year-old BMW.
No car manufacturer that examines the above performance/efficiency
comparison would chose a rotary for a modern car in a world where the
mandated fuel economy is increasing every year (2010-2025).
As I said earlier, this doesn't mean that a rotary has no merit for an
airplane, but it explains why rotaries are extinct in automobiles.
Csaba Csere
Quote Csaba;
"Observed mpg at 70 25 32 mpg" How was this done?
Quote Csaba;
"No car manufacturer that examines the above performance/efficiency
comparison would chose a rotary for a modern car in a world where the
mandated fuel economy is increasing every year (2010-2025).
As I said earlier, this doesn't mean that a rotary has no merit for an
airplane, but it explains why rotaries are extinct in automobiles."
All this is biased opinion on your part. Very clever of you to mix
opinion with facts :) I am on to you :) You get an F for this :-) (Csaba
is a former student of mine.)
http://www.zeroto60times.com/vehicle-make/mazda-0-60-mph-times/
Zero to 60 2004 RX8 5.7 seconds. Not C&D 6.5.
Perhaps Car and Driver had/has defective data taking :)
The EPA MPG numbers tell a different story.
................................RX8.....Bmw
EPA city mpg ........... 18....... 19 mpg
5% better for the BMW which is what I stated for the rotary.
................................RX8.....Bmw
EPA highway mpg .... 24....... 28 mpg
16% better for the BMW a long way from "20 to 30% better"
This is where the better gear box would help the most.
You are also ignoring the turbo compound situation which makes the
rotary uniquely qualified for application of such. All F1 engines are
turbo compound and will be for the next few years. The piston engines
will have exhaust valve problems.
BTW thousands of turbo compound DD15 engines have been sold and are
selling like hot cakes to the trucking industry. If that is not real
world I don't know what is.
Paul Lamar
Paul, Paul, Paul,
I knew I would regret poking you in your soft spot, but I must address
your scurrilous accusations.
First, Car and Driver has addressed several RX-8s over the years. The
Zero-to-60 number you quote, came from the first test in April 2003. I
know because I drove the car from a preview in Northern California to
our test in Southern California and wrote the resulting comparison test
for Car and Driver.
The figures I quoted came from the last test of the RX-8, which was
published in June 2007. I used those figures because they are the most
recent ones available and also are from a car of the same model year as
the 335i I referenced.
But using the earlier figures would not have changed the comparison
appreciably. The RX-8 is still much lighter, slower, thirstier and less
flexible.
You ask where the 32 mpg at 70 mpg comes from? That comes from my own
335i. I run with an Escort radar detector, which has a built-in GPS
speedometer, for true speed, and I have calibrated my onboard trip
computer by comparing it with 40,000 miles of actual tank mileage. I
doubt that your claimed 25 mph is any more accurate.
Then you say that 16 percent better is not 20-30 percent better. As I
recall from our discussions about vehicle dynamics three decades ago,
weight is a key factor. It certainly is in any fuel economy comparison.
The BMW weighs some 15 percent more than the RX-8. That weight
difference would suggest fuel economy at least 8 percent worse for the
heavier car compared with the lighter one.
Furthermore, there is the adjustment for the BMW's vastly greater
power, which is capable of accelerating this much heavier car, much
faster than the RX-8 can. When you combine the two aspects of heavier
weight with greater performance you do end up with an efficiency
difference in the 20-30 percent range.
Double F for you, my old friend, for ignoring these obvious physical
factors. And if you don't want to ignore them, you can compare the
RX-8's performance to a modern BMW 328i, which still beats the RX-8s
acceleration in a heavier car, while delivering an even larger fuel
economy improvement.
One final note, the engineers at Mazda are not stupid. They like to
march to a different beat and I'm sure that it kills them to not have a
rotary car in production. But the engine's efficiency is so far behind
contemporary standards that rotary cars are no longer remotely practical.
Csaba
Your opinion again.
Weight and small size is one of the major advantages of the rotary.
As you know in a frontal crash into a stationary barrier that the DOT
is so fond of, the
engine weight is a major driver of over all car weight. Low car weight
is also a major
advantage in the EPA city driving schedule. It also is an advantage in
tire rolling resistance. Tire rolling resistance is roughly half the
drag at free way speeds.
An all aluminium 13B normally aspirated would
be well under 150 pounds. Bare block is 109 pounds.
The all aluminium BMW engine probably weighs twice
that which leads to the heavier car.
I see a lot of mistakes from Mazda engineering and management.
Here is a major mistake in the RX8 structure which would have
resulted in one or two hundred pound reduction in RX8 car weight.
Colin Chapman would roll over in his grave :)
Another is never putting the rotary in the Miate. That would
have sold like hot cakes and it would be near super car status
with a turbo. Give it a different name. It seems like Mazda management
does not understand the rotary.
Did they try 4 spark plugs?
Did they try multi sparks at high RPM? (Why do I have to do that?)
Did they try a stainless steel rotor housing liner? Stainless steel has a
lower heat transfer coefficient than ordinary steel. Did they try a
stainless
steel rotor rather than cast iron? Same idea. Keep the heat in the
combustion
chamber.
Did they try turbo compound? (Why do I have to do that?)
I would be glad to loan them my 3 rotor turbo compound
unit for test on Jim Mederer's dyno.
Did they try a higher leaning angle?
The RX8 engine is identical in most fundamental respect to a 1973 13B.
The geometry is identical.
There is nothing intrinsic about the rotary that precludes significant
improvements in the BSFC.
Paul Lamar
Csaba, Csaba, Csaba,
I am going to have to come to Paul's aid here. I drive a BMW 3 series
(21 MPG for me) and have among the highest amount of hours of anyone
currently flying a rotary (9 gal/hr at 150KTAS). I am not as precise as
Car and Driver, and my bias is may be opposite to that of car and driver
(who has plenty of motivation to be biased against older cars that are
no longer in production.)
There is no doubt that the rotary is disadvantaged in most automotive
applications, but that disadvantage is no where near 30%, and it depends
very highly on the specific automotive application. Sitting at idle at a
stop light the rotary may indeed burn 20% more fuel. But in high duty
cycle and power use applications like racing or aircraft the difference
is clearly closer to 10% or less.
Lets look at automotive real world. I like a site called Fuelly. Real
world drivers report report real world mileage, fuel tank by fuel tank,
as they drive in the real world for just about any kind of car out
there. Lets compare the RX8 to the same era design (early 2000) and
similar market design (performance) and curb weight BMW M3:
http://www.fuelly.com/car/mazda/rx-8
vs.
http://www.fuelly.com/car/bmw/m3
We see that the M3 typically gets around 20mpg while the RX8 gets just
under 18 MPG, averaged over many hundreds of cars in both cases. That
is just slightly more than a 10% difference in a street driving
application that is definitely not the rotaries strong suit.
In aircraft, there are many fewer flying examples. But on many
occasions I have flown for hundreds of miles side by side with other RV
aircraft and filled fuel at the start and end of each leg. My fuel burn
is less than 10% more if we are both running lean of peak and a slightly
more than 10% more if we are both running rich of peak. But few
aircraft owners routinely fly LOP while I routinely do, which brings
real world fuel burn to about equal most of the time I fly with others.
And there is my $0.2 on fuel efficiency.
Dave Leonard
BTW Dave flies a turbo charged 13B rotary.
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