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Motorcyclist June
1996: Cylinder Head Tech
At first, the task
of clearing and recharging the cylinders in a high-speed, four-stroke
engine seems impossible. Such processes need time, and it's hard to
believe there's enough available for this one, which faces many
impediments and is crowded into the merest fragment of a clock's tick.
The intake stroke lasts for 180 degrees of crank rotation,
which is only three-thousandths of a second at 10,000 rpm. Camera shutter
openings are as brief., but light has no mass and moves at 950 million
feet per second. Air's mass makes it lag, and it hits a sonic wall about
1100 feet/second, with localized shock waves further blocking the intake
ports at much lower air speeds.
Yet cylinders get filled-with efficiencies sometimes
exceeding 100 percent-without mechanical supercharging. This is possible
because the intake process actually begins in the preceding exhaust stroke
and extends far into the following compression stroke. We've methodically
learned to make the pesky effects of inertia work for us; and minimized
the bad effects of problems that cannot yet entirely be solved.
On a cylinder head's intake side you have only atmospheric
pressure, 14.7 pounds per square inch at sea level, working to stuff air
into the cylinder. No matter how hard the descending piston tries it can't
pull air in behind it. It can only create a space for atmospheric pressure
to fill.
It's a different story over on the outlet side, where a
pressure close to six atmospheres exists when the exhaust valve opens to
begin the event called "blow down". Further, after blow-down,
pistons mechanically force exhaust products from the cylinders, and do so
against the resistance of undersized valves, badly designed headers or
steel cork mufflers.
The more important exhaust event is the high-velocity shove
the rising piston gives exhaust gases during the exhaust stroke. The shove
peaks at maximum piston speed (in most engines occurring a little less
than 80 degrees of crank rotation before the piston reaches top dead
center), where it suddenly gets yanked to a stop. But the momentum of the
gases in the exhaust pipe continues, leaving behind a partial vacuum. This
starts the air/fuel mix above the part-open intake valve moving into the
cylinder before the piston begins it's intake stroke.
Engines benefit from exhaust-augmented intake flow in two
ways; an obvious advantage is that it gives the too-brief intake period an
early start. The second effect, less obvious but also important, is that
combustion chamber cross-flow during valve opening overlap (the period
during which both intake and exhaust valves are open) clears residual
exhaust gases, which slow combustion, depress power by displacing part of
the fresh charge, and can require some weird kinks in the ignition advance
curve.
Exhaust systems primarily aid intake flow by their
manipulation of the combustion "sound wave". A sound wave
creates a disturbance ahead of it and leaves one behind; such
"positive" waves bursting from the exhaust port are followed by
negative pressures. When the strongly-positive exhaust wave emerges from
the end of a pipe, it leaves behind a negative-pressure tail, which then
reflects back toward the port. If the length of the pipe is right, the
negative wave will arrive back at the exhaust valve as the piston reaches
TDC, thus further assisting in clearing the combustion chamber.
Sound waves are reflected by any cross-section change in the
duct in which they are traveling. The sawed-off end of a pipe is one such
change; the closed end of a pipe is another. The difference is that
increases in section invert the wave while reflecting it, changing
positive waves to negative and vice-versa; section reductions reflect the
wave with the same sign.
While speaking of sonic waves, I should caution you about
confusing their behavior with that of the media in which they travel. Like
all sound-conducting media, air has mass and the other properties of
matter. Sonic waves are by contrast, purely energy and thus follow an
entirely different set of rules. Such waves make zero-radius 180 degree
turns and reversals without delay or loss of strength.
Plain pipe ends do a poor job of returning the energy of an emerging sound
wave, which is why horns have flared open end-to get better energy
recovery and thus amplitude. Megaphones, the exhaust pipe horns known in
engineering as diffusers, are vastly more efficient in this regard. Racing
two-stroke engines expansion chamber exhaust systems have elaborate
blow-down diffusers, because of their heavy reliance on this
vacuum-cleaner effect to pull air through the transfer ports.
Four-stroke engines seem perfectly happy running with plain
parallel-wall pipes, though engines developed for megaphones have to be
reworked to function well without them. Harley-Davidson's famous racing
chief, Dick O'Brien, never was totally convinced that the megaphones used
on the "low Boy" KR's did anything but make noise. At the time I
was sure he was missing something, but now I believe his reservations were
valid.
Oddly, the 45-degree cut-off at the end of KR straight pipes
did coax a tad more power out of H-D's cranky old side-valve engine;
O'Brien was at a loss to explain this oddity. I tried a 90 degree cutoff
once, and found the KR didn't like it. No coherent theory I've heard or
conceived explains why that should have been so.
It now appears exhaust pipe diameter, meaning gas velocity in
the exhaust system, is more important than sonic wave activity. Actual gas
velocities vary in ways tough to grasp and impossible to calculate, but
the nominal speed is easy to figure and provides a useful rule-of-thumb:
simply multiply piston speed by the ratio of cylinder bore and pipe areas.
Nominal gas speed were well below 200 feet/second in most
vintage bikes, but in the AJS 7R of the 50's it was up to 220 feet/second.
By 1972 the small diameter pipes on H-D's XR750 raised that engine's
exhaust velocity to just above 300 feet/sec. The Triumph 650 TT Special I
used to set a Bonneville record (and acquire an abiding dislike of
Wendover, Utah) years ago also had small pies and 300-plus exhaust gas
speeds. It had 1 3/8-inch pipes, which almost everyone thought too small.
My slide rule said they were the right size, and the larger-diameter pipes
we tried slowed the bike.
Gas velocity is even more important over the engines intake
side, where it packs air into the cylinder between the intake stroke's
ending and intake valve closing. This is crucial, since with high-speed
engines there is a significant lag between the piston beginning the intake
stroke and the flow of air into the cylinder. Outflow in the exhaust can
pull air across from the intake to give the intake process a head start,
but cylinder pressure still precipitously falls through the first half of
the intake stroke. Air simply can't keep up with the piston, which at 9000
rpm in the XR750 goes from it's stop at TDC to 80 miles per hour in 1.5
inches, reaching that speed in 0.0014 seconds.
Fortunately, the air inertia that delays air/fuel inflow
causes it to crowd in at the end of the intake stroke, and beyond. The
XR750's intake ports are small enough to raise the nominal gas speed to
370 feet/second, which gives it plenty of momentum. This is why intake
valve closing is delayed for many degrees after the piston has finished
it's intake stroke and begun compression. Closing the intake valve while
air is still flowing into the cylinder, or closing it after flow reverses,
gives less the best power. You have to close the intake valve(s) just as
the inflow slows to a stop, thus trapping the greatest weight of air/fuel
mixture in the cylinder.
Serious tuners need some means of shifting cam timing ( in
increments no coarser than 1.5 degrees) to let them experiment their way
to the optimum intake closing. This is usually done with multiple oversize
bolt holes in the driven cam sprockets and offset bushings, although my
old Aermacchi required woodruff keys with a sideways-jog at the shaft and
timing gear join to shift camshaft phasing.
High-performance engines' intake valves close typically 60 to
80 degrees after the intake stroke ends and the compression stroke begins,
so you know gas inertia is playing a major role in cylinder filling; if it
didn't there'd be no need to delay intake closing, and no sensitivity to
the timing of that event. None of the other valve actions-exhaust opening
or closing, or intake opening-are nearly as important.
Flow benches can be used to blow a lot of smoke up your shop
coat when you're looking for horsepower. You can always make air flow
numbers rise by increasing valve head diameter, or by enlarging the
passages leading from the atmosphere. But higher air flow numbers do not
necessarily translate into more power, as many in the engine development
field (including yours truly) have discovered.
Mercedes-Benz made the big-port mistake with the design of
its awesomely complex eight-cylinder M196 GP car, which had desmo valve
actuation and intake ports the size of drains. They found themselves being
out-horse powered by the British Vanwall, with an engine that was
virtually four Norton 30M Manx Cylinders and heads bolted to an aluminum
Rolls Royce armored car crankcase.
Ford's 1960's four-cam V-8 also had huge intake ports, and
while it turned more revs than the Offy four-banger engines then dominant
at Indianapolis, it was no better than a match for them. When given an
early peek at the Indy Ford's cylinder-head castings, I expressed the
thought that its ports might be too big. Ford's engineers were too polite
to tell me how absurd they considered my remark to be, but their
expressions made it plain. I was too polite to send them an "I told
you so" note after Dan Gurney sent one of the engines to Weslake
Engineering in England, where it's intake ports were made smaller and its
output got bigger.
Ford's engineers were then vastly ignorant of the world
beyond Michigan's borders. They had no idea Harry Weslake and Wally Hassan
(who created the very successful Coventry-Climax racing engines) had
learned years before not to take too literally what the flow bench said.
They were narrowing intake ports to provide nominal gas speeds in the
range of 350 to 400 feet-second, making good use of the fact that kinetic
energy packing air into the cylinders increases with the square of it's
velocity.
Harley-Davidson's experience with the highly successful XR750
should have kept it from making the big-port error in the CR1000. Yet,
that's exactly what it did: the VR's intake ports were made so big,
nominal intake velocity was down at 200 feet/second, which may explain why
it's proved sadly inferior to engines that do not test nearly as
impressively on the flow bench.
Grand prix car engines represent the pinnacle of four-stroke
development. Formula One's designers are spinning 3.0 liter V-10 engines
up to 15,000 rpm's and getting close to 800 horsepower. Ford's GP Zetec
V-8 is doing the same with 375cc cylinders, which implies that it's
possible to build a 750cc V-twin that will make nearly 200 horsepower.
Cosworth Engineering's Keith Duckworth was the creator of the modern
high-output four-stroke. Casting aside tradition, Duckworth combined
large-bore short-stroke cylinders with narrow-angle valves and a compact
combustion chamber. He didn't originate the use of high-intake port
velocities to ram-charge cylinders, but he and those he's influenced now
design for nominal intake speeds approaching 450 feet/second.
Of course, there's a lot more to cylinder gas exchange than
port velocity. But unless you've spent eons dragging air through ports,
manifolds, etc.,, at a flow bench, you probably have no real understanding
of what aids flow and what slows it. If there is any rule for the
inexperienced to keep in mind it is that everything a reasonable
intelligent person should intuitively believe to be right will probably be
totally wrong.
Take valve shape for example, these days typically an
unstreamlined disc on the end of a stick. Your eye will tell you the shape
is horrible, an example of how we've fallen into decadence since the days
of those British power plants with beautiful, deeply tuliped intake valve.
Then you hit the flow bench and find that the one with all the loveliness
of an overgrown nail is better at all lifts. And then you repeat the
experiment with another port and find it responds better to a tuliped
valve. Some ports are like that, by virtue of slightly different interior
contours or different valve angles.
Or you can try valve seating surfaces-maybe someday you can tell me why
sharp edges are better here than rounded ones. The worst valve I ever
tested was one I made the mistaken belief my eye could judge how air would
behave between the valve and seat. I ground a valve head with a radius
instead of a flat where it seated, along with a similar-shaped grinding
stone for the seat. Testing this idea required tons of work, yet my
streamlined valve and seat combination was worse at all lifts than the
typical series of abrupt, sharp-edged flats.
You'd think that getting the valve completely out of the way
while flow-testing ports would let the air really whistle on through. But
peak flow almost always occurs with the valve in place, at a lift equal to
about 30 percent of valve diameter. And this is with a manifold and
carburetor in place, and a cylinder between head and flow bench receiver (
the cylinder's adjacent walls can significantly influence flow around
intake valve heads).
Multiple valves (more than two per cylinder) actually offer
little or no real valve-area advantage. You can prove this to yourself by
drawing circles representing valves inside a larger circle signifying the
cylinder bore. Unless you fudge the whole thing with unrealistic
provisions for valve seats, clearance around the valves, etc., the total
for valve head areas is about the same for two, three or even five valve
layouts. The benefit lies in the fact that total head area counts only at
or near full lift: at lesser lifts, flow is largely limited by the valve
seat ring area, really more a function of the total of valve
circumferences than area. Viewed this way, multiple valve layouts are
better, though only Yamaha has found any gain with more than four valves.
Air flow in ports takes paths totally unlike those you would
normally envision, unless you happen to have an abundant knowledge of
compressible fluid dynamics. In your imagination, air may move in orderly
lines of travel, with particles marching along the roof of the port
staying high, those on the floor staying low, and all traveling in neat,
linear streams. The reality is a very different matter.
When flow in a duct (an intake port, for example) arrives at
a bend, it loses any semblance of orderly behavior. Particles on the
inside of the bend travel the shortest distance (offering the least
resistance to flow), so they tend to maintain speed in the downward turn
to the valve seat. But flow in the top of the port slows relative to the
floor, creating a large velocity gradient. Pressure in a moving fluid
varies inversely with it's speed, so the velocity gradient creates a lower
pressure at the port floor than at it's roof. This differential causes air
at the sides to move upward and the midstream air to move down, with the
resulting flow stream made to divide into to contrarotating vortices where
the port bends. Add to this the invisible "smoke ring" vortex
forming beneath the opening intake valve and you have enough disorder to
confound even the best of minds (or computers).
Port and valve configuration (both shapes and angles) can
profoundly influence combustion efficiency as well. Jack Williams AJS 7R
made it's best power with an intake port shape that compromised flow in
favor of creating more combustion chamber swirl and redirecting incoming
fuel droplets away from the cylinder walls. I am reliably informed that
Keith Duckworth has settled on the intake valves leaned 15 degrees from
the cylinder axis, and ports at 30 degrees from the valves in a similar
trade-off between flow and combustion.
Intake flow influences combustion because both carburetors,
and fuel-injection nozzles deliver fuel in liquid form. The best you can
hope for is a fog of droplets small enough to stay suspended in the air
while evaporating; big drops are centrifuged out of the air stream,
splatting against the intake port and cylinder walls, which is bad for
power, fuel efficiency and emissions. Fuel can't burn until it evaporates;
if you have raw fuel still trying to burn when the exhaust valve opens, it
goes out the pipe, wasting your money and polluting the air.
My experience (not the final word on anything even for me) is
that the biggest improvement in flow from a change in port shape- with the
least port enlargement and resulting velocity loss- is obtained by
widening the port floor upstream from the valve seat. Air likes to take
the most direct route, and the more you ease that route the better flow
becomes. Shaving metal out of the lower sides of the ports bend (making a
D-shaped cross-section, with the port floor on the flat side has in my
tests shown big flow improvements in sharply bent ports.
Smoothing intake flow (thereby minimizing the turbulence of
the main flow stream) is best accomplished by making sure the port's
section area decreases all the way from the carb inlet to the bend above
the valve seat. The small diameter, high-velocity section of the port
needs only a slight convergence of 1.5 degrees included angle, which
doesn't sound like much. But a 12 inch section of aluminum pipe
taper-bored for a 1.5 inch inlet and a 1.498 inch outlet flows better than
a parallel-wall pipe, and a lot better than air going from the cones'
small end to it's beg end. Sound waves love a divergent duct, air flow
does not.
I'm not convinced that polishing a port's interior surfaces
to a mirror finish does anything but look good. The problem here is that
while we know there's a degree of roughness beyond which flow suffers, we
can't agree on the limit to which polishing helps. One those rare
occasions when I do porting myself, I settle for a smooth but not polished
finish. If I were in the head porting business like my long-tie friend
Jerry Branch, I'd put a spit shine inside the ports and combustion
chamber, just as he does. The way Jerry does it, his customers never have
to wonder if the ports are smooth enough.
Jerry has discovered that some ports flow better if he cuts
tiny slots across the floor of the bend upstream from the valve. The slots
apparently act as turbulence generators that energize the air and make it
stick to the port floor, following the bend more closely. That's the
theory anyway, though like so much we believe about port air flow, it's
arguable because air hides is secrets behind a cloak if invisibility.
In time, we will know a lot more about the details of flow in
and out of cylinder heads. For decades, researchers have used smoke,
pinwheels, dye droplets, etc. in their attempts to see what air is doing.
The water-analogy method, where water substitutes for air and flow is made
visible with fine bubbles or aluminum particles, is still used in many
labs. But the growth of mystery-dispelling technologies has recently
brought Doppler-laser metering and computer imaging to the field. Maybe
one day soon we'll learn why the things a century of experience has taught
us actually do work, and why others do not.
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