Tapered intakes, port velocities etc. Part I

[Todd Knighton]

Todd Knighton wrote:
> 
> Guys,
>         Here's the article I had talked about, the thing wouldn't OCR because
> it's a low grade fax, so I typed it in.  Sorry, no pictures, the few
> that were in there weren't worth much.
> 
> Todd Knighton
> Protomotive Engineering
> 
>     ---------------------------------------------------------------
> Cylinder Head Tech:
> 
> Motorcyclist June 1996
> 
>         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 pie 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 pips, 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 crown 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 hoes 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-horsepowered 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.