Wow!
I know all this stuff but never in a month of Sundays could I put it into
words.
Thanks Darrell
----- Original Message -----
From: Ferguson, Darrell <dfergus@bactc.com>
> Depending on how rich the idle circuit is set in a particular engine will
> dictate how acrid the fumes are from the exhaust. When running gasoline,
> this setting is particularly critical. The approximate air/fuel ratio for
> gasoline is 14.7 to 1. For Methanol, the ratio drops to around 6 ~ 7 to 1.
> For Nitromethane the ratio is around 4 to 1. However there are other
> critical variables that come into play. Gasoline has a very high latent
heat
> of combustion. In other words for every ounce or lb. or gallon of gasoline
> that you burn, how much energy do you get in return. For Gasoline, this
> value is very high, around 120,000 Btu/lb. if I remember correctly (I'm
sure
> I'll get corrected on these figures but I'm at work and all of my
references
> are at home) for Methanol it's around 50,000 Btu/lb. and I believe Nitro
is
> around 80,000 Btu/lb. (again, I'm running from memory at the moment).
Here's
> where it gets tricky, and where the factors surrounding the proper design
of
> a FUEL SYSTEM come into play. Gasoline, lb. for lb. creates more energy
than
> any other fuel, drawbacks are that it's octane ratings are relatively low,
> and as the octane rating increases, the burn rate decreases. Methanol
> generates considerably less energy lb. for lb., but because of it's
air/fuel
> ratio requirements, twice as much Methanol is required for a given engine
> (right now you're probably thinking 2 lbs of Methanol at 50,000 Btu/lb. =
> 100,000 Btu vs. 120,000 for 1 lb. of Gasoline is still less energy)
however,
> since Methanol has an extremely high research octane #, you can compress
it
> much much more than Gasoline, and still not have predetonation (plus it
> burns cooler due to it's low latent heat). For this reason, typical
> naturally aspirated Methanol engines are compression ratio limited more by
> piston to valve clearance than by Octane #'s. As a result, Injected Alky
> motors don't notice an appreciable jump in power over gasoline motors.
> However, when a blower is added- VOILA ! Plenty of power can now be
> realized. Static compression ratios in supercharged Alky motors are much
> higher than for gasoline ( a competition supercharged gas motor- assuming
a
> good tight blower is used, typically has a compression ratio around 7 ~ 8
to
> 1, with perhaps 25% OD in the blower. ) An Alky motor is usually around
9~11
> to 1 with up to 40% OD (beyond that, all you're really doing is wearing
out
> the blower and losing power as you generate heat in your intake mixture)
> This is also part of the reason why Screw type superchargers are so
> successful in FM/D and FM/FC. Effective compression ratios now exceed 25
to
> 1 when the boost is factored in. Also, jetting is not as crucial when
> running methanol. If you're close, you're car will run pretty good. With
> Gas, you'll foul plugs at idle, load up the engine, or smoke a piston if
> you're even slightly off with your jetting. On Injected engines you'll
> notice a main, low speed, and high speed bypass on gas burning motors in
an
> attempt to more accurately match the fuel curve, whereas in an Alcohol
> motor, some guys don't run more than a main bypass, (or pill). The motor
> will Idle rich without any severe detriment to the motor, perhaps some
> excess oil contamination and watery eyes at the starting line, and a
little
> rich on the top end, or be right on at high rpms while a little lean in
the
> midrange. Bypasses help match the fuel curve (or a computer works pretty
> killer as I'm sure John will attest) to optimize engine efficiency
> throughout the entire rpm range, but aren't super critical, again,
Methanol
> is much more forgiving (Methanol solves Mysteries!). Plus Methanol is
around
> $1.30 a gallon versus about $4.00 a gallon for good racing gas (It's
> probably more now- it's been a while). Now on to Nitro. Nitro produces a
> relatively decent amount of energy per lb. burned, but because of it's
> air/fuel requirements approximately 3 times as much Nitro is burned as
> gasoline. In doing so, 3 lbs. X 80,000 Btu/lb. nets us 240,000 Btu from
> Nitro vs. 120,000 Btu for gasoline (again approximate figures here). This
is
> where Nitro really makes it's power, and as a result of the fuel demands,
> typical fuel pumps are the size of an aircraft starter, and fuel lines
look
> more like storm drains. For example, the fuel pump that we run on the
> AUTODESK A/FD idles at 150 lbs. of pressure and tops out over 300 lbs. of
> pressure on a typical pass. And this pump is already outdated and is being
> upgraded for the 2000 season. Similar to Methanol, Nitro would be more
> forgiving on jetting were it not one fact. As Nitro burns, it releases
it's
> own Oxygen, this is a key factor that relates to the low air/fuel ratio.
> When a gasoline engine runs, the intake charge is a finely atomized mist
of
> fuel, conversely, a Nitro motor burns an intake charge that is approaching
a
> pure liquid. Since there is so much liquid fuel in the cylinder,
compression
> ratios are very low. On the dragster we run about 10.5 to 1 compression-
> Injected- no blower. Typical supercharged Nitro Motors run about 5 to 1
> compression ratios, and rarely run more than 7 to 1 (assuming 90-100%
load).
> Here's the problem with Nitro-(besides having to upgrade every part, and
it
> costs about $19.00 a gallon if you buy it by the drum). If you can't burn
> the Nitro in your cylinder (and Nitro burns relatively slow- in fact a
Nitro
> motor won't run stable over about 7000- 7500 rpm MAX) the cylinder is full
> of almost pure liquid, and as you know, you can't compress a liquid, the
> result is your cylinder decompresses itself by blowing the motor apart
(more
> commonly known as Hydraulicing). This is why Nitro is so unforgiving in
> reference to jetting. Now that MSD has developed a Magneto that generates
44
> amps vs. 4 or 5 amps for a good Vertex or Mallory Supermag, Top Fuel and
> A/Fuel cars are making more power than ever, with fewer engine failures.
> When running two of them, that's 88 amps of burn, per cylinder- and for
the
> most part, you're burning whatever is in that combustion chamber. While
the
> Top Fuel cars are increasing in their "relative" reliability, A/Fuel cars,
> don't have the supercharger to press air into the cylinder along with the
> Nitro and therefore, are a bit more temperamental. Just as a reference as
to
> the volumes of fuel burned, we'll burn about 8 gallons of Nitro between
the
> burnout and a 1/4 mile pass. Chauvon Emmons' # 422 AFMR, prior to it's
> encounter with the dyke at Bonneville, running a comparatively
conservative
> tune-up, would consume about 34 gallons of Nitro on a typical blast down
the
> long course. At 20 dollars a gallon, I'll let you figure out the fuel bill
> for Speedweek on his machine !
>
>
> Hope this helps !
>
> Darrell Ferguson
> BLACK RADON ENGINEERING
> # 939 BBFALT
|