[Diy_efi] The Hunt effect

[dh at busb.com]

As a general rule, not to be taken all the time.  The heavier the fuel the
higher the energy available.  So yes, the heavier fuel might result in more
heat in the engine.  Diesel fuel has more BTU's / gal than gasoline.

And for a fixed ignition system - such as the 6cyl - the cheapest fuel that
won't result in audible knock is best economy and the cheaper fuel could
conceiveably give the best mileage.

In a modern high compression, computer controlled system that detects
detonation and retards the ignition,  a higher grade might be cost effective
when the price for two points of octane is less than 3% of the total fuel
cost.

dh




----- Original Message ----- 
From: "Geoff Harrison" <geoff_h at smartchat.net.au>
To: <diy_efi at diy-efi.org>
Sent: Tuesday, October 04, 2005 11:05 PM
Subject: Fw: RE: [Diy_efi] The Hunt effect


> I think a couple of points could be cleared up here, and maybe John would
be
> so kind to clarify.
> The CR of an IC engine should be adjusted to suit the fuel in use -
> Retarding ign only "masks" the issue of knock. The engine has spent the
> energy compressing the gas more than required, retarding the ign further
> reduces the amount of energy which can be sent to the crack.
>
> Alcohol has a lower calorific value than petrol, therefore you need more
of
> it for the same power. The cost saving by substituting a percentage of
> petrol for ethanol, may mean an increase on overall cost of running a
> vehicle.
>
> I have a "gut feeling" that lower octane fuels result in a higher engine
> temp. I'm not referring to alcohol blends, just straight petrol. This was
> apparent on a 64 Chrysler 225. Used standard fuel on country trips instead
> of super. (My ol man was tight, that's where I get it from)  I am sure the
> standard, poorly calibrated temp indicator in my daily driver sits a
little
> lower when on higher octane.
>
> Geoff
> ----- Original Message ----- 
> From: "John Gross" <jogross3 at hotmail.com>
> To: <diy_efi at diy-efi.org>
> Sent: Wednesday, October 05, 2005 8:24 AM
> Subject: RE: RE: [Diy_efi] The Hunt effect
>
>
> I'd have to look at the ratios, but yes, certain additives like ethyl
> alcohol have an effect on octane ratings.  HOWEVER, keep in mind that ANY
> alcohol will dry out any rubber seals, o-rings, etc in a fuel system
> designed for straight gasoline.  Also keep in mind that ethyl alcohol
burns
> at a stoich AFR of 9:1 (it's an oxygenated fuel), so if you blend it into
> gasoline, it will change the required pulse width of your injectors to
> maintain the correct lambda.  Also, it will tear up your injectors because
> they're not designed for alcohol either.  That's why you can't just run
E85
> in an everyday car.  The fuel pump, fuel lines, injectors, o-rings, seals,
> and even fuel tank liner must be designed to work with alcohol-based
fuels.
> -----Original Message-----
> From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
> Behalf Of dalemahan at charter.net
> Sent: Tuesday, October 04, 2005 4:48 PM
> To: diy_efi at diy-efi.org
> Subject: Re: RE: [Diy_efi] The Hunt effect
>
> Hi John,
>
> Ah hah - a chance to ask a real IC engineer about fuel questions!
>
> Doesn't about four ounces of acetone in 10 gallons of cheap gasoline make
it
> burn like a higher octane fuel?  Doesn't ethyl alcohol itself have a
> comparativly high octane rating? How about benzene as a fuel additive,
> ignoring its health effects?
>
> Thank you,
>
> Dale Mahan
>
>
>
> >
> > From: "John Gross" <jogross3 at hotmail.com>
> > Date: 2005/10/04 Tue PM 04:11:05 EDT
> > To: <diy_efi at diy-efi.org>
> > Subject: RE: [Diy_efi] The Hunt effect
> >
> > ;-)  Nope, but I did stay at a Holiday Inn Express last night  ;-)  All
> > kidding aside, I have a master's in Internal Combustion Engine Design.
> >
> >
> >
> > -----Original Message-----
> > From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On
> > Behalf Of Jim Butterfield
> > Sent: Tuesday, October 04, 2005 3:35 PM
> > To: diy_efi at diy-efi.org
> > Subject: RE: [Diy_efi] The Hunt effect
> >
> >
> >
> > WOW John are you a gas engineer???? great explinations of the reactions
> > inside an engine
> >
> >
> >
> > jim
> >
> >
> >
> >
> >
> >
> >
> > John Gross <jogross3 at hotmail.com> wrote:
> >
> > It takes more energy to cause the fuel to "flash" without a specific
> > ignition point.  The spark from the plug on your car is more than enough
> to
> > start the burn on just about any unleaded fuel.  I say unleaded to try
to
> > keep the discussion within reason in terms of octane ratings and
> appropriate
> > CRs for the engines.  It takes more spark energy to light off a 115
octane
> > fuel than it does an 87.  However, when talking about street-driven
cars,
> > any delay in the formation of the kernel (the initial point of
combustion
> > inside the spark plug gap) is so minute, that it is not worth
considering.
> > Additionally, what makes it harder and harder to light the air-fuel
> mixture
> > isn't just the fuel, but the dynamic compression.  That's why blower
> motors
> > typically need a more robust and powerful ignition system..the
turbulence
> > inside the cylinder in a blown motor (high dynamic compression) can
> actually
> > extinguish the kernel, thus stopping combustion.
> >
> >
> >
> > One thing to keep in mind, though, is that the kernel starts the
> combustion
> > at the plug, but once started, there are multiple turbulent pockets of
> > burning gas traveling around the combustion chamber, like a bunch of
tiny
> > eddy currents.  Quite frequently, especially in engines where the octane
> > rating of the fuel used is borderline to the dynamic compression of the
> > engine, some of these eddy currents will surround a pocket of unburnt
fuel
> > and air (end gas), and cause the local temperature and pressure to rise
to
> a
> > point where the pocket will explode, instead of burn, causing pinging.
> > Usually, this is not enough to set off the knock sensor, but when it
> > explodes like that, the release of pressure is instantaneous, and
> therefore
> > does not contribute to the overall cylinder pressure..therefore you've
> lost
> > some of the potential pressure.  So, from the uniformity of combustion
> > standpoint, running a little extra octane for WOT, max power may help
> some.
> > But also keep in mind that the fluid dynamics inside an engine are
chaotic
> > in the truest sense of the word.  A decent engine will have a
> cycle-to-cycle
> > power production variance of 5% for every given cylinder, meaning that
> > cylinder 1 on a 400hp V8 may make 50 hp on this cycle, but only 47.5 hp
on
> > the next combustion event, or it may make 52.5 hp on the next event.
It's
> > all very chaotic.  A lot of the R&D that goes into high end racing
engines
> > is in the area of reducing cycle-to-cycle variance and the overall
> > cylinder-to-cylinder balance, NOT maximizing the output of any given
> > cylinder.
> >
> >
> >
> > -----Original Message-----
> > From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On
> > Behalf Of Bret Levandowski
> > Sent: Tuesday, October 04, 2005 11:24 AM
> > To: diy_efi at diy-efi.org
> > Subject: RE: [Diy_efi] The Hunt effect
> >
> >
> >
> > But if higher octane fuel takes more energy to light off, then the
> ignition
> > point would happen later than it should causing a power loss. Yes???
No???
> > I'm thinking out loud. Or would the ECM take these factors into account
> and
> > advance the timing to compensate? This being the case, older
> non-controlled
> > motors would show a loss unless the timing were manually adjusted. As
slow
> > as the older ECM's were (80's), would they be able to adjust fast enough
> to
> > compensate? (By slow, I mean compared to todays comm speeds.)  Ski
> >
> > John Gross <jogross3 at hotmail.com> wrote:
> >
> >   _____
> >
> > Yahoo! for Good
> > Click here to donate <http://store.yahoo.com/redcross-donate3/>  to the
> > Hurricane Katrina relief effort.
> >
> >
> >
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