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Joined: Dec 2008
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'Bolter
Compression causing ignition is called DIESEL. IMHO not possible under 15:1 CR.

Joined: Sep 2011
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Lugnutz Offline OP
'Bolter
Originally Posted by panic
Compression causing ignition is called DIESEL. IMHO not possible under 15:1 CR.

My compression isn’t anywhere near that. I’m not hearing any pinging at any speed or RPM (being careful not to lug the engine of course) so I am glad that it seems to be happy with 87 octane.

Last edited by Lugnutz; Mon Sep 26 2022 11:42 PM.
Joined: Feb 2004
Posts: 25,333
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Kettle Custodian (pot stirrer)
I've seen 9:1 engines Diesel- - - -just shut the ignition switch off at around 1K RPM or a little faster.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
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Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
A number of engines, including GM ones, use an idle stop solenoid or high idle solenoid. It closed the throttle further than normal idle speed when the key was turned off to prevent dieseling.


Kevin
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
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Kettle Custodian (pot stirrer)
Originally Posted by klhansen
A number of engines, including GM ones, use an idle stop solenoid or high idle solenoid. It closed the throttle further than normal idle speed when the key was turned off to prevent dieseling.

Fuel injection solved the problem for good- - - - -no fuel to support dieseling once the ignition switch was shut off.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Joined: Oct 2018
Posts: 93
C
'Bolter
Quote
PRE-IGNITION = Squeeze the fuel molecules so tightly (high compression) that they ignite on their own (before the spark plug fires.

DETONATION = Ignition of fuel and expanding gases is early and fighting piston travel. They slam into each other making a pinging noise.

This is not quite accurate.

As Jerry mentions, detonation is caused by the heat of combustion raising the combustion chamber temperature above the auto-ignition point of the fuel, which causes unburned pocket(s) of air/fuel mixture to self ignite. The pressure waves from these flame fronts collide, causing a rapid increase in cylinder pressure. These pressure spikes actually cause the cylinder walls to resonate, resulting in the audible “pinging” sound heard from the engine.

Peak pressures during detonation occur after TDC, since the pressure/temperature rise that initiates it is a result of the combustion event. Severity of detonation can range from mild (annoying) to severe (engine damage).

One way to control detonation is to use fuel with higher octane. The octane rating of fuel is directly related to the fuel’s resistance to self-ignite. The higher the octane rating, the higher the resistance to self-ignite, which means higher resistance to detonation. Anyone old enough to remember black and white TVs or 0.30/gallon gas, knows that the go-to octane enhancer used to be tetraethyl lead. Lead was phased out of fuels in the 1970’s to support the use of catalytic converters, and for environmental concerns. Ethanol is frequently used in modern gasolines to increase octane rating.

Quote
Running a 5% Diesel fuel mix can raise the octane rating of the fuel, sometimes as much as 5 points.

Jerry, you mention that you use diesel fuel to increase octane. Since diesel engines are designed to ignite the air/fuel mixture using the heat of compression, diesel fuels have extremely low octane ratings. Their self ignition temps are low by design. I’m struggling to understand the science behind how adding diesel fuel can increase octane rating of gasoline. Please explain.

Some of the factors related to detonation are:

- Combustion chamber temp/press
- Spark advance (which affects combustion chamber temp/press)
- Compression ratio (ditto)
- Air/fuel mixture (ditto)
- Inlet air temperature (ditto)
- Engine load (ditto)

Modern engine management systems monitor for detonation using a detonation (aka “knock”) sensor. When detonation is detected, the engine controller can take corrective action to mitigate detonation, such as retarding spark advance, reducing throttle, or changing the air/fuel mixture. As was also previously mentioned, the time honored demonstration of this was to tap on the side of the block with a hammer while watching spark advance - either with a timing light or scan tool. The hammer hit causes the block to resonate at a frequency close enough to detonation to cause the engine controller to react.

In contrast, preignition, is caused by an ignition source other than the spark. Sources include: hot spots in the combustion chamber due to carbon build up, a spark plug that is too hot due to incorrect heat range selection, sharp edges on the cylinder head, valves, etc,

The bad thing about preignition is that as the name implies, it occurs well before the piston has reached TDC, causing very rapid pressure and temperature increases in the combustion chamber. It often occurs so quickly, that the engine damage is done before the driver is even aware that it is happening.

BTW, preignition is not just something to be worried about with our engines. A relatively new phenomenon called “Stochastic Preignition” is currently a source of concern among automakers designing small displacement, direct inject, turbocharged engines.

Stochastic Detonation [en.wikipedia.org]

As a quick sidebar to detonation, in the summer of 1982, I was at a party talking to a very attractive young brunette, who had just got her 1981 Olds Cutlass back from the dealer for the umpteenth time. The complaint was that the engine often made a “rattling noise” when she stepped on the gas. Each time she got it back, it seemed like the car ran worse. The dealer finally advised her to start running premium fuel, because of the “high performance” Olds V8 under the hood – which was beyond stupid, because as anyone who worked on the cars of that era knows, the compression ratio of that Olds 307 was lower than low back then.

I told her that I would look at it, and if I fixed it, she could buy me dinner.

I quick check of the ThermAC system (the thermostatic air control that preheats incoming air to the carburetor when the engine is cold) showed that the vacuum control valve was faulty, causing the system to provide heated air to the carb at all times. This super heated air, entering the combustion chamber, was enough to induce detonation under certain engine loads. I replaced the valve, and the system worked fine. A check of the ignition timing, showed that in an effort to fix the problem, the nobs at that dealer had uber-retarded the spark causing the engine to lose power and fuel economy. I reset it to factory specs and the car ran great.

I got my dinner. As of last May, we have been married 37 years.

Best Regards…

p.s. A lot of the above information was gleaned from this source. It an older textbook, but it has a lot of interesting information related to internal combustion engines and related topics. Plus, it is a freebie download.


Text [gctbooks.files.wordpress.com]

Last edited by CrowbarBob; Mon Sep 26 2022 06:33 AM. Reason: typo
Joined: Feb 2004
Posts: 25,333
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Kettle Custodian (pot stirrer)
My understanding of the advantages of adding Diesel to the gas might be in error, but I've seen a definite power increase on numerous dyno runs with doctored fuel, and less of a tendency to spark knock on long uphill pulls on repeated road tests. One theory is that the Diesel slightly increases the specific gravity of the fuel, and slows down the burn rate slightly- - - -the same thing adding an octane booster does. I don't fully understand the physics behind it, but the improvement can be documented- - - -repeatedly! Since Diesel has slightly more BTU's per gallon, I've also seen a marginal improvement in gas mileage when running Diesel-boosted fuel.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Joined: Oct 2021
Posts: 1,163
O
'Bolter
The pre detonation is actually the fuel molecules breaking apart under the force of compression. Higher octane fuels resist this breaking apart under high compression long enough for the spark to ignite the intact fuel molecule.


1939 Packard Standard Eight Coupe (The Phantom)
1950 Chevrolet 3100 (Ol' Roy)
1956 Cadillac Coupe de Ville (The Bismarck)
1966 Oldsmobile Toronado (The Purple Knif)
1966 Ford Mustang (Little Red)
1964 Ford Galaxie 500 coupe
1979 Ford F-100
1976 Ford F-150 (Big Red)
1995 Ford F-150 (Newt)
Joined: Feb 2004
Posts: 25,333
H
Kettle Custodian (pot stirrer)
Just who invented this imaginary term "Pre-Detonation"? There's "pre-ignition" which is a flame front that begins somewhere other than at the tip of the spark plug, and "detonation" which is uncontrolled, possibly explosive burning of the fuel/air mixture. How did those terms get combined? In over 60 years of tinkering with engines and spending decades teaching the mechanical trade, I've never heard that one before!
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

WAG MORE- - - - - -BARK LESS!
Joined: Oct 2021
Posts: 1,163
O
'Bolter
My bad. Typing in a hurry. I meant to type pre ignition.


1939 Packard Standard Eight Coupe (The Phantom)
1950 Chevrolet 3100 (Ol' Roy)
1956 Cadillac Coupe de Ville (The Bismarck)
1966 Oldsmobile Toronado (The Purple Knif)
1966 Ford Mustang (Little Red)
1964 Ford Galaxie 500 coupe
1979 Ford F-100
1976 Ford F-150 (Big Red)
1995 Ford F-150 (Newt)
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