better mileage with higher octane?

No it did not. It was a lubricant and prevented the valve face from "welding" to the seats. Seat recession was caused by pits being torn out of the valve seats. Stelite or induction hardened seats resist this action. It also took VERY LITTLE lead to have this effect. One gallon of leaded fuel per 100 gallons is enough to protect the valves of a "lead era" car. The result is also quite long lasting. A car that has run leaded fuel can and will survive for many many hours without valve damage on unleaded fuel.

Dieseling is part of it - but you can totally eliminate dieselling by totally closing the throttle on shut-down.Dieseling is more pre-ignition (or auto ignition) than detonation. It is still a "normal" slow burn. No air and/or no fuel means no dieseling. It is DETONATION that is prevented by higher octane fuel. Detonation, if unchecked, will almost invariably also cause pre-ignition. The combination of the two ia almost always deadly. Detonation scrubs the barrier layer off the pistons - the pistons absorb excessive heat and start to melt down. The hot piston lights the fuel before the prescribed ignition point and the pressure peaks while the piston is still coming up. This blows the weakened overheatred piston.

Funny thing - chicken and egg type - pre-ignition can cause detonation. Detonation can cause pre-ignition. Knowing which came first is essential to prevent the problem repeating itself.

OK the valves and seats had to be hardened to stop the "sticking" action of the valve due to less lubrication from the lead but not the higher temperature. My bad.

However you stated that higher octane fuels do not burn slower and then proceded to say they do again. Most articles will agree that octane stops pinging by slowing down the detonation of the fuel under heat and presure. This is not faster and the chemical reason doesn't really matter, as it appears to support my statement and disagree with yours.

When you disagree and then set out to re-enforce the other posters statement it appears as an obsession.

My understanding (not very good I admit) is that you get nothing for free. Seems to me any power produced by flashing to steam has been from heat stolen from the fuel combustion. I doubt that there is any additional power produced by steam. The basic theory seems to have been to cool the intake mixture thus getting a denser (and more fuel) charge into the cyslinder.

Harry K

They're typical numbers according to a site I found. (R+M)/2 is the same as PON btw.

Graham

Steam expanses more for a given temperature than air. And with the improved heat expansion and anti-ping control offered by water injection. You did indeed got more power and the exhaust would be half as hot at the same time. In piston engine a 1/3 more and on turbines double the horsepower.You got to remember the piston engines were running 50psi boost(22+:1 effective compression ratio or in otherwords the compression stroke would heat the air to 900F before you burned any fuel ) in the air,not automotive turbo pressures. With the exception of Stubaker ,whose car engines did with a blower on their sports model. They did water injection and avgas.....to contol the ping.and like the aircraft had a switch in the water tank/pump to kill the ignition advance and blowoff the boost psi, if the tank ran low. In both the Stubaker and the plane,if the water switch failed the engine was trash in seconds from preignition detonation.But when it worked a Stubaker Commander could hang with 427 high performance engines running half the engine displacement.The engine was dropped because of complex construction ....little did they know that modern engines now make they look simple ,50years later.

Solar Flare wrote:

Mustn't confuse detonation with normal combustion. The fuel/air mixture should burn across the chamber at about 100 feet per second. Detonation is a different process and flame fronts can reach 5000 fps or more. The enormous pressure spikes and abrupt release of heat does plenty of damage. Detonation happens when the complex fuel molecules, under the rising pressure ahead of the flame front of normal combustion, begin to break down into simpler structures that can autoignite; they don't wait for the flame front to set them off in a normal chain reaction. The whole works might go off at once just becuse they're being squeezed and their temp is rising. Detonation requires time for those molecules to break down, so low RPM is a problem, large combustion chambers (which require more flame-travel time) are a problem, lean mixtures (which burn more slowly due to the distance between molcules) are a problem. Normal combustion is not an explosion; it's a controlled burn. Detonation is an explosion, totally uncontrolled, and higher octane fuels were designed to increase the capability of handling the higher pressures and temps without breaking down into autoignitable components. In aircraft we're still using leaded fuels because the engine technology is stuck in the 1950s. Some avgas grades have large amounts of lead in them, and sparkplug fouling is a big problem, even in the heat of an aircooled engine. Those amounts of TEL are necessary for the big cylinders found on aircraft engines, their low operating RPM, and the higher operating temps (cylinder head temps to 550=B0F, depending on the engine). We've been promised unleaded avgas for some time now but haven't seen it yet.

Dan

You are being nitpicky and don't know what you are talking about. It is NOT the speed of the burning of the gasoline that is slowed down. It is the virtual explosion of the dis-associated end gasses that is ELIMINATED. Slower fuel burn INCREASES the tendancy of an engine to detonate.

Almost. Getting higher density of the air charge is no problem on a turbo engine. It is the cooling of the COMBUSTION CHAMBER that was the challenge - and water injection helped significantly. It DID have the side effect of acting a bit like an intercooler - but any heat absorbed in the intake reduced it's primary effect in the cyl.

A higher density intake charge actually INCREASED the tendancy to detonate by increasing the effective cyl compression - which was part of the problem in the first place (wouldn't ping without boost)

Right on Dan, So much misinformation flying around about Octane - good to see there are a few others who understand it. (Are you a Canuk? Member of RAA? Perhaps read my artical the other month?)

Well, it has generated about as much comment as the great toilet roll wars of the 80's;>)

I assume you mean "slowing down combustion," whch can be re-stated as "slowing down the speed of the flame front away from the spark plug in the cylinder. Higher octane fuel doesn't do that. Most "articles" are wrong ;-) Water/alcohol injection, OTOH, DOES slow down the flame front as well as pre-cooling the intake charge. Exhaust-gas recirculation slows down the flame front, but actually heats the intake charge a little bit.

You're right in that the main gain is getting a denser cooler mixture into the cylinder, but I think there is a *small* additional gain when water vapor is present in the combustion chamber because we're not dealing with ideal gasses. If all the gasses involved were truly ideal, then you would be correct. In addition, most WWII aircraft engines made some small use of the jet thrust from the exhaust stacks, and water certainly adds mass-flow to the engine and would add some jet thrust. IIRC, I've seen an estimate that a P-51's Merlin engine produces a few hundred pounds of jet thrust from its exhaust at full power. Not enough to even move the aircraft on the ground by itself, but certainly enough to offset the drag of the exhaust system itself sticking out into the slipstream, and therefore a net benefit.

Yep.

I've heard of a typical RON 4 higher than (R+M)/2. However - different fuels will have different RON/MON spreads, so it's not a given that it's going to be close to 4. Pure octane has an RON of 100 and a MON of 100. So pure octane is 100 (R+M)/2.

Or forced induction. Pretty much anything that increases the fuel/air pressure in the cylinders would require higher octane fuel.

My 2004 WRX with a 2.0L turbo four engine has a compression ratio of 8.0:1. It sounds low, but then you add the boost.

7:1 isn't uncommon when boost is anticipated. And the other cool thing about boosted engines is that they have cam profiles with very minimal valve overlap so they generally idle as smooth as a Packard v12. Whereas a normally aspirated engine with similar performance would have 10:1 compression and a wild cam that would lope at idle like a '67 426 Hemi.

Canuck. Flight Instructor. Aircraft Maintenance Engineer. Aircraft Systems instructor. Former member of EAA (1972-1992). Member of COPA. Fly my Jodel D-11 when I have the time. Building a Hummelbird. Should join RAA, but spend so much time working in aviation that by the time I get home I don't want to read any more about it. Barely have time for the H-Bird.

Dan

Wherere you located?

Well - mine is a horizontally opposed four, so I do feel a bit of shake at idle. :-(