Right. If the engine's antiknock system isn't kicking in and retarding timing, higher octane gas would be wasted.
It would be really nice what with all the brains built into ECUs if they'd add a dashboard light that would signal the opreation of the antiknock system. If you see the 'Bad Gas' light come on frequently, higher octance might be of some value.
Driving habits (and other conditions) can affect octane requirements. Some people might get away with using lower octane gas if they are easier on the gas pedal.
I notice the comment that water injection has fallen out of interest since the advent of intercooled turbo engines.
One problem with turbo'ing is the that the intake air becomes very hot due to compression and the air-fuel mixture can prematurely detonate in the cylinder.
My own Saab ( lpt ) without an intercooler is limited to about 0.3 bar boost for example to avoid such a problem..
Water injection into the hot intake air would actually cool the mixture thus resolving the problem.
However since intercoolers perform the same function without the added complexity of water or water-alcohol injection there's simply no longer any need for it.
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Graham
V
Venus                                                      Â
"dido" I heard about this gimmick high Octane, I already spent $200-$300 on all sorts of additives and high octanes gas to see if I can save moneyl. I ended up with +/- 2-3 miles. Now I am so glad I've found a better solution, now I doubled the mileage (or pay 1/2 price what I normally paid in the past.) The engineering in our car is wrong, that's what caused Global Warming, and that's what took all our dollars.
I will not give this idea away freely, perhaps some ones know about this but they just wanted to make big bugs on their Petrol stock, I don't know or perhaps they are dumb for many years.
most vehicles have a knock sensor and will try to constantly adjust the timing.If you use higher octane fuel the ecu will advance the timing further.The question is will the added mileage make up for the price difference?
Used to be add-ons sold through places like J.C. Whitney. Running the old 30s and 40s cars, there was a very noticeable improvesment in performance (don't know about economy) when the humidity was high or it was raining. Never noticed it after the advent of modern engine control systems.
Water injection was a big deal in the 1940s and was used in a lot of WWII aircraft engines. The notion here is that you get the phase change of the water during combustion, which produces additional effective power (ie. more energy into the piston stroke, less energy wasted as exhaust heat) as it turns to steam.
I don't think I have seen it on anything modern.... but it was one of the big secret features on the B-29.
Normally it's not just water either, but a water/surfactant mixture so that the fuel-air-water mixture would be more even.
They are used (beyond their designers' wildest dreams, in fact) every year at the Reno National Championship Air Races ;-)
Advancing the timing wasn't the goal, raising the BOOST PRESSURE from supercharger or turbochargers was the goal. Advancing timing is counter-productive beyond a certain point because combustion is just pushing "the wrong way" on the piston. But raising boost pressure is advantageous in many ways. The problem is that thanks to good ol PV=nRT, compressing the intake charger with a super- or turbo-charger raises the temperature of the intake charge. Too high an intake temperature results in detonation regardless of the ignition timing, and can also result in manifold explosions. Aftercoolers (air-to-air or air-to water) help, but are limited in how much heat they can reject and also add weight to the aircraft. Water injection in WWII aircraft engines primarily reduced the temperature of the intake charge to a manageable level because the evaporating water absorbs a LOT of heat (latent heat of vaporization.) Running the engine excessively rich also does the same thing using the fuel itself to absorb heat, but the drawback is that over-rich combustion produces less power than stoichiometric combustion.
In addition, the water adds inert gas to the combustion cycle, and that slows down the flame front and reduces the peak temperature in the cylinder and further suppresses harmful detonation of the type where a pocket of charge in a remote corner of the cylinder self-ignites after the primary flame-front starts increasing cylinder pressure. Modern automobile engines (since the late 70s) actually use a metered dose of their own exhaust gas (exhaust gas recirculation or EGR) to do this (slow combustion, reduce peak combustion temperature) as well as to reduce nitrogen oxide emissions.
The big drawback to water injection is maintenance. Corrosion in the plumbing, pumps, valving, etc. is a big problem. Also, the water/ethanol mixture has to be replenished along with fuel and oil and it has to be PURE water which requires reverse-osmosis, distillation, or at least deionization. Otherwise deposits build up in the system and in the engine. Water injection really isn't practical for cars, especially since most drivers can't even be persuaded to check their own oil these days.
My understanding is that the water/alcohol is fully vaporized well before it passes through the intake valve(s).
I miss that. I'm sure some folks DON'T miss the occasional crook at the pumps who made sure to put the stick in "short" so it read a quart low, the guys who dropped antacid tablets in the battery and said that the foam was due to the alternator overcharging the battery, etc. etc. etc. Gladly those guys were a small minority, but they were out there!
Not true. Higher octane fuel, generally, does NOT burn slower. In many cases it can actually burn FASTER. High octane fuel simply resists dis-associating the hydrogen radicals(H1, not H2) from the hydrocarbon fuel under heat and pressure. It is the presence of these "free radicals" which causes the fuel to detonate in the cyl.
To reduce detonation you either increase the Octane rating of the fuel or reduce the amount of heat acting on the end gas. This is done by reducing compression, retarding spark, increasing RPM, or engineering the combustion chamber for a FASTER burn.
Lead did not have a "cooling effect". The lead COATING acted as both an insulator and a lubricant on the valve seats, preventing the valve from being "hammer welded" to the seat, pulling metal from the seat and causing "seat errosion". This was a side benefit - TEL was added to act as a "negative catalyst" to prevent the breakdown of the end-gasses, reducing the production of the highly unstable hydrogen radicals.
GM did, back in '60 to '63 with the turbo Olds Jetfire and IIRC, the Turbo Monza Corvair. Don't think it was standard on the Monza, but I know some did use it (possibly a "dealer add-on" using Olds parts)
Two could play at that game. I remember when you could reset the counter on the gas pump ever $5.00 just by flipping it off and on. That way a fillup only ever costed $5.00 Oh wait a minute, it only cost $5.00 to fill up then anyway. Sorry...
High octane unleaded racing fuel supposedly burns faster once ignited. However - it has a higher resistance to detonation or preignition. A spark is usually hot enough to ignite the mixture.
I'm not sure about those numbers. The difference is highly dependent on the individual fuel. It's also called Anti-Knock Index (AKI), and on the pump the label typically says (R+M)/2, as the average of the RON and MON. I remember there was a recommended set of RON and AKI octanes in a 2001 Toyota Camry owner's manual. I don't recall off the top of my head.
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