air - fuel ratio

You do not purchases liquid fuel by weight. BUT A/F ratio is by weight. So if the specific gravity of the gas at the pump changes so does your A/F ratio unless something else changes.

Thanks, Nate!

I was all set to fire off a followup, but you beat me to it, almost point by point. A lot of the nonsense that gets posted could easily be avoided is some posters would simple go buy an engine text book and simply browse through it.

Not sure exactly why you are disagreeing with my statement. I agree that thermal efficiency is the issue. HOWEVER, I think CR is only one element of many in the equation. But you do seem to at least grasp that octane does play a role. That role is going to be much more apparent as price of fuel continues to climb.

If you look at the big picture refining higher octane base blend motor gasoline requires more barrels of crude to produce. In addition, as it happens each of those gallons of that higher octane fuel contains slightly less energy. So high octane base blend gasoline by its very nature is energy inefficient even before you get it to the engine. But the calculus is complicated. Gaining 5% energy efficiency inside the engine at a loss of 10% of the energy from the source crude is not really a winning thermal efficiency equation and it is going to inevitably cost you more. The point is these cost benefit calculations are already being made, but they are not being made with thermal efficiency in mind. That is, the tradeoffs are being calculated but the goal is to maximize the oil industry and government's interests. Unfortunately, that often works directly against thermal efficiency.

Where did you come up with that? It is my observation that 1940 cars were pretty heavy and quite fuel efficient compared to cars in the 50's of the same weight. Not that anyone cared in the 50's about fuel efficiency, but fuel efficiency was an issue before WW2 so cars were designed to be more fuel efficient. It wasn't until the early 70"s that anyone much cared about fuel efficiency. Increased Fuel efficiency in the late 70's and eighties was mostly achieved by shedding weight.

-jim

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OK, then, what *is* the object? Getting high from huffing it?

nate

Making a profit, of course. That's what ALL consumer products are about.

--scott

All you have to do is dump it on the ground and toss a match if you want to release some energy.

Out the tail pipe or radiator is where most of the energy is going. Making precise calculations of the total energy contained in a fuel is absurd - those calculations it has no direct bearing on how much is being used to move the vehicle. If you believe in those calculations, then why not put diesel in your car? That would give you the best mileage. Right?

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Here is what you said on how octane affects fuel economy.

If you read your engine textbook you will find that the RON rating is made under light load conditions just as you describe. You are correct to say driving consistently under light load is how the best fuel efficiency is achieved and because of higher fuel prices more people are driving like that. Driving at 45 mph instead of 70 mph significantly reduces engine load and may as much as double some vehicles mileage if given the right fuel. The fuels properties to knock under light loads is exactly what the RON test determines. Engines do knock under light loads even if you think they don't ( and even if you can't hear it) and that does impact on gas mileage.

The problem is you don't have any idea what the RON rating is for the fuel in your gas tank and it can vary quite a lot for any given posted pump octane rating. But in spite of the fact that you have no idea what the octane is in your gas tank under those conditions that could potentially be giving you better mileage, you are positive it has no affect on mileage cause you read something in a book.

-jim

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No it is not. Apparently you do not know under what conditions the octane ratings are obtained. Contrary to what you said both the Motor method and Research method are obtained at WOT. Do I really need to list the specifics of methods used?

Interesting assertion here. Please provide a source for this. In my 30 years of engine testing this is a new "fact" for me. And in much of my testing, I have acquired and viewed the in-cylinder pressure data and have not seen this.

This is absurd. You dismiss textbook information that has been verified over and over just because it doesn't fit your pet theory; a theory that you have yet to provide any refereed literature on. Have even read any of the books? Here's a good one: Heywood's "Internal Combustion Engine Fundamentals". This book not only explains the major phenomena but also provides literally thousands of technical journal articles as references. Can you do the same?

WOT? It's a test engine there is no throttle adjustment per se. The engine runs at 600 RPM. Does that sound like WOT? The engine load conditions are described as light (according to ASTM) as a result it yields a different (higher) octane number than the MON test which is not a light load test (again according to ASTM). And by the way the test engines that are still used today were designed something like 80 years ago - so exactly how much does an

80 year old push rod and carb engine tell you about how a modern engine will perform with respect to fuel economy. Oh that's right your textbook says it doesn't have anything to do with fuel economy . Well DUH!

The point is motorists don't know what the RON octane number is for the fuel in their gas tanks. Or more to the point they don't what the octane number is that could be useful when driving for fuel economy, Thus if you change pumps in the interest of getting better fuel economy you have no idea if you are really increasing or decreasing the knock characteristics of the fuel at light load conditions.

The RON test is done under light load. It does indeed detect knock under those conditions. How can you claim an engine can't knock under light load? The test protocol is all well documented. What isn't documented is the results of that part of the test that would actually be useful for increasing fuel economy.

What specifically is absurd? Are you saying it's absurd that I said you read a book?

It's not absurd to say that you don't know how octane interacts with fuel mileage in your own car. You have made it clear that for the specific types of driving conditions that would lead to fuel economy you don't know what the specific octane characteristics are of the fuel in your car's tank. Sure, you may know what the octane characteristics of the fuel are when you punch it to pass some guy who is going 65, but that has nothing to do with good fuel economy.

You dismiss textbook information that has been verified over and over just because it doesn't fit your pet theory.

What have I dismissed in textbooks? As far as I know they are pretty silent on the issue. The octane ratings are designed to benefit auto manufacturers so they can build cars that don't get harmed by the fuel. The octane rating system as it is now in North America makes it close to impossible for motorists to use the displayed octane numbers as a guide to purchasing for fuel economy. But that doesn't mean it has to be that way or that it always will be that way.

-jim

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Oddly enough, it does.

nate

I've always wondered: is it the higher compression ratio or the different fuel which gives diesels their greater fuel economy and their greater low-end torque. Or a combination of the two?

WOT means wide open throttle which means intake pressure is at atmospheric. That determines engine load, not the speed of the engine.

Operating conditions for research and motor methods

Research Motor ASTM D-2699 ASTM D-2700 Inlet T 52 deg C 149 deg C Inlet Pr Atmospheric The point is motorists don't know what the RON octane number

Again you confuse speed with load. They are not the same.

The purpose of the test is to determine the fuels susceptibility to detonation. For this purpose the test is perfectly valid. Sure the speed is less than that of today's vehicles. Researchers have worked on developing a "road octane rating" and what they found was that the more realistic road test method provided results the fell somewhere in between the RON and MON ratings. This is one of the reasons that things were simplified to using the pump rating of (RON+MON)/2.

What is absurd is your willingness to disregard the body of published information on this subject (and there is plenty).

The octane rating of any fuel is only used to rate the propensity of the fuel to detonate. It does not indicate the heating value of the fuel itself. This is easy to see. Commercial fuels are sold with octane ratings of 87 and 93 (pump ratings). Yet both fuels have heating values within a couple percent of one another. Another example, methanol has (R+M)/2 = 109, yet has half the heating value per kg as gasoline. The octane rating does not correlate with the energy in the fuel.

If the engine does not detonate at a particular operating condition, it does not matter what the fuel's octane rating is. It was sufficient to preclude abnormal combustion.

Diesels benefits from three main factors:

1: Higher mechanical compression ratio. Cycle efficiency is a function of expansion ratio. This is one of the biggest reasons Diesel have higher torque.

2: Unthrottled operation. In the Diesel cycle load is controlled by fuel alone. This reduces gas pumping losses incurred during the intake/exhaust strokes. In future Diesels this may fade somewhat as throttling is being used to facilitate achieving the EGR levels needed to meet oxides of nitrogen regulations.

3: Fuel heating value. Diesel fuel contains around 10% more energy per gallon than gasoline. Diesel fuel is formulated to promote autoignition, the Diesel cycle ignition method. The metric for this is called the cetane rating. The higher the cetane rating the lower the octane rating. For Diesel engines high cetane fuel is desirable. This is obviously undesirable in a spark ignition engine.

No, higher CRs and higher operating temperatures (sealed cooling systems) also contributed to higher fuel efficiency these days. Better combustion chamber design allows higher CR today without higher octane. Not many hemi head designs these days.

There actually is a fairly consistent negative correlation between octane and heating value in most motor fuel components so actually your wrong aboutr that. But your assumption that heating value alone determines fuel economy is also incorrect.

No, with almost all constituent components of motor fuels it negatively correlates, but that is also beside the point as it doesn't really say anything about octane and fuel economy.

Sure just keep repeating your mantra over and over and maybe you someday you will achieve enlightenment.

Your point isn't incorrect - it just doesn't shed any light on the relationship between fuel economy and the tendency for a particular fuel in a particular engine to detonate. If a load of fuel and air exhibits no detonation at all then that is a good indication that some of the energy that could have been converted to mechanical power failed to convert (it's wasted). How much power goes to the wheels is a function of how much pressure can be applied to the pistons over the proper period of time. The most significant limit to this pressure integral is detonation. Detonation and the measure of fuel to resist detonation is far more important than the energy content of any particular fuel because most of that energy content ends up being wasted. In other words, Octane has everything to do with fuel economy.

-jim

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One last time. WOT means the throttle is fully open with the intake manifold at atmospheric. This is exactly what the test specifies.

Say what? The speed is limited by the dynamometer.

Once you start using insults rather that verifiable data, there's no point in continuing this discussion.

Heh. that was actually a textbook for one of my college courses. Thanks for the flashback.

I think I still have it, as it was a third or fourth year class so I wasn't able to sell it to buy the next year's books...

nate

What he said. I'm not sure how much 2) contributes, but sort of as an aside, this is the reason why you see sewer-pipe sized exhaust pipes on Diesel vehicles - they are essentially pumping the full displacement of the engine (or more, if supercharged) of air through the engine every two revolutions, unlike a gasoline engine where this only happens at WOT.

nate

So having an excess of air, compared with the minimum needed to fully burn the fuel, is actually a bad thing for a diesel, is it, as regards the production of NOx?

I'd not thought about it like that, till now, but you're right. At a typical cruising speed of maybe 70 mph, that's about 2500 rpm so for a 2-litre engine that's 2500/2 * 2 = 2500 litres of air per minute or 2500/60 = 42 litres of air per second. That's quite a flow rate through a pipe that's maybe 8 cm in diameter.

Even at idling speed, there's a real draught from the exhaust pipe on my car!

How much does the effect of the turbo vary with engine speed and therefore exhaust gas flow? Presumably the faster the engine is going, the higher the inlet pressure of air that the turbo delivers to the inlet-stroke cylinder(s) and so the greater the amount of air and therefore the greater the amount of fuel that can be burned efficiently before you get partial combustion.

Yes, which is why at fairgrounds etc you often see little generators on sideshows and stands with the engine just ticking over and a tiny fuel tank on top. Interesting to hear how little the engine note changes as extra electrical load is applied, unlike petrol generators which are running much faster (to get the required torque) but which slow down as more current is drawn.