Premium Gas in a base RSX?

"Jamco" wrote in news:aO_Re.252933$on1.100933@clgrps13:

YES. you only need hi-octane for high compression motors.

Yes.

Right now you're wasting A LOT of money on premium gas. I tried a tank of premium in my base rsx once, and couldn't tell any difference.

Other posters have already noted that there's no advantage to using higher octane fuel than necessary. Here's a quick introduction to octane ratings to explain why:

Gasoline is a mixture of various hydrocarbons, each with somewhat different characteristics, including different ignition points. Since the ignition point of a type of gasoline is important to an internal- combustion engine (as I'll explain later), gasoline is rated as if it were a mixture of just two hydrocarbons, hexane and octane. Hexane has a lower ignition point than octane; octane's is close to what most car engines would want, but a little high. The octane number represents (in theory) the percentage of octane in a pure octane/ hexane mix that would have the same ignition point as that gasoline.

So "87 octane" gasoline ignites at about the same point as a mix of

87% octane and 13% hexane. The actual octane number, in the US, is the average of the Motor Octane Number and the Research Octane Number (which is why you may see the notation "(R+M)/2" on the octane sticker), which are two standard ways of determining the octane rating.

Why does the octane rating matter? In the cylinder, the fuel vapor should ignite when the spark is emitted from the plug; and the burn should propagate evenly out from the spark in a smooth wave. If the vapor ignites prematurely (before the spark, or elsewhere in the cylinder at the same time as the spark), you'll get less than ideal performance - the piston may still be on the compression stroke, for example, so the burn is forcing it the wrong way - and "knock", which is the sound of an engine that's firing at the wrong time.

In an older engine, if you get premature ignition, you hear knocking, get reduced performance, and risk engine damage. Newer engines have knock sensors, and if they detect knock they'll retard the ignition timing to compensate. That will generally eliminate the knock but reduce performance. So using a fuel with the right octane number will increase performance over one with too low a rating, but going any higher won't improve things any further.

The higher the compression in the cylinder, the more likely the fuel is to ignite, because of higher density and temperature (by Boyle's Law). So high-compression engines need fuel with a higher ignition point, which means a higher octane rating.

High-compression engines get more output per unit volume, but they're more expensive (because they have to withstand higher stresses), so they're generally found on more expensive models. Thus it's common for more-expensive cars to take more-expensive gasoline.

ok, so if that's true, how come you can't run a diesel engine on gasoline? the compression is much higher and the adiabatic heating is much greater in diesel engine....

rubbish. the compression on even the most high compression si engine is low compared to diesels. the price differential comes down to paying a premium for "performance", but that's nothing to do with having to build an engine that can cope with compression stress.

but doesn't the type S and non type S have the same engine? Ones just tuned better?

but you have no guarenteed ignition, since theres no spark. Oh, and if you do get igition, as you say, the compression is MUCH higher, its likely to be premature.

you cannot get premature ignition on a diesel.

Correct. But tuned differently is more correct than tuned better. There are some components that are different, and the ecu is set differently to take advantage of the components.

snip

You seem to be assuming that, simply because air is compressed in a diesel, with diesel fuel (ordinarily) then injected "at the right instant," that the timing could not be messed up via the use of a fuel with a much higher resistance to ignition and/or different ignition properties.

I wouldn't assume this.

The ignition properties of the two fuels are too different.

(This of course contrasts with a gasoline engine, where fuel/air mixture is compressed together, and the spark causes ignition. The timing of the spark is key. Whereas with a diesel engine, the timing of fuel injection is key.)

absolutely i'm assuming that - diesels ignite fuel instantly on injection. injection timing may be off, or there may be a fuel problem, but by definition, ignition cannot preceed injection.

You're saying any old fuel you inject into a diesel engine will ignite at the same instant as diesel fuel?

First, ignition is not "instant." Ignition may start, but full ignition of all the fuel in the cylinder at any instant takes a certain amount of time, occurring over a certain number of degrees of the diesel cycle.

There is a certain "rate of burning" (or "rate of ignition") that will vary with the fuel used.

The difference in "ignition rate" (or "rate of burning") is significant for gasoline vs. diesel fuel. Using gasoline in a diesel engine will mess up the diesel engine's timing, with a potentially (highly likely?) catastrophic outcome (massive engine damage).

As usual, this is not rocket science and is amply discussed in texts (including the net) on diesel vs. gasoline engine design.

"Elle" wrote in news:a8ETe.2176$ snipped-for-privacy@newsread3.news.pas.earthlink.net:

WHY did DIESEL come up in the first place,if you're talking about an RSX? AFAIK,there are NO diesels offered for that model.

Thus,discussion of them is NOT RELEVANT.

Does Acura/Honda even offer any diesels??

(and who's going to run gasoline in a DIESEL????)

Sheesh!

no, that's you putting words in my mouth for the sake of picking a fight.

eh? ignition /is/ instant. combustion takes time. you're confusing the two terms, seemingly because it suits your pitch.

see above.

dude, you can't use gas in a diesel because the ignition temperature of gas at those pressures exceeds the adiabatic heating temperature. unlike with diesel fuel. which is of course why "diesel" fuel is used.

yeah. i own several of those strange "text" thingies. and have bothered to read them.

When you're wrong, you're wrong, Jim.

Can't take it? Get out.

You miss the point.

People can go to authorities and verify the essence of what I posted.

Why am i getting flashbacks?

what /is/ your problem?

"flobert" wrote E wrote snip

I am not yet ready to agree with you that putting gasoline into a diesel engine is likely to result in premature ignition. Jim is right insofar as the fuel enters the diesel cylinder at the same time whether it be gasoline or diesel fuel. (I imagine you're aware of this, too, if you have any familiarity with the diesel cycle.) Gasoline, however, is designed to be more "auto-ignition resistant under pressure." Octane is a measure of its resistant to auto-ignition. Higher octane = less knock, as I think most people here know. Diesel fuel doesn't have an octane number but instead has a cetane number, which measures something different... google yada.

The timing will be off because the gasoline requires a spark or, if it does ignite, it will do so at the wrong time and burn at the wrong rate. You said similar.

Michael's wording was a little off. He didn't deserve to be jumped all over and told his comments were rubbish, etc. His post needed a bit of tweaking, though I think he would have served the group better if he simply cited a well-written web site or two, preferably one that was a dotcom or maybe dotedu, so reputations were at stake.

I've not seen propogation fronts of the different fuels in the same engines. and i've not seen, or driven a diesel now since i moved to the US, so i'm a bit rusty. Never realy been into diesel engines myselfso my basis is mainly theoretical, but my gut feeling is, were we to see the flame wave propigation in slow motion, we'd find the petrol (gasolene) irniting before, probably at the injector tip, and would continue throughout the pulse of the injector. This, to my mind, would also produce a very smokey combustion, due to the limited combustion area, and thus poor oxygen availability.

I would also be interested to see how the new highly refined, high performance diesels that are becomming not so much commonplace, as the norm (something like 33% of new cars in the UK are diesels) in comparison with the older, noisier smokier designs more commonplace in the US, would handle gasolene injection. I have a good video somewhere of a kid who's running a late 70s golf GTi (rabbit to americans) and has just put a couple of gallons of diesel in the tank, and is driving home. Lots of white smoke everywhere.