Just to slightly clarify things, it is not the compression ratio which determines engine efficiency, but the expansion ratio. Mechanically they are identical, and for some reason the compression ratio is the stated number. The effective ratios will usually be different from the mechanical ratios, as compression does not begin until the intake valve is closed and expansion ends when the exhaust valve opens. Because of differing valve timings between engines designed for different uses, the effective ratios may differ from each other in the same engine. Expansion ratio determines the efficiency, while compression ratio (along with variables such as throttle opening, engine temperature, ambient air temperature, and no doubt others) determine the temperature and pressure in the combustion chamber prior to spark firing and ignition of the fuel charge.

Wikipedia's take on octane....

http://en.wikipedia.org/wiki/Octane_rating

The chart at the bottom is quite interesting.  Diesel is 15-25 octane, while ethanol and methanol are 108 octane.

Charon wrote on 09/05/12 at 06:03:56:

C:R is always stated alone because it is a fixed variable. Everything else changes. There's no point in using any other number except for when making the fuel-map for the ECU.


SALB wrote on 09/05/12 at 09:25:06:


That's because diesel isn't ignited, it's forced into the cylinder at the top of the compression stroke and detonates due to extreme pressure and heat. Remember back when cars used to "diesel" after you'd shut them off? That was detonation keeping them running and that's exactly how a diesel engine works. There are no spark-plugs in a diesel engine.

And diesels don't "detonate" as there is no pre-mixed fuel/air just sitting there waiting for the spark to "set it all off at once".

The diesel ignition curve is an off center skewed bell curve as the very first micro-droplets of fuel ignite as they leave the ejector nozzle, rapidly growing more numerous until the injector stroke hits it's max flow rate, then tapering off as the injector stroke tails off to nothing.

This cone of fire is quite rich at the center and lean at the outer edges, and the fire cone swirls and moves around on itself intentionally as the piston starts to go down, allowing all the air that can be used to be used.

Note: the compressed air charge is of fixed volume, size and compression, nice and red hot and very consistent stroke to stroke to stroke.

The amount of fuel sprayed in per piston cycle however is a variable, and when operated at partial throttle or low speeds diesel engines can burn quite lean and hot on the initial part of the power stroke, but this quickly cools off as the charge expands as the piston goes down on the small partial fuel load of sprayed oil that had "too much" air in there and not enough fuel.

Don't look at me!    I was just making the point that diesel is such a low octane so that compression ignition could occur.    I also listed alcohol, because it's very high octane, and thus a prime choice for race cars running 13:1 or higher compression.

As for injectors, almost all manufacturers have gone to piezo  type electric injectors in the last ten years or so, and now they're starting to show up in gasoline direct injection engines.  Gotta love technology!

bill67 wrote on 09/07/12 at 10:10:55:


How's that diesel technology in your gas engine working for ya, Bill?

I just googled "Kia GDI",.. and it shows they do have it...

43 mpg's is as good as my little Geo Metro...


Bill ain't always wrong... ...

Yep.  The current issue of the American Motorcycle Association has an article that I have not read yet - but I read enough to know the article is about potential problems at the E15 pumps and leftover fuel in the hose.  On a motorcycle that only needs a gallon or two of fuel.....the ratio of ethanol could end up being too high since most are only allowed a 10% ethanol maximum.

If you need a higher octane in a motorcycle, learn to find pumps that have a seperate hose for the good stuff.....or follow the Corvettes and BMW's to the pumps so they can fill the hose with high octane for you.