gasoline compression ignition?

Jared_80

10-31-2003, 12:08 PM

Automotive engineerrs are always trying to raise compression ratios as high as they can without causing detonation with a 17:1 CR being the "perfect" compression ratio. So why cant they just inject the fuel just before TDC on a high compression ratio engine? They could even remove the spark ignition system altogether, They would only need a heating coil for coldstarts. This would also alow for a more versatile fuel control, just imagine a high reving desiel engine with a faster combustion. This would have a tramendous power potential. If any of you an find any fault in my thinking please let me know.

Mendari

10-31-2003, 12:17 PM

It might be possible for a short duration while the engine is relatively cool. Running for extended periods will require an advanced cooling system. Almost anything is possible with engines. However, when it comes time for mass production, the accounting department has the final word.

Jared_80

10-31-2003, 01:53 PM

texan

10-31-2003, 10:46 PM

I do not think that designing a cooling system for this would be that difficult, because other compression ignition engines use up to a 26:1 CR and they seem to hold up better than normel engines, especaly under long duration-high stress applacations. And if these cooling systems cannot keep up during extreamly high stress periods (aka racing) Have a water injection system as an internal auxilary cooling system.

But those "other" engines (you mean diesel) use a fuel suitable for compression based ignition. They are also throttled differently, because again the fuel is suitable for such purpose. Gasoline is much more volatile than diesel; specifically it ignites over a very large temperature range based on small changes in a/f ratio. Injecting the fuel at the last second would, among other things, result in a very non-homogenous mixture. And thus one that doesn't lend itself to efficient or accurate compression based ignition and combustion.

Moppie

11-01-2003, 05:11 AM

Please take the time to at least read the existing threads on the first page of a forum before starting another one.

This topic is already pretty well covered here only 3 threads down: http://www.automotiveforums.com/vbulletin/showthread.php?t=146815

Jared_80

11-03-2003, 02:16 PM

This non homogenous mixture that you are talking about, would that not depend on the injectors. Mitsubishi has been working on direct port fuel injecting engines for quite some time now, and from what they are saying they are spraying the fuel directly on to the sparkplug just before TDC. Their test engine is said to get 30% better fuel economy than a conventional modern engine of the same configuration. With a 30% improvment I would not think that the injectors are as ineffficient as you think. They claim that they can use as lean as a 30:1 air to fuel ratio by doing this. Another way to better mix the fuel and air is to do what the do on high output diesles use a multi-pulse injection. Some of these systems can spray five individual micro pulses of fuel for a more controled combustion, and a better mixture.

texan

11-04-2003, 01:49 AM

You're missing the point. All the direct injection gasoline motors use spark based ignition, because the advantage to using the direct injection is found in being able to vary the mixture ratio over a very large area (12:1-25-30:1). With such a huge change possible, the only ignition method that would be logical is spark based ignition. If the compression isn't variable but cylinder filling and a/f ratio are there's no way you could reliably ignite the mixture at the proper time with compression based ignition.

I wasn't aware the direct injection motors injected right at TDC, but at any rate you get my point from the above paragraph.

Jared_80

11-04-2003, 09:26 AM

Yes I understand your point, and I don't think that all direct port injectors spray on TDC; that is the only one that I have ever heard of to do that, but if you could mix it well enough it would certainly make sence.

Jared_80

11-04-2003, 09:34 AM

Two more questions what qualities does desiel have that makes it a good compression ignition fuel, that gasoline and alcohol do not have? (other than a higher BTU rating) And does the slow flame front in a desiel engine limit its power at higher RPM? (aka not combusting entirely before BDC)

Moppie

11-04-2003, 10:55 AM

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gasoline compression ignition?