I am trying to discuss with some one about the compression ratio on a n/a engine.

It's my understanding that changing the comp ratio ie raising it would affect how the ignition timing should be set, but, I'm not clever enough to explain why properly....can someone help?

Cheers

Matt

 

Hmm..
Going out soon so this is VERY tricky to explain simply and fast....

This post in fact could shoot down all those peeps i see quoting "More advance" as the reason to go low comp, so it may be usefull and educational....

Suffice it to say you ARE correct and it will need retarding, and if no ones given you a decent explanation next time im bored and have some time il have a crack for ya

Come on guys.. good techie post so get your thoughts down for us

 

stu u going out i have a piccy for u

 

Thanks, looking forward to it. I think this may be the only interesting thing I've ever asked

 

Fook.. After much thought and consideration, maybe il stay in tonight!!

 

pmsl no the pic we talked about earlier m8 (head) bastard cameras flat

 

Damn.. my pictures are at work mate.. il send a few over tomoz if ya remind me

 

where u going

 

lol, I'm totally battered, so it aint gonna be me.

 

Unsure, just fancy a spin and some chillin time....

Your house for a brew?

 

im very bad at putting things into words


higher compression ratio means a faster flame front! and as we want max cylinder pressure at the right crank angle, we have to retard our ignition to allow everythign to happen at the right time!

 

pop donw here if your bored stu

 

gareth your exactly right thus making the burn cycle/engine more efficiant with less advance the fuel burns faster

 

Ignition timing is directly linked with cylinder pressure/temperature and combustion chamber efficiency, BUT of equal significance is cylinder head design. The design and efficiency do of course go hand in hand, but there are some exceptions to the rule.

It is far to complicated to discuss as the variables are huge!!!

 

just say a raise in comp ratio with all other things remaining equal then.

In as easy terms as possible as i'm not brilliant with all this although i do have a basic grasp of the combustion cycle.

 

Generally speaking if you keep all engine variables the same and simply increase the compression ratio, this will require less ignition advance, allthough again it really is'nt as simple as that as there are far to many other variables.

 

By advance, does this mean that the spark happens before tdc..the advance bit is what confuses me a bit. Sorry for being dumb

 

yes

 

yes for example lets say u want max push on the conrod at 20degrees after tdc with a higher comp engine it may want to be ignited say 3 degrees before tdc but if you lower the comp u will then need to fire it may be at 30 degrees before tdc to get the max push to be in the same place as the fuel will burn slower in a low comp engine. hth

 

gotcha...the comp ratio changes how quickly the combustion happens.

 

Gus, ignition advance means before TDC not after.

There's no conventional engine that requires ignition after TDC.

Typically we fire the spark plug anywhere in the region of 3 Deg BTDC to 60Deg BTDC depending on engine spec/type, rpm and load, etc.

 

Sorry to jump in here, Karl you checked your pm's mate?

 

like i said for example in lame mans terms hence why he understood the reply i knnow theres no engine that fires after tdc m8 i was just simplifing it perhaps ill alter it just for u

 

Gus edit it again, you've still put AFTER tdc.

Damo yes mate just checked, will reply shortly.

 

Don't alfa twin sparks fire once just before tdc and once just after??

 

I had high hopes for this post but theyve been shattered...

A quick description in my own (very drunken) words

Spark timing is expressed in degrees before top dead center (BTDC)

We have spark timing as its necessary to light our mixture and begin the burn cycle at different times with different efficiencies and speeds to achieve our ultimate aim, maximum gas expansion rate at around 15-20degrees ATDC (Top marks Gus )

Due to the fact a given mixture will always take "X" time to complete its burn time, we need to change the time we light it in relation to our engine speed.

Example:
We have a set amount of fuel and air in our cylinder at 3000rpm, and to get maximum expansion we have to light it at say 20 degrees BTDC to get the mximum pressure from combustion to occur at our ideal 15 degrees after top dead center due to the set time it takes this mixture's combustion to progress from being unlit to maximum burn pressure.

So what happens when we have EXACTLY teh same amount and temperature of mixture in our cylinder but the engine is at 6000rpm instead of 3000 as before?

The answer is, we will have to light the mixture at least twice as early as the mixture will take the same amount of time to process in its burn, but our engine is moving twice as quickly!! So to get the same pressure at 15degrees ATDC we will have to initiate the burn twice as early at 40deg BTDC this time...

So what requires a DECREASE in spark advance?
Anything that increases the cylinder pressure and thus raises burn speed.
Examples:
Decreased Air temp. (IF large Density increase)
Decreased Air temp. (Temp threshold)
Leaner mixtures.
Higher Compression.
More air/fuel in cylinder at same air temp.
And much more.........................

The thing to remember is that Spark advance is not something we DESIRE, its something that the engine needs. The higher the compression, the more efficient the engine will be as an air pump. As we decompress the engine we will require more advance NOT because we wanted to achieve this, but because we have DECREASED the engines efficiency and the advance is now necessary to achieve PCP anywhere near 15-20deg ATDC because our mixture burns slower in our decompressed and more inefficient engine.....

Hope this helps now cos i need to sleep.. too much beer guys...

 

Stu,

I didnt elaborate because its not as simple as you have written.

A high comp engine is not more efficient than a low comp all the time. It's only at certain rpms/loads!!!!!

 

Sorry peeps, but aren't we talking N/A?

 

If it was as simple as i have written is this drunken state we would be prety in s erious trouble Karl loll

Its also very unlikely the engine will ever be more efficient in any condition apart from idle as a LC unit as you know. There are rarely higher pumping losses than gains at HC in a modern engine once stall speed is exceedef.

 

Yes. Why?

 

Agreed Stu but you'll still be suprised at the effect C.R has on the engines efficency against rpm/load.

It's too complicated to discuss here but I do NOT agree with high comp and least ignition advance as the best method.

Yes, less ignition by definition implies greater engine efficency but it does not always result in a net gain.

This is why I am a big lover of low comp engines as I have a prefered set of parameters I like to keep within.

The rest is very much secret squirrel.

 

I may well not be rthe only one who is pissed here it seems...
You are aware we are ONLY talking about High Compression NASP motors with Matt (Topic sTARTER) arent you Karl?

 

Bedtime for me karl, speak tomorrow mate

 

No mate I'm talking forced induction, seems I've lost track of the topic allready.
LOL Bed time for me.

 

We can discuss both if you want....make the thread bit more interesting for the majority on here.......

 

now that is bloody funny

 

like i said stu,, but a little better

 

Yes Gareth, just as you said matey