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Compression Ratio, Load Point, Ignition Timing, Injector Duty?

SKY 34

By SKY 34,
in Engines & Forced Induction

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SKY 34 Mentor

SKY 34

Posted
Posted

hi,

wondering what those terms means..I have a vague idea..but not so sure...

-compression ratio..it's always like 9:1 or 8.5:1

what does each side of the value represent?

and ppl tend to bore the head of the cylinder a bit to turbocharge an N/A cars...in effect lowering the compression ratio, for the turbo to kick in, how does that work? why boring the head reduce com ratio? what happened to the head gasket then?

-injector timing,

on the pfc is always measured in degrees, how does that work?

-injector duty

injector duty is shown in %...and usually at idle the % is low abt 1.4%?

so let's say I have a 440cc injector....does it mean the injector is only putting in 1.4% of the 440CC?

load point...

which I have no idea...

thanks~~

MerlinTheHapyPig Mentor

MerlinTheHapyPig

Posted
Posted (edited)

-machining out parts of the head increases the combustion chamber area which changes the compression ratio

compression ratio is volume of air/fuel before compression compared with after (i think)....

so 8.5:1 is the ratio in which the volume of air is compressed in the cylinder

-injector timing, you may be thinking ignition timing, which is where the spark occurs, angle is the crankshaft angle. BTDC is "Before top dead centre" so if timing is expressed as for example, 12degrees BTDC (which is 12 degrees before the piston reaches top of it's travel on compression stroke)

-injector duty, put simply, this is how hard the injectors are working. at 100% they are at their max capacity, if your injectors are at over 90% duty cycle (at high load), it's a good idea to get bigger injectors.

-load point, probably talking about where in the car's map the ecu is reading timing advance and fuel enrichment values from. (ECU reads data from a 3 dimentional table, RPM vs TP -- TP is the engine load (theoretical pulsewidth)

Edited by MerlinTheHapyPig
Beer Baron Grand Master

Beer Baron

Posted
Posted
compression ratio is volume of air/fuel before compression compared with after (i think)....

so 8.5:1 is the ratio in which the volume of air is compressed in the cylinder

sorry mate, so far from right it's not funny.

"the ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely."

-injector timing, you may be thinking ignition timing, which is where the spark occurs, angle is the crankshaft angle. BTDC is "Before top dead centre" so if timing is expressed as for example, 12degrees BTDC (which is 12 degrees before the piston reaches top of it's travel on compression stroke)

yeah that is pretty close.

-injector duty, put simply, this is how hard the injectors are working. at 100% they are at their max capacity, if your injectors are at over 90% duty cycle (at high load), it's a good idea to get bigger injectors.

not quite. duty cycle is the amount of time the injectors are pulsed to 'open' during a cycle. so 100% means they are open 100% of the time.

-load point, probably talking about where in the car's map the ecu is reading timing advance and fuel enrichment values from. (ECU reads data from a 3 dimentional table, RPM vs TP -- TP is the engine load (theoretical pulsewidth)

the load point description is ok, but generally mapped on RPM vs load (afm) and TP is Thottle position. i only have a small knowledge on this so someone else may correct me in a minute.

MerlinTheHapyPig Mentor

MerlinTheHapyPig

Posted
Posted (edited)
sorry mate, so far from right it's not funny.

"the ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely."

isn't that what i wrote more or less?

maybe my wording was a bit confusing, i think i said "volume of air/fuel" when i should have said "volume"

the load point description is ok, but generally mapped on RPM vs load (afm) and TP is Thottle position. i only have a small knowledge on this so someone else may correct me in a minute.

Sorry mate, so far from right it's not funny :-p (now we're even)

TP is Theoretical Pulsewidth (when talking in tuning terms)

It's a mathematically calculated value by the ecu used to determine the duration the injectors need to open to deliver a 14.7:1 A/F ratio.

It's something along the lines of...

(VQ value * Injector K value / (rpm * 256/60)) / number of injectors

^ can't remember where the 256 and 60 values came from, I was copying formula from a spreadsheet and working backwards...

Edited by MerlinTheHapyPig
Beer Baron Grand Master

Beer Baron

Posted
Posted
isn't that what i wrote more or less?

maybe my wording was a bit confusing, but that was my understanding...

no, in tuning terms, TP is Theoretical Pulsewidth

It's a mathematically calculated value by the ecu used to determine the duration the injectors need to open to deliver a 14.7:1 A/F ratio.

It's something along the lines of...

(VQ value * Injector K value / (rpm * 256/60)) / number of injectors

^ can't remember where the 256 and 60 values came from, I was copying formula from a spreadsheet and working backwards...

no, compression ratio has nothing to do with the ratio of air to fuel, either before it's burnt, after it's burnt or at any other point. it's a space thing. are you sure you read my post??? :(

edit: actually it is kind of what you were getting to, talking about cylinder volume, but it's not a raito of air/fuel. but i guess you had the idea, just didnt express it.

ok, theoretical pulsewidth is not a load axis. it's a value inputed by a tuner to tell the ecu the injector pulsewidth. it is not a load axis or load point.

MerlinTheHapyPig Mentor

MerlinTheHapyPig

Posted
Posted (edited)
btw i can tell you engine load is not expressed in pulsewidth..........

maybe, but the TP calculation includes engine load,

the engine load i guess, would be the VQ value (voltage quotient) which is calculated from the AFM voltage (looks it up in a table -- VQ table)

this value is used in the TP calculation, which one of the axis on ecu timing and fuel maps, so some people refer to it as the "load axis". The fuel map works out fuel on an enrichment/enleanment from 14.7:1 ratio. So .20 on a fuel map represents 14.7:1.2 so 12.25:1 this is where TP value is used to work out how long injectors need to open to deliver the 14.7, then the value in the fuel table scales this value to give the desired A/F ratio

anyway, most people don't really have to worry about how all this works, unless you are doing the tuning.

Edited by MerlinTheHapyPig
Abo Bob Grand Master

Abo Bob

Posted
Posted
hi,

wondering what those terms means..I have a vague idea..but not so sure...

-compression ratio..it's always like 9:1 or 8.5:1

what does each side of the value represent?

and ppl tend to bore the head of the cylinder a bit to turbocharge an N/A cars...in effect lowering the compression ratio, for the turbo to kick in, how does that work? why boring the head reduce com ratio? what happened to the head gasket then?

I stole this:

"The first thing to understand is that "compression ratio" (CR) as it is usually talked about is best termed "static compression ratio". This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 500cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 500/40, or 12.5:1. Conversely, if we hogged the chamber out to 60cc, the CR would now be 500/60, or 8.33:1."

from here

To answer the other questions you have put; People don't lower the CR 'for the turbo to kick in', they do it to lower the detonation threshold. In other words, the higher the CR the closer you are to detonation and therefore engine destruction.

Non turbo engines can handle much higher CR than turbo engines. I'm not sure that I can explain why properly. So if you are turning a non turbo engine into a turbo one you need to lower the CR to lower the threshold at which detonation becomes likely.

Yes you can use a thicker head gasket to lower the CR but this is a cheap and nasty way to do it which is why if someone is doing the job properly they will machine the combustion chamber instead (boring the head). They do this so that when the fuel and air comes into the cylinder it gets mixed together properly. You need as homogenous a mixture as possible because without it you can get hot spots which can again cause detonation.

The shape of the combustion chamber makes this happen partially by use of squish zones. This is the area between the part of the head that comes closest to the piston and the piston when it is at the top. What these do is squish the mixtures out as the piston rises mixing them better. If you use a thick head gasket instead of taking bit's of unneeded metal out from the combustion chamber you effectively delete the squish zones meaning less homogenous mixtures in the cylinder.

I'm sure it could be explained better by someone more knowledgable than me but that's the gist of it.

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