Could someone please explain compression?

I understand what technically is happening(it's the spark plug fire and subsequent movement of the piston....I think), but it's the number ratio part that I don't understand.

For example, I'm aware that our cars (3G TL) have a fairly high compression ratio. Could someone explain how that is beneficial? Does it have anything to do with performance, rev-happiness, sound?

Why do diesels have such high compression that would be "unthinkable" in a gas engine?

Any explanations would be appreciated.

 

Last first. Diesels have no spark plugs so to have the explosion, the fuel is compressed quickly under high pressure, elevates fuel temperature and explodes, so a high CR is needed.

Rather than writing a synopsis on compression in a gas engine:
http://en.wikipedia.org/wiki/Compression_ratio

 

^^^^ Good shit

 

Suck, squish, bang, and I can't remember the one for exhaust lol. It's a 4 stroke so you have an intake stroke, compression stroke, power stroke, and exhuast stroke.

The compression ratio is how much the intake charge is squeezed down on the compression stroke. Piston goes down to suck in air/fuel on the intake stroke. The piston is at the bottom. Now it starts it's way up in the cylinder compressing the charge. If your car has an 11:1 ratio, the charge is compressed down 11 times from it's original volume. The more it's compressed the more heat it makes. Too much heat and the charge ignites before the spark plug sparks while the piston is still moving up. This results in preignition and/or detonation which greatly increases cylinder pressure and stress on parts.

Diesels use this extra compression to ignite the charge. I didn't know it when I was younger but diesels don't run on detonation and they can detonate if tuned poorly.

A turbo increases cylinder filling so it increases your effective compression ratio (not your mechanical compression ratio which is always 11:1). If you're filling each cylinder at twice it's normal volume at 15psi boost, you're effective compression ratio is now 22:1. It's easy to see why boost raises octane requirements.

 

I only had about 2 minutes to write that last part.

Benefits of higher compression are better mpg, more power, and slightly quicker spool on a turbo car. Downsides are a higher octane requirement, less boost friendly, higher NoX emissions.

Diesels get away with it partially because they're direct injected. No fuel enters the cylinder until a very high pressure injector located in the head (not intake manifold like traditional port fuel injection) squirts it in making it pretty much impossible to preignite. There are some exceptions.

We're starting to see gasoline street cars with direct injection and some have 12-13:1 compression ratios to take advantage of the direct injection.

 

Thanks for the replies; I definitely have a better technical understanding of it now!

 

Another thing to understand is octane, which is the ability of the gasoline to resist detonation prior to a spark. The higher the octane, the more than you can compress and heat it before it ignites.

A third thing which can start detonation (other than temp or spark) are rough/jagged surfaces in the combustion chamber that. If you have ever heard of somebody "polishing" their heads, it is to make them as smooth as possible so that bumps and burrs don't start to ignite the gas under higher compression. The bumps send the gas flying everywhere causing more friction and thus more heat. I don't know if our TL heads are polished or not, but I am sure that the casting is better than it was years ago.

Aluminum heads/motors do a better job of getting rid of heat so that you can have a higher compression ratio.

Example of how it all comes together, I have a big block olds at about 506 cubic inches. With stock iron heads, I can run 91/92 octane and have about a 9.7 compression ratio and make about 380 horsepower - the iron does not cool all that well and the iron is rough. When I went to polished aluminum heads, the smoothness and cooler temps allowed me to go to 11:1 and make about 520 horsepower... and I might be able to go to 11.5:1 someday and make even more power. Of course, for the pros out there, they know that I also ported the heads which is another discussion for another time. By taking advantage of all of these things and using gas potential to the max, I have a motor can can get 15-16 MPG on the highway at 65 (if you are nice to the gas pedal), cruise with the air conditioner on going to a cruise night and then still go sub-12 seconds at the track.

 

I understand what you're saying but I have a feeling the majority of the extra power came from the extra airflow of the aluminum heads. The extra compression may be worth something in the 50hp-75hp range on a big block.

As I'm sure you know, if everything else was equal an iron headed motor will make more powe. You have to rum more timing, compression, or a higher coolant temp to make up for the iron heads greater thermal efficiency. Thats why I held on to stock iron heads for so long. I was going 10s with them and I was afraid to change anything. Eventually I needed the airflow of the aluminums. After getting everything sorted out I picked up about 100hp but it took a lot of tuning and another learning curve.

 

And (if I'm not mistaken), this "detonation prior to spark" is what people refer to as "backfire". One of the pros can correct me if I'm wrong on that, but that has been my understanding.

 

There's a lean pop which is when it backfires through the intake. Detonation really doesn't directly cause a backfire.

Prior to spark is preignition. Detonation is the uncontrolled explosion of the charge. It burns rapidly but is not supposed to explode. Preignition usually causes detonation. I always interchange these two words but they are slightly different.

Kind of off topic but the first ICE did not have compression so they were very limited on power and rpm.

 

Hmmm.... I'm a little confused here. So what causes backfire then? I know that fuel without enough octane will cause backfire and I thought this translated into pre-ignition (that results in detonation ?) since octane is the fuel's ability to resist detonation.

 

Unless I'm wrong, backfire is when unburned fuel is ignited in the catalytic converter.

"detonation prior to spark" is just detonation. That's all I've heard it referred to as.

 

Low octane fuel results in "pinging" which is the noise you can hear from your engine when detonation occurs.

 

I guess I cant edit messages since I'm still a newbie, but I meant knocking, not pinging, though I've heard it referred to as both.

 

I think low octane causing backfires is one of those myths that's still around.

Backfires can be several things but most commonly unburned fuel being ignited in the exhuast. A random misfire is a common cause.

If you have a very bad burned or damaged valve you can have an intake or exhaust backfire.

A lean mixture can cause a lean pop through the intake. I ran into this several times when running very high octane C-16 fuel and trying to get every last hp. With normal octane you run into detonation before you get the lean pop.

Very retarded ignition timing can cause an intake pop but it's not exactly a backfire.

On our old carbureted cars we used to rev them up to redline and with the gas pedal to the floor turn the ignition off and turn it back on right when the rpms got down to 1,000. This would usually result in a huge backfire. A friend of mine did this to his old '86 Dodge Ram van and ended up blowing the catalytic convertor and muffler up. We were on the LA freeway and people would not let him merge over so he made it backfire and everyone scattered but at the same time we had open exhaust when the engine came back on. The convertor blew apart at the seam and the muffler unwraveled itself.

Same guy with his parent's Cadillac. I was behind him and he made it backfire to scare some kids. I saw the muffler which was mounted behind the rear bumper unravel and hang down right in front of me. These backfires are unreal when you do it this way.