Whats going on guys. I currently work at a shop changing tires, changing the oil, and some engine work. My boss ( The owner) was talking trying to teach me about the compression ratios in a car and how they work, and honestly i couldn't understand maybe because of the way he talks lol. I am 18 years old and graduating this year. I am going to college for mechanical engineering, but want to have some knowledge on the way ratios works.. If you guys and gals can help me out and explain to me how it works that would be awesome!!


Thanks
-Andrew

 

It's mechanical engineering

Check out this link:

http://auto.howstuffworks.com/engine1.htm

See figure 1 for the 4 cycles :

1. intake:piston is pulled down by the crankshaft, draws in air and fuel (this is whate generates manifold vacumn)

2. compression: piston compresses the mixture on the upstroke

3. power: spark plug fires igniting the mixture, pushing the piston down causing the crankshaft to do work

4. exhaust - crankshaft pushes the piston up forcing the exhaust out

Look around that site- you can learn a lot

 

The compression stroke in a 4 cycle engine is simply pushing the air/fuel mixture from a large volume into a much smaller volume and thus compressing it. The spark then ignites it and the rapidly burning gases push the piston back down the bore.

 

hehe... i honestly completely forgot about that site..

lol thanks bro

 

thanks for your reply, but i know how the inside of a cylinder works but i am trying to figure out how the compression in a piston works such as 9:1...

 

Where you asking about compression ratios?

It is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.

 

Thanks Maui

 

A high-boost, low-compression engine will lack of low-end power until the blower boosts because the power comes from the blower. Stock supercharged cars typically have lower compression. The new ZR-1, which is blown, is 9:1 while the LS-2 car is 10.9:1. Higher compression engines need higher octane fuels to eliminate engine knock. Knock occurs when the gas is ignited by compression rather than spark. Knock is bad for the motor. Lower octane fuels are easier to ignite than higher octane fuels. The advantage of lower compression engines is the ability to use lower octane fuels (lower price). Higher compression engine will work better with a centrifugal superchargers because the blower does not really kick in until you get up to 3500 to 4000 rpms. A high compression will make up for the lack of boost on the lower end. A maggie is more suitable with a lower compression engine because the boost is instant.

 

compression ratio: the volume of the cylinder when the piston is at the (bottom of the stroke + volume of the combustion chamber) divided by the the volume of the combustion chamber.

example: 3.5" bore diameter and 4" stroke. volume of cylinder = PIxR^2
3.14159x(3.5/2)x(3.5/2)x4 = 38.4845 cubic inches

38.4845 cubic inches x 16.38706 to get 630.6481 cubic centimeters
combustion chamber volume per manufacturer lets say 68 cc

compression ratio = (630.6481 cc + 68 cc) divided by 68 cc = 10.27

thus compression ratio of 10.27:1

If you really want to get accurate, add in the volume due to the thickness of the compressed head gasket.

 

 

Is that a group hug i see?

 

very informative...great info guys.

 

There are a lot of intelligent people on this forum. But i can pretty much brake this down for you to understand it.


The way that compression ratios are numbered are quite easy to figure out once you understand.

For example, if I have a text file thats 1000K, then run bzip2 on it (probably the best widely available text compressor), it's 200K, then the compression ratio is 5:1.

That is just an example that i found on the internet so you can see how it works, basically just simple division.

How a compression ratio works:

The compression ratio for a piston internal combustion engine is computed by measuring the volume of the piston cylinder at it's greatest volume (when the piston is at it's maximum excursion) and comparing it to the lowest volume (which occurs when the piston is at top dead center, TDC).
Typical compression ratios for gasoline engines are in the neighborhood of about 9:1. A higher compression ratio gives more power out of an engine, but requires the use of a fuel with a higher octane rating. This increase in power (both horsepower and torque) causes engine components to wear more quickly and the engine must be kept in better operating condition.


It is really simple math, nothing more nothing less. When you go to school for Mechanical Engineering they will teach you all that kind of stuff.




Thanks

PS: By the way nice name (andrew) i have the same name

 

...............and then there's dynamic compression ratio