when they give a ratio of say 8:1

8 of what to one of what ?

The amount of air in the cylinder when the piston is at BDC, is compressed 8 times when at TDC.

 

Si.

Yup, or more specifically: swept volume + combustion chamber volume / combustion chamber volume = copmression ratio

Yup, or more specifically: swept volume + combustion chamber volume / combustion chamber volume = copmression ratio

 

:confused: :confused: :confused: :confused:

 

Yeah what he said :D

Doesn't piston volume, i.e. valve cutouts, or any other piston crown mods, come into the equation somwhere ?

 

Robin

Any picton crown mods or valve cut outs would affect the combustion area and so would come into both sides of pauls equation :)

 

The attached drawing may help :)

so in laymans terms the more compressed the mix is the more energy is generated ?

do turbo's have lower compression, as a portion of the compression is carried out , outside of the chamber ?

thanks for the replies

No turbos generally have lower compression ratio because there is already more air in the cylinder before it's compressed, therefore the resultant pressure at the point of combustion is greater - hence more detonation (and expense!)

so they force more in before compression is started as the inlet valve is still open ?

A turbo does kind of pre compress in that sense so as to get more O2 and fuel into the given cylinder space. But there is an upper point to the maximum pressure you can have hence the low ratio on turbos as the mix is liable to ignite itself well before the piston is all the way to the top. Detonation pinking etc. The explosion then tries to send it back the way it came with huge force shock and heat that can quickly melt pistons. So while you want as much air and fuel in the right ratio at the highest pressure short of self igniting you want it to ignite only at the right time to give an exploding mix that puts max push on the piston just as its on its way back down the cylinder. Think of it as on yer bike when you push the pedal as it goes over the top. High octane fuel makes this early igniting less likely. Timing is everything thats also why the ignition timing advance changes with engine rpm and piston speed. The exploding flame takes a certain time to reach maximum push so you have to ignite it earlier and earlier at higher rpms to be ready to catch the piston on the way back down again.

I bet he's sorry he asked!!

The normal air pressure is about 14psi. Without a turbo the cylinder is filled with air at this pressure (plus fuel) when the valves are shut and the piston starts rising this pressure wil increase to 14 * compression ratio. With the turbo the starting pressure is 14 + turbo pressure, so if you have 14 psi then you are doubling the pressure. The reason why people with more than 14 psi don't get double bhp is because the air can't get into the cylinder in time. This is probably why torque peaks at 3000rpm and then starts to reduce. Also why sticking on huge turbos and ICs is a law of deminishing returns.

 

There must be a specific pressure when a certain air fuel mixture explodes at a given temperature.

 

etc...

[pedant mode]the fuel/air mix doesnt "explode", it burns & rapidly expands![/pedant mode]

There must be a specific pressure when a certain air fuel mixture explodes at a given temperature.

 

There is a specific temperature (if you take it as given that an increase in pressure gives an increase in temperature) at which a given substance will ignite when flame is applied (A Flammable Liquid is one with a flash point of less than 55°C, a Highly Flammable Liquid is one with a flash point of less than 21°C ) and will auto-ignite (The ignition temperature of a substance is the minimum temperature required to initiate or to cause self-sustained combustion without the need for a spark or flame)

 

And you probably could work out all the variables in a theoretical situation given the data on air density, air fuel ratio etc etc, but in reality there are too many variables to give a workable answer. And I cant be arsed.

so to speak the trade off is volume and speed of air against a diminshing compression ratio ? if so what would the ideal be for maximum power at lower revs ?

A little titchy turbo like the latest generation twin turbo diesels.

 

The new Vectra twin turbo diesel for 2005 is supposed to have 220bhp from 1.9litres and 47 mpg. That equals stock TT auto performance with 47mpg or about 30mpg better than mine does.

 

The icky little turbine does from 700rpm till about 1400rpm when a big one takes over up to 5000rpm.

 

These new small turbos will revolutionise low speed torque possibilities

 

Anyone for a diesel zed?

I thought pinking was an explosion because of temperature/pressure and normal spark triggered burning was a 'controled' burn. Diesels bang petrols burn.

 

Go ahead burst my bubble.

Well you asked for it - If the timing is too advanced the nomal spark triggered burning will make it pink because the burn pressure wave is against a rising piston ;) gona burn this anorak