PDA

View Full Version : Compression ratios in NA cars versus forced induction



KnifeEdge_2K1
06-03-2004, 01:26 PM
I believe that in a forced induction engine (turbo or super charged) car, the engines have lower compression ratio's than a similarily powered NA car. I think this is because that since forced induction engines have denser fuel/air mixture they cant handle as high a ratio since there may be 1.5-2 times as much fuel and air in the piston. Does this seem right to any of you? I havn't done any research on this and im making this deduction plainly on common sense.

compare the S2000 engine and the Lancer Evolution engine. Both are 2 litres and put out similar power figures 240 and 280 (torque is way apart though).
but the s2000 engine is 11.1:1 and 8.8:1 respectively. but since the evo engine is turbo charged to a max of 1.4 bar stock. this comes out to be about equal in the amount of air/fuel in the piston. 8.8 * 1.4 = 12 (give or take)

DiabloVT
06-03-2004, 09:45 PM
sorry to put a spanner in ur works mate but u cant just multiply the compression ratio and the boost pressure to get an "overall" comp. ratio. doesnt work that way. the compression ratio is about the air AND fuel mixture in the combustion chamber. the boost readings from a turbocharger are simply air as no fuel is incorporated in the spooling process.

why NA cars have a higher compression ratio than similarly sized turbo cars (as in ur fair example) is that very simply (VERY simply) that NA cars rely on high rpm to produce the bulk of their power. the more air and fuel u can cram into a cyclinder and the more times u make it go round, naturally its going to make more power (of course the power producing stops at some stage but tahts another matter). with turbo cars, its about how much air and fuel u can cram into a cyclinder again, HOWEVER it isnt soley the amount of revs that balances the power, but the amount of boost provided by the turbocharger plays a bigger part. the lower the boost, the less air put into the 'chamber and its alot like a NA car. but the higher the boost, the more air put into the 'chamber, the less revs it needs to make the power.

so summing up, NA cars need a high compression ratio to make power as power is determined by how much air and fuel can be put into a combustion chamber and how many revs the car produces. turbo cars dont have as high a compression ratio because the added boost of air supplied by the turbo charger essentially makes up the difference and then some, not needing high revs to produce the required power.

well thats atleast what i reckon ;) :D

KnifeEdge_2K1
06-03-2004, 10:15 PM
hmm i thought multiplying would be a good rough estimate since in N/A cars you try to get complete combustion (no left over air or fuel) in a turbo car running 1.4 bar which is 1.4 times atmospheric pressure hence 1.4x amount of air, you'd need 1.4 times as much fuel (more air and same fuel isnt going to do shit)

meh maybe im wrong, guess there's a first time for everything :D

megotmea7
06-03-2004, 10:53 PM
n/a cars rely on their compression to increase their volumetric efficiency (displacment of the cylinder vs. amount of air that actualy makes it there in normal operation displayed as a percentage. i.e. 50% VE means in a 2 liter engine 1 liter of air is being displaced by the sum of the cylinders in one power stroke where 100% VE means 2 liters of air are displaced in the same situation but 100% VE and above is very rare outside of forced induction engines mostly has to do with the design of the intake plenum and intake restrictions on N/A cars) a cars highest VE usually occurs at its torque peak, a high compression N/A car sucks more air on its intake stroke(more air than a low compression engine) creating a vacuum closer to the valves pulling more air in and increasing intake tract velocitys esp. at higher RPM. on forced induction cars the turbo increases the VE buy compressing air before it reaches the cylinder pushing more air in creating 100%+ VE depending on the pressure. turbo cars dont run higher compression as their a:f mixture is already compressed and heated to a certain point, the compression by the cylinder increases heat and the instability of the fuel, if too much boost and too much compression combine it creates heat and detonation (pre-ignition) along with it, which needless to say isnt good for the engine(esp. on a rotary ;)) hope that helped, any more questions fire away!

KnifeEdge_2K1
06-04-2004, 02:48 PM
hope that helped, any more questions fire away!


so basically im right, the na cars depend on compressing the airfuel mixture in the piston while forced inductions do it before the intake manafold with the ... turbo/supercharger

Matra et Alpine
06-04-2004, 06:32 PM
so basically im right, the na cars depend on compressing the airfuel mixture in the piston while forced inductions do it before the intake manafold with the ... turbo/supercharger
erm, only a small part of the 'compression' is by the turbo/charger.
They increase the volume of air going IN to the chamber.
That volume of air can take more fuel charge.
THEN the piston does the compression, as pointed out being denser and with more fuel then it can't take a lot of compression - mainly coz it will detonate prematurely.

SilverArrowZ
06-04-2004, 07:15 PM
hmm i thought multiplying would be a good rough estimate since in N/A cars you try to get complete combustion (no left over air or fuel) in a turbo car running 1.4 bar which is 1.4 times atmospheric pressure hence 1.4x amount of air, you'd need 1.4 times as much fuel (more air and same fuel isnt going to do shit)

meh maybe im wrong, guess there's a first time for everything :D

1.4bar doesn't mean you have 1.4x the normal amount of air sucked in, it is 140% more air sucked in (minus the inefficientcy of course). I'm no expert but from common sense, with the intercooler you should know that the air that goes into the cylinder is denser, which doesn't mean more air volume, but more like more oxygen. I'm not sure about the ammount of fuel pump inside but i heard the tipical air fuel mix for turbo is 13:1 while NA is 15:1.

KnifeEdge_2K1
06-04-2004, 08:16 PM
1.4bar doesn't mean you have 1.4x the normal amount of air sucked in, it is 140% more air sucked in (minus the inefficientcy of course). I'm no expert but from common sense, with the intercooler you should know that the air that goes into the cylinder is denser, which doesn't mean more air volume, but more like more oxygen. I'm not sure about the ammount of fuel pump inside but i heard the tipical air fuel mix for turbo is 13:1 while NA is 15:1.

1.4 bar is 1.4 standard atmospheric pressure ... doesnt that mean 1.4 tiems amount of air ... after all 140% or air is the same as 1.4x ...

i never ment to imply it was volume but i ment in terms of ammount