question: Low CR vs. High CR for forced induction?

[AE82FROG]

anyway, higher SC or DC results in a steeper rise in pressure and a higher peak, whereas lower compression has slower rise and lower peak.
comparing NA vs turbo, the peak cylinder pressures might be lower in turbo.. but the area under the graph is greater, ie, there is more oomph to push the piston down for longer

Hey dude, just to clarify (some people might misunderstand): that graph is indicating both the NA and turbo setup are starting with a peak pressure of 1000psi, as explained in the article text.

Peak cylinder pressures aren't lower in a turbo setup. Peak cylinder pressure in any engine is determined completely by its state of modification no matter whether turbo or NA. It is probably safe to say though that in general turbo engines run higher peak pressures than NA - hence the greater potential for damage when modifiying turbo over NA. But again it all depends on how it is modified. A high strung NA 4AGE engine that has to run race fuel is obviously seeing higher cylinder pressures than a turbocharged one running premium.

Turbo could still have more power though because of the greater combustion chamber size

[mtp_69_i]

Sorry, didn't mean for it to be confusing. In the work i do i come across heaps of people that always state "use low comp pistons if you are going to turbocharge, otherwise the engine will blow up"

But it is not the combination of boost and high static comp that will blow the engine up, but rather making the cylinder pressures too high - as they try and chase the sort of power outputs that other people have achieved by using low comp pistons.

High comp turbocharging can be completely acceptable and safe, but you have to realise the ceiling of achievable power will always be lower than using a low static comp setup because you are working with a smaller combustion space.

so in essence, turbocharging allows you to increase the volume of the combustion chamber (your displacement?) while maintaining (or even increasing!) the compression ratio to yield higher torque/power...

yeah?

[The Witzl]

so in essence, turbocharging allows you to increase the volume of the combustion chamber (your displacement?) while maintaining (or even increasing!) the compression ratio to yield higher torque/power...

yeah?

that's kind of an arse backwards way of thinking about it, from my perspective.

Let's just simplify what an engine really is:

An engine is essentially an air pump.
Petrol is the catalyst that drives the engine.

When the piston moves downwards, it creates a lower pressure inside the cylinder, and thus for a N/A engine the greater air pressure (atmospheric pressure) present at the intake of the engine forces air into the cylinder and it is mixed with petrol.
The rotational inertia of the engine compresses this mix of fuel and air as the piston then moves back upwards.
This mix is then ignited, forcing the piston downwards.
.... you all know the rest.....

Now - in the case of a turbocharged engine, you are using the byproduct of this air pump (the exhaust) to drive a compressor turbine. This compressor turbine creates air pressure higher than atmospheric present at the intake - thus more air is capable of entering the cylinder during the intake stroke.

More air = more oxygen
More oxygen = more fuel can be added to maintain correct air/fuel mix
more fuel burnt = more energy released from cumbustion
====> MORE POWAAAHHHZZZ


This is the most simple physics method of explaining how a turbocharger increase engine power output.

from this you can then extrapolate conclusions about what happens to cylinder pressures etc etc.

[mtp_69_i]

chamber pressure/volume limits how much air can be pumped in though. by reducing compression (increasing volume) you can increase the amount of air (and therefore fuel) in.

[The Witzl]

chamber pressure/volume limits how much air can be pumped in though. by reducing compression (increasing volume) you can increase the amount of air (and therefore fuel) in.

There are many factors that determine this statement, so although as a broad statement it is generally correct, the question as to "when is too much" has too many factors involved to make this statement universally applicable.

[mtp_69_i]

sure, but your previous post didn't seem to take it into account.

[The Witzl]

yeah i spose it didnt.... i was just trying to place the seed of calculation You know, get your brain thinking on the right track, and let you take it from there.

You have done that, and so my work here is done

[mtp_69_i]

lol, thank you sensai

[mic*]

chamber pressure/volume limits how much air can be pumped in though. by reducing compression (increasing volume) you can increase the amount of air (and therefore fuel) in.

Peak chamber pressure is an operational limitation based on many factors, most directly mechanical & fuel octane rating.

In a naturally aspirated situation, by reducing the compression you reduce the motor's volumetric efficiency (simple terms; ability to "suck air") so the amount of air/fuel is reduced. Actual displacement (when revving) = static displacement X volumetric efficiency. You also reduce the peak pressure so power stroke is weaker.

The increased combustion chamber size brought about by a shorter piston/rod assembly in a forced induction situation does NOT, BY ITS SELF allow more power. It DOES allow more air/fuel mix (displacement), but it lowers the compression, which lowers the peak (& average) pressure and = weaker a power stroke. BUT, it allows the boost pressure to be increased and still be within the same peak pressure limit.

Increasing boost effectively increases displacement. Actual displcement roughly = static disp X VE X pressure ratio relative to atmospheric. So basically 1 bar boost has the effect of doubling your displacement. 2 bar boost = triple, so on and so forth.

The way i have decided i like to look at it is;

-Boost increases displacement of a cylinder at the expense of higher cylinder pressures.
-Higher static compression causes a greater "spike" in a graph of cylinder pressure.
-Lower static compression acheives a smoother curve in a graph of cylinder pressure.
-Power is a function of the area under the curve.

How you mix and match them to your peak pressure limit comes down to your intended use.
But very low comp makes starting harder. Off boost efficiency & power is out the window.

[Mr DOHC]

the new HSV VXR Astra runs 11.5:1 and 10psi, thats why it makes 340nm from a 2L

[JustCallMeOrlando]

the new HSV VXR Astra runs 11.5:1 and 10psi, thats why it makes 340nm from a 2L

No it doesnt, it runs 8.8:1 compression.

[Mr DOHC]

the article said 11.5,

[kingmick]

the new HSV VXR Astra runs 11.5:1 and 10psi, thats why it makes 340nm from a 2L

Suprising, That leaves no saftey margin at all! One dose of bad fuel and it would be all over or someone to put low octane in it because of the fuel prices! Stay tuned for some engine rebuilding and it wont be under warranty because they can get away with it easy. Intresting way for them to make money

[Mr DOHC]

oh, hang on, it also said it run a 13.4, they got the details for the BMW froim the next article and this one mixed up

[JustCallMeOrlando]

Haha, yeah I noticed that too, it's 8.8 and 320Nm.